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dimensional change—which was good news. The specific gravity of the solution used also was not significant. However, voltage used in the electropolishing process did have a significant effect on how much the height of the device changed after polishing.
What’s more, they found a statistically significant interaction between voltage and cycle time. To explore the dynamics
To produce optimal results, the design settings would need to account for the push-pull relationship between removing enough material to smooth the surface, while not removing so much that the dimensions significantly changed.
To find those settings, the team used Minitab to create an overlaid contour plot from their experimental data. He specified lower and upper bounds for their two responses, height change and roughness change.
Minitab then displayed contours for these bounds against voltage and cycle time on the plot’s axes, highlighting the region where both responses were within bounds.
This plot allowed the team to see the voltage and time settings that would produce optimal results.
Fig. 3. The white space on the contour plot identifies the region of the design space that satisfies the criteria for all responses. Setting factors at any of the levels shown in the white area should yield responses that fall within the specified ranges.
of this interaction and better understand how it related to changes in height, they used Minitab’s interaction plot.
The interaction plot made it easy to see and understand the relationship between these factors. When voltage was low, the cycle time had little effect on the response.
However, when voltage was high, a longer cycle time resulted in a much greater change in height. This interaction underscored the challenge of keeping height changes within an optimal range, while still ensuring that the device was adequately polished.
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Based on the results, Smith & Nephew determined that the electropolishing process produced results that were in specification when the voltage was between approximately 7 and 9
volts and the cycle time was between 50 and 70 seconds.
Using these settings, they minimised defects, ensured that parts were all within specification limits, and successfully demonstrated a level of dependability that satisfied the regulatory guidelines for their product. l
For more information ✔ at
www.engineerlive.com/asia
Eston Martz is Senior Creative Services Specialist, Minitab Inc, State College, PA, USA.
www.minitab.com
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