Before we address wiping techniques, we might well question the need for wiping. After all, if capillary forces hold particles to surfaces strongly enough that the particles can’t easily get airborne, why the great concern to remove the particles by wiping? The simple answer is embodied in two words: Contact Transfer. That’s the means by which surface particles are unwittingly moved from one area of the cleanroom to another. Contact transfer occurs when objects (gloved hands, notebooks, etc.) touch one surface in the cleanroom and then another. The objects will pick up particles from the first contact and transfer them to the second. Wiping surfaces prevents or minimizes the undesirable cross contamination and the inadvertent contamination of the second surface.
How best to wipe?
1. Wipe gloves before wiping surfaces, then discard the wiper.
2. In linear strokes whenever possible.
3. From known clean areas to known dirty areas.
4. From dry areas to wet areas.
5. Using a clean wiper surface for each wiping stroke.
Linear strokes with a cleanroom wiper prevent inadvertent transfer of particles and contaminants from dirty areas back to clean areas. This can be proven using a few drops of food coloring on a sheet of clean aluminum foil. Wiping the food coloring with a dampened wiper in circular strokes (the method used to wipe kitchen counters) will show the color transferred to all portions of the aluminum sheet. Wiping with linear strokes (refolding the wiper after each stroke) minimizes or eliminates the transfer of food coloring back to the clean area of the aluminum sheet.
Wiping from clean to dirty and dry to wet areas is reasonably intuitive. The reverse method would transfer contamination from dirty/wet areas to clean/dry areas. Not good.
Quarter folding a dampened 9”x9” wiper provides a convenient size. There are 8 clean wiping surfaces in each wiper (using both sides of the wiper). Exposing a fresh wiper surface after each stroke minimizes cross contamination from the previously used surfaces.
So there are optimum methods to wiping. If you have to wipe surfaces in a cleanroom – and you do, to avoid the possibility of contact transfer of contamination – it’s best to follow the 5 rules listed above.
Download Berkshire’s “Proper Cleanroom Wiper Folding & Surface Cleaning Poster”.
Next up: How to use this optimum wiping method for cleaning floors and walls.
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