A weld line, also called a knit line, is the seam that forms on a molded part where two separate flow fronts of molten plastic meet and fuse incompletely. It typically appears behind a hole or pin, or between two gates, and can be a visible cosmetic line, a structural weak point, or both.
In short: a weld line marks where the plastic split around something and rejoined, and it is often the weakest place on the part.
What Is a Weld Line in Injection Molding?
As plastic fills a mold it advances as a flow front. Whenever that front is split, by a core pin that forms a hole, by an insert, or because the part is fed from more than one gate, the separate streams have to meet again further on. By the time they do, their leading edges have started to cool, so they do not fully blend. The boundary left behind is the weld line.
"Weld line" and "knit line" mean the same thing; the terms are used interchangeably across the industry. The line forms where the melt fronts join at a sharp angle, and its position, length and angle depend on gate location, injection speed, injection pressure and mold temperature.
Weld lines matter for two reasons. They can show as a fine line or notch on a glossy or colored surface. And they reduce strength: in one study of glass-filled nylon (PA6 with 30% glass fiber), test bars with a weld line reached roughly 72–77 MPa tensile strength against the material's rated 110 MPa, about a third lower, and broke at under 5% elongation. Fiber-filled plastics are hit hardest, partly because the fiber orientation is disrupted where the fronts meet.
Where Do Weld Lines Form?
| Cause | Where the weld line appears | Typical design response |
|---|---|---|
| Two or more gates | Where the flows from each gate meet, between them | Reduce gates or move them so the meeting point falls in a low-stress, non-cosmetic area |
| Hole, slot or core pin | On the far side of the feature from the gate | Reposition the gate, or accept the line and keep it away from load paths |
| Molded-in insert | Downstream of the insert | Orient the flow so the weld does not run across a threaded or load-bearing zone |
| Long flow path from a single gate | Around the far edge from the gate | Shorten the flow length, or raise injection speed and mold temperature |
How to Reduce Weld Lines
- Choose gate locations with the weld line in mind: gate placement decides where the flows meet.
- Move the weld away from snap fits, screw bosses, hinges and other areas that will be loaded.
- Raise injection speed and mold temperature so the fronts are hotter when they meet and fuse better.
- Make sure the area where the fronts meet is vented, so trapped air does not keep them apart.
- For fiber-filled materials, allow for lower strength at the weld in the part's design margin.
Gate positions are set with the customer during DFM, the first step of a plastic injection molding project, and that is where weld line placement is decided.
Frequently Asked Questions
What is a weld line in injection molding?
It is the line formed where two flow fronts of molten plastic meet inside the mold and do not fully fuse. It appears behind holes, pins and inserts, or between gates, and it can be both visible on the surface and weaker than the surrounding material.
What is a knit line?
A knit line is another name for a weld line. Both describe the seam left where two separate streams of plastic rejoin while filling the mold; the terms are used interchangeably in injection molding.
Are weld lines weaker?
Usually, yes. The two flow fronts bond incompletely, so the weld is often the weakest point on a part. The effect is largest in glass-filled plastics; one study found glass-filled nylon lost about a third of its rated tensile strength across a weld line.
Related Services at NICE Rapid
- Plastic Injection Molding — gate location, and with it weld line placement, is reviewed during DFM before tooling starts.
- Rapid Tooling — aluminum bridge molds to check weld line position and strength on real parts before committing to steel.
- Request a Quote — share your model and mark any areas where a weld line cannot be tolerated.
Sources
- MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — weld line as "a thin line... created when different flows of molten plastics in a mold cavity meet"; "often develops around the far edge of the gate"; fixes: increase injection speed, raise mold temperature, change gate position; consider weld lines in gate positioning — ocw.mit.edu
- Weld Line Strength of Polyamide Fiberglass Composite at Different Processing Parameters in Injection Molding Technique, Polymers (PMC) — PA6 + 30% glass fiber rated at 110 MPa; weld-line samples around 72–77 MPa; elongation under 5%; higher mold temperature improves the weld — pmc.ncbi.nlm.nih.gov
- Optimizing the Tensile Strength of Weld Lines in Glass Fiber Composite Injection Molding, Polymers (PMC) — weld lines as weak areas that reduce strength; fiber orientation disorder at the weld reduces weld-line strength — pmc.ncbi.nlm.nih.gov
- Flow Disturbance Characterization of Highly Filled Thermoset Injection Molding Compounds behind an Obstacle and in a Spiral Flow Part, Polymers (PMC) — weld lines form when a flow front is separated and rejoined; position, length and angle depend on gate location, injection speed, injection pressure and mold temperature; weld line formed by melt fronts joining at a sharp angle, which reduces strength and causes cosmetic defects — pmc.ncbi.nlm.nih.gov
- Scanning acoustic microscopy investigation of weld lines in injection-molded parts manufactured from industrial thermoplastic polymer (PMC) — "line welds (or knit lines) unavoidably form... when two or more flow fronts meet in the cavity"; "Welds comprise points of weak mechanical properties" — pmc.ncbi.nlm.nih.gov
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