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1674Re: Greetings, and extrusion

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  • Roger Hamlett
    Feb 3, 2011
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      > In principle it would work with the extra air plunger but I think you are
      > unnecessarily making it more complex, creating several new variables and
      > making it harder to control. For example, the flexible hose will flex and
      > it will vary with temperature, consistency of the medium and so on. It will
      > affect both the dispensing cycle and the off cycle.
      >
      > Why not just use the rigid syringe and plunger as I suggested. Today it is
      > easy to get a backlash free linear actuator that in software can accurately
      > be controlled and if needed also reversed.
      > Bertho
      > =============================
      >
      > From: Roger Hamlett Sent: Wednesday, February 02, 2011 11:50
      >
      Problem is that you are going to need a soft seal at the tip of the
      syringe plunger (polyethylene probably), and you will be pulling at the
      'top' of a variable length (and hence variable stiction) column of clay.
      If you take a syringe of clay, and try pulling back on a nearly empty
      one, it works. But on a nearly full one, you just create a vacuum, and
      the plunger pulls away from the clay leaving a cavity (I tried...).
      It looks as if the 'pull back' really has to be done near the nozzle.

      The variability of the hose should be small in a 'room temperature'
      environment.
      I'm going to try to reduce the variability in the medium, by making some
      form of viscosity 'tester', and working to a standard on this.

      I was going to use a linear driver of some form, to give constant feed
      rates, but the idea was to give a tiny amount of suck back right behind
      the nozzle. I might think about a little brass assembly, with an O-ring
      sealed plunger that just moves a small amount (perhaps adjustable with a
      couple of screw stops), and see what the life of this is like in the
      abrasive environment, to avoid the elasticity of the pipe idea. However
      the pipe has the big advantage of being cheap, and a replaceable part. I
      was visualising it being in a shaped hole, to limit it's stretch, and
      just compressing the one section, so the variability should be small.
      This is where a few experiments are now needed.

      Best Wishes


      > Heterogeneous fluids (as in suspensions and pastes) are usually
      > non-Newtonian fluids so there will be a certain "elastic" tendency.

      >> Pneumatic control must be quite a difficult design to implement. In any
      >> case I would rather use oil pressure rather than air to actuate it.
      >> Screw drive plungers are more reliable and solid, also quicker to revert
      >> direction.
      >>
      >> On Tue, 2011-02-01 at 07:56 -0500, Boman33 wrote:
      >>
      > Yes,
      > I'd begun to suspect this was the case. So you actually need not just
      > 'removal' of pressure, but positive _subtraction_ at the tip, to prevent
      > ooze.
      > So if you have a 'pushed' syringe, feeding to a simple nozzle, with
      > control of the feed, but then have a short length of pipe between the
      > syringe and the nozzle, that is slightly squeezed at the side by an air
      > piston, when the feed is on. Cut the feed, and at the same time, the
      > feed piston stops pushing, the side pressure is relaxed (spring),
      > actually creating a small suction at the nozzle, and 'pull back'.
      > Not too complex, and seems to handle the likely problems.
      > Have ordered some pipe, nozzles, and syringes, and going to play with this.
      >
      > Best Wishes
      >
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