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CAST TIP


Metalcasting Process Control M


AMERICAN FOUNDRY SOCIETY TECHNICAL DEPARTMENT


any different processes and materials are involved in the production of a quality cast


component, and establishing good re- peatable process controls is difficult in the metalcasting environment. Good practice is the process of consistently controlling all casting parameters to minimize variations in the process and producing high quality components that meet customer needs. One method to help gain control is


the collection and analysis of process data through statistical process control charts (SPC). Although SPC is sometimes viewed as an older process control mea- sure that has been usurped by more mod- ern methods, SPC is still a valid method for quality control in the metalcasting facility and can be a valuable tool for process improvement. SPC uses process measurements and statistical methods to control a production process. Te key idea in SPC is a state of statistical process control is not natural for any manufac- turing process and can be achieved only by eliminating the process variables one by one. Overall, metalcasting is a robust process that can accept some degree of variation, so long as variation is within limits. Each process can be measured and process variations identified. When the process is under statistical process control, consistent quality parts will be produced with a more predictable num- ber of them meeting designated quality standards. As long as the process remains under control, then it should continue to


produce the same quality parts. Personnel probably have already iden- tified the processes that need to be con- trolled to produce a quality product, such as metal chemistry, pour temperature, pour rate, sand test results, among others SPC is a tool to collect process data and organize it so that it can be interpreted easily to signal when a process is veering out of control. Te most common charts used in SPC are the X-bar and R chart. Te X-bar is a plot of the average for a particular value over time. Te R chart is the range, the difference between the highest and lowest measured values. By plotting these values, businesses can see how their process varies and begin to bring it into tighter control. Data charts can be used to measure when the process has fallen out of the range specified by the metalcasting qual- ity process. However, identifying mo- ments of noncompliance is of restricted value. Te real benefit of SPC is to signal engineers the process is moving out of control. By collecting and charting data, the business can identify when a process is transitioning toward the control limits. Te metalcaster then can work to reduce variation and tighten process controls until they are fully inside the limits. Put another way, the control limits should tell when the process has failed (has gone out of control), but SPC process control lim- its are designed to prevent the failure in the first place by warning that the system is heading out of control. For example, quality and engineering


personnel collect sand test data such as compactability, permeability, and methy- lene blue clay. Control charts display test data over time so that variations from the norm are identified. In the case of a green sand system, engineers are not necessarily looking for outliers (individ- ual test results falling outside the control limits), but rather looking at the overall chart line to see if the system is trend- ing upward or downward toward one of the control limits (Fig. 1). Tis chart shows daily loss on ignition (LOI) tests. Each data point represents the average of three tests. Te red lines indicate the upper and lower process specifications. Specifications are the limits set because experience indicates that sand within these limits will produce good quality castings and sand outside those limits will likely produce casting defects. Te blue shaded area shows the limits of the current process controls. Te control limits are based on the actual data from the sand system testing. Note the current upper process control limit is outside of the specification. Tis indicates that the metalcasting facility cannot yet control LOI to the desired level and that action is required to bring the process under control. Tests also show wide variation. Once sand system engineers drive down the variation within the system, staying within the specification limits should never be a problem. Figure 2 shows the process is now in control, and the upper and lower control limits are within the process specifications.


Figure 1: This chart shows daily LOI tests. Each data point represents the average of three tests. The red lines indicate the upper and lower process specifications. The blue shaded area shows the limits of the current process controls. Note that the current upper control limit is outside of the process specification.


42 | MODERN CASTING October 2016


Figure 2: This chart shows that the process is now in control and the upper and lower control limits are within the process specifications.


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