What is gates and risers in casting

In the casting process, the gate and riser are two key functional structures in mold design, respectively undertaking the core roles of "guiding metal flow" and "compensating for metal shrinkage". They directly affect the forming quality of castings (whether the mold is fully filled and whether there are defects such as shrinkage cavities). The specific functions and design points of both are detailed as follows:

I. Functions of the Gate

The gate is the "channel" through which molten metal enters the mold cavity from the gating system (ladle, sprue, etc.), acting as a "bridge" connecting the molten metal and the casting. Its core role is to control the flow state of the molten metal to ensure the complete forming of the casting. Specific functions include:

1、Guiding the smooth filling of molten metal into the cavity

The position, shape, and size of the gate determine the direction and speed of the molten metal entering the cavity. For example:

1.For thin-walled castings, the gate should be designed to be wide and short to allow the molten metal to quickly fill the cavity (avoiding "incomplete filling" due to rapid cooling);

2.For complex cavities (such as parts with internal cavities), gates need to be distributed at multiple positions to allow the molten metal to fill uniformly from different directions, reducing eddy currents and gas entrainment.

2、Preventing impurities from entering the cavity

The gate is often matched with a "slag collector" (such as an expanded groove at the end of the gate). By utilizing the inertia of the molten metal flow, slag, oxide scales, and other impurities are retained in the gate or slag collector, preventing them from entering the casting and causing "slag inclusion" defects.

3、Regulating cavity pressure and exhaust

When the molten metal fills the cavity, the air in the cavity is discharged. A reasonable gate design (such as filling from the bottom of the cavity) allows air to be discharged orderly from the exhaust holes, reducing "gas hole" defects. Meanwhile, the cross-sectional area of the gate can control the pressure during filling to avoid damaging the mold (especially sand molds) due to excessive pressure.

4、Assisting in controlling the solidification sequence

The solidification time of the gate can be adjusted through size design: if it is desired for the gate to solidify last, its cross-sectional area can be appropriately increased (slow heat dissipation), allowing it to provide a small amount of feeding even after the casting solidifies; if it is necessary to solidify quickly to cut off feeding (avoiding the transfer of shrinkage cavities to thick parts of the casting), the gate size can be reduced.

II. Functions of the Riser

The riser is a "reservoir" in the mold located above the casting or at thick and large parts. The molten metal stored inside is mainly used to compensate for the volume shrinkage of the casting during solidification, and it is a key structure to prevent shrinkage cavities and porosity (hole-like defects inside the casting). Specific functions include:

1、Compensating for metal shrinkage

Molten metal shrinks in volume during solidification due to temperature reduction (e.g., the shrinkage rate of cast iron is about 3%-5%, and that of cast steel is about 7%-8%). If the shrinkage cannot be compensated, shrinkage cavities will form in thick and large parts of the casting (such as flanges and the center of wheel hubs). The molten metal in the riser, due to its higher position (or slower heat dissipation), fills first and solidifies later than the casting. It continuously supplements metal to the casting during its solidification and shrinkage, filling the shrinkage space.

2、Collecting slag and exhausting gas

Risers are usually set at the highest point of the casting. When the molten metal fills the mold, gas and floating slag in the cavity will rise with the molten metal and accumulate in the riser, reducing gas holes and slag inclusions in the casting.

3、Indicating casting quality

The solidification state of the riser can indirectly reflect the feeding effect of the casting: if obvious shrinkage cavities appear in the riser, it indicates insufficient feeding, and the casting may have defects; if the riser is dense after solidification, the casting is fully fed with better quality.

In summary, the gate is the "guide of molten metal", ensuring the casting is complete and free of impurities; the riser is the "supplier of molten metal", ensuring the casting is dense and free of shrinkage cavities. Although both are "auxiliary structures" of the casting, they are indispensable in the casting process. Their design needs to be comprehensively optimized based on the casting shape, material, and size (e.g., large risers for thick castings, optimized gate positions for thin-walled castings) to finally obtain qualified castings.