Analysis of the causes of poor gate cutting in injection molding

I. What is bad gate cutting?

It refers to the connection between the molded parts and the main runner when molding with a pointed gate, etc. Main runner. It is a phenomenon that the gate of the manifold is not easily cut off. If a pointed gate or a submerged gate is used, when the mold is opened and the product is connected, the gate of the main runner. The gate of the manifold will be broken automatically. However, if the shape or size of the gate is not suitable, the gate will be poorly cut and remain inside the mold.

II. Causes of bad gate cut

1. Bad balance of force

To ensure that the product part is cut off from the manifold part at the gate, the "gate strength" is maintained. It is very important to keep the balance of three forces: "gate strength", "manifold holding force" and "product holding force". When the mold is opened, if the manifold part stays on the fixed side and the product part stays on the movable side, both will be cut off at the gate. If the strength of the gate is too strong, or the holding force between the product part and the manifold is weak, poor gate cutting will occur

Generally, the manifold is held by a locking pin; the holding force depends on the shape and size of the locking pin and the temperature of the manifold section when the mold is opened. If the size or slope of the locking pin is not sufficient, the gate will fall off before it is cut, so instead of increasing the gate strength, it is better to increase the holding power of the manifold.

On the contrary, too much retention force of the manifold will make the manifold unable to break away from the mold. Also, the strength and rigidity of the resin will change with temperature, so it must be adjusted for this as well.

The product part is held by the frictional force of the side slope, or slip core. In the case of holding by slant friction, it is still necessary to reach above the gate strength. This is also influenced by the temperature.

In addition, the gate strength is of course influenced by the gate design. If the gate size is large, the strength will increase, and thus the gate will not be easily cut. If there are 2 plates and a tunnel gate is used, the angle and position of the gate will be influenced. If there are 3 plates and a pointed gate is used, it will be affected by the slope of the 2 main streams. The influence of grinding, etc.

(1) Mold temperature

Affected by the temperature of the resin after cooling. If the resin temperature changes, the strength, and rigidity will also change.
 

(2) Holding pressure and holding time

Affected by the amount of resin filling. Product. Influenced by the size of the main runner and manifold. Its size has a great influence on the side slope friction retention. When the size is too large, it may even be impossible to release the mold.

(3) Injection speed

Influenced by the amount of resin filling. Product. The size of the main runner and manifold is affected.

2. Problems inherent to the grade

For impact resistance, grades or alloy materials, etc., resins with elastomers have a slower curing speed and lower modulus of elasticity, making them more susceptible to poor gate cut than other materials. Therefore, it is necessary to fully study the countermeasures at the mold design stage.

III. Countermeasures for bad gate cut

Change the strength balance

Adjust the molding conditions according to the condition of the bad gate cut as follows

• When the manifold is left on the movable side

It can be considered that the manifold locking pin on the fixed side is weak, or the strength of the gate part is high. Therefore, increase the holding strength of the manifold locking pin or weaken the gate strength. Modifying the mold to change the size of both is also a method.

If the molding conditions are to be changed, then lowering the mold temperature to promote curing around the manifold locking pin and increasing the strength may also be effective. In the case of tunnel gates, correcting the gate section may also be considered.

• When the manifold stays on the fixed side

It can be considered that the side slope of the product part is weak, or the strength of the gate part is high. One of the countermeasures is to modify the mold to strengthen the slope or reduce the gate. Another way is to increase the holding pressure and increase the size of the manifold to improve the holding force.

• In a 3-piece mold where the product and manifold are left on the center plate

There is a high possibility that the gate is too strong, so the gate size should be slightly reduced or, conversely, the manifold locking pin should be strengthened. In terms of forming conditions, reducing the holding pressure may also be effective.