runner stick

What is the runner stick


runner stick refer to a phenomenon in which the runner of the molded article fails to separate from the mold.

If the holding pressure is applied for a long time ,so that the size of the runner is increased, or the runner path portion of the mold is scratched or undercut, resistance is generated, so that the runner cannot be peeled off when the mold is opened.

Reasons for the  runner sticking

  • Insufficient solidification of runner
When the main runner is not fully cured, the shrinkage is small at this moment, and the runner is close stick to the mold, and the strength is also low. Therefore, if the runner is to be ejected at this time, it will be very easy to break. In this way, the runner will remain on the channel. Resin temperature, mold temperature, and cycle (cooling time) have an effect on the solidification of the main flow.
  •  Excessive  pressure holding
The main runner channel is separated from the barrel nozzle directly, so it is easy to apply pressure. Therefore, after the high holding pressure is applied, the size of the runner is increased, and at the time ,the resistance of demolding is correspondingly increased, so that the main runner sticking to mold is liable to occur.
  • Mold construction
When the draft angel of the runner is small, the resistance of the fixed mold half release is increased. Although the puller is provided in order to pull out the main runner from the fixed side, if it is weak, the lock is damaged, so that the main runner is detached from the moving side. In addition, if the  lock is used, the main runner and the runner sometimes adhere to the fixed side because it is too strong.
  • Plastic material
Compared to the standard grade, the plastic material  with higher impact resistance grade shrinks less, it is easier to adhere to the mold, so less strong. As a result, the runner tick to mold is more likely to happen.

How to solve?

Allow the main runner to cool and solidify before opening the mold
Reduce mold temperature and extend cooling time. Especially for materials with low strength and slow curing, reducing the mold temperature will be an effective method.
  • Reduce the pressure holding
It is also effective to reduce the pressure holding. The holding pressure applied to the mold cavity of the product becomes 0 after the gate is closed. Therefore, if no pressure is applied thereafter, the main runner will not be subjected to useless holding pressure. However, excessive pressure reduction can easily lead to unstable injection volume.
  • Improve the mold design
The draft angles of the main runner have not been specifically specified. However, if a runner stick is likely to occur, increasing the angle is also effective. It is also effective to strengthen the puller (to increase the size and enhance the slope). On the contrary, since it is difficult to fall off due to ejection, it is necessary to perform adjustment while actually molding.

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