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Stovetop recycling of HDPE swarfSunday, May 03. 2009In which your narrator solves the problem of recycling plastic scrap, for milling... One of the great unsolved problems for Reprap has been that of either figuring out how to make plastic filament locally from scrap or how to design an extruder that can digest scrap. Reprap has successfully created a 3D printer for less than $1000 that replaces a commercial machine costing anywhere from $30-50,000. A plastic grinder and monofilament production machine runs anywhere from $75-100,000 and, unlike a commercial printer, can weigh several tons. For milling, as opposed to printing, I've discovered that recycling can become much less challenging. Years ago in South Africa, I encountered a plastics fabrication technique called Rotary Moulding or Rotomoulding. Typically, you dump a measured amount of polymer powder into a big steel mould and secure it to a motorized gimbaled drive in a heated enclosure. The steel mold becomes slightly hotter than the melting point of the plastic while the tumbling action of the gimballed drive coats the inside of the mould with plastic powder. The method is used to make everything from the petrol tank on your automobile to portable toilets and large water tanks. One guy I got to know in Pretoria back in the 1980's was using HDPE swarf to rotomold bins and water tanks. Yesterday, I got to thinking. I'm buried in fine HDPE swarf and I need HDPE sheet. Could I modify the rotary moulding technology to create it. I took an old teflon coated frying pan that I didn't need any more and heated on my stove to about 220 C, checking the evenness of the surface temperature with my handy-dandy IR thermometer. At that point I began sprinkling a couple of cups of milling swarf on the heated teflon surface. Interestingly, the swarf, being heated from below, tended to melt into the already melted surface without trapping large air bubbles like happened with my experiments some years ago to melt HDPE scrap in a toaster oven. Once I ran out of swarf I cooled the pan and plastic down in cool water, then peeled the melted swarf out of the pan and cut it in half to see how thick it was.
The bottom surface is very slightly porous, though not enough to leak water. The top surface is rough with unmelted swarf and a few bits of poplar.
Here you can see the thickness of the melt, 2 mm. It would have been thicker except that I ran out of swarf. The slightly mottled colouring of the melt reflects the mix of white and black swarf that I had. With thicker melts I could simply mill off a flat top surface. The need for that could be reduced by using a small aluminum rolling pin to flatten the top. Now I have to either buy or mill a small hand vacuum cleaner to collect the swarf in a more orderly manner than I do presently by using compressed air. Trackbacks
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sorry, I forgot you can't embed images
This tray I mean. It's a teflon tray if that turns out to be neccesary.
http://lakeland.scene7.com/is/image/Lakeland/12370?$normal$
As I said. It was a worn small frying pan that I didn't need any more.
This is quite interesting!
And this technique might be getting close to getting rid of reprappers' depencence on expensive plastic welding rod.
Imagine that one turns swarf (or pellets) into a disc of plastic 3mm thick. I'm envisioning using a large frying pan, resulting in a disk with appreciable volume of plastic.
One could then imagine a rotary "peeler" to cut the disk into a long spiral piece (~like the groove in an old vinyl record, played with a hot-wire in place of the stylus.) In cross section, this would be square, 3mm-on-a-side.
This spiral could then either be fed directly into a extruder designed to handle a square cross section (I think a pinch roller would do well here.) Or th spiral could be straightened and the cross section re-formed by gentle heating while rolling. -- And thence fed into a conventional reprap extruder.
Hmm, I was just thinking of something a bit simpler.
1: Print out a simple plastic spiral.
2: Hammer a sheet of teflon-covered metal onto the plastic spiral, producing a sort of mold.
3: Heat, sprinkle, just as above.
4: Get a long spiraled plastic rod. It's not quite cylindrical -- it has a cross-section more or less like a letter D, and it's spiraled, but it's probably close enuogh.
Interesting.
Have you tried a standard turbotherm domestic oven. Mine goes up to 220 easily (I used it to heat up brass busshings for pressing into acrylic without splitting) and can be verified using a jam thermometer.
As the heat is all over even, hot air, your plastic shouldn't heat unevenly and bubble like your toaster oven.
You want to melt the top, as you've melted the bottom? I would just flip it like a pancake.
Better still, have you seen those George Foreman grills? The concept is basically a small plug-in electrical appliance, with a teflon-coated heating surface on the bottom and another one that's hinged on top like a clamshell. You put your cheese sandwich, meat, whatever in and you close the lid. It's now being heated on both sides in a teflon-coated moulded shape. The major selling point of the George Foreman grill is that the whole thing's at a slight incline, and the runny cholesterol-filled juices run out of your food so it's not as greasy.
Imagine if the teflon surface wasn't the shape of a sandwich, but rather shaped to be a mould of a spiral. You'd presumably end up with the HDPE melting into the shape of spiral filament. If that doesn't work, I'd try something like a spaghetti maker - add in a heating element and it's basically the same process, yes?
A good idea to have cheap machinable HDPE is to mix HDPE with wax (the one my girlfriend uses to get legs waxed is cheap and can be coloured)
HDPE is found everywhere and it's cheap but I've not found a way to grind HDPE at home efficiently.
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