Thermal Cleaning
Q. We currently use a phosphate/rinse/non-chrome seal on light gage sheet metal parts. Someone has suggested we consider thermal cleaning instead of chemical cleaning as I’m getting our three-stage wash line quoted for replacement next year. How is this accomplished and how do you get the phosphate coating on material if you are using heat to cook off oils. P.D.s lemon oil to wipe down her stainless steel sink. I am anxiously awaiting your answer. J.C.
A. You hit the nail on the head with your last sentence. You cannot apply a phosphate coating with a thermal treatment. Thermal cleaning or degreasing can be used to remove the majority of oil from parts that use a light duty lubricant for a metalworking application (forming, stamping, blanking, etc.). It simply consists of heating your part up to a temperature above the boiling range of the lubricant until it evaporates from the surface. With this process, lubricant selection is key to making the process work. The lubricant has to burn off the part “ash-free” so as not to leave a residue that will interfere with subsequent operations or be aesthetically objectionable.
Metalworking that would be considered to be more severe, would have to use a heavier lubricant that would not be amenable to thermal degreasing. That sort of residue would leave a black “charred” surface from the hydrocarbon breakdown. The most common lubricants that are used in conjunction with thermal cleaning are thin mineral oils or so-called vanishing oils that are primarily a lubricity additive formulated with a carrier, commonly something like a mineral spirits.
While thermal cleaning has its advantages (minimal capital cost, little or no process control) it does have its disadvantages including limitations with lubricant selection, it cannot remove particulate, and may emit a VOC (volatile organic compound). In your case, the most significant drawback is that it is not a paint pretreatment, but simply an oil removal or degreasing method. Your manufacturing and painting process is such that it would require the continued use of a phosphate to meet paint performance criteria, so the thermal degreasing would not be a good selection for you.
