A surface defect near the gate or at the far end of a cavity is easy to label and hard to diagnose. An injection molding short shot, dark mark or snake-like flow pattern can point to how melt and displaced air moved through a particular tool, but does not prove one root cause. The useful output is a traceable question: where did it occur, under which resin and part revision, in which cavity, and what changed between acceptable and unacceptable samples?
Treat this as focused injection molding defect analysis, not a generic symptom catalogue. Jetting is mainly a flow-entry question; a burn mark often warrants an air-management question, while incomplete fill can have several contributors. Gate position, geometry, vent paths, material condition and fill profile must be considered together. The corrective action is part-specific: a small cover, thin-ribbed enclosure and deep boss present different constraints even with the same polymer family.
A dark mark alone does not establish whether the cause is trapped gas, melt degradation, contamination or another condition; evidence from the part, tool and process history is needed.
01
Separate the visual symptom from the mechanism
Jetting commonly begins as a pronounced, often matt or rough strand from the gate. BASF describes melt entering a larger open cavity at high flow rate without an immediate deflecting feature; brief contact with the mold wall can cool and crumple the strand before later melt arrives.[1] The line can therefore identify an entry condition to review against appearance and functional requirements. Photograph the mark with gate and flow direction, rather than recording only a flow mark.
Burn marks require equally careful wording. BASF lists diesel effect/burning, air entrapment and short shot as separate troubleshooting subjects, cautioning against one assumed cause.[1] Covestro explains that molten material displaces air and that an unvented last-to-fill area can trap and compress it, preventing complete fill and causing a gas burn.[2] A dark end-of-fill corner may support a venting hypothesis, but sample, cavity condition and material history still need checking.
02
Map the defect to the real fill path
Start with a marked CAD view: gate, nominal flow direction, visible faces, ribs, bosses, holes, weld-line zones and apparent last-to-fill area. Compare acceptable and nonconforming parts by cavity and, where available, by shot sequence and process record. This can show whether the issue follows one pocket, cavity or revision, and avoids hiding a geometry or air-escape question behind a broad instruction such as “increase the temperature.”
For a jetting pattern, inspect the transition just beyond the gate. BASF’s options include positioning the gate so melt meets an obstacle, enlarging its section, rounding the gate-to-part transition, reducing injection speed and avoiding abrupt wall-thickness changes.[1] They are options to assess, not a universal setting sheet: a surface improvement may alter vestige, packing, weld-line placement or a visible face. Record the constraint that rules an option in or out before requesting a tool change.
03
Test gate, flow and venting hypotheses in a controlled order
Do not treat a gate and vent as isolated features. Gate placement establishes the filling pattern and where air is driven. Covestro advises vents around runner and perimeter locations, particularly last-to-fill areas, and notes that filling speed and part volume affect venting demand.[2] Review recesses, blind-end ribs, bosses and corners as potential air traps. Whether a location can vent through a parting line, ejector, insert or another detail depends on the tool and part.
For a consequential change, use simulation or a documented trial to compare alternatives. Autodesk describes Gate Location analysis as preliminary input to Fill+Pack analysis and cautions that a fill preview should be checked with an actual Fill analysis.[3] Request the CAD revision, resin data, assumed gate and runner arrangement, predicted last-to-fill or air-trap areas, and the question the result is meant to answer.
| Observed pattern | Working question | Evidence to request before a change |
|---|---|---|
| Snake-like mark beginning at the gate | Is free jetting occurring before the melt is directed against a wall or feature? | Gate-to-part transition, local section change, fill profile, cosmetic requirement and any feasible deflection feature. |
| Dark mark at a corner or end of fill | Is air being trapped where the cavity closes out, and can it escape through the proposed tool detail? | Marked last-to-fill area, vent route and condition, cavity-specific samples, tool inspection and controlled trial record. |
| Incomplete fill near the same region | Is trapped air one contributor, or do flow resistance, feed restriction, material condition or settings require separate investigation? | Shot record, part mass where relevant, gate/runner observations, material traceability and comparison with an acceptable sample. |
| Issue changes after moving or adding a feed point | Has the change shifted a flow-front meeting, vestige, air trap or visible defect instead of resolving the product-level risk? | Before/after flow map, appearance-zone review, new last-to-fill regions and updated acceptance decision. |
04
Keep trials diagnostic rather than incremental
A useful trial changes one defined hypothesis at a time, within agreed material and product constraints. It might document vent condition and end-of-fill evidence, then compare a fill-profile adjustment or an alternate feed concept. Retain parts in sequence and record material identification, relevant handling condition, cavity, tool state and deliberately changed settings. That turns samples into evidence the product team can assess.
A less-visible mark is not automatically acceptable. Jetting can matter on a display surface, while a burn near a sealing edge, structural feature or electrical interface may need functional review even if hidden after assembly. Cosmetic acceptance belongs with the product owner. Define sample condition, viewing standard, functional checks and authority for accepting residual appearance before releasing a revised tool or process.
05
Turn the diagnosis into a buyer-controlled decision
Procurement can make defect resolution comparable by requesting a cause-and-action record rather than a promise that it is fixed. It should state controlled part and tool revision, resin grade or approved direction, defect location, affected cavities, evidence reviewed, hypotheses, actions tested and outcome against the criterion. Distinguish vent cleaning, polishing, gate modification or geometry change from a process-only adjustment, and identify effects on tool ownership, timing and approval.
Close the issue only when remaining uncertainty is explicit. A result may support a design change, venting modification, revised process window, or acceptance of a nonfunctional cosmetic condition; it may justify further investigation instead. When a report says “improved” or “eliminated,” ask which samples, cavities, settings and criteria support it. The final choice remains specific to the component, polymer, tool construction and product risk.
Practical takeaway
Before you move
to the next review.
- Mark the defect on the current CAD or part drawing, showing gate, flow direction, visible zones and apparent last-to-fill region.
- Segregate samples by cavity and sequence; retain relevant tool, material and changed-process information.
- Request a distinction between the symptom, working cause hypothesis and supporting evidence.
- Review any gate or vent proposal against cosmetic faces, interfaces, packing, mold access, maintenance and product function.
- Agree part-specific acceptance evidence and change authority before closing a trial result.
FAQ
Common questions
Are jetting and burn marks the same defect?
No. Jetting is a flow pattern in which melt can enter an open cavity as a strand; BASF treats it separately from diesel effect/burning and air entrapment.[1] They can be related through the fill pattern, but should not be assumed to have the same mechanism or correction.
Should a buyer ask for more venting whenever a burn mark appears?
Not automatically. Ask whether the mark is at a documented air-trap or last-to-fill location and whether the proposed vent route suits the resin, part and tool. Covestro notes that vents need placement appropriate to the filling pattern and poorly vented last-to-fill regions can trap air.[2] Tool condition and alternatives also need review.
Can a flow simulation confirm the final cosmetic outcome?
No. It can compare gate concepts, fill progression and areas for review under stated inputs, but is not final acceptance evidence. Autodesk describes gate-location work as preliminary to Fill+Pack analysis and recommends verifying a fill preview with an actual Fill analysis.[3] Finished parts still need agreed visual and functional evaluation.
References
Source material
- [1] BASF, “Injection Molding Problems in Engineering Thermoplastics — Causes and Solutions.”
- [2] Covestro, “Engineering Polymers: Part and Mold Design Thermoplastics — A Design Guide.”
- [3] Autodesk, “Perform a Gate Location Analysis in Moldflow Insight.”
This guide is general information for early procurement and engineering discussion. It does not replace product-specific DFM, qualification, validation, regulatory assessment or legal advice.