Worked in a Plastic injection molding plant for twenty years. High Density Polyethylene has a melting point of between 250-360 degrees F. Might want to be sure to be very careful with the melted form which will stick to skin,clothes,shoes and does not feel very good. Polyethylene can almost become liquid at around 450 to 475 degrees and does not come off of anything very easily. We melted Plastics in an long metal barrel. This barrel had a screw inside it with the incline of the threads tapered so as to increase the pressure put on the plastic which would help it to melt as it went from the feed throat to the front of the screw.

The outside of the barrel also had sections that were heated by heater bands. The screw was turned by a hydraulic vane motor. At the tip of the screw was a check valve. Plastic would go past the check valve as the screw retracted once the shot size was made the machine would push the screw forward which closed the check valve on the tip and forced the plastic into the mold through a sprue hole in the mold. So basically when the plastic is shot into the mold under pressure it is in a semi liquid state.

The molds are temperature controlled via a machine called a thermolator which would heat certain parts of the mold while cool water was also used on other parts of the mold to help the part to become solid quicker. We made plastic parts for Whirlpool refrigerators and other whirlpool products.

We also used PVC, CPVC, Polycarbonate, Styrene, Acetal Delrin, and many other plastics, each having different melt points and specs. I worked on all the machinery and kept 52 presses running the best I could along with about three other maintenance men. And one last thing we used grinders to take bad parts back down to pellet form which we loaded into a large hopper above the feed throat.