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

The gate is the engineered restriction where molten plastic leaves the runner system and enters the mold cavity. Because every cubic millimeter of polymer flows through that orifice, the gate’s type, cross-sectional depth, and location govern the entire molding cycle—dictating where weld lines converge, where cavity air gets trapped, how effectively holding pressure packs out thick sections, and what physical gate vestige remains on the finished part.

When a buyer leaves gate selection entirely unmentioned until T1 sample review, the toolmaker naturally places the gate wherever machining is simplest—which can put a trimmed scar on a Class A bezel or place a weak knit line right across a load-bearing boss.

Comparing the Main Injection Molding Gate Families

While the moldmaker calculates exact runner diameters and land lengths, buyers should understand the trade-offs between the six gate architectures most frequently proposed during pre-steel mold design review:

Gate Type	How It Feeds & Degates	Typical Vestige Profile	Best Application Fit & Trade-Offs
Edge / Tab Gate	Feeds along the parting line into the outer perimeter wall; runner remains attached at ejection and requires manual or fixture clipping.	Rectangular scar along the edge (typically 0.1–0.5 mm height depending on trim fixture).	Lowest tooling cost and easiest steel-safe depth tuning; accepts almost all unfilled and glass-filled resins, but adds degating labor.
Fan / Flash Gate	Wide, shallow edge gate that spreads melt across a broad front into flat or thin-walled parts.	Wide linear trim line along the edge requiring a fixture shear or router.	Minimizes warpage and flow hesitation on flat panels, lenses, and thin covers by creating a uniform planar flow front.
Submarine (Tunnel) / Cashew Gate	Feeds from below the parting line via an angled tapered tunnel; the runner shears off automatically as ejector pins push the part out.	Small oval shear witness mark on an internal rib, ejector pin boss, or non-cosmetic side wall.	Eliminates manual gate trimming in a standard two-plate cold-runner mold; however, overly brittle or high-glass-filled resins can chip or generate shear dust during automatic shearing.
Pin / Point Gate (3-Plate Mold)	Feeds directly onto the top or face of the part through a small pinhole that breaks free when the third runner plate separates during mold open.	Small raised or recessed point pip on the face; cold runner still requires regrinding or disposal.	Allows center-gating or multi-point feeding on deep cylindrical parts without hot-runner cost, though 3-plate molds increase press daylight and cycle time.
Hot-Runner Thermal Tip Gate	Heated nozzle tip delivers melt directly to the cavity wall without a cold runner; freezes thermally at the small gate orifice between shots.	Small conical nib or dimple (often hidden inside a recess or logo depression).	Eliminates cold-runner scrap and cycle cooling overhead in multi-cavity molds; can leave stringing or higher vestige if tip cooling fluctuates.
Hot-Runner Valve Gate	A mechanical steel pin inside the heated nozzle slides forward at the end of pack to shut off the gate orifice flush with the cavity wall.	Clean, flat circular witness ring (resembles an ejector pin mark) with near-zero raised nub.	Highest upfront tooling cost, but delivers large flow diameters, low shear, zero runner waste, and the cleanest cosmetic vestige for large or Class A parts.

Also see our companion guide on gate size and type selection for a deeper look at matching gate geometry to part volume.

Why Gate Location Drives Cosmetics, Strength, and Warpage

Approving a gate location is a three-way negotiation between cosmetic appearance, structural load paths, and dimensional flatness:

1. Thick-to-Thin Packing Physics: Molten plastic must always be gated into the thickest nominal wall section so it flows toward thinner sections. If a gate is placed in a thin wall (for example, a 1.2 mm flange) that feeds a heavy 3.5 mm mounting pad further downstream, the thin gate section freezes solid long before the heavy pad finishes cooling. Once the gate seals, holding pressure from the press barrel is cut off, leaving the distant thick section with severe sink marks, internal voids, or uncontrolled shrinkage.
2. Steering Weld Lines Away from High-Stress Features: Whenever a melt front splits around a core pin forming a hole or window, the two cooled fronts meet on the far side to form a weld (knit) line. Moving the gate 20 mm to the left shifts where that weld line lands. During DFM, ensure the gate location pushes knit lines away from screw bosses, snap-fit cantilevers, and high-gloss cosmetic zones.
3. Impinging Flow to Prevent Jetting: If a small gate fires directly into an open cavity space like a nozzle shooting into an empty room, the high-velocity polymer stream shoots across the cavity and folds onto itself—causing worm-like jetting. Good gate design angles the orifice against a core pin, lap wall, or radiused tab so the incoming melt immediately hits a surface and forms a smooth, radially expanding flow front.
4. Fiber Orientation and Anisotropic Warpage: In glass-filled resins, fibers align parallel to the direction of flow radiating out from the gate. Because glass-filled polymers shrink two to three times more across the fiber direction than along it, a single off-center edge gate on a long rectangular housing often causes twisting or banana warpage unless balanced by a fan gate or multi-drop hot runner.

