What is Sanitary / Food Grade Polishing, and how is it measured.
Surface polishing refers to the roughness of of the stainless steel surface.
We most commonly measure it using Ra, the average height of the peaks and valleys on the stainless steel surface. The lower the number, the smoother the surface. The Ra can be measured using an expensive machine called a profilometer, it works by dragging a stylus across the stainless steel surface.
The 3-A Sanitary Standards, as well as USDA guidelines specify that all sanitary stainless steel surfaces, including fabricated, welded and soldered joints, shall be at least as smooth as a 150 grit finish and shall be free of pits, folds, crevices, cracks, and misalignments in the final fabricated form.
The smooth surface of sanitary stainless steel are to ensure no entrapment areas for bacteria to accumulate.
As the table above suggests, there are plenty of different stainless steel surface polishing, they mainly belong to mill finishes and polished finishes.
Mill finishes are attained by application of rollers and mechanical abrasives to flat roll (wrought) stainless steel sheets and are the basic finishes for all flat stainless steel. Differing finishes are attained with hot or cold rolling. For example, cold rolling on polishing rollers is used to obtain a bright finish.
To get Polished Finishes, a few techniques are used to polish the stainless steel surface. For example, grinding with abrasives and/or buffing with cloth wheels may be used to achieve a reflective finish. Further polishing with progressively finer abrasives, extensive buffing with cloth rollers, or electropolishing will result in a mirror finish.
No. 4 finishes are 80 - 150 grit and most commonly used to produce sanitary tubes, fittings and valves installed in the food and beverage industry.
No. 5, 6, 7, and 8 are more expensive finishes and are at least as smooth as a No. 4 finish (150 grit).
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