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

A weld line—also called a knit line—is the faint line that appears where two flows of molten plastic meet and rejoin inside the mold. Almost every part with a hole, multiple gates, or an obstruction in the flow path has one somewhere. The buyer’s job isn’t to eliminate weld lines, which usually isn’t possible; it’s to understand where they’ll form, whether they matter for the part, and how to keep them off the surfaces and features that count. This guide is part of the injection molding defects section.

If the line sits near a screw hole, pin hole, boss, snap-fit, or another feature that carries load, use the weld line near loaded holes buyer review explainer to frame the approval questions and evidence request.

Why Weld Lines Form

As plastic fills a cavity, the flow front sometimes has to split and come back together—around a hole or pin, past a boss, or when the part is fed by more than one gate. Where the two fronts meet, they “knit” together. If they’re still hot and under pressure, they fuse almost invisibly. If they’ve cooled at the edges by the time they meet, the bond is weaker and more visible—that’s a weld line.

So a weld line is really about the condition of the plastic where the fronts meet: hotter, well-packed fronts knit better than cooler ones. This is why the same geometry can produce a barely-visible line or an obvious one depending on the process and gate setup.

Two Things a Weld Line Can Affect

For a buyer, weld lines matter in two distinct ways, and it’s worth being clear which one applies to your part:

• Cosmetic. The line can be visible on a show surface—a faint streak or witness mark. On an A-surface that may be a reject; on a hidden face it’s irrelevant.
• Structural. A weld line is often slightly weaker than the surrounding material, because the two fronts don’t bond perfectly. On a part that sees load or stress at that location, this can matter mechanically, not just visually.

Knowing whether your concern is appearance, strength, or both tells you and your supplier where to focus—and whether moving the weld line is enough or whether the area needs more attention.

What Influences Weld Lines

Factor	Effect
Gate location and count	Determines where flow fronts meet—the primary lever for where a weld line lands
Melt and mold temperature	Hotter fronts knit better; cool fronts make weld lines more visible and weaker
Injection speed and pressure	Affects how hot and well-packed the fronts are when they meet
Part geometry	Holes, bosses, and obstructions force the flow to split and rejoin
Venting	Trapped air where fronts meet worsens the weld and can scorch it

The most important of these for a buyer is gate location, because it largely decides where the weld line forms. By choosing where to gate the part, a supplier can often steer the weld line to a non-critical area—which is exactly why gate planning and weld lines are linked, and why it’s worth telling your supplier which faces are cosmetic. See gate design for how that decision is made.

What a Buyer Can Do

• Identify where weld lines will form. Anywhere the flow must split—around holes, multiple gates, large obstructions—expect one. The DFM review is the place to anticipate them, and on cosmetic or loaded parts a mold flow analysis can predict their locations before the tool is cut.
• Tell the supplier which faces are cosmetic and which features carry load, so they can place gates to move weld lines off those areas.
• Decide whether your concern is cosmetic or structural. If structural, raise it explicitly—an acceptable-looking weld line can still be a weak point that needs design or process attention.
• Resolve at sampling. Weld-line position and appearance show up at first samples, the right time to confirm they’ve landed where you can live with them.

A capable molder can also improve a weld line through process—raising temperatures and tuning fill so the fronts meet hotter—but the position is set by geometry and gating, so the realistic goal is usually placement, not elimination.

This is an independent buyer resource and not a substitute for supplier or moldflow analysis. Where a weld line forms and how much it matters depend on the specific part, gating, and resin, so confirm specifics with your molder.

Buyer FAQs

What is a weld line in injection molding?

A weld line, or knit line, is the line that forms where two flow fronts of molten plastic meet and rejoin inside the mold—typically around a hole, past an obstruction, or where the part is fed by more than one gate. If the fronts are still hot when they meet, they bond well and the line is faint; if they’ve cooled, the bond is weaker and the line is more visible.

Are weld lines a structural weakness?

They can be. Because the two flow fronts don’t always bond perfectly, a weld line is often slightly weaker than the surrounding material. On a part that carries load at that location, this can matter mechanically, not just cosmetically. If strength at a weld line is a concern, it’s worth raising explicitly so the design, gating, and process can address it rather than just hiding the line.

Can weld lines be eliminated?

Usually not entirely—wherever flow must split and rejoin, a weld line forms. The realistic goal is to move it to a non-critical location and to improve how well the fronts knit. Gate location is the main lever for placement, and higher melt and mold temperatures help the fronts bond. Telling your supplier which faces are cosmetic and which features carry load lets them place the weld line where it does the least harm.

How does gate location affect weld lines?

Gate location largely determines where the flow fronts meet, and therefore where the weld line forms. By choosing where and how many gates feed the part, a supplier can often steer weld lines away from cosmetic faces or load-bearing features. This is why weld-line concerns and gate design are decided together, and why communicating your critical surfaces up front is valuable.