Injection molding defects
Discoloration means the molded part no longer matches the intended resin or masterbatch color. Shifts can be uniform yellowing, brown or black streaks, pink or green tints from contamination, or mottled pigment. This guide separates material and colorant issues from localized burn or splay patterns and gives a practical fix order.
Defect troubleshooting
Discoloration is a color defect: the part surface or bulk color differs from the approved color standard or natural resin.
Industry troubleshooting references define discoloration as a visible change from the original material or compound color. That can mean uniform yellowing across every shot, a part that reads too dark or too light versus the color chip, mottled pigment, or localized brown and black zones. The root cause is rarely a single knob: thermal degradation, oxidative yellowing, poor colorant dispersion, contaminated regrind, cross-resin carryover, or additive breakdown can all shift hue before the part leaves the mold.
Discoloration overlaps other defects but is not the same diagnosis. Charred black at the last fill or in a trapped rib is usually a burn and venting problem, covered in the burn marks guide. Silvery flow-aligned bands are splay, covered in the silver streaks guide. When the whole lot is the wrong shade or yellowed evenly, start with resin identity, drying, residence time, and colorant mixing before you only adjust vent steel.
For new colored programs, specify the color system and cosmetic surfaces during quoting. NetProto returns DFM feedback with instant quotes so teams can align resin grade, color concentrate loading, and likely cosmetic risk on a hardened steel production tool while geometry and material choices are still flexible, depending on part size and complexity.
Root causes
Use whether the shift is global, streaky, or spotty, plus recent material or process changes, to pick the first lever.
| Likely driver | What you see | First lever to try |
|---|---|---|
| Melt overheated or held too long in the barrel, nozzle, or hot runner | Uniform yellowing or brown tone on many parts; odor or degraded specks on the same lot; worse after downtime or small shots | Reduce melt temperature and residence time within supplier guidance; purge after stops; match shot size to the machine so material does not stagnate in dead zones |
| Oxidative degradation when molten resin contacts air in the screw or hot runner | Gradual yellowing on clear or natural grades; may worsen on long cycles or open hopper exposure | Shorten open-air exposure; keep feed path sealed; reduce back pressure and screw speed trials within the process window; purge and sample after color changeovers |
| Poor colorant or masterbatch dispersion (under-mixed concentrate, wrong carrier resin) | Swirls, streaks of wrong hue, or light and dark patches on the same part; inconsistent shade shot to shot | Confirm concentrate ratio and carrier compatibility with the base resin supplier; increase mixing through controlled screw recovery trials; avoid layering concentrate in the hopper |
| Contamination or cross-resin carryover from prior runs, dirty feed lines, or wrong regrind | Sudden green, pink, or black specks after a color change; first shots good then wrong tint as purged material clears | Dedicated purge protocol between colors; clean hopper, throat, and dryer; segregate regrind by resin and color; reject unknown scrap |
| Moisture or volatiles degrading the polymer or colorant at the flow front | Brown streaks with silvery bands; onset after humid storage or skipped drying on hygroscopic grades | Dry per supplier datasheet and verify dryer performance; if streaks dominate, use the silver streaks guide for the splay branch of the diagnosis |
| Degraded additives, stabilizers, or flame retardants in the compound | Yellow or brown cast on heat-sensitive grades; lot change coincides with onset; may pair with brittleness on the same material | Confirm lot and storage age; reduce thermal exposure in the process window; escalate to the material supplier if the base resin is suspect |
| Mold deposit or corrosion staining the surface (plate-out on steel) | Repeatable discolored patch at the same mold location; wipes off early then returns; not tied to flow front on every part | Clean mold surfaces per safe practice; review vent gas and stabilizer bleed; inspect for stuck deposits in textured areas |
Diagnosis
Global hue problems usually trace to material, colorant, or melt history. Localized brown or black at ribs and last fill more often belongs in the burn marks workflow.
Troubleshooting
Photograph the color chip, a good part, and the defect before you change multiple variables at once.
Reference
Compare under agreed lighting to the approved chip or Pantone reference. Note whether the shift is uniform, streaky, or spotty and whether it is on the surface only or through the wall on a cut sample.
Material
Check bag labels, lot numbers, masterbatch ratio, and regrind source. Wrong concentrate carrier or mixed scrap is a common cause of sudden hue jumps.
Process
Confirm dryer status and time since material was exposed. Reduce melt temperature and residence time within supplier limits and purge if degradation is suspected.
Housekeeping
After a color switch, run a documented purge until transition shots stabilize. Inspect hopper, throat, and conveying path for stuck pellets of the prior color.
Machine
If pigment is streaky, adjust screw recovery and back pressure in small steps while watching melt temperature. Avoid chasing dispersion with excessive shear that yellows the base resin.
Tool
When discoloration always maps to the same steel feature, clean deposits and review whether volatiles or stabilizers are plating out. Customer-owned hardened steel tools can be maintained and modified, but mold work is scoped separately from production orders.
Prevention
Stable color is a material-handling and thermal-discipline problem as much as a pigment problem.
Lock resin grade, color concentrate, and regrind rules in the traveler before first production shots.
Dry hygroscopic grades per supplier guidance and keep feed paths closed to limit re-absorption.
Document purge shots required after every color or resin change and reject transition parts.
Avoid extended downtime with material sitting in a hot barrel; purge before restarting long stops.
Upload CAD for DFM feedback before tooling release on qualified programs so cosmetic surfaces and material notes are visible early.
Store retainers of approved first-article color so operators can separate drift from lighting or photography changes.
Color programs
Discoloration troubleshooting on the floor fixes drift. Upstream color specification and compatible resin systems reduce how often drift appears.
FAQ
Common causes include thermal or oxidative degradation of the melt, poor masterbatch dispersion, contaminated or mixed regrind, cross-resin carryover after a color change, moisture-driven degradation on hygroscopic grades, breakdown of heat-sensitive additives, and mold deposits that stain the surface at the same location every shot.
Discoloration often shifts the whole part or lot toward yellow, brown, or the wrong hue. Burn marks from trapped air are usually localized char at the last fill, rib tips, or air traps and worsen with vent and fill profile changes. Use the burn marks guide when the pattern is char at flow ends, not a uniform color drift.
Yes. Regrind from the wrong color, mixed resin, or overheated scrap can shift hue or introduce specks. Segregate regrind by resin and color, cap regrind use per your material supplier guidance, and purge thoroughly when changing color systems.
Material left in a hot barrel or hot runner can degrade and yellow, especially on small shots or heat-sensitive grades. Purge after extended stops, reduce idle time with melt in the cylinder, and confirm restart shots before running production.
Upload CAD for an instant quote and DFM feedback so resin, cosmetic surfaces, and color notes are reviewed before a hardened steel tool is built, depending on part size and complexity. The same tool is intended to run for the life of the part with no minimum order quantity on repeat production.
Ready to quote
Bring CAD, color references, and sample photos into an instant quote so material and cosmetic targets are clear before tooling. One hardened steel tool is built for the life of the part on qualified programs, depending on part size and complexity.
