Gate Design in Injection Moulds: Types, Location & Shear Kinetics

The gate is the restrictive orifice through which molten polymer enters the mold cavity from the runner. Gate geometry and placement govern injection pressure drop, melt shear rate, gate freeze time, optical orientation, and surface defects like jetting, sink marks, and blush. Selecting the correct gate type—Pin, Submarine/Tunnel, Edge, Fan, or Diaphragm—prevents premature freezing and minimizes post-moulding gate removal labor.

By completing this lesson, you will be able to:

• Select appropriate gate types based on resin viscosity, cavity count, and automated ejection needs.
• Calculate apparent vs Rabinowitsch-corrected gate melt shear rate ((\dot{\gamma}_{gate})) to prevent shear-induced degradation.
• Determine optimal gate location using thickness rules and flow path ratio ((L/t)).
• Diagnose jetting, gate blush, and excessive gate vestige.

3.1 Gate Classification

1. Submarine / Tunnel Gate: Tapered gate machined into mold plate; automatically shears during mold opening.
2. Pin Gate: Used in 3-plate molds; leaves negligible vestige ($0.8-1.5\text{ mm}$ diameter).
3. Edge Gate: Standard rectangular side entry for rigid components; requires manual trimming.

graph TD
    A["Runner System (Full Round / Trapezoidal)"] --> B["Gate Restriction Zone (High Shear Rate)"]
    B --> C["Cavity Entry (Melt Impingement against Core Wall)"]
    C --> D["Volumetric Cavity Filling"]
    D --> E["Gate Solidification / Gate Freeze Time Seal"]

Apparent vs True Gate Shear Rate Calculation

For a circular pin or submarine gate of radius $r_g$ under volumetric flow rate $Q$, the apparent wall shear rate is:

$$\dot{\gamma}_{app} = \frac{4Q}{\pi r_g^3}$$

For non-Newtonian pseudoplastic melt (e.g. Polycarbonate with power-law index $n = 0.35$), the Rabinowitsch-corrected true wall shear rate is:

$$\dot{\gamma}_{true} = \left( \frac{3n + 1}{4n} \right) \dot{\gamma}_{app} = \left( \frac{3(0.35) + 1}{4(0.35)} \right) \dot{\gamma}_{app} = 1.4643 \times \dot{\gamma}_{app}$$

Worked Numerical Example:

<div className="problem-statement">

Problem: Polycarbonate (PC) melt flows into a single cavity at a volumetric flow rate $Q = 15.0\text{ cm}^3/\text{s} = 1.5 \times 10^{-5}\text{ m}^3/\text{s}$ through a circular pin gate of radius $r_g = 0.60\text{ mm} = 6.0 \times 10^{-4}\text{ m}$. Calculate:

1. Apparent gate shear rate ((\dot{\gamma}_{app}))
2. Rabinowitsch-corrected true gate shear rate ((\dot{\gamma}_{true}))

</div> <div className="solution-step">

Solution:

1. Apparent Shear Rate:

$$\dot{\gamma}_{app} = \frac{4 \times (1.5 \times 10^{-5})}{\pi \times (6.0 \times 10^{-4})^3} = \frac{6.0 \times 10^{-5}}{6.7858 \times 10^{-10}} = 88,420\text{ s}^{-1}$$

1. Rabinowitsch-Corrected True Shear Rate ($n=0.35$):

$$\dot{\gamma}_{true} = 1.4643 \times 88,420\text{ s}^{-1} = 129,474\text{ s}^{-1}$$

Engineering Verdict: Corrected shear rate ($129,474 ext{ s}^{-1}$) highlights intense shear heating in the pin gate land, requiring melt temperature monitoring to prevent polycarbonate discoloration.

• Automotive Connectors: Multi-cavity automatic submarine gate tooling. (Illustrative Indian industry scenario based on automotive connector moulding practices in Noida).

• Gate Freeze Time: Solidification time of gate core; seals cavity pressure.
• Jetting: Melt stream snaking caused by gate un-impinged flow.

1. ISO 20457:2018 — Plastics moulded parts — Tolerances and acceptance conditions, ISO.
2. Pye, R. G. W. (2000). Injection Mold Design, Longman.