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Remelting

4 min read

To remelt salvaged plastic, shred one sorted type into 5-10 mm pieces, melt it in a steel container heated by a metal surface, oil bath or enclosed oven rather than an open flame, hold it at the low end of its working range and mold or extrude it promptly. Overheating breaks the polymer chains and produces the most toxic fume, and degradation begins 30-50 °C above the working temperature according to the carrier.

Part of Plastics and Polymers

Remelting is the heating of shredded, sorted thermoplastic until it flows, so that it can be molded or extruded into a new product.

Start From A Clean, Sorted, Uniform Feed

Everything done before the melt shows up in it. Sorted by type, washed, free of labels and glue, and shredded to an even size, the plastic melts evenly. Large lumps leave cold cores while the outside overheats. See Sorting and Identification first, and Recycling and Reprocessing for the full chain. Never melt a plastic you have not identified.

Working Ranges From The Carrier

Plastics Manufacturing gives these as the temperatures at which each plastic is melted for extrusion:

PlasticRange
Polyethylene (PE)120-180 °C
Polypropylene (PP)160-220 °C
PVC160-200 °C
Polystyrene (PS)180-260 °C

Treat them as wide ranges. Begin at the low end and raise the temperature only until the plastic flows smoothly. A range this wide cannot be an exact melting point, so the corpus gives you a window to search in, not a setting. Do not melt PVC in quantity at all; the carrier’s advice is to reuse PVC pipe whole.

Method: Melt In A Steel Container

Step 1: Heat the container, not the plastic. Use a heated metal surface, an oil bath or an enclosed oven. The carrier’s warning is that degradation begins 30-50 °C above the working temperature. The carrier says not to heat plastic over open flames, because flame touches give local overheating and ignition.

Step 2: Load in small portions. Add shredded plastic as the previous charge melts. A full cold load melts from the walls inward and chars at the contact.

Step 3: Stir gently to even the temperature and push out air. You want a uniform, glossy, flowing melt with no smoke. Visible smoke, brown colour or a sharp smell means the temperature is too high: take the heat off and ventilate.

Step 4: Mold or extrude promptly. Plastic left molten keeps degrading. See Molding Process and Extrusion.

Step 5: Keep a log of type, temperature, time and number of remelts. A batch that has been through several remelts needs checking before it carries load.

Why Each Pass Weakens The Plastic

Heat and shear break chains, and shorter chains mean a weaker, more brittle product. The carrier’s rule of thumb is to mix in virgin material or filler after 5-7 cycles. That figure is unsourced; test a molded bar from every batch by bending and see Safety and Quality. Keep regrind with its own type and cycle count.

Worked Example: Heat Needed For A Batch

The corpus gives no heat figures, so these inputs are assumed typical values for polyethylene: specific heat 2.3 kJ/(kg·K) and heat of fusion 200 kJ/kg.

  1. Mass 10 kg, from 20 °C to 180 °C: a rise of 160 K.
  2. Sensible heat = 10 × 2.3 × 160 = 3,680 kJ.
  3. Melting heat = 10 × 200 = 2,000 kJ.
  4. Total = 3,680 + 2,000 = 5,680 kJ, about 5.7 MJ, or 5,680 / 3,600 = 1.6 kWh.

That is the heat into the plastic only. Furnace and container losses are larger, so plan fuel at a multiple of it that you measure.

Common Mistakes

  • Open flame under the pot — local hot spots, char, fire.
  • Pouring in a mixed load — the higher-melting plastics stay solid in the melt.
  • Leaving melt standing on the heat — degradation and fume.
  • Water on a molten plastic fire — it can splatter; smother with sand, per the carrier.

Fume, fire and burns

Molten plastic sticks to skin and keeps burning it. If it ignites, smother it with sand and do not throw water on it. Overheated plastic gives off toxic fume, and an unidentified plastic may give off hydrogen chloride, hydrogen cyanide or a PTFE fume. Work outdoors or under strong ventilation and keep the container’s heat controllable. See Fume Safety.

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