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Lost Foam Casting: a Crankcase in Zinc Alloy

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vtsteam:
Here are some pictures from today's casting:

The shake out. I was worried I hadn't got complete fill of the molds, but was happy to see in the shakeout that both molds filled -- the brown discoloration of the plaster was a good sign. The split pipe pouring basin leaked so I didn't have as high head pressure as I'd hoped. There was no flare, and only what looked like two small bubbles during the pour.

vtsteam:
There was a small leak of metal inside the crankcase. Unfortunately this area caused shrink problems on the other side of the casting:

vtsteam:
This is what it looked like after partial cleaning. You can see where metal flows did not fuse completely. This was because the metal was too cool and also had not flashed off the foam polystyrene. The tan-bluish area on the coverplate is investment and polystyrene foam residue mixed -- it inhibited metal fusing in some areas. It's also tough to remove. The relatively thin casting also makes it hard for the metal to retain heat while traveling and melting foam out.

I'm not sure if I can pour hotter than I did this time with zinc diecast material -- normal pouring temp is only 750F -- aluminum can pour fairly easily around 1100F or more and is a material which has a lot of retained heat. It always flash burned  the foam when I used it for lost foam casting in the past.

vtsteam:
I made up a new pattern to try again today. While waiting for the investment to dry, I played around with the bad casting just to see what the final case should look like.

Some face milling and filing made it look quite a bit better -- it's thick enough that I probably could get rid of almost all the surface flaws if I kept going, but this was really a casting exercise, and I want to get a really good casting before I settle down and build an engine.

I'm tempted now to try diecast zinc one more time, tomorrow, instead of aluminum, just to confirm that it won't flash the poly. Maybe I should have used two sprue/risers for faster fill, more heat, and some air -- so I've done that for the next set of patterns.

vtsteam:
Well I went with a second try with zinc alloy lost foam today, instead of aluminum. I just had to see if maybe pouring hotter and a couple of sprues instead of one to get the metal to fill faster, and inverting the pattern to get the good face down would work. I also added a lot of little vents with a needle, hoping to let out any gasses. I sealed the split line in the hinged pouring cup so it wouldn't leak, and maintain head.

I poured at 452C which was pretty hot, I figure, for zinc alloy. And the bubbling off of the polystyrene looked much better -- more like it does with aluminum, though it still didn't flare. The pouring cup stayed full, and so added head pressure and fed the casting as it cooled. Break out looked better than it had yesterday:

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