A Midwest pharmaceutical company manufactures (in two stages) individual syringes with a self-contained, single dose of an injectable drug. In the first stage, sterile liquid drug is filled into glass syringes and sealed with a rubber stopper. The remaining stage involves insertion of the cartridge into plastic syringes and the electrical "tacking "of the containment cap at a precisely determined length of the syringe. A cap that is " tacked "at a shorter than desired length (less than4.920 inches) leads to pressure on the cartridge stop-per and, hence, partial or complete @ctivation of the syringe. Such syringes must then be scrapped. If the cap is" tacked "at a longer than desired length(4.980 inches or longer), the tacking is incomplete or inadequate, which can lead to cap loss and potentially a cartridge loss in shipment and handling. Such syringes can be reworked manually to attach the cap at a lower position. However, this process requires a 100percent inspection of the tacked syringes and results in increased cost for the items. This final production step seemed to be producing more and more scrap and reworked syringes over successive weeks.

At this point, statistical consultants became involved in an attempt to solve this problem and recommended SPC for the purpose of improving the tacking operation. The length was targeted as a critical variable to be monitored byx- andR -charts, which eventually led to identifying the root cause of the problem. The actual case history contains instances in which desired procedures were not always followed. As such, this case illustrates well the proper-ties, problems, pitfalls, and peculiarities in applying such charts, as well as the necessity of having well-trained quality specialists involved.

Operators of the final stage of this syringe assembly process were trained in the basics of process capability studies and control charting techniques. In an attempt to judge the capability of the process, the responsible technician was called in to adjust the tacking machine and to position and secure it at what seemed to be its best possible position. Then, 35 consecutive observations were taken (see Table 8.7), and a capability study was undertaken. The process had a sample mean ofx4.954 inches, which was close to the nominal aim (or target) of 4.950 inches with a sample standard deviation ofs0.0083 inches. Uppe rand lower specifications of 4.980 and 4.920 inches, respectively, gave an estimatedCpk1.03. Thus, it was determined that the process was minimally capable and could indeed produce the length desired.

To establish the control charts, the operators then collected 15 samples each of size 5 taken every 15minutes. ThexandR -charts are shown in Figure 8.53.These charts show that the process is already out of statistical control in both charts. Proper application of SPC procedures would have indicated that special causes should be identified and new control limits con-structed. Unfortunately, the operators from this shift did not plot these points but only used the control limits they obtained to evaluate future measurements. The operators from this first shift continued to collect samples of size 5 every 15 minutes, but due to their unfamiliarity with charting, they never plotted these15 new points either. At 4:00P . M . of the same day, a new shift arrived and operators did plot this second set of 15 points using the control limits obtained from the first set of 15 points as shown in Figure 8.54. These charts show clearly that the centering was out of statistical control, with the average length far greater than desired. This conclusion was substantiated by operators noticing that the caps were not being tacked properly. The maintenance technician was immediately called into adjust the machine properly.

After the first adjustment by the technician, the plot of the next sample taken 15 minutes later was already beyond the upper control limit for thex- chart. Thus, the syringes were still too long, although the technician affirmed that he had set the height lower just15 minutes earlier. The technician was recalled to read- just the machine. The second try was no better, and so the technician was called a third time to adjust the machine. This third try was successful in the sense that the length seemed to be reduced enough to have both thex- andR -values inside their control limits.

This second shift operators continued sampling and collected 15 additional samples of size 5, at 15-minuteintervals. They plotted these results (see Figure 8.54),but because no values were beyond the control limits, they took no action. It was at this point that the statistical consultants reviewed what had transpired. They not only determined that the original 15 points used to define thex- andR -charts were themselves showing process not under statistical control, but that the last 15points also showed a process not under statistical control. The second shift workers had failed to notice the string of 15 points of thex-chart all above the centerline and failed to conclude that the center was" not where you wanted it. "If they had, they would have once again called the technician to adjust the machine to lower the length of the syringes.

Fortunately, however, the consultants examined theR -chart as well as thex-chart. Again, the last14 points ofRwere all on one side of the center line, indicating a lack of statistical control. Careful examination of both charts revealed that the points ofRwere below the center and were indicating that the overall variation had been reduced by what the maintenance technician had done. Yet, in reading thex-chart (after examining theR -chart), the length of the syringes seemed to have increased. The consultants contacted both the operators and the technician in order to try to find out what had happened to cause this confusing " good and bad "thing to occur. The maintenance technician's story was most revealing.

The maintenance technician said that for his first two (unsuccessful) attempts when he was told to adjust the process center (length of syringe) down, he moved the height adjustment stop down on its threaded shaft. However, he found it was difficult to tighten the locknut for this adjustment stop. The third time (the successful one), being frustrated that the thread of the shaft was too battered at the lower end of the stud, he actually moved the adjustment stop up even though he was asked to make the syringe lengths shorter. He thought this would result in still longer syringes being produced, but at least the locknut would hold. When he was told by the shift that the process was now producing the proper length syringes and that the operators were satisfied, he was mystified. He left wondering how a machine adjusted upward (toward longer lengths) could wind up producing shorter-length syringes!

The consultants realized th4e dramatic improvement of the process variation told the important story. When the maintenance technician set the length of the adjustment cap where he was supposed to (lower), the threads were so worn as to make it impossible to hold the locknut in place. Thus, the vibration from the running machine (within about 15 minutes). Loosened the locknut and adjustment cap quickly, resulting in drifts off center, producing syringes of erratic lengths. However, when the maintenance technician set the adjustment cap higher (which would make syringes longer), the threads there were good enough for the locknut to hold the cap in place. The lengths, indeed, were a little longer than what was targeted, but the variation had been so dramatically reduced that the overall effect was one of making acceptable syringes; that is, the syringes were a tiny bit longer than desired but very consistent in their length so no plotted points were beyond the upper control limit for the x-chart.

The operators were satisfied with this situation because now the plotted points of the syringe lengths came under the upper control limit of the x-chart, which convinced them that they were making syringes to the proper length. The consultants recommended to the managers that the threaded stud on which the adjustment stop moved to be replaced. The repair work needed a special part that was fairly expensive and necessitated some downtime for the manufacturing process; nevertheless, on the strength of the control chart data and the explanation of the maintenance technician's and consultant's stories, the recommendation was implemented. Upon replacement of the threaded stud, waste and rework from the final step dropped to virtually zero over the period of many weeks.

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