This my second set of castings, I'm using Sierra pen tubes with Onlinelabels inkjet labels and Alumilite Plus casting. The first ones I did I didn't put the plugs in tight enough and some of the Alumilite got inside the tube, drill them out.
The second set I made sure the plugs were in tight and pulled the mold sides outward. Put them in a pressure pot at 45 PSI for 5 hours. When I took them out they were still a bit tacky but had these big bubbles at the ends, see the 2 attachments. There was no Alumilite in the tubes. The rest of the casting was perfect, really clear and no bubbles. I was able to turn a couple, they came out great.
What causes these bubbles? Is it the air that was in the tubes being pushed out? Someone thought it might be a reaction, from heat maybe, of the Alumilite Plus and the label, but I'm not so sure about that, I would think I'd have bubbles all along the label if it was that, these are just on the ends.
I recently had bubbles arise in a 100% of some RC-3 casting attempts. It was happening at the lowest point of an inverted dome-shaped mold, curing in the open. I preheated the mold and it still happened.
Then I realized that low spot in the mold was also a thin spot, so heat transfer could occur between the mold and what it was resting on. I placed the mold in a heavy Pyrex dish and pre-heated the mold in the dish. The bubbles stopped happening.