How to Formulate Rectification Plan for Injection Mold of Product Whitening Defect in China
Whitening defect is a common appearance defect in injection molding, which mostly appears on product surface corresponding to ejector pins and ejector blocks, showing whitish marks. In serious cases, surface tensile cracks occur. It is generally caused by excessive local tensile force of products during demolding. Form rectification plan from multiple dimensions including mold structure, surface state, ejection system, cooling and venting to reduce demolding stress and eliminate whitening defect.
Optimization of Ejection Mechanism Structure
Adjust ejector pin layout to avoid product appearance surface, thin wall and stress sensitive area, and arrange ejector pins at thicker and rigid positions of products to disperse ejection force. Increase ejector pin quantity to reduce load on single ejector pin and prevent high pressure at single point from stretching product surface. For annular or large-area products, ejector blocks or sleeve ejector pins can replace small-diameter ejector pins to increase contact area and reduce unit pressure. Check fit clearance between ejector pin and core. Too small clearance causes sticking and asynchronous ejection to pull plastic parts; too large clearance easily generates flash. Control clearance according to material characteristics. Optimize ejection stroke and ejection guide to guarantee smooth operation of ejector plate without stuck, and eliminate unilateral stress caused by tilt of ejection mechanism.
Optimization of Demolding Draft Angle and Product Fillet
Verify demolding draft angle of molding surface of product. Insufficient draft angle is a high-incidence inducement of whitening. Adjust draft angle according to plastic material, and moderately increase draft angle for high friction materials. Add fillet at inner wall corner of product to reduce stress concentration at corners and reduce the probability of material scratch and whitening. Check tiny undercut on mold surface. Polishing direction should be consistent with demolding direction. Small undercut will hook product during demolding and cause surface tensile whitening. If product structure cannot be modified, adopt local insert structure for convenient polishing and trimming of undercut area in later stage.
Polishing Treatment of Mold Cavity Surface
Carry out fine polishing on cavity area corresponding to whitening defect to reduce surface roughness of mold and adhesion friction between plastic part and mold cavity. Polishing process is carried out step by step from coarse to fine to remove tool marks and scratches. Scratches form stress concentration and scratch products during demolding to form white marks. Polishing texture must follow demolding direction. Transverse polishing grain increases friction resistance. Clean cavity surface after polishing to remove polishing paste residue, avoid sticking glue in molding process and increase demolding resistance. For materials easy to stick mold, evaluate reasonable surface coating treatment to reduce adsorption force between plastic and steel.
Rectification of Mold Cooling System
Uneven cooling causes inconsistent shrinkage between surface and inner part of plastic parts. The surface hardness of product is insufficient, and whitening easily occurs under ejection force. Add cooling channels near cavity area with whitening defect to reduce mold temperature difference and guarantee uniform cooling of products. Control cooling water flow rate and temperature to avoid high local mold temperature, softened product surface and tensile white mark after ejection. Clean scale in cooling pipeline regularly to prevent local abnormal temperature caused by pipeline blockage. Set matched mold temperature according to different plastic materials to ensure full hardening of plastic part surface before ejection action.
Optimization of Venting Structure
Trapped gas in cavity causes local scorch, vacuum adsorption between product surface and mold, sharply rising demolding resistance and inducing whitening. Add vent slots near ejector pins and product side wall. The depth of vent slots matches flash threshold of raw materials to discharge gas smoothly without flash. Check ejector pin clearance and use end face of ejector pin for auxiliary venting to eliminate vacuum adsorption effect inside cavity. Keep vent channel unobstructed, and clean carbon and glue debris regularly during production to prevent vent blockage and reform negative pressure mold sticking.
Rectification of Guide Slide and Ejection Synchronization
Check guide of ejector plate and state of return pin fitting to guarantee synchronous movement of all ejection components. Wear of return pin and stuck guide sleeve cause advance or delayed ejection of partial ejector pins and local pulling of products. Replace worn guide sleeves and reset components to ensure horizontal movement of ejector plate. Add ejection limit structure to prevent over-travel of ejection. For deep cavity products, adopt secondary ejection structure to release product wrapping force in sections. Separate product from cavity slightly at first, then eject completely. One-time ejection easily generates huge tensile stress instantly.
Mold Assembly and Daily Maintenance Control
Check height difference of ejector pins during mold assembly. End face of all ejector pins should be flush with cavity surface. Ejector pin higher or lower than cavity surface leaves stress marks on product surface. Clean oil stain and glue stain on cavity and ejector pin surface before production to reduce mold sticking risk. Establish mold inspection list, regularly check ejector pin wear and cavity surface scratch, and repair tiny damage in advance. Verify rectification effect by mold trial, compare samples before and after rectification, adjust injection molding parameters cooperatively with mold modification to reduce wrapping force and stabilize mass production.
