Plastic Material Selection Guide for Molded Parts
Materials
What the part actually does
Material selection begins with the load case: continuous stress, impact, flexure, fatigue or static load. A snap-fit clip and a structural bracket need different resins even at similar size. Define the load case and the environment before naming a material — that single step removes most over-spec and under-spec errors we see in incoming prints.
This guide is written from the engineering side: which numbers to put on the drawing, which resin class survives the environment, and where the cost really hides. Every figure below is drawn from published material data or from programs we have run and can name.
The Snapshot
- Temperature is the first filter: commodity ABS tops out near 80 °C; PA66-GF30 holds 120–140 °C; PPS runs 200–240 °C; PEEK sustains 250–260 °C.
- Flame class is a regulatory gate: enclosure and wiring parts often require UL94 V-0 — flame-out in ≤10 s with no burning drips.
- Tolerance follows resin class: general molded features land at ±0.1–0.2 mm; critical locating surfaces reach ±0.05 mm, and a connector program we ran held ±0.005 mm on critical features in glass-filled PPS.
- Volume sets the cost ceiling: the same part costs far more in an engineering resin — PEEK resin runs roughly $50–100/kg versus ABS at $1.5–3/kg.
Temperature and chemical exposure: the first filters
Under-hood, industrial and lighting parts live with heat that commodity resins cannot survive. The rule of thumb from our automotive programs: if the part sees sustained >150 °C, move from PA66 to PPS/PPA/PA46. A connector housing we molded in glass-filled PPS / PA66 for an automotive electrical program runs at 2M units/year and was qualified under IATF 16949 in 10 weeks.
Continuous-service references we design around:
- ABS / PS / PMMA / PVC: 60–100 °C ceiling — cabin, non-heat interiors.
- PP / PE / PC / POM / PA / TPU: 80–130 °C working band.
- PPS / PEI / PEEK: 200–260 °C — under-hood, medical imaging, aerospace.
A part specced in unstabilized ABS that faces sustained >80 °C will heat-age and crack inside a few years. Match the resin to the worst condition the part will see, not the average.
Fluid exposure decides survival as much as heat. PA (nylon) resists many chemicals and wear but absorbs moisture — PA6-GF30 used in a 250,000+ units/year HV busbar program needs pre-conditioning because moisture shifts dimensions. PPS and PEEK resist aggressive media (coolant, ATF, solvents) where amorphous resins swell or stress-crack.
For sealing and gasket interfaces, LSR (liquid silicone rubber) is the medical- and food-grade choice — a wearable ECG button we molded in medical-grade LSR held ±0.03 mm at 300K units/year. TPU gives soft-touch and gasket resilience; an NEV charging-port dust cover ran in custom FR-TPU (UL94 V-0) at 500,000+ units/year.
Mechanical load: impact, flex and fatigue
- Impact + transparency → PC (melt 280–320 °C, mold 80–120 °C); pair with PC-ABS for better mold flow and lower cost.
- Living hinge / fatigue flex → PP (melt 200–280 °C), the only commodity resin that survives millions of hinge cycles.
- Gears, latches, precision wear → POM (acetal, melt 180–230 °C), low friction, ±0.03 mm achievable as shown by a 5M units/year small-button program.
- Structural + heat → PA66-GF30 or PPS-GF40.
Load case first, resin second. Specifying PEEK “just in case” inflates resin cost 10–20× and lengthens cycle time with zero functional gain.
Commodity vs engineering resins
ABS and PP are commodity workhorses — cheap, easy to mold, good enough for most housings. Engineering resins (PC, PA, POM, PPS, PEI, PEEK) bring strength, heat or chemical resistance at higher cost and tighter process windows (higher melt temperatures, stricter drying).
A VR remote housing we ran in PC+ABS / TPE overmolding held ±0.04 mm at 800K units/year in 9 weeks of rapid tooling — commodity-class resin, engineering result. The resin grade, not the price tag, earned the tolerance.
