Humidity is water vapor in the air. While it can make your skin feel nice it can foam or bubble urethane castings. CO2 is formed when polyurethane resin is exposed to water or humidity resulting in bubbles or foam in your parts. Even when vacuumed and pressure casted once the resin is contaminated bubbles can keep forming. Resins are moisture sensitive and do not like water or humidity; see below for causes and cures.
The 4 most common sources of contamination to urethane resin are:
1. Humidity In The Air Is Enough to Affect Your Castings – Check the weather. 50% relative humidity or less is best for using and storing Magikast and other polyurethane resins.
2. Opening And Sealing Containers – Humidity in the air will get trapped in your container as you seal it. The next time you use it you may think it has gone bad. It’s the moisture that was trapped when you shut the container. Use a Dry Gas Blanket every time you close a container to greatly extend the life of your unused resin. The dry gas is heavier than air so it sits on the resin separating and protecting it from the moist air. Just a quick squirt is all that is needed.
3. Fillers Need To Be Dry – Magikast and other resins can take a lot of filler for effects. Fillers included powdered stones, powdered metals and more. Some powdered fillers should be baked in an oven to drive off the moisture assuring a bubble free casting.
4. Paper And Wood Accessories – These can retain moisture and cause bubbles in your castings. These should be replaced with metal or plastic buckets and mixing sticks. Silicone molds are the best for urethane castings.
It is best practice to close containers as soon as you’re done pouring and use the dry gas blanket for extended shelf life. Do not over apply mold release as that can also be a source of contamination causing Champagne like bubbles or pinholes in your casting. Using an air conditioner and or a dehumidifier in the humid summer months can be a great help in reducing bubbles and pinholes.
Detroit gets an honorable mention because I am from there and know how humid it can be. The relative humidity typically ranges from 40% (comfortable) to 95% (very humid) over the course of the year, rarely dropping below 25% (dry) and reaching as high as 100% (very humid).
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