Summary of Common Defects and Solutions for China Injection Molding

In the production process of China injection molding, various product defects are mostly related to raw material status, mold structure and injection molding process parameters. A single defect may have multiple causes. Troubleshooting step by step from simple to complex can improve defect handling efficiency and stabilize product yield.

Sink Marks

Sink marks usually appear on the back of thick rubber positions and rib structures. After the melt cools and shrinks, the surface caves inward. The main cause is insufficient rubber replenishment in thick areas when molten material shrinks during cooling. Appropriately increase packing pressure and extend packing time to enhance material feeding capacity at the gate. Reduce barrel temperature and mold temperature to lower melt shrinkage. At the mold level, optimize the gate position to shorten the flow distance of melt to thick rubber areas. Reduce wall thickness at thick sections where possible, and add local exhaust if necessary.

Weld Lines

Weld lines form where two or more melt streams converge. The position has weak mechanical strength and thin visible lines on the surface. The core inducement is temperature drop before melt convergence and poor air exhaust. Raise barrel and mold temperature to improve melt fluidity. Increase injection speed to shorten filling time. Adjust the number of gates to reduce melt convergence points. Open exhaust slots at weld line positions to discharge trapped air inside the cavity. Modify product structure to avoid weld lines appearing on stress-bearing areas if product structure permits.

Flash

Excess rubber overflowing at parting surfaces, inserts and ejector pin gaps forms flash. Insufficient clamping force, excessive injection pressure and large mold cavity gaps easily trigger this problem. Prioritize increasing clamping force and reduce injection and packing pressure. Check whether the mold parting surface has foreign matter or crush damage, grind and repair damaged sealing surfaces. Control material temperature to prevent melt from being too thin. Inspect fitting gaps of ejector pins and slides, and replace parts when gaps exceed standards.

Silver Streaks and Voids

Silver streaks distribute on the product surface while bubbles hide inside the product. Most of them come from raw material moisture and decomposed gas inside the barrel. Raw material moisture is the most common factor. Dry raw materials strictly according to material requirements. Reduce barrel temperature to prevent gas generated by high-temperature decomposition of plastic. Slow down screw retraction speed to reduce entrapped air. Add exhaust slots on the mold to discharge gas out of the cavity. Distinguish bubble types. Internal bubbles of thick parts are mostly shrinkage bubbles, which need to be optimized by packing adjustment.

Ejector Whitening and Ejector Cracking

Whitening or even cracking occurs at the ejection position of products, caused by excessive wrapping force and uneven ejection force. Reduce mold temperature to lower wrapping force of products on the mold. Increase draft angle and polish cavity surface to reduce demolding resistance. Adjust ejector pin layout and add ejection points to disperse ejection force. Reduce ejection speed. If ribs are too deep, add lifter structure on the mold to assist demolding.

Warpage Deformation

Plastic parts bend or twist after demolding, resulting from uneven shrinkage in different regions. Adjust mold temperature to keep consistent cooling speed on both sides of products. Reduce packing pressure to cut internal stress. Optimize product wall thickness to reduce sudden thickness change. Reserve pre-deformation compensation at the mold design stage. For glass-filled materials, pay attention to fiber orientation and adjust gate direction to reduce deformation tendency.

Short Shot

Melt cannot fully fill the cavity, resulting in incomplete filling at local product positions. Raise injection pressure and injection speed, increase barrel and mold temperature to improve fluidity. Enlarge gate and runner size to reduce flow resistance. Check exhaust slots. Gas trapped in the cavity will block melt filling. Confirm whether raw material fluidity meets molding requirements when replacing materials.

Black Spots and Burning Marks

Partial black burning is mainly caused by compressed heating of trapped gas in the cavity or carbonization of materials at high temperature. Optimize exhaust, widen and deepen exhaust slots appropriately. Reduce injection speed to cut heat generated by gas compression. Lower barrel temperature and clean carbon deposits accumulated in the barrel. Check dead corners of runners, where carbonized material is easy to accumulate, and polish these areas.

Floating Fiber

Glass fiber exposed on the surface of glass-filled plastic affects appearance. Raise barrel and mold temperature to improve resin wrapping performance on glass fiber. Reduce injection speed to decrease shear fracture of glass fiber. Optimize gates to control melt flow direction. Polish the cavity to improve surface coating effect. Low floating fiber modified grades can be selected in raw material selection.

Stress Cracking

Products crack after being placed for a period of time due to high residual internal stress. Reduce packing pressure to lower internal stress generated during filling. Add fillets to products to reduce stress concentration. Add tempering treatment after molding to remove stress. Optimize gate position to avoid high pressure locally on melt.