Plastic Mold Rectification Ideas for Stress Whitening of Conical Cap Plugs

Stress whitening is a typical molding defect of conical cap plugs. Under the action of ejection force, residual stress inside the product causes local whitening on the surface, which affects appearance and structural performance. Most residual stress originates from unreasonable mold structure and poor flow conditions during filling. The mold rectification work should focus on product structure, gating system, ejection and cooling, to reduce melt flow resistance and uneven stress distribution and eliminate whitening issues.

1. Product Wall Thickness and Structural Optimization

Check wall thickness uniformity of the conical cap plug first. Abrupt wall thickness transition will cause uneven melt cooling and residual stress. Smooth the sharp transition positions, add rounded corners at the junction of cone and cylinder to reduce stress concentration. Avoid overly deep reinforcing ribs. If the inner cone draft angle is insufficient, increase the draft angle properly, which can lower the friction force between product and core during ejection and reduce surface tensile stress leading to whitening.

2. Gate and Runner System Adjustment

Improper gate type and position are common causes of stress whitening. Move the gate away from the whitening area. For conical cap plugs, side gate or submerged gate is preferred instead of gate directly on the outer cone surface. Increase gate cross-section moderately to reduce shear stress during melt passing. Optimize runner balance to ensure uniform filling of the whole cavity, prevent local over-packing. Reduce sharp corners inside runners to avoid high shear heating and material molecular orientation.

3. Ejection Mechanism Modification

Over-concentrated top force will directly pull the product and form whitening. Expand the ejection contact area. Add more ejector pins or adopt sleeve ejection to disperse ejection pressure. Ensure all ejector pins keep synchronous movement. Polish core surface to reduce demolding friction. Check for rough marks or tiny scratches on the core surface. If the core surface is too rough, polish it to lower the friction resistance when the product separates from the mold.

4. Cooling System Optimization

Uneven cooling creates inconsistent shrinkage and residual stress. Adjust cooling water channel layout, make cooling channels close to the cone forming surface, keep uniform mold temperature on inner and outer sides of the cap plug. Eliminate local hot spots. Control mold temperature properly, raise mold temperature appropriately to slow down melt cooling speed, reduce internal stress caused by rapid cooling. Avoid local overcooling at the position where whitening appears.

5. Exhaust Structure Improvement

Poor exhaust leads to trapped gas and high local pressure, resulting in stress abnormality. Add exhaust slots at melt filling end and conical surface fusion line position. Set reasonable exhaust depth to prevent flash. Clean exhaust slots regularly to avoid blockage by gas residue. Good exhaust helps complete cavity filling and reduces unstable pressure fluctuation inside the plastic part.

6. Mold Trial Verification and Fine Tuning

After mold modification, carry out trial molding step by step. Start with low injection speed and low packing pressure to reduce molecular shear stress. Observe the whitening position of the product, check whether the defect is weakened. If whitening still exists, recheck ejection synchronization and mold temperature distribution. Record rectification measures and product changes, form standardized reference data for subsequent mold maintenance and mass production adjustment.