Injection Molding Materials Guide | Plastic Resin & Mold Steel Selection Reference | JBRplas
Complete engineering reference for injection molding material selection — plastic resin properties, mold steel grades, compatibility matrix, and industry-specific material recommendations from JBRplas.
Part 1: Plastic Material Selection
Selecting the right plastic resin is the single most consequential material decision in any injection molding project. The resin determines part performance, tooling requirements, cycle time, and unit cost — and changing materials mid-program is expensive. This reference covers the resins JBRplas processes daily, organized to support engineering decisions.
Decision Framework
Work through these questions before opening a data sheet:
- Mechanical — What loads, impacts, fatigue cycles, and creep conditions does the part face?
- Thermal — Continuous and peak service temperature?
- Chemical — Exposure to solvents, cleaners, fuels, body fluids, UV?
- Regulatory — UL 94, FDA, USP Class VI, RoHS, REACH, EN71?
- Cosmetic — Class A surface, optical clarity, colour accuracy, texture?
- Production — Annual volume, target cycle time, tolerance requirements?
- Cost — What is the resin cost envelope for the part?
Engineering Resins — Properties at a Glance
| Resin | Density (g/cm³) | Tensile (MPa) | Flex Mod (GPa) | HDT (°C) | Impact (J/m) | Shrinkage (%) | Relative Cost |
|---|---|---|---|---|---|---|---|
| ABS | 1.03–1.06 | 40–50 | 2.0–2.8 | 85–100 | 150–250 | 0.4–0.7 | $$ |
| PC | 1.20 | 55–65 | 2.3–2.4 | 125–135 | 600–900 | 0.5–0.7 | $$$ |
| PC/ABS | 1.12–1.16 | 50–60 | 2.3–2.8 | 105–120 | 350–550 | 0.5–0.7 | $$$ |
| PA6 | 1.13 | 70–80 | 2.5–3.0 | 65 (dry) | 50–150 | 0.8–1.5 | $$ |
| PA66 | 1.14 | 75–90 | 2.8–3.3 | 90 (dry) | 40–100 | 0.8–1.5 | $$$ |
| PA66-GF30 | 1.37 | 140–170 | 7.0–9.0 | 250 | 80–120 | 0.2–0.5 | $$$ |
| POM (Acetal) | 1.41 | 60–70 | 2.5–3.0 | 110 | 50–100 | 1.8–2.2 | $$$ |
| PBT-GF30 | 1.53 | 100–120 | 5.0–7.0 | 200–210 | 50–80 | 0.2–0.5 | $$$ |
| PPO/PPE | 1.06–1.10 | 50–65 | 2.3–2.5 | 120–140 | 150–250 | 0.5–0.7 | $$$$ |
| PMMA (Acrylic) | 1.19 | 65–75 | 3.0–3.5 | 85–100 | 15–25 | 0.3–0.6 | $$$ |
| PEEK | 1.30–1.50 | 90–170 | 3.5–11.0 | 250–315 | 40–80 | 1.0–1.4 | $$$$$ |
| TPE / TPU | 0.90–1.25 | 5–40 | 0.01–0.50 | 60–120 | No break | 1.0–2.5 | $$–$$$$ |
Commodity Resins
| Resin | Density (g/cm³) | Tensile (MPa) | Flex Mod (GPa) | HDT (°C) | Shrinkage (%) | Relative Cost |
|---|---|---|---|---|---|---|
| PP (homopolymer) | 0.90 | 25–40 | 1.0–1.7 | 100–115 | 1.0–2.5 | $ |
| PP (copolymer) | 0.90 | 20–30 | 0.8–1.4 | 90–105 | 1.0–2.5 | $ |
| PP-GF20 | 1.04 | 60–80 | 3.5–4.5 | 140–155 | 0.3–0.7 | $$ |
| HDPE | 0.95 | 20–30 | 0.8–1.4 | 65–80 | 1.5–3.0 | $ |
| LDPE | 0.92 | 10–15 | 0.2–0.5 | 40–50 | 1.5–3.5 | $ |
| GPPS | 1.04 | 40–50 | 3.0–3.5 | 70–85 | 0.4–0.7 | $ |
| HIPS | 1.04 | 20–35 | 1.8–2.5 | 75–90 | 0.4–0.7 | $ |
| SAN | 1.08 | 65–75 | 3.5–3.9 | 85–95 | 0.3–0.6 | $$ |
| PVC (rigid) | 1.40 | 40–50 | 2.5–3.5 | 65–75 | 0.2–0.5 | $ |
| PVC (flexible) | 1.25–1.35 | 10–20 | — | 50–65 | 1.0–3.5 | $ |
Specialty Grade Quick Reference
| Requirement | Available Grades |
|---|---|
