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Best way to raise and lower a plunger

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  • maxsthekat
    Hi all, I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air
    Message 1 of 18 , Oct 20 10:49 PM
      Hi all,

      I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold.

      Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this?

      Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts?

      Thanks!

      -Sean

      *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.
    • Max Cato
      Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)
      Message 2 of 18 , Oct 20 11:09 PM
        Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)


        Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)


        From: maxsthekat <maxsthekat@...>
        To: SeattleRobotics@yahoogroups.com
        Sent: Saturday, October 20, 2012 10:49 PM
        Subject: [SeattleRobotics] Best way to raise and lower a plunger

         
        Hi all,

        I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold.

        Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this?

        Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts?

        Thanks!

        -Sean

        *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.



      • Peter Balch
        How accurate does it need to be when it delivers its gunk? Does it need to suck too? I d go for a lead-screw. If you google syringe pump lead screw and click
        Message 3 of 18 , Oct 21 9:04 AM
          How accurate does it need to be when it delivers its gunk?

          Does it need to suck too?

          I'd go for a lead-screw.

          If you google "syringe pump lead screw" and click images, you get some
          expensive versions of what you want.

          I'd go for a really cheap version. Any big DIY store should sell M4 or M5
          threaded rod which will work well enough as a lead screw..

          Imagine a cage like this one
          http://puzzlerookie.blogspot.co.uk/2012/08/lion-in-cage-puzzle.html
          but with only 4 bars - one at each corner. The lead screw passes through a
          nut in the floor of the cage. It somehow pushes the syringe.

          The top of the lead screw is attached to the axle of a gearmotor. You could
          use a servo that has been modified for continuous rotation. Or a cheap one
          like this
          http://i00.i.aliimg.com/photo/228157753/Gear_Motor_dc_motor_robot_toys_toys_motor.jpg

          The gearmotor is free to move up/down along the bars of the cage but can't
          rotate and can't move far from side-to-side.

          The motor turn the lead screw. The lead screw screws itself through the
          floor.

          It's the sort of thing you could make with hand-tools in an afternoon.

          OR

          Get a hollow cylinder about the same diameter and twice the length of the
          syringe plunger. Glue (?) it onto the top of the syringe plunger. Fix a M4
          nut onto the upper end of the cylinder (probably use a wingnut). I'd go into
          a DIY store and look at the plumbing fittings.

          Thread the lead screw into the cylinder.

          Attach the top of the lead screw to the axle of the gearmotor.

          Fix the body of the gearmotor and the body of the syringe to a piece of
          wood.

          As the gearmotor turns, the leadscrew unscrews and forces the plunger into
          the syringe. Or pulls the plunger out.


          I think the second design is easier to build but won't work as well. The
          problem is that the force pushing the plunger is taken by the bearings of
          the gearmotor. And a cheap gearmotor won't be designed to take an axial
          force like that.

          Peter
        • robotMaker
          I ve done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam. Regarding the mold making and castings, I do the
          Message 4 of 18 , Oct 21 11:31 AM
            I've done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam.

            Regarding the mold making and castings, I do the same. I keep the cost low by using, caulking silicone to create my molds. Very cheap stuff and cures in an hour. For my casting material I use, 15 minute, 20 minute and 30 minute epoxy resin and other resins. I usually make gears and other small parts. When I make my molds I do not have any problems with air bubbles, but I do with the casting, regardless of what kind of resin I use.  I built a small vacuum chamber using a pickle jar, and an auto vacuum test pump. But it hardly helps removing the trapped air in the mold. So when the situation is right, I use other tricks to get rid of the air bubble deformities out of the part.

            This is what I started with to make the silicone molds, and refined the process. I also use another simple trick, soap and water on the silicone, and this works just fine for me, better than the paint thinner.

            http://mechanicalmashup.tv/?woo_video=episode-26-how-to-make-a-silicone-mold-in-your-kitchen-on-the-cheap

            Cesar


            From: Max Cato <maxsthekat@...>
            To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
            Sent: Sunday, October 21, 2012 1:09 AM
            Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



            Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)

            http://www.seattlerobotics.org/encoder/sep97/motors.html

            Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)


            From: maxsthekat <maxsthekat@...>
            To: SeattleRobotics@yahoogroups.com
            Sent: Saturday, October 20, 2012 10:49 PM
            Subject: [SeattleRobotics] Best way to raise and lower a plunger

             
            Hi all,

            I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold.

            Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this?

            Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts?

            Thanks!

            -Sean

            *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.







          • K Maxon
              Hello Cesar & Sean,   I cast parts in Polyurethane plastic, RTV urethane rubber and silicone rubber and haven t had any problems with air bubbles in the
            Message 5 of 18 , Oct 22 1:55 AM
               
              Hello Cesar & Sean,
               
              I cast parts in Polyurethane plastic, RTV urethane rubber and silicone rubber and haven't had any problems with air bubbles in the cosmetic / surface finish of a part unless there is a particularly rough spot in the mold.  If you have very rough spots in the mold condusive to trapping pinpoint air bubbles, a extra hit of mold release seems to do the trick well.  Another way to release them in all three mediums is to use the much longer cures versions (24+hrs) for each.  The air bubbles are released by 45seconds of vacuum followed by vigorous agitation and another 45 seconds of vacuum.  I use extremely low vacuum pressure (only a couple pounds below atmospheric pressure) provided by a venturi hooked to the air compressor and a plastic bucket.  The reason to use the longer cure versions of each material, is that upon inital mix and during its pot/working life it has a much lower viscosity which is condusive to releasing air bubbles.   Once the air is release, cure the part in a heated box (I use a rubber maid storage box) at 140F which greatly accelerates the cure time.  For heating I use a set of 250W power resistors and a DC power supply.  Not high tech but it gets the job done and makes reasonably good looking parts without pinpoint voids from air bubbles.
               
              I'm not sure that is exactly an answer to the question being asked, possibly it is.  Hope this helps... 
              -Kenneth


              From: robotMaker <robotmeiker@...>
              To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
              Sent: Sunday, October 21, 2012 11:31 AM
              Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



              I've done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam.

              Regarding the mold making and castings, I do the same. I keep the cost low by using, caulking silicone to create my molds. Very cheap stuff and cures in an hour. For my casting material I use, 15 minute, 20 minute and 30 minute epoxy resin and other resins. I usually make gears and other small parts. When I make my molds I do not have any problems with air bubbles, but I do with the casting, regardless of what kind of resin I use.  I built a small vacuum chamber using a pickle jar, and an auto vacuum test pump. But it hardly helps removing the trapped air in the mold. So when the situation is right, I use other tricks to get rid of the air bubble deformities out of the part.

              This is what I started with to make the silicone molds, and refined the process. I also use another simple trick, soap and water on the silicone, and this works just fine for me, better than the paint thinner.

              http://mechanicalmashup.tv/?woo_video=episode-26-how-to-make-a-silicone-mold-in-your-kitchen-on-the-cheap

              Cesar

              From: Max Cato <maxsthekat@...>
              To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
              Sent: Sunday, October 21, 2012 1:09 AM
              Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



              Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)

              http://www.seattlerobotics.org/encoder/sep97/motors.html

              Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)

              From: maxsthekat <maxsthekat@...>
              To: SeattleRobotics@yahoogroups.com
              Sent: Saturday, October 20, 2012 10:49 PM
              Subject: [SeattleRobotics] Best way to raise and lower a plunger

               
              Hi all,

              I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold.

              Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this?

              Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts?

              Thanks!

              -Sean

              *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.











            • Peter Balch
              ... Just plain epoxy glue or metal-powder-filled epoxy? How useful are the gears? What size? Peter
              Message 6 of 18 , Oct 22 4:54 AM
                > For my casting material I use, 15 minute, 20 minute and 30 minute
                > epoxy resin and other resins. I usually make gears and other small parts.

                Just plain epoxy glue or metal-powder-filled epoxy?

                How useful are the gears?

                What size?

                Peter
              • robotMaker
                Peter: Just plain epoxy glue or metal-powder-filled epoxy? Just plain epoxy (two part), the kind that one can buy at anytime anywhere. I use the quick setting
                Message 7 of 18 , Oct 22 1:05 PM
                  Peter:
                  Just plain epoxy glue or metal-powder-filled epoxy?

                  Just plain epoxy (two part), the kind that one can buy at anytime anywhere. I use the quick setting when I just want the part now, and I use the longer curing epoxy resin when I want it to be free from air bubbles. My molds are simple and the longer setting epoxy resin lets the bubbles rise to the top. I also use other resins. I tried one that is called Everlasting Elegance, and it takes a few days for it to cure, and the piece is completely air bubble free. It looks like glass and it's tough.

                  How useful are the gears?

                  The gears are useful for me to use on my bots, nothing commercial. The gears are as good as the original. I can drill them and file or modify them as needed. If I were to use them for commercial use, I would use Everlasting Elegance.

