Plastic processing is one of the most commonly used manufacturing processes in industry. Even when the technology itself is refined to near perfection, one classic quality issue keeps coming back in real life: scratches on a plastic part. Sometimes they appear right after demolding, sometimes during transportation, packaging, or inspection—which is exactly why the root cause is often not obvious.
Fishbone Diagram for Scratches on a Plastic Part
Below you’ll find 44 potential causes (Fishbone / Ishikawa diagram) of this quality issue: scratches on a plastic part. I’ve grouped them into the typical categories (6M plus a few additional ones commonly seen in the automotive industry).
If you want a slightly more “automotive” wording, I can also swap “plastic part” to “plastic component” and “demolding” to “part removal from the mold,” depending on your audience.
People (Human Factors)
The operator handles parts while wearing jewelry (rings/watches) or with long fingernails. This also links to H&S: there is a risk of ring avulsion injury (a “degloving” injury) when rings get caught during work.
Different part-handling techniques across shifts, for example:
- Gripping in different areas: one shift holds the part by ribs/functional areas, another touches the visual surface (A-surface).
- Placing vs. “dropping”: one person gently places the part, another releases it from 2-3 cm (scuffing/abrasion).
- Sliding on a table/belt instead of lifting (especially at inspection).
- Different stacking methods in containers: tight vs. loose, overlapping, flat vs. vertical, with separators or without.
Rushing due to takt time → careless placement/handling.
Not following the “no sliding” rule (parts are dragged across surfaces).
Lack of training on visually critical surfaces (A-surface).
Improper glove use (dirty gloves or the wrong glove type).
Scratches during rework (trimming, deburring, cleaning).
Maintenance personnel scratch parts during interventions.
A forklift hits containers → vibration/impact and potential part-to-part rubbing inside.
Machine (Equipment)
A worn or gritty conveyor belt surface between the injection molding machine and the inspection (or assembly) station.
Guide rails set too tight or misaligned. This refers to rails on a conveyor, transfer track, chute, or within a device that are intended to keep the part “on-axis” while moving through the process. If they are set too narrow or are skewed/asymmetrical, the part starts rubbing against the rails – creating scratches, usually in a repeatable location.
Worn robot gripper jaws or sharp edges.
Missing protective covers/padding on the gripper, or the padding has hardened (i.e., insufficient TPM / lack of preventive maintenance).
Excessive drop height on a slide/chute.
Ejector pins cause scuff marks (when the part sticks and is “dragged” during ejection).
Damaged injection mold surface transferring scratches onto the part.
Fixtures/holders have burrs or rough surfaces.
The automatic degating/deflashing unit contacts the A-surface.
Method (Process / Work Instructions)
Parts slide against each other (one on top of another) during transport between operations. This should typically be considered and addressed during the Process FMEA (PFMEA) development.
Incorrect stacking/arrangement in intermediate Work In Progress (WIP) containers.
Too many parts per container → compression and rubbing/abrasion.
No defined orientation (e.g., “cosmetic side up/down”). Verify whether the part is sometimes placed with the cosmetic surface against: a workbench/tabletop, inside a container or carton, or on a conveyor.
Excessive cleaning/wiping (over-rubbing).
Use of abrasive materials (cloths/paper towels).
Poorly defined rework method (knife angles, tools, work technique).
Insufficient cooling time → the surface remains soft and scratches easily.
Material (Raw Material / Consumables / Packaging)
Plastic pellet contamination (hard particles) → can scratch the surface during demolding / part removal from the mold.
Excessive regrind content → increased surface sensitivity (higher risk of scuffing/scratches).
Changes in additives/fillers increase brittleness or scratch susceptibility.
Excessive mold release agent attracts dust and particles.
Incorrect gloves (e.g., powdered gloves) leave abrasive residue.
Dirty packaging materials (dusty trays, reused cardboard).
Returnable packaging with worn or sharp internal surfaces.
Missing separators/interleaves or the wrong type of separators.
Low-quality or contaminated protective film.
Measurement (Inspection / Acceptance Criteria)
No consistent scratch reference / acceptance criteria / defect catalogue (visual standards are not aligned).
The inspection table has a rough or dirty surface.
No standard for viewing angle/distance → repeated flipping/rotating and rubbing the part during inspection.
Different criteria between inspectors → more handling, sorting, and re-checking.
Environment (Surroundings)
Dust and particles in the area (e.g., open warehouse curtains/doors).
Static charge on plastic attracts dust → particles on the surface act as an abrasive.
Temperature too low → the plastic becomes more brittle and marks can appear like scratches.
Humidity too low → increased static electricity.
Chips/swarf/metal filings on workbenches and in trays.
Finally, the fishbone diagram is one of the simplest tools for organizing information when a quality issue appears on the shop floor – such as scratches on a part. Instead of guessing, you break the problem down into logical areas (People, Machine, Method, Material, Measurement, and Environment) and quickly see where friction, contact, contamination, or unnecessary handling may actually be happening.
The key takeaway is simple: scratches rarely appear “out of nowhere.” In many cases, you’re not dealing with a single root cause but with a combination of contributing factors occurring at the same time. If you approach the issue methodically, you narrow down the list of causes, run quick verification tests, and only then implement corrective actions.
If you want to apply this example in practice, start with one step: pick the 3-5 most likely branches of the fishbone diagram and verify them in a short test series. This is the fastest way not only to “put out the fire,” but also to reduce scratches in the process permanently.
You are also welcome to join our training on plastic processing assessment based on CQI-23 guidelines: Injection Process Assessment (CQI-23).
Author: Dariusz Kowalczyk
