4

Critical to Quality Metrics

In this chapter, we will be exploring quality metrics of the product. These metrics are either defined from the industry we serve or are inherent from the way the product performs. Later in this chapter, these product properties will be prepared for use in House of Quality 1 (HOQ1). In Chapter 5, these Critical to Quality (CTQ) metrics will be ranked based on the order of importance, using product performance as the standard for ranking.

4.1   Define Quality Metrics

4.1.1   Industry Metrics

Quality metrics can be imposed by the specific industry we serve or performance testing done on the products being made. For example, if you are a producer of electrical metallic tubing or EMT steel conduit, these tubes are used in building infrastructure encasing electrical wires. Therefore, your product will need to follow the standards and guidelines of NEMA, the National Electrical Manufacturers Association. If you are in another industry, such as kitchen cabinets, the KCMA (Kitchen Cabinet Manufacturers Association) can provide product certifications based on quality tests and eco-friendly products. These certifications will differentiate your product since they have been tested according to the rigorous standards defined by KCMA. Additional steps may be obligatory for your industry or may be an optional choice to enhance your product offering. Whichever the case may be, it can provide the end user a level of competitiveness that may be worthwhile for your company to take the extra effort.

Sometimes these quality standards and certifications may not be enough to distinguish your product from others, you may choose to participate on having performance evaluation and ranking done by an organization that provides unbiased testing, certifications, or rankings. Of course, these types of tests need to be carefully selected and determined if it is viable for the type of product you offer. Some of these organizations may invite your company to participate, and you may choose to commit to these extra rigorous tests against your competitors. In addition, these qualifying bodies may offer certifications for that industry. Two prominent organizations that provide these types of services are Consumer Reports and UL (known formally as Underwriters Laboratories). Let’s say, you are a producer of paint and you comfortably meet the industry standards and would like to have an extra advantage to have your product ranked with your competitors. Consumer Reports does precisely that, and they will rank from best overall performance paint to the least. They will even split it between interior paint and exterior paint. This ranking is done using many performance statistics and price, those values that have the best balancing of all these metrics make it to the top of the ranking. Needless to say, this type of unbiased accolade on your product will play very well for the discriminating end user.

Let us return to our case study, we are the supplier of paint coatings, and our customer is the one applying the brightly colored paint on plastic tubes. Since we are using a fictitious product and customer, we will also use fictitious industry standards. Our company, Liquid Paint Specialists, is part of the Liquid Paint Coating Industry, LPCI, and needs to follow the industry norms and standards of crack rates and cure time dictated by LPCI. Our customer, Tubing Experts, needs to follow the Plastic Tubing Association’s (PTA) standards. PTA requires three different industry metrics: crack rates, cure time, and tubing flexibility. These metrics must be front and center when making these products since they are imposed by the industry that the product is made for. They are the first three parameters that define the CTQ metrics.

To know more about these CTQs, each one of these three metrics has a procedure that defines the acceptable levels of performance provided by LPCI. Starting with crack rates, we learn that it follows standard LPCI-123, which specifies the Standard Operating Procedure (SOP) on how the crack rate test is performed and evaluated. The acceptable crack rate test is 1 crack every 1,000 ft of production. The crack rate is determined by bending the painted tube 180° and evaluate if the paint cracked. This test becomes a specification we need to adhere to.

Moving along with the second CTQ, cure time, it follows standard LPCI-456. Similar to the previous standard, it contains the SOP and desired performance characteristics. In this case, the amount of time stipulated for the paint to cure is 120 seconds or less.

The third CTQ, tubing flexibility, uses standard PTA-007, and it has an SOP with its specification. This spec is a little more complex and it reads, “…a batch of tubes bent 180 degrees should have 90% of tube integrity…” This standard needs more interpretation. A batch of tubes is referring to a sampling technique. In the case of Tubing Experts, they gather ten tubes periodically from production to evaluate if at least nine tubes pass the 90% threshold. Tube integrity refers to several potential failures. The unpainted tube must maintain its cross-sectional round shape and not collapse during the bend. In addition, it must maintain a smooth clean bend with no cracks, splits, or tents. A summary of these results is shown in Table 4.1.

TABLE 4.1
CTQs from Industry Standards

Critical to Quality	Metric	Standard
Crack rates	1 crack per 1,000 ft	LPCI-123
Cure time	120 seconds or less	LPCI-456
Tube integrity	90% or better	PTA-007

4.1.2   Customer Metrics: You Don’t Know What You Don’t Know

Measurements create commitments. When you measure, you start to know, and that knowledge can provide valuable information for a decision. Let’s take that thinking to a different level. You step on a scale to reveal your weight. Now you know! That piece of information can take you two paths, either you gained weight or lost weight. If you gained weight, you had “too much fun” during the Thanksgiving holidays, you are surprised by the unexpected weight gain. The second reflects the new exercise and diet routines started 2 weeks ago; it is taking effect and you are happy. As shown in these two outcomes, the measurement helped confirm the assumptions of that elusive number, revealing and helping us know more about the decision we took or shouldn’t have taken.

