In electronic manufacturing plants, products undergo quality inspection (QC) before warehousing to ensure compliance with quality standards. Functional quality is typically verified through 100% testing, and judgments are usually straightforward unless intermittent defects occur.
However, cosmetic (appearance) inspections often lead to disputes, where production supervisors may approve a product while quality inspectors reject it due to minor scratches or slight color variations. When disagreements persist, quality engineers and process engineers may step in, and if no resolution is reached, higher management may need to arbitrate.
Key Parameters for Plastic Parts Appearance Inspection
Before defining defect acceptance criteria, the following inspection conditions must be standardized:
Lighting Conditions
80–120 lux illumination (typical office lighting)
Avoid direct sunlight or overly dim environments, which can mask defects.
Viewing Distance
30 cm (≈12 inches) from the inspector’s eyes.
Viewing Angle
Tilt the product up to 45° (front, back, left, right).
Inspection Duration
5 seconds per surface (longer inspections may lead to over-scrutiny).
How to define and standardize product appearance standards?
When defining cosmetic inspection standards, the following parameters must first be established: lighting conditions, viewing distance, viewing angle, and inspection duration.
Our standard specifies inspection under 80–120 lux illumination, with the sample held approximately 30 cm from the inspector’s eyes. The product should be tilted up to 45 degrees (front, back, left, and right) and examined for 5 seconds. If a defect is visible under these conditions, it should be evaluated against the cosmetic standard; otherwise, the part is acceptable.
Common cosmetic defects include:
Stains/contamination
Scratches
Flash/burrs
Sink marks
Printing defects
Color deviation/discoloration
Contamination (Stains/Spots):
Acceptance criteria are typically defined based on the size and quantity of allowable stains. For example:
≤0.2mm stains: Maximum of 3 allowed
≤0.3mm stains: Maximum of 1 allowed
In stricter standards, additional requirements may specify:
Minimum separation distance between multiple stains (e.g., no two stains within 5mm of each other).
This ensures defects are neither clustered nor overly noticeable.
Scratches:
Scratches are classified based on their severity:
Minor Scratch: Visible but not detectable with a fingernail.
Moderate Scratch: Detectable with a fingernail but not with a fingertip.
Severe Scratch: Clearly palpable with a fingernail or fingertip.
Acceptance criteria are further categorized by the location of the scratch:
Front surface (most critical)
Side surfaces
Back surface (least critical)
Common standards include:
Maximum allowable scratch dimensions (e.g., ≤0.1mm width × ≤5mm length).
Total scratch area limit (e.g., ≤1mm² cumulative in a defined inspection zone).
These criteria ensure consistency while accounting for visibility and functional impact.
Color Deviation (Discoloration):
For reference: Discussion on Plastic Part Color Variation (Plastic Discoloration)
For other subjective cosmetic issues that are difficult to define quantitatively, golden samples (limit samples) are typically used as reference standards. Examples include:
Parting line (location, size)
Flash/burrs (size)
Gate marks (location, size)
Sink marks (depth)
With advancements in digital photography, most appearance criteria can now be documented through high-resolution photos and incorporated into specifications. Recommendation: Whenever possible, replace physical golden samples with photographic evidence in the specification documents to minimize reliance on physical samples, as their long-term preservation is challenging.
This approach ensures consistency while reducing storage and maintenance difficulties.
Conclusion: Best Practices for Cosmetic Standards
Define inspection conditions first (lighting, distance, angle, time).
Categorize defects by severity and location (A/B/C surfaces).
Use objective measurements where possible (e.g., spectrophotometer for color).
Document subjective defects with photos instead of physical samples.
Train inspectors consistently to reduce human judgment variability.
By implementing these standards, manufacturers can reduce disputes, improve consistency, and enhance product quality.