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An idea for recycling HDPE and the mysteries of steam!

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John Hill:
Hello gentlemen!  It seems years since I last visited this forum but the mind never stops wandering!

Now then,  I would like to recycle HDPE at home and I do not want to invest in shredders and extruders etc.

I have a system that sort of works.  I heat HDPE scrap in an oven and when it is hot  (175-200C) I stuff it in to a metal mold made from a length of 2.5" water pipe. Then give it a good squeeze in my press and wait for it to cool.

This process gets me useful ingots of brightly coloured HDPE that I can put in my lathe to produce such things as light-duty gears and fancy-scamcy gear lever knobs, etc.   The problem is air entrapment.

The mold is closed at one end with a removable cap and a piston at the other end.

Back to the air entrapment,  I am thinking I could drill a vent hole through the piston and fill the mold with water before putting the scrap bits in.   I.E. filling the mold with water would mean no air lurking among the scrap bits!

I would put the mold filled with water and HDPE scrap in the oven with the piston in place and the vent hole open.  The oven temperature will be about 200C so the water will boil and evaporate and steam pass out the vent hole!  When the mold eventually reaches 200C all the water will be gone and the scrap will be near 200C.

At that point I will transfer the mold to my press and squeeze down on the molten plastic.

Now the question is will there be any water entrapped in the plastic ingot?

BillTodd:
 Yes.


Plastic, loves water  . Water molecules love to embed themselves in the poly carbon structure and wil change the dimensions of the plastic. 

Some plastics are worse than others , Nylon is incredibly hydrophilic ,  part will visibly change with humidity.

I suggest you look into a vacuum pump to remove any gas as you compress the pellet, if you require maximum density.

RussellT:
I am also wondering about how dry the inside of your tube will get and what is preventing the gaps just being refilled with air.  I am thinking about how long it takes to dry out stuff being cooked in the oven.

I second Bill's suggestion of a vacuum pump.

Russell

David Jupp:
HDPE is hydrophobic.

BUT when you've boiled off the water - there won't be a vacuum in any voids, it'll be either air or water vapour.  So other than use a lot of energy boiling of the water, I don't see that the proposed scheme will do any good at all.

A vacuum pump alone may not help enormously as the polymer melt is viscous - there has to be some transport mechanism to get the bubbles to where the vacuum can access them.

In a traditional reprocessing system, air can escape backwards along the extruder screw (whilst the polymer is being melted and compressed) and/or there may be a vacuum port to remove volatiles part way along the extruder where the melt has been stretched out to expose lost of surface.  The engineering of vacuum vented extruders is not simple.

John Hill:
Air inclusion is the biggest problem I have with home shop recycling of HDPE and the idea is that packing the scrap pieces underwater will exclude air.  The water is then boiled off leaving voids filled with water vapour at atmospheric pressure (1bar, approx).


I expect the voids will be filled with water vapour and I do not understand what would cause that to leave to be replaced by air.

So assuming I have a hot mould filled with melted HDPE  interspersed with pockets of water vapour at 200C and 1 bar. 

When that hot mould is compressed (piston forced down) the pressure in those pockets will rise but will fall again as the mould cools. 

Eventually the HDPE becomes one solid mass containing whatever water is left from the condensation of the water vapour.  If my maths are correct that is about 0.001 grams of water for every cc of void space.

Of course I do not know how big the voids may be but just assuming the initial packed scrap and water mix to be 50/50 and the volume of the mould to be 2 litres,  1 litre of scrap and 1 litre of water.

At the end of the heating, 1 litre of HDPE and 1 litre of water vapour.  When that is compressed and cools there is still 1 litre of HDPE and 1 gram of condensed water spread through it.   That seems much better than air pockets!

That is the idea and I just hope my mathematics is correct!

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