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.

Urethane Rubber Mold

FAQ: 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 . . . 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 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 dririte tubes to attach to your 5 gallon or 55 gallon containers, or a tank of nitrogen to cap your container after dispensing material.