Liquid Silicone Rubber (LSR) injection molding is a highly specialized process requiring precision molds that can withstand high temperatures, provide exceptional surface finishes, and maintain dimensional stability over long production runs. One of the most critical factors in mold performance is the steel material used in its construction.
The right steel material affects mold longevity, product quality, maintenance costs, and production efficiency. Choosing poorly can lead to premature wear, frequent repairs, and inconsistent product quality.
In this guide, we’ll explore the better steel materials for LSR injection molds, their pros and cons, and how to select the most suitable one for your manufacturing needs.
1. Why Steel Material Matters in LSR Injection Molds
Unlike thermoplastics, LSR is injected at lower pressure but cured at high temperatures (typically 160°C–200°C). This means the mold steel must resist:
- Thermal fatigue from repeated heating and cooling cycles.
- Corrosion from silicone additives and curing agents.
- Surface wear from high-precision demolding.
- Dimensional changes under prolonged exposure to heat.
In short, the mold steel must deliver:
- High hardness for precision.
- Good polishability for smooth product surfaces.
- Corrosion resistance for longevity.
- Thermal stability for consistent part dimensions.
2. Common Steel Materials for LSR Injection Molds
Below are the most commonly used steel materials for LSR mold making, along with their characteristics:
2.1 P20 Pre-Hardened Steel
Properties:
- Hardness: ~28–32 HRC
- Good machinability
- Moderate corrosion resistance
Advantages:
- Lower cost
- Shorter lead time
- Suitable for low to medium production volumes
Limitations:
- Less wear resistance than hardened steels
- Not ideal for highly corrosive silicone compounds
Best Use: Prototypes and low-volume production molds.
2.2 H13 Hot-Work Tool Steel
Properties:
- Hardness: 48–52 HRC (after heat treatment)
- Excellent thermal fatigue resistance
- Good toughness
Advantages:
- Withstands high curing temperatures
- Resists cracking under thermal cycling
- Can be nitrided for higher surface hardness
Limitations:
- Moderate corrosion resistance (requires protective coatings for some LSR formulations)
Best Use: High-volume production of general LSR products.
2.3 Stainless Steel (420, 440C)
Properties:
- High corrosion resistance
- Hardness: up to 50–55 HRC (420), ~58 HRC (440C)
- Good polishability
Advantages:
- Prevents rust and corrosion in humid environments
- Excellent for high-polish optical parts
- Suitable for medical-grade silicone molding
Limitations:
- Higher cost
- Requires careful machining due to hardness
Best Use: Medical devices, optical silicone components, food-grade products.
2.4 S136 Stainless Tool Steel
Properties:
- Equivalent to AISI 420 stainless, but with enhanced purity
- Excellent corrosion resistance
- Exceptional surface finish capability
Advantages:
- Ideal for molds exposed to corrosive silicone chemicals
- Provides mirror-like polish for transparent silicone parts
Limitations:
- More expensive than standard tool steels
Best Use: Optical-grade silicone products, high-precision medical parts.
2.5 Maraging Steel (e.g., 1.2709)
Properties:
- Very high strength and toughness
- Good machinability before aging
- Used in additive manufacturing of molds
Advantages:
- Excellent for complex mold inserts
- Maintains precision over long production runs
Limitations:
- Expensive and requires specialized heat treatment
Best Use: Complex geometries, high-precision molds with long life cycles.
3. Factors to Consider When Choosing Steel Material
When deciding on the best steel material for an LSR mold, consider the following:
1. Production Volume
- Low-volume runs: P20 steel may be cost-effective.
- High-volume production: H13 or stainless steel ensures longer tool life.
2. Part Requirements
- Optical clarity: S136 or 420 stainless for high-polish finishes.
- Biocompatibility: Stainless steels to meet medical standards.
3. Corrosion Exposure
- Some silicone grades release byproducts that corrode steel; stainless is better for such cases.
4. Mold Complexity
- Complex shapes with fine details may require maraging steel or premium stainless for precision.
5. Budget Constraints
- While premium steels have higher upfront costs, they reduce maintenance and downtime.
4. Surface Treatments to Improve Mold Steel Performance
Even the best steel material can benefit from surface treatments that extend mold life:
- Nitriding – Improves surface hardness and wear resistance.
- Hard chrome plating – Adds corrosion and wear protection.
- PVD coatings (TiN, CrN) – Reduces sticking and improves mold release.
- Polishing – Critical for optical or medical-grade silicone parts.
5. Maintenance Practices for LSR Mold Steel
Regular maintenance ensures the chosen steel material performs for years:
- Clean after each production run to remove silicone residue.
- Inspect for wear and corrosion periodically.
- Re-polish or re-coat surfaces when performance declines.
- Proper storage to avoid humidity-related corrosion.
6. Recommended Steel Material Choices by Application
| Application | Recommended Steel Material | Reason |
|---|---|---|
| Medical-grade LSR parts | S136 / 420 Stainless | High polish, corrosion resistance |
| Optical silicone components | S136, 440C | Mirror finish capability |
| General consumer products | H13 | Good thermal fatigue resistance |
| Low-volume prototypes | P20 | Cost-effective |
| High-volume industrial seals & gaskets | H13 or S136 | Long tool life, dimensional stability |
Conclusion
Selecting the right steel material for LSR injection molds is not just about hardness — it’s about balancing thermal stability, corrosion resistance, surface finish, and production economics.
- P20 is great for low-volume, cost-sensitive projects.
- H13 is the workhorse for high-volume production.
- S136 or stainless steels are the top choice for medical, optical, and food-contact parts.
- Maraging steel suits highly complex, long-life mold applications.
By understanding your production requirements and matching them with the right steel material, you can maximize mold life, minimize downtime, and achieve consistent product quality in LSR injection molding.
