Vacuum Casting
Problems to Pay Attention to in Silicone Compounding: A Practical Guide for Flawless Molds
Making a great silicone mold is the most critical step in vacuum casting. If your mold is flawed, every single part you cast will be flawed too. The process of mixing liquid silicone with a curing agent is called silicone compounding. It sounds simple: just mix two parts together. But small mistakes here can ruin your mold. Too much hardener makes it brittle. Air bubbles create holes. Poor mixing leaves sticky spots.
Key takeaways
- The curing agent (also called hardener) is what turns liquid silicone into a solid, rubbery mold.
- Air bubbles are the enemy of a good mold. If bubbles are trapped in the silicone, they create voids or pockmarks on the surface of your mold.
- This problem is frustrating. You demold your part, and one side is perfect, but the other side is sticky or soft. This is called uneven curing.
- You might be tempted to add silicone oil to your mixture. People do this to make the silicone thinner and easier to pour. This is a big mistake.
1. Curing Agent: How Much Is Just Right?
The curing agent (also called hardener) is what turns liquid silicone into a solid, rubbery mold. Getting the amount wrong is the number one mistake beginners make. The right ratio depends on your workshop temperature.
Why Temperature Changes the Ratio
Heat speeds up the chemical reaction. Cold slows it down. You must adjust the amount of curing agent based on the temperature.
| Workshop Temperature | Recommended Curing Agent Ratio | Expected Cure Time | What Happens If It’s Wrong |
|---|---|---|---|
| 25–30°C (77–86°F) | 1 – 1.5% | 4 – 6 hours | Too much (>2%): Mold is brittle and cracks.Too little (<0.5%): Takes 8-12 hours, stays sticky. |
| 18–24°C (64–75°F) | 1.5 – 2% | 6 – 8 hours | Too much (>2.5%): Mold shrinks >1.5%, parts are undersized.Too little (<1%): Never fully cures, peels apart. |
| 10–17°C (50–63°F) | 2 – 2.5% | 8 – 10 hours | Too much (>3%): Hardens unevenly, has lumps.Too little (<1.5%): Cures very soft, tears easily. |
A Real-World Mistake
A workshop in Guangzhou was making a mold for a phone case. It was a hot day, about 32°C. They used their standard ratio of 2% curing agent, which is perfect for 20°C. The silicone hardened in just 3 hours. But it was so brittle that when they tried to remove the master pattern, the mold cracked in half. They wasted $20 of silicone and lost two days.
The Fix: On the next attempt, they reduced the curing agent to 1% for the hot weather. The mold cured perfectly in 5 hours and was flexible and strong, lasting for over 15 casts.
2. Bubbles: How to Get Them Out?
Air bubbles are the enemy of a good mold. If bubbles are trapped in the silicone, they create voids or pockmarks on the surface of your mold. These defects then show up on every single part you cast.
Method 1: Using a Vacuum Chamber (Best Results)
A vacuum chamber is the most reliable tool for removing bubbles.
- Mix your silicone and curing agent thoroughly.
- Place the container inside the vacuum chamber.
- Turn on the pump. You want a vacuum of at least -0.095 MPa.
- Watch the mixture. It will foam up and then collapse. Keep the vacuum on for 5 to 8 minutes, until you see no more bubbles rising.
- Pour the degassed silicone gently into your mold frame.
Success Rate: Over 95% bubble-free.
Method 2: Without a Vacuum Chamber (Good Alternative)
If you don’t have a vacuum pump, you can still get good results. It just takes more care.
- Adjust the Ratio: Reduce the curing agent by a tiny amount (0.3% to 0.5%). This gives you more working time.
- Stir Slowly: Mix the silicone slowly. Fast stirring whips air in. Aim for about 100-150 RPM (stirring speed). Mix for 3-4 minutes.
- Let It Sit: After mixing, let the bucket sit for 10 to 15 minutes. This allows most of the air bubbles to rise to the surface and pop.
- Pour Carefully: Pour the silicone in a thin stream against the wall of the mold frame, not directly onto the master pattern.
Success Rate: Around 80-85%.
A Quick Pro Tip
Even after pouring, you might see a few tiny bubbles on the surface. Use a toothpick to gently pop them. Do this within 5 minutes of pouring, before the silicone gets too thick.
3. Uneven Curing: Why Is My Mold Part Hard and Part Soft?
This problem is frustrating. You demold your part, and one side is perfect, but the other side is sticky or soft. This is called uneven curing. It happens about 20% of the time with beginners, and it is almost always a mixing or measuring error.
The Main Causes
- Bad Measurements: Using a cheap kitchen scale that is not accurate. You need precision to 0.1 grams.
- Poor Stirring: Only stirring the top of the mixture. The curing agent is heavy and sinks to the bottom.
