Introduction
Elastomers and flexible materials are everywhere. They seal your car doors. They cushion your shoes. They grip your toothbrush. They stretch, compress, and recover—cycle after cycle.
But molding these materials is not like molding rigid plastics. They flow differently. They stick to molds. They demand precise control over hardness, compression set, and elasticity. A 5-point variation in Shore hardness can render a seal ineffective. A part with high compression set fails to rebound, losing its sealing function.
This guide covers the essential aspects of injection molding elastomers and flexible materials: properties, process parameters, mold design, quality control, and applications.
What Defines Elastomers and Flexible Materials?
Elastomers and flexible materials are defined by their ability to deform under stress and return to their original shape—elasticity .
Key Properties
| Property | Typical Range | Significance |
|---|---|---|
| Elongation at break | 100% – 1000% | Stretches and recovers; gaskets stretch to fit and seal |
| Shore hardness | 30A (soft) – 70D (rigid) | Determines feel and function; medical gloves: 30–40A; door seals: 60–70A |
| Compression set | <20% (after 22h at 70°C) | Measures permanent deformation after compression; critical for seals |
| Tear resistance | Varies by material | Withstands repeated stretching without splitting |
| Thermal stability | -50°C to 200°C | Silicone excels in high-heat applications (oven gaskets) |
Material Types
Thermoplastic Elastomers (TPEs): Can be melted and reshaped. Efficient for injection molding. Recyclable.
Thermoset elastomers: Undergo vulcanization, becoming permanently cross-linked. Better thermal stability but cannot be recycled. Examples: rubber, silicone (in thermoset form).
Specialized elastomers:
| Material | Key Properties | Applications |
|---|---|---|
| Silicone | High thermal stability (-50°C to 200°C), biocompatible | Medical devices, oven gaskets |
| Nitrile rubber | Oil resistance | Fuel seals, industrial components |
| EPDM | UV resistance, weatherability | Outdoor seals, automotive weatherstripping |
| Polyurethane | Abrasion resistance | Conveyor belts, wheels, industrial parts |
How Do You Injection Mold Elastomers?
Key Process Parameters
Melt temperature:
| Material | Range |
|---|---|
| TPEs | 160 – 200°C |
| Silicone | 170 – 200°C |
Exceeding these temperatures causes degradation—reduced elasticity, increased brittleness.
Injection pressure: Lower than rigid plastics—30 to 80 MPa . Elastomers have low viscosity; excessive pressure causes flash.
Injection speed: Moderate—20 to 50 mm/s . Too fast: material folds over itself, creating weak spots. Too slow: premature cooling, short shots.
Cooling time: 10 to 20 seconds for most elastomers. Thicker parts need longer to prevent warpage.
Cycle time: 20 to 40 seconds. Balance fill and pack phases—avoid overpacking (flash) or underpacking (voids).
Material Handling
Most elastomers have minimal drying requirements. Hygroscopic TPEs should be dried at 60°C to 80°C for 2 to 4 hours to prevent surface defects.
The table below summarizes process parameters:
| Parameter | Range | Notes |
|---|---|---|
| Melt temperature (TPE) | 160 – 200°C | Higher degrades material |
| Injection pressure | 30 – 80 MPa | Lower than rigid plastics |
| Injection speed | 20 – 50 mm/s | Moderate to control flow |
| Cooling time | 10 – 20 seconds | Longer for thick parts |
| Cycle time | 20 – 40 seconds | Optimize fill/pack balance |
How Should Molds Be Designed for Elastomers?
Venting
Elastomers have low viscosity during processing. Stringent venting prevents air traps, which cause voids and weak spots.
Vent depth: 0.02 to 0.05 mm at flow ends.
Cooling Channels
Uniform cooling prevents inconsistent Shore hardness. Place channels 10 to 15 mm from the cavity. Water temperature: 40°C to 60°C for most materials.
Draft Angles
Larger than rigid plastics—2° to 5° . Elastomers stretch and stick to mold surfaces. Insufficient draft causes tearing during ejection.
Ejector Pin Design
Use multiple small pins or stripper rings to distribute force. Critical for thin-walled parts like diaphragms—prevents tearing.
Parting Lines
Must be tight—gap <0.02 mm —to prevent flash, a common issue with low-viscosity elastomers.
Hot Runner Systems
Suitable for TPEs. Reduce waste, improve consistency. Require precise temperature control.
Mold Materials
| Material | Application |
|---|---|
| P20 steel | General use |
| Stainless steel | Corrosive environments (silicone molding) |
Mold surfaces should be smooth—Ra 0.8 to 1.6 μm —to prevent sticking.
What Defects Occur and How to Prevent Them?
| Defect | Cause | Solution |
|---|---|---|
| Flash | Excessive pressure or mold gap | Reduce pressure; tighten mold clamping |
| Voids | Trapped air or underpacking | Add vents; increase hold pressure |
| Warpage | Uneven cooling | Balance cooling channels; adjust cooling time |
| Poor elasticity | Material degradation | Lower melt temperature; check residence time |
| Inconsistent hardness | Uneven cooling or material mix | Improve temperature control; verify material batches |
Quality Control Methods
Mechanical property testing:
- Tensile tests: Measure elongation at break, tear resistance
- Compression set tests: Ensure long-term performance
Shore hardness: Check with durometers. Allowable variation: ±3 points for critical parts.
Statistical Process Control (SPC): Monitor melt temperature (±2°C), pressure (±5 MPa). Tight tolerances maintain consistency.
Inspection techniques:
- Visual checks for flash, surface defects
- Dimensional measurements with calipers (tolerances often ±0.1 mm)
Where Are Elastomers Used?
