Plastics : Statistical Process Control Integration with AQL Systems

Statistical Process Control (SPC) Integration with AQL Systems:

 
Core Integration Concepts:

Statistical Process Control (SPC) is a quality control methodology that uses statistical methods to monitor, control, and improve production processes

AQL (Acceptable Quality Limit or Acceptable Quality Level) is a statistical quality control measurement that defines the maximum number of defective units considered acceptable during random sampling inspection

 
Complementary Roles:

• AQL:  End-product inspection methodology
• SPC:   In-process control methodology
• AQL Focus:  Accept/Reject decisions on finished lots 
• SPC Focus:  Real-time process monitoring and adjustment 

Quality Control Hierarchy:

Integration Points:

Sampling Alignment:

 
SPC Sampling:

• Frequent small samples
• Real-time data collection
• Process-focused measurements

AQL Sampling:

• Larger batch samples
• End-of-process inspection
• Product-focused measurements

Control Limits vs. Specification Limits:

SPC Control Limits: 

• X̄ ± 3σ (Process Variation)

X̄      :   (Mean) represents the average value of the process
σ       :  (Sigma) represents standard deviation ( how much values spread from mean) ±3σ  :  means 99.73% of all measurements fall within this range

AQL Limits: 

• Based on specification requirements

Below is a little reminder of AQL levels:

Selection Method:
- Lower AQL means stricter quality requirements
- Common AQLs are 0.65% (Critical), 1.5% (Major), 4.0% (Minor)

Implementation Framework:

Process Monitoring Phase:

SPC Metrics:

• X-bar and R charts
• Individual measurements
• Process capability (Cp, Cpk)

X-bar and R charts are statistical process control (SPC) tools used to monitor and control processes

Cp and Cpk - key process capability indices are used in statistical process control:

• Cp (Process Capability) measures potential capability
• Cpk (Process Capability Index) measures actual capability

AQL Integration:

• Adjust sampling frequency based on SPC results
• Modify inspection levels based on process stability

Decision Matrix:

Action Levels:

 
High-Risk Conditions (Immediate Action):

• SPC signals out-of-control
• AQL rejection occurs

Immediate actions to be taken:
1. Stop production
2. Investigate process
3. Implement corrections
4. Verify effectiveness and resume production

Medium-Risk Conditions (Heightened Monitoring):

• SPC shows trending towards limits
• AQL results near rejection threshold

Immediate actions to be taken:
1. Increase sampling frequency
2. Review process parameters
3. Preventive maintenance check
4. Operator verification

Low-Risk Conditions (Standard Operations):

• SPC shows stable process
• AQL results consistently acceptable

Immediate actions to be taken:
1. Maintain standard monitoring
2. Regular maintenance
3. Routine operator checks

Integration Benefits:

 
Early Detection:

SPC: Identifies process shifts before defects occur
AQL: Verifies effectiveness of process controls

Cost Reduction:

Reduced inspection costs through:
- Optimized sampling
- Fewer rejections
- Targeted interventions