Flame class and regulatory
Enclosure, connector and wiring parts frequently require a UL94 rating. The classes in ascending severity:
- HB: horizontal burning, slowest requirement.
- V-2 / V-1 / V-0: vertical burn; V-0 means flame-out in ≤10 s with no burning drips — the common bar for enclosures near electronics.
- 5VA / 5VB: highest, for critical barriers.
An NEV charging dust cover was specced FR-TPU, UL94 V-0; a medical MRI balun ran in unfilled PEI for zero RF leakage at 15,000+ units/year. Flame class is a line item on the material cert, not a surprise at qualification.
Moisture, drying and the process window
Absorbent resins (PA, PBT, PC) need drying before molding or you get splay and brittle parts. PA66 equilibrium moisture can shift critical dimensions by 0.2–0.5% — pre-condition test parts before measuring. POM and PPS are dimensionally stable but UV-sensitive (interior only unless stabilized).
Shrinkage also varies by class: amorphous resins (PC, PMMA, PS) shrink 0.2–0.7% and hold tighter tolerances; semi-crystalline (PP, PE, POM, PA) shrink 1.0–3.0% and need more generous draft and gate control.
Each resin has a narrow process window that drives cycle time and defect rate. Getting melt and mold temperature right is the difference between a clean shot and splay, short shots or burn marks.
| Material | Melt temp (°C) | Mold temp (°C) | Drying need |
|---|---|---|---|
| ABS | 200–260 | 40–80 | Yes (80 °C, 2–4 h) |
| PP | 200–280 | 20–60 | Low |
| PC | 280–320 | 80–120 | Yes (120 °C, 4 h) |
| PA66 | 260–300 | 60–100 | Yes (80 °C, 4 h) |
| POM | 180–230 | 80–120 | Yes (80 °C, 2 h) |
| PPS | 300–340 | 120–160 | Yes (150 °C, 3 h) |
| PEEK | 350–400 | 160–200 | Yes (150 °C, 3 h) |
| PMMA | 220–260 | 60–90 | Yes (80 °C, 3 h) |
| TPU | 190–220 | 20–60 | Yes (100 °C, 2 h) |
| LSR | n/a (cures) | 170–200 | None (2-part) |
Resin cost per kg by grade
Resin price is the recurring cost driver at volume, not the mold. Typical spot ranges:
| Material | Typical price (USD/kg) |
|---|---|
| PP / PS | $1–2.5 |
| ABS | $1.5–3 |
| PC / POM / PBT | $3–5 |
| PA66 | $3–6 |
| PPS | $8–15 |
| TPU / LSR | $3–30 |
| PEEK | $50–100 |
At 5M units/year the resin spread between ABS and PEEK dwarfs any mold-price difference. Pick the grade the function justifies.
Real program examples and the final checklist
| Program | Material | Tolerance | Volume | Lead time |
|---|---|---|---|---|
| Automotive connector | Glass-filled PPS / PA66 | ±0.005 mm critical | 2M / yr | 10 weeks |
| VR remote housing | PC+ABS / TPE | ±0.04 mm | 800K / yr | 9 weeks |
| Small button | POM / ABS | ±0.03 mm | 5M / yr | 12 weeks |
| Medical ECG button | LSR (medical) | ±0.03 mm | 300K / yr | 7 weeks |
| NEV dust cover | FR-TPU (UL94 V-0) | — | 500K / yr | 6 weeks |
| HV busbar | PA6-GF30 + Cu | — | 250K / yr | 8 weeks |
These are not hypotheticals — they are the programs the numbers in this guide came from.
Final checklist before you send the print:
- Worst-case sustained temperature → set the resin class.
- Fluid/chemical exposure → confirm resistance or move up a class.
- Flammability regulation → specify UL94 level on the print.
- Load type → impact, hinge, wear or structural.
- Moisture → drying and pre-conditioning plan.
- Annual volume → does an engineering grade pay back?
Pick the resin with engineering
Bring the load case, the worst-case temperature, the fluid exposure and the required flame class. We return a grade matched to function and budget, with lot-level material certification and DFM notes before any steel is cut.