Hello
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.
Mukund, fir me, usually the first place to look is spruing. Inadequate, or too small sprues can cause havok. The problem with a fine surface but porosity just below the surface can be when metalinitially enters the mold cavity and freezes, and sprues start to solidify before the main cavity fully fills, so you get shrinkage porosity in the body of the metal. Try raising flask temperature 20 - 50 degrees and see if ghat helps. But then you say your metal cannot be the problem because 80% of the time its fine. That kind if problem solving just with logic but no real testing can easily hide problems. Even with 18 Or 20K alloys, every time you melt it, some alloy components burn off, especially grain refiners and antioxidants. But even copper to silver ratios can change. Plus, dissolved gasses and oxides dont just magically disappear. Try only reusing metal once, and with new metal the majority of your casting. Part of your problem could have been hiding behind that assumption that it couldn’t be that…
Thanks Peterrowe,
Please see that the porosity is random, there are many times it occurs with fresh metal too. Thats why we have rules out that point.
I am completely confused as i am unable to find any solid reason behind it. Please check that the porosity is so bad that the surface becomes very rough to touch, or sometimes very tiny holes sort distributed randomly on surface.
About sprue , we use thick medium length.
Regards
The shape of the porosities will help you figure it out. Spherical is often gas. Jagged can be investment breakdown, contamination inclusion or hot tearing. Spongy microporosity can be “shrink spot porosity “ which is at times a result of improper sprucing, too high a flask for temp (at casting time) or a cooling problem.
I will share the macro clear pic of the surface tomorrow. For me it seems like Shrink spot …still you let me know. I am unable to understand the cooling problem, please elaborate.
not sure about your higher karats, I usually cast 14K, Metal issue does not seem to be the problem after reading the discussion.
I would suggest to try lowering your flask temps, the only thing I cast higher than 1100F is platinum. On heavier pieces I will go as low as 1000 F