The Step-by-Step Fix
- Weigh Accurately: Use a digital scale that measures to 0.1g. If you have 100g of silicone and need a 1.5% ratio, you must add exactly 1.5g of curing agent. Not 1.4g, not 1.6g.
- Stir Thoroughly: Stir for a full 3 to 5 minutes. Use a figure-8 motion. Scrape the bottom and sides of the container with your stirring stick. This brings the curing agent from the bottom back into the mix.
- The “Streak Test”: Lift your stirring stick. Look at the silicone dripping off. It should look completely uniform. If you see streaks or swirls of different colors, you are not done mixing. Stir for another 1-2 minutes.
A Medical Device Example
A team was making a mold for a small sensor. They used a kitchen scale to weigh 1.5g of curing agent for 100g of silicone. But their scale was only accurate to 1g, so they actually added about 1.2g. They also stirred quickly for only one minute.
The result? The mold cured unevenly. The top was hard, but the bottom, where the un-mixed curing agent settled, was a sticky mess. They had to throw it away.
The Fix: They bought a proper digital scale (0.1g accuracy) and stirred for a full 5 minutes, scraping the bottom. The next mold was perfect and lasted for 18 casting cycles.
4. Silicone Oil: Why You Should Never Add It
You might be tempted to add silicone oil to your mixture. People do this to make the silicone thinner and easier to pour. This is a big mistake. It will ruin your mold.
The Risks of Adding Oil
| Issue | What Happens to Your Mold | The Data |
|---|---|---|
| Lower Strength | The mold tears easily. It might only last 5-8 uses instead of 15-20. | Adding just 5% oil can reduce tear strength by 40%. |
| Poor Adhesion | The silicone might not stick to the mold frame properly. It can leak out, ruining the mold’s shape. | Over 60% of molds with added oil leak during pouring. |
| Soft Spots | The hardness becomes inconsistent. Parts cast from it will be uneven. | Hardness can vary by over 10 Shore A points across the mold. |
What to Do Instead
If your silicone feels too thick, do not add oil. Warm it up.
- Place the un-mixed silicone container in a warm water bath.
- Warm it to about 25–30°C.
- This lowers the viscosity and makes it flow better, without changing its chemical properties.
Warning: Do not heat it above 35°C. If it gets too hot, it might start curing in the container.
How to write a spec that gets priced correctly
A drawing is a contract. If it leaves a number open, the shop fills that number in with its own default. The default is rarely the one you had in mind. Put these five lines on every RFQ and most of the back-and-forth disappears before it starts.
- A master pattern printed or machined to the final finish
- Cast resin grade and the colour you need
- Number of parts required from each mould
- Tolerance on the features that mate
- Finish: as cast, primed or painted
The point is not paperwork. Each line removes a decision the shop would otherwise make for you. A complete RFQ comes back priced. An incomplete one comes back guessed.
What actually drives the price
Buyers usually push on material because it is the line they can see. In practice the material is rarely the biggest number on a quote. These five lines decide most of it:
- master pattern quality
- silicone mould life
- cast material grade
- colour and finish
- number of parts per mould
Ask for the quote split by line. A supplier who cannot break it down is guessing, and you will pay for the guess. The line you can usually renegotiate is the tolerance band, not the raw material.
Problems you will meet, and the fix
Every process has a failure mode it prefers. The same four show up again and again on this kind of work, and none of them are mysteries once you know what to look for.
- Bubbles trapped against a vertical face
- Dimensional drift after the tenth pull from one mould
- Colour mismatch between two moulds
- Shrinkage on a thick section that cools last
Catch the fault on the first part, not the hundredth. A first-article check costs an hour. A bad batch costs the whole order. Write the check into the purchase order so nobody has to remember it.
What to check on the first article
The first article is the cheapest inspection you will ever buy. Use it properly and the rest of the run becomes boring, which is exactly what you want.
- Every dimension on the drawing, with the measured value written beside it.
- The material certificate, matched to the heat or lot number on the parts.
- The finish against the sample you approved, under the same light.
- Fit against the mating part, assembled rather than measured.
- Packaging, because parts that arrive damaged were never in tolerance.
Check in this order: datums, then mating features, then cosmetic faces. If the datums are wrong, every number after them is noise. Keep the report on file. When a problem shows up in month six, that document is the only thing that tells you what changed.
How to compare two quotes fairly
Two numbers are not comparable unless the scope matches. Line them up before you decide, because a low number with three gaps is not a low number.
Same drawing, same material, same finish, same inspection. Anything else is a different job wearing the same part number. Then look at what each quote leaves out. Freight, packaging, certificates and tooling are the usual gaps.
Ask each supplier to state their assumptions in writing. The one who does is usually the one who has run the process before.
Scaling from one part to a batch
The first part teaches you the process. The hundredth teaches you the process cost.
Watch three things as volume grows: setup amortisation, tool wear, and how much hand work the finish still needs. If the unit price barely moves between ten and a hundred, the cost sits in material or in hand work, not in setup. Ask which, because only one of them is negotiable.
