If it’s the same plastic, why the two different densities?
The first thing we need to understand: What is density? Density can be thought of as how many grams of a plastic will fit into a 1cm x 1cm x 1cm cube as illustrated in Figure 1. When working with plastics, there are two types of densities that need to be considered: solid density and melt density. But why the two different densities? After all, it’s the same plastic we are talking about, right? True, but we need to understand what happens to the molecular chains when we apply heat to plastic. The chains expand and separate, thus increasing the volume that will be occupied.