ABS, PA, POM, PEEK… : choosing a plastic for a technical part

An engineering plastic is chosen like any other component: you start from what it has to do. A single acronym covers dozens of grades, filled or unfilled, for injection moulding or for extrusion, and two ABS parts can behave very differently. Before naming a material, answer these questions:

• What loads? Constant or occasional load, impacts, vibration, friction. Under constant load a plastic slowly deforms (this is called creep): a part that passes testing can distort after months in service.
• What temperature? Continuous operating temperature, any peaks, hot and cold cycles. Plastics soften well before metals do.
• What environment? Moisture, hot water, oils, detergents, solvents, UV radiation for an outdoor part.
• What appearance? Transparency, colour, surface finish, whether it can be bonded, painted or marked.
• What regulatory requirements? Food contact, medical use, fire performance.
• What manufacturing route? Quantity, planned process and tolerances required.

These answers already narrow the list to two or three candidates, which the workshop can help you decide between. Describe them in your request rather than imposing a material: a plastics processing or machining workshop may suggest a more suitable or more economical grade.

Thermoplastics fall into three tiers:

• Commodity plastics (PE, PP, PVC, PS): inexpensive and light, for lightly loaded parts (trays, covers, guides).
• Engineering plastics (ABS, PC, PA, POM, PBT): better mechanical and thermal behaviour, for functional parts.
• High-performance plastics (PEEK, PPS, PEI, PTFE): high heat and chemical resistance, but the material and its processing are considerably more expensive.

A second distinction helps to anticipate behaviour. Amorphous plastics (ABS, PC, PMMA) shrink little in moulding, bond easily and can be transparent, but resist chemicals less well and can crack under stress. Semi-crystalline plastics (PA, POM, PP, PE, PEEK, PTFE) have better resistance to wear, fatigue and chemicals, but shrink more and are harder to bond.

Finally, fillers change the picture: glass fibres increase stiffness and temperature resistance, at the cost of a material that is more abrasive on tools and often more brittle; sliding fillers (PTFE, graphite, molybdenum disulphide) reduce friction.

• ABS: a good compromise between impact resistance, stiffness and ease of processing. It can be injection moulded, machined, bonded and painted easily, but its temperature resistance is limited and it ages badly in sunlight without protection. Housings, covers, prototypes.
• PC (polycarbonate): very impact resistant, transparent, more heat tolerant than ABS. It scratches easily and can crack on contact with certain solvents or oils.
• PMMA: very transparent, good UV resistance, but brittle. It machines well with sharp tools and can be polished. Glazing, diffusers, display stands.
• PA 6 and PA 66 (polyamides): resistant to wear, impact and fatigue. They absorb moisture from the air: the part swells and its properties change, which complicates tight fits. PA 12 absorbs significantly less. Gears, rollers, bushes.
• POM (acetal): rigid, stable, low friction, very insensitive to moisture and very pleasant to machine. It is often the best choice for a precision machined mechanical part. It comes as a copolymer (POM-C, the most common) and as a homopolymer (POM-H). It bonds poorly and does not tolerate strong acids well.
• PP and HDPE: light, inexpensive and highly chemical resistant, but not very rigid, with high thermal expansion and difficult to bond. PP lends itself to integral hinges.
• PEEK: around 250 °C in continuous use, excellent chemical resistance, sterilisable, with biocompatible grades. It is very expensive and demanding to injection mould: reserve it for needs that other materials cannot meet (medical, aerospace, chemicals).
• PTFE: very low friction and chemical inertness, but soft and prone to creep. It cannot be injection moulded conventionally: it is machined from bars, tubes or sintered sheets. Seals, linings, sliding parts.

Qualitative guidelines, to be confirmed against the datasheet of the grade you select. The “Injection” column shows whether the material is commonly injection moulded; relative cost refers to the material (€ economical, €€€€ very high).

Machining: semi-finished stock, no tooling

Machining starts from bars, sheets or tubes and needs no tooling: it is the natural route for a prototype or a small batch. Plastic heats up during cutting, expands and sometimes releases its internal stresses: the workshop will often plan roughing, stabilisation and finishing. Tight tolerances are harder to hold than in metal: limit them to functional dimensions (see the guide on materials for machined parts).

Injection moulding: a mould, then production runs

Plastic injection moulding forces molten material into a mould. The tooling is an investment that only pays off over a production run; in exchange, the price of each part falls. The material must exist in an injection grade (PTFE, for example, has none), and its shrinkage, higher for semi-crystalline plastics, affects accuracy and warping. Moulding tolerances are generally looser than machining tolerances, and the part must follow design rules (draft angles, uniform wall thicknesses). The guide on the price of an injection mould details the cost of tooling.

3D printing: useful for testing, but not equivalent

Additive manufacturing produces plastic parts without tooling, notably in polyamide. But a printed part does not have the properties of an injection moulded or machined part in the same family: it is often weaker in one direction. It validates shape and assembly, not always mechanical strength (see 3D printing or CNC machining).

There is no universal threshold between machining and injection moulding: if your quantity falls between the two, have both processes quoted (see from prototype to series production). If the final run is to be injection moulded, prototype in a similar material, then validate a few parts from the mould before launching production.

• Expansion. A plastic expands several times more than steel: a tight fit at 20 °C can seize or loosen at 60 °C. Allow for clearance, especially in a metal frame.
• Creep. A part under permanent load deforms over time. On a screwed assembly, prefer a metal insert or a wide washer to spread the load.
• Moisture. Polyamides swell and lose stiffness; POM is far more stable. For a part that must hold its dimensions, this is often the deciding criterion.
• Chemicals. Compatibility depends on the material, concentration, temperature and stress. PC and PMMA can crack on contact with certain solvents. Ask the supplier for the chemical resistance of the grade, or run a test.
• UV and outdoor use. Without a stabiliser, several plastics yellow or become brittle. State outdoor use: stabilised grades exist.
• Fire performance. In electrical or transport applications, a fire rating may be required (for example UL 94). It restricts the choice: state it from the outset in your request.
• Food or medical contact. Only certain grades are declared compliant: ask for the supplier’s declaration and have it validated by your regulatory manager. See also the medical sector.

A poorly specified material leads to questions, quotes that miss the mark or an unpleasant surprise on delivery. State:

• the precise designation: “natural POM-C”, “PA 6 with 30% glass fibre”, “black ABS” rather than “plastic” or “nylon”;
• whether it is mandatory or not: “or equivalent” lets the workshop propose the most economical grade available;
• the colour and appearance: natural, black, through-coloured, any polishing;
• the conditions of use: temperature, substances in contact, moisture, outdoor use;
• the documents expected (material certificate, declaration of conformity), to be requested only if they serve a purpose;
• the quantity and, for a prototype, the production run and process targeted afterwards.

Ready to ask for prices? Post your request on Meaflow: it is free and commission-free, and you compare quotes from several workshops.