Weld Line / Confluence Mark

Line-shaped marks are formed where multiple melt streams converge. Weld lines appear as line-shaped marks along the flow convergence direction, with darker or lighter color and poor surface gloss; fine grooves can be seen under magnified observation. Mechanical tests show that the tensile strength at the weld line is only 60%-80% of the base material, and the notched impact strength can drop below 40%. When the part is subjected to bending or impact loads, fracture often starts at the weld line. For transparent parts, the refractive index difference at the weld line causes the area to show obvious strip-shaped optical distortion.

1. Process parameter factors: low melt convergence temperature, poor venting, slow injection speed, improper gate position. 2. Mold comprehensive factors: in addition to venting, mold design parameters such as cooling water channel layout, gate position and size, and demolding draft angle all have a systematic impact on part quality, and optimization is required from the source of mold design.

1. Process parameter optimization and adjustment: increase melt temperature and mold temperature, speed up injection speed, add overflow wells, optimize gate layout. 2. Systematic troubleshooting method: troubleshoot item by item in the order of "parameters -> material -> mold -> equipment". First adjust the parameters that are easiest to change, such as injection pressure and temperature, and observe the effect; if there is no improvement, check the drying and fluidity of the raw material; further check mold venting and gate design; finally check the mechanical condition of the equipment (check ring, screw). Change only one variable at a time to determine the root cause. 3. Preventive management: establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluation. For recurring problems, use fishbone diagram or 5Why analysis to trace the root cause, and fix the countermeasures as PFMEA control points.

Weld lines cannot be completely eliminated but can be greatly reduced. The core strategy is to let the two melt streams converge under conditions of high temperature and low air resistance — increase mold temperature to slow down front cooling, improve venting to eliminate air cushion barrier, and optimize gate layout to improve the convergence angle.