1. Now, consider the case where extrusion is a Six Sigma process. In this case, the extruder output should have a standard deviation of 1.667 thou. What proportion of the rubber extruded will be within specifications in this case?

Six Sigma Quality at Flyrock Tires Susan Douglas, vice president of quality at Flyrock Tires, was wondering how to explain the value of Six Sigma quality to the people within her organization. Flyrock, a major manufacturer of tires in the United States, had five manufacturing facilities where tires were made and another 20 facilities for various components and material used in tires. Quality had always been emphasized at Flyrock, but lately quality was a bigger issue because of recent fatal accidents involving tires made by other manufacturers due to tread separation. These tire problems had received significant coverage in the popular press and all manufacturers were under pressure to show they were working to ensure that problems did not arise in the future. Several lawsuits had been filed on behalf of the families of victims in these accidents and much of the discussion in these lawsuits was likely to involve the way these tires were built. Douglas wanted to push through a Six Sigma program and significantly improve the level of quality at Flyrock, and she needed to build a solid case for it first. Manufacturing Tires While tire technology had made many advances over the decades from old-fashioned, inexpensive, bias-ply tires to today's steel-belted radials, the basic process of making tires had essentially remained unchanged and was shared by all manufacturers. Rubber, the major component of tires, arrived in bales at the tire factory where it was mixed with chemicals and other ingredients. Each manufacturer had its own recipe for this process. Large blades in a machine broke down the raw rubber and mixed the chemicals, much like a kitchen mixer would. The rubber compound was then rolled into a milling machine where it was forced back and forth through a series of rollers to more thoroughly mix the chemicals added earlier. Tread referred to the part of the tire that touched the road, while sidewall referred to the sides of the tire. Tread and sidewall rubber went through the same process but with a different chemical mix. After further processing, each type of rubber was extruded into a single continuous strip. These strips were put onto a conveyor system with the strips of tread rubber in the middle and the sidewall rubber on either side. The long strips of tread and sidewall rubber were then sent through a coating apparatus that applied adhesive. After the strips had cooled, they were shrunk and cut to the exact size needed. While the rubber was cooling, strips of fabric, usually polyester, were coated with another rubber blend. These strips were used to make the body of the tire after the layering process began