MOLDITQUICK
MOLDITQUICK

Plastic Material Selection Guide for Molded Parts

RCRay Chan·2026-08-18·20 min read
Table of Contents

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.

The 2026 environment makes the discipline more important. Resin prices swing with feedstock and logistics; halogen-free flame-retardant demand is reshaping the FR grade list; and programs increasingly demand a documented material decision — the “why this grade” note — for the audit file. A selection method beats a selection habit.

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.
  • Extreme-temperature headroom exists beyond PEEK: PBI sustains 300–370 °C, PEI is rated 170 °C long-term / 510 °C short-term, PTFE spans −196 to 260 °C, and UHMWPE survives liquid-nitrogen impact (FirstMold materials data, https://firstmold.com/materials/injection-molding-materials/).

Table of Contents

  1. The selection workflow: filters in order
  2. Temperature and chemical exposure: the first filters
  3. Mechanical load: impact, flex and fatigue
  4. Commodity vs engineering resins
  5. Fillers and reinforcements: the glass-fiber ladder
  6. Flame class, regulatory and material certification
  7. Moisture, drying and the process window
  8. Overmolding and material pairing
  9. Additives, color and masterbatch
  10. Datasheet numbers vs molded reality
  11. Resin cost per kg by grade
  12. Real program examples and the final checklist
  13. Frequently Asked Questions
  14. Sources
  15. Pick the resin with engineering

The selection workflow: filters in order

The fastest way to the right resin is to run the filters in a fixed order, so each decision shrinks the shortlist:

  1. Worst-case sustained temperature → sets the resin class. Below 80 °C: commodity (ABS, PP, PS). 80–130 °C: PC, POM, PA, TPU. 120–150 °C with load: PA66-GF30, PPA. 150–240 °C: PPS, PEI. 240 °C+: PEEK, PBI.
  2. Chemical / fluid exposure → confirms the class or moves it up. Coolant, ATF, solvents and flame-retardant environments favor PPS/PEEK over amorphous grades.
  3. Mechanical load → picks the grade. Impact: PC-rich blends. Hinge/fatigue: PP, POM. Wear: POM, PEEK. Structural + heat: PA66-GF30, PPS-GF40.
  4. Flame / regulatory gate → forces FR grades (UL94 V-0, 5VA) or certified grades (USP Class VI for medical, food-contact, RoHS/REACH compliance).
  5. Cosmetics and surface → narrows to moldable, paint-able, texture-friendly classes (ABS, PC+ABS) or forces surface treatment (PP/PE).
  6. Volume and cost → the last filter. An engineering grade only pays back if the environment or regulation actually demands it.
Filter Question Decides
1. Temperature Worst sustained °C? Commodity vs engineering vs high-performance class
2. Chemicals Fluids, solvents, moisture? PA vs PPS/PEEK; need for conditioning
3. Load Impact, hinge, wear, structure? Grade within class (GF%, blend)
4. Flame/regulatory UL94 level, USP, food? FR or certified grade
5. Cosmetics A-surface, finish — see our detailed guide: surface finishing stack? Substrate for decoration
6. Volume/cost Units per year? Value engineering vs premium grade

Running the filters in this order is exactly what we do in DFM review — the print arrives, and the material callout is checked against all six before a quote is built.

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.

Beyond the main classes, the published extreme-temperature table (FirstMold materials page) is worth keeping on the wall: PBI 300–370 °C long-term with no breakdown at 538 °C; PEI 170 °C long-term / 510 °C short-term; PEEK 260 °C stable / >300 °C short-term; PI −240 to 290 °C (480 °C short); PTFE −196 to 260 °C; UHMWPE impact survival at −269 °C; TPU brittle point −60 °C with >90% elasticity at −40 °C. When the application is a cryogenic valve or an engine-bay sensor, the answer is on this table.

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 — hinge thickness 0.25–0.5 mm with a transition radius ≥0.5 mm (FirstMold PP design handbook, https://firstmold.com/pp-injection-molding/).
  • 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; FirstMold publishes a <1% mass-production defect rate on POM parts (https://firstmold.com/pom-injection-molding/).
  • Structural + heat → PA66-GF30 or PPS-GF40.
  • Wear + heat + chemicals together → PEEK: tensile 90–100 MPa, wear resistance ~4× PTFE (https://firstmold.com/peek-injection-molding/).

