Injection Moulding: Plastic Materials

Injection moulding allows manufacturers to create precise and consistent plastic parts for a wide range of applications. One of the key factors that determine a part’s performance is the type of plastic material used. Understanding the basic plastics helps designers and engineers select the right material for strength, flexibility, durability, and cost.

1. Thermoplastics

Thermoplastics are the most commonly used plastics in injection moulding. They can be heated, melted, and re-melted multiple times, making them versatile and recyclable.

Common Thermoplastics:

  • Polypropylene (PP): Lightweight, chemically resistant, and ideal for automotive parts, packaging, and consumer goods.
  • Polyethylene (PE): Highly flexible, impact-resistant, and used in containers, tubing, and caps.
  • Acrylonitrile Butadiene Styrene (ABS): Strong, rigid, and heat-resistant, commonly found in electronics, housings, and toys.
  • Polystyrene (PS): Economical, easy to process, and used in disposable containers, trays, and housings.
  • Polycarbonate (PC): Transparent, high-strength, heat-resistant, ideal for lenses, medical devices, and safety equipment.
  • Polyamide (PA / Nylon): Strong, wear-resistant, and suitable for mechanical parts, gears, and automotive components.

Advantages of Thermoplastics:

  • Recyclable and reusable
  • Wide range of mechanical and thermal properties
  • Compatible with overmoulding and multi-material processes

2. Thermosetting Plastics

Thermosets are plastics that harden permanently after being heated and cured. Unlike thermoplastics, they cannot be remelted.

Common Thermosets:

  • Epoxy Resins: Excellent chemical resistance and electrical insulation; often used in electronics and adhesives.
  • Phenolic Resins: High heat resistance, flame retardant, used for electrical components and automotive parts.
  • Urea-Formaldehyde (UF) / Melamine-Formaldehyde (MF): Hard, durable, used in kitchenware, laminates, and decorative parts.

Advantages of Thermosets:

  • High dimensional stability
  • Excellent heat and chemical resistance
  • Strong and rigid, ideal for load-bearing applications

3. Semi-Crystalline vs. Amorphous Plastics

Plastic behavior can also be categorized as semi-crystalline or amorphous:

  • Semi-Crystalline Plastics (e.g., PE, PP, PA) have defined melting points, high chemical resistance, and are often used for structural components.
  • Amorphous Plastics (e.g., ABS, PC, PS) soften over a temperature range, are easier to process, and provide good dimensional stability.

4. Factors to Consider When Choosing Plastic Materials

  • Mechanical Requirements: Strength, flexibility, impact resistance
  • Thermal Resistance: Can the part withstand high temperatures during use or sterilization?
  • Chemical Resistance: Exposure to solvents, oils, or cleaning agents
  • Cost and Availability: Balancing material properties with budget constraints
  • Sustainability: Use of recycled materials and recyclability

5. Conclusion

Choosing the right plastic material is essential for the performance, durability, and manufacturability of injection-moulded parts. Understanding thermoplastics, thermosets, and material behaviors allows designers and engineers to make informed decisions and optimize their products for function, cost, and sustainability. If you need help with this, do not hesitate to reach out to FPI.