Plastic Bags into Plastic Blocks: Revisited

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Plastic Bags into Plastic Blocks: Revisited

final product

For the last several years I’ve experimented with turning common HDPE plastic bags into solid materials at home without creating caustic fumes. My first attempt was made before I even started writing for MAKE, where I stewed the bags in canola oil before pressing them into blocks. The project worked after a fashion, but was impractical since the blocks were impregnated with oil.

Then a couple of years ago I used canola oil again to regulate temperature, but tried mixing the plastic in a double boiler set-up. Again, it was partially successful, but lacked the homogeneity I was aiming for.

Over the weekend I made another attempt, and it looks promising. I want to share my process in the hopes that the community can experiment and help with it’s refinement.

HDPE has a melting point of 266 degrees Fahrenheit. When it gets toward that level, it gets tacky but doesn’t completely melt, and can be formed into solid blocks without causing fumes. I used a barbecue thermometer in my oven to steadily increase the temperature and find the right amount of time to bake the plastic over several hours of testing.

I started with a wooden box (look familiar?) that was stuffed with cut-up bag pieces (keeping my eye out for a shredder to do this in the future.) I stuffed the box, put in another piece of wood that nested perfectly within the box, and compressed it with c-clamps.

wood box

What I found was that while the plastic fused, it didn’t do so uniformly. I had a feeling that the different types of material had something to do with this, so I switched to a steel box I had welded together a while back. I used an angle grinder to make a lid and tried the same c-clamp method.


I baked it at 450 degrees for 40 minutes, re-tightened the clamps, and let it all cool for about an hour and a half. There were no fumes as far as I could tell, but I cannot verify the safety of this method (any chemists in the house?)

No matter how hard I tried, I simply could not lift the lid off the box, so I resorted to cutting the box open. Inside was a solid block of rigid plastic.

box break

rough cut

It hadn’t spread uniformly throughout the container, so I trimmed it down to a 2.5″ x 6″ rectangle. It was encouraging to find that it was easy to cut and sand.

When I trimmed the edge off, I noticed the two pieces had symmetrical relief. I don’t have a band saw, but would love to run a piece through lengthwise for a bookmatching effect.


I think this material has a lot of potential for makers. It’s an example of point-of-use recycling, is strong, and I’m anticipating that it can be easily shaped by power tools and CNC routers.

Here are the issues I currently have. I would love if you’d sound off in the comments about them or do some experimentation on your own.

1. Scaling up. Obviously a larger container would help with this, but as you can see, I stuffed that box full of plastic pieces and ended up with a comparatively small block. I suppose a multi-step process could help this, but that would be time consuming.

2. Separating from the mold. Is there a safe releasing agent that we can coat the container with? Perhaps a box that could be dismantled would help.

3. Uniform clamping. My piece has a slight angle to it. How can we compress the plastic so it’s completely flat?

I’ll continue working on this and post updates to the MAKE blog when I have them. Thanks for your help.

72 thoughts on “Plastic Bags into Plastic Blocks: Revisited

  1. Donald says:

    Very Cool. I’d caution you about plastic in your oven at 450 F. It could give off flammable fumes, which could mix with the air in your oven, ignite, and blow the door off of your oven. It may sound far-fetched, but I know of a case where this happened in an industrial depyrogenation oven that was accidentally loaded with some plastic material.

    1. Michael Colombo says:

      I know, I was worried about that, but I kept doing tests and raised the temp incrementally. It didn’t fuse until I hit 450. I might try lowering the temperature and increasing the bake time instead.

      1. juannavarro says:

        Is there any dangers to toxins that could be left behind in the oven afterwards? Some residue or gases?

        1. Michael Colombo says:

          I honestly don’t know. I (nor my wife) smelled anything as it was baking. Even so, I’d use caution until we know for sure.

      2. Bob Howie says:

        HDPE 2 melts at 150C/300F but I bake at 180C/360F as it gives a better solid block

        1. Michael Colombo says:

          Hi Bob. Thanks for the comments! What are the largest blocks you have made? What do you use them for? Do you have any photos or other documentation online?

  2. trkemp says:

    If you don’t want to have the plastic stick to the box, lining the box with a layer of aluminium foil would probably work.

