Engineering plastics

Engineering plastics (also called engineering thermoplastics or engineering polymers) are polymers with better mechanical, thermal and chemical properties than commodity plastics such as PE, PP, PS or PVC. They are used where a plastic part carries a load, moves, runs at elevated temperature or comes into contact with chemicals. Typical examples are polyamides (PA6, PA66, PA12), POM, polycarbonate (PC), PBT and PET; ABS and PMMA (acrylic) are often included as well.

Above them sit high-performance plastics such as PEEK, PPS, PEI, PSU or PTFE, which typically withstand 150 to 250 °C continuously and in many cases also resist aggressive chemicals, but cost several times more. Standard engineering plastics have a continuous service temperature on the order of 80 to 120 °C.

By their internal structure they fall into two groups. Amorphous plastics (PC, ABS, PMMA, PEI) can be transparent and have low shrinkage and good dimensional accuracy, but they are more sensitive to chemicals and environmental stress cracking. Semi-crystalline plastics (PA, POM, PBT, PEEK) resist chemicals and wear better and have good sliding properties.

Fillers change the properties considerably. Glass fibers (most often 30 % by weight, designated for example PA66-GF30, in US shops simply 30 % glass-filled nylon 66) increase strength and stiffness, carbon fibers (CF) raise stiffness at lower weight, and PTFE or graphite reduce friction. Unfilled engineering plastics typically have a modulus of elasticity of 1.5 to 3.5 GPa, with 30 % glass fiber roughly 6 to 10 GPa, whereas aluminum is around 70 GPa and steel around 210 GPa. In production they are processed mainly by injection molding, in small batches by machining sheet and rod, and for prototypes also by 3D printing, for example PA12 by SLS and MJF.

Consider an engineering plastic instead of metal where low weight, quiet running, sliding without lubrication, electrical insulation, corrosion resistance or a low unit cost in volume thanks to injection molding matter most: gears, plain bearings, rollers, instrument housings, brackets, connectors, pump parts or components of food processing machinery. Choose one over a commodity plastic when the part carries a sustained load or operates at elevated temperature.

Start the selection from the requirements: service temperature including short-term peaks, the magnitude and duration of the load, the required stiffness, friction and wear, chemicals and cleaning agents, UV exposure, transparency, flammability (UL 94 rating) and any food contact. The most expensive plastic is not necessarily the best: in many applications PA66 or POM does the job instead of PEEK.

The most common mistake is sizing a plastic part from datasheet values the same way as a metal one. Those values come from a short-term test at 23 °C; at higher temperatures strength and stiffness drop significantly, and under sustained load the plastic slowly deforms (creep). Allow for thermal expansion, which for unfilled plastics is five to ten times that of steel, and for polyamides also for dimensional changes after moisture absorption.

Fillers cause anisotropy: a glass-filled part is stronger and shrinks less in the direction of melt flow than across it, which leads to warping. Sharp notches and inside corners without a radius are typical fracture points. Amorphous plastics can crack after contact with solvents, adhesives or cleaning agents. So do not write just "plastic" or "PA" on the drawing, but the specific grade including filler and color (for example PA66-GF30, black), and for a prototype check how far its material and process differ from those of the production part.