Common Mistakes to Avoid When Using Liquid Silicone Rubber
Excellent question! Working with Liquid Silicone Rubber (LSR) is a precise science, and avoiding common pitfalls is key to achieving high-quality, consistent parts. Here's a comprehensive guide to the most frequent mistakes and how to avoid them.
1. Inadequate Material Preparation (Mixing & Degassing)
Mistake: Not mixing Part A and Part B in the exact 1:1 ratio (for most LSRs), mixing unevenly, or skipping degassing. This leads to uncured spots, soft areas, or severe air bubbles in finished parts.
Solution: Use precision metering and mixing equipment. For manual mixing, scrape sides and bottom thoroughly. Always degass the mixed material under vacuum before molding, especially for critical applications.
2. Ignoring Moisture Contamination
Mistake: LSR is extremely sensitive to moisture before curing. Exposure to humid air or contaminated tools causes "popping" or bubbling (from water vapor) or inhibition of the platinum catalyst, leading to tacky, uncured surfaces.
Solution: Store components in sealed containers. Keep lids on Part A/B pots. Use a dry air or nitrogen purge in the material holding tanks and on the mold surface. Ensure pigments/additives are anhydrous.
3. Incorrect Mold Temperature
Mistake: Curing LSR at the wrong mold temperature. Too cold = incomplete cure and long cycle times. Too hot = premature "scorching" or curing in the runners, leading to short shots and flow marks.
Solution: Know your LSR material's specific curing profile. Maintain a consistent, evenly distributed mold temperature (typically 150-200°C / 300-390°F). Use reliable mold heaters and controllers.
4. Poor Mold Design & Venting
Mistake: Inadequate venting traps air, causing air traps, short shots, and burn marks. Sharp corners create stress points. Incorrect draft angles make demolding difficult, leading to part tearing.
Solution: Design molds with proper vent channels (typically 0.005-0.015 mm deep) at the last points to fill. Use generous radii on corners. Include sufficient draft angles (≥1° is common). Consider vacuum venting for complex parts.
5. Injection Speed/Pressure Issues
Mistake: Injecting too fast creates turbulent flow, trapping air. Injecting too slow can cause premature curing in the flow front ("scorch").
Solution: Use a multi-stage injection profile: Fast fill to just before the cavity, then switch to a slower, controlled fill/pack stage to allow air to escape through vents. Optimize pack pressure to minimize flash.
6. Improper Post-Curing
Mistake: Skipping post-cure when material properties require it, or doing it at the wrong temperature/time. This leaves parts with inferior mechanical properties, higher compression set, or residual odors.
Solution: Check the material data sheet. Post-cure (e.g., 2-4 hours at 150-200°C) is often essential for achieving final tensile strength, thermal stability, and reducing volatiles. It's critical for medical and food-grade parts.
7. Contamination from Other Silicones
Mistake: Using tools, releases, or seals contaminated with peroxide-cure silicones (like RTV). Even tiny amounts of tin or amine inhibitors can poison the platinum catalyst, causing areas that won't cure.
Solution: Dedicate tools and equipment for LSR. Use only platinum-cure compatible release agents (usually water-based). Isolate LSR production areas from RTV silicone work.
8. Using the Wrong Release Agent (or Too Much)
Mistake: Spraying a silicone- or oil-based release agent into the mold. This can contaminate the part surface, preventing secondary bonding or painting, and build up in the mold.
Solution: If needed, use an extremely thin, platinum-compatible, water-based release. Apply sparingly and infrequently. The best practice is to design a mold that runs without any release agent (self-releasing surfaces like nickel-plated cavities).
9. Neglecting Gate and Runner Design
Mistake: Gates that are too small cause excessive shear heating (scorching) and high injection pressure. Runners that are too large waste material and increase cycle time.
Solution: Use full-round runners to minimize pressure drop. Size gates appropriately-typically 0.2-0.5 mm thick and as wide as practical. Pin or tunnel gates are common. Simulate fill patterns if possible.
10. Insufficient Process Control & Documentation
Mistake: Assuming LSR processing is "set and forget." Variations in ambient conditions, material batch, or machine settings can affect quality.
Solution: Monitor and document key parameters: material temperature, mold temperature, injection speeds/pressures, and cure time. Implement a robust Quality Control (QC) check for dimensions, durometer, and visual defects.
Summary Checklist for Success:
✅ Mix & Degas meticulously.
✅ Control Moisture with dry air/nitrogen.
✅ Maintain precise, stable mold temps.
✅ Design molds for venting and easy demolding.
✅ Optimize injection speed/profile.
✅ Post-cure when required by the spec.
✅ Prevent cross-contamination (especially from RTV).
✅ Minimize or eliminate release agents.
✅ Design efficient gating/runner systems.
✅ Document and control the process.
By understanding and avoiding these common mistakes, you can harness LSR's excellent properties-like high temperature resistance, biocompatibility, and durability-to produce flawless, high-performance parts.