Guide to Selecting TPE Materials for Injection Molding

Imagine needing a new product that combines the flexibility of rubber with the moldability of plastic, while avoiding the complex processing and recycling challenges of traditional thermoset rubber. Thermoplastic elastomers (TPEs) have emerged as the solution, offering both rubber-like elasticity and plastic-like injection molding capabilities, along with recyclability. This article provides a comprehensive analysis of TPE materials, comparing them with thermoset rubbers and detailing six major TPE types—from chemical properties to performance parameters and typical applications—to help engineers and designers make informed decisions for injection molding projects.

The most significant distinction between thermoplastic elastomers (TPE) and thermoset elastomers (like liquid silicone rubber, LSR) lies in their molecular bonding. TPEs soften when heated and harden when cooled—a reversible process similar to thermoplastics. In contrast, thermoset rubbers undergo chemical cross-linking during molding, forming permanent three-dimensional networks that cannot be remelted once cured. This difference directly impacts processing methods, performance characteristics, and recyclability.

Key differences include:

• Processing: TPEs can be processed using standard injection molding machines. The resin is heated until molten, injected into molds, and cooled to set. Thermoset rubbers require lower-temperature molding followed by high-temperature chemical cross-linking for curing.
• Recyclability: Since TPEs don't cross-link, they can be remelted and recycled, aligning with sustainability goals. Thermoset rubbers cannot be recycled and typically end up incinerated or landfilled.
• Process Optimization: TPEs can be processed using the same equipment as standard thermoplastics, enabling process optimizations like overmolding or multi-color injection molding to streamline production and reduce costs.

TPEs aren't a single material but comprise six primary categories with distinct chemical compositions and structures that lead to different properties and applications. Selecting the right TPE requires considering part lifespan, cost, operating environment, and specific performance needs.

The table below summarizes key characteristics of the six major TPE types, each explored in detail in subsequent sections:

TPVs consist of hard thermoplastic materials with dispersed cross-linked rubber regions, yielding a soft touch, matte surface, and excellent compression set resistance. Transparent grades are typically unavailable.

Key Properties:

• Good chemical resistance
• Strong dimensional stability
• Higher density
• Medium tensile strength
• Shore hardness: 40A - 50D
• Maximum continuous use temperature: 135°C

Applications: Primarily used for sealing, damping, and protection in:

• Seals (fluid/gas containment)
• Grommets and protective sleeves (wire/cable protection)
• Automotive bumpers (impact absorption)
• Underhood components (heat/chemical resistance)

TPUs are block copolymers with alternating hard/soft segments containing urethane linkages. Notable for higher hardness, good transparency, moderate compression set, and outstanding abrasion/tear resistance. Suitable for outdoor use but requires pre-drying before molding. Currently the only 3D-printable TPE.

Key Properties:

• Exceptional chemical resistance (especially polyester-based)
• Good dimensional stability (enhanceable with additives)
• Higher density
• High tensile strength
• Shore hardness: 65A - 80D
• Maximum continuous use temperature: 120°C

Applications: Ideal for high-strength, wear-resistant products:

• Protective cases (electronics)
• Sports equipment (shoes, protective gear)
• Medical devices (catheters, films)
• Footwear components
• Skate wheels (high wear/impact resistance)

TPOs blend hard polyolefins (typically polypropylene) with soft non-crosslinked rubber regions. Their high hardness provides strong impact resistance, with some weather-resistant grades available. Compared to TPU, TPOs have lower compression set and reduced costs.

Key Properties:

• Good chemical resistance
• Strong dimensional stability
• Lower density
• High tensile strength
• Shore hardness: 75A - 80D
• Maximum continuous use temperature: 120°C

Applications: Predominantly automotive:

• Interior components (dashboards, door panels)
• Exterior bumpers (impact/weather resistance)
• Roof liners (lightweight durability)

SBCs combine rigid styrene blocks with soft elastomer regions, often blended with harder polymers like polypropylene. The softest, most elastic TPE category features glossy surfaces, high elongation, good transparency, and decent abrasion resistance. Performance varies significantly by formulation.

Key Properties:

• Limited chemical resistance
• Good dimensional stability
• Lower density
• Low-medium tensile strength
• Shore hardness: 15A - 50D
• Maximum continuous use temperature: 110°C

Applications: Preferred for soft-touch applications:

• Ergonomic handles (tools, appliances)
• Control buttons/knobs (electronics, appliances)
• Grips (bicycles, motorcycles)
• Gel pads (insoles, mouse pads)

COPEs combine crystalline polyester hard segments with amorphous soft segments, excelling in heat resistance, tear strength, and impact resistance. They also demonstrate good creep resistance and low moisture absorption.

Key Properties:

• Good chemical resistance
• Strong dimensional stability
• Higher density
• High tensile strength
• Shore hardness: 90A - 80D
• Maximum continuous use temperature: 140°C

Applications: Demanding environments requiring:

• Furniture components (durability/aesthetics)
• Automotive dust boots (contaminant protection)
• Bumpers (impact absorption)
• Prosthetics (biocompatibility/durability)

PEBAs alternate polyamide hard blocks with soft elastomer blocks, offering exceptional flex fatigue resistance, creep resistance, and impact strength. They perform well at elevated temperatures with low compression set.

Key Properties:

• Good chemical resistance
• Strong dimensional stability
• Lower density
• High tensile strength
• Shore hardness: 80A - 75D
• Maximum continuous use temperature: 170°C

Applications: High-performance uses requiring:

• Medical devices (catheters, inflatable components)
• Sports equipment (ski boots, running shoes)
• Electronics (cable jackets, connectors)

Selecting the appropriate TPE requires comprehensive evaluation of each type's properties, applications, and cost structures. By thoroughly understanding the advantages and limitations of different TPEs, engineers and designers can optimize product performance, reduce production expenses, and ultimately achieve commercial success. For practical applications, detailed technical consultations with material suppliers are recommended to ensure accurate, reliable material selection.