Overview of design principles

The process of developing thermoplastic parts requires a full understanding of typical material properties under various conditions. Thermoplastics can be categorized by their molecular structure as either amorphous, semi-crystalline plastics, or liquid crystal polymers (LCPs). The microstructures of these plastics and the effects of heating and cooling on the microstructures are shown in the figure below. Molecular structure of thermoplastics.

Amorphous thermoplastics

Amorphous polymers have a structure that shows no regularity. In an unstressed molten state polymer molecules are randomly oriented and entangled with other molecules. Amorphous materials retain this type of entangled and disordered molecular configuration regardless of their states. Only after heat treatment some small degree of orientation can be observed (physical aging). When the temperature of the melt decreases, amorphous polymers start becoming rubbery. When the temperature is further reduced to below the glass transition temperature, the amorphous polymers turn into glassy materials. Amorphous polymers possess a wide softening range (with no distinct melting temperature), moderate heat resistance, good impact resistance, and low shrinkage.

Semi-crystalline thermoplastics

Semi-crystalline plastics, in their solid state, show local regular crystalline structures dispersed in an amorphous phase. These crystalline structures are formed when semi-crystalline plastics cool down from melt to solid state. The polymer chains are partly able to create a compacted structure with a relatively high density. The degree of crystallization depends on the length and the mobility of the polymer segments, the use of nucleants, the melt, and the mold temperatures.

Liquid crystal polymers

Liquid crystal polymers (LCPs) exhibit ordered molecular arrangements in both the melt and solid states. Their stiff, rod-like molecules that form the parallel arrays or domains characterize these materials.

The difference in molecular structure may cause remarkable differences in properties. Various properties are time or temperature dependent. The shear modulus, for instance, decreases at elevated temperatures. The shear modulus curve illustrates the temperature limits of a thermoplastic The shape of the curve is different for amorphous and semi-crystalline thermoplastics. Glass transition temperature (Tg) and melt temperature (Tm) are indicated in figure below.

The following graph demonstrates time dependent creep moduli.  In general semi-crystalline materials have lower creep rates than amorphous materials. Glass reinforcement generally improves the creep resistance of a thermoplastic material.