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How can I prevent the formation of bubbles in my castings?


Casters often struggle with the presence of bubbles in urethane resin castings. Urethane resins are sensitive to moisture, and the appearance of bubbles in cured plastic is often due to a reaction between the liquid plastic and moisture from the environment.

Common Sources of Moisture that Can Cause Reaction with Liquid Urethane Plastic.

1.Humidity – Urethane plastic can react with moisture in the air, especially when the plastic has a long working time. It is recommended to store and use urethane plastics in an environment with a relative humidity of 50% or less.

2.Adding Fillers to Urethane Resins – Many casters mix fillers with urethane resins for various effects or cost reduction. However, some fillers absorb moisture, which can cause the resin to bubble or foam. To remedy this, spread the filler on a cookie sheet and bake it in an industrial oven at 150°F/60°C for at least four hours to allow the moisture to evaporate.

3.Moisture from Mixing Tools and Containers – Wood stirring sticks and paper buckets can introduce moisture into the polyurethane rubber or plastic. It is recommended to use plastic or metal mixing tools and containers to minimize moisture absorption.

4.Opening and Closing of Parts A and B – Containers should be sealed as soon as possible after dispensing to prevent moisture from entering on humid days.

5.Layer Casting – Casting thin sections using slow-setting liquid plastic can lead to moisture absorption. It is advised to cast slow-setting resins in a controlled environment or consider using a faster setting casting compound.

6.Over-application of Release Agent – Excessive use of release agent can result in champagne bubbles or pinholes in the cured plastic. It is best to use the “spray-brush-spray” application method to prevent over-application.

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