                  What size?

                  I've been using 32 and 48 pitch. I'm currently preparing to make a mold of this one:

                  http://buy.traxxas.com/advanced_search_result.php?partnum=1884&partsearch.x=0&partsearch.y=0&partsearch=partsearch

                  this it the largest one that I've made:

                  http://buy.traxxas.com/advanced_search_result.php?osCsid=sg2gg1pf1pbauvbqg6gga5h535&partnum=4472&partsearch.x=0&partsearch.y=0&partsearch=partsearch

                  this is the smallest one:

                  http://buy.traxxas.com/product_info.php?products_id=3897

                  and this one is the smallest one, from a servo, I have not as yet 'used' the casting from this mold. Still experimenting, since my resin casting piece comes out too soft.

                  http://www.globalhobby.com/public/gallery/444950.asp

                  Cesar



                  From: Peter Balch <peterbalch@...>
                  To: SeattleRobotics@yahoogroups.com
                  Sent: Monday, October 22, 2012 6:54 AM
                  Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger

                  > For my casting material I use, 15 minute, 20 minute and 30 minute
                  > epoxy resin and other resins. I usually make gears and other small parts.

                  Just plain epoxy glue or metal-powder-filled epoxy?

                  How useful are the gears?

                  What size?

                  Peter





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                • Peter Balch
                  Silicone stays rigid enough to keep the gear circular? Interesting. ... Imperial presumably. So 48 pitch means the teeth are around 1.7mm apart. That s quite
                  Message 8 of 18 , Oct 22 3:38 PM
                    Silicone stays rigid enough to keep the gear circular? Interesting.

                    > I've been using 32 and 48 pitch.

                    Imperial presumably.

                    So 48 pitch means the teeth are around 1.7mm apart. That's quite fine.

                    > http://www.globalhobby.com/public/gallery/444950.asp

                    That looks even finer.

                    What about the set-screws? Do you have to include a metal bush?

                    Peter
                  • Max Cato
                    Kenneth,   What is the smallest part you ve casted using polyurethane? I ask becuse I am trying to use this to create 28mm scale miniatures for board games,
                    Message 9 of 18 , Oct 22 5:07 PM
                      Kenneth,
                       
                      What is the smallest part you've casted using polyurethane? I ask becuse I am trying to use this to create 28mm scale miniatures for board games, which have details as fine as 0.1mm. I typically use the OOMOO 30 stuff from Smooth-On, and the medium setting (10 min) urethane they offer.
                       
                      I figured that if the model has too many defects caused by air pockets, then I can simply evacuate all of the air! (Insert maniacal laugh). I'll have to give your method a try. I'm not quite up to the level where I can build an injection mold machine from scratch like you can ;)
                       
                      -Sean

                      From: K Maxon <k_maxon23@...>
                      To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                      Sent: Monday, October 22, 2012 1:55 AM
                      Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                       
                       
                      Hello Cesar & Sean,
                       
                      I cast parts in Polyurethane plastic, RTV urethane rubber and silicone rubber and haven't had any problems with air bubbles in the cosmetic / surface finish of a part unless there is a particularly rough spot in the mold.  If you have very rough spots in the mold condusive to trapping pinpoint air bubbles, a extra hit of mold release seems to do the trick well.  Another way to release them in all three mediums is to use the much longer cures versions (24+hrs) for each.  The air bubbles are released by 45seconds of vacuum followed by vigorous agitation and another 45 seconds of vacuum.  I use extremely low vacuum pressure (only a couple pounds below atmospheric pressure) provided by a venturi hooked to the air compressor and a plastic bucket.  The reason to use the longer cure versions of each material, is that upon inital mix and during its pot/working life it has a much lower viscosity which is condusive to releasing air bubbles.   Once the air is release, cure the part in a heated box (I use a rubber maid storage box) at 140F which greatly accelerates the cure time.  For heating I use a set of 250W power resistors and a DC power supply.  Not high tech but it gets the job done and makes reasonably good looking parts without pinpoint voids from air bubbles.
                       
                      I'm not sure that is exactly an answer to the question being asked, possibly it is.  Hope this helps... 
                      -Kenneth


                      From: robotMaker <robotmeiker@...>
                      To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                      Sent: Sunday, October 21, 2012 11:31 AM
                      Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                      I've done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam. Regarding the mold making and castings, I do the same. I keep the cost low by using, caulking silicone to create my molds. Very cheap stuff and cures in an hour. For my casting material I use, 15 minute, 20 minute and 30 minute epoxy resin and other resins. I usually make gears and other small parts. When I make my molds I do not have any problems with air bubbles, but I do with the casting, regardless of what kind of resin I use.  I built a small vacuum chamber using a pickle jar, and an auto vacuum test pump. But it hardly helps removing the trapped air in the mold. So when the situation is right, I use other tricks to get rid of the air bubble deformities out of the part.

                      This is what I started with to make the silicone molds, and refined the process. I also use another simple trick, soap and water on the silicone, and this works just fine for me, better than the paint thinner.

                      http://mechanicalmashup.tv/?woo_video=episode-26-how-to-make-a-silicone-mold-in-your-kitchen-on-the-cheap

                      Cesar

                      From: Max Cato <maxsthekat@...>
                      To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                      Sent: Sunday, October 21, 2012 1:09 AM
                      Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                      Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)

                      http://www.seattlerobotics.org/encoder/sep97/motors.html

                      Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)

                      From: maxsthekat <maxsthekat@...>
                      To: SeattleRobotics@yahoogroups.com
                      Sent: Saturday, October 20, 2012 10:49 PM
                      Subject: [SeattleRobotics] Best way to raise and lower a plunger
                       
                      Hi all, I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold. Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this? Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts? Thanks! -Sean *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.
                    • Max Cato
                      Peter,   Silicone is used to form the mold. The actual cast (the piece you use) is made typically of a polymer (polyurethane, epoxy, etc). Here s a good video
                      Message 10 of 18 , Oct 22 5:22 PM
                        Peter,
                         
                        Silicone is used to form the mold. The actual cast (the piece you use) is made typically of a polymer (polyurethane, epoxy, etc). Here's a good video showing the process: http://www.youtube.com/watch?v=FQ1A7ZjTsx8
                         
                        -Sean

                        From: Peter Balch <peterbalch@...>
                        To: SeattleRobotics@yahoogroups.com
                        Sent: Monday, October 22, 2012 3:38 PM
                        Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger for silicone casting
                         
                        Silicone stays rigid enough to keep the gear circular? Interesting.

                        > I've been using 32 and 48 pitch.

                        Imperial presumably.

                        So 48 pitch means the teeth are around 1.7mm apart. That's quite fine.

                        > http://www.globalhobby.com/public/gallery/444950.asp

                        That looks even finer.

                        What about the set-screws? Do you have to include a metal bush?

                        Peter

                      • K Maxon
                          Hello Sean,   I am wondering if that was a type-O?  0.1mm is about 4 thousandths of an inch.  In either epoxy or polyurethane a 0.004-in thick feature
                        Message 11 of 18 , Oct 22 6:35 PM

                           
                          Hello Sean,
                           
                          I am wondering if that was a type-O?  0.1mm is about 4 thousandths of an inch.  In either epoxy or polyurethane a 0.004-in thick feature breaks off with the brush of a finger if you can keep  if it is even possible to get it out of the mold.   Possibly I am missunderstanding and you are trying to imprint textures of this size, which will show up in the final part.
                           
                          I have cast parts that come down to a "knife edge" to the best that I can push the material without vacuum assist.  The part shown in the photo below is magnified many, many times.  For a sense of scale those are #4 screws...  I rarely take the time to vacuum degas the material.  Only if I have a highly visible shiny part in black rubber.  In plastic, or with colored parts or matt-finishes, it tends to be not worth the extra time and work as the process in most molds tends to not leave bubbles in the first place.  (low viscosity - slow cure materials.)
                           
                          As a hopefully helpful debugging step I would offer up that if you are getting significant volume of air bubles in your part (not just tiny cosmetic pin points here and there stuck to the surface.)  Then you might want to review your process and materials.  On the process side, evaluate how you are mixing the materials that might be allowing air bubbles to become trapped in the material in the first place.  On the materials side, check that you are using compatible mold releases and cleaning agents.  The air bubbles might not be air in the conventional sense at all, but a side effect of a liquid detergent, used for mold release, interactiving with the chemical processes of curing the plastic and forming gas bubbles.  Lastly if you are embedding objects in your parts for strength or other (plastics, rubber, wood, etc...) It is important to know their composition, as they will impart a change to the reaction during cure and can also contribute to out-gassing / bubbles.
                           
