The causes of adhesion and poor ejection in injection molding are multifaceted, and mold failure is one of the primary factors.
1. Rough Mold Cavity Surface:
Cause: If the mold cavity and runner surfaces have defects like nicks, scratches, dents, the plastic part will easily adhere to the mold, leading to ejection difficulties.
Solution: Improve the surface finish of the mold cavity and runners as much as possible. Chrome plating of the mold cavity interior is recommended. When polishing, the direction of the polishing tool’s movement should be consistent with the melt flow direction during filling.
2. Mold Wear, Scratches, or Large Gaps Between Inserts:
Cause: When flash is generated in the worn or scratched areas of the mold or in the gaps between inserts, it can also cause ejection problems.
Solution: Repair the damaged areas and reduce the gaps between inserts.
3. Insufficient Mold Rigidity:
Cause: If the mold cannot be opened at the beginning of the injection process, it indicates that the mold is deforming under the injection pressure due to insufficient rigidity. If the deformation exceeds the elastic limit, the mold will not return to its original shape and cannot be used further. Even if the deformation does not exceed the elastic limit, the plastic part, cooled and solidified under high pressure in the mold cavity, will be trapped by the rebound force after the injection pressure is removed and the mold returns to its deformed state. This makes it impossible to open the mold.
Solution: Sufficient rigidity and strength must be designed into the mold during the design phase. During the trial run, it is best to install dial indicators on the mold to check if the mold cavity and mold frame deform during the filling process. The initial injection pressure during the trial run should not be too high. Observe the amount of mold deformation while gradually increasing the injection pressure, keeping the deformation within a certain limit. When mold clamping failure occurs due to excessive rebound force, simply increasing the opening force is not enough. The mold should be immediately disassembled, and the plastic part should be heated and softened for removal. For molds with insufficient rigidity, a frame can be inlaid on the outside of the mold to increase rigidity.
4. Insufficient Draft Angle or Poor Parallelism Between Moving and Fixed Plates:
Cause: Sufficient draft angles must be ensured during mold design and fabrication; otherwise, the part will be difficult to eject, and forced ejection often results in part warping, whitening at the ejection points, or cracking.
Solution: The moving and fixed plates of the mold must be relatively parallel; otherwise, the mold cavity will be misaligned, leading to poor ejection.
5. Unreasonable Gating System Design:
Cause: If the runner is too long or too small, the connection between the main runner and sub-runners is weak, there is no cold slug well in the main runner, the gate balance is poor, the diameter of the main runner does not match the nozzle hole diameter, or the spherical surface of the sprue bushing does not match the nozzle, adhesion and poor ejection can occur.
Solution: Appropriately shorten the runner length and increase its cross-sectional area. Strengthen the connection between the main runner and sub-runners. A cold slug well should be provided in the main runner. When determining the gate location, methods such as adding auxiliary gates can be used to balance the filling rates of each cavity in a multi-cavity mold and reduce the pressure in the mold cavity. Generally, the small-end diameter of the main runner should be 0.5~1mm larger than the nozzle hole diameter, and the concave radius of the sprue bushing should be 1~2mm larger than the radius of the nozzle sphere.
6. Unreasonable Ejection System Design or Improper Operation:
Cause: If the ejection device has insufficient stroke, uneven ejection, or malfunctioning ejector plates, the part will fail to eject.
Solution: Whenever possible, increase the effective ejection area of the ejector pins to ensure sufficient ejection stroke. The ejection speed of the part should be controlled within an appropriate range, not too fast or too slow. The main reason for ejector plate malfunction is the adhesion between sliding parts. For example, when the ejector plate actuates a slide core, because there is no cooling device at the slide core, its temperature is higher than that of other core pins. During continuous operation, the clearance between the slide core body and the core pin is very small, which often causes adhesion and leads to poor slide core movement. Another example is when the parallelism between the ejector pin hole and the guide pin of the ejector plate is poor, or the ejector pin is bent, the ejector plate will malfunction. If there is no stop pin in the ejection mechanism, and there are foreign objects between the ejector plate and the mounting plate, the ejector plate will tilt, resulting in subsequent malfunction. In medium and large molds, if there is only one ejector pin acting, the ejector plate will not be able to push evenly, which will also cause malfunction.
7. Poor Mold Venting or Lack of Air Intake for the Core:
Cause: Poor mold venting or lack of air intake for the core can also cause adhesion and poor ejection.
Solution: Improve the mold’s venting conditions, and provide air intake holes at the core.
8. Inappropriate Mold Temperature Control or Cooling Time:
Cause: If it is difficult to eject from the parting line, the mold temperature can be appropriately increased, and the cooling time can be shortened. If it is difficult to eject from the mold cavity, the mold temperature can be appropriately decreased, or the cooling time can be increased. In addition, too high a temperature in the fixed half of the mold can also lead to poor ejection. If the mold cavity material is a porous, soft material, it can cause adhesion.
Solution: Use a harder steel or apply a surface plating treatment.
