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DIY injection molding... plunger leakage?

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  • DIY injection molding... plunger leakage?

    I think I want to mess around with ABS injection molding. Building a press seems pretty straightforward but most of the self made hand operated machines I see on online have molten plastic leaking out around the top of the plunger. I'm not scared of a goopy mess but felt it was worth some investigating. Could silicone o-rings be used, or is there a reason that many DIY builds don't incorporate such a thing? Perhaps just tight tolerances to reduce leakage, or is there a reason to leave slop, such as to prevent binding? Thank you.

  • #2
    I'm now most of the way through building the machine with the remaining items being the plunger and wiring. The whole time I've been considering the question I posted regarding leakage around the plunger. It seems that air bubbles would need to be able to escape topside and air needs a way to enter when the plunger is withdrawn otherwise it would enter through the nozzle at the bottom. Perhaps some plunger slop is needed to keep a simple build... well, simple.

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    • #3
      I do low pressure hand injection all the time of another media. I design the molds I use with vents for air bubbles. Air trapped in the mold to begin with is the biggest risk of air contamination in the injected item. I also have a small lever actuated plastic injection machine on my work bench (that I have not used yet), and I keep meaning to walk out and look at it to see if the plunger is a loose fit or a sliding fit.
      *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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      • #4
        The issue could be one of tolerance with the piston and cylinder constantly changing dimensions during temperature cycles.
        I just need one more tool,just one!

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        • #5
          Originally posted by wierdscience View Post
          The issue could be one of tolerance with the piston and cylinder constantly changing dimensions during temperature cycles.
          I'd think the answer to that would be to use a piston ring with an overlap. Most small machines feed the cylinder with a funnel at the top, and the piston comes all the way out to reload the cylinder. Bell mouth the top of the cylinder and it will compress the ring as it slides in.
          *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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          • #6
            ANYWAY.... I just went back and took a look at my hand lever plastic injector. It just has a plane close fit piston. Looks like a few thousandths clearance. Added some pictures below. Now that I think about it viscosity of the plastic itself will likely prevent much flashing up past the piston. The piston will quickly heat sink any plastic that does since it will be cooler than the cylinder and that thin bit of plastic will then seal the piston to the cylinder. I'd suggest not leaving the piston in the cylinder at any time other than when applying pressure to the plastic.






            Last edited by Bob La Londe; 02-24-2019, 09:17 PM.
            *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

            Comment

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