Suppliers Selection, Process Capability, Statistical Process control, Sigma Level of Process, Defect per Million of Opportunities, DMAIC
Onix is a factory that provides metallic components, some of them are cans for canned food. These cans are used by several clients that are big manufactures of conserve meat and fish. In the last weeks, Onix is receiving a lot of complaints about the quality of products supplied, most of them around the sealed parts and nicks on the body. For that reason the factory will employ the Six Sigma DMAIC procedure in order to find and eliminate the principal root cause of the problem.
When the Onix management group receives the mentioned group of complaints, immediately activate its Quality Committee to investigate the main root causes of these problems. This team find out two possible and antagonistic root causes of this problem, as shown in the next chart.
After this causes analyses, the work team has to answer the following question: What is the real root cause of this problem? To solve this problem, what we must do first is check the problems with suppliers, (the process stability and capability).
In this case we can’t check the process stability and capability before is discarded the problems with the raw materials, because problems with raw materials affects the process stability because it is a variation cause.
Specifically for this problem, the quality characteristic that we must take into account is the tinplate hardness; for that reason it was carried out specific controls for that characteristic, which have to be higher than 90 N/m2. The data sets obtained with the Vickers test are presented following:
In order to have a better interpretation of the hardness results, we assess the supplier capability, seeing that vendor 2 is the better, because have a good values of process capability, vendor 3 can accomplish the harness requirements but only with 3 sigma level and vendor 1 are not capable for our requirements.
After cheeked the quality of raw materials, is needed to verify the process statistical control and capability. For accomplish this goal the production is started using only the best quality raw materials, provided by supplier 2. During this production the work team verifies every lot of 10 000 units, with the objective of do not supply any lot with 0.5 % of defects; the obtained data is presented following.
Parting on the obtained data for the lot defects, is executed a quality capability and process stability study, obtaining that in general the process is stable, despite it have two points out of control limits, besides the process is capable to accomplish the clients requirements . The 26 lots have 0.1984 % of defects; that means 1984 defects per million. See next graphic.
The Z metric converts the average percent of defects in a capability index similar to the other that can be calculated with continuous data. In most cases is desired a Z value higher that 4, in this case is obtained 2.8, for that reason we can assume that the process must be improved if we want to obtain world class capability indexes.
The tolerance limits show that is expected less of 95 % of lots with 27 defects.
It was checked that the principal problem was the poor quality of raw materials, obtained from 3 different vendors that have significant differences between the hardness of tinplate provided. It is needed a rigorous control of raw materials to prevent this problem.
It is strongly recommended to maintain commercial relationships only with supplier 2 until the other suppliers can meet the quality requirements needed by Onix.