Original Date: 07/25/2005
Revision Date: 09/12/2006
Best Practice : Statistical Process Control Application in Plating
Northrop Grumman Electronic Systems has developed a Qualified Manufacturing Line for its chromate conversion plating line. The controls established and maintained for this process typically yield Cpks of 5.41 to 10.15. This relates to a reject rate of less than 0.001%.
Northrop Grumman Electronic Systems (NGES) has been applying and using the tools of Statistical Process Control (SPC) to many of their work cells and processes since the late 1980s. In particular, NGES established SPC processes in its metal plating facility in 1989 and continue to improve its plating process. NGES maintains an Automated Chromate Conversion Line that can process parts as large as 18' x 4' x 5' a manual plating facility for cleaning, chromating, and anodizing aluminum parts and passivation of stainless steel parts. NGES also maintains a plating laboratory that handles small quantities of the electroplating of copper, nickel, silver, tin and gold as well as electroless plating of copper and nickel. NGES has controlled and repeatable processes to regulate the chemistry and temperature of the plating baths for the wide variety of surface finish protections and service-life requirements of its products.
The SPC approach applied by NGES yielded Cpks typically ranging from 5.41 to 10.15 that resulted in the development of a Qualified Manufacturing Line (QML) in October 1998. Having a QML allows NGES to avoid the expense of in-process inspections of each lot of parts being plated. In order to attain and maintain the Cpks demonstrated, the process must be continually monitored and chemical concentrations in the plating baths adjusted. NGES has automated the process of pulling plating bath samples, having them analyzed by laboratory personnel, and having new chemical concentrations requirements calculated and added to the plating baths. The data collected by the laboratory personnel is used to automatically create SPC control charts that are used for Cpk calculation. This process provides tighter control limits on chemical additions, reduces chemical consumption, and provides instant online feedback to plating personnel.
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