Knit Lines (Weld Lines)

A knit line forms wherever two melt fronts meet and fail to fully merge. The line is always weaker than the surrounding plastic. In the worst case it holds about 20% of the base strength. In the best case it reaches around 80%. It also leaves a witness mark on the surface. You cannot wish knit lines away on complex parts. You can decide where they land and how strong they are.

Where knit lines come from

Two sources cause most of them. Multiple gates split the fill into fronts that must reunite. Flow around a core pin or a molded hole does the same on a smaller scale. The stream divides, wraps the pin, and knits downstream. That downstream region is weakened. This is why holes in stressed areas are often machined after molding instead of molded in. Gate position and part geometry write the knit line map before the first shot. Flow simulation earns its fee here. It shows where the fronts will meet while the steel is still soft.

Put the line where it can do the least harm

When multiple gates are unavoidable on a large or complex part, place the knit where stress is lowest. Keep knit lines out of high-stress zones and away from edges. The same rule applies to gates. Roughly two-thirds of fracture lines run through a gate, so a knit near a gate doubles the risk. A knit line across a snap fit or a mounting boss is a field failure waiting for a date. Move the gate and you move the line.

Temperature is the other half of strength. Hotter melt fronts knit better because the polymer chains interdiffuse across the interface. Running melt and mold temperature at the high end of the material window is the cheapest strength upgrade available. Venting matters too. Air trapped at the knit keeps the fronts from meeting intimately, so vent the knit region generously. Shallow vents of 0.01–0.04 mm clear the air without flashing.

A real tradeoff: the textured lid

A consumer device lid shows how these decisions play out. The show surface carried a medium sandblast texture. The mold builder chose two edge gates on the non-cosmetic side, and filling drove that call. The part was fairly large and fairly thin, with some thicker sections. The gate vestiges had to stay hidden.

The knit line landed in the textured region and stood out. Under raking light the line itself was visible. The bigger problem was the texture around it. The cooler leading fronts pressed into the mold texture differently than the hotter material behind them. The thin part froze before the texture could even out. The result was a visible band of altered texture straddling the knit.

The team weighed four options. One edge gate would erase the knit but risked a short fill. Thickening the part would make single gating viable, at the cost of weight, extra material, redesign time, and mating-part headaches. Moving a gate toward the knit would push the cold-front region along with the flow. But the gate vestige would then land on the cosmetic face. Working with the customer engineer, they chose a hot tip gate feeding straight through the A side into the part. No sprue, no runner. The tradeoff was a small hot-tip vestige and slight blushing against a knit line gone. The customer took the vestige.

For multi-gate parts where the knit cannot be relocated, sequential valve gating is the modern answer. The gates open in cascade, so the second melt front arrives while the first is still fully molten. The two merge instead of knitting cold. It costs a valve-gate hot runner system, but on cosmetic housings the math usually works.

Design rules that prevent weak knits

• Gate so that knits form in low-stress, non-cosmetic areas. Simulate before cutting steel.
• Never place a knit across a snap fit, living hinge, or loaded boss.
• Run melt and mold temperatures at the high end of the material window to strengthen the knit.
• Vent the expected knit region. Trapped air guarantees a weak line.
• For holes in stressed zones, mold small and ream, or machine after molding.
• On textured show surfaces, remember the knit changes texture replication, not just strength. The lid case above is the pattern to avoid.

When the knit still fails at trial

Raise melt temperature first. A 10–20°C increase at the nozzle often lifts knit strength a full grade with no tool change. Next, slow the fill slightly through the knit region so the fronts arrive hot. If the line persists, check venting at the knit. A blind pocket of trapped air guarantees a cold shut no process tweak can fix. Add vents or move an ejector pin to break the pocket. When none of that works, the gate scheme is wrong for the geometry. Sequential valve gating or a relocated gate solves it at the design level. Do not accept a weak knit on a loaded feature and hope inspection catches it.

A flow mark costs appearance. A knit line in the wrong place costs the part. When knit strength is safety-relevant, prove it with molded test bars from the actual gate scheme, not with the resin datasheet.

If knit lines keep surprising you at trial, the gating scheme is the suspect. A defect root-cause analysis of the fill pattern usually finds the front collision. A simulation would have shown it before steel was cut.