A couple of miscellaneous foundry issues...

[Chet]

1. I've successfully poured nearly a dozen 3/8 x 10" diameter disks in aluminum. But in my latest pour, the molten aluminum lifted up the sand on the top of the foam pattern rather than back fill the pouring cup with the excess as usual. The sand is simple dry sand, no green mix. I think I've read an explanation about this problem but I can't remember what it is...

2. I try to monitor my propane use with weighing in before and after each session. I was down to 22 pounds (gas & tank) and figured I could easily cover the 2 pounds I'd need for my pour. Not so... As I approached the 20# weight, my pressure dropped radically from 12 lbs to about 4 lbs pressure. Is there a rule of thumb for determining at what poundage of gas you might lose USEFUL pressure (eg 12 psi)?

3. For this melt I used larger pieces from earlier sessions instead of my usual pieces of old propane grills. I expected the same melt times, but not so. Using the larger chunks resulted in a nearly 30% increase in melt time. In retrospect I guess it's not that surprising, but I sure wasn't expecting it...

[dly31]

As long as there is any liquid propane in the tank, the pressure is pretty constant and varies only with the temperature of the tank. Once the last of the liquid boils into vapor, the pressure begins to drop quickly, as you found. There are some temperature indicators that stick to the outside of the tank and can give you a rough idea of the contents but the very best way is just to monitor the weight of the tank. A bathroom scale works pretty well for smaller tanks.

Don Young

[steamin10]

As you have found the pressure can drop severely when you get down to the last third or so, and have a heavy draw for your burner. As the tank temperature drops so can your draw pressure, and your burner can starve putting out less btu's. One answer is a bigger tank, or two tanks siamesed on a 'T' line. Another answer is to float the emptying tank in a tub of warm water, as water will add heat about 7 times faster than air, eliminating freezup or frosting of the tank, keeping pressure higher, to be regulated.

I use a 100lb bottle. It is good to the last 4 inches or so before I get frosting.

As for the first problem of bloating the mold, there are a coupla of ways around this. As the metal fills the cavity, it will cool at the extreme edges first, and stay liquidous more at the center. As the edges get slushy and start to cool, new metal rams into the center, and like blowing up a water bottle, can move the unstable sand. If you are using sugar sand, you are lucky the foam doesn't callapse before the fill, allowing sand to sink into the mold space. Top weights and top boards are used to stop molds from floating, a common problem in Iron casting, as the iron weighs so much more than the sand, and can lift the cope or parts of it.

[steamin10]

At Blaw Knox, I witnessed a blunder similar to yours , only bigger.

A mill bearing housing was being poured, and one of the cores were set late. It was a rebar reinforced slug of sand about 8 feet across that looked like a pink marshmellow, with the core wash painted on. The foreman put a scrap tank hull on it, about 22K, or 11 tons. I told him he had to bar it down, and he said "no time!" As the pour reached about 2 feet from the top, the marshmallow began to float up, because the metal it displaced was heavier, and it just began to float out. It was only a 50 ton casting or so, but it was lost to the movement of the corepiece.

Foundry bars, in this case are 20 feet long, with 4 crown ends, and lightening holes along its length. with one or two of these, foundry bar calmps and wedges could be used to secure anything to the floor mold clamp rings, in the floor, or drag frame parts. Then no floating.

In Brass fouding, steel 1/2 inch plates are often used, called top weights, to keep from breaking thin cope tops and losing chunks of mold floating up, or even floating the cope up out of position, allowing it to flash or runout at the mating line. The problem is proportion, and weights are usually used when the part is half or more of the exposed flask area, as a rule of thumb.