Bubbles on the working surface of a cured rubber mold can negatively affect the details and render the mold unusable. These bubbles can stem from various sources, and there are several controllable factors that influence the size and quantity of bubbles in your mold.
Urethane Rubber Mold
FAQ:
“I made a mold using urethane rubber. I followed the mixing instructions, but when I demolded it the next day, my mold had many air bubbles, including on the working surface, ruining the detail and making the mold unusable. What went wrong?”
Answer:
Urethane mold rubbers are sensitive to moisture, and the bubbles often found in cured urethane rubber are typically caused by a reaction between the rubber and moisture during the curing process. This reaction can sometimes be severe, causing the cured rubber to foam, depending on how much moisture was introduced into the mixture.
Common sources of moisture that may cause a reaction with liquid urethane rubber:
- Humidity – Urethane rubbers generally have a long pot life, which gives ample time for a moisture reaction to occur in a humid environment.
Remedy: Work in a humidity-controlled space (using air conditioning) with as low a relative humidity as possible. - Wet Model – Applying liquid urethane rubber over a model that contains moisture (such as freshly cast plaster, gypsum, or concrete) can cause a moisture reaction.
Remedy: Seal models containing moisture with a high-quality spray shellac and apply a release agent before using the mold rubber. - Mixing Containers and Sticks – Wooden or paper mixing containers, as well as wooden stir sticks, may absorb moisture from the environment, which reacts with urethane rubber.
Remedy: Use mixing containers made from plastic, metal, or glass, and use mixing utensils made from plastic or metal. - Repeatedly Opening Containers – Frequent opening of parts A and B can allow moisture from the air to enter and react with the unused material.
Remedy: After dispensing, immediately close the lids of the containers and store them in a dry, cool place. Consider using a dry gas blanket to displace air and extend the shelf life of moisture-sensitive urethane products. For larger quantities, you may want to use desiccant tubes or nitrogen tanks to protect the material after dispensing.
By addressing these factors, you can reduce the likelihood of bubbles forming in your urethane rubber molds and improve the overall quality of your mold-making process.