Color Process Control Charts

9. When the source of the out-of-control condition is found and corrected, repeat procedures in steps 1 through 6. LIMITS AND TOLERANCES Once the mean or standard has been established, action and control limits are set according to the appropriate monitoring manual. The action limits act as "early warning" limits. Production work can still be processed when the action limit is exceeded, but this indicates that a condition exists that needs to be corrected or the process may drift out of control. Once the process drifts out of control, you should stop production until the problem is corrected. If a control strip shows that the control limits have been exceeded, confirm this with a second strip; then refer to the appropriate monitoring manual and stop production until the trouble is corrected. As you continue to plot control values, you will see a random variation around the process mean over which you have little control. As long as the control limits are not exceeded, acceptable quality can be expected. COLOR PROCESS CONTROL CHARTS A good control program uses control charts and subjective print quality analysis in decision making. A control chart provides a tool to avoid situations where a serious processing error is compensated for in printing to keep print quality acceptable. An imaging facility that is compensating for errors near or beyond the control limits is certain to have more quality problems than a facility operating within acceptable standards. Preparing a control chart, such as the Kodak Color Process Record Form, No. Y-55, is quite easy. Follow this 8-step procedure: 1. Use a separate chart for each processing machine. 2. Record the reference strip code number and the reference values in the appropriate places. Use color pencils to distinguish the red, green, and blue densities when recording the reference values. 3. Draw horizontal lines to represent the mean, action limits, and upper- and lower-control limits. 4. Record the process and machine number. 5. When plotting the control values, record the date and time that the control strip was processed (not read), and note any chemical or mechanical changes made as a result of the plots. 6. Plot control values having a plus sign above the line that represents the reference value, and plot control values with a minus sign below the reference value line. 7. Connect the points to provide a continuous graph. 8. When changing to a new control strip code number, note it on the chart. Record the date and the new reference values and limits. The control charts shown in figure 2-18 are not intended to represent an actual control film or process. It is used for illustration purposes only to show patterns that can occur on actual control charts. Processing Control Strips Once control charts are established, control strips must be processed on a regular basis. Each color product has a particular control strip with a particular format. In a sink-line process, a control strip should be processed with each run of production film because of human variables. In continuous (machine) processors, control strips should be processed as follows: Before processing production work at the beginning of the workday, or shift, or after a long shutdown, such as a weekend. Along with production work, at various times throughout the day. At the end of each workday, or shift. After any chemical or mechanical change. Be sure to indicate this change on the control chart. Whenever fresh chemicals are used in the process. Make a note of this on the control chart. 2-34