casting 101 C Metalcasting Process Control
establishing good repeatable process controls is diffi cult in the metalcast- ing environment. Good practice is the process of consistently controlling all casting parameters to minimize variations in the process and produc- ing high quality components that meet customer needs.
M One method to help gain control is
the collection and analysis of process data through statistical process con- trol 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 pro- duction process. T e key idea in SPC is a state of statistical process control is not natural for any manufactur- ing 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 varia- tion is within limits. Each process can be measured and process variations identifi ed. 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
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any diff erent pro- cesses and materials are involved in the production of a quality cast component, and
control, then it should continue to produce the same quality parts. Personnel probably have already
identifi ed the processes that need to be controlled to produce a quality prod- uct, such as metal chemistry, pour tem- perature, pour rate, sand test results, etc. SPC is a tool to collect process data and organize it so that it can be interpreted easily to signal when a pro- cess is veering out of control. T e most common charts used in SPC are the X-bar and R chart. T e X-bar is a plot of the average for a particular value over time. T e R chart is the range, the diff erence 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 specifi ed by the metalcasting quality process. However, identify- ing moments of noncompliance is of restricted value. T e real benefi t 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 transition- ing toward the control limits. T e 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 limits are designed to prevent the failure in the fi rst place by warning that the system is heading out of control.
For example, quality and engineer-
ing 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 identifi ed. In the case of a green sand system, engineers are not necessarily looking for outliers (individual test re- sults falling outside the control limits), but rather looking at the overall chart line to see if the system is trending up- ward or downward toward one of the control limits (Fig. 1). T is chart shows daily loss on ignition (LOI) tests. Each data point represents the average of three tests. T e red lines indicate the upper and lower process specifi ca- tions. Specifi cations 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 de- fects. T e blue shaded area shows the limits of the current process controls. T e control limits are based on the ac- tual data from the sand system testing. Note the current upper process control limit is outside of the specifi cation. T is 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 specifi cation limits should never be a problem. Figure 2 shows the process is now in control, and the up- per and lower control limits are within the process specifi cations.
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 specifi cations. The blue shaded area shows the limits of the current process controls. Note that the current upper control limit is outside of the process specifi cation.
46 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2016
Figure 2: This chart shows that the process is now in control and the upper and lower control limits are within the process specifi cations.
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