Parts are removed from our vibratory finisher with an internal separation screen, and they fall into a wire tote. Full totes are taken to the inspection and packaging area where they are allowed to dry for at least a day before inspection. We are surprised to find that on hot days the parts may take as much as a shift longer to dry than on mild days. Often, this extra drying time results in rust forming on part surfaces. Can you explain this? D.B.
Consider the following two scenarios: In the first example, the plant temperature is 86ºF, plant dew point is 74ºF and part temperature after mass finishing is 66ºF. In the second example the respective temperatures are 72ºF, 62ºF and 66ºF.
In the first case, the humidity in the plant will condense on the parts until they warm to 74ºF. That can take several hours during which time the condensed moisture—distilled water—is diluting the residual rust inhibitors on the part surfaces.
In the second case, the parts begin to dry immediately, allowing the rust inhibitor residue to set up on the part surfaces, providing the designed protection. Many water-based inhibitors are resistant to water dilution once they are thoroughly dry.
Evaporation from a surface that is below the ambient dew point does not occur at a useful rate, if at all.
One answer is to dip the parts in a water-displacing solvent inhibitor. This is becoming unpopular as companies try to get away from the environmental and safety problems associated with petroleum oils and solvents. There are, however, several options using water-based cleaners and inhibitors.
I infer from the question that the vibratory solution provides adequate in-house protection of the parts, except in the stated case of hot days. The least expensive step may be to increase the concentration, and I am assuming you already tried that without success. You can also warm the finishing solution, so parts are heated higher than the dew point. (One customer, using well water in the vibratory machines, runs the water through a water heater set at 90ºF before mixing the solution.) Another possibility is to dip the parts in a heated solution after the finishing operation, or the parts can be partially dried and heated using a warm air blow off on the discharge screen. These suggestions are aimed at just getting the parts somewhat warmer before they set in the tote box.
More expensive answers include equipment such as spin dryers or heated tunnel dryers.