The opening size is the number one criteria for selecting screens as it impacts the separation quality of the screening process. However, when choosing the opening size, you must first know what specifications are for the final product, and that depends on whether you choose to use the screener for scalping, fines removal or grading:
In this post, we’ll describe how to choose the woven wire screen that is optimal for you, including choosing the best method for determining screen opening size, wire diameter, and attachments, as well as the best practices for vibrating screen maintenance.
Some Screener Basics
A vibrating screening machine is also called a separator or sifter, and its purpose is to separate dry, free-flowing materials of various particle sizes through vibration. The vibrating action of the screening machine separates the particles, discharging the smaller unwanted materials through “holes” of specified shape and diameter in the screen and retaining the larger particles for further processing. The screens are round or rectangular and are attached to a frame within an assembly, called a screen deck.
Screeners operate in a wide range of applications, including chemicals, pharmaceuticals, food, plastics, minerals, and pigments. The scalp material, remove fines, and grade materials. For any of these uses, choosing the right screen is the difference between achieving high-quality, efficient separation performance and inefficient or poor performance.
The majority of screeners use woven wire screens with square openings. When choosing a screen, you’ll need to specify at least two of four key parameters — mesh count, wire diameter, opening size, and open-area percentage — and they will differ depending on whether you choose imperial or metric units. Mesh count and wire diameter are the specifications used in the U.S. Mesh count equals the number of wires per linear inch or the number of openings per linear inch, depending on your preference, and wire diameter is determined in inches.
In countries using the metric system, screens are specified by their opening size in microns and the wire diameter or open-area percentage. The opening size by itself isn’t enough to specify wire screens accurately because the difference in diameters causes the screens to perform differently during the screening process.
Choosing Between Screen Types
Woven wire screen cloth is manufactured in high volume and in various combinations of mesh count and wire diameter. Dry bulk material applications require one of the following types of screen cloth: market grade, mill grade, and tensile bolting cloth. Market-grade screens have the largest wire diameters for given opening sizes, and tensile bolting cloth screens have the smallest diameters.
Some screens use Type 430 stainless steel, which is suited for applications that have stringent product quality requirements. Unlike other stainless steel materials, Type 430 is magnetic and is prone to be picked up by the downstream magnetic separators, improving the odds of capturing and preventing stray wires that separate the screen from falling into the screened material. On the other hand, type 430 stainless steel is more brittle than other stainless steel matrials, reducing the life of the screen.
Choosing the Screen Opening Size
The opening size is the number one criteria for selecting screens because it impacts the separation quality of the screening process. However, when choosing the opening size, you must first know what specifications are for the final product, and that depends on whether you choose to use the screener for scalping, fines removal or grading:
- Scalping :- In the scalping process, the screener consists of a single screen and “takes a little off the top” — it removes a small number of large particles from the surface of a material that’s almost entirely within a particle size spec (or on-size). Typically, the material that enters the screener contains 95 percent or more on-size particles. Work backward from the final product spec to arrive at the proper screen opening size. For example, if your scalping spec is 0% +60 U.S., there should be no material retained on the number 60 U.S. Standard test sieve when you conduct your evaluation. The sieve has a 250-micron opening size, and you should select a process screen with an opening that does not exceed 250 microns. Recommended practice is to choose a screen that has an opening size smaller than the opening of the specified test sieve because of variations in the wire-weaving process (called weaving tolerances). If you refer to a particle-size-to-screen-mesh conversion chart, you’ll find that a 60-mesh market-grade screen is a good choice for this application.
- Fines Removal :- Choosing a screen is more complicated when removing fine particles, especially if the screener has to remove particles smaller than a number 60 U.S. Standard test sieves’ opening size. Due to the inherent inefficiencies of the screeners, it is not possible to remove all fine particles, so it is important to allow tolerances when creating specs for the fines removal. For example, a reasonable spec for allowable fines in the final product is five percent maximum – 60 U.S, which means that no more than five percent of the sample should pass through the number 60 U.S. Standard test sieve during evaluation. To achieve the five percent maximum – 60 US product spec, choose a market-grade or TBC screen with an opening size somewhat larger than the 60-mesh market-grade screen. This will enable near-size fine particles to pass through the screen and make it more likely that the final product will meet allowable fines tolerance. A cautionary note: If the opening size is too large, large numbers of on-size particles may start passing through the screen and discharging with the fines.
- Grading :- In grading applications, also called sizing or classifying, the screening machine separates matter into distinct discharge streams via the use of multiple stacked screen decks. Each stream has its own particle size distribution, and typically one or more of the discharge streams constitute the final product, which usually has a spec limiting the amount of oversize or fine particles. For grading applications, choosing the right screen opening size for a replacement screen is especially critical because any change in the opening size can affect not just one, but multiple final products. Different opening sizes will affect the particle size distributions of the final products, which in turn affect the fines content, product yields, and production rates. Therefore, it’s good to choose the screen opening size based on the results of screening tests conducted at the screener manufacturer’s laboratory.
- The screener’s impact on opening size :- Other factors related to selecting the size of the screen opening have more to do with how the screener operates. While the material in a lab test sieve shaker spends several minutes in the shaker, material spends only seconds in the process screener. In addition, while the sieve shaker only moves horizontally — effective for passing small batches of material through the sieve stack — a process screener’s motion can also move vertically. That’s because the machine operates continuously, so screened material is removed by moving to the discharge across the screen. The screener’s vertical screen motion combined with the slope of the screen conveys the material out of the machine. The particles in the process screener move differently and spend less time on the screen, meaning they have more of a chance to move through a test sieve in a lab test shaker than through the screen. This makes the screen’s apparent opening seem smaller than the measured opening size, which means the actual screen opening size should be up to ten percent larger than in a corresponding test sieve.
Choosing the Proper Wire Diameter
Screener capacity, screen flexibility, and screen durability are important considerations when determining which wire diameter is the best for your screen:
- Screener capacity :- Selecting the correct wire diameter has a major impact on your screener’s capacity. It affects the screen’s open-area percentage and therefore, how quickly material is screened. Typically, the larger open-area percentage will yield higher depending on the capacity. Let’s look at two screens with the same dimensions and opening size: an 80-mesh large wire diameter market-grade screen with a 178-micron opening size, and a 94-mesh small wire diameter TBC screen with 180-micron opening size. The first example has a 31.4 percent open area, while the second has a 45 percent open area. So, the 94-mesh TBC screen with a smaller wire diameter has 43 percent more opening than the 80-mesh market-grade screen for the same screen dimensions, resulting in a much higher screening capacity.
- Screen flexibility :- A smaller diameter screen also tends to be more flexible, which makes it more resistant to blinding. Blinding occurs when particles that are slightly larger than the screen opening become stuck in the screen. This phenomenon mostly occurs in sizing applications involving materials containing high percentages of near-size particles. Screens with larger diameter wires are more likely to trap a particle stuck in the opening, making it more difficult to dislodge. A smaller diameter screen tends to yield a little so its natural movement can easily dislodge the particle. Anti-blinding devices, such as vibrating sliders or bouncing balls, can be mounted under the screen, but they also work better with flexible screens.
- Screen durability :- Despite the capacity and flexibility advantages of smaller diameter wires, there is a downside. They are less durable, which translates into short service life in some applications that handle larger, heavier material loads or abrasive materials. In these situations, it might be more practical to select a large wire diameter screen, but if there are concerns about blinding problems, the best option might be a screen with a smaller wire diameter, although you’ll need to replace it more frequently.
Choosing the Right Screen Attachment
There are two methods used to attach screens to frames and ensure proper screen tensions. You can use mechanical fasteners and hook strips or pre-tensioned screens on rigid frames. VibraScreener™ experts can help you choose the best method for your application:
- Mechanical fasteners :- Mechanical fasteners such as tension bolts, typically for round or rectangular screens or spring clips for rectangular-shaped screens, is the simplest method for attaching screen frames. The screen has a metal, cloth or plastic edging on the periphery punched with holes. Bolts or spring clips are inserted into the holes to fasten the screen to the frame. Then the bolts are adjusted to provide the right screen tension. The benefits of this method include lower costs and, because the screen can be rolled up, easy transportation, and storage. One disadvantage of mechanical fasteners is that it is very difficult to achieve proper, consistent tension of the screen because tension can be added to the flexible edging only at the bolt or spring clip locations. In addition, the screen is held tightly to the frame only at certain points around the screen edging. This allows some coarse materials to escape under the edging between these points and contaminate the screened material. For larger screeners, dozens or sometimes hundreds of bolts or spring clips are required, making screen changes very slow and labor-intensive. Screening vibrations can also loosen the bolts or break or dislodge the spring clips, which can loosen the screen tension and contaminate the screened material with metal bits. These disadvantages limit the mechanical method to coarse screens and noncritical applications, such as scalping minerals.
- Hook Strips :- This is a more advanced method used with rectangular screens that consists of securing screens to frames via hook strips. These are rigid metal edgings that form lips around the sides of the screens. Tension bolts are mounted on the frame to engage the hook strips and then tighten the bolts so they apply a predetermined amount of pressure to the screen. Screens using hook strips are faster to install than those with mechanical fasteners, and they can be rolled up, making it easier for easier transportation and storage. The downside is that screens with hook strips aren’t well suited for fine screening applications. It’s almost impossible to sustain the right amount of tension on the screen, and the poor seal that exists between the tension bolts can allow material to bypass the screen.
- Pre-tensioned screens :- Pre-tensioned screens affixed to rigid frames are an excellent attachment method for round and rectangular screens. The screens are manufactured by stretching the cloth using an adjustable tensioning tool that can provide optimal tension for the screen. Once the appropriate tension is achieved, the screen’s perimeter is bonded with epoxy to a rigid screen frame (typically constructed of square or rectangular stainless steel tubing. When the epoxy sets, the entire screen-and-frame assembly is removed from the tensioning tool, and any extra screen cloth is trimmed away. Pre-tensioned screens usually cost more than other screens discussed in this post, but they provide a number of advantages as well. The uniform tension across the screen’s surface results in accurate, efficient separation performance — especially in fine screening applications. Pre-tensioned screens are also more effective than other screens in sanitary applications that have stringent product safety requirements, including food and pharmaceuticals.
Protecting Against Vibrating Malfunction
To protect against vibrating screen problems and ensure your screener provides the best separation performance over the long haul, you need to ensure proper transport, storage, installation, and maintenance. Here are some tips:
- Transport your screen in the original sturdy corrugated carton and don’t remove the screen until you’re ready to do the installation.
- Store the screen in a vertical position to conserve your plant floor space. This will protect against stacking other items on the screen.
- If mechanical fasteners or hook strips are used to attach the screen to its frame, be careful not to dent or crease the loose screen to avoid creating stress points that can lead to tears or holes. Also, be careful not to under- or over-tension screens that use fasters or hook strips. This will impede separation and shorten service life.
- Regular, ongoing maintenance is critical to keeping your screener running efficiently in the long term. Some routine maintenance steps include checking screen tension, inspecting for material buildup or blinding, reviewing the condition of anti-blinding devices, and checking each screen for tears or holes.
Contact us today for more information about selecting and maintaining a vibrating screener.