Weld Line

Formation Mechanism

Split melt flows cool down through continuous contact with cold cavity steel before meeting on the far side of an internal core pin. As the fronts collide, thin frozen layers at the front edges limit mutual molecular interdiffusion, preventing the polymer chains from fully entangling across the seam. When the collision occurs head-on, the convergence creates a butt weld, which exhibits noticeably weaker mechanical integrity than a melding line formed by parallel streams sliding alongside each other.

Trapped gases that fail to escape through tool vents get compressed between the incoming melt fronts, scorching the polymer and preventing molecular fusion. Low melt temperatures, inadequate injection velocities, and insufficient packing pressure exacerbate this poor molecular knitting, locking permanent structural weak points into the finished part.

Structural Penalty

Tensile strength across the weld line can drop by fifty percent compared to the bulk resin, while impact toughness plummets even more severely. In short-fiber reinforced grades, the fibers align parallel to the advancing melt fronts, remaining oriented along the interface rather than bridging across the seam. This orientation leaves the knit zone devoid of fiber reinforcement, turning the seam into an unreinforced resin plane prone to brittle fracture under minor impact.

Amorphous resins knit together more successfully than semi-crystalline polymers, as the latter demand precise thermal retention to achieve proper crystalline growth across the boundary. Cosmetic defects also multiply at these interfaces, where differences in crystal packing or pigment orientation create visible lines that ruin high-gloss, painted, or textured component aesthetics.

Tooling Prevention

Tool designers alter gate locations, revise part wall thicknesses, or implement sequential valve gating to steer melt fronts into non-critical zones. Adding overflow wells pulls the weak, chilled melt front out of the critical structural area, concentrating the defective seam in a scrap tab that is trimmed off after demoulding. Heating the tool steel locally via dynamic mould temperature control keeps the polymer front molten until convergence finishes, boosting molecular diffusion across the boundary.

Venting the mould steel along the parting line exactly where the fronts converge lets trapped air escape, preventing micro-dieseling and incomplete fusion. Regrind materials containing thermal degradation by-products degrade weld line strength further, because particulate contaminants accumulate directly at the advancing melt boundaries and disrupt molecular entanglement.