Injection Molding Surface Gloss Defects: Causes and Solutions

I. Common Types of Surface Gloss Defects

II. Gloss Defects Caused by Raw Materials

1. Moisture absorption or insufficient drying

• Dry materials fully in accordance with material specifications;
• Do not arbitrarily reduce drying temperature or duration;
• Increase inspection frequency of dehumidifying dryers in humid seasons;
• Prevent dried pellets from prolonged exposure to ambient air.

2. Thermal degradation or overheating

• Lower barrel temperature, especially the nozzle and front zone temperature;
• Shorten material residence time inside the barrel;
• Avoid excessive backpressure and high shear;
• Clean the barrel regularly to prevent accumulated thermal history.

3. Poor dispersion of fillers, additives and color masterbatch

• Verify compatibility between color masterbatch carrier and base resin;
• Appropriately raise backpressure to enhance mixing;
• Optimize screw configuration and process parameters;
• Switch to color masterbatch or additives with better dispersibility.

4. Poor surface replication due to material shrinkage

• Increase packing pressure and packing time;
• Optimize injection speed and switchover position;
• Improve mold venting to avoid gas blocking melt contact with cavity surface;
• Adjust gate location and runner size if necessary.

III. Gloss Defects Caused by Molds

1. Poor cavity surface condition

• Finish cosmetic surfaces with mirror or textured treatment as required;
• Inspect cavity surface condition periodically;
• Clean and apply anti-rust treatment before and after production;
• Avoid scratching the cavity with hard tools.

2. Improper gate and runner design

• Enlarge gate size appropriately;
• Optimize gate location to reduce melt impingement on cosmetic surfaces;
• Shorten runners to minimize pressure drop;
• Adopt multi-stage injection speed to reduce shear near gates.

3. Poor venting

• Add vents at melt end positions, weld lines and thick sections;
• Check the depth and width of vent slots;
• Clear carbon deposits and plastic residues inside vents;
• Reduce injection speed to avoid adiabatic gas compression.

4. Uneven cooling

• Optimize cooling channel layout to minimize mold temperature difference;
• Check cooling channels for blockage and scaling;
• Stabilize mold temperature and avoid temperature fluctuation from frequent mold opening/closing;
• Apply zone temperature control for large flat parts.

5. Improper release agent application

• Minimize the use of release agent whenever possible;
• Select release agents compatible with the plastic material;
• Apply thinly and evenly;
• Clean molds thoroughly to prevent release agent residue buildup.

IV. Gloss Defects Caused by Molding Process

1. Unsuitable injection speed

• Use multi-stage injection: fast filling at early stage, slow packing at later stage;
• Reduce injection speed around the gate;
• Find a balanced speed window for filling and shear control;
• Adjust speed switch points according to cosmetic surface layout.

2. Insufficient packing or early switchover

• Increase packing pressure;
• Extend packing time;
• Optimize the switch point from injection to packing;
• Confirm gate freeze-off.

3. Low or unstable mold temperature

• Raise mold temperature appropriately;
• Use a mold temperature controller for stable temperature control;
• Reduce idle waiting time between mold cycles;
• Guarantee stable cooling water flow and temperature.

4. Too high or too low barrel temperature

• Set barrel temperature within the recommended range for the specific resin grade;
• Prevent overheating at the nozzle;
• Reduce backpressure and screw speed to cut shear heating;
• Refer to actual melt temperature instead of only set values.

5. Excessive backpressure and screw speed

• Lower backpressure;
• Reduce screw rotation speed;
• Maintain sufficient homogenization while avoiding excessive shear;
• Strictly control thermal history for heat-sensitive resins.

V. Quick Troubleshooting Guide for Common Gloss Defects

Scenario 1: Bright band near the gate

• Is injection speed too high?
• Is gate size too small?
• Is nozzle temperature too high?
• Does the material suffer from shear overheating?

Scenario 2: Overall dullness or haze

• Is the material fully dried?
• Is mold temperature too low?
• Is mold venting inadequate?
• Is release agent oversprayed?

Scenario 3: Local bright spots

• Do spots correspond to ribs or screw bosses on the opposite side?
• Is there localized shear overheating?
• Are sink marks or material accumulation present?
• Is color masterbatch poorly dispersed?

Scenario 4: Flow marks or light-dark stripes

• Does melt flow cross cosmetic surfaces?
• Is injection speed unstable?
• Is mold temperature evenly distributed?
• Is gate location poorly designed?

Scenario 5: Gloss inconsistency between batches

• Are drying conditions consistent?
• Does barrel temperature fluctuate?
• Is mold temperature stable?
• Are resin batches or color masterbatch ratios changed?

VI. Summary of Comprehensive Solutions

VII. Preventive Recommendations

1. Evaluate material and mold at the early design stage For parts with strict gloss requirements, confirm material shrinkage, cavity texture, gate layout, venting and cooling plans during material selection and mold review.
2. Establish standardized process parameter sheets Lock critical parameters including drying condition, barrel temperature, mold temperature, injection speed, packing and backpressure for each product to minimize arbitrary adjustments.
3. Strengthen first-article inspection Compare the first shot with the approved standard sample before mass production, to confirm gloss, color and texture consistency.
4. Perform regular mold maintenance Clean cavities, vents and cooling channels periodically to prevent gloss defects caused by oil, carbon deposits and scaling.
5. Control raw material and color masterbatch batches Conduct trial molding when switching resin grades or masterbatches to avoid gloss variation from batch differences.

Conclusion