Load case first, resin second. Specifying PEEK “just in case” inflates resin cost 10–20× and lengthens cycle time with zero functional gain. The mechanical table above is a filter: if the part does not actually flex, wear or carry sustained load, the premium grade is not buying anything.

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).

FirstMold’s published five-category classification is the cleanest mental model (https://firstmold.com/materials/injection-molding-materials/):

Category Examples Where they earn their place
Commodity thermoplastics PP, PE, PS Low-cost, high-volume, non-demanding
Engineering plastics ABS, PC, Nylon (PA), POM Housings, structural, precision
High-performance PEEK, PPS, PEI Heat, chemicals, flame, low outgassing
Thermosetting Epoxy, silicone (LSR) Seals, potting, cure-in-place
Modified compounds Glass-filled nylon, FR grades Stiffness, flame class, dimensional control

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.

The economics of the split matter at volume: FirstMold’s cost structure for a molded part is raw material 40–60%, processing 20–35%, mold 15–25%, post-processing 5–20% — so the resin class is the biggest single lever on part cost, and high-MFI grades can cut production time ~30% while reinforced grades accelerate mold wear ~3× (same source).

Fillers and reinforcements: the glass-fiber ladder

Glass fiber is the standard way to buy stiffness and heat without switching classes. The published glass-filled-PP ladder (FirstMold PP page) shows the pattern that repeats across PA, PPS and PEEK:

Grade Tensile strength HDT Mold shrinkage
PP unfilled ~30 MPa (typical) ~100 °C (typical) ~1.5–2.0% (typical)
PP-GF10 ~45 MPa ~145 °C ~0.8%
PP-GF20 ~60 MPa ~152 °C ~0.7%
PP-GF30 ~80 MPa ~160 °C ~0.6%
PP-GF40 ~95 MPa ~165 °C ~0.5%

GFPP values per FirstMold (https://firstmold.com/pp-injection-molding/); the unfilled row is typical published datasheet range for comparison.

What the ladder teaches: each 10% of glass adds stiffness and HDT, but shrinkage drops and becomes anisotropic — the part shrinks differently along and across flow. Design consequences: gates must be placed to control orientation, weld lines become strength lines (a weld line in a GF grade is a crack-initiation point), and draft must be generous (glass doesn’t let go of the core). PA66-GF30 and PPS-GF40 follow the same logic at higher temperature; PEEK-GF30 at the top. If the requirement is 160 °C HDT at commodity cost, GFPP30 is the answer; if it is 200 °C in coolant, that is PPS-GF40 territory.

Flame class, regulatory and material certification

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 — PEI is inherently V-0, which is why it shows up in medical and aerospace enclosures. Beyond flammability, the regulatory stack includes USP Class VI (medical — FirstMold lists PP, ABS and COC as USP Class VI workhorses, https://firstmold.com/industries/medical/), food-contact grades, and RoHS/REACH restricted-substance compliance for anything shipped into the EU.

Flame class is a line item on the material cert, not a surprise at qualification. Two buying habits that prevent trouble: (1) specify the UL94 level on the print, and (2) require the FR data sheet — V-0 grades vary in smoke, heat release and how much the FR package degrades mechanicals, so “V-0” alone is not a complete spec.

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). PVC is not hygroscopic — it only needs surface-moisture removal (Kemal, https://www.kemalmfg.com/pvc-injection-molding/).

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. Protolabs’ published shrink data makes the same point in imperial units: ABS 0.003 in/in, PP 0.018 in/in — a tool cut for ABS switched to PP yields parts ~0.015 in/in smaller (https://www.protolabs.com/resources/blog/injection-molding-tolerances/).

