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Metalcasting process control: American Foundry Society Technical Department.

Many different processes and materials are involved in the production of a quality cast component, and establishing good repeatable 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 measure that has been usurped by more modern 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. The key idea in SPC is a state of statistical process control is not natural for any manufacturing 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 number 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 identified the processes that need to be controlled 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. The most common charts used in SPC are the X-bar and R chart. The X-bar is a plot of the average for a particular value over time. The 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 quality process. However, identifying moments of noncompliance is of restricted value. The 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. The metalcaster then can work to reduce variation and tighten process controls until they are folly 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 limits 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 methylene 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 (individual test results falling outside the control limits), but rather looking at the overall chart line to see if the system is trending upward or downward toward one of the control limits (Fig. l).This chart shows daily loss on ignition (LOI) tests. Each data point represents the average of three tests. The 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. The blue shaded area shows the limits of the current process controls. The 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. This 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.


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Title Annotation:CAST TIP
Publication:Modern Casting
Date:Oct 1, 2016
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