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Nice “short form” Instructable from user mfoster, who cuts up milk jugs and cleans and dries the pieces before heating them to 350°F, loose, in a pot of canola oil. The temperature is monitored with a candy thermometer. The HDPE pieces soften into a clear “gel” at this temperature, but do not come apart, and are removed from the hot oil with a slotted spoon. They are transferred to a simple press made from a few scraps of lumber, and the press is closed and operated using a set of three manual clamps. The cooled blocks can be shaped with normal woodworking tools.

Making Blocks out of HDPE milk jugs

Sean Michael Ragan

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I am a long-time contributor to MAKE magazine and My work has also appeared in ReadyMade, c’t – Magazin für Computertechnik, and The Wall Street Journal.

  • jamesbx

    Interesting. I would have expected the plastic to stick to the slotted spoon.

    • Sean Ragan

      I can see why you would. It may be the oil that makes the key difference in that respect.

  • onlinefilmler

    Interesting. I would have expected the plastic to stick to the slotted spoon.

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  • Slarti Bartfast

    This instructable caught my eye, as I wanted a cheap source of machinable plastics, some of which I use for the creation of boomerangs.

    You probably dont need oil – both HDPE and polypropylene melt non-toxically in a normal oven; The actual melting temperature seems to vary quite a bit, but my HDPE milk bottles melted at around 180-200 degrees celsius, although other bottles needed more – I didnt have time to find out how much higher. My polypropylene (ice-cream!) containers needed 200-210 degrees. PP also seems to re-melt more reliably, and since literature seems to point to it being a bit tougher it is the material I am leaning to.

    Both solidify very quickly on cooling, so need quick handling if you are pressing them into moulds, or to flatten evenly and remove air bubbles.

    By the way PET is non-toxic too, but the melting point is about 100 degrees higher, so not suitable for me.