                          -Kenneth
                           
                           

                          From: Max Cato <maxsthekat@...>
                          To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                          Sent: Monday, October 22, 2012 5:07 PM
                          Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                          Kenneth,
                           
                          What is the smallest part you've casted using polyurethane? I ask becuse I am trying to use this to create 28mm scale miniatures for board games, which have details as fine as 0.1mm. I typically use the OOMOO 30 stuff from Smooth-On, and the medium setting (10 min) urethane they offer.
                           
                          I figured that if the model has too many defects caused by air pockets, then I can simply evacuate all of the air! (Insert maniacal laugh). I'll have to give your method a try. I'm not quite up to the level where I can build an injection mold machine from scratch like you can ;)
                           
                          -Sean

                          From: K Maxon <k_maxon23@...>
                          To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                          Sent: Monday, October 22, 2012 1:55 AM
                          Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                           
                           
                          Hello Cesar & Sean,
                           
                          I cast parts in Polyurethane plastic, RTV urethane rubber and silicone rubber and haven't had any problems with air bubbles in the cosmetic / surface finish of a part unless there is a particularly rough spot in the mold.  If you have very rough spots in the mold condusive to trapping pinpoint air bubbles, a extra hit of mold release seems to do the trick well.  Another way to release them in all three mediums is to use the much longer cures versions (24+hrs) for each.  The air bubbles are released by 45seconds of vacuum followed by vigorous agitation and another 45 seconds of vacuum.  I use extremely low vacuum pressure (only a couple pounds below atmospheric pressure) provided by a venturi hooked to the air compressor and a plastic bucket.  The reason to use the longer cure versions of each material, is that upon inital mix and during its pot/working life it has a much lower viscosity which is condusive to releasing air bubbles.   Once the air is release, cure the part in a heated box (I use a rubber maid storage box) at 140F which greatly accelerates the cure time.  For heating I use a set of 250W power resistors and a DC power supply.  Not high tech but it gets the job done and makes reasonably good looking parts without pinpoint voids from air bubbles.
                           
                          I'm not sure that is exactly an answer to the question being asked, possibly it is.  Hope this helps... 
                          -Kenneth


                          From: robotMaker <robotmeiker@...>
                          To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                          Sent: Sunday, October 21, 2012 11:31 AM
                          Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                          I've done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam. Regarding the mold making and castings, I do the same. I keep the cost low by using, caulking silicone to create my molds. Very cheap stuff and cures in an hour. For my casting material I use, 15 minute, 20 minute and 30 minute epoxy resin and other resins. I usually make gears and other small parts. When I make my molds I do not have any problems with air bubbles, but I do with the casting, regardless of what kind of resin I use.  I built a small vacuum chamber using a pickle jar, and an auto vacuum test pump. But it hardly helps removing the trapped air in the mold. So when the situation is right, I use other tricks to get rid of the air bubble deformities out of the part.

                          This is what I started with to make the silicone molds, and refined the process. I also use another simple trick, soap and water on the silicone, and this works just fine for me, better than the paint thinner.

                          http://mechanicalmashup.tv/?woo_video=episode-26-how-to-make-a-silicone-mold-in-your-kitchen-on-the-cheap

                          Cesar

                          From: Max Cato <maxsthekat@...>
                          To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                          Sent: Sunday, October 21, 2012 1:09 AM
                          Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                          Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)

                          http://www.seattlerobotics.org/encoder/sep97/motors.html

                          Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)

                          From: maxsthekat <maxsthekat@...>
                          To: SeattleRobotics@yahoogroups.com
                          Sent: Saturday, October 20, 2012 10:49 PM
                          Subject: [SeattleRobotics] Best way to raise and lower a plunger
                           
                          Hi all, I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold. Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this? Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts? Thanks! -Sean *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.




                        • Max Cato
                          Yes, you are correct-- they are textures. I make miniatures very akin to
                          Message 12 of 18 , Oct 22 7:17 PM
                            Yes, you are correct-- they are textures. I make miniatures very akin to these: http://www.games-workshop.com/MEDIA_CustomProductCatalog/m1252489_99120101034_SMAssaultSqdmain_873x627.jpg (each one of those guys, minus the base they are on, are approximately 28mm tall). Try not to laugh-- it's a fun little hobby :) The detail on certain items (such as the eagle's crest under the straps on the one in the middle or the where the legs meet the torsos on all of them) can get very detailed. (Note: these are multi-part figures-- they certainly don't come as one piece.) Typically, you make one very good master mold, create a few models from that, and they create production molds.

                            In addition, each of the parts is very "organically" (irregularly) shaped. So, this creates all sorts of places in the mold where air can become trapped during a gravity feed. I've tried cutting more vents in strategic places, but I tend to end up with voids, no matter what I do. So, I figured it's come down to either getting an injection mold machine setup, akin to how the commercial companies do, or try to simply do my gravity feed in the absence of air. No air would mean no voids possible, right? :) I'm not certain if the polyurethane will simply vaporize, but I figured it might be worth a shot.

                            But, you are certainly correct, and a review of my process is warranted. I will definitely look into ways to get any bubbles/voids removed from each cast... You and Cesar both have very good results, and perhaps this can be done as a gravity feed. 

                            -Sean


                            From: K Maxon <k_maxon23@...>
                            To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                            Sent: Monday, October 22, 2012 6:35 PM
                            Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger

                             

                             
                            Hello Sean,
                             
                            I am wondering if that was a type-O?  0.1mm is about 4 thousandths of an inch.  In either epoxy or polyurethane a 0.004-in thick feature breaks off with the brush of a finger if you can keep  if it is even possible to get it out of the mold.   Possibly I am missunderstanding and you are trying to imprint textures of this size, which will show up in the final part.
                             
                            I have cast parts that come down to a "knife edge" to the best that I can push the material without vacuum assist.  The part shown in the photo below is magnified many, many times.  For a sense of scale those are #4 screws...  I rarely take the time to vacuum degas the material.  Only if I have a highly visible shiny part in black rubber.  In plastic, or with colored parts or matt-finishes, it tends to be not worth the extra time and work as the process in most molds tends to not leave bubbles in the first place.  (low viscosity - slow cure materials.)
                            http://nikita.argia.net/kmaxon/q40_132_0021273.jpg
                             
                            As a hopefully helpful debugging step I would offer up that if you are getting significant volume of air bubles in your part (not just tiny cosmetic pin points here and there stuck to the surface.)  Then you might want to review your process and materials.  On the process side, evaluate how you are mixing the materials that might be allowing air bubbles to become trapped in the material in the first place.  On the materials side, check that you are using compatible mold releases and cleaning agents.  The air bubbles might not be air in the conventional sense at all, but a side effect of a liquid detergent, used for mold release, interactiving with the chemical processes of curing the plastic and forming gas bubbles.  Lastly if you are embedding objects in your parts for strength or other (plastics, rubber, wood, etc...) It is important to know their composition, as they will impart a change to the reaction during cure and can also contribute to out-gassing / bubbles.
                             
                            -Kenneth
                             
                             

                            From: Max Cato <maxsthekat@...>
                            To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                            Sent: Monday, October 22, 2012 5:07 PM
                            Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                            Kenneth,
                             
                            What is the smallest part you've casted using polyurethane? I ask becuse I am trying to use this to create 28mm scale miniatures for board games, which have details as fine as 0.1mm. I typically use the OOMOO 30 stuff from Smooth-On, and the medium setting (10 min) urethane they offer.
                             
                            I figured that if the model has too many defects caused by air pockets, then I can simply evacuate all of the air! (Insert maniacal laugh). I'll have to give your method a try. I'm not quite up to the level where I can build an injection mold machine from scratch like you can ;)
                             
                            -Sean

                            From: K Maxon <k_maxon23@...>
                            To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                            Sent: Monday, October 22, 2012 1:55 AM
                            Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                             
                             
                            Hello Cesar & Sean,
                             
                            I cast parts in Polyurethane plastic, RTV urethane rubber and silicone rubber and haven't had any problems with air bubbles in the cosmetic / surface finish of a part unless there is a particularly rough spot in the mold.  If you have very rough spots in the mold condusive to trapping pinpoint air bubbles, a extra hit of mold release seems to do the trick well.  Another way to release them in all three mediums is to use the much longer cures versions (24+hrs) for each.  The air bubbles are released by 45seconds of vacuum followed by vigorous agitation and another 45 seconds of vacuum.  I use extremely low vacuum pressure (only a couple pounds below atmospheric pressure) provided by a venturi hooked to the air compressor and a plastic bucket.  The reason to use the longer cure versions of each material, is that upon inital mix and during its pot/working life it has a much lower viscosity which is condusive to releasing air bubbles.   Once the air is release, cure the part in a heated box (I use a rubber maid storage box) at 140F which greatly accelerates the cure time.  For heating I use a set of 250W power resistors and a DC power supply.  Not high tech but it gets the job done and makes reasonably good looking parts without pinpoint voids from air bubbles.
                             
