Plastics : A general overview of the Acceptable Quality Level (AQL)

 

Overview of AQL:

Acceptance Quality Level (AQL) represents the maximum number of defects acceptable during random sampling inspection of plastic products. It is a fundamental measure of quality control standards in plastic manufacturing.

Standard AQL Levels for Plastic Products:

Critical Defects:

AQL 0.065% includes the defects that make the product unsafe or violate regulations:

• Sharp edges or burrs that could cause injury
• Toxic material contamination
• Structural integrity issues
• Non-compliance with safety standards
• Chemical composition errors

Major Defects:

AQL 1.5% includes the defects that significantly affect product function or appearance:

Dimensional Issues:

• Size variations beyond ±0.5 mm
• Wall thickness variations >10%
• Warpage exceeding 0.5 mm/100 mm 

Visual Defects:

• Color variations from standard
• Flow marks visible from 30 cm
• Burn marks
• Obvious sink marks

Functional Issues:

• Assembly problems
• Improper fit with mating parts
• Surface finish problems affecting function

Minor Defects:

AQL 2.5% include defects that slightly affect appearance but not function.

Surface Imperfections:

• Light scratches
• Minor flow lines
• Slight color variations
• Small gloss variations
• Cosmetic Issues:
• Minor flash < 0.2 mm
• Light gate marks
• Slight texture inconsistency
• Small surface blemishes

Specific Tests and Measurements:

Dimensional Inspection:

• Critical Dimensions
• Use calibrated measuring tools

Record measurements for:

• Overall dimensions
• Wall thickness
• Hole diameters
• Critical features

Visual Inspection:

• Lighting: 800-1000 lux
• Distance: 30-40 cm
• Inspection time: 30 seconds per part
• Rotation: 360° examination

Material Properties Testing:

Physical Properties:

• Density
• Hardness
• Impact strength
• Tensile strength
• Environmental Testing:
• Heat resistance
• UV stability
• Chemical resistance
• Weathering performance

Documentation Requirements:

Quality Control Records:

Inspection Reports:

• Date and time
• Lot number
• Sample size
• Defect counts by category
• Pass/fail determination
• Inspector identification

Test Results:

• Material certifications
• Dimensional measurements
• Physical property test results
• Environmental test results

Corrective Action Documentation:

For Rejected Lots:

• Root cause analysis
• Corrective actions taken
• Prevention measures
• Re-inspection results

Industry-Specific Requirements:

Medical Plastics:

• Zero defect policy for critical aspects
• 100% inspection for certain features
• Enhanced cleanliness requirements

Automotive Plastics:

• Extended durability testing
• Specific environmental resistance
• Enhanced traceability requirements

Food Contact Plastics:

• Additional chemical testing
• Enhanced surface quality requirements
• Specific migration testing

Process Monitoring:

In-Process Checks:

• Regular machine parameter verification
• Process capability studies
• Statistical process control (SPC) : is a method of quality control which uses statistical methods to monitor and control a process. It helps prevent quality issues by detecting process variations early.

Environmental Controls:

• Temperature and humidity monitoring
• Contamination prevention
• Clean room requirements where applicable

Training Requirements:

Inspector Qualifications:

Required Knowledge:

• Material properties
• Common defect types
• Measurement techniques
• Quality standards
• Skills Verification
• Annual certification
• Regular performance evaluation
• Continuous training updates

Review and Updates:

Regular Review:

• Annual standard review
• Customer requirement updates
• Industry standard changes
• Process improvement incorporation

Documentation Updates:

• Revision control
• Change notification
• Training updates
• Implementation verification