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HDPE plastic recycling. How to avoid voids?

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  • HDPE plastic recycling. How to avoid voids?

    The basic softening and kneading into a mass is easy enough but how can I avoid voids when I press it into my mould (2.5" water pipe)? I have a 10 ton press and I put on as much pressure as I dare.

  • #2
    How big is your mould?

    I'm assuming that you are starting with effectively liquid HDPE.

    Try to maintain the mould at or slightly above the melting point temperature of the HDPE (248 - 355 C).

    Cool the mould after the pour is done.


    • #3
      Get it hotter and more "liquid-ey"

      Like bob says .. more of a pour than a softening and kneading.
      John Titor, when are you.


      • #4
        When I tried to make sheet I rolled it from one end with a wooden roller so the air bubbles had an escape route. I can't think of an equivalent for a tube. However you might be able to roll up sheet in the open so the air escapes as you roll it up, into a longer thinner roll which you then insert it into a long tube. Then evacuate the tube while there are still gaps for the air to flow out and finally compress it lengthways.


        • #5
          I inject HDPE in the plastic injection machine around 1500psi depending on venting in the mold. You might need an aluminum pipe to get good density.


          • #6
            It is not easy to get HDPE to a near liquid without some surface charring. My technique is to knead as much air out as possible then pushing it into the pipe mould. I then put the mould in the oven for half an hour to get everything to the same temperature before moving it to the press.

            Last edited by The Artful Bodger; 10-25-2016, 08:20 PM.


            • #7
              How about adding heaters to the mould and a lid with a vacuum source?Maybe an old refrigeration pump?
              I just need one more tool,just one!


              • #8
                I found casting HDPE and TEPE more trouble than it's worth. I had fantastic success with LDPE, found on flexible (steel bodied) coffee can lids. I made a coutnersink handle out of it. It's not as chemical resistant as HDPE, and much softer. But, for most plastic applications it's just fine around the home shop. I melted mine on a can on a hot plate, no surface charring and it actually turned liquid.

                LDPE is a softer version of HDPE, low versus high density.


                • #9
                  Try to reduce the size of the initial feed source, chip, shred or grind.
                  The smaller the initial particle/flake size will result in less entrapped gases, and melt in a more controlled uniform fashion.
                  I use an aluminum crucible on an electric hot plate supplemented by a propane torch to get the melt initiated (No direct application of the flame on the plastic!).
                  Some folks suggest using a very small amount of vegetable oil to promote uniform heating of the melt - somewhat less than a tablespoon per two cups of fine flaked material (6x6mm). I find that this is okay for LDPE, but very smokey when doing HDPE.


                  • #10
                    Maybe I should take my stash of HDPE bottles out to the farm and put them through the old chaff cutter!


                    • #11
                      I saw something on YouTube where the fellow re-purposed an old meat grinder to grind HDPE.

                      It was your typical environmentalist kludge! Any halfways mechanically inclined ten year old could devise a much better solution.