    To not have to break your box, drill some holes in the bottom and put in a false bottom. Then you should be able to flip the box over and push the false bottom and plastic block out.

    1. Michael Colombo says:

      Oh those are great ideas! I especially like the one about drilling holes. It might come out looking like a lego block but it could be easily smoothed down.

      1. stephen paulger says:

        The false bottom would cover the holes.

  3. Chris says:

    What about an extruder that extrudes these blocks? Then you might have solved all three? I guess the new problem would be the extrusion mechanism that feeds the plastic but that might be an easier task?

    1. Michael Colombo says:

      I think an extruder might be a little complex. Then you’d have to make a heating element and a press… the spirit of the project (sorry if I didn’t make it clear) is to make this easy for anyone to do in their own home. Almost everyone has an oven and some basic tools, I’d rather not ask folks to build an entire machine to do this.

      1. Chris says:

        Fair! :) It’s easy to make things complex.
        Then I would try using a glass box, I always seem to have trouble getting plastics to stick on glass surfaces. Might be simple to try with a glass oven bowl. Maybe use some silicone or ptfe spray to line the glass with? Or just cooking spray? Cheers!

  4. Joel says:

    Several thoughts.
    1. Flat steel will flex under your clamps, that’s why you don’t have uniform thickness. Stiffeners in the box bottom and the compression lid will fix that.
    2. Think of this as a casting situation and you’ll build a better box/mold. You need somewhere for the plastic to “flow” in order for it to completely fill the mold. You cannot hope to get 100% utilization of the material. You need some squeeze out waste in order to get a full part. Look at how gates and risers are designed in a green sand casting process. What you need is a riser to allow material to flow into and let it become the imperfection in the part.
    3. Draft. In order to get a permanent metal mold to separate you need draft in the design. Draft is the slight taper in the mold so that as you separate it the mold moves away from the part instead of sliding in friction the entire time.
    4. I think you might be able to do two shallow open top boxes that have some type of alignment pin arrangement. Fill them with enough material that when you initially clamp them together they won’t come completely together. As the material melts some will squeeze out the gap. This would be your “riser” and it would be around the entire perimeter which would promote material to flow to all four corners of the box. You might be able to gauge how much gap to start with by how much you were able to tighten the clamps at the mid point of your test. I’m going to guess that you probably need to start with something like a 1/2″-1″ gap. Material that squeezes out would be considered “flashing” in the traditional casting process.
    5. Silicone mold liners and maybe even a silicone grease or spray might work as a release agent. It can usually handle 450 F just fine. You could also try chilling the mold with either a salt water ice bath or dry ice as the plastic will shrink more than the steel for sure and might pull away from the mold.
    I have quite a bit of foundry and component design experience if you want to send me an e-mail. I’m curious about how you choose your stock material. I might want to try this myself.

    1. Bob Howie says:

      Old baking tins or frying pans that are non-stick work well, I use them.

  5. Joel says:

    Oh, one more thing. Heat and pressure (i.e. compression molding) is exactly how they make UHMW-PE sheets and parts. They start with powder/pellets for the base stock though.

  6. J. Steve White says:

    I’d wager that parchment paper sprayed with a silicon mold release might work, though you’d probably better weld on a handle if you wanna get that plate out the next time. I’m thinking about trying this – it’s brilliant!, thanks!. I’m trying to figure out how to heat a cinder block oven outside and control the temperature.

    Can you provide references for your 266F number? I’ve been looking for something detailing that but everything I find says ~400F for HDPE fusion. I’m thinking about a larger press and longer, temp controlled bake times.

    I’ve been getting my experimental HDPE material from the dollar store in the form of HDPE cutting boards, but I would love to be able to recycle plastic bags here at home. Thanks for the ideas!

    1. Michael Colombo says:

      The 266F number was from the wikipedia page on HDPE. I didn’t know there’s info on it fusing at 400. I’ll try that next time instead of 450.

      For temp. control I’m using an aftermarket BBQ thermometer that goes in 10 degree increments.

    2. Marc Schaefermeyer says:

      I agree with the parchment paper. I have used just a few bags and a clothes iron. I put the bags between the parchment paper and they never stuck to the paper.

