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 Plastic Bags into Plastic Blocks: Revisited

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 Plastic Bags into Plastic Blocks: Revisited

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

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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