GlossaryKind of term: Manufacturing termEngineering plastics
What are engineering plastics?
Engineering plastics are thermoplastics such as POM-C, PA6, UHMW-PE and PEEK for functional parts; they expand more than metal and some absorb moisture.
What makes them engineering grades is their mechanical and thermal properties, and each grade has its own field: POM-C for guides, gears and insulators, PA6 and PA66 for wear parts, UHMW-PE for sliding surfaces, and high-performance plastics such as PEEK and PTFE where temperature or chemical resistance matters. For machining they come as stock shapes: sheet, rod, tube or block, extruded, compression moulded or cast. Glass- or carbon-fibre-filled grades behave differently under the cutter and wear tools faster.
What does the dimensional stability of engineering plastics depend on?
Unfilled plastics expand with heat far more than metals: POM-C in the order of 0.1 mm per metre per degree, aluminium about a fifth of that; fibre-reinforced grades expand less. Over 300 mm that is roughly 0.03 mm per degree for POM-C. Polyamide also absorbs moisture and changes size with it. Extruded stock can hold residual stress that is released when one side is machined, and low stiffness makes the workpiece sensitive to clamping and cutting forces. How well a dimension can be held therefore depends on grade, stock form, wall thickness, clamping method, and the temperature and moisture content at machining and at inspection.
How do engineering plastics work in practice?
Engineering plastics machine with sharp, positive cutting edges, high speeds and good chip evacuation; too much heat in the cut can cause melting, burrs or dimensional error. For tight tolerances or flatness it helps to use stress-relieved stock, let the part rest after roughing, clamp lightly and measure at the 20 °C reference temperature or under the conditioning the drawing specifies. Price per kilo says little for plastics without the density and the form you buy in: POM-C weighs about half as much as aluminium, and PEEK usually costs several times as much per kilo as POM-C. A tight metal-style tolerance on a plastic part is a reason to agree with the customer what the function actually needs.
What are engineering plastics often confused with?
An engineering plastic is not a soft metal: cutting geometry, clamping force, thermal behaviour and dimensional stability all differ, and starting from metal values leads to wrong estimates of both time and achievable tolerance. Nor is a machined plastic part an injection moulding: tolerances and design rules for moulding do not apply to a part milled or turned from stock.
How do you see engineering plastics in Blake?
Metals and plastics share one materials catalogue, each with your own purchase price, source and date. Material and machining appear as separate lines, and the outcome is labelled as an estimate. More on materials.
Frequently asked questions
Why are tight tolerances difficult in plastics?
Because the material moves: it expands strongly with heat, springs back after clamping, can release residual stress, and polyamide absorbs moisture. A dimension that is right on the machine can be off at another temperature or a day later.
Should plastic parts rest before finishing?
For tight tolerances or flatness, often yes: residual stress can be released after roughing and distort the part. How long depends on grade, stock form and shape. Stress-relieved stock reduces the effect.