How to Select Plastic Materials for Injection Molding

Material selection drives everything downstream: tooling steel, gate design, cycle time, part cost, and whether the product survives in the field. This guide compares the six most commonly injection-molded thermoplastics and provides a selection framework you can apply to any project.

Engineering plastics material selection comparison chart

Figure 1. Engineering plastics material selection comparison chart.

Quick Comparison Table

Property	PP	ABS	PC	PA66 (GF30)	POM	TPU
Tensile strength (MPa)	30–40	40–50	60–70	150–180	60–70	30–50
Max continuous use temp (°C)	90–100	70–85	120–130	120–150	90–100	80–90
Impact resistance	Good (thin)	Good	Excellent	Good	Moderate	Excellent
Chemical resistance	Excellent	Moderate	Moderate	Good	Excellent	Good
Relative cost	$	$	$$$	$$$	$$	$$–$$$
Shrinkage (%)	1.2–2.0	0.4–0.7	0.5–0.7	0.2–0.5	1.8–2.5	0.8–1.5

Material Profiles

### PP (Polypropylene) — The Cost Workhorse Best for living hinges, food containers, battery boxes, and anything cost-sensitive with moderate demands. Excellent chemical resistance and fatigue performance at zero hinge thickness. Weaknesses: low-temperature brittleness (unless copolymer), UV sensitivity without additives, and high shrinkage that challenges tight tolerances.

### ABS — The Appearance Champion The default for consumer product housings: good gloss, easy painting, plating, and texture, stable dimensions. Combines well in PC/ABS blends when more impact or heat resistance is needed.

### PC (Polycarbonate) — Impact and Transparency When parts must not break — safety glasses, lamp lenses, enclosures for power tools — PC is the answer. Watch for: hydrolysis during molding if not dried properly (splay and brittleness), stress cracking in contact with adhesives and cleaners, and UV stabilization requirements for outdoor use.

### PA66 (Nylon, often GF30) — Structural and Heat 30% glass-filled PA66 replaces metal in brackets, gears, and housings under the hood. Key considerations: it absorbs moisture (changing dimensions and stiffness — design with conditioned properties), needs thorough drying before molding, and glass fill introduces anisotropic shrinkage and warp.

### POM (Acetal) — Precision Mechanisms Low friction, high stiffness, excellent fatigue for gears, slides, and springs-in-plastic. Very low water absorption versus nylon makes it more dimensionally predictable. Its high, sharp shrinkage demands careful gate and cooling design.

### TPU — Flexible and Abrasion-Resistant Shore hardness 60A–75D. Used for overmolding grips, seals, cable jackets, wheels. Overmolding onto PC or PA requires bond-compatible grades; shim back TPU sections to avoid long, difficult flexible flow paths.

A Five-Step Selection Framework

List the load cases. Static or impact? Continuous or occasional? At what temperature?
Define the environment. UV, chemicals, moisture, temperature range, flame requirements (UL94 rating).
Define cosmetic and assembly requirements. Texture, paint, transparency, snap fits, overmolding.
Check regulatory needs. Food contact (FDA/EU 10/2011), medical (ISO 10993), electrical (UL yellow card).
Optimize cost at target wall thickness. A cheaper resin at 3 mm often loses to a pricier resin at 1.8 mm once cycle time and part weight are counted.

Common Mistakes

Designing around unfilled data sheets when the part is glass-filled (shrinkage and stiffness differ radically)
Ignoring moisture-conditioning effects on nylon dimensions
Choosing PC without a UV package for outdoor enclosures
Specifying tight tolerances on high-shrinkage materials (PP, POM)

Need Help Choosing?

We mold all of the materials above daily and can provide material recommendations with cost and cycle-time comparison as part of our free DFM review. Send your application requirements and 3D files to get started.

Quick reference - key checks and values for How to Select Plastic Materials for Injection Molding

Figure 2. Quick reference: the key checks and reference values from this guide.