Southern California Section of the Society of Plastics Engineers

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SIX SIGMA – AN IMPROVEMENT SYSTEM By Victor Okhuysen, PhD, Prof of Manufacturing Engineering Cal Poly Pomona University

     Continuous improvement has become a mode of life for any manufacturing enterprise including plastics manufacturing. In this mode of continuous improvement many approaches and trends (and some would call fads) have emerged and become part of our manufacturing lexicon. For instance, philosophies such as Total Quality Management, Just In Time, Lean Manufacturing and Six Sigma as well as more specific tools and methods such as Poka-Yoke, SPC, Single Minute Exchange Die (SMED), DMAIC cycles, fishbone diagrams, etc. In this article I will explore what Six Sigma (6) is and how it has evolved and been used.


     Six Sigma (6) started as a statistical quantification of process capability. Simply put, your process variation at 6 should be equal or smaller than the tolerance band for the feature. This would lead to a defect rate of less than 3.4 parts per million. In order to achieve six sigma levels of quality, thus, it is necessary to improve process capability to this level. In reality, a manufacturing facility will determine the proper ‘sigma level’ for the desired capability based on product specifications. Six sigma techniques are then applied to reduce process variability


 A six sigma effort will typically include a DMAIC (Define, Measure, Analyze, Improve and Control)



- In the Define step the project goals are determined. These can be reduction in variability, production

time, cost, etc.

- In the Measure step the relevant raw data will be collected. For instance, scrap rates, time studies, part weights, cycle times, melt temperatures, etc.

- In the Analyze step the raw data will be analyzed and there are many tools that are used in this process. The most common tools include statistical (ANOVA, Gage R&R, Taguchi methods), graphical (pareto charts, flowcharts, ishikawa/cause-effect/fishbone diagrams), “5 why” analysis, Root cause analysis, Value Stream Mapping, among many others. It is important to note that many of these tools are used in improvement efforts independent of six sigma efforts as well, which leads to confusionof what six sigma is.


-In the Improve step the process is changed based on the results of the Analyze step. This could be a change in raw materials, procedures, equipment, training, facility layout, etc. The solution to be implemented in the improve step will generally provide a return on the investment that is justified with the data and results from the previous two steps. In this manner it is easier to avoid money losing initiatives, particularly when equipment purchase or major layout changes are executed. In fact, GE claimed to have executed six sigma projects that contributed over one billion dollars to profits.

-In the control step the systems and tools are put in place to ensure that the changes will remain effective. For instance fool-proofing (poka-yoke) and SPC (Statistical Process Control) tools are put in place.


     The body of knowledge for application of these tools is extensive and led to the development of specialists within organizations. Certifications that cover this body of knowledge have been developed by various entities including by some companies for internal purposes. It is important to note that for this reason the standards for each certification will vary. However, they generally do overlap. The American Society for Quality ( offers a set of certifications related to six sigma. The most common certification levels are a “Green Belt” and “Black Belt”. The Green Belt typically involves the basic tools used in Six Sigma including some of the statistical tools as well as in depth knowledge of the DMAIC process. Green Belts are expected to work under the supervision (in six sigma issues) of a Black Belt. The Black Belt certification includes team and leadership concepts as well as more depth in the general body of knowledge with regards to tools.


     Six sigma is often confused with other process improvement and system management philosophies such as TQM and Lean Manufacturing. For example, cycle time reduction is often a goal of six sigma efforts, so what does cycle time have with the original definition of reducing product variability? Well, not much, but the tools used in six sigma efforts can be readily used for other goals thus leading to “lean-sigma” efforts. In these cases companies have undertaken lean manufacturing improvements by using six sigma tools and methodologies. Again, some of the tools are complementary and commonly shared with the goal of improving processes. The reality is that many of the process improvement

methodologies such as TPM, QRM, Lean, Six Sigma and TQM have overlapping methods, tools and objectives.


     In conclusion, six sigma is an improvement system based on the DMAIC cycle with many associated tools. These tools can be used to improve many processes and products within a manufacturing facility. If these skills do not exist within your facility to pursue these continuous improvement projects hiring or developing green/black belt certified personnel can be a route to acquire these skills. My recommendation would be to someone who is already a plastics processing professional obtain these certifications if needed.


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