Gating

The Gating is one of the most critical steps in the process to achieve quality castings.  There are a number of considerations that must be addressed for a successful casting.  First, let’s look at a diagram of a simple gating assembly

The first part of the gating system is the pouring cup. This is where the molten bronze first enters the mold. It needs to be large enough to catch the bronze and force metal into the mold. The next part is the Sprue. I use a tapered, square sprue to minimize turbulence and created more pressure at the bottom of the mold for a complete fill. The sprue carries the metal downward and feeds metal to the Runners that are connected to the part being cast. Runners are smaller than the sprue and are located about every 6" for good metal distribution. The Vents are for the air that is being expelled as the metal fills the mold. Vents also serve as a way to remove the wax from the mold.

Sometimes more than one part can be attached to a common gating system. This is called a tree. There are four parts on this tree. Generally waxes are cut into pieces before being cast. This is done so the interior of the part or core can be coated and dried during the shell building process. After casting the parts are welded back together

These are the basic elements of a cup/sprue system. I make my own cups and sprues. The metal object in the foreground is EMT tubing cut to length with a lag screw welded on the end. I insert it into the sprue to support the wax assembly during fabrication and mold making.

This is the cup/sprue system assembled. Note the tubing projects above the cup and serves as a handle.

Here we see a section of the wax for Birdform 1, some 1" diameter wax rods for runners and the cup sprue assembly. I weighed the wax section to estimate the amount of bronze needed to fill this mold. The multiplier is approximately 9. This part weighed 4 pounds so it will weigh 36 pounds not including the gating.

The wax rods in the openings are to support the wax so it will not distort. They will also serve as runners. Note the uneven erratic cut lines. This is done so the parts will fit back together like a jigsaw puzzle and have perfect registration when they are reassembled following casting.

The gating is in place. This part is large enough to gate on the inside. This reduces bronze chasing later on. Note the multiple runners and a generous number of vents. Some runners are angled upward so metal will rise inside the part and not blast in. This reduces surface defects caused by metal erosion. Vents are placed on the high points so air can be released and wax can be completely evacuated during de-waxing.

A picture of the large base gated and ready for ceramic shell investing.

Here are all the wax forms gated and moved to the shell room. The EMT rods fit into holes on my drying rack to support the assemblies. Next we will make the ceramic shell molds.

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