Injection molding defects
Flow lines are visible wavy or streaky bands that follow the path molten plastic took through the cavity. This guide explains how they form, how to separate them from knit seams and jetting, and what to change in process, material prep, or part and gate design.
Defect troubleshooting
Flow lines are cosmetic surface marks that track how the melt front advanced, not a single seam where two fronts met.
During filling, plastic enters as a fountain flow: material freezes in a thin skin against the mold while hotter melt continues through the core. When injection speed, temperature, or geometry changes how that front moves, the frozen skin can record bands of different gloss, color, or texture. Those records appear as wavy flow lines, flow marks, or read-through patterns that follow flow direction rather than a straight knit seam.
Thermoplastic troubleshooting references tie flow lines to cold or viscous melt, slow or uneven fill, abrupt wall thickness steps, restrictive gates or runners, and moisture that disrupts homogeneity in hygroscopic resins. The defect is most obvious on glossy, transparent, or pigment-sensitive surfaces where small changes in orientation or shear show as light and dark tracks.
For new programs, gate placement, uniform wall thickness, and fill profile set much of the risk before steel is cut. NetProto returns DFM feedback with instant quotes so teams can review fill 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 temperature, fill rate, gate size, venting, drying, or CAD transitions.
| Likely driver | What you see | First lever to try |
|---|---|---|
| Melt or mold temperature too low for the resin window | Wavy bands that worsen away from the gate; dull surface with stiff flow | Raise melt and mold heat within supplier limits; confirm barrel profile matches the approved setup sheet |
| Fill too slow so the skin freezes before the cavity is uniformly wetted | Ripple or ring patterns along flow; semi-solid layers pushed by later melt | Increase injection speed during fill in controlled stages; avoid a single slow first stage on long flow paths |
| Abrupt wall thickness changes or sharp corners in the cavity | Lines that track thin-to-thick transitions; hesitation marks near ribs or bosses | Smooth transitions in CAD; add radii on cores; use fill simulation to preview hesitation before tooling changes |
| Undersized gate or runner restricting flow | High shear near the gate plus downstream waves; pressure spikes on the machine log | Enlarge gate or runner in the next tool revision; balance restriction with jetting risk at the gate |
| Moisture or inconsistent melt density in hygroscopic resins | Flow lines plus splay or silver streaks; defect worsens after material lot change | Verify drying time and temperature; stabilize back pressure for homogeneous melt before chasing mold polish |
| Poor venting or trapped air disturbing the flow front | Waves at end of fill; burn or blush adjacent to flow tracks | Clean and deepen vents at last-to-fill regions; confirm parting line alignment before texture or polish work |
Troubleshooting
Confirm the mark follows flow direction before you treat it as a weld line or ejection scratch.
Flow versus knit versus handling
Mark the defect under raking light and trace it back toward the gate. Flow lines wander with shear history; weld lines sit on a fixed meeting plane downstream of holes or splits. Scratches from handling do not repeat identically on every shot from the same cavity.
Remove viscosity drift
Match melt temperature, mold heat, fill, pack, and back pressure to the last good run. Confirm drying records for nylon, polycarbonate, or other moisture-sensitive grades. Flow lines that appear after a resin change often trace to moisture or lot viscosity, not worn cavity texture.
Keep the front hot and moving
Increase injection speed during fill and raise melt or mold temperature within material limits so the cavity wets out before the skin freezes. Change one variable per experiment and keep labeled samples. Industry guides note that faster fill can improve knit fusion, but for flow lines the goal is uniform shear without opening jetting at the gate.
Geometry sets the pattern
Check whether waves start at a restrictive gate, a sharp corner, or a thickness step. Clean vents at end-of-fill areas. If jetting appears as a snaking stream ahead of the waves, reduce initial gate velocity or redirect melt with gate style and location before adding pack pressure.
When waves sit on a show face
Relocate the gate, enlarge runners, or smooth thickness transitions in the next revision so flow crosses the cosmetic face with steadier shear. 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 flow marks trace to gate strategy and thickness uniformity rather than a single out-of-spec cycle.
Place the gate so cosmetic faces fill with a steady, expanding front instead of a long, slowing tail across high-gloss surfaces.
Keep wall thickness uniform within practical DFM limits and radius sharp core corners that stall flow.
Size gates and runners for the resin and flow length so fill completes without excessive restriction or uncontrolled jetting.
Run mold-flow or fill simulation on complex parts to preview shear and hesitation before tooling release on qualified programs.
Document drying, lot, and setup so a sudden wave pattern is not mistaken for mold texture wear.
Upload CAD for DFM feedback before a hardened steel tool is ordered so gate and thickness changes stay low cost.
Specifications
Flow-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
No. Flow lines are wavy or streaky bands that follow how melt moved through the cavity, usually from varying shear or early skin freeze. Weld or knit lines are junctions where separate flow fronts met. Fix flow lines with temperature, fill profile, and geometry; fix weld lines by relocating or improving that meeting point.
High-gloss and transparent surfaces amplify small differences in orientation, texture, and freeze timing. The same shear history that is invisible on a matte texture can read as light and dark tracks under raking light on a polished cavity.
Usually not as a first move. Slow fill lets the surface skin freeze early, which can make flow marks more visible. Increase melt and mold temperature within resin limits and use a faster, controlled fill so the cavity wets uniformly, while watching for jetting at the gate.
Gate location sets flow length, shear at entry, and which faces see the slowing tail of fill. Moving or resizing the gate is often required when waves sit on a cosmetic datum, but process tuning and thickness uniformity still matter.
Upload CAD for an instant quote and DFM feedback. Engineers can flag gate location, thickness steps, 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.
