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An idea for recycling HDPE and the mysteries of steam!

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John Hill:
Thanks...

As always, entrapped air is the problem.

The opening mould would be good but it might cause problems as I have to compress the mass as it cools and it likes to squirt out the most tiny gaps.

Yea, heating the plastic while in the mould (which is a length of water pipe) is something I have tried and is not a really bad idea but for a 2.5" pipe it takes more than an hour to heat through and I cannot really get the air out that gets trapped as I add more pieces.  Vibration does not work, something about Newtownian fluids or so I have been told.

I will definitely keep your piece of wood floating on top in mind.  I have a few pieces that have been in the mould but did not properly bond probably due to the surface chilling between wrestling off the oven tray and rolling to go in the mold so your piece of wood is timely as I have found trying to reheat chunks in the mould it does get charred after an hour in the oven.

This is summer in NZ and I am still waiting for a good day to drag everything outdoors to have a 'plastic session'.  I might try a new idea and that is to soften the scraps and add them to the mould a few at a time giving a 10 ton squeeze as I do so, maybe another hour at high heat will give me a bonded mass.

John

vtsteam:
I'd love to see your plastic contraption/session when the time comes, John.  :coffee:

Some more thoughts:

What about working the air out by folding and hand kneading a stiffer (less melted) mass, then forming into shape as these guys do, and/or placing semi-softened kneaded plastic in the clamshell mold (last post)?

I realize it still shrinks, but maybe less, and maybe either just give a shrink allowance for turning true, or maybe a little piston pressure in the clamshell mod on what is a lot firmer starting mass. It shouldn't flow through cracks because of the higher viscosity.....?

vtsteam:
These guys give a pretty good run down on the types of HDPE that work the best, the differences in how they melt and flow, how to combine them to reduce the difficult properties of bottle types, and how they remove bubbles by manipulation, and the use of compression starting roughly at 9 minutes in here:



Also they did one on a large round blank like you want (used by them making a mallet) early on in their career, this one appears to combine difficult bottle HDPE (chopped up) with bottle cap (easier) HDPE, an interesting point in the video also is the use of silicone spray as a release agent:

vtsteam:
John, not to ignore your original question, my thoughts (on the negative side) are:

1.) that water might prevent adhesion between layers of plastic
2.) that you might get pops or "explosions" like when water gets under oil in a hot cooking pan (prior to compressing while evaporating the water)
3.) that pressure from compressing might condense remaining steam pockets back into water, causing 1.)

Also, beaded polystyrene foam is actually created by exposing molten solid polystyrene beads to steam. Different plastic, I know, but might have a similar expansion effect...

John Hill:
I tried the procedure used to make the mallet, you can see it is a pretty slow job putting one layer in the mould, tramp it down and heating, rinse and repeat.

I make one ingot by my usual method** and followed with two small diameter bars using the  procedure shown for the mallet.  Those two bars are still in the oven so I cannot comment on them just yet.

It is really slow adding caps as they take up a lot of space, but I have another idea!  I am thinking of boiling the caps etc in a saucepan so they will be soft(ish) and easier to pack in the mould. 



**my usual method, heat a mass in the oven then stuff that as best as possible in to the mould.


{Later}  The two bars  of the heat in mould process came out OK but I would not recommend that process as it is just too tedious!

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