Introduction
Rubber prototypes are essential when your product needs to stretch, seal, grip, or absorb shock. Unlike plastic or metal parts, rubber components behave differently—they compress and recover, they flex without breaking, and they provide friction against smooth surfaces. Making a rubber prototype requires specialized processes because you cannot just print or machine it like a rigid material. You need molds, careful material selection, and precise control over curing. This article walks you through the entire process, from defining requirements to final testing, so you can create rubber prototypes that accurately represent your production parts.
Pre-Production: Define Requirements and Plan Design
Before you make anything, you need to know exactly what the prototype must do. Rubber parts serve specific functions, and your testing goals determine material and process choices.
Clarify Core Requirements
Rubber prototypes validate three main types of performance. Be clear about which matter for your part.
| Requirement Category | What to Test | Real-World Example |
|---|---|---|
| Functional validation | Elasticity, sealing, slip resistance | A silicone waterproof ring for a phone must seal the charging port during 30 minutes underwater |
| Material property testing | Temperature resistance, wear resistance, hardness | A TPE grip for outdoor gear must stay flexible after 1,000 hours of UV exposure |
| Structural compatibility | Assembly with other parts, thickness optimization | A silicone tube for medical use must not kink when bent at 90 degrees |
Hardness matters: Rubber hardness is measured on the Shore scale. Typical ranges are 30 to 70 Shore A. Softer rubbers seal better but wear faster. Harder rubbers last longer but require more force to compress.
Design 3D Models with Rubber-Specific Rules
Rubber shrinks as it cures. Your CAD model must account for this or your parts will come out undersized.
Shrinkage rate:
- Silicone: approximately 3% shrinkage
- Polyurethane: approximately 2% shrinkage
- Natural rubber: variable, typically 2% to 5%
If you need a final part that is 100mm long, model it at 103mm for silicone. Measure a test sample first to confirm actual shrinkage with your specific material and process.
Thickness and chamfers:
- Keep wall thickness between 0.5mm and 5mm. Thinner parts tear easily. Thicker parts take longer to cure and may develop internal voids.
- Add 0.5mm to 1mm chamfers on sharp edges. A silicone shock pad with 0.8mm chamfers survived 50 compression cycles. A sharp-edged version cracked after 50 presses.
Anti-slip patterns and holes:
- Mark texture areas clearly in your CAD. Grid patterns 0.5mm deep improve grip significantly.
- For assembly holes, model them slightly undersized. Rubber stretches, so a 2mm hole will accept a 2.5mm pin if needed.
Split Complex Parts
If your part has undercuts, thin walls, or sharp curves, consider splitting it into sections. Each section gets its own simpler mold, which reduces demolding problems and trapped air.
Example: A curved silicone earbud tip is split into the ear tip and the body. Two small molds produce better results than one large mold with complex geometry.
Mold Making: Choose the Right Method
Rubber prototypes need molds. You have two main options, depending on precision needs and quantity.
| Mold Type | Process | Advantages | Limitations | Best For |
|---|---|---|---|---|
| CNC-machined mold | Machine steel or aluminum, polish to Ra 0.8-1.6 | High precision (±0.05mm), reusable for 500+ cycles | High cost (4,000-15,000 CNY), 3-5 day lead time | 10-100 high-precision parts like medical seals |
| 3D-printed mold | Print resin or nylon, sand smooth | Fast (1-2 days), low cost (800-2,500 CNY), handles complex shapes | Short life (20-50 cycles), limited to soft rubbers | 1-10 units, complex shapes, quick iterations |
Practical tip: Start with a 3D-printed mold for your first one to five prototypes. Test the design. When you are confident, invest in a CNC mold for larger batches.
Molding Process: Select Based on Rubber Type and Part Size
Three common processes cover most rubber prototype needs. Match the process to your material and application.
Liquid Silicone Injection Molding (LSR)
This process is for high-precision parts made from liquid silicone.
How it works:
- Mix liquid silicone Part A and Part B in equal parts.
- Inject the mixture into a heated mold at 150°C to 200°C.
- Cure for 5 to 15 minutes depending on thickness.
- Demold and trim excess material.
Advantages: High precision for small parts, excellent elasticity, no post-curing needed.
Best for: Transparent medical parts like infusion tube fittings. High-precision seals for electronics.
Solid Rubber Pressing
This process uses pre-cut rubber sheets and pressure to form parts.
How it works:
- Cut rubber sheets to approximate shape.
- Heat sheets to 120°C to 180°C to soften.
- Press into the mold with 10 to 20 MPa pressure.
- Cool for 10 to 20 minutes, then demold.
Advantages: Low cost, fast for large parts, works with aging-resistant materials like EPDM.
Best for: Sealing strips and door gaskets. Large shock-absorbing pads.
Polyurethane Casting
This process uses liquid polyurethane that cures at room temperature or with mild heat.
How it works:
- Mix resin and hardener in equal parts.
- Degas under vacuum to remove air bubbles.
- Pour slowly into the mold to avoid trapping air.
- Cure at room temperature for 24 hours or 80°C for 2 hours.
Advantages: Mimics rubber flexibility well. Perfect for overmolding onto metal or plastic cores.
Best for: Overmolded grips where rubber bonds to a rigid handle. Prototypes with metal inserts.
