Solutions for Stress Whitening of Conical Plug in Plastic Mold Modification

Stress whitening often occurs on conical sealing plugs at cone surfaces, root corners and thin-wall transition areas during ejection, assembly or service. This defect originates from concentrated residual stress inside polymer materials. Microcracks form under external force and create a whitish visual appearance. Apart from raw material and process adjustment, targeted mold modification can reduce internal stress and relieve stress whitening of molded plastic parts.

Optimization of molded part structure on mold

Modify sharp corners at stress concentration zones into rounded transitions to lower stress during ejection and assembly. Keep uniform wall thickness on the conical surface and avoid abrupt wall changes, which easily generate residual stress during cooling shrinkage. Adjust the draft angle properly. Insufficient draft increases friction during demolding and pulls the surface of the plastic part, leaving residual stress and whitening. Thin sealing lips cannot be designed too thin to prevent deformation during ejection and uneven cooling that causes inconsistent shrinkage between inner and outer layers.

Modification of gate and runner system

Set the gate away from conical sealing surfaces and root corners with high stress. Prevent molten material from directly impacting thin-wall features and reduce internal stress caused by polymer chain orientation. Enlarge gate size moderately to shorten gate freeze time and decrease residual pressure from excessive packing pressure, a major source of residual stress. Select gate types that reduce shear to lower shear heating and molecular orientation. For multi-cavity molds, keep symmetrical runners to balance filling and avoid overpressure in single cavities. Polish runner inner walls smoothly and apply arc transitions at corners to reduce flow resistance and required injection pressure.

Optimization of ejection system

Unreasonable ejection structure is a common cause of stress whitening on conical plugs. Arrange ejector pins on rigid zones instead of thin conical surfaces to prevent concentrated ejection force from stretching and whitening parts. Add more ejector pins to share ejection load and ensure even force release. Inspect ejection clearance and eliminate pin sticking or unsynchronized ejection, which pulls the plastic surface and builds tensile stress. Adjust ejection draft and polish cavity surfaces to reduce demolding friction. Use segmented ejection speed with low initial speed to avoid instant tensile damage on the plastic surface.

Adjustment of mold cooling system

Uneven cooling leads to inconsistent shrinkage rates and residual stress. Add cooling channels near high-stress areas such as cone surfaces and root corners to achieve uniform mold temperature and reduce temperature difference inside molded parts. Maintain small temperature difference between inlet and outlet cooling water. Over rapid cooling locks polymer chains and accumulates heavy residual stress. Clean cooling channels regularly to avoid scale and cooling failure. Raise mold temperature properly according to material characteristics to slow cooling and allow polymer chains to relax and reduce residual stress.

Surface treatment of parting surface and cavity

Polish the conical sealing area of the cavity carefully, with polishing lines parallel to demolding direction to reduce scraping during ejection. Remove scratches and tool marks on cavity surfaces, which act as stress concentration points and scratch products during demolding. Check tiny undercuts on drafted surfaces. Minor undercuts scrape parts and induce stress whitening. Fit parting surfaces precisely to stabilize clamping and prevent local flash, as trimming flash easily damages edges and creates weak stress zones.

Optimization of venting system

Trapped gas creates uneven filling pressure, burn marks and stress concentration that worsen whitening. Add vent slots at melt end and corner positions with proper depth to exhaust trapped gas without generating flash. Keep vents away from functional sealing surfaces. Adequate venting lowers required injection pressure and reduces residual stress caused by high packing pressure.

Mold trial verification and long-term control

Verify modification results via mold trials and match stable molding parameters with structural changes. Immerse samples in solvent after molding to quickly test residual stress and check whether whitening zones are improved. Perform assembly tests to simulate working conditions and check reoccurrence of whitening under compression on conical surfaces. Build regular mold inspection checklist to monitor cavity roughness and ejector movement and avoid demolding scratches after mold wear for stable mass production.