Gate Design in Injection Molds: Types and Selection Guide

The gate is one of the smallest features in an injection mold, but it plays an outsized role in determining part quality. It is the point where molten plastic enters the cavity, and its size, shape, and location influence everything from fill pattern to cosmetic appearance to residual stress in the finished part. Getting the gate right can mean the difference between a part that runs consistently across shifts and one that produces rejects no matter how much the process engineer tweaks the parameters.

Mold designers have a range of gate types to choose from, and each has its own set of tradeoffs. The selection depends on part geometry, material properties, cosmetic requirements, and production volume. There is no single best gate type. The right choice is the one that balances these factors for a specific application.

Gate Location Principles

Before discussing gate types, it is worth covering the principles that apply to any gate location. The gate should be placed in the thickest section of the part so that material flows from thick to thin. This allows the thicker sections to be packed while the thinner sections are still filling, reducing sink marks and internal voids. Placing the gate in a thin section forces the material to flow into thicker areas, which is much harder to pack effectively.

Another general rule is to avoid placing the gate near areas that will be subjected to high stress in service. The gate area has a distinct molecular orientation and sometimes a visible witness mark that can act as a stress concentration point. Gates should also be positioned so that the flow front does not split around a core or insert and then rejoin, because that creates a weld line. If a weld line is unavoidable, the gate should be placed to push the weld line to a low-stress or low-visibility area of the part.

For multi-cavity molds, gate location must be identical across all cavities to ensure balanced filling. Even small differences in gate position can cause one cavity to fill before the others, leading to overpacking in some cavities and short shots in others. Balanced runner systems with identical gate geometry are essential for consistent parts from every cavity.

Edge Gate

The edge gate is the most common and straightforward gate type. It is located at the parting line on the edge of the part. The gate is essentially a small rectangular opening that connects the runner to the cavity. Edge gates are simple to machine and easy to modify if adjustments are needed during mold trials.

The main drawback is that the gate leaves a small tab on the part where it was sheared off. For parts where appearance matters, this may require a secondary trimming operation. Edge gates work well for two-plate molds and are suitable for a wide range of materials and part sizes. Gate dimensions typically range from one to two millimeters deep and two to six millimeters wide, depending on part thickness and material viscosity.

Pinpoint Gate

Pinpoint gates are used in three-plate molds and cold runner systems where the gate needs to be automatically separated from the part during ejection. The gate is very small, typically 0.5 to 1.5 millimeters in diameter, which means it leaves a minimal vestige on the part surface. This makes pinpoint gates a good choice for cosmetic parts where gate removal marks need to be as inconspicuous as possible.

Because the gate is so small, pinpoint gates generate high shear rates as the material passes through the constriction. This can cause degradation in shear-sensitive materials like PVC or some nylons. The small gate size also means higher injection pressure is required to fill the cavity, which may be a limitation on machines with lower pressure capacity. The gate is usually positioned at the top of the part and feeds through a runner system in a separate plate.

Fan Gate

Fan gates gradually widen from the runner into a thin, wide opening that spreads the material across the cavity entrance. They are used when the designer wants to reduce flow marks and improve material distribution across a wide part. The fan shape reduces the injection speed at the gate entrance, which lowers shear stress and minimizes gate blush and flow marks.

Fan gates are commonly used for large flat parts like panels and covers. The gate thickness should be about seventy to eighty percent of the part wall thickness at the entrance. The width can be as wide as the part itself. One drawback is that degating requires a secondary operation, usually a trimming jig or hand cutting.

Tab Gate

A tab gate is essentially an edge gate with a small tab or land added to the part. The tab serves as a sacrificial area where high shear stresses are concentrated, preventing them from affecting the main part. The tab is trimmed off after molding. This gate type is useful when the material is sensitive to shear or when the fill pattern near the gate could cause cosmetic defects on the visible surface.

Tab gates are common in applications where the part will be painted or plated, because the surface quality near the gate area is protected by the tab. The additional material consumption and trimming step increase part cost, so this gate type is reserved for applications that genuinely need it.

Submarine Gate

Submarine gates, also called tunnel gates, are located below the parting line and feed the cavity through a small tunnel. During ejection, the gate automatically shears off, eliminating the need for a separate degating operation. This makes submarine gates ideal for fully automatic molding cycles.

The gate is machined into the mold steel as a small angled tunnel. The tunnel diameter is typically 0.8 to 2.0 millimeters, and the angle is usually thirty to forty-five degrees. The gate wears over time and may need periodic dressing. Submarine gates work best with flexible materials that can deform during ejection without breaking.

Gate Vestige and De-Gating

The size of the gate vestige left on the part depends on the gate type and the material. For appearance parts, the gate vestige must be minimized or hidden in a non-critical area. Sharp gates produce cleaner breaks than round gates. Gate hardness also affects break quality, so gate steel selection matters.

For manual degating, the operator uses cutters or a trimming fixture. For automatic degating, the gate breaks during ejection as the part is pushed off the runner. Semi-automatic degating uses a robot to separate the gate from the part after the mold opens but before the part is fully ejected.

Conclusion

Gate design deserves careful attention during the mold design phase because it is much easier to cut a gate larger or reposition it before the mold is built than after. The cost of modifying a gate in a hardened tool is significant, and the schedule impact can delay production by weeks. Taking the time to select the right gate type and location during the design review pays off in faster mold trials and more consistent production runs.

Gate freeze time is another important consideration. The gate must freeze before the screw retracts for the next shot; otherwise, material can flow back into the runner, reducing packing pressure and causing sink marks. Larger gates take longer to freeze, which means longer cooling times. For thin-wall parts, small gates that freeze quickly allow faster cycles. For thick-wall parts that need extended packing time, larger gates that stay open longer are necessary to maintain packing pressure.

Material viscosity directly affects gate sizing. High-viscosity materials like polycarbonate need larger gates than low-viscosity materials like polypropylene. A gate that works well for one material may cause flow marks or short shots in another. When in doubt, start with the gate at the upper end of the recommended size range and reduce it during mold trials if needed. Cutting a gate smaller is always easier than making it bigger.

Shear heating at the gate can actually help fill thin sections by reducing the viscosity of the material as it passes through the constriction. The temperature rise at the gate can be ten to twenty degrees Celsius depending on the flow rate and gate geometry. This effect is sometimes exploited deliberately in thin-wall molding applications where the material would otherwise have difficulty filling the cavity.

Gate sealing is the final check during mold validation. The gate seal test involves weighing parts produced at different holding times. When the part weight stops increasing with longer holding times, the gate has sealed. This determines the minimum holding time needed. If the gate seal time is longer than the cooling time, the gate is too large and needs to be reduced. If the gate seals too early, the part may have sink marks or insufficient packing.