How to Address Main Runner Sticking in Injection Molding?
Runner sticking in injection molding occurs when the cooled molten plastic gets stuck in the runner and cannot be easily ejected. This issue can impact production efficiency and damage the mold or the product.
Optimize Mold Design
1. Increase Draft Angle
Design an appropriate draft angle (typically 1°~3°) on the inner wall of the runner to reduce friction between the plastic and the mold wall, making demolding easier.
2. Apply Coatings or Surface Treatments
Polish the surface of the runner or apply hard chrome plating, Teflon coating, or similar treatments to reduce surface roughness and friction.
3. Improve Main Runner Structure
Spherical Transition: Design a smooth, spherical transition between the runner and the sprue bushing to avoid sharp edges that could cause plastic sticking.
Tapered Runner: Create a tapered runner to allow the cooled plastic to naturally shrink, facilitating demolding.
4. Inspect Sprue Bushing Material
Ensure sprue bushing is made of wear-resistant, high-temperature-resistant, low-friction materials, such as SKD61 or other high-quality steel grades.
Optimize Injection Molding Process
1. Control Cooling Time
Excessively long cooling times can cause the plastic to shrink or harden excessively, making demolding harder. Adjust the cooling time appropriately to mitigate sticking issues.
2. Adjust Injection Pressure and Speed
High injection pressure and speed can cause the plastic to adhere more tightly to the runner walls. Reduce these parameters to alleviate sticking.
3. Optimize Holding Pressure and Back Pressure
Excessive holding pressure can increase residual stress in the runner. Lower the holding pressure or adjust its duration to reduce sticking.
4. Increase Mold Temperature
Raising the mold temperature can reduce thermal shrinkage of the plastic, making it easier to demold.
