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.

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.

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.