Front page / What is a weld line, and how much weaker does it make a part?

What is a weld line, and how much weaker does it make a part?

A weld line, also called a knit line, forms where two melt fronts meet inside the mould, for example after flowing around a hole or from two gates. Plastics Technology reports that strength at a weld line can be as little as 20 percent of the part's nominal strength or as high as 100 percent. Glass-filled grades usually suffer most. You cannot always remove weld lines, but you can usually move them away from areas that carry load or must look good.

Weld lines and meld lines

Autodesk's Moldflow help separates the two by the angle at which flows meet. Weld lines form when fronts meet head-on or at a steep angle; meld lines form when parallel flows merge, and they tend to be stronger and less visible. Machine Design describes the same split as fronts meeting from opposite directions versus the same direction.

How much strength is lost

Machine Design explains the glass-fibre loss: fibres at the meeting point turn across the flow, so they add no strength there, and the knit line behaves roughly like unfilled resin.

Counting them before the mould is built

Machine Design gives a simple estimate: N = G + Co − 1, where N is the number of weld lines, G the number of gates and Co the number of through cores or pins that split the flow. One gate and two through holes therefore give about two weld lines; two gates and two holes give about three. A mould flow analysis shows where they will fall.

What helps

• Move them with the gate. Plastics Technology's first rule is to gate so the weld line lands in an area that is not highly stressed in use, and where fronts can keep flowing together after they meet.
• Keep walls even. The same article gives maximum nominal wall variation of 25 percent for amorphous or low-shrink resins and 15 percent for semi-crystalline or high-shrink resins.
• Vent the meeting point. Trapped air and volatiles can hold the fronts apart; Plastics Technology lists venting at the weld line, a flow tab that is trimmed off later, porous steel inserts, vented core pins and vacuum venting.
• Temperature and pressure where the fronts meet. Autodesk says a good weld typically forms when the melt is no more than 20 °C below the injection temperature at that point, and that pressure until freeze-off matters. Plastics Technology adds that raising melt temperature can release volatiles that weaken the weld, so it treats that as a last resort. It points first to faster injection, higher and longer pack and hold, and raising mould temperature by about 10 °C.
• Choose the resin with weld lines in mind. If a glass-filled part must carry load across an unavoidable weld line, discuss alternatives with the resin supplier.

Strength figures here are published examples. Actual weld-line strength depends on the grade, design, mould and process, and is confirmed by testing moulded parts.

Questions buyers ask

Can the moulder simply process the weld line away?

Usually not completely. Plastics Technology notes that processing can improve strength and appearance but cannot remove root causes in material, part or tool design.

Is a visible line always a weak line?

No. Some lines are mainly cosmetic, and meld lines are generally stronger than weld lines. Load testing tells you whether the line matters.

What should I ask for before steel is cut?

Ask for a mould flow report showing predicted weld line positions, the gate options considered, and venting at those positions.

How do I check weld lines on samples?

Mark predicted positions on the drawing, inspect first samples there, and include those areas in any strength or drop test.

Sources

• Plastics Technology, What to do about weak weld lines
• Machine Design, The weak link in reinforced nylons
• Autodesk Moldflow help, Weld and meld lines result