REFINED METHOD OF CONTROL OF CORDINESS AND WORKABILITY OF GLASS DURING PRODUCTION*

Many glass plants obtain daily measurements of the density or specific gravity of the glass in each furnace. Small daily fluctuations of about ±0.0010 density unit are usually taken for granted, while pronounced changes within a two- or three-day period are a matter of concern; but heretofore neither criteria of permissible variability nor rules for interpretation of the data have been in general use. In the present work, the control-chart method of statistical analysis of past data has been applied to data from ten glass furnaces. Small daily fluctuations of density are found to be statistical in character, and the predominant cause of large variations is found to be in the batch house. The rational subgroup sample to be used in analyzing such variations and in operating a control chart is found to be a subgroup of three consecutive daily density values obtained from a particular furnace. Using this subgroup, the average 3-day range of density for the ten furnaces varied from 0.0006 to 0.0023, and the corresponding 3-sigma limits for daily variation from the central line density were ±0.0011 to ±0.0040. A typical value for the average 3-day range of density is 0.0012 and a value no larger than this is a reasonable goal for a glass container plant. The use of control charts for maintaining a state of statistical control of density during production is illustrated for four furnaces over a 2- to 6-month period. Many assignable causes of variation were found in the batch house, usually in the scales; other assignable causes were changes in cullet and in raw materials, changes in firing of the furnace, and laboratory errors in measurement of the density. Present experience indicates that it is difficult to maintain a state of statistical control with the types of batch-weighing equipment in use in some plants. The importance of control, however, was demonstrated for two furnaces in two different plants by the fact that cordiness increased with increasing 3-day range of density. When the density was not maintained under statistical control in one plant, trouble was experienced with checks in the ware. The use of control charts for keeping lack of control within tolerable limits is discussed for one furnace where the variations were small and the control limits narrow. The range was held under control, but the density showed “trends” and went out of control. In this instance, the 3-sigma control limits for variation of daily values from the central line density were ±0.0011, corresponding to ±0.09% replacement of lime by silica. Inasmuch as composition changes in excess of ±0.09% are tolerable in the present state of the art, a modified control limit corresponding to a composition change of approximately ±0.25% is suggested, the corresponding density limits being ±0.0030. When the 3-sigma limits for density are less than, this value, modified limits may be used, although the 3-sigma limits for range are retained. When the 3-sigma limits are greater than ±0.0030, it is most desirable to maintain strict statistical control, and efforts should be made to reduce the variability; otherwise there may be excessive cordiness and other difficulties in fabrication of the ware. In some instances, a reduction in variability will require major repair of batch handling and weighing equipment or a new batch-house weighing installation. Other subgroup methods and other sources of variability are also discussed. Control charts on density are of practical utility to plants. “Assignable-cause” variations are easily distinguished from unimportant, normal variations. The use of 3-sigma action limits keeps investigation of fluctuations to a minimum, and sets troubleshooting, when it is necessary, on the right track. The charts, furthermore, are a useful guide toward a permanent reduction of the variability. They should be helpful to management in striking an economic balance among tonnage pulled, glass quality, and capital expenditures for improvement of batch mixing and handling and other changes. The time required to maintain a chart for one furnace is about one day for past-data analysis, one minute each day for plotting, and not more than one day per month for current analysis, review, and adjustment.