What are Screens?
Screens consist of a surface with openings of specific sizes. Screens are commonly used to provide open but secure separation for a clean environment and flying objects, debris, or insects. Heavy duty screens can be used as a grate or grid, which may provide flooring on stairways or catwalks, or as security grates over open floor vents or drains.
Screens are also used in bulk goods processing, dewatering, water treatment, food processing, and other processes that require straining or sorting. Screens may be used to separate components or materials of different sizes and may be used in conjunction to offer refined filtering or separating. In applications such as dewatering, screens are used to wash solids with water, and may also be connected to motors that vibrate or shake the screens.
Selecting screens requires the buyer to identify the required physical specifications of the screen, the material(s) of construction, the base construction, and special construction features. This selection guide is designed to help buyers with this process.
Opening size, overall width, and wire diameter are the most important characteristics of screens. Distributors will also often provide a mesh size in characterizing screens to relate opening size and wire diameter.
- Opening size is the actual clear opening or space between the inside edges of two parallel wires or the diameter of each hole in a perforated sheet. For rectangular openings, enter theopening width(smallest side of the rectangular opening) in the 'at least' field;enter the opening length(longest side of the rectangular opening) in the 'no more than' field.
- Overall width is the width of the entire length of sheet. Specifying the correct width is important for screen installation or fitting a screen as part of a larger system. Screen length is usually determined by the amount of screen a buyer needs or is predetermined by the distributor to set lengths that they wish to sell.
- Wire or strand diameter is the diameter of the wire, strand, or filament before it is woven to create the screen. Wire diameter affects the durability and cost of the screen, as it determines the amount of material used in its construction.
Mesh size is also used as a way of specifying screen size, but is different than opening size. The difference between mesh and opening size is how they are measured. Mesh is measured from the centers of the wires while opening size is the clear opening between the wires. A two-mesh cloth and a cloth with 1/2-inch (1/2") openings are similar. However, because mesh includes the wires in its measurement, two-mesh cloth has smaller openings than a cloth with an opening size of a 1/2-inch.
Materials of Construction
It may be important to select a screen based on the materials used in its construction, depending on the demands and specifics of the application. Material selection determines a screen's strength, chemical and corrosion resistance, hardness, and electrical conductivity. Materials can be classified as metals, plastics, natural fibers, or other unique materials.
- Metals and metal alloys tend to be strong and durable materials, and have the ability to be deformed under stress without breaking or cleaving. These qualities are important for screens that could be subject to different forms up stress and also allows for some types of metals screens to be rolled up for transport of longer lengths. They also tend to have higher electrical conductivity and thermal conductivity than other materials. For more detailed information on specific types of metals, please visit the "Materials of Construction" section of the "Search by Specification" page, or explore the Metals and Alloys section of GlobalSpec's Product & Suppliers database.
- Plastics and polymers include a wide range of synthetic and semi-synthetic solids. Two classifications of plastics are thermoplastics and thermosetting polymers. Thermoplastics do not undergo chemical change when heated and can be molded again. Thermosets stay solid after taking shape initially. Depending on the type of polymer, plastics offer a range of advantages such as low cost, corrosion resistance, low friction, and flexibility. For more detailed information on specific types of plastics, please visit the "Materials of Construction" section of the "Search by Specification" page, or explore the Plastics, Elastomers, and Polymers section of GlobalSpec's Product & Suppliers database.
- Natural fibers are softer, naturally occurring fibrous materials. They include cotton, hemp, and silk. Strands from these materials are woven to create more durable wires and ropes. Natural fibers are used to design decorative screens, but tend to not have the strength or resilience for most standard applications.
- Fiberglass is a unique material consisting of numerous extremely fine fibers of glass. It is lightweight, extremely strong, and robust material. Its bulk strength and weight properties are also very favorable when compared to most metals, but it can also be more expensive. Fiberglass mesh tends to be more opaque than standard mesh materials, allowing for greater visibility and light allowance.
Base construction is an important selection consideration for screens, as it determines the way a screen will function. Screens can be made by perforating a sheet, weaving metal wire or synthetic fibers, photochemical etching, extrusion, or electroforming.
- Perforation incorporates punching holes or shapes in a metal sheet to create a screen mesh. Because it is based off of a metal sheet, usually steel or stainless steel, perforated screen is appropriate for high strength applications.
- Weaving is a process wheremetal wire or fibers are interlaced to form a mesh or cloth. The threads which run lengthways are called the warp and the threads which run across from side to side are the weft or filling. The strength of a weave depends on the fiber or wire material and the type of weave pattern used.
- Photochemical etching, also known as chemical milling, is the process of using acids, bases, or other chemicals to dissolvematerials such as metals into a specific shape or part. The etching process has a number of advantages, including no mechanical stresses built into the product, no burns or rough edges, and unaltered material properties.
- Extrusion is a high volume manufacturing process in which rawmaterial is melted and formed into a continuous profile. The process is usually performed with a plastic or polymeric material. Extrusions offers high production stability, but attributes such as strand diameter and material thickness tend to vary more than other construction processes.
- Electroforming is a metal forming process that forms thin parts through the electroplating process. The part is produced by plating a metal skin onto a base form, known as a mandrel, which is removed after plating. Some of the benefits of electroforming include repeatability of aperture dimensions and location, shape and smoothness of aperture walls, low surface tension, and smooth surface finish.
Construction features that may be important when selecting a screen include crimping and calendering.
- Crimping is a process that transforms a wire or fiber from a straight form to a wavy, often sawtooth, form.
- Calendering is a finishing processwhere fabric or other materialispassed under rollers at high temperatures and pressures to reduce the thickness of the material, increase its bulk density, and/or create a smoother surface.