Mold Gate Type and Dimension Design

Mold Gate Type and Dimension Design

The gate, also known as the feed inlet, is a short, narrow channel connecting the runner to the mold cavity (except for direct gates) and is a critical part of the gating system. Its main functions are:

1. After the cavity is filled, the melt solidifies first at the gate, preventing backflow.

2. It facilitates the removal of the solidified material from the gating system. The gate's cross-sectional area is typically 0.03 to 0.09 times that of the runner. The gate length is usually 0.5 mm to 2 mm; specific dimensions are generally determined by experience, starting with the lower limit and then adjusted during mold trials.

When the plastic melt passes through the gate, the shear rate increases, intensifying internal friction, raising the melt temperature, and reducing viscosity. This improves flowability and benefits mold filling. However, an excessively small gate increases pressure loss, accelerates solidification, hinders packing, and may even cause jetting, affecting part quality.

Types of gates include:

(1) Direct Gate (Sprue Gate) Also known as a center gate, it offers low flow resistance and fast filling, commonly used in single-cavity molds for large, deep parts. It suits various plastics, especially those with high viscosity and poor flowability, like PC and PSF. When molding shallow, flat parts, direct gates can cause warping and distortion. Gate removal is inconvenient, leaving a noticeable mark. Heat concentration at the gate can delay cavity sealing, increase internal stress, and lead to cracking. Therefore, the gate should be as small as possible. For thin-walled parts, the gate root diameter should not exceed twice the part wall thickness.

(2) Edge Gate (Side Gate) Also known as a side gate, its cross-section is rectangular, typically located on the parting line, feeding from the part's side. Its position can be chosen flexibly, especially suitable for multi-cavity molds. Generally, width B = 1.5–5 mm, thickness h = 0.5–2 mm (or 1/3–2/3 of the part wall thickness), and length L = 0.7–2 mm.

(3) Pin Gate (Point Gate) A pin gate, or pinpoint gate, is a very small gate. The plastic melt experiences a high shear rate when passing through it. It is widely used for various shell-type parts. During mold opening, the gate can break off automatically. To prevent damage to the part when the pin gate breaks, a small boss can be designed at the gate connection. Due to its small cross-sectional area, the pin gate solidifies quickly, making packing difficult. It is not suitable for parts with thick walls.

(4) Submarine Gate (Tunnel Gate) Also known as a shear gate, evolved from the pin gate. While pin gates are used in three-plate molds, submarine gates are used in two-plate molds, simplifying the mold structure. The submarine gate is placed in a concealed area on the inside or outside of the part, preserving its aesthetic appearance. The gate is severed during part ejection but requires strong ejection force and is not suitable for tough, resilient plastics.

Common empirical values for gate dimensions

Typical dimensions for direct gates for common plastics

Recommended dimensions for edge gates and pin gates