How to Choose the Right Plastic Resin for Injection Molding
With hundreds of plastic resins available for injection molding, selecting the right material is one of the most consequential — and most frequently rushed — decisions in product development. Choose wrong and you face field failures, processing problems, regulatory non-compliance, or cost overruns. Choose well and your part performs reliably, processes efficiently, and meets every requirement.
This guide provides a practical framework for resin selection, with a focus on the materials most commonly used in industrial injection molding.
Step 1: Define Your Requirements Before Looking at Data Sheets
Before opening a material database, define what your part must do. Work through these categories:
Mechanical Requirements
- Maximum sustained load (tensile, compressive, bending)?
- Impact events (drop test, assembly snap-in)?
- Fatigue cycles (snap-fit open/close, vibration)?
- Creep under sustained load?
- Required stiffness (flexural modulus)?
Thermal Requirements
- Continuous service temperature?
- Peak temperature (brief exposure)?
- Does the part need to survive autoclave sterilization?
- Will it see engine bay temperatures (>120°C)?
Chemical Exposure
- Solvents (acetone, IPA, petrol, hydraulic fluid)?
- Cleaning agents (bleach, hospital disinfectants)?
- Body fluids?
- UV and outdoor weathering?
Regulatory & Compliance
- UL 94 flammability rating (V-0, V-1, V-2)?
- RoHS / REACH compliance?
- FDA food contact or medical biocompatibility (USP Class VI)?
- EU toy safety (EN 71)?
Appearance
- Class A cosmetic surface required?
- Optical clarity needed?
- Colour match to Pantone / RAL?
- Texture to be applied in-mold?
Only after defining these requirements should you look at resin candidates.
Step 2: Match Requirements to Resin Families
ABS (Acrylonitrile Butadiene Styrene)
Best for: Consumer electronics enclosures, power tool housings, appliance panels, automotive interior trim.
| Property | Value |
|---|---|
| Density | 1.03–1.06 g/cm³ |
| Tensile strength | 40–50 MPa |
| Flexural modulus | 2.0–2.8 GPa |
| Heat deflection temperature | 85–100°C |
| Notched Izod impact | 150–250 J/m |
| Shrinkage | 0.4–0.7% |
ABS strengths: Excellent impact resistance, easy to process, good surface finish, paintable and plateable, wide colour range, cost-effective.
ABS limitations: Not suitable for outdoor weathering without UV stabilization; limited chemical resistance (attacked by acetone, ethyl acetate, strong acids); service temperature ceiling around 85°C continuous.
Key grades:
- Standard ABS — general purpose
- High-impact ABS — improved low-temperature impact
- ABS FR (V-0) — flame retardant for electrical enclosures
- ABS + PC alloy — improved heat and impact vs. standard ABS
- ABS plating grade — copper-nickel-chrome plating applications
PC (Polycarbonate)
Best for: Lighting lenses, safety equipment, electronic display covers, medical device housings, automotive lighting.
| Property | Value |
|---|---|
| Density | 1.20 g/cm³ |
| Tensile strength | 55–65 MPa |
| Flexural modulus | 2.3–2.4 GPa |
| Heat deflection temperature | 125–135°C |
| Notched Izod impact | 600–900 J/m |
| Shrinkage | 0.5–0.7% |
PC strengths: Outstanding impact resistance (one of the best of any thermoplastic), optical clarity, high heat resistance, dimensional stability.
PC limitations: Susceptible to stress cracking with certain chemicals (including many cleaning agents); requires careful drying before processing; higher cost than ABS; not suitable for applications with aggressive chemical exposure.
Key grades:
- Optical PC — lens-grade clarity (Covestro Makrolon, SABIC Lexan)
- PC FR V-0 — electrical and electronic enclosures
- PC medical grade — USP Class VI biocompatibility
- PC/ABS blend — cost/performance balance for automotive and electronics
PP (Polypropylene)
Best for: Automotive under-bonnet components, food containers, packaging, living hinges, chemical-resistant enclosures.
| Property | Value |
|---|---|
| Density | 0.90–0.91 g/cm³ |
| Tensile strength | 25–40 MPa |
| Flexural modulus | 1.0–1.7 GPa |
| Heat deflection temperature | 100–115°C (unfilled) |
| Notched Izod impact | 20–80 J/m |
| Shrinkage | 1.0–2.5% (high — design accordingly) |
PP strengths: Lowest density of common thermoplastics (cost-efficient by volume); excellent chemical resistance; natural living hinge capability; FDA-compliant grades available; low cost.
PP limitations: High shrinkage (warpage risk for large flat parts); lower stiffness than ABS or PC; limited UV resistance without additives; weaker weld lines vs. amorphous materials.