Automotive Components
| Component | Material | Properties Used |
|---|---|---|
| Door seals | EPDM, Shore A 60 | Elasticity, chemical resistance, weatherability |
| Grommets | TPE, Shore A 50 | Flexibility, tear resistance |
| Shock absorbers | Polyurethane | Abrasion resistance, energy absorption |
Medical Devices
| Component | Material | Properties Used |
|---|---|---|
| Syringe gaskets | Silicone, Shore A 40 | Biocompatibility, low compression set |
| Tourniquets | TPE, 300% elongation | Elasticity, tear resistance |
Consumer Products
| Component | Material | Properties Used |
|---|---|---|
| Toothbrush grips | TPE, Shore A 30 | Soft feel, comfortable grip |
| Phone cases | TPU, Shore A 80 | Flexibility, shock absorption |
Industrial Seals
| Component | Material | Properties Used |
|---|---|---|
| O-rings | Nitrile, Shore A 70 | Oil resistance, low compression set |
| Valve diaphragms | EPDM | Elasticity, chemical resistance |
Wearable Devices
Smartwatch bands use TPE (Shore A 50)—flexibility with UV resistance for daily use.
What Post-Processing Options Exist?
Trimming Flash
Use sharp blades or cryogenic methods—freezing hardens material for clean edges without damaging elasticity.
Surface Treatments
Plasma etching improves adhesion for painting and coating. Many elastomers are colored during molding to avoid post-processing.
Adhesive Bonding
Cyanoacrylates or hot-melt adhesives work. Surface preparation is critical for strong bonds.
Ultrasonic Welding
Effective for TPEs. Creates strong joints without heat damage—ideal for assembling gaskets into rigid plastic housings.
Vulcanization
Required for thermoset elastomers like rubber. Heating at 150°C to 200°C cross-links molecules to achieve final elasticity.
Design for Manufacturing Tips
- Use generous radii to avoid stress concentration
- Avoid thick sections (cause sink marks)
- For seals, design with 10% to 20% interference fit to ensure proper compression
What Does a Real-World Example Look Like?
A manufacturer of automotive door seals needed EPDM parts with consistent Shore A 60 hardness and compression set below 20%. Previous supplier delivered parts with hardness varying ±5 points and compression set above 25%.
The solution involved:
- Mold design: Tight parting lines (<0.02 mm) to prevent flash; uniform cooling channels 12 mm from cavity; draft angles 3°
- Process control: Melt temperature 180°C, injection pressure 50 MPa, cooling time 15 seconds
- Material handling: EPDM stored in sealed containers; pre-heated before molding
The result: Shore hardness consistent at 60 ±2 points. Compression set reduced to 18%. Flash eliminated. Parts passed 100,000-cycle durability tests.
Conclusion
Injection molding elastomers and flexible materials requires understanding their unique properties: elasticity (elongation up to 1000%), Shore hardness (30A–70D), and compression set (<20% for quality seals).
Process parameters differ from rigid plastics:
- Lower injection pressure (30–80 MPa)
- Moderate injection speed (20–50 mm/s)
- Melt temperature 160–200°C (TPE) or 170–200°C (silicone)
Mold design must account for low viscosity: tight parting lines (<0.02 mm), proper venting (0.02–0.05 mm), larger draft angles (2–5°), and multiple ejector pins.
Quality control includes Shore hardness verification (±3 points), compression set testing, and SPC monitoring of melt temperature (±2°C) and pressure (±5 MPa).
Applications span automotive seals, medical devices, consumer products, industrial components, and wearables.
When materials, process, and mold align, elastomer injection molding delivers reliable, high-performance flexible parts.
FAQ
What's the difference between TPEs and thermoset elastomers?
TPEs (thermoplastic elastomers) can be melted and reshaped. They are efficient for injection molding and recyclable. Thermoset elastomers (rubber, thermoset silicone) undergo vulcanization, becoming permanently cross-linked. They offer better thermal stability but cannot be recycled.
How can I improve the compression set of elastomer parts?
Choose materials with low compression set ratings—silicone <20%. Optimize mold cooling to ensure uniform curing. Avoid overpacking during molding—excessive pressure introduces internal stress that reduces rebound. Ensure complete curing; under-cured parts have higher compression set.
What causes elastomer parts to harden over time?
Hardening is often due to oxidation or UV degradation. Use antioxidants in material formulation. Add UV stabilizers for outdoor applications. For high-temperature applications, ensure the material’s thermal stability matches the operating environment—silicone maintains properties up to 200°C.
Why do elastomers require larger draft angles than rigid plastics?
Elastomers stretch and stick to mold surfaces during ejection. Insufficient draft causes tearing. Larger draft angles—2° to 5° —allow the part to release without deformation. For rigid plastics, 1° may suffice.
What is the typical tolerance for Shore hardness in molded elastomer parts?
For critical applications, Shore hardness should be controlled within ±3 points . A 5-point variation can render a seal ineffective—too soft leaks; too hard fails to conform. Tight process control—uniform cooling, consistent material batches—maintains hardness within specification.
Contact Yigu Technology for Custom Manufacturing
At Yigu Technology , we specialize in injection molding elastomers and flexible materials. Our molds feature tight parting lines (<0.02 mm), proper venting, and uniform cooling channels. We control melt temperature, injection pressure, and cooling time to deliver consistent Shore hardness and low compression set.
We work with TPEs, silicone, EPDM, polyurethane, and nitrile rubber. Applications include automotive seals, medical devices, consumer products, and industrial components.
Contact Yigu Technology today to discuss your elastomer injection molding project.
