There are different densities and compound configurations. Also, there is a difference between a plastic compound and polymers.

If the object is big enough to have a stamp, you will find an indicator of the type of material it is made from. For example, most 5 gallon buckets are HDPE 2 (High Density Poly Ethylene), and some deli containers are PP5 (Polypropylene). Each has a different melting point, and other tolerances.