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Expertise needed on melting and remolding plastic (PP, HDPE etc.) at home with hand made molds... there must be a way! Answered

Hi, I want to make a rough casing for a project out of plastic of some kind. I tried the potato plastic instructables, but I'm not really happy with the result. So I attempted to melt down some plastic chips broken from old plastic casings (I think the plastic was polypropylene) in an old can on a barbeque (good as it was outside). This didn't work as I kept heating the plastic chips even when they became a bit soggy as I expected them to become much more liquid. This is because I hoped to 'pour' the molten plastic into a mold. Before this happened however the plastic caught fire. So, I was wondering if I heated them in a more sealed environment would the lack of oxygen make it possible to achieve a more molten consistency without the plastic igniting? Some plastics on consumer electricals have flame retardants... surely these would stop the plastic from igniting? If the plastic won't ever become liquid/molten, then I assume to mold it you have to have very high pressures (like commercial injection molding machines)... any ideas as to how to acieve these kind of pressures easily at home? I think with so much 'waste' plastic out there it'd be great to work out some easyish way of remolding it at home! Any help appreciated! Thomas


Toaster set at the correct temperature for the plastic that you are melting, (ex. ~350F for HDPE, #2). Once it is melted to the consistency you want it, transfer it quick to a mold held within a shop press, or a vice grip, or something similiar.

'melted' plastics arent much for calling them 'liquid' at any point during that journey. Injection molded parts are done so at hundreds, if not thousands of pounds of pressure to for the relatively unwilling plastic to do your bidding.

Used to work as a die setter in an injection moulding factory, the smaller presses inject at around 8 tons, the larger presses can go up to 50 tons.

Actually, that plastic smithing instructable does say you can use #2 or #4 plastic instead of bags. If you shred it and cook it in oil, it becomes very gooey moldable. I have made a chunk of pink plastic like this from a bubble soap container.

I know it's been 7 years, but don't cook it in oil.

That will change how the plastic behaves when it's dry, as the oil mixes with the plastic,.

Instead heat up ya oven to three hundred n fifty degrees, and let it warm up fir 10 minutes. Then stick a drink can with cut up ♻2 plastic in it, it will meet, then you take it out with a sealed back pencil. Thee YouTube videos on how to do this now

I'll second Lemonie's suggestion re the "melty-plastic" Instructable, Stasterisk's Plastic Smithing:https://www.instructables.com/id/HomemadePlastic/
She melted HDPE plastic bags in a pan on the stove, and successfully molded them into a wheel for a robot.

HDPE melts at 265 F (often a bit lower), while PP doesn't melt until 320 F. But the plastic chips you were using might not have been polypropylene. If they were one of the "thermosetting" polymers, they wouldn't ever melt at all, just like you can never melt a hard-boiled egg.

Any of the plastics with recycling triangle numbers from 1 through 6 are "thermoplastic" polymers; and can (at least in theory) be melted, molded, cooled, remelted, etc. indefinitely, like a ice cube rather than a hard-boiled egg.

HDPE (#2) and LDPE (#4) have the lowest melting points and emit little or nothing in the way of fumes. And since they are both the same polymer (polyethylene), they can be melted together.

PP (#5) melts at 320 F, but may sometime have additives that emit harmful fumes.

PET or PETE (#1) has little or nothing in the way of fumes, but doesn't melt until 500 F.

PS (#6) melts at 465 F, but may often have additives that produce fumes that are much better avoided.

PVC or V (#3) melts around 220-265 F, but always produces very nasty, harmful, dangerous fumes. Don't try this at home, kids.DANGER, WILL ROBINSON!AVOID AVOID AVOID.

#1 PET gets soft and stretchy at relatively low temperatures, less than 100C. I've had success re-shaping PET bottles by carefully heating with a hot-air gun, and re-forming. Your description sounds like you want to cast (pour into a mold) rather than form (force against a mold), but forming is a lot easier.

PVC in the form of pipe doesn't get workable in larger thicknesses until 400F and at 500F it will reluctantly take impression from screw heads. however, it will only soften, its liquefication point is higher than the combustion point, so for higher temps it requires an oxygenless environment


5 years ago

Would a pressure cooker work? It would increase the temperature a bit and as the atmosphere inside the cooker would be steam rather than air, there should be little risk of fire. If I tried this, I'd use HDPE or LDPE, as I don't think ordinary pressure cookers would get up to the temperatures required for polypropylene - indeed a rough calculation suggests that it would only just work for HDPE and LDPE.


6 years ago

Polystyrene plastic, number 6, forms when dipped in acetone. White EPS foam, coffee cups, solid plastic cups, yogurt containers, all will turn to playdoh, then solidfy when the aceton evaporates. You can hand form it, stuff it into a mold, extrude it like playdoh, etc. Wear latex gloves.

Look at the extruders on the RepRap home 3d printing machine project pages. Also, you would take a steel pipe, coil nichrome wire around it, cover that in plaster, then nest the whole assembly in another pipe with an extruder tip. Put your plastic chips in the tube. Force it through the tip with a plunger of some sort. Maybe try using a hydraulic arbor press like those used in the automotive trade.

Otherwise, consider fusing plastic into sheets, then creating a 2-part mold and forming the hot sheet.

Sounds like you tried to work with thermosetting plastics, i.e. they don't liquify as they get hotter. You can only reheat and reform thermoplastics. Google for specific lists, there are too many to discuss here. I would not use a direct heat source like a grill, but a smoker might work if you can build a small capsule that keeps the smoke from reacting with the polymers. When you heat up the plastics, you increase the rate at which reactions occur, so you want to limit the interaction with contaminants. Any type of plastic used in an environment where heat is generated, i.e. electrical, are usually thermosetting. Don't waste your time. Good luck