Choosing the right plastic material for injection molding isn’t just about what flows well into a mold – it’s about how that material behaves once it’s molded, how it holds up under stress and how it fits into your budget and production timeline. One of the most important decisions you’ll make is whether to use a thermoplastic or a thermoset polymer.
Both thermoplastics and thermosets have their place in injection molding, but each comes with trade-offs. Understanding how they differ – and what each material is best suited for – will help you make smarter, more scalable decisions.
What’s the Difference?
Let’s start with the basics. Thermoplastics and thermosets are both polymers, but they behave very differently under heat.
Thermoplastics soften when heated and harden when cooled – again and again. This means you can re-melt and reshape them, which offers recyclability and process flexibility. Think of materials like ABS, polycarbonate and nylon.
Thermosets, on the other hand, cure irreversibly. Once they’re molded and set through a chemical reaction, they become rigid and heat-resistant – but can’t be re-melted. Common thermosets include epoxy, phenolic and certain polyurethane resins.
Understanding this fundamental difference sets the stage for how each material performs in manufacturing and end-use environments.
Performance in Use
So how do thermoplastics and thermosets compare when your part is actually in the field?
Thermoplastics are generally tough, flexible and resistant to impact. They perform well in dynamic applications, especially those requiring elasticity or repeated stress. For example, a polycarbonate housing on a medical device can take knocks and drops without cracking.
Thermosets shine in environments where structural integrity, chemical resistance or heat stability is critical. Once cured, they’re hard, dimensionally stable and less likely to deform under high pressure or temperature. That’s why they’re commonly used in electrical insulators, automotive under-the-hood components or aerospace systems.
If your part needs to flex, bend or take abuse, a thermoplastic might be ideal. If it needs to maintain its shape in harsh environments, consider a thermoset.
Thermosets also offer excellent resistance to creep and deformation over long periods, making them a preferred option in applications that demand structural longevity. Their cross-linked structure provides lasting dimensional precision, which is crucial in mission-critical components where failure is not an option.
Cost Considerations
Cost isn’t just about the material – it’s also about how long it takes to mold and how much tooling is required.
Thermoplastics usually win when it comes to production speed. Their shorter cycle times allow for faster turnaround and higher throughput. They’re also more forgiving in terms of mold complexity, which can reduce upfront costs.
Thermosets typically require longer cure times and more precise temperature and pressure control. While this can add cost per unit, their superior heat and chemical resistance may eliminate the need for secondary operations or additional components – offsetting the initial investment.
Material cost also varies. Commodity thermoplastics like polypropylene or ABS are generally more affordable, while specialty thermoplastics (e.g., PEEK) and thermosets (e.g., epoxy compounds) can be more expensive.
The real question is: where does the value come from? If long-term performance or compliance is mission-critical, paying more up front for a thermoset might make sense.
Tooling and Processing Requirements
If you’ve worked with injection molding before, you know that not all materials behave the same in the mold.
Thermoplastics are easier to process. They require simpler molds and can be reheated and reshaped if needed. That flexibility also makes them a good choice during the prototyping and low-volume production phases.
Thermosets need specific curing conditions and tighter control over mold temperatures. Once cured, any design flaw or dimensional mistake can’t be undone. The tooling often needs to withstand higher temperatures and provide uniform heating, which can increase complexity.
In terms of waste, thermoplastics offer recyclability – you can regrind and reuse sprues and runners. Thermosets generate more waste due to their irreversible curing.
For lean manufacturing or sustainability goals, thermoplastics often align better. But when the mold integrity and precision matter most, thermosets bring unmatched dimensional control.
Common Use Cases for Thermoplastics
Thermoplastics are found everywhere, especially in consumer and medical applications.
You’ll see them in:
- Wearable health devices
- Packaging components
- Enclosures and housings for electronics
Because thermoplastics can be injection molded quickly and in complex shapes, they support rapid iteration. They’re also easy to color, texture or customize, which is ideal for customer-facing products.
In regulated industries, many thermoplastics meet ISO 10993 or USP Class VI requirements, making them suitable for non-implantable medical devices.
Where Thermosets Excel
Thermosets aren’t as commonly used – but where they show up, they matter.
They’re the go-to for:
- High-temperature environments
- Structural composites in aerospace
- Medical devices requiring dimensional stability
Thermosets also offer excellent electrical insulation properties, which makes them invaluable in switchgear, connectors and transformers.
One of their biggest strengths is creep resistance. If your part is under constant mechanical load or pressure, thermosets can maintain their shape and function over time better than thermoplastics.
They’re also widely used in applications where fire resistance is critical. Many thermosetting resins have self-extinguishing properties and can be engineered to meet stringent flame retardancy standards, making them ideal for use in public transportation, industrial panels or energy equipment.
Which Should You Choose?
The decision between thermoplastic and thermoset isn’t black and white. It comes down to priorities: flexibility vs. rigidity, speed vs. stability, cost vs. performance.
Ask yourself:
- Will this part face high heat or chemical exposure?
- Does the design require tight tolerances?
- Is recyclability or speed a key factor?
- Will the part need to flex or absorb impact?
Also consider where you are in the development cycle. Thermoplastics are ideal for rapid prototyping and small-batch production. If you’re heading into high-performance production and want long-term reliability, thermosets may be the better fit.
Still unsure? Work with a partner who understands both material families and can align your selection with your performance, compliance and manufacturing goals. The right choice doesn’t just improve your part – it improves your whole production outcome.