  7. juannavarro says:

    What if the box was the oven? Running some wire in a homemade box could give you a uniform amount heat all around for a better melt.? I know that could be a whole other project

    Also the use of plates to sandwich them, maybe would be more rigid in keeping pressure? As for release, maybe a scored pattern on the steel of wood? give it a texture for gripping while helping it release?

  8. flamedryad says:

    great idea. here maybe another path to try that will save your oven.

    1. Michael Colombo says:

      Yeah these are way cool. My wife has made wallets and handbags with this method. But what I’m doing is different. I want to be able to do something like build a shelving unit out of plastic bags.

      1. flamedryad says:

        Well can’t you just keep stacking them and heating them till it forms a brick?
        Also what about a solar oven? Just trying to figure out alternatives cause I can’t get away with cooking bags in the oven.

        1. Bao-Khang Luu says:

          Thanks for referencing my site, flamedryad! Michael, instead of making thick blocks or planks of plastic for a shelving unit or other bigger projects, what if you tried to create corrugated boards instead. I’m not sure what facilities ITP affords you, but if you have to do this in a small NYC apt this could be more practical. Sheets of plastic would be significantly thinner and require less heat. I imagine you could construct it in a manner similar to hollow core doors.

          1. Michael Colombo says:

            That’s a good idea too. I actually graduated ITP a year and moved to the west coast, so they’re no help in that dept anymore :) – with all these ideas on here though, I’m thinking of moving operations to a local hackerspace.

          2. relevedesign says:

            Yeah, I realized that after I sent the reply. :D

            Well, more room and equipment for you! Though it would be doubly awesome if you could figure out a method that anyone can do in their own home. I’m sure you’ll figure out a solution. Good luck!

  9. srt19170 says:

    Here’s guys who did something similar adding sawdust:

    1. Rob G says:

      I generate a ton of sawdust doing woodworking and I’m constantly looking for ways to get rid of it. I imagine the woodworker’s clubs in your area would love to help you out there.

  10. Bruce Miller says:

    Plastic bottles shape with just boiling water?

  11. terrefirma says:

    I love that everyone is so collaborative. Please dont use your oven anymore though- it sounds like the ‘teflon’ syndrome. If you can’t cook with something and not kill your bird, don’t do it! How about a rocket mass stove outside? Much more energy efficient, too. (and if anyone wants to help start a maker space in Tampa PLEASE contact me- I have an empty 3bedroom house with big yard that I will let you use for FREE!) Don’t get mad for me posting here, I’m desperate.

  12. Sean Harrison says:

    Maybe making a steel mold box and using a vulcanizer press would work better than C clamps.

    You’d have well regulated heat and even pressure up to several tons. That would be able to compress the plastic better and much more evenly.

    Brand new they can be a bit pricey, but you can find them used for around $1500-$2000

  13. mr wizard says:

    if you want to go oven-less, try using a hotplate (or two depending on how big your box is) underneath the box to heat it up.

  14. Don Pancoe says:

    I know your goal was to develop a method for at-home use, but this also seems like a good project for a hacker or maker space, where a bit larger scale operation could be set up. Members could bring in their old plastic bags – something many people struggle to dispose of conscientiously anyway, and the collective could make blocks or sheets of raw material that would be sold back to members for a nominal fee (still cheaper than commercial material), raising funds for the space. You could probably even make an arrangement with a local grocery store that has a bag recycle bin if the membership alone can’t provide enough raw material.

    1. Michael Colombo says:

      That would be a great project for a hackerspace, actually. Sorry, I didn’t mean to stifle ideas or anything – I think this is just how I envisioned it. I’d love to see similar projects done in different/larger ways.

  15. raster says:

    Last year I turned a bunch of plastic bags into an art project by using a heat gun… I like the idea of pressing them into sheets though. I thought I’d just toss out the heat gun idea in case someone else had a good idea that could build upon that.

  16. Jake Spurlock says:

    What about using a hot plate with a temperature controller?

    1. juannavarro says:

      Seems to me you need pressure to mak the form, and heat to make it come together. COuld a pressure pot be of some use?