| Flame retardant (UL 94 V-0) | ABS FR, PC FR, PC/ABS FR, PA66 FR, PP FR, PBT FR |
| Flame retardant (UL 94 V-1/V-2) | ABS V-1, PP V-2, PA6 V-2 |
| Glass-filled (stiffness + dimensional stability) | PA6-GF15/30/50, PA66-GF15/30/50, PP-GF20/30, PBT-GF15/30, POM-GF20, PEEK-GF30 |
| UV-stabilized (outdoor) | PP UV, ABS UV, PC UV, PA66 UV |
| Food-contact (FDA / EU 10/2011) | PP, PE, PC, ABS (food-grade), POM |
| Medical-grade (USP Class VI) | PP, PE, PC, ABS, PEEK, TPE |
| Biocompatible (ISO 10993) | PC (medical), PEEK, medical-grade TPE/TPU |
| Conductive / ESD-safe | Carbon-filled ABS, carbon-filled PC, carbon-fibre reinforced PA |
| Wear-resistant | POM, PA+PTFE, PEEK, PA66+MoS2 |
| High-gloss / optical | PMMA, PC (optical grade), SAN, GPPS |
| Soft-touch / elastomeric | TPE (Shore 30A–70A), TPU (Shore 60A–90A), TPV |
Material Recommendations by Industry
| Industry | Primary Resins | Key Requirements |
|---|---|---|
| Automotive interior | PC/ABS, ABS, PA66-GF30, POM | Heat resistance, UV stability, low VOC, PPAP documentation |
| Automotive under-bonnet | PA66-GF30, PA6-GF15, PP-GF20, PBT-GF30 | High-temp resistance, chemical resistance, validated Cpk |
| Medical device | PC (medical), ABS (medical), PP (medical), PEEK | USP Class VI, ISO 10993, ISO 13485 traceability, clean room molding |
| Health care consumer | ABS, PC, PP, TPE | Skin-contact rated, IP-rated, cosmetic Class A, colour consistency |
| Smart home / IoT | ABS FR, PC FR, PC/ABS FR | UL 94 V-0, cosmetic Class A, RoHS/REACH, ΔE < 1.5 colour |
| Consumer electronics | ABS, PC, PC/ABS | Impact resistance, cosmetic finish, snap-fit durability, thin-wall capability |
| Financial devices | PC/ABS FR, ABS FR (carbon-filled), PA66 FR | ESD-safe, tamper-evident, EMV slot precision, UL 94 V-0 |
| Branded toys | ABS, PP, HDPE, HIPS, TPE | EN71, ASTM F963, CPSIA, phthalate-free, colour accuracy ΔE < 1.5 |
| Industrial equipment | PA66-GF30, POM, PC, ABS | High stiffness, wear resistance, chemical resistance, long service life |
Part 2: Mold Steel Selection
The mold steel determines tool life, part quality, cycle time, and maintenance cost. Selecting the right steel requires balancing the resin being molded, the production volume, and the cosmetic and dimensional requirements of the part.
Steel Grade Reference
| Steel | Hardness (HRC) | Best For | Shot Life | Polishability | Corrosion Resistance | Relative Cost |
|---|---|---|---|---|---|---|
| P20 (718H) | 28–34 | General purpose: ABS, PP, PS, PC | 300K–500K | SPI B1 | Low | $$ |
| H13 | 48–52 | High-temp resins: PA, POM, PBT, PEEK | 500K–1M+ | SPI B1 | Moderate | $$$$ |
| S136 (420SS) | 48–52 | Corrosive resins: PVC, PC, FR grades. Optical parts. Medical. | 500K–1M+ | SPI A1 | High | $$$$$ |
| NAK80 | 38–43 | Mirror finish: clear parts, optical lenses, high-gloss cosmetic | 300K–500K | SPI A1 | Low | $$$$ |
| 2344 (H13 eq.) | 44–52 | High-pressure, automotive, abrasive GF resins | 800K–1M+ | SPI B1 | Moderate | $$$$ |
| Aluminum (7075-T6) | — | Prototype / bridge tooling, <5K shots | 1K–5K | SPI B1 | Low | $ |
Steel vs Resin Compatibility Matrix
This matrix answers: “I’m molding [resin]. What steel should I use?”
