It refers to a phenomenon in which gate connecting molded product, main runner, and branch runner is not easily cut off when molding with a point gate. If you use point gates or latent gates, when mold is opened and product is connected, gates of main runner and runner will automatically break. However, if shape or size of gate is not appropriate, gate will not be cut properly and will remain inside mold.

In order to ensure that gate cuts off product part and runner part, it is very important to maintain balance of three forces of "gate strength", "retention force of runner" and "product retention force". When mold is opened, if runner part is left on fixed side and product part is left on movable side, both will be cut off at gate. If strength of gate is too large, or holding force between product part and runner is weak, poor gate cutting will occur.
Generally speaking, runner is held by locking pin; its holding force depends on shape and size of locking pin and temperature of runner part when mold is opened. If size or slope of locking pin is insufficient, gate will fall off before being cut. Therefore, instead of increasing strength of gate, it is better to increase retention of runner.

 

On the contrary, if holding force of runner is too large, runner cannot be separated from mold. Strength and rigidity of resin will also change with temperature, so it must be adjusted according to this point.
Product part is maintained by friction of side slope, or sliding core. In the case of relying on slope friction to maintain, gate strength must still be reached. At this time, it will also be affected by temperature.
In addition, gate strength will of course be affected by gate design. If gate size is too large, strength will increase, so that gate is not easy to be cut. If there are two mold plates, tunnel gate is used, it will also be affected by angle of gate and setting position. If there are three mold plates and point gates are used, they will also be affected by inclination and grinding of secondary main runner.

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

Affected by amount of resin filling, size of product, main runner and runner. Its size has a great influence on slope friction retention of side. When size is too large, it may even fail to demold.

Affected by amount of resin filling, size of product, main runner and runner.

In terms of impact resistance grades or alloy materials, resins added with elastomers have slow curing speed and lower elastic modulus of material, so they are more prone to poor gate cutting than other materials. It can be seen that relevant countermeasures need to be fully studied in mold design stage.

Change balance of intensity
According to poor cutting condition of gate, adjust molding conditions as follows

It can be considered that runner locking pin on fixed side is weak, or strength of gate part is too strong. Therefore, measures can be taken to increase retention strength of runner locking pin, or to reduce strength of gate. Modifying mold to change size of both is also a method.
If you want to change molding conditions, lowering mold temperature, promoting curing around runner locking pin, and increasing strength may also be effective. If it is a tunnel gate, you can also consider modifying gate part.

It can be considered that side slope of product part is weak, or strength of gate part is too large. One of countermeasures is to modify mold to strengthen slope or reduce gate. Another method is to increase holding pressure and increase size of runner to increase holding force.

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