Resolving Shiny Unevenness in Injection Molded Parts: Comprehensive Solutions

1. Optimization of Injection Molding Process Parameters

1.1 Adjust Injection Speed and Pressure

• Reduce the filling speed at the gate (e.g., from 20mm/s to 10mm/s) to minimize local shine caused by excessive shear stress.

• Adopt low-pressure and low-speed injection mode to avoid overall gloss inconsistency due to high-speed and high-pressure impact, which is particularly critical for precision injection molding of complex structural parts.

• Ensure the injection speed is uniform throughout the filling process to prevent uneven shear effects on the melt surface.

1.2 Control Mold Temperature

• Maintain balanced mold temperature (e.g., 80–90°C for the cavity side and 60–70°C for the core side), with a temperature difference between the two sides not exceeding 20°C.

• Uniform mold temperature ensures consistent cooling and shrinkage of the plastic part, avoiding gloss differences caused by uneven cooling—an essential control point in precision injection molding.

• Use a high-precision mold temperature controller to monitor and adjust the temperature in real time, ensuring stability within the set range.

1.3 Optimize Holding Pressure and Time

• Shorten the holding time appropriately to reduce molecular orientation stress during the material replenishment process.

• Ensure the residual pressure in the mold is close to atmospheric pressure when opening the mold, avoiding surface gloss variations caused by uneven internal stress.

• Adjust the holding pressure according to the part structure: use lower pressure for uniform wall thickness to prevent over-compression, and moderate pressure for thick-walled areas to ensure compactness without affecting gloss.

2. Improvement of Mold Design

2.1 Enhance Ventilation System

• Add vent grooves near the flow end and parting line to ensure rapid discharge of trapped air, preventing local high-temperature shine caused by air compression.

• The depth of the vent groove should be close to the material's flash value (generally 0.02–0.05mm) to balance exhaust efficiency and anti-flash effect.

• For complex cavities, design distributed vent grooves to avoid air accumulation in dead corners, a key detail in precision injection molding mold design.

2.2 Adjust Gate and Runner Design

• Increase the gate size to reduce the shear rate of the melt passing through, avoiding local overheating and gloss differences caused by excessive shear.

• Adopt a multi-gate design for large or complex parts to shorten the melt flow distance, ensuring uniform filling and consistent gloss across the entire part surface.

• Optimize the runner cross-section (prefer circular or trapezoidal) to reduce flow resistance and ensure stable melt flow, which is conducive to improving the surface consistency of precision injection molding products.

2.3 Address Uneven Wall Thickness and Structural Details

• Ensure the slope length of the wall thickness transition area is 5–7 times the thin wall thickness (thin → thick) or 3–5 times (thick → thin) to avoid abrupt thickness changes that cause uneven cooling and gloss.

• Use fillet transitions at corners to reduce stress concentration and ensure smooth melt flow; the thickness of ribs should not exceed 0.5–0.7 times the main body thickness, and hollowing treatment can be adopted to reduce local material accumulation.

• Disperse weld lines through throttling and exhaust design to avoid concentrated stress leading to gloss differences, which is an important optimization direction for precision injection molding molds.

3. Material Selection and Treatment

3.1 Improve Material Fluidity

• Choose materials with higher melt flow index (MFI) (e.g., adjust from 18 g/10min to 22 g/10min) to improve filling uniformity and reduce gloss differences caused by poor flow.

• Reduce the content of matting agents (e.g., from 4% to 2%) if necessary, as excessive matting agents can easily cause uneven dispersion and lead to shiny unevenness.

• For materials with poor fluidity, add an appropriate amount of compatibilizers or lubricants to optimize melt flow performance, which is especially important for precision injection molding of thin-walled parts.

3.2 Reduce Internal Stress

• For materials with high internal stress (such as PC), perform annealing treatment after molding to eliminate residual stress and avoid surface gloss inconsistencies caused by stress release.

• Add nucleating agents or reinforcing fibers to the material to disperse internal stress, improve the uniformity of the material structure, and enhance surface gloss consistency.

• Ensure the raw materials are fully dried to avoid moisture-induced bubbles or decomposition, which can affect surface gloss— a basic requirement for precision injection molding material preparation.

4. Conclusion