10
Create the Control Plan
The results from House of Quality 3 (HOQ3) are the foundation in developing a Control Plan. This last house focuses on key Process Variables (PVs) that aid the manufacturing of high-quality product. Moreover, all those manufacturing metrics are leading indicators, ensuring a suitable product for our customer. Identifying PVs is not enough, and we need more definitions on how to control and maintain those operating ranges from HOQ3. The outcome of this chapter is to manufacture a consistent predictable quality product, in other words, sustain the gains.
10.1 Sustain the Gains
What happens if the variable is not within the defined values? How much variation is acceptable before a change is needed? The key PVs, as defined in Chapter 8, need to be maintained at a target value within specified ranges. We learned about the sensitivity of the PVs during the scale-up process. There, we not only learned which variables are impactful to product quality but also how much change affects the quality of the product. During the scale-up process, we gathered data which helped us understand the relationships and interactions of the PVs with the Quality Control (QC) metrics. Those building blocks of knowledge will help us put together the pieces needed to start making the Control Plan and sustain the PVs at or near their targets.
The Control Plan is a document that captures the acceptable ranges of operating parameters during the manufacturing of the product, and also it helps adjust that PV back to the acceptable range. The Control Plan is made up of five information parts: process step, measuring type, PV, measurement information, and reaction plan. Figure 10.1 shows those parts.
The boxes indicate the location where these variables are in the manufacturing plant, how these are measured, and what to do if these are not within an acceptable range. The expanded parts are shown in Figure 10.2.
FIGURE 10.2
Parts of the Control Plan with details.
To understand how these expanded parts work, we will use a PV from Liquid Paint Specialists. This example becomes the source for developing a complete Control Plan with a reaction plan. The eight PVs identified from the Pareto chart in HOQ3 (Figure 10.3) have the variables sorted from the most important to least and will be used to create the Control Plan.
FIGURE 10.3
PVs rankings Pareto chart.
The best way to populate the Liquid Paint Specialist Control Plan document is filled out one part at a time starting with Process. The following figures (10.1 through 10.4) show step-by-step how the highest ranked PV (complete order and in time) is filled out.
FIGURE 10.4
Control Plan with one PV and its reaction plan.
We now start to fill the Control Plan. Process has two sub-parts regarding manufacturing location and measurement type. The first sub-part is Process Step Metric which is the measured PV (complete order and shipped on time). The Unit Operation is where the process step metric is located; in this case, it is in the shipping area. The Measurement Type is shown as an Output variable and the unit Type is % (percent). The Process Variable LSL (Lower Specification Limit) is the lowest acceptable value while still making high-quality product. In our current example, we chose 95%. Going below the 95% LSL triggers the reaction plan on how to rectify this issue (described later in this section). Target is the desired value we are striving to obtain, typically the midpoint value between the LSL and the USL (Upper Specification Limit). The target here is 97.5%. The USL for production is 100%, defining the maximum value we can have before triggering the reaction plan. In this case, 100% complete order and the order to be 100% on time cannot go beyond those values which defines a boundary. Therefore, this PV is allowed to operate between the range of 95% and 100%. Anything below 95%, the reaction plan is triggered. It is important to note that these are production specifications and not customer specifications. This difference is very important and will be expanded in more detail in the next section.
The Measurement Method is how each PV is taken, and the Frequency defines how often the measurement is done. In our example, each purchase order is counted. In the Reaction Plan, the Current Control Method explains how the PV is captured: verify that each purchase order is completely fulfilled and shipped on time. If the value is not within the stated LSL and USL, the Reaction Plan is now triggered. This contingency strategy returns the PV to the normal operating range which has been developed during scale-up. The details of the Reaction Plan are on the far right column, and they explain, in our example, how to react when the value is not above the 95% range triggering an investigation.
The remaining PVs in the Control Plan with their respective reaction plans are shown in Figure 10.5. The details shown in the Control Plan figure follows the same logic as explained in the previous section.
FIGURE 10.5
Control Plan with all PVs and reaction plan.
10.1.1 Are These Customer Specifications or Production Specifications?
Measurements that are taken internally in the manufacturing process can sometimes be passed onto the customer. The customer notices that the values did not match the expected target, becoming concerned. The confusion may be caused by subtle differences on the way the producer measures internal product and how the customer measures it. Since we are evaluating the same product, the two observations may have different interpretations. Looking back at HOQ3, the PVs affect the manufacturing conditions and they are tested using the QC metrics. Some of those QC metrics are strictly internal production metrics, and they anticipate, as a leading indicator should do, how the product may perform at the customer. Providing to the customer, our internal QC metrics may cause contradictory results. For example, a company supplies oil at a certain viscosity that they measure internally. The customer receiving the oil also measures the viscosity using their own methods and instruments. When the customer saw the internal values of viscosity from the supplier to be lower than the one they measure, they rejected it immediately. However, the product was never out of spec on viscosity. They both were not using the same viscosity instrument and method of measurement. These metrics were not equal and led to confusion of whether the oil was out of spec or not. It is important to identify clearly which metrics are for internal use and which are shared externally. In addition, if the viscosity metric is shared externally and the customer would like to double check the values with their own testing, make sure that the same procedure and instrument is communicated to the customer, ensuring consistency between their measurement values and the suppliers.
10.2 Control Plan Revisions and Training
The Control Plan is a living document. It should not be viewed as something done once and shelved. The document must be reviewed to ensure it captures the current state of the process. When process changes are considered, proper management of change procedures should be applied, and a line item needs to be added to include updating the Control Plan. Another simple approach would be to have an annual review to capture subtle changes of the process or other dependent procedures.
The Control Plan can also be the foundation for training new employees on how to operate the different unit operations and how to react when the PVs are not operating within the recommended ranges. Consider refresher training sessions for employees as this approach will force the document to be reviewed annually. This approach will maximize the effort in a threefold manner, not only having employees using a consistent method of operating the process, but also training employees on a periodic basis and finally helping to keep the documentation current.
We have reached the end of the Control Plan process and documentation. Hopefully, using the topics covered in this chapter may aid you in sustaining the gains.