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Author: Linda Publish Time: 2026-11-05 Origin: Site
Flow marks in injection molding are visible ripples, bands, or changes in gloss left on a plastic part as the mold fills. They can form when the advancing melt slows, changes direction, or cools unevenly, often near a gate or a change in wall thickness. To reduce them, identify where the pattern starts, then test the relevant fix. Typical options include adjusting the fill-speed profile, stabilizing mold temperature, improving the gate layout, and smoothing abrupt wall-thickness transitions.
Start with a full-part photo that shows the gate, the affected area, and nearby openings or ribs. Mark where the pattern begins and record the cavity number. For a glossy part, take a second photo at an angle that makes the gloss change visible. Those two views give the toolmaker more to work with than a close-up labeled “surface defect.”
Several injection molding surface defects look similar under shop lighting. Use their location and shape to decide what to investigate first.
What you see | Likely condition | What to check first |
Repeating arcs or alternating gloss bands across the fill path | Flow marking | Compare where the bands begin with the fill sequence and any wall-thickness transition. |
A narrow seam downstream of a hole, slot, or insert | Weld line | Check where the melt fronts split and rejoin. |
A hazy patch confined to the gate area | Gate blush | Check the gate geometry and initial fill speed. |
A snake-shaped trace extending from the gate | Jetting | Check whether the melt enters open cavity space before contacting a wall. |
Fine, silvery streaks that vary from shot to shot | Possible splay | Verify resin drying and handling, then investigate other sources of gas or degradation. |
Treat these visual clues as hypotheses. A seam behind an opening deserves particular attention if it crosses a loaded clip or boss. In that situation, inspect the appearance and verify the feature’s strength separately.
As the melt advances, it deposits material against the cooler cavity wall. That surface layer starts to solidify while the core continues to flow. A change in how the melt advances or cools can change how the molded surface reproduces the cavity finish. On a high-gloss face, the result can be visible even when the part feels smooth.
· A thick section fills ahead of an adjacent thin section. The melt can favor the thicker path and hesitate at the thin section. On a faceplate, compare the start of the band with features on the reverse side, including a recessed area, rib, or boss.
· The gate changes how the melt enters the cavity. A restrictive gate or an entry aimed into open space can produce a localized blemish. If the defect stays close to the gate, review the entry geometry and initial fill speed first.
· The mold or insert changes local cooling. Compare parts made at startup with parts made once the tool reaches steady conditions. Around a metal insert, check both local cooling and where the split flow fronts rejoin.
· The pattern repeats in only one cavity. Compare cavity samples from the same shot. If the mark consistently follows one cavity, prioritize that cavity’s gate, fill balance, and cooling before changing settings for the entire tool.
Mark the cosmetic surfaces and installed viewing direction on the drawing. Then compare gate options against the wall transitions and openings on the part. A gate hidden behind a bezel can keep the vestige out of sight while shifting a weld line onto the visible face. Where geometry allows, taper abrupt thickness changes and core out heavy sections behind the show surface. Use mold-flow analysis to compare gate layouts and locate likely hesitation or weld-line areas before cutting steel.
Run a series of short shots by stopping the fill at progressively later stages. Photograph each incomplete part and compare the exposed flow front with the mark on a fully molded sample. If the melt repeatedly hesitates at the same thin section where the gloss band starts, test a fill-profile change through that section. If the blemish appears as soon as the melt enters, focus on the gate and initial fill speed. Keep the resin grade and drying conditions consistent, and evaluate temperature changes within the material supplier’s processing guidance.
Compare parts from each cavity over a representative run. If the original defect appeared after startup or a pause, include a restart in the check. Reinspect critical dimensions and assembly fit after process changes. If the surface passes only at a narrow setting, use the trial evidence to decide whether the gate, cooling layout, or part geometry needs revision. Record the conditions that produced acceptable parts so the production team can reproduce them.
PlasticNavi published a case involving a white polycarbonate switch panel for German customer Klein GmbH. The customer specified a mirror-gloss face, making visible imperfections a concern. PlasticNavi says it reviewed gate placement, ejector locations, and cooling through DFM and mold-flow analysis. The supplier reports that the resulting samples met the customer’s expectations. The case gives no process settings, measured gloss values, or production reject rates.
How to apply the case to a similar panel. On a faceplate with switch openings, review where the melt fronts rejoin for each candidate gate location. During the first trial, put the short-shot sequence beside the finished part. A fine line directly behind an opening gives the team a reason to investigate a weld line. A broad band starting where the panel gets thinner gives the team a reason to investigate hesitation. Those observations narrow the next trial to a specific feature and a specific process change.
For consumer electronics, inspect display bezels and control panels alongside the adjacent housing. A gloss change that is easy to miss on a loose part may stand out next to a matching finish. For automotive switch surrounds and console trim, include the installed orientation in the review so the acceptance decision reflects the surfaces occupants see.
“Free of visible flow marks” is only as clear as the inspection method behind it. Put the cosmetic requirements in the RFQ or drawing package before tooling approval, while gate locations and tooling changes are still open for discussion.
Item | What to agree before production |
Cosmetic zones | Mark primary visible faces, secondary faces, and hidden surfaces. Define where flow marks are rejectable and where a boundary sample governs. |
Viewing conditions | Specify the light source, distance, angle, viewing time, and part orientation. Include an assembled view if neighboring surfaces affect the assessment. |
Boundary samples | Approve physical samples for the specified resin, color, and finish. Use photos to document the affected location, and use the samples to judge subtle gloss differences. |
Sampling and traceability | Agree on cavity coverage for first-off approval, lot and cavity identification, and checks after restarts or material changes where relevant. |
Functional acceptance | Inspect fit and critical dimensions separately. If a weld line crosses a loaded clip, boss, or insert interface, specify the applicable functional test and pass criteria. |
For a multi-cavity mold, ask for cavity-identified samples during first-off approval. If only one cavity produces a visible band, those samples let the supplier trace the problem to the relevant cavity while the buyer evaluates it against the same agreed limit.
Dawang Precision has 27 years of manufacturing experience and operates more than 400 advanced machine tools, including Röders and Mazak five-axis machines. Our engineering team can review cosmetic surfaces, gate options, and wall-thickness transitions before tooling release.
Send us your STEP model and PDF drawing for a free DFM review. Include the target resin, color, and finish, and mark the surfaces visible after assembly. If you’re troubleshooting an existing part, include a full-part photo that shows both the defect and the gate. We’ll reply within 24 hours. Request a free DFM review
