Does melting HDPE plastic produce toxic fumes? (melting, not burning) ● The question isn't about burning HDPE but melting it at the proper temperature. (At 120 to 180°C depending on it's density, it becomes gooey. According to the source below the "extrusion" temperature range of HDPE is 176 to 260°C ― at 260°C, I guess it becomes a very viscous liquid). 

Is it perfectly safe (eg: indoor melting, in a kitchen oven). Or does it produce toxic fumes ― before 180°C? Before 260°C? If so which one? 

● And what about melting it canola oil? (This technic seems more energy efficient, and easier to control the temperature).
This experimenter tried with Canola oil, which has a smoke point of over 204 °C. He melted the HDPE in the oil at 176°C (the output needs to form a block and expresses the oil).

Is it safer to melt HDPE in oil ? 

○ Bonus: does the Canola oil degrade the HDE? (The experimenter found out that the oil "gets really thick when cool" and he thinks it's because some HDPE dissolves on it. Someone comment "The canola oil probably acts like that because some of the HDPE is converted into paraffin wax which is soluble in canola oil. HDPE isn't lipid soluble". 

Any information on the topic is welcome



It's not a duplicated:
This question has been asked about specifically melting empty silicone tubes (made of HDPE). It's off topic since not about HDPE only (it's a complex issue since there are many different types of silicone on the market)
This table indicates that polyethylene (HD/LD not specified) "melts at" 135°C, decomposes in the range 335-450°C, and produces vapors that will ignite between 341-357°C.

I'm sure melting/softening temperature is more complex than that, but I'm not sure that you should expect much decomposition into toxic gases at temperatures below 260°C.

However, if you're melting it in an oven, it's possible to overshoot the setpoint temperature. Your oven's thermostat might be slow to kick on and off. Direct thermal radiation from the heating element can heat an object in the oven well beyond the air temperature. Your plastic can even spill/drip onto the heating element.

I wouldn't like the idea of disposing of canola oil contaminated with unknown HDPE breakdown products. I'd be happier using some sort of double-boiler arrangement, where the HDPE is in a container that's heated by a bath of some other fluid. I'd suggest paraffin as a heat-transfer fluid, but its flash point is a bit low (200-240°C?)