  17. Dale Giermann Jr. says:

    I think it might have been said already but i didnt read all the replies…but in my mind i see a metal container (probably cylindrical) with wiring wrapped around it to “cook” the bag low and slow with a metal top on rails that can slide down while gravity feeds it and on top of it all it would be vacuumed from under the bags so basically kinda like a vacuum former that way you have even heating (maybe not so much on the inside) constant even pressure and the vacuum will remove any imperfections.

  18. Dan says:

    Why waste so much energy to heat all the plastic flakes when you could just bond them together with a smaller amount of pre melted plastic? Heat, mix, press. If you can use solar thermal as your energy source that would be ideal.

  19. soulsatzero says:

    These plans for a powdercoating oven may help in scaling up the process.

  20. Mitchell Cottew says:

    has anyone ever tried such a thing with childrens toys? I’ve been taking the electronics from these things and saving the shells until i can figure out how to smelt them or something

  21. Mr wizard says:

    Are you sure these bags are HDPE? I think LDPE is more common for plastic bags. Not a big deal, but they do have different properties.

    1. Michael Colombo says:

      Yes they are. If they’re not labeled you can tell because HDPE is more “crinkly” and LDPE is smoother and stretchier. In early experiments with this I made the mistake of mixing them.

  22. Eric says:

    Its interesting, but makers tend to always think that they are always the first people smart enough to broach a topic, so they always start at first principles without looking to see if anyone has done work on it first.

    Out in the midwest, there are people that have been doing this for years using a propane burner system to turn waste bags and packaging into 4×8 sheets of paneling for hog and chicken sheds, as it is cheap and doesn’t rot, and can be heat seamed to be waterproof.
    So, its already been figured out how to practically do this – don’t look to reinvent the wheel, look at how to improve it.

    Then, do exactly what they have already been doing for years – making cheap, waterproof shelters for instance, just claim to have invented a brand new solution for something currently trendy (re;the third world or something), slap the name ‘open source’ on it, and rake in the grant bucks.
    After all, most successful makers aren’t actually makers per se, they are people successful at marketing and/or patenting ideas they stole from others (i.e. Makerbot). Making is about repackaging and cashing in, not actually making things. Get with the program.

    1. Michael Colombo says:

      Hi Eric. Thanks for commenting. Wow! I had no idea people have been doing that in the midwest. Do you have documentation for it? I’ve done quite a bit of research on this and haven’t come across much besides plastic lumber production that’s done on an industrial (and proprietary) scale.

      btw, I never claimed to be the first one to try this. In fact if you look at my first MAKE post about it, I mentioned that I got the idea off Instructables – I’ve just been looking into refining the process. I hope this has “gotten me into the program.”

      1. eric says:

        Sure, I’ll try to find some phone numbers. Guys in Iowa use laundry detergent bottles and milk jugs and bags and stuff. Makes for really bright and ugly multicolored sheets, but the local farmers all use it because its cheap and panels seam up together with just a big soldering iron with a tile cutter tip. That isn’t the only place i’ve seen the panels, just the only place where I saw guys actually make them. Been a few years since I lived in farm country, but I will try to see if anyone still back there knows who is still making the stuff.

        1. Michael Colombo says:

          Eric if you get in touch with someone who does that and send me some video documentation I will love you forever and send you a prize in the mail. For real! Part of what we try to do at MAKE is find makers in crevices and bring them into the light. There are so many people around doing cool stuff on their own and deserve some recognition.

          1. Eric says:

            No video, but I found a source for you to contact to get one;
            Though it seems the old guys I know seem to have stopped doing this, there is another even bigger operation right nearby in Iowa called ReWall that makes this same product under the name ‘Nakedboard’ (may be the same business just that grew, don’t know).

            There is also company in the UK called Smile Plastics doing just this for more mainstream uses. Their stuff is pretty cool, it looks just like the old paneling from those old dudes Iowa (especially the stuff made from yogurt containers).

            Hope this helps. I was being a bit crass in my previous post, but I was serious in that you should re-purpose this idea and pull in grant money. No kidding its out there. “Turning a third-world trash disposal issue into a sustainable housing bounty” sounds like a surefire grant proposal title to produce a lot of column inches of press and research dollars towards something that could really help people!

            Good luck!