Lead time behaves the same way. A one-off is scheduled around other jobs. A repeat order is scheduled around your forecast. Give the shop a forecast and the lead time gets shorter, not longer.
How to read a supplier's quote
A quote is a short document with a long shadow. Four things in it decide whether the job runs smoothly, and only one of them is the price.
- The scope line. It should name the process, the material and the finish, not just the part.
- The tolerance basis. General tolerance or the drawing, and which standard it follows.
- The exclusions. Tooling, freight, certificates and inspection are the usual omissions.
- The validity date. Material prices move, and an open-ended quote is a promise nobody can keep.
Read the exclusions before you read the total. A quote that leaves out tooling is not cheaper. It is a quote for a different job.
What to send with the first order
The first order sets the pattern for every order after it. Send a complete package once and the shop files it, and the second order takes a fraction of the time.
- A STEP or native model plus a PDF drawing carrying the same revision letter.
- The material callout by standard number, not by trade name.
- The finish and the exact faces it applies to.
- Any test the part must pass: fit, seal, drop, salt spray, dielectric.
- A contact who can answer a question inside one working day.
Then write down what was agreed. An email that confirms the material, the finish and the date is worth more than a longer purchase order that leaves them open.
5. Mold Storage: Why You Must Wait 24 Hours
You have mixed the silicone perfectly. You have removed the bubbles. The mold has cured and you have demolded it. You are eager to start casting parts. Wait.
A freshly cured mold is not ready to use. You must store it for 24 hours first.
Why Is This Waiting Period Critical?
- Final Curing: The chemical reaction continues slowly even after the mold feels hard. Giving it 24 hours allows it to reach its final, stable strength.
- Stress Relief: The mold material relaxes and any internal stresses from the curing process disappear. This prevents the mold from warping later when you pour hot resin into it.
How to Store It Properly
- Keep it in a cool, dry place (18–24°C is ideal).
- Keep it out of direct sunlight. UV rays damage silicone.
- Place a sheet of parchment paper between the mold and any surface it touches. This prevents it from sticking to things while it finishes curing.
A Toy Manufacturer’s Story
A toy company made a mold for a small action figure. They were in a rush to meet a deadline, so they used the mold just 2 hours after it hardened. They poured their plastic resin. As the resin cured, it got warm. The mold, which hadn’t fully relaxed, warped under the heat. The final parts were 0.5mm smaller than the master prototype. The parts did not fit together.
They learned their lesson. They made a new mold, stored it for 24 hours, and the next batch of parts was perfect.
Silicone compounding is a skill that rewards patience and precision. By paying attention to just a few key details, you can avoid the most common and costly mistakes. Remember to adjust your curing agent for the temperature, always remove bubbles with a vacuum or careful pouring, mix thoroughly to avoid soft spots, never add silicone oil, and wait 24 hours before using your new mold. Master these five points, and you will create flawless, long-lasting silicone molds every time.
How we can help
YiguProto runs vacuum casting in house, from a single prototype to a production batch. We quote the process, the material and the finish as separate lines, so you can see where the money goes instead of arguing with one number.
Send a STEP file or a drawing and you get a DFM report in one to two business days. It flags the features that add cost and the features that will not hold tolerance on the process you picked.
Every order ships with an inspection report on the features you call out. If a part misses, we make it again. Ready to price a part? Start with a free quote and DFM review, or read more about our vacuum casting. You can also talk to an engineer before you commit to a drawing.
Frequently asked questions
What should I check before approving the parts?
Datums first, then the features that mate, then the cosmetic faces. Ask for a dimensional report, a material certificate and a finish sample. A part can be inside every tolerance and still not fit, so assemble one before you sign.
Does the process work for prototypes as well as production?
Yes, and that is usually the point. Running the prototype on the production process removes a whole class of surprises, because the part you test is made the same way as the part you sell.
What material should I specify?
Specify by standard, not by trade name. A trade name can change under you between two orders; a standard number cannot. Then add the one property that actually decides the part, whether that is strength, heat resistance or chemical exposure.
Is there a minimum order quantity?
No hard minimum. One part is fine. What changes with volume is which costs dominate, so a single unit is priced mostly on setup and a thousand units are priced mostly on cycle time and material.
What files do you need in order to quote?
A STEP file plus a PDF drawing that carries the same revision letter. The model carries the geometry and the drawing carries the intent. When the two disagree, the shop picks one, and it is never the one you meant.
How do I compare two quotes fairly?
Match the scope first: same drawing, same material, same finish, same inspection. Then read the exclusions. Freight, tooling and certificates are the usual gaps, and a low number with three gaps is not a low number.
The short version
Good parts reward the buyer who reads the data sheet and the drawing together. Pick the option for the property that matters, not for the price per kilo.
Tighten only what mates. Test a sample before the run. Then let the process do the rest.