I changed my design of the chimney cap and tried casting it repeatedly. The new design has ears for 2 springs instead of one as a hinge. Mostly just to make it easier to build although probably not as nice as one attached with a hinge.
The pattern has a deeper gasket pocket which also made the part thinner. I was having trouble getting the mold to fill before cooling. I knew what the problem was but was trying to avoid another riser or vent.
This was my last attempt. Hopefully I can clean this one up good enough to use. My sand might not be the best mixture or I’m just not good enough to get perfect molds. I might need more clay but I had too much clay in my green sand years ago when I was trying to cast metal and it made lumps of sand that were like bricks every time I made a mold and needed forced through a screen to be usable again. My mixture now breaks up fairly easily by hand.