Weld lines, known as knit lines, weld lines are linear traces on the surface of plastic parts, primarily formed by the convergence of several flow fronts during injection molding or extrusion molding. The strength of weld lines typically ranges from 40% to 95% of the strength of the surrounding plastic, significantly impacting the design and lifespan of the product.
Causes of Weld Lines
Convergence of two or more streams of molten plastic at different temperatures.
Injection speed too fast or too slow.
Low fluidity of the material.
Nozzle temperature set too low.
Poor or inadequate venting.
Improper gate size or location.
Remedies for Weld Lines
Reduce the number of gates and adjust their positions.
Increase injection speed and pressure.
Raise material and mold temperatures to enhance plastic flow.
Improve the design of the mold venting system.
Reduce the use of mold release agents.
Shorten the distance between the gate and weld area.
