(53) Poor gate cutting Injection Molding Process Defect Handling Manual (65 types)

1) What is Bad Gate Cutting
It refers to a phenomenon that the gate connecting the molded product, the main flow path, and the shunt path is not easily cut when using a point gate for molding. If a point gate or a latent gate is selected, the gates of the main flow path and shunt path will automatically break when connecting the product while opening the mold. However, if the shape or size of the gate is not appropriate, poor gate cutting can occur and remain inside the mold
(2) Reasons for poor gate cutting
(2-1) Poor balance in force
In order to ensure that the product portion is cut off from the diverter portion at the gate, it is important to maintain a balance between the three forces, namely, "gate strength", "diverter retention force", and "product retention force.". When opening the mold, if the runner portion remains on the fixed side and the product portion remains on the movable side, both will be cut off at the gate.
If the strength of the gate is too large, or the retention force between the product part and the splitter is weak, poor gate cutting can occur. Generally, the splitter is maintained through a locking pin; The retention force depends on the shape and size of the locking pin and the temperature of the runner portion during mold opening.
If the size or inclination of the locking pin is insufficient, the gate will fall off before it is cut off. Therefore, instead of increasing the gate strength, it is better to enhance the retention force of the splitter. On the contrary, excessive retention of the runner will prevent the runner from leaving the mold. And the strength and rigidity of the resin will also vary with temperature, so adjustments must also be made based on this.
The product part is maintained by the friction of the side slope, or sliding core. It is still necessary to achieve a gate strength above that maintained by slope friction. It is also 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, making it difficult to cut the gate. If 2 templates are used and the tunnel gate is used, it will also be affected by the gate angle and setting position. If three templates are used and a point gate is used, it will also be affected by the slope and grinding of the secondary main runner.
(1) Mold temperature
Affected by the resin temperature after cooling. If the resin temperature changes, the strength and rigidity will also change.
(2) Pressure holding pressure and pressure holding time
It is affected by the resin filling amount, product, main flow path, and shunt path dimensions. Its size has a significant impact on the slope friction retention of the side. When the size is too large, it may even occur that the mold cannot be released.
(3) Injection speed
It is affected by the resin filling amount, product, main flow path, and shunt path dimensions.
(2-2) Problems inherent in grades
In terms of impact resistance grades or alloy materials, resins added with elastomers are more prone to poor gate cutting than other materials due to their slower curing speed and lower elastic modulus. Therefore, it is necessary to fully study relevant countermeasures during the mold design stage
(3) Countermeasures for Bad Gate Cutting
(3-1) Changing the strength balance
Adjust the molding conditions according to the condition of poor gate cutting as follows
★ When the shunt is left on the movable side
It can be considered that the runner lock pin on the fixed side is weak or the strength of the gate portion is too large. Therefore, countermeasures can be taken to increase the retaining strength of the diverter lock pin, or to reduce the gate strength. Modifying the mold to change the size of both is also a method. If forming conditions are to be changed, methods of reducing mold temperature, promoting solidification around the runner lock pin, and improving strength may also be effective. If it is a tunnel gate, you can also consider modifying the gate section.
★ When the shunt remains 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 countermeasure is to modify the mold to strengthen the slope or reduce the gate. Another method is to increase the pressure holding and increase the size of the shunt channel to improve the retention force
★ When the product and flow path remain on the middle plate in a 3-piece mold
The possibility of a gate being too strong is high, so the gate size should be slightly reduced, or in turn, the runner lock pin should be strengthened. In terms of forming conditions, reducing packing may also be effective.