gate

In sand and permanent mould casting, the gate is the deliberate channel cut or formed in the mould that directs molten metal from the pouring cup (or sprue) down into the casting cavity itself. It is a controlled passageway, not an accident of the mould design, and its dimensions and angle directly determine how fast the metal fills and how cleanly it enters the cavity.

Gate geometry matters enormously. A gate that is too narrow causes the metal to cool and solidify before the mould is full, leaving shrinkage voids in the casting. A gate that is too wide allows the metal to enter with such velocity that it splashes, traps gases, or erodes the mould walls. The cross-sectional area is usually sized to match the pouring temperature and the thickness of the thinnest section of the casting. Gate length and angle also influence metal velocity and turbulence. In high-volume production, multiple gates are sometimes used on a single cavity to ensure even fill.

Different casting processes employ different gate strategies. In sand casting, the gate is often formed by removing a core or by packing sand to create a narrow restriction at the point where the sprue meets the cavity. In permanent mould and die casting, the gate is machined or cast as part of the die itself. In investment casting, the gate is part of the wax pattern and burns away with the mould. Squeeze casting and low-pressure casting may use gates sized and positioned to exploit gravity or applied pressure for directional solidification.

Gate design and defects

Poor gate design is responsible for a significant share of casting defects. Gates positioned opposite a thin section cause the metal to impinge on the far wall and create turbulent zones that trap oxide films and gas. Gates positioned too low in a cavity allow metal to pile up before spreading laterally, again trapping defects. Casting engineers use flow simulation software and trial pouring to optimise gate location and section. After casting, the gate is removed by cutting, grinding, or chipping, and the scar must be dressed smooth if appearance or stress concentration matters.

The term ingate is often used synonymously, particularly in older foundry practice, emphasising the idea of metal entering the cavity. The term tedge appears in some historical texts and refers to the small fins or excess metal left at the gate line. Modern foundries distinguish between the sprue (the main vertical channel) and the gate (the final restriction leading into the cavity), making the distinction clearer for both design and troubleshooting.