                            I'm not sure that is exactly an answer to the question being asked, possibly it is.  Hope this helps... 
                            -Kenneth


                            From: robotMaker <robotmeiker@...>
                            To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                            Sent: Sunday, October 21, 2012 11:31 AM
                            Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                            I've done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam. Regarding the mold making and castings, I do the same. I keep the cost low by using, caulking silicone to create my molds. Very cheap stuff and cures in an hour. For my casting material I use, 15 minute, 20 minute and 30 minute epoxy resin and other resins. I usually make gears and other small parts. When I make my molds I do not have any problems with air bubbles, but I do with the casting, regardless of what kind of resin I use.  I built a small vacuum chamber using a pickle jar, and an auto vacuum test pump. But it hardly helps removing the trapped air in the mold. So when the situation is right, I use other tricks to get rid of the air bubble deformities out of the part.

                            This is what I started with to make the silicone molds, and refined the process. I also use another simple trick, soap and water on the silicone, and this works just fine for me, better than the paint thinner.

                            http://mechanicalmashup.tv/?woo_video=episode-26-how-to-make-a-silicone-mold-in-your-kitchen-on-the-cheap

                            Cesar

                            From: Max Cato <maxsthekat@...>
                            To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                            Sent: Sunday, October 21, 2012 1:09 AM
                            Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                            Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)

                            http://www.seattlerobotics.org/encoder/sep97/motors.html

                            Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)

                            From: maxsthekat <maxsthekat@...>
                            To: SeattleRobotics@yahoogroups.com
                            Sent: Saturday, October 20, 2012 10:49 PM
                            Subject: [SeattleRobotics] Best way to raise and lower a plunger
                             
                            Hi all, I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold. Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this? Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts? Thanks! -Sean *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.






                          • K Maxon
                              Hello Sean,   Thank you for taking a minute to explain.  I think I understand just a bit better the problem you are facing.   If I take a minute to state
                            Message 13 of 18 , Oct 22 7:41 PM
                               
                              Hello Sean,
                               
                              Thank you for taking a minute to explain.  I think I understand just a bit better the problem you are facing.
                               
                              If I take a minute to state a hypothetical, it may nail it down for me...   I believe this would be represented by attempting to cast a part in the shape of a capital letter "J" by gravity feeding material down the stem.  If one were using vacuum evacuation to get the air out of the "toe" then a little bit of material will easily work itself up into that area as long as there is at least the tiniest set of channels for the vaccum to evacuate the air there.
                               
                              The challenge that you appear to be facing is not so much working against the forces of gravity, but because the parts you are working on are so small and that there is such a small volume (weight) of material pushing in, that surface tension is becoming a significant contributing force factor working against the gravity feed.  (Lower viscosity material would help this along with degasing.) 
                               
                              To offset this, you are then envisioning using the plunger / injector to physically push the material into the tip / toe of the letter "J" shape and use the material pushing back through the cavity to evacuate the air.  Reasonably, that sounds like it may work as well.
                               
                              -Kenneth


                              From: Max Cato <maxsthekat@...>
                              To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                              Sent: Monday, October 22, 2012 7:17 PM
                              Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                              Yes, you are correct-- they are textures. I make miniatures very akin to these: http://www.games-workshop.com/MEDIA_CustomProductCatalog/m1252489_99120101034_SMAssaultSqdmain_873x627.jpg (each one of those guys, minus the base they are on, are approximately 28mm tall). Try not to laugh-- it's a fun little hobby :) The detail on certain items (such as the eagle's crest under the straps on the one in the middle or the where the legs meet the torsos on all of them) can get very detailed. (Note: these are multi-part figures-- they certainly don't come as one piece.) Typically, you make one very good master mold, create a few models from that, and they create production molds.

                              In addition, each of the parts is very "organically" (irregularly) shaped. So, this creates all sorts of places in the mold where air can become trapped during a gravity feed. I've tried cutting more vents in strategic places, but I tend to end up with voids, no matter what I do. So, I figured it's come down to either getting an injection mold machine setup, akin to how the commercial companies do, or try to simply do my gravity feed in the absence of air. No air would mean no voids possible, right? :) I'm not certain if the polyurethane will simply vaporize, but I figured it might be worth a shot.

                              But, you are certainly correct, and a review of my process is warranted. I will definitely look into ways to get any bubbles/voids removed from each cast... You and Cesar both have very good results, and perhaps this can be done as a gravity feed. 

                              -Sean

                              From: K Maxon <k_maxon23@...>
                              To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                              Sent: Monday, October 22, 2012 6:35 PM
                              Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger

                               

                               
                              Hello Sean,
                               
                              I am wondering if that was a type-O?  0.1mm is about 4 thousandths of an inch.  In either epoxy or polyurethane a 0.004-in thick feature breaks off with the brush of a finger if you can keep  if it is even possible to get it out of the mold.   Possibly I am missunderstanding and you are trying to imprint textures of this size, which will show up in the final part.
                               
                              I have cast parts that come down to a "knife edge" to the best that I can push the material without vacuum assist.  The part shown in the photo below is magnified many, many times.  For a sense of scale those are #4 screws...  I rarely take the time to vacuum degas the material.  Only if I have a highly visible shiny part in black rubber.  In plastic, or with colored parts or matt-finishes, it tends to be not worth the extra time and work as the process in most molds tends to not leave bubbles in the first place.  (low viscosity - slow cure materials.)
                              http://nikita.argia.net/kmaxon/q40_132_0021273.jpg
                               
                              As a hopefully helpful debugging step I would offer up that if you are getting significant volume of air bubles in your part (not just tiny cosmetic pin points here and there stuck to the surface.)  Then you might want to review your process and materials.  On the process side, evaluate how you are mixing the materials that might be allowing air bubbles to become trapped in the material in the first place.  On the materials side, check that you are using compatible mold releases and cleaning agents.  The air bubbles might not be air in the conventional sense at all, but a side effect of a liquid detergent, used for mold release, interactiving with the chemical processes of curing the plastic and forming gas bubbles.  Lastly if you are embedding objects in your parts for strength or other (plastics, rubber, wood, etc...) It is important to know their composition, as they will impart a change to the reaction during cure and can also contribute to out-gassing / bubbles.
                               
                              -Kenneth
                               
                               

                              From: Max Cato <maxsthekat@...>
                              To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                              Sent: Monday, October 22, 2012 5:07 PM
                              Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                              Kenneth,
                               
                              What is the smallest part you've casted using polyurethane? I ask becuse I am trying to use this to create 28mm scale miniatures for board games, which have details as fine as 0.1mm. I typically use the OOMOO 30 stuff from Smooth-On, and the medium setting (10 min) urethane they offer.
                               
                              I figured that if the model has too many defects caused by air pockets, then I can simply evacuate all of the air! (Insert maniacal laugh). I'll have to give your method a try. I'm not quite up to the level where I can build an injection mold machine from scratch like you can ;)
                               
                              -Sean

                              From: K Maxon <k_maxon23@...>
                              To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                              Sent: Monday, October 22, 2012 1:55 AM
                              Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                               
                               
                              Hello Cesar & Sean,
                               
                              I cast parts in Polyurethane plastic, RTV urethane rubber and silicone rubber and haven't had any problems with air bubbles in the cosmetic / surface finish of a part unless there is a particularly rough spot in the mold.  If you have very rough spots in the mold condusive to trapping pinpoint air bubbles, a extra hit of mold release seems to do the trick well.  Another way to release them in all three mediums is to use the much longer cures versions (24+hrs) for each.  The air bubbles are released by 45seconds of vacuum followed by vigorous agitation and another 45 seconds of vacuum.  I use extremely low vacuum pressure (only a couple pounds below atmospheric pressure) provided by a venturi hooked to the air compressor and a plastic bucket.  The reason to use the longer cure versions of each material, is that upon inital mix and during its pot/working life it has a much lower viscosity which is condusive to releasing air bubbles.   Once the air is release, cure the part in a heated box (I use a rubber maid storage box) at 140F which greatly accelerates the cure time.  For heating I use a set of 250W power resistors and a DC power supply.  Not high tech but it gets the job done and makes reasonably good looking parts without pinpoint voids from air bubbles.
                               
                              I'm not sure that is exactly an answer to the question being asked, possibly it is.  Hope this helps... 
                              -Kenneth


                              From: robotMaker <robotmeiker@...>
                              To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                              Sent: Sunday, October 21, 2012 11:31 AM
                              Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                              I've done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam. Regarding the mold making and castings, I do the same. I keep the cost low by using, caulking silicone to create my molds. Very cheap stuff and cures in an hour. For my casting material I use, 15 minute, 20 minute and 30 minute epoxy resin and other resins. I usually make gears and other small parts. When I make my molds I do not have any problems with air bubbles, but I do with the casting, regardless of what kind of resin I use.  I built a small vacuum chamber using a pickle jar, and an auto vacuum test pump. But it hardly helps removing the trapped air in the mold. So when the situation is right, I use other tricks to get rid of the air bubble deformities out of the part.

