Material selection is the quietest decision in an injection molding project and the one with the longest shadow. The resin decides strength, temperature resistance, appearance, compliance and cost, and by the time the wrong choice becomes visible in production, the cavity has already been cut for the wrong shrinkage rate.
This guide compares the seven resins that cover almost every application, then gives you the four questions that narrow the field in minutes.
1. Start With Four Questions
| Question | Why It Decides |
|---|---|
| Load? | Structural parts need PA or PC; housings and covers are fine in ABS or PP |
| Temperature? | Above ~100°C, ABS is out, PC, PA or POM are the candidates |
| Appearance? | Transparent needs PC or PMMA; cosmetic textured parts favor ABS or PC/ABS |
| Compliance? | Food contact, medical, flame retardant, outdoor UV, each narrows the list hard |
2. The Seven Workhorse Resins
| Resin | Strengths | Limits | Typical Use |
|---|---|---|---|
| ABS (e.g. PA-757) | Balanced strength/toughness, excellent surface, paint & texture friendly, low cost | ~80–95°C service limit, poor UV, solvent-sensitive | Enclosures, appliance parts, consumer products |
| PC | High impact, ~120°C service, transparent, dimensional stability | Cost, stress cracking in chemicals, needs drying | Transparent parts, impact-critical, electrical |
| PP | Lightest, chemical resistant, living hinges, lowest cost | Low stiffness, poor paintability, UV-sensitive | Living hinges, food contact, chemical-exposed parts |
| PA6 / PA66 | Strength, wear resistance, ~120–150°C service | Moisture absorption (dimensional drift), needs drying | Gears, bearings, engine-bay components |
| PA6-GF30 | High stiffness at temperature, creep resistant | Anisotropic shrinkage (warpage), abrasion, cosmetic limits | Structural automotive, brackets, under-hood |
| POM (acetal) | Low friction, dimensional stability, excellent machinability | Brittle at notches, poor UV, gassing during molding | Gears, slides, precision moving parts |
| PMMA | Optical clarity, UV resistance, surface hardness | Brittle, scratch-sensitive, solvent-sensitive | Lenses, light guides, displays |
3. The Head-to-Head You Will Actually Face: ABS vs PC
Most material debates reduce to ABS versus PC. Both mold well, both cover the general housing and enclosure space, and the differences are consistent:
| Property | ABS | PC |
|---|---|---|
| Impact strength | Good | Excellent, the impact benchmark |
| Service temperature | ~80–95°C | ~120°C |
| Transparency | Opaque | Transparent (up to ~89% light transmission) |
| Surface & paint | Excellent, texture and paint friendly | Good, but harder to paint (low surface energy) |
| Chemical resistance | Moderate | Prone to stress cracking in some chemicals |
| Cost | Lower | Higher (roughly 1.5–2.5x ABS) |
| Processing | Forgiving window | Needs drying, higher melt temperature |
The pragmatic answer for most housings: ABS unless you need PC's temperature, impact or transparency. And when you need a middle ground, PC/ABS blends combine PC heat resistance with ABS processability, the standard choice for automotive interiors and thin-wall housings.
4. The Glass-Filled Trade-Off
Glass fiber is the classic strength-for-headache trade. PA6-GF30 roughly doubles stiffness versus unfilled PA6 and holds it at temperature, which is why under-hood and structural automotive parts specify it. But three consequences must be designed in:
- Anisotropic shrinkage, the part shrinks differently along versus across fiber flow, warping flat plates and large surfaces unless flow direction is managed
- Abrasion, glass erodes gates, runners and cavity surfaces; hardened H13 steel or nitrided surfaces become standard, not optional
- Cosmetic limits, fiber bloom and weld-line weakness rule filled grades out of most cosmetic applications
5. Procurement Notes That Matter
- Grade substitutability: PA-757 is a specific Chi Mei grade with equivalents from most major ABS producers. When cost or supply forces a switch, verify shrinkage rate and flow behavior, drop-in equivalents exist, but they are verified, not assumed.
- Drying is not optional: PC, PA6/PA66, PMMA, PET and PBT absorb atmospheric moisture. At melt temperature the moisture reacts with the polymer and permanently degrades it, silver streaks and brittleness in production trace back to a dryer setting, not the resin.
- Regrind discipline: regrind is normal and economical, but uncontrolled ratios shift shrinkage, strength and color. Define the ratio in the process spec, not on the shop floor.
- Shrinkage is resin-specific: cavity dimensions are cut for one resin's shrinkage rate. Changing material mid-program is a dimensional change, not a parameter change, decide before the tool, not after.
6. The Decision Path
- Transparent part? → PC or PMMA (PMMA for optical clarity, PC for impact + heat)
- Living hinge or chemical exposure? → PP
- Above ~100°C service? → PC, PA66, POM or PC/ABS blend
- Structural with stiffness at temperature? → PA6-GF30 (design for warpage)
- Gears, slides, precision motion? → POM
- General housing, balanced properties, best surface? → ABS (PA-757 class)
If you are weighing two candidates for a part right now, the fastest resolution is a five-minute conversation with the molder: describe the load, temperature, appearance and compliance requirements, and the shortlist writes itself. Send us the part function and environment, we will recommend the resin on engineering merit, including when the answer is the cheaper one.