Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Weld lines, also known as knit lines or meld lines, are surface defects that appear on molded plastic parts during the injection molding process. 

They form when two or more fronts of molten plastic (the "melt") converge and attempt to fuse together, but the resulting joint is often weaker than the surrounding material. 

How Weld Lines Form

In injection molding, molten plastic is injected into a mold cavity under high pressure. Weld lines typically occur in these scenarios:

1.Flow around obstacles: The melt splits to flow around features like holes, cores, pins, inserts, or bosses, then rejoins on the opposite side. 

2.Multiple gates: When the mold uses two or more injection gates, the melt fronts from each gate meet in the cavity. 

3.Flow front convergence: At the meeting point, if the fronts collide at a sharp angle (often >90°), the plastic doesn't fully knit together due to insufficient time, temperature, or shear for molecular entanglement. 

Visually, weld lines often appear as faint lines, seams, or color variations on the part's surface, though they can sometimes be glossy or V-shaped.

Types of Weld Lines

Meld lines: Form when flow fronts meet at shallow angles (<90°), allowing better fusion and strength closer to the bulk material. 

Knit lines: Occur at steeper angles, leading to poorer bonding and more noticeable weaknesses. 

Effects and Importance

Weld lines are a common challenge in injection molding because they create potential failure points:

Mechanical weakness: The joint has reduced tensile strength (up to 50-80% weaker) due to incomplete molecular orientation and fewer entanglements at the interface. 

Aesthetic issues: They can mar the appearance of visible surfaces, especially in clear or colored parts.

Functional risks: In load-bearing parts (e.g., automotive components or medical devices), they may lead to cracks or leaks under stress.

While unavoidable in complex molds, they can be minimized through design changes (e.g., relocating gates or obstacles), process adjustments (e.g., higher melt/mold temperatures, faster injection speeds), or material selection. 

Simulation software is often used to predict and avoid them during mold design.