Gallery, Projects and General > How do I??

HDPE Recycling, centrifuge and all that jazz!

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David Jupp:
In a typical plastic processing operation, an extruder would be used, the extruder screw has variable depth (and sometimes variable pitch) flights.  Though it looks deceptively simple, the extruder screw does several things as it turns

* Conveys polymer forwards
* ensures contact with heated surfaces to melt the polymer
* mixes molten polymer
* allows air to escape backwards to the feed hopper
* compresses the solid/molten bedNow 'molten' polymers are typically very viscous, and also non-Newtonian.   It's difficult for trapped bubbles to escape once in the melt.  You really need to get air out during the melting stage.

Vacuum de-gassing extruders include a special section where vacuum is applied and the molten polymer is repeatedly mixed to expose fresh surface to the vacuum.  These are complex and expensive beasts.

All the above should tell you that bubbles won't be removed just by buoyancy, and simply applying a vacuum won't do much either.

John Hill:
I had another play in my shed last evening and built a fairly promising vibrator.   A strong magnet sitting on the remains of an old bulk tape eraser that had served an earlier life erasing reels of inch mag tape.

A tin can sitting on the magnet gets a very thorough shake up but whether that will be enough to release the air bubbles is yet to be seen.

ddmckee54:
You are going to have to get the HDPE rather hot, and keep it that hot, to get it fluid enough to be able to vibrate the bubbles out of it.  Unless you are planning on vibrating this in the oven while it's heating I don't think you've got much of a chance of removing the bubbles.

I've seen Youtube videos of people making hammers out of molded HDPE.  If you use their techniques, or at least modify those techniques, I'd think that you should be able to make your billets.

David Jupp:
A spinneret into a vacuum chamber might be a possibility - get many strand of melt (lots of surface area, with short diffusion paths) to allow gas to escape...

Is the HDPE from injection moulded or blow moulded items?  It will make a big difference to the viscosity of the melt.

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