Injection molding defects
Weld lines are visible or weak regions where separate melt fronts meet inside the cavity. This guide explains how they form, how to tell them from nearby surface defects, and what to change in process, material prep, or part and mold design.
Defect troubleshooting
Weld lines mark where two or more flow fronts recombine during filling. They are also called knit lines in shop talk.
Molten plastic advances through the cavity in fountain flow: a skin freezes at the mold wall while fresh melt moves through the core. When flow splits around a hole, rib, insert, or thickness change, the separated fronts meet again downstream. If those fronts have cooled or oriented differently, the junction can show as a line, notch, or color shift and can be weaker than the surrounding bulk.
Simulation and molding references distinguish weld lines from meld lines by how sharply the fronts meet, but on the production floor both show up as knit-like seams. The practical goal is the same: move the junction to a low-stress, low-visibility zone, improve fusion at the meeting point, or redesign so only one dominant flow path exists.
For new programs, gate location and part geometry set most weld-line risk before steel is cut. NetProto returns DFM feedback with instant quotes so teams can review fill pattern assumptions before a hardened steel tool is released, depending on part size and complexity.
Root causes
Use this table to decide whether the next change belongs in fill profile, temperature, venting, or CAD and gate strategy.
| Likely driver | What you see | First lever to try |
|---|---|---|
| Flow splits around cores, holes, or bosses | Predictable line downstream of the obstacle; repeats on every shot in the same cavity | Relocate obstacle or gate in CAD when possible; otherwise tune fill so fronts meet warmer and with higher shear at the junction |
| Multiple gates or balanced fills that converge late | Seam on a cosmetic face; cavity-to-cavity shift when one gate lags | Sequence fills or reduce to a single dominant gate in the next tool revision; adjust hot-runner timing with process engineering |
| Cold or stiff flow fronts at the meeting point | Deep notch or silver line; poor strength in a bend test at the seam | Raise melt and mold temperature within material limits; increase injection rate during fill; verify resin is dry and lot-stable |
| Hesitation or race tracking on uneven stiffness | Weld line wanders shot to shot; line tracks thin-to-thick transitions | Smooth thickness transitions in CAD; adjust fill speed profile; use mold-flow review to see hesitation before tooling changes |
| Jetting that breaks into separate fronts | Wavy surface upstream plus a knit seam; turbulence marks near the gate | Reduce initial gate velocity or change gate style and location; fix jetting before chasing pack pressure |
| Trapped air at the weld junction | Burned streaks adjacent to the line; short fill at the end of flow | Improve venting at the last-to-fill region; confirm vents are clean and not polished shut during maintenance |
Troubleshooting
Confirm the seam is a true weld line before you change pack pressure for an unrelated defect.
Locate the meeting point
Mark the line on the part and trace backward to holes, ribs, gates, or thickness steps. Photograph the same cavity across five consecutive shots. If the line moves, suspect hesitation or vent issues rather than a fixed obstacle weld.
Remove viscosity drift
Match the approved setup sheet for melt temperature, mold heat, fill, pack, and back pressure. Confirm drying and lot match the last acceptable run. Weld lines that worsen after a resin change often trace to moisture or viscosity, not worn cavity polish.
Hotter, faster meeting fronts
Increase injection speed during fill and raise melt or mold temperature within supplier limits so fronts interfuse before the skin freezes. Change one variable per experiment and keep labeled samples. Avoid fixing a knit problem only with higher pack after the gate has frozen.
Air steals fusion
Inspect vents at weld-adjacent end-of-fill areas. Partial vacuum or compressed air traps leave weak knit zones and can add burn or blush nearby. Clean vents and confirm steel alignment before reprofiling the parting line.
When the line must move
If the seam sits on a load-bearing or cosmetic datum, relocate the gate or obstacle in the next revision so the junction moves to a rib side or internal wall. Mold-flow simulation helps compare gate options before cutting steel on a hardened production tool.
Diagnosis
Mislabeling the defect sends you to the wrong lever. Use this comparison at the inspection bench.
Prevention
Most recurring knit problems trace to gate choice and obstacle layout rather than a single out-of-spec cycle.
Place gates so the last-to-fill region is a low-stress, non-cosmetic wall when possible; avoid forcing multiple fronts to meet on a show surface.
Minimize flow splits by consolidating holes near ribs, using core pins only where function requires, and smoothing thickness transitions that cause hesitation.
Run mold-flow or fill simulation on complex parts to preview weld location before tooling release on qualified programs.
Specify cosmetic acceptance on drawings with photos or SPI finish class references; tie strength-critical seams to structural test coupons when the part flexes at the knit.
Keep setup sheets and lot records so a sudden knit change is not mistaken for mold wear.
Upload CAD for DFM feedback before a hardened steel tool is ordered so gate and obstacle changes stay low cost.
Specifications
Weld-line acceptance is program specific. For general molded dimension bands tied to surface finish class, use the published commercial tolerance table rather than inventing part-specific limits in a troubleshooting guide.
FAQ
In most molding shops the terms refer to the same phenomenon: a junction where separate melt fronts meet. Some simulation tools label sharper meetings as weld lines and shallower mergers as meld lines, but troubleshooting and prevention focus on relocating or improving that junction.
Any part with holes, multiple flow paths, or more than one gate will form at least one knit junction somewhere. The practical target is to move the line to a hidden area, improve fusion with process tuning, or redesign flow so only one dominant front reaches critical surfaces.
Not always visibly, but research on thermoplastic molding shows knit regions can be weaker than bulk material, especially with fibers or sharp meeting angles. Validate strength with bend or tensile tests on production coupons when the seam sits in a loaded area.
Pack mainly affects density after the gate freezes. Knit quality is mostly set during fill when fronts meet. Raise fill rate or temperature first, then verify venting. Higher pack without a fill fix can add flash risk without moving the seam.
Upload CAD for an instant quote and DFM feedback. Engineers can flag gate location, obstacle splits, and cosmetic faces while the hardened steel tool is still in planning, depending on part size and complexity. There is no minimum order quantity once the tool is running.
Ready to quote
Bring CAD and cosmetic requirements into an instant quote so DFM review and gate strategy align before steel is cut. One hardened steel tool is built for the life of the part and can typically achieve 100,000+ parts over its service life when the process window is proven.
