Poor gate cutting
It refers to a phenomenon in which the gate connecting the molded product, the main runner, and the sub-runner is not easily cut off when molding with point gates. If you use point gates or latent gates, when the mold is opened and connected with products, the gates of the main runner and sub-runner will automatically break. However, if the shape or size of the gate is not appropriate, the gate will be poorly cut and will remain inside the mold.
Causes of poor gate cutting
Poor balance of power
In order to ensure that the gate cuts off the product part and the runner part, it is very important to maintain a balance of the three forces including “gate strength”, “management force of the runner” and “product retention force”. When opening the mold, if the runner part remains on the fixed side and the product part remains on the movable side, both will be cut off at the gate. If the strength of the gate is too large, or the holding force between the product part and the sub-runner is weak, poor gate cutting will occur.
Generally speaking, the sub-runner is held by the locking pin. Its holding force depends on the shape and size of the locking pin and the temperature of the runner part when the mold is opened. If the size or slope of the locking pin is insufficient, the gate will fall off before being cut. Therefore, instead of increasing the strength of the gate, it is better to increase the retention of the runner.
On the contrary, if the holding force of the runner is too large, the runner cannot be separated from the mold. And the strength and rigidity of the resin will also change with temperature, so it must be adjusted.
The product is maintained by the friction of the side slope, or the sliding core. In this situation, it’s still necessary to reach the gate strength. At this time, it will also be affected by temperature.
In addition, the gate strength will of course be affected by the gate design. If the gate size is too large, the strength will increase. So that the gate will not be easily cut. If there are two templates and a tunnel gate is used, it will also be affected by the gate angle and setting position. If there are three templates and a point gate is used, it will also be affected by the inclination and grinding of the secondary main runner.
(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, the size of the product, the main runner and the sub-runner. Its size has a great influence on the slope friction retention of the side. When the size is too large, it may even fail to demold.
(3) Injection speed
Affected by the amount of resin filling, the size of the product, the main runner and the sub-runner.
The inherent problems of the grade
In terms of impact resistance grades or alloy materials, after adding elastomers, the resins have slow curing speed and lower elastic modulus of the material. So they are more prone to poor gate cutting than other materials. Therefore, relevant countermeasures need to be fully studied in the mold design stage.
Countermeasures for poor gate cutting — Change the intensity balance
Adjust the molding conditions as follows according to poor gate cutting situations:
★When the runner is left on the movable side
It can be considered that the locking pin of the fixed side runner is weak, or the strength of the gate part is too strong. Therefore, we can increase the retention strength of the runner locking pin, or to reduce the strength of the gate. Modifying the mold to change the size of both is also a method.
If you want to change the molding conditions, you can lower the mold temperature. Also, you can promote the curing around the runner locking pin and increase the strength. Those may also be effective. If it is a tunnel gate, you can also consider modifying the gate part.
★When the runner is left 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 too large. One of the countermeasures is to modify the mold to strengthen the slope or reduce the gate. Another is to increase the holding pressure and increase the size of the runner to increase the holding force.
★When the product and the runner are left on the middle plate in the 3-piece mold
There is a high possibility that the gate is too strong. So the gate size should be slightly smaller, or conversely strengthen the runner locking pin. In terms of forming conditions, reducing the holding pressure may also be effective.