How to Design the Gate Correctly for Products?

The gate is a short groove with a small cross-sectional area to connect the runner and the cavity. Therefore, the cross-sectional area should be small in order to achieve the following effects:

  • Shortly after the cavity is injected, the gateiscold
  • Simple dewatering port
  • The dewatering port is completed, leaving only a few traces
  • Make the filling of multiple mold cavities easier to control
  • Reduce excessive packing

Gate location and size

1) Place the gate at the thickest part of the product. And ejection from the thickest part can provide better filling and pressure-holding effects. If the holding pressure is insufficient, the thinner area will solidify faster than the thicker area. Don’t place the gate where a sudden thickness change occurs to avoid hysteresis or short injection.

2) If possible, eject from the center of the product, and placing the gate in it can provide equal length flow. The flow length will affect the required injection pressure. The central ejection makes the injecting pressure uniform in all directions and it can avoid uneven volume shrinkage.

3) When the melt plastic flows into the runner, the melt plastic which it approaches the mold surface is the first to reduce heat (cooling) and solidify. When the plastic flows forward, it just only flows through from this solidified plastic layer. And because plastic is a low heat transfer material, outside solid plastic forms an insulating layer and inside a retaining layer that can still flow.

Therefore, under ideal circumstances, the gate should be set at the position of the cross-runner layer to achieve the best plastic flow effect. This situation is most common in circular and hexagonal cross flow runner. However, the trapezoidal cross flow runner cannot achieve this effect, because the gate cannot be set in the middle of the flow channel.

Principles for deciding gate location

When deciding the gate location, we should adhere to the following principles:

A. The plastic injected into each part of the cavity should be as even as possible.

B. The plastic injected into the mold should maintain a uniform and stable flow frontline at all stages of the injection process.

C. Weld marks, bubbles, cavities, vacant positions, insufficient injection, and spray should be considered.

D. The operation of take-off should be made as easy as possible, preferably automatic operation.

E. The location of the gate should match all aspects.


Basic elements for gate design

There are no hard and fast rules for the method of gate design, mostly based on experience. But there are two basic elements that must be compromised:

A. The larger the cross-sectional area of the gate, the better. The shorter the length of the runner, the better. So the larger cross-sectional area and the shorter runner can reduce the pressure loss when the plastic passes through.

B. The gate must be thin and narrow so as to be easy to freeze and prevent excessive plastic from flowing back. Therefore, the gate is in the center of the runner, and its cross-section should be as circular as possible. However, the gate switch is usually determined by the switch of the module.

The gate size can be determined by the cross-sectional area and gate length. The following factors can determine the best gate size:

  • Rubber flow characteristics
  • The thickness of the module
  • The amount of rubber injected into the cavity
  • Melting temperature
  • Mold temperature

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