# HDPE Cooling/Pressure-Molding Question



## Lacumo (Aug 18, 2013)

When you take your full-temperature, just-baked HDPE blob out of the oven and quickly get it into whatever kind of pressure mold you use to force out the air pockets and form it into a board&#8230; How long do you need to keep it "under full pressure" before it's pretty much hardened up into the board it's going to be?

I suspect the full pressure is only necessary for 10 minutes or less until it's as hardened and compressed as it'll get and after that it's just a matter of cooling down and contracting a little bit from then on...

Any of you more experienced HDPE guys have any insights on this one? Thanks for your help...


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## ghost0311/8541 (Jan 6, 2013)

i do mine in a 9x9 cake pan with a board on top and a weight and let it cool it shrinks as it cools and pulls a way from pan and then i trim on a table saw i would give it 30 min because it holds heat for awhile


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## stej (Jan 16, 2013)

I don't know exactly how long we should wait, but .. is there any reason why to hurry up? I know that my slab takes at least 2 hours to cool down and that's pretty short time for me..


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## Lacumo (Aug 18, 2013)

I only have a limited amount of weight to put on the board I put on top of the plastic, so I have to stand on it until it hardens up. Standing on it for 2 hours isn't going to happen.


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## GrayWolf (May 14, 2012)

How about a couple of 2 x 10s cut to size, put one on top and one under the pan and put in a vise or use some clamps.

Just a thought.

Todd


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## ghost0311/8541 (Jan 6, 2013)

i use a rail road track that is used for a anvil.


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## GrayWolf (May 14, 2012)

I have a small piece of track. Is that enough pressure to keep it flat?


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## ghost0311/8541 (Jan 6, 2013)

thats all i use but i heat mine with a torch i dont know if there is a diffrents how the people do it in a toaster oven.


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## Lacumo (Aug 18, 2013)

In my first try, 75 pounds didn't do a good enough job of forcing the air pockets out and smoothing off the top surface. On the second try, I stood on top of the weights and that upped the ante to >270 pounds for >10 minutes. I think by that time the plastic had hardened up and all it had left to do was cooling, which takes at least a couple hours like Stej said. On that second try, it appeared to do an OK job of getting the air pockets out and at least somewhat smoothing off the top. I don't have a jack or any scrap wood, so I've been trying to avoid putting the time and $$ into building a bottle jack powered press.


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## stej (Jan 16, 2013)

Ok, just my experience - air bubbles will be there no matter how much you press the slab. It much more depends on how you mix the pieces of HDPE and whether you use big pieces or small ones etc. I tried to avoid bubbles a lot, because they scared me (as they weaken the slingshot), but now I just gave up. I concentrate on the procedure itself. So e.g. if I fold the heated HDPE (to make the slab thicker, but narrower), I pay attention to do it well so that there is no air in the folded part.

The smooth surface.. I think it's possible, but you need to press it heavily and do it immediatelly after you take the slab from the oven.

Just my :twocents:


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## Lacumo (Aug 18, 2013)

Stej--Thanks for the thoughts.

Out of curiosity --

1-Do big pieces or small pieces of HDPE cause less air pockets?

2-How do you mix the pieces to make less air pockets?


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## stej (Jan 16, 2013)

I think there are better experts than me to answer your question.. 

Anyway, small pieces are better for sure. I also tried to stack plates, but it was quite tricky.









As you see the new one starts bending - the edges get hot much quicker so they start bending first. With time it introduces possibilty that it catches some air inside. I hope it is clear from the picture.

I was quite paranoic about the air bubbles so I several times took the slab from the oven and tried to press it with something. I also squeezed the slab with hands (with gloves) which might have helped a bit..


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## V-alan-tine (Nov 12, 2013)

stej is correct because the only way to remove all the air is to use a vacuum system to suck them out.

One way you could try to minimize bubbles is to try what is called "soaking " , this means keeping it at the most liquid state you can reach

and keep it there long enough for the larger bubbles to float out on their own and then cooling gradually to prevent any stress flaws.

To keep it flat try clamping a piece of wood on it, just keep an eye on the tension as it cools.


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## stej (Jan 16, 2013)

The problematic part for me is "most liquid state"  The HDPE that I used most of the time is much more like clay or something like that. I noticed that some bottle caps are much softer (and maybe more luquid) and sticky. So it all might also depend on some other stuff that is mixed in the HDPE to (for example) add the color to the HDPE or to add some other property.


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## ghost0311/8541 (Jan 6, 2013)

the best one i made i had a soap bottle that i dis not clean out really good and it come out really good and you could see the soap cooking off and it was really good board the other two i made that came out good was where i cooked it three times they both come out good.


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## Lacumo (Aug 18, 2013)

Stej, I think you're right&#8230; The HDPE I've used (liquid laundry detergent bottles) never liquifies--it only softens and gets able to be fused if put under serious pressure.

Oh, well--looks like I'll have to open my wallet up, let the moths out and spring for the materials to make a good hydraulic jack powered press.


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## stej (Jan 16, 2013)

Lacume, any pics? I'm curious about experience of any other HDPE figther 

I collected a lot of bottle caps, so I have to make a mold nad then persuade my wife to use the oven.


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## ghost0311/8541 (Jan 6, 2013)

that where the torch comes in it all melts and if you want color save come and push down into the melt it will blend in.


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## Lacumo (Aug 18, 2013)

Stej--- I've only made 2 boards so far. One's an ugly pukey orange/beige color Tide laundry detergent jug failure. The other's a marginal white one and the top got browned up a bit because I had the oven a little too hot. Each needs to be reworked to get it up to OK-looking and usable. Nothing worth pix yet. Also&#8230; I've never posted a photo I took myself. I need to do a little self-education in that area. With luck, maybe by January (?). Right now, I'm still at the student-trainee-intern stage.


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## ash (Apr 23, 2013)

I think a used sandwich press is a better buy than a hydraulic jack. Start off with a thin layer and just keep adding chopped material to the melt and folding it until you have it thick enough.


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## Greavous (Sep 29, 2013)

Is there any reason you could not speed up the cooling time by soaking the heat out with water in some way?


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## stej (Jan 16, 2013)

My mold is from wood, that's why it's not possible (or desirable) to soak it in water. In other cases it might work imho.


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