Weld Lines (Knit Lines) in Injection Molding: What Buyers Should Know

A weld line (knit line) forms where two flows of molten plastic meet inside the mold. Almost every part with a hole, multiple gates, or an obstruction will have one. Your goal is not to eliminate weld lines; that is rarely possible. Move them off critical structural features and show surfaces. Related pages: injection molding defects.

If a weld line sits near a screw hole, pin hole, boss, or snap-fit, use the weld line near loaded holes buyer review explainer to frame approval questions.

Why Weld Lines Form

As plastic fills a cavity, the flow front splits around holes, pins, or bosses, or when multiple gates feed the part. Where those fronts meet, they knit. Strength and visibility depend on the plastic’s condition at that meeting point. Hot, well-pressurized fronts fuse almost invisibly. If the fronts have cooled, the bond is weaker and more visible.

The same geometry can yield a nearly invisible knit or a glaring weak point depending on process and gate placement.

Cosmetic vs. Structural Weld Lines

Weld lines create two different risks. Be explicit with your supplier about which applies:

• Cosmetic: A faint streak on an A-surface is often a reject. On a hidden face, it may be irrelevant.
• Structural: Flow fronts never bond perfectly, so the weld is a localized weak point. If the part sees load at the weld line, that is a structural risk even when the line looks faint.

How Much Weaker Are They?

“Slightly weaker” is a dangerous understatement for reinforced materials.

A 2024 study in Materials tested PA6 with 30% glass fiber. Samples without a weld line measured 110 MPa ultimate tensile strength. Samples with a weld line averaged 66.02 MPa.

That is roughly 60% of the base strength, about a 40% loss, driven by where the flow fronts met.

The study also showed elongation below 5% at the weld: brittle failure without the warning deformation of the bulk material. Electron microscopy showed remaining strength relied heavily on fiber bridging: glass fibers crossing the weld.

Because fiber orientation at the weld drives integrity, weld-line strength in filled resins is highly process-sensitive. Packing pressure was the most influential parameter in the study, followed by melt temperature. Hotter, well-packed fronts knit better.

Unfilled resins generally lose less strength, but the practical rule stays: if a weld line lands where the part carries load, it is a design problem. Ask what retained strength is at that location. If nobody knows, nobody has checked.

What Influences Weld Lines

Factor	Effect
Gate location and count	Determines where flow fronts meet. The primary lever for placement.
Melt and mold temperature	Hotter fronts knit better; cool fronts make welds weak and visible.
Injection speed and pressure	Dictates the heat and packing pressure at the flow front interface.
Part geometry	Obstructions like holes and bosses force flow fronts to split.
Venting	Trapped air at the meeting point worsens the weld and can cause scorch marks.

Gate location is the buyer’s most important placement lever. Adjusting gates can steer the weld away from high-stress or cosmetic areas. Review gate design together with cosmetic requirements.

What a Buyer Can Do

• Anticipate locations. Expect welds wherever flow splits. A DFM review and mold flow analysis can predict them before steel is cut.
• Define critical zones. Tell the supplier which faces are cosmetic and which features carry load.
• Differentiate the risk. If the concern is structural, say so. An acceptable-looking weld can still fail mechanically.
• Verify at T1 sampling. Position and severity will be obvious. Confirm they sit in acceptable locations.

A molder can improve a weld by raising temperatures and optimizing pack pressure. The physical location is still fixed by geometry and gate placement.

Buyer FAQs

What is a weld line in injection molding?

A seam where two molten flow fronts meet and rejoin, typically around holes, past obstructions, or with multiple gates. Hot fronts bond stronger and look fainter; cooled fronts look worse and hold less. In the RFQ, name cosmetic faces and loaded features so gate planning has somewhere to steer the weld.

Are weld lines a structural weakness?

Yes. Flow fronts never bond perfectly, so a weld is weaker than bulk material, and in glass-filled resins the gap can be large. The PA6 + 30% glass study measured 66.02 MPa average UTS across weld-line samples versus 110 MPa without a weld (about a 40% loss), with elongation below 5% at the weld. Remaining strength depended heavily on fiber bridging. On a load-bearing part, treat a weld at the load path as a design issue: require retained-strength evidence or move the weld before production release.

Can weld lines be eliminated?

Rarely. If flow must split and rejoin, a weld forms. Goal: move it to a non-critical location and improve knitting through process. Do not accept “we will process it out” as a substitute for gate or geometry changes when the weld sits on a loaded feature.

How does gate location affect weld lines?

Gate placement sets the melt path and therefore where fronts meet. Suppliers use gates to steer welds off cosmetic faces or loaded features. Communicate critical surfaces early so the tool is gated accordingly before steel is cut.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Confirm defect causes and corrective actions with your supplier against your specific part, tool, and process.