“Surface defect” is a description, not a root cause. Silver streaks, dark or brown streaks, sink marks, gloss variation, weld lines, drag marks and contamination can look different and require different investigations. First classify the appearance and location; then compare the material, mold, machine and process evidence before selecting a corrective action.
Document the defect and acceptance criteria
Use photographs under the agreed lighting and viewing conditions. Record the part and drawing revision, defect location, cavity, resin and lot, color, production time, machine, mold and whether the issue appears at startup or during a stable run. Identify the applicable appearance zone, sample limit or customer acceptance standard. Without a shared visual criterion, “improved” may mean different things to different reviewers.
Separate the main defect signatures
- Silver or white streaks: investigate moisture, trapped air and material handling; confirm the material supplier’s requirements and the actual drying records.
- Dark or brown streaks: examine both air traps and possible thermal degradation. Review venting, flow restrictions, residence and process evidence before changing speed or temperature.
- Sink marks or internal voids: compare the defect location with thick sections, ribs, bosses, packing and gate-freeze evidence.
- Gloss or flow marks: compare surface texture, flow behavior, cooling and the location of the appearance change.
- Weld lines or drag marks: map the mark to flow fronts, part geometry, ejection direction and the affected surface.
More than one factor may be present. Autodesk’s molding references distinguish, for example, moisture- or air-related silver streaks from dark streaks associated with trapped air or thermal damage; the visible mark should not be treated as proof of one cause.
Use evidence to choose the next check
Compare conforming and affected parts from the same lot or production window. Review resin identification and preparation, machine and tool records, vent and gate condition, cooling behavior, maintenance and process changes. If the issue is limited to one cavity or one area, map the evidence to that cavity and flow path. If it appears across several cavities, also examine material, machine and process conditions shared by those cavities.
Confirm a fix with a controlled trial
Record the original setup, the evidence behind the proposed change, the changed condition and the sample results. Avoid applying a generic temperature, injection-speed or pressure recipe to an unknown resin and part. A change that reduces one appearance issue may alter fill, dimensions, residual stress, cycle or another defect. Recheck the agreed visual standard and functional criteria before approving the change.
For a related supplier inspection framework, see DAYIN’s injection-molded part inspection guide and custom injection molding service. Technical reference: Autodesk’s molding-defect glossary distinguishes white/silver streaks from dark streaks and describes possible contributing mechanisms.