What Causes Part or Sprue Sticking in Injection Molding?

Rough Surface of the Mold Cavity or Sprue Bushing

If the mold cavity and sprue bushing have surface defects such as grooves, notches, scratches, dents, etc., the plastic part and gate can easily stick to the mold, making ejection difficult, and easy to case part and sprue sticking in injection molding. Therefore, the surface finish of the cavity and gate should be improved as much as possible. During polishing, the movement direction of the polishing tool should align with the mold-filling direction.

Mold Wear, Scratches, or Excessive Gaps

When molten material leaks into scratched areas or gaps between mold inserts, it can cause flash, leading to ejection difficulties. Damaged mold areas should be repaired, and gaps between inserts should be minimized.

Insufficient Mold Stiffness

If the mold cannot open at the beginning of injection, it indicates that the mold deforms under injection pressure due to insufficient rigidity. If the deformation exceeds the elastic limit, the mold cannot return to its original shape and becomes unusable. Even if the deformation stays within the elastic limit, the molten material cools and solidifies under high cavity pressure. When the injection pressure is released and the mold rebounds, the plastic part gets clamped by the rebound force, preventing mold opening.

Therefore, mold design must ensure sufficient stiffness and strength. During trial molding, a dial indicator should be installed to check for deformation in the cavity and mold base during filling. Initial injection pressure should not be too high—pressure should be gradually increased while monitoring deformation to keep it within an acceptable range.

If the mold fails to close due to excessive rebound force, simply increasing clamping force is insufficient. Instead, the mold should be disassembled immediately, and the plastic part should be heated and softened before removal. For molds with insufficient rigidity, an external frame can be installed to enhance stiffness.

Insufficient Draft Angle or Poor Parallelism Between Core and Cavity Plates

Sufficient draft angles must be ensured during mold design and manufacturing; otherwise, plastic part ejection will be difficult. Forced ejection often causes warping, ejection marks, or cracking. The core and cavity plates must remain parallel to avoid misalignment, which can lead to part sticking.

Improper Gating System Design

Issues include excessively long or undersized runners, weak connections between the main and branch runners, lack of a cold slug well in the main runner, unbalanced gates, mismatched nozzle and sprue diameters, or poor alignment between the sprue bushing and the spherical nozzle. To address this, runner length and cross-sectional area should be optimized, connection strength between main and branch runners should be increased, and a cold slug well should be added to the main runner.

For multi-cavity molds, gate positioning should balance filling rates across cavities, and auxiliary gates can be added to reduce cavity pressure. Typically, the sprue’s small-end diameter should be 0.5–1 mm larger than the nozzle diameter, and the sprue bushing’s radius should be 1–2 mm larger than the nozzle radius.

Improper Design or Operation of the Ejection Mechanism

If the ejection mechanism has insufficient stroke, uneven ejection, or poor movement of the ejector plate, the plastic part cannot be ejected properly.

Incorrect Process Parameter Settings

Over-packing: Causes excessive adhesion of the part to the mold.
Excessive holding pressure: Leads to strong adhesion, making demolding difficult.
Overlong holding time: Results in tight adhesion, causing sticking.
Excessive ejection speed: May cause warping or cracking of the part.

Material Issues

Excessive material temperature: Causes the part to adhere too strongly to the mold.
Poor material flowability: Makes it difficult to eject the part from the mold.