                              This is what I started with to make the silicone molds, and refined the process. I also use another simple trick, soap and water on the silicone, and this works just fine for me, better than the paint thinner.

                              http://mechanicalmashup.tv/?woo_video=episode-26-how-to-make-a-silicone-mold-in-your-kitchen-on-the-cheap

                              Cesar

                              From: Max Cato <maxsthekat@...>
                              To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                              Sent: Sunday, October 21, 2012 1:09 AM
                              Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                              Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)

                              http://www.seattlerobotics.org/encoder/sep97/motors.html

                              Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)

                              From: maxsthekat <maxsthekat@...>
                              To: SeattleRobotics@yahoogroups.com
                              Sent: Saturday, October 20, 2012 10:49 PM
                              Subject: [SeattleRobotics] Best way to raise and lower a plunger
                               
                              Hi all, I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold. Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this? Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts? Thanks! -Sean *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.










                            • Max Cato
                              Hi Kenneth, Yes, you ve hit the nail right on the head. I ve tried to find the lowest viscosity I can with the right properties for the hardness of the
                              Message 14 of 18 , Oct 22 8:32 PM
                                Hi Kenneth,

                                Yes, you've hit the nail right on the head. I've tried to find the lowest viscosity I can with the right properties for the hardness of the urethane. So far, I've found the Smooth Cast 300 from Smooth-On is about as low as I can find (80 cps) http://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1209/index.html 

                                That particular urethane works pretty well, but like you said, I need to fill the toe of the "J". So, the machine I'm envisioning has a twofold purpose: 

                                1) Push the urethane along to fill cavities. This would be a gentle push (very scientific terms here), since too much force will easily deform the finer parts within the mold.
                                2) Be my "hand" on the plunger, since I can't operate on a mold while it is in a vacuum. 

                                So, I need to set about making this! Commercial vacuum chambers are hideously expensive, so I might try the "cheap" route for my first setup and build one out of PVC and acrylic plexiglass for a "window". I'm not sure how great of a vacuum this will yield, and hopefully, it won't implode, either. Then, it's time to build the injector, which, given the plans on the website for a leadscrew based method, should be straightforward. I'm fairly proficient on Arduinos, and have a couple of XBee modules laying around I plan on using for wireless communication.

                                My next step up from this will be to obtain one of the "hobbyist" size injection mold machines I've seen for around $3k. These aren't anything nearly as fancy as yours, but they should get the job done for a 2oz shot of polystyrene. I would love to eventually build a fully robotic machine such as yours, but that remains far off for the moment.

                                If I may ask a question though, one thing I didn't see on your website was how you are injecting the plastic into the cavity. As I understand it, most commercial machines use a specially tapered screw that serves not only to help force the pellets along the path to be extruded, but also acts as a ram, when hydraulic force is applied behind the screw assembly to drive the plastic into the mold cavity. Does your machine do something similar to this, or do you simply use the force from the screw's motion to force the material into the cavity? For that matter, are you using a commercial screw, or did you fabricate one of your own?

                                Thanks again for your time!

                                -Sean


                                From: K Maxon <k_maxon23@...>
                                To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                Sent: Monday, October 22, 2012 7:41 PM
                                Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger

                                 
                                 
                                Hello Sean,
                                 
                                Thank you for taking a minute to explain.  I think I understand just a bit better the problem you are facing.
                                 
                                If I take a minute to state a hypothetical, it may nail it down for me...   I believe this would be represented by attempting to cast a part in the shape of a capital letter "J" by gravity feeding material down the stem.  If one were using vacuum evacuation to get the air out of the "toe" then a little bit of material will easily work itself up into that area as long as there is at least the tiniest set of channels for the vaccum to evacuate the air there.
                                 
                                The challenge that you appear to be facing is not so much working against the forces of gravity, but because the parts you are working on are so small and that there is such a small volume (weight) of material pushing in, that surface tension is becoming a significant contributing force factor working against the gravity feed.  (Lower viscosity material would help this along with degasing.) 
                                 
                                To offset this, you are then envisioning using the plunger / injector to physically push the material into the tip / toe of the letter "J" shape and use the material pushing back through the cavity to evacuate the air.  Reasonably, that sounds like it may work as well.
                                 
                                -Kenneth


                                From: Max Cato <maxsthekat@...>
                                To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                Sent: Monday, October 22, 2012 7:17 PM
                                Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                                Yes, you are correct-- they are textures. I make miniatures very akin to these: http://www.games-workshop.com/MEDIA_CustomProductCatalog/m1252489_99120101034_SMAssaultSqdmain_873x627.jpg (each one of those guys, minus the base they are on, are approximately 28mm tall). Try not to laugh-- it's a fun little hobby :) The detail on certain items (such as the eagle's crest under the straps on the one in the middle or the where the legs meet the torsos on all of them) can get very detailed. (Note: these are multi-part figures-- they certainly don't come as one piece.) Typically, you make one very good master mold, create a few models from that, and they create production molds.

                                In addition, each of the parts is very "organically" (irregularly) shaped. So, this creates all sorts of places in the mold where air can become trapped during a gravity feed. I've tried cutting more vents in strategic places, but I tend to end up with voids, no matter what I do. So, I figured it's come down to either getting an injection mold machine setup, akin to how the commercial companies do, or try to simply do my gravity feed in the absence of air. No air would mean no voids possible, right? :) I'm not certain if the polyurethane will simply vaporize, but I figured it might be worth a shot.

                                But, you are certainly correct, and a review of my process is warranted. I will definitely look into ways to get any bubbles/voids removed from each cast... You and Cesar both have very good results, and perhaps this can be done as a gravity feed. 

                                -Sean

                                From: K Maxon <k_maxon23@...>
                                To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                Sent: Monday, October 22, 2012 6:35 PM
                                Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger

                                 

                                 
                                Hello Sean,
                                 
                                I am wondering if that was a type-O?  0.1mm is about 4 thousandths of an inch.  In either epoxy or polyurethane a 0.004-in thick feature breaks off with the brush of a finger if you can keep  if it is even possible to get it out of the mold.   Possibly I am missunderstanding and you are trying to imprint textures of this size, which will show up in the final part.
                                 
                                I have cast parts that come down to a "knife edge" to the best that I can push the material without vacuum assist.  The part shown in the photo below is magnified many, many times.  For a sense of scale those are #4 screws...  I rarely take the time to vacuum degas the material.  Only if I have a highly visible shiny part in black rubber.  In plastic, or with colored parts or matt-finishes, it tends to be not worth the extra time and work as the process in most molds tends to not leave bubbles in the first place.  (low viscosity - slow cure materials.)
                                http://nikita.argia.net/kmaxon/q40_132_0021273.jpg
                                 
                                As a hopefully helpful debugging step I would offer up that if you are getting significant volume of air bubles in your part (not just tiny cosmetic pin points here and there stuck to the surface.)  Then you might want to review your process and materials.  On the process side, evaluate how you are mixing the materials that might be allowing air bubbles to become trapped in the material in the first place.  On the materials side, check that you are using compatible mold releases and cleaning agents.  The air bubbles might not be air in the conventional sense at all, but a side effect of a liquid detergent, used for mold release, interactiving with the chemical processes of curing the plastic and forming gas bubbles.  Lastly if you are embedding objects in your parts for strength or other (plastics, rubber, wood, etc...) It is important to know their composition, as they will impart a change to the reaction during cure and can also contribute to out-gassing / bubbles.
                                 
                                -Kenneth
                                 
                                 

                                From: Max Cato <maxsthekat@...>
                                To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                Sent: Monday, October 22, 2012 5:07 PM
                                Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                                Kenneth,
                                 
                                What is the smallest part you've casted using polyurethane? I ask becuse I am trying to use this to create 28mm scale miniatures for board games, which have details as fine as 0.1mm. I typically use the OOMOO 30 stuff from Smooth-On, and the medium setting (10 min) urethane they offer.
                                 
                                I figured that if the model has too many defects caused by air pockets, then I can simply evacuate all of the air! (Insert maniacal laugh). I'll have to give your method a try. I'm not quite up to the level where I can build an injection mold machine from scratch like you can ;)
                                 
                                -Sean

                                From: K Maxon <k_maxon23@...>
                                To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                Sent: Monday, October 22, 2012 1:55 AM
                                Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                                 
                                 
                                Hello Cesar & Sean,
                                 
                                I cast parts in Polyurethane plastic, RTV urethane rubber and silicone rubber and haven't had any problems with air bubbles in the cosmetic / surface finish of a part unless there is a particularly rough spot in the mold.  If you have very rough spots in the mold condusive to trapping pinpoint air bubbles, a extra hit of mold release seems to do the trick well.  Another way to release them in all three mediums is to use the much longer cures versions (24+hrs) for each.  The air bubbles are released by 45seconds of vacuum followed by vigorous agitation and another 45 seconds of vacuum.  I use extremely low vacuum pressure (only a couple pounds below atmospheric pressure) provided by a venturi hooked to the air compressor and a plastic bucket.  The reason to use the longer cure versions of each material, is that upon inital mix and during its pot/working life it has a much lower viscosity which is condusive to releasing air bubbles.   Once the air is release, cure the part in a heated box (I use a rubber maid storage box) at 140F which greatly accelerates the cure time.  For heating I use a set of 250W power resistors and a DC power supply.  Not high tech but it gets the job done and makes reasonably good looking parts without pinpoint voids from air bubbles.
                                 