          2. Eric says:

            Sorry, the ‘Nakedboard’ is not the right one. they have “Nakedboard+” which appears to be plastic cored. The plain Nakedboard is paper product. Looks like they do both.
            The Smile UK stuff is what you are looking at still, though.

        2. Bob Howie says:

          If you use laundry detergent bottles your room will smell lovely, anything pungent that penetrates the plastic will last, I use old milk bottles as they do not have an ingrained smell.

  23. jay says:

    I say leave the C clamps off and heat the thing through then onto a shop press.

  24. Rob G says:

    For the mold, you might want to sue a fiberglass release agent, which is basically just several coats of wax. You can get better flow by shredding up the bags, and filling the mold more regularly prior to molding. TO take care of the non-planar top surface, you might use a flat weight instead of a C-clamp, or use a bowed top plate to compensate for the curve from the clamps.

  25. Michael says:

    Cartridge heaters are pretty cheap in case you want to make a heated mold. Make a hydraulic press from a bottle jack and some steel tubing.

  26. Keisar Betancourt says:

    This will never be a viable product until you can control with some certainty that it will be internally stable, share attributes from test point to test point both in density, size, and relative strength (separation, shear, etc.) even as bricks. so… get to it! lots of potential.

    1. Michael Colombo says:

      Oh definitely. At this point I want to be able to make a process that that produces something of a decent size and consistency. After that the stress testing shall begin :)

  27. James Patrick says:

    Try a blender for shredding the bags. If I were you, I wouldn’t use your food oven for chemical experiments. That’s what Craigslist ovens are for.

    Has anyone tried this yet?: Melt bags in a cooking pan until they start to stick together. Then put the mass into a warm mold and compress into blocks.

  28. xaqfixx says:

    If concerned about fumes – have you checked the MSDS for the HDPE in the bags? It should provide handling risks, tips, and chemicals. Search for HDPE Film MSDS.

    I looked at several and didn’t find any flammable off-gasses, some dangers of irritation from prolonged exposure to above-melting temperature heat – similar to the risk from prolonged exposure to dust.

    Bering a thermoplastic it may be finicky to machine, for example it can be cut with a laser cutter but melts horribly. I’ve not tried to cut HDPE on a bandsaw or scrollsaw but have had very little luck with other thermoplastics – friction heat re-welds the cut I’d just made.

    1. Bob Howie says:

      It does tend to melt if cutting with rotary cutters but I found sanding works well except for the extremely fine dust that gets everywhere so a good face mask is recommended.
      I use a Moldex 9003 with dust and fume filters as they can be stacked.

  29. Bob Howie says:

    I make my plastic blocks from old milk bottles (HDPE 2), I use an old pan or an old baking tin, usually cut the bottom off the bottle at the bottom ridge this gives me sides to stack the material in.
    I then chop the rest of the milk bottle into strips and fill the space I require. If the bottom is full I will cook it like that if it is thinner I cut the sides so they fold inwards then I secure with tie-wraps. Remember if you do not apply pressure to the top the shrinkage will be greater, I have found tie-wraps to be best, as they melt at a much higher temperature and do not stick to the plastic.
    For a piece about 20mm thick, I cook at 180C for 20 minutes, longer the thicker it is, then let it cool naturally. So far I have not encountered any problem with air bubbles being trapped. I have also been able to cook one piece then put it into another bake and increase it’s size without there being any dividing line.
    The only caution….when sanding, the powder is extremely fine, almost like talcum powder, so a good mask, hood and gloves is recommended.

  30. Bob Howie says:

    I use a combination microwave but only use the fan oven function so the fumes do not gather but also open a window when doing it for added safety, although I haven’t found the fumes to be that bad.

  31. Jean Danniele Salindong says:

    Greetings! May i ask if I can just use a grill instead of an oven or a hot plate? And also, what is a hot plate? I apologize for the ignorance; i am just a neophyte with these stuff. Please reply as soon as possible. Thank you.

  32. John Klatt says:

    I was thinking something more like the metal box when I read through your first post with the wooden box.

    My comments and what I will be doing.

    Metal box with possible slight taper to aid in release, just in case things get wonky.

