Causes and Solutions for Poor Gate Cutting

1. What is poor gate cutting?

Poor gate cutting refers to a phenomenon where the gates that connect the molded product, the main runner, and the sub-runner are not easily separated when using methods like point gating during molding.

When using point gates or submerged gates, the gates connecting the product, main runner, and sub-runner should automatically break during mold opening.

However, if the gate’s shape or size is not appropriate, it can result in poor gate cutting, leaving residue inside the mold.

2. Causes of Poor Gate Cutting

(1) Imbalance in Forces

To ensure that the gate separates the product portion from the sub-runner portion and maintains a balance of three forces: “gate strength,” “sub-runner retention force,” and “product retention force” are crucial.

When the mold opens, if the sub-runner portion remains on the fixed side of the mold while the product portion stays on the movable side, they will be separated at the gate.

Poor gate cutting can occur if the gate’s strength is too high, or if the product portion has a weaker retention force compared to the sub-runner.

Generally, sub-runners are retained by locking pins, and their retention force depends on the shape and size of the locking pins, as well as the temperature of the sub-runner portion during mold opening.

If the locking pins are too small or have insufficient slope, the gate can fall off before it is properly cut.

So it’s more beneficial to enhance the retention force of the sub-runner rather than increasing gate strength.

On the contrary, excessive retention force in the sub-runner can prevent it from detaching from the mold. Additionally, the plastic‘s strength and rigidity can vary with temperature changes, so adjustments must be made accordingly.

The part of the product is retained by friction on the side due to its slope, or by the use of core slides. When relying on slope friction, the gate strength must still exceed a certain level. This method is also influenced by temperature.

Furthermore, gate strength is naturally influenced by gate design.

  • If the gate size is too large, the strength increases, making it harder for the gate to be cut.
  • In the case of a two-plate mold with a tunnel gate, it’s also affected by the gate angle and its placement.
  • For a three-plate mold with a point gate, it’s influenced by the slope of the two main runners, grinding, and other factors.
Poor Gate Cutting

★ Mold Temperature

Affected by the temperature of the plastic after cooling. Changes in plastic temperature result in corresponding variations in strength and rigidity.

★ Holding Pressure and Holding Time

Influenced by the resin filling volume, product dimensions, and the size of the main and sub-runners. The dimensions significantly affect the frictional retention on the side. When the dimensions are too large, it can even lead to demolding difficulties.

★ Injection Speed

Affected by the resin filling volume, product dimensions, and the size of the main and sub-runners.

(2) Inherent mold Grade-Related Issues

Regarding factors like impact resistance grades or alloy materials, resins with added elastomers tend to cure more slowly and have a lower elastic modulus, making them more prone to poor gate cutting than other materials.

Hence, it is evident that thorough consideration of relevant countermeasures is required during the mold design.

3. Countermeasures for Poor Gate Cutting

Adjusting the Strength Balance. Based on the conditions of poor gate cutting, the molding conditions can be adjusted as follows:

★ When the sub-runner remains on the movable side of the mold

This may be due to weak locking pins on the fixed side of the sub-runner or excessive strength in the gate portion.

Therefore, measures can be taken to increase the retention strength of the sub-runner locking pins or reduce the gate strength. Modifying the mold to change their sizes is also an option.

If changing the injection molding conditions, lowering the mold temperature to promote curing around the sub-runner locking pins and enhance strength may also be effective.

In the case of tunnel gates, considering modifications to the mold gate.

★ When the sub-runner remains on the fixed side

This may be due to weak side slope on the product portion or excessive strength in the gate portion. One countermeasure is to modify the mold to enhance the slope or reduce the gate size.

Another approach is to increase the holding pressure and enlarge the sub-runner size to enhance retention.

★ When the product and sub-runner remain on the middle plate in a three-plate mold

The gate may be too strong in this case, so it’s advisable to slightly reduce the gate size or conversely reinforce the sub-runner locking pins.

Regarding molding conditions, reducing the holding pressure may also be workable.

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