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

The gate is the restricted opening where molten plastic enters the cavity. It is the origin of flow, so it sets how the cavity fills. That pattern decides where weld lines form, where air traps, how evenly the part packs, and where the physical gate mark lands.

A gate chosen only for tooling convenience can lock a cosmetic or dimensional defect into the part.

Gate Families and Tooling Implications

Gate type, count, and location balance fill, cosmetics, and tooling cost. You do not design the gate. You should understand the families suppliers propose:

Edge gates: Feed from the part edge along the parting line. Simple and inexpensive to tool, but leave a visible nub that usually needs manual trimming.

Sub / Tunnel gates: Feed from below the parting line and shear off automatically at ejection. Avoids manual trimming, but restricts tool geometry and limits gate size.

Pin / Point gates: Feed on the face through a small point. Used in three-plate and hot-runner tools. Leave a small vestige and work well for feeding large parts from multiple central locations.

Hot-runner gates: Deliver plastic through a heated manifold directly to the part, cutting or eliminating runner waste. Higher initial tooling cost. Common for high-volume or multi-cavity work.

Why Location Drives Outcomes

Gate location sits between tooling design and buyer requirements.

Cosmetics: Every gate leaves a mark (vestige, scar, or trimmed nub). Place it where the mark can be tolerated, usually a hidden non-A surface.

Weld lines: Flow fronts that split around a core and meet again form a weld (knit) line. That line is a cosmetic blemish and a structural weak point. Gate location steers where those lines land.

Warpage and packing: Gate location controls how packing pressure reaches the cavity. A poorly placed gate can freeze off before distant thick sections pack, which drives uneven shrinkage and warpage.

Sizing is Proportional, Not Absolute

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

If you change wall thickness late, you silently invalidate gate size. The same guidance sets a practical floor: for Eastman’s polyester materials, gate thicknesses below 1.65 mm (0.065 in) should be avoided to limit shear.

That proportionality has a commercial consequence for material swaps. A tool built for a low-viscosity resin and later run with a high-viscosity replacement will often have an undersized gate. A resin swap on an existing tool is a tooling question, not only a purchasing one.

The Consequence of Undersized Gates: Shear

Forcing melt through a restriction heats and stresses it. That energy shows on the part at the feed point as gate blush, a brittle region, or a visible halo.

Eastman’s guidelines say low-shear gates are “essential” for appearance parts and call for radiused sharp corners in the gate and runner. They recommend gating into continuous flow paths rather than into a notch or rib.

If a supplier proposes shrinking a gate to shrink the witness mark, ask what that does to shear and packing there. Trading a visible mark for an invisible weak spot is rarely a good deal.

RFQ Strategy

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

• Mark all cosmetic and functional surfaces where a gate mark is unacceptable.
• Identify critical dimensions so the supplier can judge fill and packing effects.
• Ask for the proposed gating approach (type and location) and what mark it will leave.
• For high-risk, tight-tolerance parts, request mold flow analysis to predict weld-line locations and 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.