Every caster has seen his or her fair share of investment inclusions. You’ll be polishing a casting when suddenly a whole will pop up with a bit of investment inside. You then have to dig out the investment and fill in the hole with wire, using a laser welder. It’s time consuming – not to mention an inferior product.
During some recent experiments that I was conducting on burnouts, I captured an investment inclusion in the making. It occurred when I was cutting platinum flasks down the middle to study various stages of burnout.
The photo here is a cutaway view of a flask, showing the investment after the wax has been mostly removed with a partial burnout cycle. You can see at the openings – the junctions of the feed sprues and the main sprue – tiny pieces of investment hanging off the sides. When a centrifugal casting machine shoots molten metal into that hole at high velocity, the investment will break off and become a problematic inclusion.
Avoiding the number one cause of this problem is a simple fix: Take care to seal and create a fillet between the feed sprues and the main sprue in your casting tree. Spending a little more time on the front end will avoid nasty surprises on the back end.