Quality Assurance and Standards
Quality Assurance and Standards examines how textile processes and products are controlled against specifications. The exam tests QA versus QC, tolerances, control charts, acceptance sampling, capability and management-system standards.
Core concepts
- QA is process-oriented prevention; QC is inspection and testing of output.
- Specifications set nominal values and tolerance limits; capability indices compare process spread and centring.
- Control charts separate common-cause variation from special-cause variation.
- AQL sampling plans accept or reject lots by attribute; AQL is an acceptance limit, not a target defect rate.
- Standards include ISO 9001 for quality management, ISO 105 and AATCC methods for colourfastness, and buyer chemical standards.
Worked example
A fabric length is specified as 20.0 ± 1.0 cm. Process mean is 20.2 cm and σ is 0.3 cm. Cp = (USL − LSL)/(6σ) = 2.0/1.8 = 1.11. Cpk = min[(21.0 − 20.2)/(0.9), (20.2 − 19.0)/(0.9)] = min(0.89, 1.33) = 0.89. The process is off-centre; moving the mean to 20.0 would raise Cpk to 1.11 without changing spread.
Traps
- Treating Cp as sufficient when the process is off-centre.
- Reading AQL as zero defects or as a defect target.
- Confusing product standards with test methods.
- Using tightened inspection without adjusting the sampling plan.