Bumps on Bronze Cast in Ceramic Shell
Hi everyone! I work at a bronze foundry that exclusively uses ceramic shell molds and recently we have been getting this strange casting defect. Raised bumps have been appearing on the majority of our pieces that are inconsistent with the normal boogers that you would get from air bubbles in the slurry. They are forming on both high and low texture/relief areas, on the face and inside of the piece, as well as on the cup and sprue bars. We burn out in a flash kiln and there are no visible defects on the interior surface of the ceramic after they come out of the kiln. We thoroughly blow out the molds and they are properly preheated prior to pouring. This issue started maybe 3 months ago, before that we had pretty flawless surface texture aside from the occasional burn in or cold spot. But apparently this problem has happened several different times before I worked here and nobody seems to be able to tell me how it was fixed, just that it eventually went away. We use Ransom & Randolph Primcote Binder for 2 primary coats and Customcote Binder for the 8-10 backup coats. We stay on top of the PH and the mixers are on an automatic timer. We pay special attention on the primary coats, making sure to brush away excess sand and using tools to lightly scrape any crevices to make sure air bubbles don’t get trapped for the first few dips. I am personally convinced that this is an issue with the structural integrity of the primary coat breaking down when the molten metal hits the mold. I feel like I have tracked down every other variable but I would really appreciate any advice I can get. Has anyone else ever had this problem?