Silicone compounding is the backbone of silicone duplicating—it’s how you create the flexible molds that replicate prototypes into functional parts. But even small mistakes in compounding (like wrong curing agent amounts or poor bubble control) can ruin molds, delay projects, and waste money. Below, we break down the most critical problems to watch for, with data, real-world examples, and step-by-step fixes to keep your silicone compounding on track.
1. Curing Agent Usage: Avoid Too Much or Too Little
The curing agent is what turns liquid silicone into a solid mold—but getting its quantity right is tricky. It depends entirely on temperature, and even a 5% off ratio can cause major issues.
Key Rules for Curing Agent Amounts
- Temperature-Based Adjustment: When temperatures rise, reduce the curing agent—when they drop, increase it slightly. The table below shows standard ratios for common workshop temperatures (based on industry data from silicone manufacturers like Dow Corning):
| Workshop Temperature | Curing Agent Ratio (vs. Silicone) | Expected Cure Time | Potential Outcome If Ratio Is Wrong |
| 25–30°C (77–86°F) | 1–1.5% | 4–6 hours | Too much (2%+): Mold becomes brittle; cracks after 5–8 uses. Too little (0.5%–): Cure time doubles to 8–12 hours; mold stays sticky. |
| 18–24°C (64–75°F) | 1.5–2% | 6–8 hours | Too much (2.5%+): Mold shrinks by 1.5%+; doesn’t match prototype size. Too little (1%–): Mold never fully cures; peels off prototypes. |
| 10–17°C (50–63°F) | 2–2.5% | 8–10 hours | Too much (3%+): Mold hardens unevenly; has lumps. Too little (1.5%–): Cure time extends to 12–15 hours; mold is soft and tears easily. |
Real-World Example
A small electronics workshop in Guangzhou tried making a silicone mold for a phone case prototype on a hot day (32°C). They used the standard 2% curing agent ratio (for 18–24°C) instead of reducing it to 1%. The mold hardened in 3 hours but was so brittle that it cracked when they removed the prototype—wasting 500g of silicone (costing ~$20) and delaying the project by 2 days. After adjusting to 1% curing agent, the next mold was flexible and lasted 15 uses.
2. Bubble Treatment: Prevent Air From Ruining Molds
Bubbles in silicone molds create holes or uneven surfaces in your cast parts—this is one of the most common (and avoidable) problems in silicone compounding.
Two Effective Fixes for Bubbles
| Scenario | Solution Step-by-Step | Success Rate | Time Added to Process |
| You have a vacuum extractor | 1. Mix silicone and curing agent thoroughly. 2. Pour the mixture into a vacuum chamber. 3. Run the extractor at -0.095 MPa for 5–8 minutes (until no bubbles rise). 4. Pour the bubble-free silicone into the mold frame. | 95%+ | 10–15 minutes |
| You don’t have a vacuum extractor | 1. Reduce the curing agent ratio by 0.3–0.5% (e.g., from 1.5% to 1.2% at 20°C). 2. Stir the mixture slowly (100–150 RPM) for 3–4 minutes (fast stirring creates bubbles). 3. Let the mixture sit for 10–15 minutes before pouring—air rises to the surface. 4. Pour the silicone slowly along the edge of the mold frame (not directly onto the prototype). | 80–85% | 20–25 minutes |
Pro Tip
If you still see small bubbles after pouring, use a toothpick to pop them—do this within 5 minutes of pouring, before the silicone starts to thicken.
3. Uneven Curing: Ensure Silicone and Curing Agent Mix Perfectly
Uneven curing (where parts of the mold are hard and parts are soft) happens when silicone and curing agent aren’t mixed correctly. This ruins 20% of beginner-made silicone molds, but it’s easy to fix with proper mixing.
Why It Happens & How to Fix It
- Main Causes:
- Inaccurate weighing (e.g., using a kitchen scale instead of a digital scale that measures to 0.1g).
- Poor stirring (e.g., stirring only the top layer, not the bottom of the container).
- Step-by-Step Fix:
- Weigh Accurately: Use a digital scale with 0.1g precision. For example, if you have 100g of silicone and need a 1.5% curing agent ratio, measure exactly 1.5g of curing agent (not 1.4g or 1.6g).
- Stir Thoroughly: Stir the mixture in a figure-8 pattern for 3–5 minutes. Scrape the bottom and sides of the container with a spatula—this ensures no unmixed silicone or curing agent is left.