    On the lid a thought would be to have a threaded hole at near an edge or corner. Then you could thread a bolt in to help it release, if just having foil didn’t work.

    For the clamping, if it will be enough, I have a 100lb anvil I’ll be setting on top of it.

    If you had a rough idea of your compression rates on the materials you could have some stop blocks set to the side. For instance, get some big plate weights have them so they are sitting well above the box, then as they lower at some point they hit a stop. That would obviously affect your thickness and compression so it would take some playing around to get used to, but if your stops were all even eventually it should settle out the high spots and be level. That’s just in theory of course.

    Another thing would be to add integral stops inside the box. This would work if you wanted to consistently produce the same sized block of material. Weld some bits of metal to add 45 degree bevels to each corner. Say all 1″ tall. Compress and melt, add more material and compress and melt. With some high temp gloves you could undo the mold while it’s still very hot, add more material, and then reclamp and remelt. Would add more steps to it but potentially yield a more uniform brick.

    Just my thoughts on it. My eventual goal is to build a steel box to work as an oven and use a fresnel lens to heat it.

  33. Natelea says:

    Could you do this in an electric kiln do you think? To help better regulate the temperature? Also, what about using oil as the mold release? I will be trying this to mold up a carved figure for a sculpture I am working on at the moment and I think there are some really great possibilities here.

  34. Jon Durfee says:

    I have been experimenting with the HDPE with some decent success. I am currently trying to figure out the best molding proceedure. I use mostly milk jugs as my source of material, but I may try plastic bags soon.
    After reading this article, I wanted to pass on a few tips.
    1) Try using a CONVECTION OVEN at 350 degrees to melt the material. It’s constant flow of air cuts the melt time down dramatically. I used a regular toaster oven at first but it was much slower and because of it’s directional heat it tended to char the parchment paper.
    2) Use Oven safe Wax or Parchment paper. Covering cooking pans and molds with nonstick paper will allow for easy release. When the plastic is molten it will adhere to the paper but as soon as it cools to a tacky temperature it will stop sticking.
    3) Don’t be afraid to melt in stages, small pieces of HDPE and Plastic bags are bulky. Melt a small amount at a time and then add more and more until you have the amount you need. Folding and mixing different colors will make cool patterns

    Check out and his plastic molding projects section. He has probably done more work with HDPE recycling at home than anyone else. His blog taught me alot. I will be experimenting on some heated molds soon and will post updates as they come.

  35. Andy Lee Graham says:

    I think your ideas can help save babies from dying of malaria. Michael, I am from Orland, Indiana, but hang around in Togo, West Africa, and have become obsessed with stopping malaria. Children under 5 die after being bitten by malaria parasite infested mosquitoes. The mosquitoes breed in water, the plastic bags here make dams in river, are on the ground in piles, they hold water creating breeding grounds.

    Your concepts are BRILLIANT

    1. Compression — This is the one I was lacking.

    2. Heat – We have an overabundance of solar, so I hope to do this with the sun.

    3. Break away steel box — Super easy here in Togo, West Africa, strange as it gets, the cost of welding is half the price of carpenters. I can get Ameyo to make this easy. They use this device to break tires from rims of cars here, it is big lever you put your weigh on, and it compresses the tire. I can modify this, make it so we load with rocks, and cook with solar… I hope.

    4. Need for uniformity — No need for a few products. I am thinking of using for walls of septic tanks, they need cheap tanks for the water from toilets. Maybe bricks of plastic.

    I would super appreciate if we could talk on Skype, or write emails. I am hoboontheroad AT AT / or Skype or 1-260-624-4414 that rings on Skype after 10-15 rings here in Togo, West Africa, a country to the right of Ghana.

  36. jeroen says:

    Hey man,

    You have some cool ideas. Note that a different plastic bags are produced in different factories, with different filling material. Polymers also have a melting-range, the melting temperature of a polymer is multiply defined. The melting temperature (Tm) is also depending on the speed of heating etc. There are a lot of factors that play a role. The fact that the plastic does not fuse uniformly is indeed because of different types of material. I see a picture with a scissor, have you ever tried a blender or something similar?

    I am looking to melt down PET-flasks, do you got any experience with that?