                                I'm not sure that is exactly an answer to the question being asked, possibly it is.  Hope this helps... 
                                -Kenneth


                                From: robotMaker <robotmeiker@...>
                                To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                Sent: Sunday, October 21, 2012 11:31 AM
                                Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                                I've done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam. Regarding the mold making and castings, I do the same. I keep the cost low by using, caulking silicone to create my molds. Very cheap stuff and cures in an hour. For my casting material I use, 15 minute, 20 minute and 30 minute epoxy resin and other resins. I usually make gears and other small parts. When I make my molds I do not have any problems with air bubbles, but I do with the casting, regardless of what kind of resin I use.  I built a small vacuum chamber using a pickle jar, and an auto vacuum test pump. But it hardly helps removing the trapped air in the mold. So when the situation is right, I use other tricks to get rid of the air bubble deformities out of the part.

                                This is what I started with to make the silicone molds, and refined the process. I also use another simple trick, soap and water on the silicone, and this works just fine for me, better than the paint thinner.

                                http://mechanicalmashup.tv/?woo_video=episode-26-how-to-make-a-silicone-mold-in-your-kitchen-on-the-cheap

                                Cesar

                                From: Max Cato <maxsthekat@...>
                                To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                Sent: Sunday, October 21, 2012 1:09 AM
                                Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                                Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)

                                http://www.seattlerobotics.org/encoder/sep97/motors.html

                                Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)

                                From: maxsthekat <maxsthekat@...>
                                To: SeattleRobotics@yahoogroups.com
                                Sent: Saturday, October 20, 2012 10:49 PM
                                Subject: [SeattleRobotics] Best way to raise and lower a plunger
                                 
                                Hi all, I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold. Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this? Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts? Thanks! -Sean *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.












                              • robotMaker
                                Peter: The more silicone mass makes up the mold, the more stable it is. To be safe I put the mold in the container that I used to make it, which is rigid. What
                                Message 15 of 18 , Oct 22 9:06 PM
                                  Peter:

                                  The more silicone mass makes up the mold, the more stable it is. To be safe I put the mold in the container that I used to make it, which is rigid.

                                  What about the set-screws? Do you have to include a metal bush?

                                  So far the gears have been idler gears. But for a servo output shaft I used a gear and put a pin thru it and into the servo arm (the circular kind) and for the driven gear, I epoxied it to the brass shaft.

                                  As I said, I'm still experimenting with the servo gear mold and casting resin. The mold is fine (like the teeth), but the resin casting is coming out soft. I'm starting to suspect that the mass of the mold must be acting like a heat sink.

                                  Cesar


                                  From: Peter Balch <peterbalch@...>
                                  To: SeattleRobotics@yahoogroups.com
                                  Sent: Monday, October 22, 2012 5:38 PM
                                  Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger for silicone casting

                                  Silicone stays rigid enough to keep the gear circular? Interesting.

                                  > I've been using 32 and 48 pitch.

                                  Imperial presumably.

                                  So 48 pitch means the teeth are around 1.7mm apart. That's quite fine.

                                  > http://www.globalhobby.com/public/gallery/444950.asp

                                  That looks even finer.

                                  What about the set-screws? Do you have to include a metal bush?

                                  Peter



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                                • K Maxon
                                    Hi Sean,   Just an observation, given the density of polystyrene a 2-oz shot of that material is physically an large part compared to the size of the leg
                                  Message 16 of 18 , Oct 22 9:33 PM
                                     
                                    Hi Sean,
                                     
                                    Just an observation, given the density of polystyrene a 2-oz shot of that material is physically an large part compared to the size of the leg or body of one of these 28mm tall guys...  The part in the following photo is a 2-in diameter part and when measured before cutting the sprue off, these parts are less than a 2oz shot.  These are made from a material more dense than polystyrene and as such the same part in that material would be ratiomatically larger.
                                     
                                    Back to the issue at hand.  5-gallon home improvement general purpose buckets make great vacuum chambers when flipped upside down and set on a ring of plumbers putty.  Not as much fun to fabricate since you just buy it or salvage it at the dump but functional and fast to get things moving.
                                     
                                    One last note, I don't know if you caugth this, I certainly don't want to push if you got this the first time...  If you fill the mold (gravity feed), throw it into the vacuum for 45seconds to degas and "fill the toe" and then pull it out and top it off, this would completely remove the need for an injector.  Holding vacuum is not required for duration, only enough time is needed to degas the material qhich is pretty quick.  You may be thinking of building the injector for other parts that we aren't talking about here,  ...  just thought I'd point that out...
                                     
                                    To answer your question about the machine I built.  The 5-in diameter cylider at the top of the machine (see photo) is driving all the way through the center of the machine to drive a 2-in diameter brass piston that injects material into the mold.  During operation, only the last 1/2in of travel (out of 4-in travel) is used to lift the nozzel off of the parting line.
                                     
                                    So, ...  there are no screws involved.  Screw driven machines have been around for quite a long time now, however prior to this design improvement, early machines were piston driven.  There are a few machines still sold today that are piston driven however these don't drive the huge dollar price tags as the hundred++ ton specialty machines so they aren't the focus these days.
                                     
                                    -Kenneth


                                    From: Max Cato <maxsthekat@...>
                                    To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                    Sent: Monday, October 22, 2012 8:32 PM
                                    Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                                    Hi Kenneth,

                                    Yes, you've hit the nail right on the head. I've tried to find the lowest viscosity I can with the right properties for the hardness of the urethane. So far, I've found the Smooth Cast 300 from Smooth-On is about as low as I can find (80 cps) http://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1209/index.html 

                                    That particular urethane works pretty well, but like you said, I need to fill the toe of the "J". So, the machine I'm envisioning has a twofold purpose: 

                                    1) Push the urethane along to fill cavities. This would be a gentle push (very scientific terms here), since too much force will easily deform the finer parts within the mold.
                                    2) Be my "hand" on the plunger, since I can't operate on a mold while it is in a vacuum. 

                                    So, I need to set about making this! Commercial vacuum chambers are hideously expensive, so I might try the "cheap" route for my first setup and build one out of PVC and acrylic plexiglass for a "window". I'm not sure how great of a vacuum this will yield, and hopefully, it won't implode, either. Then, it's time to build the injector, which, given the plans on the website for a leadscrew based method, should be straightforward. I'm fairly proficient on Arduinos, and have a couple of XBee modules laying around I plan on using for wireless communication.

                                    My next step up from this will be to obtain one of the "hobbyist" size injection mold machines I've seen for around $3k. These aren't anything nearly as fancy as yours, but they should get the job done for a 2oz shot of polystyrene. I would love to eventually build a fully robotic machine such as yours, but that remains far off for the moment.

                                    If I may ask a question though, one thing I didn't see on your website was how you are injecting the plastic into the cavity. As I understand it, most commercial machines use a specially tapered screw that serves not only to help force the pellets along the path to be extruded, but also acts as a ram, when hydraulic force is applied behind the screw assembly to drive the plastic into the mold cavity. Does your machine do something similar to this, or do you simply use the force from the screw's motion to force the material into the cavity? For that matter, are you using a commercial screw, or did you fabricate one of your own?

                                    Thanks again for your time!

                                    -Sean

                                    From: K Maxon <k_maxon23@...>
                                    To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                    Sent: Monday, October 22, 2012 7:41 PM
                                    Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger

                                     
                                     
                                    Hello Sean,
                                     
                                    Thank you for taking a minute to explain.  I think I understand just a bit better the problem you are facing.
                                     
                                    If I take a minute to state a hypothetical, it may nail it down for me...   I believe this would be represented by attempting to cast a part in the shape of a capital letter "J" by gravity feeding material down the stem.  If one were using vacuum evacuation to get the air out of the "toe" then a little bit of material will easily work itself up into that area as long as there is at least the tiniest set of channels for the vaccum to evacuate the air there.
                                     
                                    The challenge that you appear to be facing is not so much working against the forces of gravity, but because the parts you are working on are so small and that there is such a small volume (weight) of material pushing in, that surface tension is becoming a significant contributing force factor working against the gravity feed.  (Lower viscosity material would help this along with degasing.) 
                                     
                                    To offset this, you are then envisioning using the plunger / injector to physically push the material into the tip / toe of the letter "J" shape and use the material pushing back through the cavity to evacuate the air.  Reasonably, that sounds like it may work as well.
                                     