Let’s expand on that cause-and-effect relationship between measurement and decisions with more examples. I look at my watch to know the time (measure). Will I make it on time to the meeting on the fifth floor? (decision) What’s the temperature outside (measure), and will I need a coat and gloves (decision)? As seen in these events, measurements and decisions are everywhere. Most companies will measure their manufactured article along the way as it is being produced. The sampling and measurement techniques may occur from the very early stages of product creation all the way to the final test. Learning about the key steps and metrics that the customer tracks when using our supplied raw materials reveals what is important to them; it becomes a key metric.

Tubing Experts very closely track five metrics in their manufacturing line that relates to our supplied paint for their tube manufacturing. These five are: plugged spray nozzles, consistency of cured film build, sticky tubes, plugged filters, and holiday testing. Each variable is monitored and tracked by our technical sales support team during the routine visits at the customer’s plant. Keeping a close eye on these five key numbers gives us a good glimpse of their production process, ensuring high-quality painted tubes emerge from the finishing line of their factory.

Let’s learn how each of these customer variables is defined. Figure 4.1 shows the process steps and where along that process the measurement is taken as the tube is being manufactured.

FIGURE 4.1
Customer’s process block flow diagram.

Starting with the spray booth, we find two variables being measured in this area: plugged spray nozzles and plugged filters. The tubes are made in the extruder and are being formed in a continuous manner without interruption, like a garden hose albeit more rigid. Immediately after the tube is coated, it passes through the paint booth, and the paint spray nozzle coats the tube in a nonstop manner which enters to the next unit operation, the oven, where the freshly coated tube will get cured. Since this tube is moving constantly, the nozzles need to provide a uniform spray pattern; in addition, the paint filters collect any debris that may cause a change in the pattern or potentially restrict the flow of paint. The evaluation of the clogging of the spray nozzles is done visually and given a three-level rating of cleanliness using the stop-light visual cues: red = partly clogged, yellow = minimum clogging, and green = clean nozzle. These stop-light work instructions are on the factory floor with sample pictures of the nozzles to show the three levels of defects. In a comparable manner, the paint filters have a dial that indicate when these filters need to be changed. These units have a pressure dial across the filter, and on the dial, there is a red arrow at the value of 2 psi, as shown in Figure 4.2. When the filter reaches this value, it is time to change the filter. This pressure dial is monitored routinely as part of the paint nozzle checks.

FIGURE 4.2
Pressure dial to change filters.

Moving along the block diagram, the tube cutter will cut the tubes to the desired length. Typically, the tube is cut to the length of 10 ft. To perform this cut, the tube is being held by an automated jaw that will travel along at the speed of the tube while the saw cuts it and drops after a fraction of a second. After that very quick cut, the 10-ft tube rolls to the next unit operation for packaging and palletizing. Sticky tubes occur when the painted tube didn’t dry or cure well enough. While the jaw holds it during the cutting process, several issues may contribute in not having the paint to be fully cured by the time it reaches the cutter. This is one of the main reasons we look at a key variable that consolidates many dependent conditions which could affect sticky tubes. This value is binary, either it is sticky showing signs of the tube adhering to the jaws of the cutting mechanism or it is not sticking, and the tube is released freely, rolling down to the packaging and palletizing area. If there is an issue with sticky tubes, then an investigation of the many potential causes needs to be conducted.

The last two metrics from our customer process measured in the Quality Control lab are consistency of cured film build and holiday testing. The first one, consistency of cured film build, is measured under the microscope by looking at a cross-sectional cut of the tube and measured at 90° angles. These four values are measured in mils. The specification for each measurement is 1.0 mils (0.025 mm) with a tolerance of ±0.3 mils. The second QC lab metric is the holiday testing. Holidays are defined as pinholes and voids on a painted thin-film coating. Typically, these pinholes can range from 1 to 10 mils (0.025–0.254 mm) in diameter. These holidays are counted and recorded under a microscope over a fixed surface area, expressing the results as a percent of pinholes or voids present. The acceptable range is between 0% and 1% maximum. Table 4.2 summarizes all the metrics from the customer’s process.

TABLE 4.2
CTQs from Tubing Experts’ Process

Critical to Quality	Metric	Process Area
Plugged spray nozzles	Red = partly clogged Yellow = minimum clogging Green = clean nozzle	Spray booth
Plugged filters	0–2 psi (>2 change)	Spray booth
Sticky tubes	Yes or no	Tube cutter
Consistency of cured film build	1.0 ± 0.3 mil	Quality control lab
Holiday testing	0% and 1%	Quality control lab

4.2   Organize CTQs

All CTQ metrics have been gathered from the standards imposed by the industry and from the customer manufacturing process. A summary of all CTQs is shown in Table 4.3.

TABLE 4.3
All CTQs for Tubing Experts

Critical to Quality	Metric
Crack rates	1 crack per 1,000 ft
Cure time	120 seconds or less
Tube integrity	90% or better
Plugged spray nozzles	Red = partly clogged Yellow = minimum clogging Green = clean nozzle
Plugged filters	0–2 psi (>2 change)
Sticky tubes	Yes or no
Consistency of cured film build	1.0 ± 0.3 mil
Holiday testing	0% and 1%

As mentioned earlier, these CTQs will be used in building HOQ1. Figure 4.3 shows a flow diagram on how this will be used later in Chapter 5 to build HOQ1.

FIGURE 4.3
Organizing the CTQs for HOQ1.