Injection Mould Runner Systems, Gate Design & Feed Balancing

The feed system—comprising sprue, runners, and gates—delivers molten polymer from the machine nozzle into individual mold cavities. Properly engineered cold or hot runner systems ensure balanced melt filling, minimize pressure drops, control shear heating, and enable clean part ejection without gate vestige defects.

By completing this lesson, you will be able to:

• Design naturally balanced runner layouts (H-bridge, star layout) vs artificially balanced systems.
• Calculate empirical runner diameter ((D)) starting values and verify pressure drop.
• Select appropriate gate types (Pin, Submarine/Tunnel, Edge, Fan, Diaphragm) based on resin viscosity.
• Troubleshoot gate freeze time and jetting.

Comparison of Runner Geometries:

1. Full Round: Ideal hydraulic efficiency (lowest surface area to volume ratio), lowest pressure drop.
2. Trapezoidal: Easiest to machine in single mold plate; 80% hydraulic efficiency.

graph TD
    A["Machine Nozzle"] --> B["Sprue Bushing (d_s >= d_nozzle + 1mm)"]
    B --> C["Primary Runner (Full Round / Trapezoidal)"]
    C --> D["Secondary Branch Runners (Balanced Length & Diameter)"]
    D --> E["Gate Entry (Edge / Submarine / Pin Gate)"]
    E --> F["Mold Cavity"]

Empirical Runner Diameter Formula & Operating Assumptions

$$D = \frac{\sqrt{W} \cdot L^{1/4}}{3.5}$$

Worked Numerical Example:

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Problem: For the Polycarbonate part with mass $W = 49.0 ext{ g}$ and runner length $L = 81.0 ext{ mm}$, calculate the empirical full-round runner diameter ($D$).

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Solution:

$$D = \frac{\sqrt{49.0} \times (81.0)^{1/4}}{3.5} = \frac{7.0 \times 3.0}{3.5} = 6.0\text{ mm}$$

Design Rule: Use 6.0 mm full-round runner as empirical starting size to avoid premature gate/runner freeze.

• Submarine (Tunnel) Gates: Automatic gate shearing during mold opening. (Illustrative Indian industry scenario based on automotive connector tooling practices).
• Valve-Gated Hot Runners: Elimination of runner scrap in 64-cavity PET preform molds.

• Naturally Balanced Layout: Equal flow distance and diameter from sprue to every cavity.
• Gate Freeze Time: Time required for gate center to solidify, sealing cavity pressure.
• Empirical Sizing: 6.0 mm runner is an initial estimate requiring melt pressure drop validation.

1. Pye, R. G. W. (2000). Injection Mold Design, 4th Ed., Longman Scientific & Technical.
2. Moldflow Design Guide — Runner and Gate Optimization Techniques.