                                    -Kenneth


                                    From: Max Cato <maxsthekat@...>
                                    To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                    Sent: Monday, October 22, 2012 7:17 PM
                                    Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                                    Yes, you are correct-- they are textures. I make miniatures very akin to these: http://www.games-workshop.com/MEDIA_CustomProductCatalog/m1252489_99120101034_SMAssaultSqdmain_873x627.jpg (each one of those guys, minus the base they are on, are approximately 28mm tall). Try not to laugh-- it's a fun little hobby :) The detail on certain items (such as the eagle's crest under the straps on the one in the middle or the where the legs meet the torsos on all of them) can get very detailed. (Note: these are multi-part figures-- they certainly don't come as one piece.) Typically, you make one very good master mold, create a few models from that, and they create production molds.

                                    In addition, each of the parts is very "organically" (irregularly) shaped. So, this creates all sorts of places in the mold where air can become trapped during a gravity feed. I've tried cutting more vents in strategic places, but I tend to end up with voids, no matter what I do. So, I figured it's come down to either getting an injection mold machine setup, akin to how the commercial companies do, or try to simply do my gravity feed in the absence of air. No air would mean no voids possible, right? :) I'm not certain if the polyurethane will simply vaporize, but I figured it might be worth a shot.

                                    But, you are certainly correct, and a review of my process is warranted. I will definitely look into ways to get any bubbles/voids removed from each cast... You and Cesar both have very good results, and perhaps this can be done as a gravity feed. 

                                    -Sean

                                    From: K Maxon <k_maxon23@...>
                                    To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                    Sent: Monday, October 22, 2012 6:35 PM
                                    Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger

                                     

                                     
                                    Hello Sean,
                                     
                                    I am wondering if that was a type-O?  0.1mm is about 4 thousandths of an inch.  In either epoxy or polyurethane a 0.004-in thick feature breaks off with the brush of a finger if you can keep  if it is even possible to get it out of the mold.   Possibly I am missunderstanding and you are trying to imprint textures of this size, which will show up in the final part.
                                     
                                    I have cast parts that come down to a "knife edge" to the best that I can push the material without vacuum assist.  The part shown in the photo below is magnified many, many times.  For a sense of scale those are #4 screws...  I rarely take the time to vacuum degas the material.  Only if I have a highly visible shiny part in black rubber.  In plastic, or with colored parts or matt-finishes, it tends to be not worth the extra time and work as the process in most molds tends to not leave bubbles in the first place.  (low viscosity - slow cure materials.)
                                    http://nikita.argia.net/kmaxon/q40_132_0021273.jpg
                                     
                                    As a hopefully helpful debugging step I would offer up that if you are getting significant volume of air bubles in your part (not just tiny cosmetic pin points here and there stuck to the surface.)  Then you might want to review your process and materials.  On the process side, evaluate how you are mixing the materials that might be allowing air bubbles to become trapped in the material in the first place.  On the materials side, check that you are using compatible mold releases and cleaning agents.  The air bubbles might not be air in the conventional sense at all, but a side effect of a liquid detergent, used for mold release, interactiving with the chemical processes of curing the plastic and forming gas bubbles.  Lastly if you are embedding objects in your parts for strength or other (plastics, rubber, wood, etc...) It is important to know their composition, as they will impart a change to the reaction during cure and can also contribute to out-gassing / bubbles.
                                     
                                    -Kenneth
                                     
                                     

                                    From: Max Cato <maxsthekat@...>
                                    To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                    Sent: Monday, October 22, 2012 5:07 PM
                                    Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                                    Kenneth,
                                     
                                    What is the smallest part you've casted using polyurethane? I ask becuse I am trying to use this to create 28mm scale miniatures for board games, which have details as fine as 0.1mm. I typically use the OOMOO 30 stuff from Smooth-On, and the medium setting (10 min) urethane they offer.
                                     
                                    I figured that if the model has too many defects caused by air pockets, then I can simply evacuate all of the air! (Insert maniacal laugh). I'll have to give your method a try. I'm not quite up to the level where I can build an injection mold machine from scratch like you can ;)
                                     
                                    -Sean

                                    From: K Maxon <k_maxon23@...>
                                    To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                    Sent: Monday, October 22, 2012 1:55 AM
                                    Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                                     
                                     
                                    Hello Cesar & Sean,
                                     
                                    I cast parts in Polyurethane plastic, RTV urethane rubber and silicone rubber and haven't had any problems with air bubbles in the cosmetic / surface finish of a part unless there is a particularly rough spot in the mold.  If you have very rough spots in the mold condusive to trapping pinpoint air bubbles, a extra hit of mold release seems to do the trick well.  Another way to release them in all three mediums is to use the much longer cures versions (24+hrs) for each.  The air bubbles are released by 45seconds of vacuum followed by vigorous agitation and another 45 seconds of vacuum.  I use extremely low vacuum pressure (only a couple pounds below atmospheric pressure) provided by a venturi hooked to the air compressor and a plastic bucket.  The reason to use the longer cure versions of each material, is that upon inital mix and during its pot/working life it has a much lower viscosity which is condusive to releasing air bubbles.   Once the air is release, cure the part in a heated box (I use a rubber maid storage box) at 140F which greatly accelerates the cure time.  For heating I use a set of 250W power resistors and a DC power supply.  Not high tech but it gets the job done and makes reasonably good looking parts without pinpoint voids from air bubbles.
                                     
                                    I'm not sure that is exactly an answer to the question being asked, possibly it is.  Hope this helps... 
                                    -Kenneth


                                    From: robotMaker <robotmeiker@...>
                                    To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                    Sent: Sunday, October 21, 2012 11:31 AM
                                    Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                                    I've done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam. Regarding the mold making and castings, I do the same. I keep the cost low by using, caulking silicone to create my molds. Very cheap stuff and cures in an hour. For my casting material I use, 15 minute, 20 minute and 30 minute epoxy resin and other resins. I usually make gears and other small parts. When I make my molds I do not have any problems with air bubbles, but I do with the casting, regardless of what kind of resin I use.  I built a small vacuum chamber using a pickle jar, and an auto vacuum test pump. But it hardly helps removing the trapped air in the mold. So when the situation is right, I use other tricks to get rid of the air bubble deformities out of the part.

                                    This is what I started with to make the silicone molds, and refined the process. I also use another simple trick, soap and water on the silicone, and this works just fine for me, better than the paint thinner.

                                    http://mechanicalmashup.tv/?woo_video=episode-26-how-to-make-a-silicone-mold-in-your-kitchen-on-the-cheap

                                    Cesar

                                    From: Max Cato <maxsthekat@...>
                                    To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                    Sent: Sunday, October 21, 2012 1:09 AM
                                    Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                                    Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)

                                    http://www.seattlerobotics.org/encoder/sep97/motors.html

                                    Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)

                                    From: maxsthekat <maxsthekat@...>
                                    To: SeattleRobotics@yahoogroups.com
                                    Sent: Saturday, October 20, 2012 10:49 PM
                                    Subject: [SeattleRobotics] Best way to raise and lower a plunger
                                     
                                    Hi all, I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold. Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this? Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts? Thanks! -Sean *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.
















                                  • robotMaker
                                    Sean: My son makes those figures too. He makes his molds using the silicone caulking method, cheap and practical. His two part molds are about the size of a
                                    Message 17 of 18 , Oct 23 11:07 AM
                                      Sean:

                                      My son makes those figures too. He makes his molds using the 'silicone caulking' method, cheap and practical. His two part molds are about the size of a lemon, since his figure parts are small. He too had problems with the air bubbles getting trapped in some places of the figures. So he tried something that I did not think that could be done. When he pours his slow curing casting resin, he carefully squeezes the mold to expel the bubbles. The silicone mold is rigid enough to return to its original shape, and most of his castings come out bubble free. When the parts do have some deformities from trapped air bubbles, he fixes them, by doing something else. He retouches them with fresh resin and puts the casting part back into the mold, and since the silicone is smooth, the part slides in very easily. When the part is fully cured, there are no signs that there were ever any air bubbles trapped. His hardest part was the Wolverine head, since it has pointed ears and nose. I too use the same method to save my gears with air bubble deformities, but my mold are simple one sided molds.

                                      I should also mention that the Silicone Caulking molds that we are making and using, are clear, shinning an led flash light on one side shows when air bubbles are trapped.