- Check for Uniformity: After stirring, lift the spatula— the mixture should be smooth and consistent (no streaks or clumps). If you see streaks, stir for another 1–2 minutes.
Example
A medical device team was making a mold for a small sensor prototype. They used a kitchen scale (measures to 1g) to weigh 1.5g of curing agent for 100g of silicone—but the scale was off by 0.3g, so they only used 1.2g. The mold cured unevenly: the top was hard, but the bottom was sticky. After switching to a digital scale and stirring for 5 minutes, the next mold cured perfectly and worked for 18 uses.
4. Material Additives: Never Add Silicone Oil (Here’s Why)
It’s tempting to add silicone oil to silicone compound to make it easier to pour—but this is a big mistake. Silicone oil changes the physical properties of the mold, making it weak and short-lived.
Risks of Adding Silicone Oil
| Issue | Impact on the Mold | Data from Industry Tests |
| Reduced Tensile Strength | Mold tears easily when removing prototypes—lasts only 5–8 uses (vs. 15–20 normal). | Silicone with 5% oil has 40% lower tensile strength than pure silicone. |
| Poor Adhesion | Mold doesn’t stick to the mold frame properly—silicone leaks out, ruining the shape. | 60% of molds with silicone oil leak during pouring. |
| Inconsistent Hardness | Mold has soft spots that deform under pressure—cast parts are uneven. | Silicone with 3% oil has a hardness variation of Shore A 10+ (vs. 2–3 for pure silicone). |
What to Do Instead
If your silicone is too thick to pour, warm it slightly (to 25–30°C) using a water bath—this makes it more fluid without changing its properties. Never heat it above 35°C, though—high heat can start curing the silicone early.
5. Mold Storage: Wait 24 Hours Before Using (Critical!)
After compounding and curing the mold, it’s important to store it for 24 hours before using it. Rushing to use the mold right after curing leads to poor performance.
Why 24 Hours Matters
- Final Curing: The mold continues to cure slowly after the initial hardening—24 hours ensures it’s fully stable.
- Stress Relief: The mold cools and releases internal stress during storage—this prevents warping when you pour casting materials (like ABS or PC).
Storage Best Practices
- Store the mold in a cool, dry place (18–24°C, 40–60% humidity).
- Keep it away from direct sunlight (UV rays break down silicone over time).
- Place a piece of parchment paper between the mold and any surfaces to avoid sticking.
Example
A toy manufacturer made a silicone mold for a small action figure prototype. They used the mold 2 hours after curing to meet a deadline—but the mold warped when they poured the plastic casting material. The cast parts were 0.5mm smaller than the prototype. After storing the next mold for 24 hours, the cast parts matched the prototype size exactly.
Yigu Technology’s Perspective
At Yigu Technology, we see silicone compounding mistakes as avoidable with attention to detail. We always advise clients to prioritize accurate weighing (digital scales are non-negotiable) and proper bubble treatment—these two steps fix 70% of mold issues. For teams without vacuum extractors, reducing curing agent and slow stirring works well, but we often recommend investing in a small extractor for high-volume projects. We also warn against silicone oil—its short-term convenience leads to long-term costs (reworking molds). By following these rules, clients get molds that last 15–20 uses and produce consistent, high-quality parts.
FAQ
- I used the right curing agent ratio, but the mold is still sticky—what’s wrong?
This is likely due to poor stirring. Even if the ratio is correct, unmixed curing agent means parts of the silicone don’t cure. Try stirring for 5 minutes (scraping the container bottom) and using a digital scale to recheck the ratio. If it’s still sticky, the silicone may be expired—check the manufacturer’s expiration date.
- Can I reuse a mold that had bubbles?
It depends on the bubble size. Small bubbles (less than 1mm) are okay for non-critical parts (e.g., a decorative prototype). Large bubbles (1mm+) create holes in cast parts—you’ll need to make a new mold. To fix small bubbles, fill them with a tiny amount of uncured silicone (mixed with curing agent) and let it cure for 4–6 hours.
- How do I know if my silicone mold is fully cured (even if I waited 24 hours)?
Press your finger gently on the mold—if it’s firm and doesn’t leave a mark, it’s fully cured. If it’s soft or sticky, it needs more time (12–24 hours). You can also check the hardness with a Shore A durometer—most silicone molds should measure Shore A 20–30 when fully cured.
