Manufacturing Process Capability & Statistical Quality Control (Cpk / Six Sigma)

Quality in tire manufacturing is not inspected in - it is built in through process control. A tire that fails a uniformity test, exhibits force variation above specification, shows belt edge separation in end-of-life durability testing, or generates a consumer warranty claim for irregular wear has typically failed at a compound mixing, building or curing process step - not at the final inspection stage. The role of Statistical Quality Control (SQC) and process capability analysis is to move quality assurance upstream, to the point where variation is generated, and to bring that variation under statistical control before it propagates into finished product.

Tire manufacturing processes have well-defined critical-to-quality (CTQ) characteristics at every production stage. In compound mixing, CTQ parameters include Mooney viscosity, bound rubber content, dispersion quality of carbon black or silica, and cure scorch time. In tire building, CTQ parameters include component width and gauge, splice overlap, bead seating concentricity and ply turn-up height. In curing, CTQ parameters include cure temperature uniformity across the mold cavity, internal pressure profile during vulcanization and post-cure inflation pressure. At final inspection, CTQ parameters include radial force variation (RFV), lateral force variation (LFV), conicity, balance mass and geometric dimensions.