                                      Cesar


                                      From: Max Cato <maxsthekat@...>
                                      To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                      Sent: Monday, October 22, 2012 9:17 PM
                                      Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                                      Yes, you are correct-- they are textures. I make miniatures very akin to these: http://www.games-workshop.com/MEDIA_CustomProductCatalog/m1252489_99120101034_SMAssaultSqdmain_873x627.jpg (each one of those guys, minus the base they are on, are approximately 28mm tall). Try not to laugh-- it's a fun little hobby :) The detail on certain items (such as the eagle's crest under the straps on the one in the middle or the where the legs meet the torsos on all of them) can get very detailed. (Note: these are multi-part figures-- they certainly don't come as one piece.) Typically, you make one very good master mold, create a few models from that, and they create production molds.

                                      In addition, each of the parts is very "organically" (irregularly) shaped. So, this creates all sorts of places in the mold where air can become trapped during a gravity feed. I've tried cutting more vents in strategic places, but I tend to end up with voids, no matter what I do. So, I figured it's come down to either getting an injection mold machine setup, akin to how the commercial companies do, or try to simply do my gravity feed in the absence of air. No air would mean no voids possible, right? :) I'm not certain if the polyurethane will simply vaporize, but I figured it might be worth a shot.

                                      But, you are certainly correct, and a review of my process is warranted. I will definitely look into ways to get any bubbles/voids removed from each cast... You and Cesar both have very good results, and perhaps this can be done as a gravity feed. 

                                      -Sean


                                      From: K Maxon <k_maxon23@...>
                                      To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                      Sent: Monday, October 22, 2012 6:35 PM
                                      Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger

                                       

                                       
                                      Hello Sean,
                                       
                                      I am wondering if that was a type-O?  0.1mm is about 4 thousandths of an inch.  In either epoxy or polyurethane a 0.004-in thick feature breaks off with the brush of a finger if you can keep  if it is even possible to get it out of the mold.   Possibly I am missunderstanding and you are trying to imprint textures of this size, which will show up in the final part.
                                       
                                      I have cast parts that come down to a "knife edge" to the best that I can push the material without vacuum assist.  The part shown in the photo below is magnified many, many times.  For a sense of scale those are #4 screws...  I rarely take the time to vacuum degas the material.  Only if I have a highly visible shiny part in black rubber.  In plastic, or with colored parts or matt-finishes, it tends to be not worth the extra time and work as the process in most molds tends to not leave bubbles in the first place.  (low viscosity - slow cure materials.)
                                      http://nikita.argia.net/kmaxon/q40_132_0021273.jpg
                                       
                                      As a hopefully helpful debugging step I would offer up that if you are getting significant volume of air bubles in your part (not just tiny cosmetic pin points here and there stuck to the surface.)  Then you might want to review your process and materials.  On the process side, evaluate how you are mixing the materials that might be allowing air bubbles to become trapped in the material in the first place.  On the materials side, check that you are using compatible mold releases and cleaning agents.  The air bubbles might not be air in the conventional sense at all, but a side effect of a liquid detergent, used for mold release, interactiving with the chemical processes of curing the plastic and forming gas bubbles.  Lastly if you are embedding objects in your parts for strength or other (plastics, rubber, wood, etc...) It is important to know their composition, as they will impart a change to the reaction during cure and can also contribute to out-gassing / bubbles.
                                       
                                      -Kenneth
                                       
                                       

                                      From: Max Cato <maxsthekat@...>
                                      To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                      Sent: Monday, October 22, 2012 5:07 PM
                                      Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger



                                      Kenneth,
                                       
                                      What is the smallest part you've casted using polyurethane? I ask becuse I am trying to use this to create 28mm scale miniatures for board games, which have details as fine as 0.1mm. I typically use the OOMOO 30 stuff from Smooth-On, and the medium setting (10 min) urethane they offer.
                                       
                                      I figured that if the model has too many defects caused by air pockets, then I can simply evacuate all of the air! (Insert maniacal laugh). I'll have to give your method a try. I'm not quite up to the level where I can build an injection mold machine from scratch like you can ;)
                                       
                                      -Sean

                                      From: K Maxon <k_maxon23@...>
                                      To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                      Sent: Monday, October 22, 2012 1:55 AM
                                      Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                                       
                                       
                                      Hello Cesar & Sean,
                                       
                                      I cast parts in Polyurethane plastic, RTV urethane rubber and silicone rubber and haven't had any problems with air bubbles in the cosmetic / surface finish of a part unless there is a particularly rough spot in the mold.  If you have very rough spots in the mold condusive to trapping pinpoint air bubbles, a extra hit of mold release seems to do the trick well.  Another way to release them in all three mediums is to use the much longer cures versions (24+hrs) for each.  The air bubbles are released by 45seconds of vacuum followed by vigorous agitation and another 45 seconds of vacuum.  I use extremely low vacuum pressure (only a couple pounds below atmospheric pressure) provided by a venturi hooked to the air compressor and a plastic bucket.  The reason to use the longer cure versions of each material, is that upon inital mix and during its pot/working life it has a much lower viscosity which is condusive to releasing air bubbles.   Once the air is release, cure the part in a heated box (I use a rubber maid storage box) at 140F which greatly accelerates the cure time.  For heating I use a set of 250W power resistors and a DC power supply.  Not high tech but it gets the job done and makes reasonably good looking parts without pinpoint voids from air bubbles.
                                       
                                      I'm not sure that is exactly an answer to the question being asked, possibly it is.  Hope this helps... 
                                      -Kenneth


                                      From: robotMaker <robotmeiker@...>
                                      To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                      Sent: Sunday, October 21, 2012 11:31 AM
                                      Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                                      I've done things like this before, and also used 2-56 threaded rod, but in my case I was pan-tilting a cam. Regarding the mold making and castings, I do the same. I keep the cost low by using, caulking silicone to create my molds. Very cheap stuff and cures in an hour. For my casting material I use, 15 minute, 20 minute and 30 minute epoxy resin and other resins. I usually make gears and other small parts. When I make my molds I do not have any problems with air bubbles, but I do with the casting, regardless of what kind of resin I use.  I built a small vacuum chamber using a pickle jar, and an auto vacuum test pump. But it hardly helps removing the trapped air in the mold. So when the situation is right, I use other tricks to get rid of the air bubble deformities out of the part.

                                      This is what I started with to make the silicone molds, and refined the process. I also use another simple trick, soap and water on the silicone, and this works just fine for me, better than the paint thinner.

                                      http://mechanicalmashup.tv/?woo_video=episode-26-how-to-make-a-silicone-mold-in-your-kitchen-on-the-cheap

                                      Cesar

                                      From: Max Cato <maxsthekat@...>
                                      To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                                      Sent: Sunday, October 21, 2012 1:09 AM
                                      Subject: Re: [SeattleRobotics] Best way to raise and lower a plunger
                                      Arg.. Or there could be this great little write-up on just what I need! (This always happens after I fire off an email)

                                      http://www.seattlerobotics.org/encoder/sep97/motors.html

                                      Still, feel free to chime in if you have other or better ways! Sharing info is a good thing :)

                                      From: maxsthekat <maxsthekat@...>
                                      To: SeattleRobotics@yahoogroups.com
                                      Sent: Saturday, October 20, 2012 10:49 PM
                                      Subject: [SeattleRobotics] Best way to raise and lower a plunger
                                       
                                      Hi all, I do a little mold making and casting in my spare time. The best molds are made by pulling out nearly all of the air in the chamber, so to prevent air pockets causing defects and ruining your cast. I'd like to do something similar for the casting process. However, you can't just fill 'er up and suck all of the air out of the mold this time-- you'd end up with a less than filled mold, which would be just as bad! So, I need some way to add my casting material (urethane) to the mold. Enter robotics! :) What I need to do is raise and lower a plunger (think syringe) which contains my material. Unfortunately, I'm more of a computer than mechanical engineer, and so, I ask, what do you think is the best way to go about this? Originally, I thought maybe I could make a cam with cam follower, but then I have to worry adding a spring* being embedded in urethane, which would set up and harden. Then, I thought, well, maybe a piston and crankshaft, but that seems like a complicated linkage for a simple idea. I ruled out solenoids because of too small of a stroke. I also ruled out linear actuators because, let's face it, I'm not rich. Finally, I'm thinking maybe I should go with a rack and pinion sort of setup, but I wanted to get your take on it first. Surely, one of you has encountered turning rotary motion to linear motion. What are your thoughts? Thanks! -Sean *For the return, to push the cam follower back against the cam, since the plunger must point down because gravity is what pulls the urethane into the mold.










                                    • Peter Balch
                                      Kenneth ... Such as? Cesar ... Of course, I hadn t thought of that. I was imagining the difficulties of casting a bush accurately into the gear. You just drill
                                      Message 18 of 18 , Oct 23 12:52 PM
                                        Kenneth
                                        > check that you are using compatible mold releases and cleaning agents.

                                        Such as?

                                        Cesar
                                        > I epoxied it to the brass shaft.

                                        Of course, I hadn't thought of that. I was imagining the difficulties of
                                        casting a bush accurately into the gear. You just drill out a hole and glue
                                        it in!

                                        Peter:
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