Abstract
Process capability analysis combines statistical tools and control charts with good engineering judgment to interpret and analyze the data representing a process. This work analyzes the process capability of a polypropylene bag producing company. The case study organization uses two plants for production and data was collected over a period of nine months for this study. Analysis showed that the output spread of plant 1 was greater than the specification interval spread which implies poor capability. There are non-conforming parts below the Lower Specification Limit (LSL: 500,000 metres) and above the Upper Specification Limit (USL: 600,000 metres) and that the output requires improvement. Similarly, the capability analysis of plant 2 shows that the overall output spread is greater than the specification interval spread (poor capability). The output centre in the specification and overall interval are vertically aligned, thus specifying that the output from plant 2 is also process centered and requires improvement. Recommendations were made to improve the outputs from each production plant.
Highlights
All processes have inherent statistical variability which can be evaluated by statistical methods
These analyses use indices such as the process capability and potential process capability index as quantitative measures of assessing whether a production process under study is capable of delivering products that meet the required specifications (Dharmasena and Zeephongsekul, 2016)
A process capability index uses both the process variability and the process specifications to determine whether a process is "capable"
Summary
Data Acquisition: The case study organization is a foremost producer of polypropylene bags in Nigeria. The aim is to assess production capability of the two plants (Plants 1 and 2) used in the case study. For purpose of this study, production data was collected over a period of 9 months. The data collected includes plant production outputs, rework and scrap data from the plants. Process capability indices are dimensionless measures that relate the output dispersion of a process allowed by its specification tolerance limits to the actual dispersion of the process (Erameh et el., 2016). These indices allow the comparison across a whole range of processes, industries, and countries. The Potential process capability index (Cp) is defined as the ratio of specification width over the process spread
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