I wanted to make some HDPE blocks to use with my CNC router and wood shaping tools. I’m not able to make the shredder and press machines, so I needed another way. I saw a lot of YouTube videos about recycling HDPE by heating it up, and then clamping it into a wood form. I figured… why not clamp in in a form, and THEN head it up. The only problem was… I didn’t have a welder. But, now I do, so I got some metal and made a thing! The top plate has another plate sticking off of it that pushes down into the walls of the form. I stuck in some plastic from an HDPE barrel I found in the woods, tightened it all down, and stuck it in the toaster oven. I was happy to see plastic oozing out after a while. Well, it sucked that it was oozing, because it was due to the bottom plate bending, but it was good to see the plastic was flowing. I did end up with one void on the very bottom, but overall, the material bonded well.
Btw, I say “easy” in quotes, because the plastic spilled out around the… plunge plate? I don’t know what to call it. I had to use a block of wood to beat it out. Also, here is the void at the bottom. I think it’s because the plastic pieces I put in were curved.
So far using this mold to clamp down the plastic before cooking it seems to be working fine. My only issue so far is that the toaster oven I am using seems to be on it’s way out, and doesn’t always reach the temperature I set it to, and then shuts off.
When I use materials with a lot of voids between the pieces, I press them down into flatter pieces first, and then later combine them with other ones I have done so they can get clamped down nice and tight, and then the air, so far, appears to be forced out. I need to figure out the weight of a good solid brick, and use that as a basis of how much material needs to go in.
I have been having some luck with HDPE thick plastic bags, what I do is put them in a pan and slowly melt them, when they are sticky and not 100% liquid I roll them into a ball that I keep in the pan, then put more pieces in. I continue this cycle, and I end up with this dough ball which I kneed/move around and continue to shape. Then when I have enough I put the blob into my molds.
Took some practice to eliminate blob voids, basically I had shape the the dough to the shape of the mold, the less the plastic has to move to fit in the mold, the better.
I think you can do this technique with your bottom of your pan, this will get rid of the majority of the voids and air bubbles, then clamp it down for your last bake, or even clamp it down outside the oven. You will get a good fart of the air as it escapes when you first put it in. and the clamps will ensure you get the remaining out.
There is a community of people who make slingshots out of HDPE. They buy 5 gallon buckets and turn them into blocks to carve the sling shots from. All they do is heat the plastic in an oven, then clamp a board over it to give it pressure to get the air bubbles out. It seems to work fine. I just took it a step further and clamped the HDPE down first, then heated it up. It expands when it heats up, so the clamping pressure is amplified. I could be wrong, but I assume the big void on the bottom of my block was because the barrel pieces I used were curved, and I placed them in the mold with the concave side down. I plan to try again with the concave side up so that air can roll out the sides better. Other than that, the last photo on the first post is a cross section after I cut the block in half with my bandsaw. The total block was 7.5″ square, and 3/4″ thick. I used large nuts as spacers on the press plate so I could make them thinner or thicker as needed.
Nice! the problem with larger blocks of hdpe is they end up with voids inside. This is due gas bubbles and the only way to get rid of them seems to be to squish the plastic with such enormous force that they compress until invisible. You might be able to reduce their size by adding stiff springs to the bolts so that the plate can move down further as the plastic melts in the mould.
I also wanted to create blanks, to turn on a lathe, but the obstacle of voids made me choose a different method, as I cannot find an efficient way to generate enough force to compress blanks of the size I need. I’m looking into 3d printing with PET instead, because the shape of my product would require a complicated and expensive mould.