Gate Design and Gate Types in Injection Molding: A Buyer's Guide

The gate is the restricted opening where molten plastic enters the cavity. Because it is the origin point of flow, the gate determines how the entire cavity fills. That filling pattern dictates where weld lines form, where air traps occur, how evenly the part packs out, and where the physical gate mark lands.

A gate placed solely for tooling convenience can engineer a cosmetic or dimensional defect into the part.

Gate Families and Tooling Implications

Gate type, count, and location are chosen to balance fill requirements, cosmetic constraints, and tooling cost. Buyers do not need to design the gate, but they should understand the tooling families suppliers propose:

Edge gates: Feed the part from its edge along the parting line. They are simple, robust, and inexpensive to tool, but they leave a visible nub that usually requires a manual trimming operation.

Sub / Tunnel gates: Feed from below the parting line and are designed to shear off automatically as the part ejects. This avoids manual trimming but restricts the geometry of the tool and limits the gate size.

Pin / Point gates: Feed the part directly on its face through a small point. Used in three-plate and hot-runner tools, they leave a small vestige on the surface and are excellent for feeding large parts from multiple central locations.

Hot-runner gates: Deliver plastic through a heated manifold directly to the part, reducing or eliminating runner waste. They carry a high initial tooling cost but are standard for high-volume or multi-cavity production.

Why Location Drives Outcomes

Gate location sits at the intersection of tooling design and buyer requirements.

Cosmetics: Every gate leaves a mark—a vestige, a scar, or a trimmed nub. Placing the gate where this mark can be tolerated (e.g., a hidden non-A surface) is the primary goal of gate planning.

Weld lines: When flow fronts split around a core and meet again, they form a weld (knit) line. This line is both a cosmetic blemish and a structural weak point. Gate location steers where these lines land.

Warpage and packing: The gate location dictates how pressure is transferred into the cavity during the packing phase. A poorly placed gate will seal off before distant, thicker sections of the part can pack out, leading to uneven shrinkage and warpage.

Sizing is Proportional, Not Absolute

A gate is not a fixed dimension; it scales with the wall it feeds. According to Eastman’s mold design guidelines, a gate should generally be 50–80 percent of the part’s wall thickness at that location.

If a buyer changes a wall thickness late in the design process, they have silently invalidated the gate size. Furthermore, the guidance establishes a practical floor: for Eastman’s polyester materials, gate thicknesses below 1.65 mm (0.065 in) should be avoided to prevent excessive shear.

This proportionality has a specific commercial consequence for material substitutions. If a tool built for a low-viscosity resin is later run with a high-viscosity replacement, the gate will likely be undersized. A resin swap on an existing tool is a tooling question, not just a purchasing one.

The Consequence of Undersized Gates: Shear

Forcing melt through a restriction heats and stresses it. That energy surfaces on the part exactly where the gate feeds—presenting as gate blush, a brittle region, or a visible halo.

Eastman’s guidelines state that low-shear gates are “essential” for appearance parts, requiring sharp corners in the gate and runner to be radiused. They recommend gating into areas where the flow path is continuous rather than into a notch or rib.

If a supplier proposes shrinking a gate to reduce the size of the witness mark, the buyer must ask what that reduction will do to shear and packing at that location. Trading a visible mark for an invisible weak spot is rarely a good deal.

RFQ Strategy

Do not wait until T1 samples to discuss the gate. In the RFQ:

• Explicitly mark all cosmetic and functional surfaces where a gate mark is unacceptable.
• Identify critical dimensions so the supplier can evaluate how filling and packing will affect them.
• Ask the supplier for their proposed gating approach (type and location) and what kind of mark it will leave.
• For high-risk, tight-tolerance parts, request a mold flow analysis to predict weld-line locations and evaluate gate seal times before steel is cut.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, provide engineering services, or certify suppliers. Design and tooling decisions are part-specific, confirm them through your supplier’s and moldmaker’s engineering review.