Why thermoplastic and thermoset materials in injection moulding perform differently from each other
Cooking an omelette is an apt analogy to illustrate how thermoplastics and thermosets can be differentiated based on their behaviour in the presence of heat.
Two broad categories of plastic materials exist: thermoplastics and thermosets, which are differentiated by their behaviour when heated. This guide offers considerations to keep in mind when choosing between these material types for injection molding.
One way to explain the difference is to consider an omelette. Start with an egg, a slice of cheese, and a warm pan. The egg begins as a liquid which, when heated, becomes a solid. The cheese begins as a solid but becomes a viscous liquid when heated. After heating, the egg will never return to its liquid state – it remains solid, just like thermoset polymers. But if you cool the melted cheese, it regains its solid form; reheat it and it flows again – just like thermoplastics.
Guidelines for Using Thermoplastics
A well‑designed part can fail if manufactured from the wrong material. Carefully consider factors such as strength, impact resistance, performance at high temperatures, and other elements when selecting materials for injection moulding.
Review the properties of common resin types (see table below). For more details, consult our Materials Comparison Guide → /resources/material-comparison-guide/]. If a standard material does not provide everything you need, a blended resin may meet your requirements. For example, when polycarbonate (PC) and ABS are combined, the new material is stronger and more dimensionally accurate than ABS alone.
| Resin Type | Strength | Impact Resistance | Dimensional Accuracy | Capability to Fill Small Features | Performance at High Mould Temps | Cost |
|---|---|---|---|---|---|---|
| Acetal (POM) | Medium | Medium | Fair | Fair | Fair | Medium |
| Acrylic (PMMA) | Medium | Low | Good | Fair | Good | Medium |
| ABS | Low to medium | High | Good | Fair | Good | Low |
| HDPE | Low | High | Fair | Excellent | Good | Low |
| Polycarbonate (PC) | Medium | High | Good | Fair | Good | Medium to High |
| PC/ABS alloy | Medium | High | Good to excellent | Fair | Good | Medium |
| Polypropylene (PP) | Low | High | Fair | Excellent | Good | Low |
| Polystyrene (PS) | Low to medium | Low | Good | Good | Good | Low |
For detailed data sheets on these and other thermoplastics, visit our Materials Resource Center.
Considerations for Thermosets
For thermosetting polymers, Konlida offers a variety of liquid silicone rubber (LSR) materials.
LSR is a common thermoset valued for its superior flexibility, heat resistance, and chemical resistance. Typical applications include soft‑touch surfaces, gaskets, seals, and heat insulation. Two components of uncured LSR are mixed to form a low‑viscosity liquid rubber solution. The material cures in the presence of heat, forming a bond that cannot be undone.
The following table shows our general‑purpose LSR materials, plus special grades for fluorosilicone, medical, and optical applications. Detailed data sheets are available in our Materials Comparison Guide.
| Type | Material |
|---|---|
| LSR (standard) | Elastosil 3003/30 A/B, 3003/40 A/B, 3003/50 A/B, 3003/60 A/B, 3003/70 A/B, 3003/80 A/B |
| LSR (Fluorosilicone) | Silastic FL 60‑9201 |
| LSR (Medical) | Dow Corning QP1‑250 |
| LSR (Optical) | Dow Corning MS‑1002 |
When working with LSR materials, consider the following design factors:
- Wall and rib thickness: LSR typically fills thin wall sections with minimal difficulty. Walls as thin as 0.25 mm are possible, depending on size and adjacent thicker sections. Rib thickness should be 0.5 to 1.0 times the adjoining wall thickness. LSR accommodates wall thickness variations well, and sink marks are almost non‑existent.
- Shrinkage and flash: LSR has a fairly high shrinkage rate, approximately 2.5%. It also tends to flash easily during moulding (in gaps as small as 0.005 mm). Konlida reduces flash by incorporating additional features into the mould design.
- Parting lines: Simplifying and minimising parting lines in your design helps achieve cleaner LSR parts more quickly.
- Undercuts: LSR can be moulded to accommodate undercuts, which are manually removed after moulding. Mechanical tooling actions for undercuts are selectively available.
- Part ejection: Ejector pins are normally not used in LSR moulding due to the flexible, low‑viscosity nature of the material. Parts should be designed so they remain on one half of the mould when opened, then manually de‑moulded (often with air assistance).
Cross‑linking determines many characteristics of thermosets, making them strong, dimensionally stable, and highly resistant to heat and chemicals. A familiar example is silicone bakeware: cross‑linking allows it to easily withstand oven temperatures up to 200 °C and gives it a non‑stick surface. However, harder thermosetting plastics are not as impact‑resistant as thermoplastics and can tend to shatter.
For more design guidance on LSR moulding, see our LSR Design Guide → /resources/lsr-design/].
Choosing the Right Material for Your Project
Every material type has rules and guidelines. We encourage you to refer to our Materials Comparison Guide, where you can review and choose from hundreds of manufacturing materials.
At Konlida, we do not make resin choices for you, but we are always available to help you evaluate the characteristics when deciding between thermoplastic and thermoset injection moulding.
Contact our team to consult with one of our application engineers.