The Nature of Injection Molding Defects: Five Major Process Imbalances
Injection molding usually goes through stages such as plasticizing and metering, injection filling, speed-pressure switching, pressure maintaining and shrinking, cooling and shaping, and ejection and demoulding. Abnormalities at any stage may cause defects on the product.
Based on physical mechanisms, common Injection Molding defects can be summarized into the following five categories.
Typical defects include short shots, jet marks and flow marks.
- Insufficient melt flow capacity;
- Excessive runner, gate or cavity resistance;
- The injection speed curve is unreasonable;
- Melt front temperature drops too fast;
- Local wall thickness changes leading to stagnation, jetting, or sudden changes in flow rate;
- Cavity exhaust resistance prevents continued filling of melt.
Typical defects include trapped air, scorch, silver streaks, bubbles and localized surface whitishness.
- Original air in the cavity;
- Moisture and volatiles in raw materials;
- Air entrapped during plasticization;
- Gases produced by thermal degradation of materials;
- Release agent, oil, or other contaminants evaporate due to heat.
Typical defects include flash, dents, shrinkage cavities and localized dimensional instability.
- The cavity pressure at the filling end is too high;
- The speed-pressure switching position is unreasonable;
- Insufficient holding pressure or effective holding time;
- Premature freezing of gates;
- Thick-walled areas do not have consistent access to feed;
- Insufficient mold sealing ability or local stiffness.
Typical defects include delamination, brittleness, and insufficient weld line strength.
- Compatibility between materials;
- Dry state of raw materials;
- Resin thermal history;
- The proportion of recycled materials and the degree of contamination;
- Degradation of molecular chains;
- Temperature, pressure and exhaust conditions when two melt fronts meet.
Typical defects include warpage, dimensional drift and stress cracking.
Injection molded parts don’t simply shrink uniformly as a whole, but shrink in different areas at different speeds, degrees and directions. When the cooling, holding pressure, molecular orientation, or fiber orientation of various areas of the part are inconsistent, internal stress will be released through warping, deformation, or cracking.
Therefore, the first principle of defect diagnosis is to first identify the dominant mechanism, and then decide whether to start with materials, product structures, molds, equipment or process conditions.
