Plastics : AQL (Acceptable Quality Limit) for plastics inspection

AQL is a grading system for product quality that tells you:

• How many defects are acceptable in a batch of products
• How many items to check (sample size)
• When to pass or fail a batch

The below diagram explain the AQL process:

AQL (Acceptable Quality Limit) calculation for plastics inspection: 

Basic Components:

• Lot size (N)
• Sample size (n)
• Inspection level
• Defect classification
• Acceptance/rejection numbers

AQL Levels: 

Critical Defects:

• AQL 0.065% - 0.10%
• Life safety issues
• Regulatory compliance

Major Defects:

• AQL 1.0% - 2.5%
• Function impairment
• Performance issues

Minor Defects:

• AQL 2.5% - 4.0%
• Appearance issues
• Minor variations

Sample Size Determination: 

Based on lot size:

• Small (N < 150) :  Lower sample size
• Medium (N = 151-500) : Medium sample size
• Large (N > 501) : Larger sample size

Inspection Levels: 

General Inspection Levels:

• Level I (reduced inspection)
• Level II (normal inspection)
• Level III (tightened inspection)

Special Inspection Levels:

• S-1 (smallest sample)
• S-2
• S-3
• S-4 (largest sample)

Key Differences between the special inspection levels:

Going from S-1 to S-4: 

• Sample size increases
• Inspection detail increases
• Cost increases
• Risk decreases

Acceptance/Rejection: Acceptance Number (Ac):

• Maximum allowable defects
• Based on AQL level
• Sample size dependent

Rejection Number (Re):

• Minimum defects for rejection
• One more than acceptance number
• Triggers lot rejection

AQL (Acceptable Quality Limit) Calculation Example:

• Lot size:                   3000
• Inspection level:    II 
• AQL:                         2.5%
• Sample size =          200
• Ac =                           10
• Re =                           11

Switching Rules:  

Normal to Tightened:

• 2 rejected lots in 5
• Consecutive rejections

Tightened to Normal:

• 5 consecutive acceptances
• Process improvement verified

Normal to Reduced:

• 10 consecutive acceptances
• Stable process demonstrated