  37. Will says:

    Anyone tried using oven safe silicone molds? That way you could avoid contaminating your HDPE with oil. They’d have the benefit of being reusable and not require baking paper, which I imagine is fiddly to cut to the correct size. If you got 2 you could stack and clamp them… would probably need some board to give even compression. I’ve talked myself into giving it a go!

  38. Jeff Ax says:

    I’ve recently been playing around with HDPE also, and I’ve quickly learned a few things that I want to pass on.

    1) HDPE sticks to metal REALLY, REALLY WELL!!! Using a plain steel box of any type as your melting vessel or mold will ensure a NIGHTMARE when you try to retrieve the finished piece!!
    Solution: Non-stick baking sheets, cake pans, etc. – These work awesome as long as you have patience as well. If you work quickly and carefully, you can peel the hot molten stuff out using a very stiff plastic putty knife or something similar. (Leather gloves required, and be careful not to scratch the non-stick coating.) Otherwise, just let it cool completely and it will easily pop right out. Because it shrinks as it cools though, this is really only good for making chunks that you plan on milling further in some fashion.

    2) Melamine is excellent both in the oven and out. – My very first attempt at melting HDPE was for the purpose of making guide rails for my table saw sled. I wanted to make pieces of material a few inches wide that I would then run through the saw to the proper width. I wanted them to be 3/8inch thick right out of the mold though, so I used a sheet of 3/4″ melamine, (particle board with melamine on both sides, found at almost any home improvement store), to build a long narrow box about 4″ x 22″ (as wide as would possibly fit inside the oven), and a couple inches deep. I then cut a lid from the same material so that it fit WITHIN the sides nice and snug. The plan was to drop that in on top of the molten plastic and use lots of clamps to compress it. I drilled 4 or 5 random holes through that lid using an 1/8″ bit to give the air a way to escape.
    -To achieve the thickness I wanted: – Keeping in mind that it’ll shrink as it dries, I cut a couple of narrow (aprox 1/2″ wide) strips of wood about 19″ long and about 7/16″ thick. I placed the strips inside the box, one on each side, so that I could compress the lid down on top of them and get the thickness I wanted, (at least 3/8 thick after it cooled). And I purposely cut them shorter then the full inside length of my box so that the extra material would have a place to go as I compressed the top. This actually worked out really well although I would do it a little differently next time…
    -Notes: – Since I used a raw softwood for those thickness strips, I had to figure out how to ensure they wouldn’t permanently stick to the HDPE. I decided to try an easy solution first and just coated them with vegetable cooking oil a couple of times over about a half-hour period and then, because that mostly soaked in, brushed one more coat on right before I put it all in the oven. I got lucky and that actually worked out really well.
    Next time, I wouldn’t bother building a box. As it turns out, just a base plate with those thickness strips down the length on either side, and then the top clamped down when the time came, is all it really needed. The rest was pointless.

    3) Pyrex is great as a melting vessel also, with one very big caveat! – So, I saw a video where another guy melted the HDPE in a Pyrex dish and then flipped the dish over and let it cool upside down. As it cooled, the HDPE separated from the glass and the cooled brick simply landed on the table. How much easier could it get?!?! WOOHOO! I figured “Game on, there’s the obvious solution!”. So I went and bought a 9×9 Pyrex dish, cut a piece of Melamine to fit nicely inside it, and layed out my clamps to compress it once I had my molten mass.
    This is a no-brainer. Piece of cake, right? ……… WRONG!!
    Remember that part about the brick of plastic separating from the glass and falling once it cooled? Ya… turns out that that’s possible due to the “shrinking as it cools” factor. The HDPE sticks tenaciously to the glass when it’s hot, (nearly impossible to get it out). The slow cooling and shrinking, as it turns out, is what causes it to separate. My melamine and clamps, (which I dutifully tightened a couple of times as it was cooling for the first 10 minutes or so), made it impossible for that separation to happen. The repercussions of this were pretty amazing!!! LOL
    After about a half hour I heard some crackling and a quick *POP*. Yep, that was the Pyrex breaking due to the plastic PULLING IT INWARD ON ITSELF!! That glass is probably 1/4″ thick, but it was being shattered due to that plastic pulling the entire bottom surface inward, laterally, as it cooled!! And I do mean “shattered”!! Over the next 5 or 10 minutes cracks continued to shoot out in every direction until the entire thing was pieces no bigger than about 1″ in any direction, yet all still held together perfectly due to being so thoroughly stuck to the glass!! LOL It was amazing! I had to put on a full-face shield and use a hammer to remove (most of) the glass from the plastic, and it still left a thin film of glass on the plastic that I still haven’t figured out how I’m going to remove. HAHAHA
    Soooooooo… I would strongly suggest NOT clamping the plastic to the glass. Only use the Pyrex method for making “blanks” that you can later mill down as needed… hehehe

