When tiny balloons appear on your castings, there are several paths to troubleshoot.

   

While this could be a problem with mixture of the investment for the outer ring and incorporating too much air in the mixture, I would rather suspect it's a problem with the technicians' preparation of the master model along with model drying technique; and their sealing technique of the master model. The typical problem is refractory models that was not sealed properly. Sealing materials include: paraffin, rosin dip, beeswax, safe dip or some-other substitute for sealing the refractory model. Determine if the sealer has penetrated or soaked into the surface of the refractory model. The model must be sealed well enough so that when the waxer begins waxing and using instruments, the surface of the refractory model is not exposed. Furthermore, when they put a pattern down and then try to lift it making readjustments, they end up removing the little surface that was sealed, consequently when you pour the outer ring, the air from within refractory model seeks an escape route; thus the bubbles you see on the castings.

This problem could be from the mixture of the outer ring investment. Sometimes these phenomena occur when one does not use the Green Paint On or some other paint-on materials properly. If a paint-on material is used and not allowed to dry properly this type of phenomenon may occur, however, that is a very rare occurrence. The abuse of the paint-on, still leads back to improper dehydration and sealing of the refractory model.

To resolve this, one must go back to the basics in terms of sealing (times and temperatures) for the refractory model, making certain you have enough time and the correct temperature in the model drying oven and the correct temperature of the sealer. Secondly, caution should be observed in terms of using very sharp instruments during the waxing process, and the cutting process of the patterns on the refractory model.

 

 

 

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