When people think of plastics, packaging comes to mind first; yet the plastics working under a car's hood, inside an electrical panel or in a gearbox belong to an entirely different class. This article brings together in one place what engineering plastics are, which one is chosen for which part, and what to watch out for when processing them.
What is an engineering plastic?
Plastics roughly form a three-tier pyramid. At the bottom are high-volume, cheap and easy-to-process commodity plastics; in the middle are engineering plastics that carry mechanical loads and withstand temperature and chemicals; and at the top are high-performance plastics that can replace metals and are used in aircraft and implant parts.
| Tier | Examples | Typical use | Continuous service temperature |
|---|---|---|---|
| Commodity | PP, PE, PS, PVC | Packaging, household goods, pipes | 60 - 80 °C |
| Engineering | PA6, PA66, POM, PC, PBT, ABS, PMMA | Gears, connectors, headlamps, housings | 80 - 130 °C |
| High performance | PPS, PEEK, PEI, PSU, LCP | Metal replacement, engine surroundings, medical | 150 - 260 °C |
As you go up the tiers the price per kilogram rises steeply — but if the right choice is made, the part becomes smaller, its life longer and the cost of machining metal disappears. A choice in the wrong direction burns money in two ways: buying a material that is more expensive than necessary, or a cheap material breaking early in the field.
Which engineering plastic for which part?
Polyamide (PA6 / PA66) is the workhorse of engineering plastics: gears, clips, cable ties, radiator tanks, covers around the engine. Its toughness and wear resistance are high. Its best-known trait is moisture absorption — as it takes up water its toughness increases but its rigidity falls; that is why dimensionally sensitive PA parts are delivered conditioned. PA66 withstands hotter environments than PA6.
POM (acetal), with its low coefficient of friction and springy structure, is the material of precision mechanisms: gear wheels, snap-fit locks, hinges, fuel system parts. Thanks to its slippery surface, it needs no lubrication in most applications. Its sensitivity on the processing side is different: if held in the barrel for a long time it degrades quickly, so attention is paid to the shot-to-barrel ratio when choosing a machine.
Polycarbonate (PC) is the only address for parts that require transparency and impact strength at the same time: car headlamps, protective visors, lighting covers. It looks like glass and takes impacts many times greater than glass. The price is processing discipline — PC processed damp both shows visual defects and quietly loses its impact strength. Contact with solvents on the surface can trigger stress cracking.
PBT and PET are the materials of the electrical and electronics world: connectors, coil bodies, fuse boxes. Their electrical insulation and dimensional stability are high; they are almost always used glass-fiber reinforced.
ABS and PC/ABS are the class for visible parts: appliance housings, car interior trim parts, bodies under plating. ABS is easy to process and can be painted; the PC/ABS blend adds impact and temperature margin to the same ease.
PMMA (acrylic) offers the highest optical clarity — lenses, tail lamps, light guides. It is not as tough as PC; where there is a risk of impact PC is chosen, and where optical quality is the priority, PMMA.
In the high-performance tier, PPS withstands a continuous 200 °C and aggressive chemicals; it replaces cast metal in parts around pumps and engines. PEEK sits at the top of the pyramid with continuous service above 250 °C, resistance to sterilization and implant approval. PEI (Ultem) is transparent and flame-retardant; LCP is the only material that can fill the thin walls of micro-electronic connectors.
Quick selection from need to material
| Your priority | Materials to look at first |
|---|---|
| Wear and friction | POM, PA66, (PEEK under extreme load) |
| Transparency + impact | PC; PMMA if optics are the priority |
| Continuous temperature above 120 °C | PPS, PEEK, PEI |
| Electrical insulation | PBT, PET, PA (in the dry state) |
| Chemical contact | PPS, PP (on the commodity side), POM |
| Visible surface / paintability | ABS, PC/ABS |
| Spring effect, snap-fit | POM, PA |
| High cost pressure, medium load | PA6 GF, PBT GF |
The glass fiber reality
Most engineering plastics are used reinforced in the field: PA66 with 30% glass fiber is many times more rigid than the unreinforced grade and holds its shape much better under temperature. But reinforcement has three costs: the part shrinks anisotropically (little in the fiber direction, much perpendicular to it — a risk of warpage), the surface becomes matte, and because the viscosity rises, higher injection pressure and therefore more clamping force are needed. You can see the effect of the glass fiber ratio on tonnage by entering the additive type and percentage in our calculator; for the effect of abrasive fiber on screw and barrel life, see our wear guide.
Processing requirements: engineering plastics demand discipline
Commodity plastics forgive, engineering plastics do not. Three points are decisive:
1. Drying is not negotiable. PA, PC, PBT, PET and PEI are hygroscopic — if processed damp, hydrolysis breaks the chains; the part looks sound but its impact strength has dropped. The visual symptom may be moisture streaks, but the real danger is the invisible loss of strength. For details, see our drying guide and the dry air dryers page.
2. Mold temperature rises. Whereas commodity plastics work with a 30 - 50 °C mold, engineering plastics require 80 - 140 °C, and PEEK 180 °C. In a crystalline material a cold mold leaves crystallization unfinished: the part comes out of the mold fine but shrinks later and warps. Above 90 °C means a water circuit, above 150 °C an oil circuit — this is where choosing a mold temperature control unit comes in.
3. Temperatures and residence tolerance narrow. Typical values:
| Material | Melt | Mold | Drying |
|---|---|---|---|
| PA6 | 255 °C | 80 °C | 80 °C / 4 hours |
| PA66 | 290 °C | 80 °C | 80 °C / 4 hours |
| POM | 200 °C | 90 °C | generally not needed in sealed packaging; if opened/stored damp, 100 °C / 2 – 3 hours |
| PC | 300 °C | 90 °C | 120 °C / 3 hours |
| PBT | 250 °C | 70 °C | 120 °C / 3 hours |
| PPS | 320 °C | 140 °C | 140 °C / 3 hours |
| PEEK | 380 °C | 180 °C | 150 °C / 3 hours |
The full processing values for 45 materials are in our processing values table; our residence time calculator calculates the safe residence time in the barrel with material-specific zones.
The equipment side: what does processing engineering plastics require?
For a plant moving to engineering plastics, the auxiliary equipment is put to the test as much as the machine: a dry air dryer is no longer an option but a necessity, the mold temperature cannot be held without a temperature control unit, and in glass-fiber recipes an investment in a bimetallic screw and barrel pays for itself in wear life. In medical-grade production, repeatability comes to the fore — machines designed for this job, such as the MX series, are preferred.
If you want to determine the right machine and tonnage for your part, our calculators recommend models taking the material and additive ratio into account; for an assessment specific to your application, you can contact our technical team.