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.
Pp and Ppk
Process Index
Statistical ratios measure the overall performance of a manufacturing process without regard to its long-term stability or centering. In the quality control of paper converting, the metrics of pp and ppk compare the total variation of the output against the engineering specification limits. These numbers are calculated during the initial product trials to determine if the equipment is capable of running the specified cartonboard grade.
Calculation Method
The math is based on the total standard deviation of all samples collected over a specified period, capturing both short-term noise and long-term process shifts. While the first index evaluates the total spread of the process, the second ratio adjusts for any deviation of the mean from the target center. When calculating pp and ppk, the sample size is usually larger than that used for standard control charts, providing a realistic view of overall batch variation.
A low value in these ratios indicates that the process requires mechanical adjustment or that the tolerance limits are too narrow for the current machinery.
Supplier Selection
High performance values in these indices give print buyers confidence that the converter can maintain uniform quality across different production runs. This consistency is vital for maintaining brand standards across global markets.