Weld Lines and Knit Lines in Injection Molding — Why Do Two Flow Fronts Meet Weakly, and How Do You Move or Strengthen the Join?

Last updated: 2026-09-04 · Reading time: about 6 min

Weld lines form where two flow fronts converge and fail to fuse fully. A head-on merge (convergence angle > 135°) forms a weld line; an angled merge (< 135°) forms a knit line. At the join, polymer chains do not interdiffuse across the interface — the result is a local strength drop of 10–40 % and a visible cosmetic line. Move the line away from high-stress areas by relocating the gate; strengthen what remains by raising melt temperature, injection speed and packing pressure at the merge point.
See also related article.

Key facts

A weld line is a structural discontinuity where flow fronts meet head-on. Polymer chains do not entangle across the interface, so the local tensile and impact strength drops below the bulk value — typically 10–25 % for amorphous resins, up to 40 % for filled grades.

A knit line forms where fronts meet at an angle and partially overlap. It is cosmetically less visible and mechanically stronger than a weld line, but still weaker than the bulk material.

Three variables control weld-line strength: melt temperature at the merge (hotter = better fusion), packing pressure transmitted to the merge (higher = more compression), and flow-front velocity at the merge (faster = less cooling before contact).

Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. See also related guide.

Why do two flow fronts meet weakly?

The answer is molecular. When two fronts touch, the polymer chains on each side have already begun to cool and orient. They do not have time or thermal energy to interdiffuse across the interface, so the join remains a plane of weakness.

Factor at the Merge	Effect on Weld Strength	How to Improve It
Melt temperature	Lower temperature = less chain mobility = weaker bond	Raise barrel and mold temperature to keep the front hot at the merge
Packing pressure	Low pressure = less compression of the interface	Increase packing pressure and hold time to compress the join
Flow-front velocity	Slow front = more cooling before contact = weaker bond	Increase injection speed or resize the flow path to maintain velocity
Convergence angle	Head-on (< 135°) = weld line, weakest; wide angle = knit line, stronger	Relocate the gate or add an overflow well to change the merge geometry
Filler content	Fibers align parallel to the flow front and do not bridge the join	Reduce filler locally (not practical) or accept lower strength and design around it

Weld line. A linear weakness on or below the surface of an injection molded part, formed where two flow fronts converge and the polymer chains fail to interdiffuse across the interface. A weld line forms at a head-on merge; a knit line forms at an angled merge. Both are weaker than the bulk material, but a knit line is cosmetically less visible and mechanically stronger because the fronts partially overlap.

The practical implication is that every hole, every second gate and every obstacle that splits the flow creates at least one weld line. You cannot eliminate them entirely — you can only move them to low-stress areas and strengthen them through process and tool design. The goal is not a weld-free part; it is a part where every weld is located, predicted and engineered to carry its share of the load.

How do you move or strengthen a weld line?

Four steps, applied in order. The first two move the line; the next two strengthen whatever remains.

Relocate the gate to move the merge away from high-stress areas. The gate position determines where the flow splits and where it reunites. A mold-flow analysis predicts every weld location before the tool is cut — move the gate so merges fall on non-structural faces, behind ribs or in areas where the cross-section is thick enough to tolerate a local strength drop. This is the single highest-leverage decision in the DFM review.
Add overflow wells to capture the cold, poorly fused material. If the weld line must remain in a critical area, place an overflow well (a small blind pocket) at the merge point. The first material to arrive — coolest, most oriented, weakest — flows into the well and is trimmed off after molding. The material that remains in the main cavity is the hotter, better-fused portion. Overflow wells add material cost and a secondary trimming operation, but they rescue a weld that would otherwise fail.
Raise melt temperature and mold temperature at the merge. A hotter front has more chain mobility when it meets the opposing front, so the interdiffusion is deeper and the bond is stronger. Increase barrel temperature within the resin’s processing window, and raise the mold temperature locally at the weld location — a heated mold insert or a localized mold-temperature controller channel can make the difference between a failing and a passing weld.
Increase injection speed and packing pressure to compress the interface. A faster fill means the front arrives at the merge hotter and with more energy for chain interdiffusion. Higher packing pressure compresses the interface and reduces the void content at the join. Both changes must be balanced against flash risk and gas-trap concerns — vent the merge location to prevent diesel burns while increasing speed.

“You cannot eliminate weld lines — every hole and every second gate creates one. The goal is to predict every merge in mold-flow, move the joins to low-stress areas, and then engineer the process to make the remaining welds carry their load.”

— Tooling Engineer, aCatMould

Frequently asked questions

Key parameter identified — critical variable confirmed against spec.

Tolerance within achievable range — commercial ±0.15mm or tighter as needed.

Material matched to application — resin grade selected for end-use environment.

Process window validated at T1 — parameters locked after first trial.

Inspection method defined — CMM, visual, or functional test specified.

How much weaker is a weld line compared to the bulk material?

It depends on the resin and the processing.
Unfilled amorphous resins (ABS, PC) typically see a 10–20 % drop in tensile and impact strength at the weld. Semi-crystalline resins (PA, POM) show a 15–30 % drop because crystallization at the interface is disrupted. Glass-filled grades can lose 30–40 % because fibers align parallel to the front and do not bridge the join.

The actual number for your part depends on the temperature, pressure and velocity at the merge — which is why a mold-flow prediction paired with a T1 validation on the actual tool is the only reliable way to set the design margin.

Can sequential valve gating eliminate weld lines entirely?

It can, in specific geometries. Sequential valve gating closes and opens hot-runner gates in a timed sequence so that only one front advances at a time — the second gate opens after the first front has passed the merge location, so the two fronts never meet. The result is a part with no visible weld line on the cosmetic surface.

The trade-off is cost and complexity: sequential valve gating requires a hot runner with individually controlled valves, a timing controller and process validation. It pays off on high-volume cosmetic parts where a weld line is unacceptable; on technical parts where the weld can be moved to a non-visible face, gate relocation is simpler and cheaper.

Does the weld line always show on the surface?

Not always. A well-processed weld in a non-cosmetic resin may be structurally present but visually invisible. A weld in a glossy or light-colored part, or one where the merge is cool and the material is slightly degraded, will show as a visible line — sometimes a color shift, sometimes a surface depression. If the weld must be cosmetically clean, move it to a non-visible face with gate relocation, or raise the merge temperature so the surface heals fully.

Texture can also hide a weld line. A fine grain texture on the surface masks the visual line even when the structural weakness remains — so a cosmetically clean weld is not necessarily a structurally sound one. Always validate strength separately from appearance.

Key takeaways

Every hole and second gate creates a weld line — predict all merges in mold-flow before the tool is cut, and relocate gates to move joins to low-stress areas.

Weld strength depends on three variables at the merge: melt temperature, packing pressure and flow-front velocity.
Raise all three within the processing window to strengthen the join.

A cosmetically clean weld is not necessarily a structurally sound one — validate strength separately from appearance, especially on textured or colored parts.

Weld lines are not a defect — they are a geometric consequence of how the cavity fills.
The discipline is in predicting every merge, moving the critical ones away from load paths, and engineering the process to make the remaining welds strong enough for the application. Get the gate location right at DFM, then tune temperature, speed and pressure at T1 — and the weld stops being a surprise.