Process Selection Guide
| Part Type | Recommended Process | Typical Material | Lead Time |
|---|---|---|---|
| Micro-seals under 5mm | LSR | Liquid silicone | 1-2 days |
| Large parts over 200mm | Solid rubber pressing | EPDM, natural rubber | 2-3 days |
| Overmolded grips | Polyurethane casting | TPU, TPE | 1-3 days |
Post-Processing: Refine and Enhance Quality
Rubber parts almost always need some finishing after demolding.
Basic Trimming and Deburring
Use a sharp blade to remove flash—the thin layer of rubber that seeps between mold halves. For small parts like silicone earbuds, use tweezers to peel fine burrs. Avoid sanding soft rubber; it damages the surface.
Secondary Vulcanization for Silicone
Bake silicone prototypes at 150°C to 200°C for two to four hours. This improves temperature resistance by about 30% and extends lifespan by 50%.
Real example: A silicone automotive seal after secondary vulcanization withstood 150°C for 1,000 hours without hardening. An untreated version failed at 500 hours.
Surface Treatment
Add finishes that improve function or appearance.
- Anti-slip coating: Spray epoxy-based paint on grips to increase friction. Reduces slip by 60% on wet surfaces.
- Silk screen printing: Add logos, hardness ratings, or warning text.
- Material bonding: Use rubber-specific adhesives to attach rubber to metal or plastic.
Testing and Optimization: Validate Performance and Fix Issues
Test your prototype against the requirements you defined at the start.
Key Test Items
| Test Type | How to Perform | Pass Criteria |
|---|---|---|
| Elasticity | Compress to 50% thickness, release, measure recovery | Recovers to original shape in under 1 second |
| Sealing | Submerge in water or apply air pressure | No leaks after 30 minutes |
| Dimensional accuracy | Measure key features with calipers | Error within ±0.1mm |
Solve Common Defects
| Defect | Causes | Fixes |
|---|---|---|
| Demolding difficulty | Rough mold surface, no draft angle | Polish mold to Ra 0.8 or smoother. Add 1 to 3 degree draft. Use release agent. |
| Bubbles | Air trapped during pouring, no degassing | Degas material under vacuum. Pour slowly. Add air vents to mold. |
| Dimensional deformation | Uneven mold temperature, wrong shrinkage assumption | Heat mold evenly. Adjust CAD model based on measured shrinkage. |
Conclusion
Making rubber prototypes requires a different approach than plastic or metal. You must account for shrinkage in your CAD model. You need molds, which means choosing between fast 3D-printed molds for early iterations and precision CNC molds for larger batches. The molding process—LSR, solid pressing, or polyurethane casting—depends on your material and part geometry. Post-processing refines the part, and testing confirms it meets requirements. Rubber prototypes reward attention to detail. Small adjustments like adding a 2-degree draft angle or adjusting shrinkage compensation can turn a frustrating process into a reliable one.
Frequently Asked Questions
Can I use 3D printing directly to make rubber prototypes without molds?
Rarely. Most 3D-printed “rubber-like” materials lack the elasticity and sealing performance of true molded rubber. TPU filaments can work for simple concept models where exact rubber properties do not matter. But for functional validation of seals, grips, or shock absorbers, you need molded prototypes.
How do I choose between silicone and TPE for my prototype?
Choose silicone for high temperature resistance up to 200°C, transparency, and medical-grade non-toxicity. Silicone works for baby bottle nipples and phone waterproof rings. Choose TPE for better wear resistance and lower cost. TPE works for grips and toy parts that do not see extreme temperatures.
Why does my rubber prototype have a sticky surface?
Sticky surfaces usually mean incomplete curing. The mold temperature may have been too low, or curing time too short. For LSR parts, cure at the recommended temperature for the full time. For silicone, secondary vulcanization often eliminates stickiness by removing residual compounds.
How do I account for rubber shrinkage in my CAD model?
Start with the published shrinkage rate for your material—typically 2% to 5%. Model your part larger by that percentage. Then make one test part, measure the actual shrinkage, and adjust your model for production runs. Shrinkage varies with part geometry, so always validate with your specific design.
What hardness should I choose for a rubber prototype?
Match the hardness you expect in production. For seals and gaskets, 40 to 60 Shore A provides good compression. For grips, 50 to 70 Shore A balances comfort and durability. For shock absorption, softer materials around 30 Shore A work best. Your supplier can provide test samples in different hardnesses if you are unsure.
Can I overmold rubber onto a plastic or metal insert in a prototype?
Yes, polyurethane casting works well for overmolded prototypes. Place the insert in the mold before pouring. The rubber bonds to the insert during curing. For production, this process scales to injection molding, so prototype results transfer directly.
Discuss Your Projects with Yigu Rapid Prototyping
At Yigu Technology, we help product teams navigate the complexities of rubber prototyping. Our capabilities include 3D-printed molds for quick iterations, CNC-machined molds for precision parts, and all three molding processes—LSR, solid pressing, and polyurethane casting. We guide you through material selection, shrinkage compensation, and post-processing to ensure your prototypes perform as intended. Located in Shenzhen’s manufacturing hub, we combine technical expertise with competitive pricing and fast turnaround. If you need rubber prototypes for seals, grips, or custom components, reach out to us. Let’s discuss how we can turn your design into a functional rubber part ready for testing.