A Beginner's Guide to Mixing Tin-Cure Silicone (Condensation-Cure Silicone) with a 100:2-3 Ratio

1. Introduction

Tin-cure silicone—also known as condensation-cure RTV silicone rubber—remains one of the most widely used elastomers for mold making, prototyping, casting, and industrial replication. Its affordability, forgiving curing behavior, and room-temperature crosslinking make it ideal for workshops, studios, and large-volume manufacturers.

This guide provides an industry-level explanation of how to properly mix tin-cure silicone at the standard 100:2–3 ratio, covering tools, procedures, troubleshooting, and expert-level optimization tips.

2. What Is Tin-Cure Silicone?

Tin-cure silicone is a two-component RTV elastomer that cures through a condensation reaction in which Part B (catalyst) reacts with hydroxyl-terminated silicone polymers in Part A.

Key Characteristics

• Economical: lower cost than platinum-cure
• Versatile: compatible with most resins (epoxy, PU, polyester)
• Good tear resistance
• Acceptable shrinkage (0.3–0.8%)
• Cures at room temperature

3. Why the 100:2–3 Ratio Matters

Tin-cure silicone is designed to cure optimally when mixed by weight at a ratio of:

• 100 parts Part A
• 2–3 parts Part B

If catalyst is too low (<2%)

• Incomplete cure
• Sticky or soft surface
• Weak tear strength

If catalyst is too high (3%)

• Faster curing but poor mechanical performance
• Brittleness & cracks
• Reduced working time
• More shrinkage

Pro Tip: For your first try, use 2.5% (100g A : 2.5g B). This middle ground offers a great balance of working time and cure speed.

4. Tools and Safety Precautions

Essential Tools

• Digital Scale (Accuracy to ±0.1g is crucial!)
• Mixing Cups (Disposable plastic or reusable silicone)
• Stirring Sticks (Flat-ended sticks are best for scraping)
• Disposable Gloves

Safety Notes:

• Tin-cure catalyst may release alcohols or acetic acid → ensure ventilation
• Avoid skin contact with uncured material
• Store Part B tightly sealed to prevent moisture absorption

5. Technical Parameters to Understand

6. Step-by-Step Mixing Instructions

Step 1: Weigh Part APrecisely

• Place your mixing cup on the scale and tare it to zero.
• Add exactly 100 grams of Part A.

Step 2: Measure Part B with Care

• Tare the scale again so it reads zero with the cup and Part A.
• For 100g of A, add between 2g and 3g of Part B. We recommend 2.5g.

Step 3: Mix Until Flawless

• Slow and thorough wins the race. Stir for a full 2-3 minutes.
• Scrape the sides and bottom of the cup repeatedly. The color must be perfectly uniform with no streaks.

Step 4:Degas for Premium Results (Optional)

• For bubble-critical applications (e.g., optical clarity), use a vacuum chamber.
• For most crafts, careful pouring is sufficient.

Step 5: Pour and Cure

• Pour in a thin, steady stream into your mold to minimize new bubbles.
• Cure at room temperature (20-25°C / 68-77°F). It will be touch-free in 30-90 minutes and fully cured in 4-12 hours.

7. Common Problems & Solutions

❌ Problem 1: Silicone stays soft or sticky

Cause:ratio too low, humidity too high

Fix:ensure accurate 100:2–3 ratio, work in <60% humidity

❌ Problem 2: Bubbles inside the mold

Cause:fast stirring or high-viscosity silicone

Fix:mix slowly, pour thinly, degas if needed

❌ Problem 3: Shrinkage or warping

Cause:excessive catalyst (over 3%)

Fix:use max 3%

❌ Problem 4: Tear during demolding

Cause:wrong hardness or weak tear resistance

Fix:choose Shore A 20–25 for high undercuts

8. Advanced Tips for Optimal Results

• Customizing Hardness:Adjust Part B within the 2–3% range. More catalyst increases hardness; less enhances elasticity.
• Adding Pigments:Use silicone-compatible colorants (<5% by weight) and mix them into Part A before adding Part B.
• Post-Curing:Bake cured silicone at 60–80°C for 1–2 hours to improve mechanical properties.

9. Applications of Tin-Cure Silicone

• Mold Making: For resins, wax, or low-melt metals.
• Encapsulation: Protecting electronic components from moisture and dust.
• Prototyping: Creating flexible parts or seals.

10. How Tin-Cure Silicone Compares to Platinum Silicone

11. Conclusion

Tin-cure silicone remains an excellent choice for cost-effective mold making, industrial prototyping, and general-purpose casting. Mastering the 100:2–3 ratio ensures consistent curing, minimal defects, and predictable mechanical performance.

By following this guide—accurate weighing, thorough mixing, proper curing conditions—you can achieve professional-grade results whether you are a manufacturer, artist, engineer, or OEM partner.