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Preventing Formation of Bubbles in Castings

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FAQ:  How can I prevent bubbles in my castings?

A:  Bubbles experienced while casting urethane resins are the nemesis of casters everywhere.  As you may know, urethane resins are moisture sensitive, and often bubbles found in cured urethane plastic are a result of a reaction between the plastic in its liquid state and moisture coming from somewhere.

Common sources of moisture that might react with liquid urethane plastic;

1.  Humidity – urethane plastic will react with moisture in the air, especially if the plastic has a long pot life / working time. 

Remedy: work in a climate-controlled environment (air conditioning) with as low a relative humidity as possible.  Also, if possible, use a plastic with a shorter pot life.  It will have less time to react with ambient moisture. 

2.    Adding Fillers To Urethane Resins – many people will mix fillers (play sand, calcium carbonate, Urefil fillers, micro balloons, metal powders, etc.) with urethane resins to achieve different effects, lower the cost of their castings, etc.  Many fillers absorb moisture and, when mixed with the moisture sensitive urethane, cause the resin to bubble or even foam.  Remedy; spread your filler out on a cookie sheet to a depth of 3/8” (1 cm) and bake in an industrial oven at 150°F/60°C for at least four hours.  Let cool before using. This will allow moisture to evaporate.

3.  Mixing sticks and mixing containers, depending on what they are made of can introduce moisture.  Stirring sticks made of wood can absorb moisture and will transfer that moisture into polyurethane rubber or plastic.   Paper buckets will also absorb moisture.   Remedy:  A humidity-controlled environment (air-conditioned) will help minimize moisture absorption by these elements.  Better yet, use only plastic or metal mixing tools and plastic or metal buckets.  These do not absorb moisture that could be introduced to your mix.  

4.   Repeated opening and closing of part A and B container can introduce moisture from the air to the unused portions, especially on humid days. 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.

5.   Layer casting: casting in thin sections using a slow setting liquid plastic (SC 310) gives the plastic time to absorb ambient moisture.  Remedy: Cast slow setting resins in a temperature and humidity controlled environment.  Or, try using a faster setting casting compound (SC 300 or 305).  This way the material will cure before it has a chance to react with the moisture in the air, thereby minimizing bubble . 

Undercuts In The Mold

A:  Undercuts and deep detail will entrap air; they allow air bubbles to “cling” or “hover” in the recessed areas of an undercut, which leave voids in a finished casting.  Remedy: Start by adding a small amount of material then slush coat your mold by tilting it from side to side.  Add more material while continuing to slush coat your mold.  This allows the material to flow into the trouble spots by forcing out entrapped air and eliminating surface voids.

Another option is to build air vents into the mold to vent air during casting.

Over-Application Of Release Agent

A:  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 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 and 3. Apply another light mist coating and let dry for 15 minutes.