Statistical Process Control Fundamentals in Packaging Substrate Converting
Statistical process control in substrate converting relies on sub-grouping across machine direction variance and isolating non-normal barrier defects.
Cp and Cpk
Statistical Capability
Statistical indices evaluate the inherent ability of a manufacturing process to produce output within specified tolerance limits. When monitoring paperboard converting, the twin metrics of cp and cpk provide a numerical comparison between the natural spread of a production line and its engineering limits. The first of these numbers indicates the potential performance if the process were perfectly centered, while the second adjusts for any actual deviation of the average from the midpoint.
Mathematical Computation
Calculation of these ratios relies on dividing the allowable tolerance spread by the estimated standard deviation of the process. For cp and cpk to be calculated accurately, a production run of several thousand sheets must first establish a stable baseline. The standard deviation, derived from the subgroup ranges, determines the denominator, representing six times the process spread.
A lower value in either number signals that the process variation exceeds the permitted tolerances, which leads to defective creasing or misaligned folds. When the process average shifts towards one of the limits, the second metric drops below the first, exposing the risk of producing out of specification material.
Industrial Performance
Achieving a ratio above a value of one point thirty-three represents the standard target for high-speed cartonboard packaging runs. This benchmark ensures that the variability in crease stiffness remains narrow enough to prevent jams on automated cartoning machines. Higher values protect the converter from losses due to material rejection.