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Re: [SeattleRobotics] Best way to raise and lower a plunger

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  • 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 1 of 18 , Oct 23 11:07 AM

      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.


      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. 


      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.)
      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.

      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

      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 ;)

      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... 

      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.



      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)


      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 2 of 18 , Oct 23 12:52 PM
        > check that you are using compatible mold releases and cleaning agents.

        Such as?

        > 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!

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