Key grades:
- Homopolymer PP — higher stiffness, lower impact
- Copolymer PP — better impact, slightly lower stiffness
- PP-GF20/GF30 — glass-filled for stiffness and dimensional stability
- PP UV-stabilized — outdoor applications
- PP FR — flame retardant
PA6 / PA66 (Nylon)
Best for: Gears, structural brackets, clips and fasteners, automotive under-bonnet, power tool housings.
| Property | PA6 | PA66 |
|---|---|---|
| Tensile strength | 70–80 MPa | 75–90 MPa |
| Flexural modulus | 2.5–3.0 GPa | 2.8–3.3 GPa |
| Heat deflection (1.8 MPa) | 65°C dry | 90°C dry |
| Moisture absorption | Higher | Lower |
| Processing temp | 230–260°C | 260–290°C |
PA strengths: High strength and stiffness, excellent wear and fatigue resistance, good heat resistance (especially PA66), natural lubricity.
PA limitations: Absorbs moisture from air — parts must be conditioned before dimensional measurements; properties change significantly with moisture content; higher processing temperature than ABS/PP; drying critical before molding.
Key grades:
- PA6-GF30 / PA66-GF30 — most commonly used structural grade
- PA66 FR V-0 — electrical connectors, relay housings
- PA12 — flexible, lower moisture absorption, fuel-contact applications
- PA6 medical grade — USP Class VI
POM (Polyoxymethylene / Acetal)
Best for: Precision gears, cams, bearings, valve components, fasteners, sliding mechanisms.
| Property | Value |
|---|---|
| Tensile strength | 60–70 MPa |
| Flexural modulus | 2.5–3.0 GPa |
| Heat deflection (1.8 MPa) | 110°C |
| Coefficient of friction | Very low (0.2–0.35) |
| Shrinkage | 1.8–2.2% |
POM strengths: Excellent dimensional stability, lowest coefficient of friction of any thermoplastic, fatigue resistance, chemical resistance (except strong acids/bases), low creep.
POM limitations: Cannot be bonded with most adhesives; poor UV resistance; flammable (no standard V-0 grade available); cannot be plated.
Two types:
- POM homopolymer (Delrin) — higher strength and fatigue resistance
- POM copolymer — better chemical resistance, more processable
PBT / PET (Polybutylene / Polyethylene Terephthalate)
Best for: Electrical connectors, sensor housings, switch bodies, automotive under-bonnet electrical.
| Property | PBT-GF30 |
|---|---|
| Tensile strength | 100–120 MPa |
| Flexural modulus | 5.0–7.0 GPa |
| Heat deflection | 200–210°C |
| Dimensional stability | Excellent |
Key advantage: PBT-GF30 offers the best combination of electrical properties, heat resistance, and dimensional stability in its cost class. It is the default choice for electrical connector bodies.
PEEK (Polyether Ether Ketone)
Best for: Aerospace, medical implants, semiconductor equipment, extreme environment applications.
When to use PEEK:
- Continuous service temperature >200°C
- Chemical resistance to virtually all solvents
- Steam autoclave sterilization required
- Regulatory requirements (FDA, USP Class VI, biocompatibility for implants)
Cost note: PEEK resin is 20–50× the price of ABS or PP. It is the material of last resort — specified only when no lower-cost alternative meets the requirements. Ensure you genuinely need PEEK before specifying it.
Step 3: Consider Processing and Cost
Two materials may both meet your technical requirements, but differ significantly in:
- Processing ease — some resins require tight temperature control (PC, PEEK), extensive drying (PA, PC, PBT), or specific screw geometry
- Material cost per kg — a 5× cost difference in resin translates directly to part cost
- Tooling implications — high-shrinkage materials (PP, PA) require larger cavity dimensions; corrosive resins (PVC) require hardened/corrosion-resistant steel
Always discuss material selection with your mold supplier before finalizing — they may have experience with your specific application that suggests a better or more economical option.
Quick Reference: Resin Selection Matrix
| Requirement | Best Resin Choices |
|---|---|
| Lowest cost | PP, PE, ABS |
| Best impact resistance | PC, PC/ABS, ABS |
| Highest temperature resistance | PEEK, PEI, PPS, PA66-GF |
| Best chemical resistance | PEEK, POM, PP, HDPE |
| Optical clarity | PC, PMMA, SAN, PS |
| Lowest friction / wear | POM, PA+PTFE, PEEK |
| Best dimensional stability | POM, PBT-GF30, PA66-GF30 |
| UL 94 V-0 flame retardant | PC FR, ABS FR, PA66 FR, PBT FR |
| FDA / USP Class VI | PP, PE, PC (medical), PEEK |
| Living hinge | PP homopolymer |
| Softest / elastomeric | TPE, TPU, TPV, silicone |
JBRplas provides material selection guidance as part of every DFM review. If you’re uncertain which resin is right for your application, send us your part design and requirements — we’ll recommend the most suitable options with cost and performance trade-offs.