Bubbles that appear on the working surface of a cured rubber mould can compromise the fine details and render the mould unusable.

Bubbles can originate from various sources, and several variables (most of which can be controlled) influence the size and quantity of bubbles produced.

Q: I created a mould using urethane rubber. I followed the rubber mixing instructions, but when I demoulded it the next day, the mould had numerous air bubbles, including on the working surface, which ruined the detail, making the mould unusable. What went wrong?
Answer: Urethane mould rubbers are sensitive to moisture, and often bubbles in cured urethane rubber result from a reaction between the liquid rubber and moisture from some source. Depending on the amount of moisture introduced to the urethane rubber mix, this moisture reaction can be severe, causing the cured rubber to appear foamy.

Common sources of moisture that may react with liquid urethane rubber include:
Humidity – Urethane rubbers typically have a long pot life, allowing for moisture reactions in humid environments.

Remedy: Work in a humidity-controlled environment (with air conditioning) with the lowest possible relative humidity.

Moist Model – If liquid urethane rubber is applied over a model containing moisture (such as freshly cast plaster/gypsum or concrete), a moisture reaction occurs.

Remedy: When using urethane rubber for mould making, seal a model containing moisture with a high-quality spray shellac followed by a release agent before applying mould rubber.

Mixing containers and mixing sticks – Wooden or paper mixing containers and wooden mixing utensils (paint stirrers) stored in a humid environment may absorb moisture, leading to a reaction with urethane rubber.

Remedy: Use plastic, metal, or glass mixing containers and plastic or metal mixing utensils.

Repeated opening and closing of parts A and B can introduce moisture from the air to the unused material.

Remedy: After dispensing, promptly replace the lids on the containers and store them in a dry, cool place. You can also consider using Smooth-On's XTEND-IT®, a dry gas blanket designed to extend the shelf life of moisture-sensitive polyurethane products by displacing the air in the container. For larger quantities of urethane rubber or plastic, you might want to consider using Drierite® tubes attached to your 5-gallon or 55-gallon containers, or a nitrogen tank to seal the container after dispensing the material.

Q: I've noticed tiny "champagne" bubbles on the working surface of my finished mould, which are now appearing in my castings. What causes these bubbles?
Answer: Describing the bubbles as "pinholes" or "champagne" indicates that too much mould release agent was applied to the rubber mould before casting resin. People often think that more release agent is better. However, this not only wastes release agent but also leads to the formation of pinhole bubbles in the plastic.

Remedy: Use the "Spray-Brush-Spray" technique to apply the mould release agent.

1. Spray a light mist coating on the mould surface.
2. Use a soft bristle brush to evenly spread the release agent over all surfaces, including deep details and undercuts.
3. Apply another light mist coating and let it dry for 15 minutes before mixing and applying the mould rubber.

Q: After demoulding a one-piece block mould (poured), I noticed a single large air bubble on the working surface of the mould. What caused it, how can I prevent it in the future, and can I repair the bubble?
Answer: As mentioned before, air bubbles can come from various sources. In this case, air may have originated from a few places:

1. When you poured rubber over the model, liquid rubber may have found its way underneath the model, displacing air that ended up as one or more large bubbles in your cured rubber.

Remedy: The model must be securely attached to a platform or base, and a bead of hot melt glue or modelling clay material should be applied at the interface between the model and the platform to create an airtight seal. The goal is to prevent the liquid rubber from flowing underneath the model. Most liquid rubbers (even high-viscosity silicones) can penetrate even the smallest openings (including pinholes!).

2. Highly Porous Models – Models made from highly porous materials like dry plaster, concrete, or limestone contain a lot of air. When liquid rubber is poured over these models, the air contained within them needs to escape somewhere. If left unaddressed, the air can rise through the liquid mould rubber and occasionally become trapped in the mould rubber as it cures. This trapped air manifests as air bubbles of varying sizes in different areas of the mould.

Remedy: Create an air vent by drilling a hole up the back of the base or platform on which the model will be mounted. Slightly elevate the base in all four corners with modelling clay or wooden pieces to ensure it is level. Then, secure the model to the base as usual. When liquid rubber is poured over the model, the air can escape through the vent hole under the model rather than rising through the liquid rubber and potentially getting trapped.

Bubbles in Silicone Rubber Mould
Many silicone rubbers are thick, and most have a high viscosity. These silicones do not naturally de-air themselves as easily as urethane rubbers and require vacuuming.

Therefore, if you mix and pour high-viscosity silicone rubber that has not been vacuumed over a model, air may become trapped in the mixture as it solidifies, resulting in bubbles appearing on the working surface of the mould.

For More Information, Refer to FAQ: Vacuuming Silicone Rubber

Q: My brushed-on mould rubber dripped off my model, and when it cured, I ended up with tiny bubbles on the working surface of my mould, which are now visible in my castings. What am I doing wrong?
Answer: You are probably attempting to apply too much mould rubber to the model's surface. When making a brush-on rubber mould, avoid over-applying the rubber, particularly on the first coat.

More Is Not Better – People often assume that brush-on mould rubber needs to be applied generously. Applying large quantities of rubber to your model's surface traps air and wastes valuable material.

In general, it is better to apply four thin layers than three heavy layers. Thin layers are particularly effective, especially for the first layer or "detail coat."

Go Easy! A thin initial coat reduces the amount of trapped air and captures the best detail from the model's surface.

The key is to apply the first layer thinly using stippling (dabbing strokes) to ensure that rubber covers and penetrates all surface details. Over-application of rubber traps air, which can be reflected in the finished mould and castings as air bubbles.

Applying rubber in this way allows bubbles to move away from the model's surface and rise through the rubber, where they can pop on the surface. Rubber applied too heavily will trap air. After the first coat, you can apply heavier layers, but avoid going too heavy.

Allow the first coat to become tacky. Then mix and apply the second coat, adding a small amount of liquid pigment or tint to distinguish it from the first coat. Applying white on white can be challenging, making it difficult to determine where you have applied the rubber.

Adding colour (pigment/tint) to every other layer helps prevent thin spots in the finished mould