An idea for recycling HDPE and the mysteries of steam!

[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!

[AdeV]

There's another option - mix it with candle wax (in a 4 parts wax, 1 part HDPE) and heat to around 175-200C. Mix thoroughly. Allow to set (it will shrink massively). Tada - machinable wax!

I used LDPE for my machinable wax, but HDPE should do just fine - it'll probably make it a little harder, which isn't a bad thing.

If you can find scrap candle wax for cheap, that'd be ideal; but failing that, the cheapest candles you can find in bulk will work just as well.

[vtsteam]

Welcome back John! 

I've seen polyethylene scrap from plastic bottles melted in (I think) an open top bread loaf pan and I believe the air bubbles just slowly rose to the surface, and were then popped with a wire. If I remember correctly it was in a video about a guy who made slingshots.

[vtsteam]

Here it is:

[John Hill]

Thanks, that is good but not want I want!  He is making flat pieces  but I want to make rounds  for playing in my lathe!

His method of heating in the oven pan is pretty much what I do but I have to use something like a silicone oven mat under the plastic otherwise it is almost impossible to get out when hot, which is what I must do if putting it in my cylindrical moulds.

With this method by the time I have got the hot plastic off the silicone mat and tried to roll it to go in the mould the surfaces have chilled so I get air inclusions weakness where layers fail to bond.

[vtsteam]

Well John a couple of (possibly worthless) suggestions, not having tried them on plastic.

1. Split the tube mold and braze on a cabinet hinge so it can be opened and closed to release the part. Tack on some rod handles so it is easier to open and close. Use a clip over the handles to hold them closed. The rods will apply spring tension.

2.Try soot on the inside of the tube mold as a release agent. (I use a candle held close to the surface for this as a release for casting aluminum over a releasable steel mandrel.)

3.Heat the tube itself to melt the plastic

There still might be bubbles that don't make it to the top in time to open them up with picks, it depends on how deep your mold is and how long you give it. Air is the main cause of scorching (besides heat and time). If you can exclude it somewhat you can heat for longer. I guess a deep mold should scorch only at the top. Maybe add a piece of wood to float on top?

Vibrator to help move the bubbles?

BTW what are the dimensions of the finished ingot?

(photos from long gone but not forgotten, brilliant "Build Your Idea" website.

[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. 

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!

[vtsteam]

In their video about 11 things they wished they knew earlier, they use a panini press with two layers of teflon sheet to fairly quickly melt those bottle caps because it's direct heat from both directions, above and below. They build up a hot mass by adding more material and closing the press again, eventually fold it out of there. They are handling a pretty big chunk of taffy like material (silicone oven gloves). Maybe then you could put that into a mold. You're not adding little hot bits at a time to a mold that way. Seems quicker. The mallet thing was an early attempt maybe 5 years before.

[awemawson]

One of the stages in processing natural rubber is to pass it back and forth between heated rollers to make it a homogeneous mass excluding air - I wonder if something similar would work for HDPE etc once initially warmed?

[vtsteam]

Andrew, I think that would work. In a way, the videos showing those two brothers kneading the molten plastic is essentially the same thing. A hot rolling pin on a heated platten? Use a hot air gun to warm them up?