Metal Casting Zone

Hi,guys. I'm new to this casting lark. I've had reasonable success with smaller alli, bronze, brass & iron castings, but I'm having real trouble with this one. It's a loco cylinder, cast in gunmetal. Weighs about 6 kilos with sprues etc.



It looks OK from the outside but inside the metal has shrunk away from the core:



Anyone got any suggestions?

Ian

I haven't cast anything that large but I think you need a shrink bob. A large cavity that will fill with bronze and stay molten while the part sets up. As the part cools it draws metal in from the bob and keeps the pattern full. There's a good thread on this if I can find it.

I thought the spue & the feeder on the top would do that. Looking at it again, I may have poured from the wrong end (the right hand side in the picture) while the largest bulk of metal is on the other side.

The sprue and the riser can act as a shrink bob. You just need them to be fatter and throw some sand on after the pour to hold the heat.
Another thing you may need to look at is if you core was completely dry and did you poke vent hoses in the sand so gases wouldn't build up around the core. That might be what caused the shortness. Just an idea.

The core was dry. It had been baked for several hours. It also had a 3/16 hole though for venting. The sand on top as an insulator is a good idea. I'll have another go in a few days.

Sounds like you were set. Maybe try to put the sprue in the heavy side and make it a little bigger so it can feed. Good luck with it.

Sorted!
thinking about the problem, the area around the core would be the last to solidify, so I needed to get the metal into the mould as close as possible to the core. I put the sprue at one end of the core & the riser at the other, feeding over the top of the core. The next thing is that the metal in the sprue & riser will still cool quicker than in the centre of the mould, as they are surrounded by sand. The answer was insulating sleeves! A piece of 3mm boiler lagging rolled into a double thickness tube inserted into the spue & riser with a pad at the bottom of each. I was amazed at how long the metal remainded liquid.
Ian

excellent! any chance of a pic???? did you remelt the cylinder??? the pics looked to me as if the brass itself was the cause. did the metal have "black threads" through it??? by remelting the cylinder the brass would have changed it specs.

Yes, I did re-melt the bronze (many times!) I don't think gunmetal changes as much as brass (very little zinc to burn out). The metal was clean. When I cut thought the faulty castings, the metal was fine, just shrinkage. I'm told that a foundry would use cast iron chill plates in the bore to force the centre to solidify first.
here's the insulated sprue & riser.

& here's the finished casting.