Plastic Material Selection Guide for Injection Molding

Guide to choosing the right plastic for injection molded parts: amorphous vs semi-crystalline, engineering plastics, and property comparisons.

Plastic Classification

Injection molding plastics fall into two categories: amorphous and semi-crystalline.

Amorphous Plastics

Random molecular arrangement. Clear, low shrinkage, good dimensional stability, lower chemical resistance.

• ABS: Tough, impact-resistant. Consumer goods, automotive interiors.
• Polycarbonate (PC): High impact strength, clear. Safety glasses, housings, medical.
• Acrylic (PMMA): Excellent optical clarity, UV stable. Lenses, displays.
• Polystyrene (GPPS/HIPS): Low cost. Packaging, toys.
• PVC: Fire-retardant, chemical resistant. Pipes, fittings.

Semi-Crystalline Plastics

Ordered molecular structure. Opaque, higher chemical resistance, better fatigue properties.

• Polypropylene (PP): Low cost, chemical resistant. Living hinges, containers.
• Nylon (PA6/PA66): Strong, wear resistant. Gears, bearings, under-hood parts.
• Acetal/POM (Delrin): Low friction, high stiffness. Precision mechanical parts.
• Polyethylene (HDPE/LDPE): Low cost, impact resistant. Bottles, caps.
• PEEK: High temperature (260°C continuous), strong. Aerospace, medical implants.

Key Selection Factors

• Operating Temperature: PP (100°C), PC (130°C), PEEK (260°C)
• Impact Strength: PC (highest), ABS (good), PS (brittle)
• Chemical Resistance: Semi-crystalline (better), Amorphous (worse)
• Cost: PP/PE/PS ($1-3/kg), PC/ABS ($3-5/kg), PEEK ($50-100/kg)