If you think back to all the discussions you have had with your foundry, I’m sure the subject of the gating system has probably come up. Perhaps the conversation could have sounded like this: “The last time we ran your casting we encountered a higher-than-normal scrap rate, but after we made a few gating changes, the problem was resolved.” That’s good news for you, but in the back of your mind, you are probably wondering what a gating system is and what does it have to do with my casting?

Let’s begin with a very high-level definition of the gating system: It is the network of channels that allow molten metal to be poured into a mold and fill the mold cavity in a precisely-engineered manner to produce a sound and defect-free casting. Easy to understand, right? But there is much more involved in a gating system, so let’s take a deeper dive into the detail to help you gain a better understanding.

Getting Metal into the Mold 

Molten metal is poured into a mold via the sprue (or down sprue), generally a tapered round tube running from the top of the mold to the parting line. The area at the base of the sprue is where the runner bar system begins. The runner bar system is a collection of square or rectangular channels that distribute the molten metal into the casting cavity. The area where the runner bar attaches to the casting cavity is called the in-gate (or gate). 

Gating systems are characterized as pressurized or non-pressurized, depending on the location of the choke. A choke controls the velocity at which the metal can enter the mold, and is defined as the area of the gating system that contains the smallest cross-sectional area. If the choke is located at the in-gate or riser connection, then the system is considered pressurized. Chokes located in the runner bar at or near the base of the sprue are considered non-pressurized. In my experience, a non-pressurized system provides better metal flow and is less likely to cause turbulence or sand erosion.

Gating is almost always permanently mounted to the pattern to eliminate any variation in this engineered system that could cause defects to occur. In the production of some large castings, ceramic tile gating is used, and these are assembled and attached to the mold cavity separately.

Making Sure the Metal Is Clean  

Properly designed gating systems can help eliminate the formation of various defects such as slag, dross, and sand erosion in the casting. For example, the sprue is tapered to follow the natural shape the metal takes when it leaves the pouring ladle, which helps eliminate air from being aspirated into the metal stream. 

Near the base of the sprue, the foundry will often place a ceramic filter in the runner bar to help ensure metal cleanliness by filtering out any slag particles that may have entered during pouring. Ceramic filters also reduce turbulence in the metal––which has the tendency to form slag and dross––by creating what is referred to as a laminar flow. Slag also floats on liquid metal, so having enough runner bar length will help trap slag particles before they enter the casting cavity.  

Making Sure the Casting Is Sound  

Many casting designs are structural in nature and require that any internal porosity is limited or eliminated. Since liquid metal will shrink during solidification, the foundry will address the shrinkage by adding risers to the casting in these critical areas. A riser is a specifically-engineered reservoir of liquid metal that provides feed metal to counteract volumetric shrinkage as the casting solidifies. 

Most often, the flow of metal goes from the runner bar into the riser in order to provide the hottest metal possible to the riser. Risers can be referred to as:

1) Side risers––located alongside the casting on the parting line.
2) Top risers––located on the top of the casting when sections requiring feed metal can’t be reached by side risers. 
3) Open risers––the top of the riser is open to the top of the mold surface.
4) Or cold risers––not attached directly to the runner bar.

The humble gating system is vital in the production of a defect-free casting. Correctly designed systems not only distribute metal but also reduce casting stress and strain, minimize defects, and improve the physical properties. The next time you visit your casting supplier, be sure to check out the gating system.