Flow marks are visible bands, ripples, or halos that show the melt front did not create a uniform surface as it filled the cavity. They may appear near a gate, around an obstacle, at a wall-thickness transition, or toward the end of fill.
The right fix depends on the symptom and the location. Increasing temperature or speed can help a cold flow front, but the same change can create flash, burn, shear, or a different cosmetic defect. Diagnose the material, machine, mold, and part together.
What flow marks look like
- Concentric rings near a gate: investigate gate freeze, cold material, gate geometry, and the initial fill profile.
- Waves along a wall: check hesitation, wall-thickness transitions, injection speed, mold temperature, and flow length.
- Marks around a hole or insert: review the split flow, weld line, venting, and insert temperature.
- Random streaks or splay-like lines: check moisture, contamination, degraded resin, and shear-sensitive additives before changing the mold.
Root-cause workflow
- Photograph the defect and record its position relative to the gate, parting line, vents, ribs, bosses, and cooling channels.
- Run short shots to see the fill-front sequence and identify hesitation or an unstable fountain flow.
- Check material lot, drying, regrind, melt quality, nozzle, and barrel-temperature stability.
- Review gate, runner, cold-slug well, vent depth, and the end-of-fill location.
- Change one process variable within a controlled window and record the result.
- Confirm the fix with dimensional, cosmetic, and repeatability checks rather than one good shot.
Material and process checks
| Check | Why it matters | Typical action |
|---|---|---|
| Drying and material condition | Moisture or degradation can create streaks that look like flow marks | Verify the grade-specific drying record and purge quality |
| Melt and mold temperature | A cold front can freeze or hesitate before the cavity fills | Adjust within the supplier window and watch for flash or degradation |
| Injection-speed profile | Too slow may freeze the front; too fast may increase shear or jetting | Use a staged profile and compare short-shot patterns |
| Packing and gate freeze | Early gate freeze can preserve a nonuniform surface and underpack the part | Check gate size, hold time, pressure, and weight stabilization |
Mold and part design checks
Flow marks often reveal a design issue rather than a controller setting. Review abrupt wall changes, thin sections, ribs, bosses, flow around inserts, gate position, runner restriction, cold-slug wells, and end-of-fill vents. A more balanced gate or a gradual transition may widen the process window more effectively than repeated temperature adjustments.
Cooling also matters because a nonuniform mold surface temperature can make the flow front freeze unevenly. Check circuit balance, blocked channels, insert heat transfer, and local hot spots during production.
What not to do
- Do not raise barrel temperature indefinitely without checking residence time and material degradation.
- Do not increase injection speed blindly; it can move the defect or create burn and flash.
- Do not polish a visible mark before proving that the defect is not caused by filling or venting.
- Do not change several variables at once if the goal is to identify the root cause.
RFQ and process-development information
For a new tool or a difficult cosmetic part, provide resin grade, color, wall map, gate proposal, surface-finish standard, expected cycle, photos of the defect, short-shot data, and the current process window. The [engineering team](/engineering/) can review flow and venting, while the [injection molding team](/injection-molding/) can help structure the trial plan. [Contact Cavity Mold](/contact/) with photos and sample data for a focused review.
Technical references
Autodesk Moldflow’s flow-mark troubleshooting guidance connects flow marks with temperature, speed, runner, gate, and packing conditions. Eastman’s troubleshooting guide provides an example of symptom-to-cause process control for molded parts.