Six Sigma – Process Selection and Monitoring

In order to achieve six sigma results, one must evaluate, measure and statistically control ALL phases of the cleaning process.  A typical cleaning process is comprised of three main stages – – washing, rinsing and drying.  Each of these must be addressed and controlled separately as any one of the three being out of control will jeopardize the overall process.

Process Selection –

Not all cleaning methods lend themselves to six sigma process control.  Let’s look at a couple that do.  Spray washing is by far the most common method of washing parts and can in some cases meet six sigma capabilities depending on the part and the cleaning specification. Parts are typically conveyed through a washer with a halo of spray covering them. Spray is a directional process and is best suited for cleaning external surfaces but, with properly directed sprays, internal passages may be cleaned as well. Spray is quite versatile in its application because many features can be varied such as pressure, direction, nozzles and flow to meet the cleaning needs. Pressures can vary from 30psi for general cleaning to more than 3,000psi for de-chipping and deburring applications. Performing trial testing can determine the optimum spray pattern to meet a specific need. In addition to spray, immersion cleaning with ultrasonics is particularly well-suited to meeting six sigma quality levels as well as meeting even more stringent cleaning requirements. The cavitation intensity achieved in the cleaning tank scrubs the surface of parts and allows the cleaning solution to be more effective than it would be using only immersion, spray, agitation or brush methods. The cleaning action of almost any detergent, soap or solvent improves through the application of ultrasonics making it a natural choice for meeting six sigma quality standards. Ultrasonics can reach into the smallest cavity or blind hole, cleaning a part inside and out. For proper cavitation to occur the fluid must not have much movement. Select a system with slow part movement through the bath. The ultrasonic solution should be filtered when there are not parts in the bath. An important attribute of ultrasonics based cleaning that makes it so well suited for six sigma cleaning is that it produces consistent results over an extended period of time.

Selection of a cleaning method alone, however, does not by itself assure six sigma results.  Assuring proper performance requires monitoring and control of a number of variables within each process.  This can be achieved by analyzing the process upfront using Process Failure Mode Effects Analysis (PFMEA) on the parts washer, which helps to identify the variables that need to be eliminated or reduced to maintain a consistent cleanliness level.

Monitoring Variables –

As we have stated before, the “big four” of cleaning are time, temperature, chemistry and agitation.  A thorough PFMEA will identify means to monitor and control ALL of these variables.  In many cases, there may be several factors that contribute to proper monitoring and control of each of these “big four.” To achieve six sigma, feedback on “health” of the cleaning system must be acquired regularly to assure that any deviation is corrected BEFORE it reaches a point that jeopardizes the overall process.  For example, let’s say that we have a process that we have determined through testing requires a temperature of between 120 and 140°F to be effective.  A higher or lower temperature will produce unacceptable results.  A start, of course, is to provide sensor-controlled heaters set to maintain a temperature of 130°F.  To achieve six sigma, however, a secondary sensor is required that will indicate any deviation from the ideal 130°F temperature before it reaches the process limits to allow for corrective action.  Such a device, for example, might indicate that the temperature is 5 degrees over or 5 degrees below the target of 130°F thereby sounding an alarm before further deviation results in ineffective cleaning.

to be continued

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