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)

Overmolding and material pairing

When a part combines hard and soft material, the pairing rules decide whether it bonds or delaminates:

  • Bonding needs polarity match: TPE bonds well to PP and ABS; TPU bonds to PC-rich substrates; LSR bonds to most plastics with proper surface prep. Non-polar substrates (PP, PE) need plasma or flame treatment — treated surfaces reach ~72 mN/m (FirstMold materials page).
  • Melt-temperature compatibility: the overmold resin must process without degrading the substrate; a high-melt TPU over a low-softening ABS core needs a temperature-staged process.
  • Mold-in-place beats assembly: FirstMold’s insert — see our detailed guide: insert molding-molding data cites 50% bond-robustness improvement and 30% fewer assembly steps versus post-molded hardware (https://firstmold.com/insert-molding/), and two-shot molding cuts assembly cost up to 40% (https://firstmold.com/two-shot-injection-molding/).
  • Softness range: overmold grips run Shore A 40–90; sealing gaskets and medical parts step down to LSR.
  • Substrate thickness: keep the plastic wall ≥1.5× the insert diameter around metal inserts with ~120% packing so the plastic contracts onto the insert (FirstMold insert practice).

The pairing table in practice: VR remote (PC+ABS core, TPE grip), charging-port cover (FR-TPU over ABS), ECG button (LSR over medical-grade substrate), tool handles (PP core, TPE grip). Same rules, different classes.

Additives, color and masterbatch

The base resin is rarely the final compound. Additives decide real-world survival:

  • UV stabilization: PP and POM degrade under UV — HALS stabilizers and carbon black extend outdoor life (FirstMold’s PP guidance cites 2,000+ QUV hours with proper stabilization, https://firstmold.com/pp-injection-molding/).
  • Flame retardants: halogenated and halogen-free FR packages; the FR choice trades flammability against mechanicals and smoke.
  • Fillers and reinforcements: glass fiber, talc, minerals — stiffness and HDT up, shrinkage anisotropic (see the GF ladder above).
  • Color masterbatch: color is compounded at a controlled ratio; color matching is verified with color controllers in production (the QA stack FirstMold documents on its PBT page: CMM, height gauges, moisture analyzers, pressure gauges, color controllers — https://firstmold.com/pbt-injection-molding/).
  • Regrind: runners and gates re-enter the shot at a controlled ratio — typically 10–30% — and color/impact are monitored; uncontrolled regrind is how a batch drifts.
  • Special-effect packages: anti-static, anti-fingerprint (★ on FirstMold’s surface-cost scale), conductive fillers for ESD parts.

Additive decisions belong on the material spec, not the shop floor — the molder should never be deciding UV stabilization for a sun-exposed part on your behalf.

Datasheet numbers vs molded reality

Datasheets are lab data measured under standardized conditions (ISO/ASTM test specimens). Molded reality differs systematically, and knowing the delta prevents surprises:

  • Shrink is directional: molded parts shrink differently along and across flow, especially with glass fill — the datasheet number is an average, not a map.
  • Weld lines knock down strength: a weld line in a reinforced grade can lose a significant share of tensile strength (typical published range for reinforced grades) — the datasheet tests a weld-free specimen.
  • Moisture moves dimensions: PA parts change 0.2–0.5% with moisture content — the datasheet value assumes a conditioning state.
  • Post-shrink continues: semi-crystalline parts keep shrinking after ejection — PP shows ~0.3% post-shrink, handled with an 80 °C / 2 h anneal and ~0.2% design allowance (FirstMold PP page).
  • Process window is the real spec: melt and mold temperature bands (table above) define what the molder can actually hold; a “tight-tolerance” resin run on a drifting mold temperature is worse than a commodity resin run tight.

The discipline: design with the datasheet, then verify with the first-article dimensional report and a capability study (Cpk ≥ 1.33 on critical characteristics). The datasheet starts the conversation; the Cpk file ends it.

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.

Two cost levers worth naming: mold cost runs $3,000–6,000 for simple tools and from ~$7,000 for complex steel/multi-cavity (HLH Rapid, https://www.hlhrapid.com/capabilities/injection-molding/), and tooling lead time runs 8–12 weeks for production with rapid-tooling first parts in 3–9 weeks depending on program. The resin class decision sits on top of both: an engineering grade with a narrow process window costs more per shot and per tool hour, which is why the six-filter workflow ends on cost rather than starting there.

