Last updated: 2026-09-02 · 5 min read

Injection Mold Gate Types — How Do You Select the Right Gate for Your Part?

The gate is the narrowest point in the runner system where molten plastic enters the cavity — and its type, size and location determine fill balance, weld line position, gate vestige appearance, packing pressure transmission and part ejection quality. The five most common gate types — edge gate, sub-gate (pin-point), fan gate, diaphragm gate and hot runner valve gate — each serve different part geometries, resin families and cosmetic requirements, and the selection is made during the DFM review before the mold is cut.

Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. See also related guide.

Key Facts

  • Gate type selection is driven by four factors: resin viscosity, part wall thickness, cosmetic requirement (visible vs. hidden surface) and automated degating (does the runner separate cleanly from the part at ejection).
  • Gate size affects shear rate, fill pressure and packing: too small a gate causes excessive shear heating and material degradation; too large a gate leaves a prominent vestige and makes automated degating difficult.
  • Gate location is determined by mold flow analysis — the goal is balanced fill, minimal weld lines in structural areas, and gate vestige on non-cosmetic surfaces.

    • Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. The related guide covers adjacent topics.

    Gate Type Comparison

    Gate Type Best For Limitation
    Edge gate Flat parts, thick walls, general-purpose resins Visible gate vestige; requires secondary trimming for cosmetic parts
    Sub-gate (pin-point) Multi-cavity molds, automated degating, small parts Small gate size limits high-viscosity resins; gate freeze-off can cause short shots
    Fan gate Wide flat parts, reduces jetting and weld lines Larger gate vestige than sub-gate; not suitable for thin walls
    Diaphragm gate Cylindrical parts with center hole (e.g., bushings, cups) Limited to axisymmetric geometries; vestige around the entire inner diameter
    Hot runner valve gate Cosmetic surfaces, multi-cavity balance, expensive resins Adds $5,000–$30,000 to mold cost; requires maintenance of valve pins and heaters

    How Gate Location Affects Part Quality

  • Weld lines form where two melt fronts meet — gate location determines where they occur. Structural weld lines (in load-bearing areas) reduce part strength by 10–30%; the DFM review repositions the gate or adds overflow wells to move weld lines to non-critical areas.
  • Air traps form at the end of fill — if the gate location causes air to be trapped in a deep cavity or boss, the result is a burn mark or short shot. Venting or gate repositioning resolves the issue before T1.
  • Packing pressure transmission depends on gate freeze-off timing: a gate that freezes too early prevents packing pressure from reaching the far ends of the part, causing sink marks and dimensional variation; a gate that freezes too late causes over-packing and flash.

    • What This Means for Your Quotation

    The DFM report should specify gate type, location and size — with mold flow analysis showing fill balance, weld line positions and air trap locations — before the mold is cut. When the gate selection is documented and agreed, the T1 trial confirms the prediction; when it is left to the mold maker’s judgment without review, the first trial often reveals gate-related defects that require mold modification and delay the program.

    Frequently Asked Questions

    Can gate location be changed after the mold is built?

    Yes, but at significant cost — moving a gate requires welding and re-machining the cavity or adding a new gate insert. The cost ranges from $500 to $3,000 depending on the mold size and steel hardness. This is why gate location is finalized during the DFM review, not after T1.

    What gate type produces the cleanest cosmetic result?

    Hot runner valve gates produce the smallest, cleanest gate vestige — essentially a flat circular mark under 1mm in diameter. They are the standard choice for cosmetic surfaces (enclosures, consumer electronics, medical device housings) where gate blush or vestige is unacceptable. The trade-off is the added hot runner cost and maintenance.

    How does gate size affect injection pressure?

    A smaller gate increases shear rate, which heats the melt and reduces viscosity — but if the gate is too small, the pressure drop across it exceeds the available injection pressure, causing short shots. The gate size is optimized in mold flow analysis to balance fill pressure against shear heating, typically 0.5–1.5mm deep for sub-gates and 1.0–3.0mm for edge gates.

    Key Takeaways

    1. Gate type is selected by resin viscosity, wall thickness, cosmetic requirement and automated degating — the five common types (edge, sub-gate, fan, diaphragm, valve gate) each serve different geometries.
    2. Gate location determines weld line position, air trap location and packing transmission — all resolved in mold flow analysis before the mold is cut.
    3. The DFM report should document gate type, location and size with flow analysis results — changing gate location after mold build costs $500–$3,000 and delays the program.

    Send aCatMould the part drawing, and the DFM review specifies gate type, location and size — with mold flow analysis showing fill balance, weld line positions and air trap locations — so the gate decision is made on data before the mold is cut.

    Sources
    Plastics Technology — gate design and selection
    SPE — injection molding gate types and applications
    MoldMaking Technology — gate location optimization

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet