Figure 3 shows a laboratory wire-mesh sieve screen used for 140-mesh powder. If atomized powder is not able to go through this screen because the openings in the screen are smaller than the size of those powder particles, then those large particles will remain on top of the 140-mesh screen and would be called “plus (+)” sized powders because they sit on top of the screen and cannot go through. Powder particles that can go through the screen are given a “minus (-)” particle-size designation, indicating that they can go through that screen.
The 140-mesh screen size is only one of many different-sized screens that can be used to segregate different-sized powder particles from one another. Table 1 shows a number of other screen sizes commonly used. Notice in the table that the 140-mesh screen (in the U.S Std. Sieve column) is the same as a 150-mesh screen in the Tyler Std. Sieve series and that each opening in such a screen is just over 0.004 inch (0.10 mm) in size. Notice that as the mesh-size number itself gets larger (such as 325-mesh, which means that there are about 325 openings per linear inch), the size of each opening has to get smaller and smaller. For a 325-mesh screen, each opening is only 0.0015 inch (0.038 mm) in size.
Fig. 3. In a 140-mesh screen, there will be 140 wires per linear inch. Powder sitting on top of that screen is known as a “+140-mesh powder,” and powder going through that screen is called “-140 mesh powder.”
People may ask, “I understand that powders are classified by which mesh screens they can go through, but doesn’t all 325-mesh powder also go through a 140-mesh screen? Why then isn’t all that 325-mesh merely classified as a -140 mesh powder?”
Yes, all the 325-mesh powder will technically go through a 140-mesh screen, but it cannot be labeled as -140 mesh powder since each particular mesh-size designation has additional requirements that must be met for it to be properly classified by a given mesh number.