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:

  1. Worst-case sustained temperature → set the resin class.
  2. Fluid/chemical exposure → confirm resistance or move up a class.
  3. Flammability regulation → specify UL94 level on the print.
  4. Load type → impact, hinge, wear or structural.
  5. Moisture → drying and pre-conditioning plan.
  6. Annual volume → does an engineering grade pay back?
  7. Additives → UV, FR, color, regrind policy on the spec, not the shop floor.
  8. Verification → first-article dimensional report plus Cpk ≥ 1.33 on criticals.

Frequently Asked Questions

1. What is the first thing to decide in material selection? The worst-case sustained temperature. It splits the resin universe into commodity (ABS, PP — up to ~80–100 °C), engineering (PC, PA, POM — 80–130 °C), and high-performance (PPS, PEI, PEEK — 200–260 °C) before any other consideration.

2. ABS vs PC vs PA — which do I pick? ABS for cheap, easy-molding shells below 80 °C. PC (or PC+ABS) when you need impact and transparency or dimensional stability. PA (nylon) when you need toughness, wear and heat up to ~130 °C — accepting its moisture behavior (0.2–0.5% dimensional shift).

3. When is PEEK worth $50–100/kg? When the environment demands it: sustained heat above ~200 °C, aggressive chemicals, low outgassing, or wear-plus-heat combinations. Below those conditions, PPS ($8–15/kg) or PA66-GF30 covers the zone at a fraction of the cost.

4. What does UL94 V-0 actually mean? A vertical-burn test where the specimen stops burning within 10 seconds after two 10-second flame applications, with no burning drips that ignite cotton below. It is the common bar for enclosures near electronics; 5VA/5VB are higher-severity barrier classes.

5. How much does glass fiber change a resin? Each ~10% of glass adds stiffness and heat deflection (PP-GF30: ~80 MPa tensile, ~160 °C HDT vs ~30 MPa/~100 °C unfilled typical) but shrinkage drops to 0.5–0.8% and becomes anisotropic — orientation, weld lines and draft all become design constraints.

6. Why do nylon parts change size? PA absorbs moisture — equilibrium moisture can shift dimensions 0.2–0.5%. Parts are dried before molding and conditioned before measurement; this is a handling discipline, not a material defect.

7. Can TPE overmold onto any plastic? No. Bonding needs polarity match and clean surface energy: TPE bonds well to PP and ABS, TPU to PC-rich substrates; PP/PE need plasma or flame treatment (~72 mN/m) for reliable adhesion.

8. What is the cheapest resin that survives 160 °C? Glass-filled PP (GF30) reaches ~160 °C HDT at commodity prices; PA66-GF30 covers 120–140 °C continuous with better toughness. Above that, PPS-GF40 is the value answer in the 200 °C zone.

9. How do I know if a datasheet number is achievable on my part? Cross-check the datasheet against molded reality: directional shrinkage, weld-line strength loss in reinforced grades, moisture effects, and post-shrink. The first-article dimensional report and a Cpk ≥ 1.33 capability study are the verification, not the datasheet.

10. What certifications should the material carry? UL94 for flammability; USP Class VI for medical; food-contact grades for kitchen/food; RoHS/REACH for EU shipments; and a per-lot material certificate (melt-flow, tensile, flame class) with traceability back to the resin lot.

11. What is the difference between a commodity and an engineering resin? Commodity (PP, PE, PS, ABS) — cheap, wide process window, good enough for most shells. Engineering (PC, PA, POM, PPS, PEI, PEEK) — strength, heat or chemical resistance at higher cost and tighter process windows. The grade must earn its place through the environment or load case.

12. What tolerance can each resin class hold? General features: ±0.1–0.2 mm for most classes. Critical locating surfaces: ±0.05 mm with steel-safe tooling. Amorphous resins hold tighter (shrink 0.2–0.7%) than semi-crystalline (1.0–3.0%). Our glass-filled PPS connector program held ±0.005 mm on critical pins at 2M units/year.

Sources

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.

NEXT STEP

Ready to Start Your Project?

Send us your requirements — our team responds within 24 hours with pricing and lead time.

RC

Written by

Ray Chan

Manufacturing Engineer · Custom Manufacturing Specialist. Ray helps global importers and integrators source factory-direct plastic parts and tooling.

← Back to Blog