Why are there bubbles on the surface of my rubber mold?

Bubbles that show up on the working surface of a cured rubber mold can ruin detail and result in a mold that is unusable.

Bubbles can come from different sources and there are different variables (most controllable) that will affect the size and quantity of bubbles generated.

Q: I made a mold using urethane rubber. I mixed the rubber as directed, but when I demolded next day, my mold had a lot of air bubbles throughout including (worst of all) on the working surface, which ruined all of my detail and the mold is unusable. What went wrong?

Answer: Urethane mold rubbers are moisture sensitive, and often bubbles found in cured urethane rubber are a result of a reaction between the rubber in its liquid state and moisture coming from somewhere. A moisture reaction can be severe (cured rubber will look like foam), depending on the amount moisture introduced to the urethane rubber mix.

Common sources of moisture that might react with liquid urethane rubber:

  1. Humidity – urethane rubbers generally have a long pot life, which gives plenty of time for a moisture reaction in a humid environment.

    Remedy: work in a humidity-controlled environment (air conditioning) with as low a relative humidity as possible.

  2. Wet Model – If liquid urethane rubber is applied over a model that contains moisture (such as newly cast plaster/gypsum or concrete), you will experience a moisture reaction.

    Remedy: when using urethane rubber to make your mold, seal a model containing moisture with high quality spray shellac followed by a release agent before applying mold rubber.

  3. Mixing containers and mixing sticks – mixing containers made of wood or paper as well as wooden mixing utensils (paint stirrers) stored in a humid environment may absorb moisture that will react with urethane rubber.

    Remedy: use mixing containers made of plastic, metal or glass. Also, use mixing utensils made of plastic or metal.

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

    Remedy: After dispensing place the lids back on the containers as soon as possible and store in a dry cool place. Also, try 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. If using larger quantities of urethane rubber or plastic, you might want to consider buying Drierite® tubes to attach to your 5 gallon or 55 gallon containers, or a tank of nitrogen to cap your container after dispensing material.

Q: I've noticed tiny "champagne" bubbles on the working surface of my finished mold, which are now being reflected in my castings. What causes these bubbles?

Answer: The description of bubbles as "pinholes" or "champagne" is an indicator that too much mold release was applied to the rubber mold prior to casting resin. People tend to get carried away, thinking that more release agent is better. This is not only a waste of release agent, it will also cause pinhole bubbling to occur in the plastic.

Remedy: use the "Spray-Brush-Spray" technique for applying mold release agent.

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

Q: After demolding a one piece block mold (poured), I noticed a single large air bubble on the working surface of the mold. What caused it, how do I avoid it in the future and can I repair the bubble?

Answer: As noted before, air bubbles can come from different sources. In this case, the air could be coming from a few places:

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

    Remedy: the model must be tightly secured to a platform or base and a bead of hot melt glue or modeling clay material should be applied at the interface between the model and the platform, providing an airtight seal. The goal is to prevent the liquid rubber from going underneath the model. Most liquid rubbers (even high viscosity silicones) will find there way into the smallest holes (even pinholes!)

  2. Highly Porous Models – Models that are highly porous (made from such materials as dry plaster, concrete or limestone) contain a lot of air. When liquid rubber is poured over these models, the air contained in these models has to go somewhere. If nothing is done, the only place the air can go is up through the liquid mold rubber, and occasionally the air becomes trapped in the mold rubber as it is curing. This air is reflected in the cured mold as air bubbles of different sizes located in different areas in the mold.

    Remedy: Drill an air vent up the back of the base or platform on which the model will be mounted. Elevate the base slightly in all four corners with modeling clay or wood pieces – make sure the base is level. Then, secure the model to the base as usual. When the liquid rubber is mixed and poured over the model, the air will escape through the vent hole underneath the model rather than go up through the liquid rubber and possibly be trapped.

Bubbles in Silicone Rubber Mold

Many silicone rubbers are thick and most have a high viscosity. These silicones do not de-air themselves as readily as urethane rubbers and need to be vacuumed.

Consequently, if you mix and pour high-viscosity silicone rubber that has not been vacuumed over a model, air may be trapped in the mixture as it turns from liquid to solid and these bubbles may show up on the working surface of the mold.

For More Information, Consult FAQ: Vacuuming Silicone Rubber

Q: My brushed on mold rubber dripped off my model and when it cured, I had tiny bubbles on the working surface of my mold that are reflected in my casting. What am I doing wrong?

Answer: You are probably trying to apply too much mold rubber to the model surface. When making a brush-on rubber mold do not over apply rubber, especially the first coat.

More Is Not Better – People assume that brush-on mold rubber needs to be applied in quantity. Applying globs of rubber to your model surface entraps air and wastes your valuable material.

Generally, it is better to apply 4 thin layers than 3 heavy layers. Thin layers work best, especially for the first layer or "detail coat."

Go Easy! A thin initial coat will minimize entrapped air and capture the best detail from the model surface.

The secret is to apply the first layer thinly with dabbing strokes (stippling) that will get rubber over and into all of the surface detail. Over applying rubber entraps air that is then reflected in the finished mold and castings – air bubble city!

Applying rubber in this manner also lets bubbles move away from the model surface and up 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 not too heavy.

Let your first coat become tacky. Mix and apply the second coat; this time, add a small amount of liquid pigment or tint to the mixture. This will help you distinguish the second coat from the first coat. Ever try brushing white on white? It is impossible to tell you where you have been.

Adding color (pigment/tint) to every other layer will help avoid thin spots in the finished mold. Smooth-On Brush-On® Series mold rubbers were carefully formulated to address several problems experienced with brushable mold rubbers of the past. High tear strength and abrasion resistance are but two attributes that have made these products so popular. How they perform when actually making the mold is equally as important.

This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.