Key Differences Between EPDM and Thermoplastics in Injection Molding

In the world of injection molding, selecting the right material is critical to a product's performance, durability, and cost-effectiveness. Among the many material options available, EPDM rubber and thermoplastics are two widely used categories—but they serve very different purposes and behave quite differently during the injection molding process.

This comprehensive guide explores the key differences between EPDM and thermoplastics in injection molding, comparing their physical properties, molding behavior, typical applications, cost considerations, and more. Whether you're a product designer, engineer, or manufacturer, understanding these differences can help you make better decisions when specifying materials for molded parts.

1. What Is EPDM?

EPDM stands for Ethylene Propylene Diene Monomer, a type of synthetic rubber. It's known for excellent weather resistance, flexibility, and resilience. EPDM belongs to the family of elastomers, which are polymers with viscoelasticity—commonly referred to as “rubbery” materials.

Key Properties of EPDM:

• Excellent UV and ozone resistance

• High resistance to heat and cold

• Superior weatherability

• Outstanding flexibility and compressibility

• Good electrical insulation properties

• Inherently non-polar (resistant to water and steam)

• Typically requires vulcanization (curing) to set

2. What Are Thermoplastics?

Thermoplastics are a class of polymers that become soft and moldable when heated and harden when cooled—a process that is reversible and repeatable. They are among the most commonly used materials in plastic injection molding because of their versatility and recyclability.

Common Types of Thermoplastics:

• Polypropylene (PP)

• Polyethylene (PE)

• Acrylonitrile Butadiene Styrene (ABS)

• Polycarbonate (PC)

• Nylon (PA)

• Polystyrene (PS)

• Thermoplastic elastomers (TPE/TPU)

3. Injection Molding Process: EPDM vs. Thermoplastics

The injection molding process varies significantly depending on whether you are using EPDM or thermoplastics.

Molding EPDM

EPDM is a thermoset elastomer, which means it does not melt but undergoes a chemical curing reaction (vulcanization) when heated. Once cured, it cannot be re-melted.

Key Characteristics:

• Requires rubber injection molding machines

• Material is injected in an uncured (plastic-like) state

• Curing (cross-linking) occurs inside the mold under heat and pressure

• Typically longer cycle times due to vulcanization

• Mold temperatures often range from 150–200°C (302–392°F)

• Cannot be remolded or recycled once cured

Molding Thermoplastics

Thermoplastics, on the other hand, melt when heated and solidify when cooled. The process is physical, not chemical.

Key Characteristics:

• Uses standard plastic injection molding machines

• Material is melted and injected into the mold

• Cooling and solidification occur in the mold (no chemical change)

• Faster cycle times compared to EPDM

• Mold temperatures vary based on material (often 50–120°C)

• Thermoplastics can be reprocessed and recycled

4. Material Behavior Comparison

Property	EPDM Rubber	Thermoplastics
Elasticity	Very high	Varies (moderate in rigid plastics, high   in TPE)
Deformation	Recovers fully	Permanent in rigid types
Curing	Chemical (irreversible)	Physical (reversible)
Temperature Resistance	-50°C to 150°C	Varies by type (-40°C to 130°C average)
Recyclability	Not recyclable once cured	Recyclable and reprocessable
UV/Weather Resistance	Excellent	Moderate (depends on additives)
Chemical Resistance	Good (especially to water/steam)	Varies by resin type
Abrasion Resistance	Good	Varies
Hardness Range	Shore A 40–90	Shore D 20–80
Surface Finish	Matte or textured	Can be glossy, matte, or textured

5. Applications: EPDM vs. Thermoplastics

EPDM Applications

EPDM is widely used in applications that require weather resistance, sealing, or high flexibility over time.

Common Uses:

• Automotive weatherstripping and seals

• HVAC gaskets and seals

• Roof membranes

• Electrical insulation

• Industrial hose and tubing

• Washing machine door seals

Thermoplastics Applications

Thermoplastics are used in a much broader range of applications because they can be engineered to exhibit a wide range of properties.

Common Uses:

• Consumer electronics housings

• Automotive interior and structural parts

• Medical devices

• Packaging

• Household goods

• Toys

• Appliances

6. Design Considerations

EPDM Design Guidelines

Designing for EPDM injection molding requires attention to cure time, venting, and parting lines.

• Avoid sharp corners to minimize stress points.

• Include vents to allow gases to escape during curing.

• Allow for shrinkage and post-curing expansion.

Thermoplastic Design Guidelines

Designing for thermoplastics focuses on wall thickness, draft angles, and material flow.

• Maintain uniform wall thickness to prevent warping.

• Use proper draft angles for easier demolding.

• Include ribs and gussets for strength without added weight.

7. Cost Comparison

Factor	EPDM	Thermoplastics
Raw Material Cost	Generally lower	Can be higher depending on resin
Tooling Cost	Higher (special molds for curing)	Lower (widely available tooling)
Cycle Time	Longer	Shorter
Production Volume	Medium to high	Very high
Recycling Cost	Not recyclable	Recyclable, reducing waste

While the per-part cost of EPDM may be lower in raw material terms, the longer cycle time and complex tooling requirements often make thermoplastics more cost-efficient for high-volume production.

8. Environmental Impact

EPDM:

• Not recyclable after curing

• Less favorable from a sustainability standpoint

• However, EPDM has a long lifespan, reducing replacement waste

Thermoplastics:

• Thermoplastics are recyclable, which is a major advantage

• Post-consumer recycling infrastructure is well-established

• Biodegradable and bio-based alternatives are emerging (e.g., PLA)

9. Hybrid Materials: TPE as the Middle Ground

If you’re looking for rubber-like flexibility with thermoplastic processability, TPEs (Thermoplastic Elastomers) offer a viable compromise. These materials can be injection molded like thermoplastics while offering some of the softness and elasticity of EPDM.

• Advantages: Faster cycle time, recyclable, no curing needed

• Limitations: May not match EPDM's heat and weather resistance

10. Summary of Key Differences

Category	EPDM Rubber	Thermoplastics
Type	Thermoset Elastomer	Thermoplastic Polymer
Processing	Chemical cross-linking (vulcanization)	Melting and solidifying
Equipment	Rubber injection molding machine	Standard plastic injection molding   machine
Recyclability	Non-recyclable post-cure	Recyclable
Flexibility	High	Varies
Environmental Resistance	Excellent	Varies (can be improved with additives)
Speed	Slower cycle times	Fast cycle times
Design Freedom	Limited by curing requirements	High
Application	Seals, weatherstripping, insulation	Housings, mechanical parts, packaging

11. Choosing Between EPDM and Thermoplastics

The decision comes down to performance requirements, production volume, budget, and end-use environment.

Choose EPDM if:

• You need long-term weather, ozone, or UV resistance

• You need a flexible seal or gasket

• The part will experience compression and rebound

Choose Thermoplastics if:

• You need high-volume, low-cost production

• The part requires precision and a smooth finish

• Recycling or reusability is important

Conclusion

Both EPDM and thermoplastics have critical roles in injection molding, but they are suited to very different applications. EPDM offers durability, flexibility, and weather resistance that thermoplastics often can't match—especially in seals and outdoor environments. However, thermoplastics win in terms of speed, design flexibility, cost-efficiency, and sustainability.

Choosing the right material involves balancing functional requirements with manufacturing realities. By understanding the key differences outlined in this article, you can make more informed decisions that optimize product performance, cost, and longevity.