HDPE Recycling, centrifuge and all that jazz!

[John Hill]

Right, so I want to recycle HDPE and I want to produce luverly  plastic billets  that I can use to make stuff.

I have an oven,  a big stash of suitable HDPE scrap and some short lengths of steel water pipe (2.5" or so) which I have used to mould the HDPE.

The problem with my operation is how to get pure plastic without air inclusions?   My thought is to put the hot plastic in the water pipes and centrifuge them to expel the air.

The hot plastic is not exactly fluid but it is almost pour able.


That is the idea, so far!

[ddmckee54]

A vacuum chamber might work better.  Unless you put the centrifuge in the oven the HDPE will cool and be set up before you could get the air spun out of it.

You've already got the steel pipe so you're half way to a vacuum chamber. Clamp the pipe between a couple of steel plates with some silicone sheets for gaskets, a vacuum port on one end, and Bob's your uncle - you've got a vacuum chamber.  You could even wrap some heaters around the pipe to keep the HDPE hot while you apply the vacuum.

If you're really serious about this, have you thought about an extruder?  The extrusion process removes the entrained air and you can mold whatever shaped billet you want.  You probably already know about it, but if not check out the Precious Plastic website. https://preciousplastic.com/

[awemawson]

There is a thread on the forum about machineable wax made from HDPE & MDPE

https://www.madmodder.net/index.php/topic,13394.msg161114.html#msg161114

There are others

[John Hill]

A vacuum chamber might work better.  Unless you put the centrifuge in the oven the HDPE will cool and be set up before you could get the air spun out of it.

You've already got the steel pipe so you're half way to a vacuum chamber. Clamp the pipe between a couple of steel plates with some silicone sheets for gaskets, a vacuum port on one end, and Bob's your uncle - you've got a vacuum chamber.  You could even wrap some heaters around the pipe to keep the HDPE hot while you apply the vacuum.

If you're really serious about this, have you thought about an extruder?  The extrusion process removes the entrained air and you can mold whatever shaped billet you want.  You probably already know about it, but if not check out the Precious Plastic website. https://preciousplastic.com/

I have discussed a vacuum chamber on another site and the message I got is that a very high vacuum would be needed and even then there was doubt that the air would flow in the plastic.

The Precious Plastics site is very interesting but it looks like I would be up for kilobux by the time I had bought/built shredder, extruder, press etc and that makes me hesitate.

[John Hill]

It is interesting that no one on any site is supportive of the centrifuge idea so I will just leave that aside for the meantime.

[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 bed
Now '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.