How to Choose the Right Injection Molding Material

Three Constraints That Decide a Program

Every material decision in injection molding comes down to three questions, in order. Heat: what temperature will the part see continuously and at peaks? Commodity resins (PP, ABS, PS) top out around 60–90 °C; engineering grades (PC, PA6/66, POM, PBT) cover 90–140 °C; high-heat polymers (PPS, PEEK) go well beyond. Chemical/environment: what does the part touch — oil, solvents, UV, humidity, disinfectants? PA absorbs moisture and hydrolyzes in hot wet service; POM survives fuels and solvents but not strong acids; PBT resists oils but not hot water long-term. Wear: what mechanical abuse happens in service — impact, abrasion, flex fatigue, creep under load?

Answer those three and the candidate list collapses from hundreds of grades to a handful. Then cost, moldability and regulatory requirements pick the winner. The same ranking works for a medical housing (heat and chemical sterilization + biocompatibility → PC or medical PP), a connector (heat + oil + dimensional stability → PBT or PA-GF), or a caster wheel (abrasion + impact → TPU or PA).

A Fast Selection Flowchart

• Step 1 — Service temperature above 100 °C? No → consider PP, ABS, PS, PE (commodity, cheapest). Yes → continue.
• Step 2 — Chemical exposure (oil, solvent, fuel)? Yes → PA (dry service), POM (fuels), PBT (oils); avoid PS and PC. No → continue.
• Step 3 — Hot, humid or hydrolytic environment? Yes → prefer PBT, PPS or ether-based TPU over unfilled PA. No → PA is a strong, cost-effective choice.
• Step 4 — Continuous heat above 140 °C? Yes → PPS or PEEK territory. No → PC, PA-GF or PBT cover most engineering applications.
• Step 5 — Needs to flex, seal or absorb impact? Yes → TPU, TPE or silicone; consider overmolding onto a rigid carrier.
• Step 6 — Regulatory (UL, food, medical)? Confirm the grade carries the required listing or biocompatibility data before tooling.

Fillers change the math: 30% glass fiber in PA or PBT roughly doubles stiffness, cuts shrinkage and raises heat deflection — at the cost of surface finish and tool wear. Our materials index and guides cover the individual resins in depth.

Common Resins at a Glance

• ABS: tough, cheap, great finish — interiors, housings, toys; poor UV and chemical resistance.
• PC / PC-ABS: transparent, impact-resistant, 120 °C+ — housings, lenses, automotive interior.
• PA6/PA66 (GF): strong, stiff, wear-resistant — structural, under-hood, gears; hygroscopic, needs drying.
• POM: low friction, excellent dimensional stability, fuel/solvent resistant — gears, bearings, clips.
• PBT: stiff, oil-resistant, good electricals — connectors, coil formers, automotive electrical.
• PPS: 200 °C+ continuous, chemical-resistant, inherently flame-retardant — high-heat electrical, pump parts.
• TPU / TPE / silicone: flexible, sealing, soft-touch — gaskets, grips, overmolds.

Shrinkage is the hidden variable: every resin shrinks differently as it cools, so mold cavity dimensions are cut for the specific grade. That is why switching materials after tooling is expensive — and why we lock the grade in DFM before cutting steel. For a reference on industry-standard material ranges, First Mold advertises 500+ engineering-grade thermoplastics with ±0.01 mm molding tolerance (firstmold.com); we quote the grades we run daily and verify properties on first articles.