Gating and Risering
Gates and risers are part of the system to provide molten metal to the part you're casting. This page will provide a starting point for you, it is certainly not the definitive collection on the subject. There are whole books devoted to this subject. On my LINKS page there are some book titles for those who may want more information. That being said, it is possible to make good castings without following, or even knowing these rules. When I first started casting I just cut my gates and runners with an old kitchen spoon so that they looked right to me. If you remember to gate to the largest part of your casting and round all the corners in the system you will likely have good luck with your castings, particularly simple ones. If you have a casting that is giving you problems with shrinkage defects, misruns, etc. you may want to have a look at the information below to see if you can try any of it to help solve the problem. If your pattern is on a matchplate, once you find a gating system that works well you can build that onto the matchplate as well so it is molded at the same time as your mold cavity.
First some definitions will make it easier to understand this page.
The goals for the gating system are;
The design of the pouring basin and sprue can affect turbulence. For best results you want to design your pouring basin and sprue so that you can keep the sprue full of molten metal throughout your pour. A sprue tapered to a smaller size at its bottom will create a choke which will help keep the sprue full of molten metal. If you don't use a tapered sprue you can put a choke in when you are making the runners, you will want to have the choke as close to the bottom of the sprue as possible. The choke will also increase the speed of the molten metal, which is undesirable. To address this problem you can create an enlarged area at the bottom of the sprue, called a sprue base. This decreases the speed of the molten metal. There are two basic types of sprue bases, enlargement and well.
The general rules of thumb for enlargement bases are;
The general rules of thumb for well bases are;
The bottom of the sprue base should be flat, not rounded like a bowl. If it's it will cause turbulence in the metal.
One of the most important things to remember in your runners and gates is to avoid sharp corners. Any changes in direction or cross sectional area should make use of rounded corners. Also make sure the runners and gates are well rammed and smooth. This will help avoid sand erosion and turbulence.
To ensure that the metal is not flowing too fast in the runners the rule of thumb is that the cross sectional area of the runners should be greater than the area of the choke. The walls of the runners should be as smooth as possible to avoid causing turbulence. The runners should be filled with metal before the gates are, one way to ensure this happens is to put the runners in the drag and the gates in the cope. If you need to have a choke in the runner to restrict flow it should be at least 6" from the first gate.
The cross sectional area of the runners should decrease as the gates come off them to keep the the same gating ratio. A good gating ration for aluminum is 1:4:4. The 1 is for the cross sectional area of the choke. The first 4 is the total cross sectional area of the runners (measured after the choke but before the first gate) and the final 4 is total cross sectional area of the gates. For example, say you have a tapered sprue with an exit area of 0.5 sq. in., two runners with 2 gates off of each runner. The total runner area should be 2 sq. in so each runner would be 1 sq. in. The total gate area should be 2 sq. in., there are 4 gates so each gate would have an area of 0.5 sq. in. The gate calculation only works this way if there are an equal number of gates on each runner. If that is not the case divide the area of the runner by the number of gates on that runner to get the area of each gate.
The area of the runners should be reduced just after a gate by an amount equal to the area of that gate. This will insure that each gate in the system will have the same flow of metal, even if it's farther from the sprue. The first bit of metal poured is most likely to be contaminated by air and sand entrapment. To prevent this metal from going into the mold cavity you use a runner extension. That first bit of metal will flow to the end of this dead end and be trapped there, where it can't harm the piece you're trying to cast.The runner extension will have the same area as that of the last gate on that runner.
Risers are important to ensure a flow of molten metal to the part being cast as it's starting to solidify. Without a riser heavier parts of the casting will have shrinkage defects, either on the surface or internally.
As molten metal solidifies it shrinks. If it does not have a source of more molten metal to feed it as it shrinks you will get defects in your casting. A risers purpose is to provide that extra molten metal. Basically a riser is a vertical portion of the gating system, similar to a straight sprue, that stores the molten metal until it is needed by the casting. This means the metal in the riser must stay liquid longer than the metal in the part being cast.
A riser may be required for every hot spot in your cast part. In other words the part of the casting that solidifies last, usually an are with a larger volume of metal. The risers can either be attached to the top or the side of a part. They may also be blind risers. A blind riser is completely contained in the mold, not exposed to the air. Since it's not open to the air this type of riser cools slower and thus will stay liquid longer. It's important that no matter where it's located the gate that connects the riser to the casting is not too small and as short as possible or else the gate will solidify too soon and prevent the metal in the riser from reaching the casting, try and keep the length to 1/2 the diameter of the riser.
Risers may be upstream from the casting in the runner/gate system. In this case the metal must flow through the riser prior to reaching the casting and after the pour is completed the metal in the riser will be hotter than the metal in the casting. They may also be placed downstream, after the casting. This means the metal flows through the casting to get to the riser so the metal in the riser will be cooler than the metal in the casting. This could cause the metal in the casting to feed the riser as it cools, definitely not desired.
You want the metal in the riser to solidify last, after the part being cast. Since the more surface area something has the faster it cools you want to minimize the surface area of the riser for a given volume. Because of this the optimum shape for a riser would be a sphere, however that is not an easy shape to mold. The next best alternative is a cylinder, which is easy to make. Ideally the cylinders height should be some where between 1/2 and 1 1/2 times the diameter. If possible the bottom, and top if it's a blind riser, should be spherical, or bowl shaped. This will also help the metal stay molten longer.