Thermoplastic polymers can be categorized as semi-crystalline or amorphous. Semi-crystalline polymers possess long range molecular order, or crystal structures, while amorphous polymers lack crystal structures. This video will explore the differences between the properties and processing requirements for these two types of thermoplastic materials.
Semi-crystalline polymers are characterized by ordered arrangement of the polymer chains, allowing for close packing of the molecules. These resins also incorporate some level of amorphous content between the crystalline regions. The traits of semi-crystalline polymers will be dependent on the degree of crystallinity. Traits include streamlined molecules with no bulky side-groups, the presence of both a melting temperature and a glass transition temperature, good chemical resistance in most cases, and higher use temperature than comparable amorphous resins. Because semi-crystalline resins shrink more than amorphous resins, they typically require looser part tolerances.
Amorphous polymers are characterized by random entanglement of the individual polymer chains and due to bulky side groups, do not allow close packing of the chains. Large spaces between polymer chains allow light to pass easily and chemicals to penetrate. Because of this, amorphous resins tend to be transparent and solvent sensitive. Amorphous resins can be characterized by their glass transition temperature. They do not have a melting temperature, and they soften over a broad temperature range. They exhibit lower mold shrinkage, so parts made from amorphous resins can be held to tighter tolerances.
While semi-crystalline and amorphous resins are both thermoplastics, they can behave very differently. When designing a new part, it is important to understand these differences, so you can choose the right material for your application.
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