    4) You can’t simply push the bubbles out via clamping. – This stuff is so thick when molten, and so sticky, that compressing will not (CAN NOT) possibly push the air out. How will the air possibly go anywhere when you’re also pushing down on the plastic all around it?! You’ll only end up with a smaller bubble containing compressed air. The only way to remove the bubbles is to work them out via kneading the plastic like bread dough, which is pretty damn tricky due to the temp of it.
    -Method #1 – Thick leather gloves and multiple bursts of kneading it really quickly (10-15 seconds tops) before tossing it back into the oven because it’s already becoming to stiff and you’re hands are burning! Repeat ten minutes later, once it’s molten again, as needed…
    -Method #2 (the no burn method) – Compressing a small section of the plastic in the middle will successfully remove the bubbles because the air has a place to go, (outward, toward the edges). I decided to try using the head of a 3lb sledge hammer, pushing down as hard as I could right in the middle, and then working my way outward. The cold hammer almost instantly cooled the plastic though, so I didn’t get far before having to throw it back in the oven. The solution was to put the hammer in the oven with it! The hot metal head of the sledge allowed me to work the plastic a lot more before it stiffened up. It still took multiple tries, but also resulted in a much more consistent finished piece.

    And that’s what I’ve learned so far. I want to try to make some “wheel bearings” (basically) next. My next project is a home-made benchtop belt sander and I’m curious about impregnating the HDPE with oil and then using it with a stainless steel rod running through it, essentially like a wheel bearing. Will melting it down in a pan of hot vegetable oil result in the HDPE having slipperier properties once finished? I’m thinking it probably will, but any input would be appreciated!! Thanx!!

  39. Bruce Miller says:

    I stumbled upon this while pondering the environmental issues of HDPE, and it is great! I will try baking my first brick this afternoon and report the results.

  40. MaFu says:

    Had an idea for the release problem on the metal box. I don’t know if anyone has tried it yet, but maybe worth a go? You can buy teflon in a spray can form from several different places. They used to sell it at my local restaurant supply place back in virginia. I’m sure you can also order it online somewhere. Try coating the inside of your box and lid with the teflon just like a non stick baking pan. Should make release much easier next time.

  41. C Squared says:

    Use springs between the clamp and lid so you won’t have to adjust them

  42. joezen777 says:

    I experimented with a hot air gun. In order to get any appreciable difference in hardness from the plastic bag I was working with I had to heat the area to 150 Celsius (measured with IR gun). Heating to 130 Celsius or just below still resulted in only a smaller but still flimsy and brittle fabric. In order to get consistent shrinkage from my hot air gun (the type you use for soldering) I set the temperature to 260 Celsius and the air flow speed to max. That was to get an area temperature up to 150 Celsius on the plastic. Earlier I had tried between 225 and 130 on my air gun but those were the temperatures that resulted in the plastic not going above 130 Celsius. I think next step for me would be to see how I could get the plastic to shrink to a mold. I was trying to melt within a paper origami box or around one. My next guess would be treating it like canvas and see if I can fold multiple layer around an origami paper box and clamp in place and see if it allows me to mold and heat into small plastic bins for my electronics.

  43. ada says:

    I am trying to use plastic bags into toy molds but I am having problems with melting the bags enough for enough time as well

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In addition to being an online editor for MAKE Magazine, Michael Colombo works in fabrication, electronics, sound design, music production and performance (Yes. All that.) In the past he has also been a childrens' educator and entertainer, and holds a Masters degree from NYU's Interactive Telecommunications Program.

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