Multivariate control charts of MDF and OSB vertical density profile attributes

The vertical density profile of wood composite panels is influenced by many variables in the manufacturing process and is an important product attribu te for composite panel end-users. Monitoring the quality of product attribu tes, such as the vertical density profile (VDP), is a critical step in ensu ring the delivery of product value to the end-user and for maintaining the competitive position of a firm. In this study, multivariate control chartin g procedures using Hotelling's T-2 statistic for correlated VDP variables w ere compared with univariate control charts derived from the same VDP varia bles. Comparisons were based on samples obtained from typical production ru ns of a manufacturer of medium density fiberboard (MDF) and a manufacturer of oriented strandboard (OSB). The MDF samples were taken from 3/4-inch sto ck and the OSB samples were taken from 23/32-inch stock. Statistical proces s control (SPC) is intended to prevent the manufacture of defective product that may otherwise occur using traditional quality-control procedures. The Shewhart control chart is the primary tool of SPC, which separates variati on as either "special cause" or "random." Even though Shewhart control char ts provide a sound method for detecting problems in manufacturing processes that may otherwise go undetected, such charts are univariate in nature and have limitations for both uncorrelated and correlated variables. The typic al analysis of univariate control charts for correlated VDP variables revea led that false signals of statistical control occurred for both the MDF and OSB samples. Multivariate control charts of Hotelling's T-2 statistic prov ided a more robust (less false-signals) method for control charting of corr elated variables.