Casting Problems

Sand is too wet. If the sand is too wet it will bubble in the pouring sprue and trap gas inside the casting. Torch molds before you close, if you suspect too much moisture.


Sand is too dry. If the sand is too dry it will get more bumpy when you try to smooth it with a spoon or trowel. When you blow the loose pieces, the mold gets worse. Dump the mold back into the sand container. Sprinkle with 2 cups of water per 25 lbs of sand, mix and let sit for about 1 hour to let moisture even out.



Metal is too cold. With aluminum, metal will be at pouring temp 3 minutes after the last piece has turned to liquid. When melting brass, 5 minutes.

Metal is too hot. Aluminum absorbs gas as soon as it turns liquid, do not overheat aluminum and it will not have a chance to absorb any gas. Brass and bronze start to loose their low melting elements (lead, tin, etc.) from the alloy after they turn molten, they start to smoke when this happens. Smoking brass or bronze indicates it is close to or at pouring temperature.


 The most common casting problems involve thickness of the casting and thickness of the gating system.

Castings of aluminum, brass and bronze should be 3/16" thick or more. If your pattern is too thin, you can tape cardboard to the back for a quick fix. Castings can be made 1/8" thick, but can be no larger than 4"x 4".

When making your gating system, the runner must be at least 3/4" thick and 3/4" wide.

1. Metal flows down the sprue
2. Into the runner
3. Across the gate
4.Fills the casting
5. Up the pop-up when full

This casting is 11" long and 2.5" wide.
Note distance from runner to casting (gate) is only about 1/2".

Deep 3/4" runner allows the metal to stay molten longer. This casting is 1/8" thick.

Runner is almost as long as the casting.
A pop-up vent is located near the end of the runner, but not at the end. The pop-up tells you when the mold is full.

Gate is the same thickness as the casting to allow metal to flow in as fast as possible. Gate to runner (square edge) must also be rounded to prevent erosion, use a molding spoon or your finger.

No vents are needed on the casting.

Even with small castings, a thick 3/4"x 3/4"  runner will keep the metal hot until it reaches the casting.

Gating is wide, deep and smooth.