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.

clamp

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.

bookmatched

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.

Michael Colombo

Michael Colombo

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.


  • Donald

    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.

    • http://pushtheotherbutton.wordpress.com Michael Colombo

      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.

      • http://www.fwacata.com juannavarro

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

        • http://pushtheotherbutton.wordpress.com Michael Colombo

          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.

      • Bob Howie

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

        • https://www.facebook.com/michaelcolombo Michael Colombo

          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?

  • http://trkemp.wordpress.com trkemp

    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.

    • http://pushtheotherbutton.wordpress.com Michael Colombo

      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.

      • stephen paulger

        The false bottom would cover the holes.

  • Chris

    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?

    • http://pushtheotherbutton.wordpress.com Michael Colombo

      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.

      • Chris

        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!

  • Joel

    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.

    • Bob Howie

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

  • Joel

    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.

  • http://www.facebook.com/jstevewhite J. Steve White

    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!

    • http://pushtheotherbutton.wordpress.com Michael Colombo

      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.

    • http://mtschaef.blogspot.com Marc Schaefermeyer

      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.

  • http://www.fwacata.com juannavarro

    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?

  • flamedryad

    great idea. here maybe another path to try that will save your oven.
    http://www.relevedesign.com/how-to-fuse-plastic-bags/

    • http://pushtheotherbutton.wordpress.com Michael Colombo

      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.

      • flamedryad

        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.

        • http://gravatar.com/relevedesign Bao-Khang Luu

          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.

          • http://www.facebook.com/michaelcolombo Michael Colombo

            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.

          • http://relevedesign.wordpress.com relevedesign

            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!

  • srt19170

    Here’s guys who did something similar adding sawdust:

    http://www.designboom.com/design/kulla-design-studio-50-sawdust/

    • Rob G

      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.

  • Bruce Miller

    Plastic bottles shape with just boiling water?

  • http://gravatar.com/terrefirma terrefirma

    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.

  • http://www.facebook.com/sean.harrison.5076 Sean Harrison

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

    http://www.contenti.com/products/spin-casting/175-312A.html

    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

  • mr wizard

    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.

  • http://donpancoe.wordpress.com Don Pancoe

    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.

    • http://www.facebook.com/michaelcolombo Michael Colombo

      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.

  • http://rasterweb.net/raster/ raster

    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. http://rasterweb.net/raster/projects/plasticsun/

  • http://jakespurlock.com/ Jake Spurlock

    What about using a hot plate with a temperature controller?

    http://blog.makezine.com/2011/09/15/jakes-laser-cut-sous-vide-controller/

    • http://www.fwacata.com juannavarro

      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?

  • http://www.facebook.com/profile.php?id=100005639952925 Dale Giermann Jr.

    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.

  • Dan

    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.

  • soulsatzero

    These plans for a powdercoating oven may help in scaling up the process. http://www.powdercoatoven.4t.com/Index.html

  • http://www.facebook.com/Vincent777at Mitchell Cottew

    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

  • Mr wizard

    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.

    • http://www.facebook.com/michaelcolombo Michael Colombo

      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.

  • Eric

    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.

    • http://www.facebook.com/michaelcolombo Michael Colombo

      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.”

      • eric

        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.

        • http://www.facebook.com/michaelcolombo Michael Colombo

          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.

        • Bob Howie

          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.

  • jay

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

  • Rob G

    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.

  • http://www.bustedbricks.com Michael

    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.

  • Keisar Betancourt

    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.

    • http://www.facebook.com/michaelcolombo Michael Colombo

      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 :)

  • http://jamesrpatrick.com James Patrick

    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.

  • http://gravatar.com/xaqfixx xaqfixx

    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.

  • Bob Howie

    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.

  • Bob Howie

    p.s.
    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.

  • https://www.facebook.com/jeandanniele.salindong Jean Danniele Salindong

    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.

  • John Klatt

    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.

  • http://www.roderickandbell.com Natelea

    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.

  • Jon Durfee

    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 atomicshrimp.com 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.