We are doing casting since 2014, and have read lots of stuff regarding high quality Casting finish. We are doing casting in a close vaccume chamber and pressure. It’s almost been year now, porosity has become a nightmare for me!! we follow almost every step to avoid it and 80% times the castings are flawless, i mean the surface finish is excellent. But there are sometimes when we get small bubbles all over one side face of rings, and it happens on only one sideface, the other side face, top and even from inside its clean. Worst part is that we only comes to know about porosity at the time of polish. To clear further about porosity, it appears like tiny cavities and sometime bubbles…when we touch that area its very rough to feel. The temperature of flask and gold we keep is as follows
18k : 1080 degree celcius for thin rings, 1050 and flask at 630 degree celcius.
20k : plus 20 degree from 18 and flask 650
22k : plus 20 from 20k and flask 680
I will share any more details if needed, please help me, i am unable to get the reason why sometimes it happens !! One thing i like to mention here that we do use old scrap metal multiple times, but i rule out that point because most of the time surface comes neat.
Waiting for the reply.