Rick's Column

Bubbles

Champagne, beer, bath beads, soufflés and expanding foam…..in these products, bubbles are considered a good thing.  In our casting resins – not so much!

Where do bubbles come from and why do they show up?  In most cases they can be mechanically generated or chemically produced.

When mixing our resin components together, it’s sometimes easy to stir a bit too vigorously, allowing large gulps of air to get caught in the folds of the liquids.  Lighter, low viscosity fluids will allow the bubbles to rise to the surface and pop out, but higher viscosity or fast set resins may be too thick for the bubble to force itself out and it gets entrapped.  Chemically, some very fast acting resins can create their own bubbles due to high heat and very slow resins may actually have more time to interact with any moisture that might be present in the mould (or even in some wooden stir sticks).  Internal bubbles are obviously more noticeable in clear resins.

Bubble avoidance:

-                     Keep moulds dry, so there’s no moisture to affect resins.

-                     Watch mould geometry to avoid undercuts or air entrapment areas and fill it in such a way that air can most easily escape.

-                     Use of a mould release can help keep bubbles from adhering to the sides of moulds.

-                     Pour resin gently and smoothly so entrapped air can escape the pour stream.

-                     Pre-dry any filler materials to ensure they’re moisture-free and premix them with the resin Part B to get it fully wet before adding Part A and going on the “pot life clock”.

-                     Try to keep working temperatures close to the recommendations.

-                     Some resins have enough pot life that they can be vacuum degassed prior to pouring into the mould.  Timing has to be watched closely and there is still the risk of trapping air during the pour.

A proven method for bubble control in casting is the use of a pressure chamber.  The resin is mixed and poured into the mould, following all bubble precautions, and the mould is placed in a vessel that can be pressurized with compressed air to a maximum of 60 PSI (30 PSI is probably adequate) until the resin can set.  The bubbles are still there, but the pressure has kept them from growing and joining their buddies, so they remain microscopic.  (This method works best with void-free rubber type moulds.)

Ask us about our 5 Gallon Capacity Pressure Chamber for your intricate pours.