Causes and Elimination Methods of Weld Lines in Injection Molding

Weld lines are seam marks formed after two or more melt flows converge and cool inside mold cavity. It is a common appearance and structural defect in injection production. Weld lines form when melt converging position suffers insufficient pressure, temperature drop or poor exhaust. Weld lines not only leave visible linear marks on plastic part surface, but also reduce local mechanical strength of products, and the part is easy to crack at this position under stress, which may result in batch scrap in serious cases. Multi-dimensional inspection from raw materials, mold structure and injection molding process can reduce or eliminate adverse effects brought by weld lines and stabilize appearance and structural performance of products.

1. Raw Material Factors and Improvement Measures

Drying state of raw materials directly affects melt flow performance. Moisture inside raw materials vaporizes under high temperature and forms bubbles at melt converging area, worsening weld lines. Some raw materials have low fluidity and cool quickly during filling. Two melt flows cannot fully fuse when meeting and form obvious seams. For modified plastics added with glass fiber or mineral filler, solid fillers accumulate at melt converging interface and further reduce bonding strength. Complete raw material drying in accordance with material specifications before production and control moisture content within standard range. Select raw material grades with better fluidity on the premise of meeting product performance requirements. For filled materials, appropriately increase additive ratio to improve filler dispersion and reduce filler accumulation at converging interface.

2. Influence and Optimization of Mold Structure

Gate quantity and position are key factors leading to weld lines. Multi-gate layout inevitably produces melt convergence lines. Improper gate positions may leave weld lines on appearance or key stress areas. Holes, pillars and ribs inside products block melt flow and force melt to converge after bypassing obstacles, generating weld lines behind barriers. Insufficient cavity exhaust prevents trapped gas at melt converging area from discharging, forming burn marks and hindering full melt fusion. Optimize gate scheme, reduce gate quantity on the premise of meeting filling requirements and adjust gate positions to transfer weld lines to non-appearance and low-stress areas. Split hole and pillar structures into inserts as much as possible, open micro exhaust grooves at positions corresponding to weld lines to discharge trapped gas rapidly. Reasonably adjust runner size to lower pressure loss of melt in runners and ensure melt has sufficient pressure when reaching converging positions.

3. Adjustment Methods of Injection Molding Process Parameters

Low melt temperature and mold temperature cause plastic to cool down before reaching converging positions, resulting in insufficient entanglement of molecular chains and weak seam strength. Slow injection speed makes the front of melt solidify in advance, so two melt flows cannot fully fuse. Insufficient injection pressure fails to compact converging interface and forms loose seams. Improper pressure holding maintenance also amplifies weld marks. Raise barrel temperature and mold temperature to maintain high activity of melt. Improve injection speed to make two melt flows converge under relatively high temperature. Appropriately increase injection pressure and packing pressure to compact melt converging area and promote molecular bonding. Optimize screw back pressure to improve melting uniformity and reduce melt temperature difference. Control cycle time to avoid material degradation caused by long residence in barrel.

4. Optimization Points of Product Structure Design

Excessive wall thickness difference leads to uneven melt flow speed. Melt in thin wall positions cools in advance and obvious weld lines tend to appear. Too small wall thickness accelerates melt cooling and increases probability of weld lines. Keep uniform wall thickness transition in product structure design to reduce sudden wall thickness change. Add local fillets at melt converging positions to reduce temperature loss caused by melt detour. Appropriately increase local wall thickness at weld line positions without affecting assembly to improve melt fusion effect. Reduce isolated columns, small holes and other melt-blocking structures inside products to lower the chance of flow bypass and convergence.

5. Other Auxiliary Improvement Measures

Add overflow wells on mold corresponding to weld line positions to let cooled melt front flow into overflow wells, so pure melt can converge. Cut off overflow well material after molding. Clean mold exhaust grooves regularly in production to prevent carbon deposition from blocking exhaust effect. For appearance parts, cooperate with mold polishing after process optimization to weaken visual perception of weld lines and improve appearance quality.