- November 01, 2010, By Mark Miller, Coating Tech Service
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Six Sigma is a quality control system utilized for everything from discrete parts to organizational efficiency. The question is: How do you best utilize Six Sigma to reduce web defects?
Utilization in the converting industry hinges on applying discrete measures to a continuous product. Applications can be as varied as optical film coating to adhesive tape manufacturing. Regardless of the product being made, Six Sigma can be applied to web-coated products.
Like the scientific method, Six Sigma follows a road map: Define, Measure, Analyze, Improve, and Control the problem. Six Sigma is derived from the standard deviation, a statistical analysis parameter, and is defined as defect levels below 3.4 defects per million opportunities. It is important that the defect and the opportunity are correctly defined or the measurement is useless.
Six Sigma doctrine holds to continuous efforts to achieve stable results that are of vital importance to business success; manufacturing processes have characteristics that can be affected; and achieving sustained quality improvement requires commitment from the entire organization.
Define | It is important that a goal is defined and “entitlement” is understood. Entitlement is a stretch goal defined by the subject matter experts. This goal should have a continuous process focus but a discrete product plan. The measurement needs to focus on improving the performance of the product within the confines of the process.
It is important that the human side of the operation is understood. Emphasis on support from management is just as vital as an operator's understanding of the project. Team buy-in from the top down and the bottom up is critical. Never assume the project will work because people “do what they are told.”
If we take an existing product and process, individuals who have been involved with this system can be tapped to identify where the weaknesses and strengths of the process are as applied to the product. For a new product on an existing line, the borders of the process and materials need to be the constraints for the product. Ultimately, for a new product with an unidentified process, the world is wide open (which may be a blessing or a curse).
The process map needs to be drawn up regardless of the product constraints. This is a unit operations diagram taking into account human interaction with equipment, raw material variations, and measurement system limitations.
Measure | The key consideration in measuring a web process is the final product use. The difficulty in measuring a product in roll form is that it cannot be “fully” measured. In theory, each discrete widget can be measured for quality if the test is not destructive. Not so with a continuous web. Improvements can be made at the main variable locations to reduce defects and improve quality.
One simple example is to have idler rolls of different diameters grouped by sections throughout the process. Therefore, if you see a repeating defect, you can isolate it to the section of the correlating diameter. The unit operations include the following:
Fluid and fluid delivery: In fluid delivery, the equipment variables to be monitored include mixers, pumps, flow meters, and hoses. The material characteristics to be measured include viscosity. The issues (typically downweb in form) include contamination, air entrapment, and temperature control.
Substrate and substrate delivery: In substrate delivery, the equipment variables to be monitored include web cleaners, steering, tension, static control, and temperature control. The material characteristics to be measured include surface energy and substrate manufacturing data. The issues (typically downweb in form) include wrinkles and debris. Communication with the upstream provider of substrate is critical in understanding what is available for use.
Coating head: In the coating head (specifically in a slot die), the equipment variables to be monitored include attack angle (positioning), temperature control, roll total indicated runout (TIR), and chatter. The material characteristics at this point are a function of the roll throughput yield (failures combined up to this point in the process) of the interfaces. The issues (typically crossweb in form) include air entrainment and edge effects.
In the coating head, interfaces are the name of the game. Fluid distribution within the coating station can be simulated prior to manufacturing or operation. In a production setting, the improvement of edge effects needs to be balanced against the overall cost of waste.
Curing: In curing operations, equipment variables to monitor include energy into the web, drying, and stability. Issues include temperature control, rate of cure, and effect on substrate. There are various cure methods and each has different effects. A preventive maintenance schedule helps maintain stability of an operation.
Measurement systems: It is critical to measure the process at the intersection of the unit operations. This means monitoring the incoming substrate, fluid, coating head, and the outgoing web, in addition to ensuring the measurements capture crossweb, downweb, and machine chatter defects. Remember continuous changes and defects occur over time, so steady state operation is important to accurate measurement and reaction.
Analyze | Analysis of the variables (causes) and the effect on the defects is the next step. Once the subject matter experts have identified what to measure to improve product performance, it is again time to get management on board. A key issue is the time it will take to measure the process accurately.
An example is when a consumable backing roll was made to order and had a limited lifespan. Although this Six Sigma project took months to analyze, the root cause was identified, and the problem was solved. We need to narrow the scope so the team can identify the unit operation upon which to work. The team should be focusing on having “no wasted effort.”
Improve | There are two types of experiments that are important: designed of experiments (DOE) and keeping a process journal. DOE allows focus on the identified variables. In a 2(5-1) factorial design, all five variables can be studied with minimal work. Process journals allow previously unidentified variables to uncover new root causes.
A critical idea is Roll Throughput Yield (RTY), which is defined as the combined variability of the process. I worked with a coater that had a product within spec for the discrete sample but in roll form had a hard band that made it unacceptable. This caliper variation did not show until the on-line gauge and process journal pointed the engineer in the right direction.
Control | The goal has been met for product improvement and the changes need to be implemented. Make sure the changes are understood and upheld by having a brief description of the “why” behind these new control measures.
This stage usually means a redefinition of the product and process. So in the end, “control” means a new beginning.
Remember to utilize the Six Sigma methodology and identify the important differences when you consider webs over widgets.
Readers can look forward to learning more about coating technology starting in January 2011 with Miller's new monthly column: Coating Matters.
Mark is an independent consultant in the roll-to-roll coating industry and owner of Coating Tech Service, Eau Claire, WI.
As a certified Six Sigma Black Belt for 3M, Mark has been integral to new developments and technology in web coating applications. He holds a Masters degree in Polymer Science and Engineering from Lehigh Univ. and a Bachelors degree in Chemical Engineering from the Univ. of Wisconsin.
Reach him at firstname.lastname@example.org or 715-456-9545.
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