Semi-crystalline thermoplastic polymers show more order than amorphous thermoplastics. The molecules align in a structured crystalline form. The size of the crystals or spherulites is much larger than the wavelength of visible light, making semi-crystalline materials translucent, rather than transparent. Crystalline regions are small with molecular chains comprised of both crystalline and amorphous regions. The degree of crystallinity in a typical thermoplastic will vary from grade to grade, as for example in polyethylene, where the degree of crystallinity depends on the branching and the cooling rate.
Thermoplastics are divided into two material classes (amorphous and semi-crystalline). Their classification is determined by both their transition temperatures and their structure in different states, which works great for molded silicone parts.