| Resin | P20 (718H) | H13 | S136 (420SS) | NAK80 | Notes |
|---|---|---|---|---|---|
| ABS | ✅ | ✅ | ✅ | ✅ | P20 adequate for standard. S136 for medical/optical. |
| ABS FR (V-0) | ⚠️ | ✅ | ✅ | ⚠️ | Halogenated FR grades produce corrosive off-gas. Use S136 if part is cosmetic and volume exceeds 100K. |
| PC | ✅ | ✅ | ✅ | ✅ | S136 or NAK80 for optical clarity. P20 adequate for structural. |
| PC (optical) | ❌ | ⚠️ | ✅ | ✅ | Only S136 or NAK80 for lens-grade parts. |
| PC/ABS | ✅ | ✅ | ✅ | ✅ | P20 adequate for standard. H13 for high volume. |
| PP | ✅ | ✅ | ✅ | ⚠️ | P20 adequate for all standard applications. |
| PP-GF (glass-filled) | ⚠️ | ✅ | ✅ | ❌ | GF is abrasive — use H13 for volume >100K shots. P20 adequate for low volume only. |
| PA6 / PA66 | ✅ | ✅ | ✅ | ⚠️ | P20 for low volume. H13 for production volumes. PA absorbs moisture — S136 reduces corrosion risk. |
| PA66-GF30 | ❌ | ✅ | ✅ | ❌ | GF30 is abrasive — H13 or 2344 required. P20 wears rapidly. |
| POM (Acetal) | ⚠️ | ✅ | ✅ | ⚠️ | POM processing temperature is near P20’s practical limit. H13 recommended for production. S136 for corrosive-grade POM. |
| PBT / PBT-GF | ❌ | ✅ | ✅ | ❌ | High processing temp + abrasive when GF. H13 minimum. |
| PMMA (Acrylic) | ✅ | ✅ | ✅ | ✅ | NAK80 for optical; S136 for medical; P20 for general. |
| PPO/PPE | ✅ | ✅ | ✅ | ⚠️ | Standard requirements. P20 adequate in most cases. |
| PEEK | ❌ | ✅ | ✅ | ❌ | H13 minimum due to extreme processing temperature (360–400°C). S136 if medical. |
| PVC (rigid / flexible) | ❌ | ❌ | ✅ | ❌ | PVC produces HCl at processing temperature — highly corrosive. S136 mandatory. |
| TPE / TPU | ✅ | ✅ | ✅ | ⚠️ | P20 adequate. S136 if medical grade. |
| HDPE / LDPE | ✅ | ✅ | ✅ | ⚠️ | P20 adequate for all standard applications. |
Legend: ✅ Recommended · ⚠️ Acceptable with limitations · ❌ Not recommended
Surface Finish by Steel Grade
| Finish | SPI Code | Ra (μm) | Achievable With |
|---|---|---|---|
| Diamond polish | A1 | 0.012 | S136, NAK80 |
| High-gloss polish | A2 | 0.025 | S136, NAK80, H13 |
| Fine polish | A3 | 0.05 | S136, NAK80, H13, P20 |
| Semi-gloss | B1 | 0.10 | All grades |
| Fine stone | B2 | 0.16 | All grades |
| Medium stone | B3 | 0.25 | All grades |
| Coarse stone | C1 | 0.28 | All grades |
| Dry blast | D1 | 0.80 | All grades |
| VDI texture | VDI 12–45 | varies | All grades (deeper VDI on harder steel requires more passes) |
| Laser texture | Custom | varies | P20, H13, S136 |
Shot Life by Steel Grade and Application
| Steel | Prototype | Low Volume | Production | High Volume |
|---|---|---|---|---|
| Aluminum 7075 | 1K–5K | — | — | — |
| P20 (718H) | — | 50K–100K | 100K–300K | 300K–500K |
| NAK80 | — | — | 100K–300K | 300K–500K |
| H13 | — | — | 300K–500K | 500K–1M+ |
| S136 | — | — | 300K–500K | 500K–1M+ |
| 2344 | — | — | 500K+ | 800K–1M+ |
How to Use This Guide
- Start with your resin. Identify the plastic material from Part 1.
- Check the compatibility matrix. Find your resin in the Steel vs Resin table. Eliminate incompatible steels.
- Filter by volume. Match your annual volume against shot life expectations.
- Factor in cosmetics. If your part requires optical clarity or Class A high-gloss, restrict to S136 or NAK80.
- Consider the budget. P20 is often adequate for low-to-medium volume standard resins. Don’t over-specify steel — but don’t under-specify on abrasive or high-temperature materials where tool wear becomes a quality risk.
JBRplas provides steel selection recommendations as part of every DFM review. If you are unsure which steel grade is right for your mold, submit your part design and we will recommend the appropriate grade based on your resin, volume, and cosmetic requirements.
Request a free DFM review with material and steel recommendations →