When looking at the screen specifications, you can see how different they are (Figure 1). When comparing opening size, the fabrics vary by up to 30 percent. This is due to the fiber diameters that are used to weave the fabric.
The essential specs on all screens are mesh count (per inch or cm), opening size, fiber diameter and open area. With two of these four specifications you can identify the other specifications to the fabric. For example, if you know the mesh and the opening size, you can calculate your open area and thread diameter. With these specifications on the fabric known, you can accurately compare one screen to another.
Fabric SelectionDue to the large range of fabrics that are available, it is easy to adjust production by varying the screen specifications. Choosing the fabric that gives your plant an ideal mix of throughput (particles through the screen) and screen life are key.
Fabrics that are woven with a smaller thread diameter will have a higher percentage of open area. These screens will offer much more product through the screen if the particles are under-sized. Unfortunately, with the increase in potential throughput, there is a decrease in screen life as the threads are much less resistant to abrasion.
In turn, fabrics that are woven with a thick wire diameter will have a much lower percentage of open area. This will decrease the amount of product that could pass through the screen, but offer a very long screen life.
For instance, the fabric samples shown in Figure 2 each has an equal pore or hole size; what varies is the fiber diameter. Screen A has a fine thread, which yields a high mesh count and high open area. Screen B is an example of a thick fiber, which yields a lower mesh count, and lower percentage of open area. Screen C is a medium size thread with will give you a mesh count and percentage of open area somewhere in between that of sample A and sample B.
Particle Distribution Relating to Screen SelectionIn every application, understanding your particle distribution is essential. Particle distribution will give good insight to whether it will be necessary to select a fabric based on its potential throughput or the screen's longevity.
With a particle distribution mostly smaller than your selected pore size, it is necessary to offer the highest number of holes for your product to pass through. This will mean that a small fiber diameter or high open area fabric will be necessary. This will offer the most efficient screen use, getting the maximum undersized particles through. The small amount of oversized particles will pass over the screen and because of the light load it will still allow for a longer screen life.
With a particle distribution mostly larger than your selected pore size, throughput is not necessary. A small number of holes will effectively remove the undersize particles, while the larger thread diameter will hold up much longer to the added abrasion of the over sized particles passing across the screen.
Choosing the Correct MaterialThe most common materials used in precision sifting applications are woven screens. The screens are typically woven with a nylon (polyamide) fiber, polyester fiber, or stainless steel wire. All can offer unique properties that should be taken into consideration.
Nylon is a durable fiber that is very resistant to abrasion. Nylon also accepts a fair amount of moisture that will allow for some dissipation of static. Unfortunately when the fiber picks up moisture it can cause swelling. This will slightly impact hole size and open area of the fabric. Swelling can cause slight deviations in particle sizing and throughput from the variations in the moisture content of the batch or simply changes in air humidity.
Polyester is slightly less resistant to abrasion; however, it absorbs much less moisture. This will result in consistent product despite batch moisture content and humidity. Polyester is also less resistant to elongation, meaning the fabric will not stretch as much as a Nylon mesh. This can cause more abrasion issues with incorrect fit, but can lead to a very consistent cut with accurate pore sizes despite loading and pressures.
Stainless steel fabrics are much different than synthetics. Stainless steel screens are rigid and resistant to both high temperatures and abrasion. Woven wire screens are susceptible to "work hardening" or premature failure due to wire breakdown. Flat sifting applications may see advantages with the dissipation of static charges and high percentage of open area. The stainless steel is also less resistant to blinding but very tough to fabricate for the rotary screen applications.
Rotary Screen Fabrication OptionsOnce again there are options when it comes to fabrication of cylindrical screens used on rotary style sifters. These sifters primary use synthetic screens and have a couple choices in fabrication style.
The most basic is a sewn seam and sewn cloth edge. This screen is the lowest in cost but has a high failure rate. The stitched seam prematurely breaks due to the abrasion from the paddles in the sifter and product. This screen also directional or needs to be installed so the sifter paddles go with the seam. If installed backwards the seam will rip shortly after startup.
A similar product to the basic screen is offered with the exception of the seam. The seam can be made by molding urethane through the mesh to form a solid seam. This will allow the seam to last longer due to better resistance to abrasion. Also a welded seam screen can be installed in either direction, as there is no noticeable difference in profile of either side of the seam.
The cylindrical screen can also be manufactured with no sewing. The edge and seam would be made from welded urethane. This has the lowest issue of contamination because you are not dealing with any threads or fibrous material. Also the surface tension on the urethane edge will reduce the slippage of screen off the collar of the sifter.
When choosing a screen for your application, it is important to understand all of the options. For instance, SaatiTech weaves more than 200 specifications of synthetic fabric and has about the same number of specs in wire cloth. The ability to compare screens will allow you to correctly identify the best fabric for your product and plant. In each situation, the preferred screen will vary. Your screen supplier should be able to suggest options based on your production goals.