A concept hardly taken into account in much of the hot-dip galvanized industry is the (apparently obvious) absence of normality of the process.
Please look at the image below: it corresponds to an analysis of the observed growth of the galvanizing layer in steelwork of more than 6 mm section, with measurements taken in three shifts in a galvanizing plant with standardized processes.
For the time being, don’t worry about the graphs and stick with the observed growth model for the averaged values (µ) and the time values per shift (µ1, µ2, µ3).
If the process were sufficiently standardized, analogous growth patterns between shifts and consistent with the average (top left) could be expected. But it was not like that. In the first shift, a µ1 thickness model was observed whose growth rate was 20,7 microns per minute. Things were different on the second shift: before introducing the material into the molten zinc, the minimum thickness was already 112 microns and growth occurred at a rate of 2,9 microns per minute. In the third shift, the values were 92,6 microns for the minimum thickness and a growth rate of 6,2 microns per minute. However, what the plant detected in its product inspections was that this type of material started with a minimum thickness of 90 microns and a growth rate of 6,7 microns per minute. These were the average values between shifts.
Of course, the solution was to apply Six Sigma to control the process (reduce variability) and decrease thickness.