10/07/2026
In injection molding, weld lines form when separated melt fronts reunite after flowing around an obstruction or entering through multiple gates.
As the fronts cool, polymer diffusion and entanglement across the interface can be reduced, creating a localized mechanical weakness.
In glass-filled materials, the risk can increase because:
• Fiber orientation changes at the interface, often leaving fewer fibers effectively bridging the weld region
• Load transfer across the weld region is reduced
• Crack initiation may occur under cyclic loading
Common high-risk locations include:
• Holes and cutouts
• Multi-gate designs
• Insert-molded features
• Abrupt wall-thickness transitions
Weld lines can substantially reduce tensile and fatigue performance. The magnitude depends on the material, reinforcement, weld-line orientation, geometry, and processing conditions.
For structural components, weld-line location should be considered during part design, not simply evaluated after tooling is complete.
Engineering takeaway:
A weld line in a highly loaded region should be treated as a structural design condition, not a cosmetic artifact.
During DFM, Davies evaluates:
• Gate location
• Flow-path balance
• Venting strategy
• Mold-filling behavior
• Weld-line location relative to highly stressed features
Address weld-line risk before tooling begins.
Talk with Davies about your part design: https://ow.ly/nbHa50ZUzGO