Gate Sizing Rules: Proportional to Wall Thickness and Shear Limits

A gate is never sized by guesswork; its depth scales directly with the wall thickness it feeds and the shear sensitivity of the polymer.

Eastman Chemical’s Processing and Mold Design Guidelines establish two foundational engineering benchmarks:

• Proportional Gate Depth (50% to 80% of Wall): For standard edge, fan, and tab gates, the gate depth (thickness) is typically machined to 50% to 80% of the adjacent nominal wall thickness—often starting steel-safe around 50% at T1 so the toolmaker can grind the gate deeper after running a gate-seal study.
• Minimum Thickness Floor to Prevent Shear Degradation: For shear-sensitive amorphous polyesters and copolyesters, Eastman warns against gate thicknesses below 1.65 mm (0.065 in) where geometry allows, calls for radiused transitions between the runner and gate, and stresses that low-shear gating is essential on clear or appearance parts.

When a gate is cut too shallow or too narrow in a misguided attempt to leave a tiny witness mark, forcing the same volume of plastic through that microscopic restriction spikes the shear rate. Excessive shear overheats the polymer skin, breaks down molecular chains, and leaves frosted gate blush, splay streaks, or a brittle zone around the gate that cracks on impact.

Why Switching Resins Often Invalidates an Existing Gate

Because gate sizing depends on polymer viscosity, changing resin families on an existing mold is a tooling event, not just a purchasing swap. A small 0.8 mm submarine gate that fills cleanly with low-viscosity unfilled Nylon 6 will act as a severe high-shear choke point if you try to mold high-viscosity polycarbonate or 30% glass-filled PBT in the same cavity. Always confirm gate depth and land length with the moldmaker before approving an alternate resin grade.

What Buyers Should Specify in the RFQ and DFM Review

Prevent gate disputes before steel is ordered by putting four explicit rules into your injection molding RFQ package:

• Zone Cosmetic Surfaces on the 2D Print: Mark Class A faces where gates and parting lines are strictly prohibited, and indicate preferred non-cosmetic edges or internal pockets where a gate witness mark is acceptable.
• State Maximum Permissible Gate Vestige: Specify a numeric protrusion limit (for example, “Gate vestige must be recessed or trimmed flush within 0.15 mm max on Surface B”) so the supplier prices either a submarine gate, a recessed gate well, or a dedicated trim fixture.
• Require Gate Proposal Sign-Off Before Cutting Steel: Make written approval of gate type, dimensions, and location a mandatory milestone in the pre-steel DFM review.
• Request Gate-Seal Verification at Qualification: Ask the molder to document a scientific gate-seal (gate-freeze) study during T1/T2 trials to prove that the gate stays open long enough to pack the part to stable weight.

Buyer FAQs

What is the best gate type for a cosmetic injection-molded part?

For high-volume Class A cosmetic enclosures where no trimmed scar is acceptable on the outer edge, a submarine (tunnel) gate or curved cashew gate that feeds into an internal rib or hidden ejector boss on the B-side—or a hot-runner valve gate placed on a hidden floor—generally provides the cleanest exterior appearance. If a part must be gated on an exterior edge, recessing an edge or hot-tip gate inside a small stepped pocket protects the mating assembly clearance from any residual vestige nub.

Why do moldmakers say they are cutting the gate “steel-safe” for T1?

“Steel-safe” means the toolmaker machines the gate opening at the smaller end of the recommended range (for example, 50% of the wall thickness rather than 75%) before the first T1 trial. Because enlarging a gate only requires grinding away a few thousandths of an inch of steel on the bench—whereas making a gate smaller requires TIG or laser welding the mold cavity and re-machining—starting steel-safe lets the molder observe actual fill balance and gate-blush behavior at T1 and open the gate incrementally for T2.

Can a small gate cause weak parts even if the cavity fills completely?

Yes. If a gate is too thin relative to the part wall, the thin polymer skin inside the gate freezes solid within a second or two after the cavity fills, while the thicker part walls downstream are still molten and shrinking. Once the gate freezes (“seals”), holding pressure from the injection screw can no longer push additional plastic into the cavity to compensate for volumetric shrinkage—leaving the part under-packed, prone to sink marks and internal voids, and weaker along weld lines.

How do you avoid gate blush or halo marks around the gate?

Gate blush (a dull, frosted, or scaly halo radiating out from the gate) occurs when molten plastic is forced through a restricted or sharp-cornered gate at excessive velocity, fracturing the melt front and creating high shear stress. To eliminate it, profile the injection velocity so the screw fills slowly across the gate until the melt front clears the orifice and then accelerates for main cavity fill, radius the transition between the runner and gate, and enlarge the gate depth toward 60%–80% of the wall thickness to lower shear.

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.