The secondary motion of the Armaflex screen sections effectively handles the clumping effect created by dust suppression systems.
In the last year or so, Polydeck Screen Corp. has introduced three new products that have proven to be very popular in prep plants. Polydeck specializes in the manufacture of screening panels using injected molded polymers. The three new products are the Armadex line of bolt-down rubber panels, and the polyurethane VST and Cutter screen panels. Each offers plant operators a way to improve performance and, in some cases, reduce operating costs.
At the center of all this activity is Steve Parsons, one of Polydeck’s regional managers who services customers in the heart of coal country. With more than 30 years of experience, Parsons is also heavily involved in the company’s ongoing R&D efforts serving as one of Polydeck’s product managers, recommending, developing and testing new designs and innovations. He not only understands the screen manufacturing process, he also benefits from the one-on-one interaction he has with dozens of plant superintendents, all of whom have different issues. “Each prep plant has to deal with site specific conditions and most of the improvements that we’ve made as a company are the result of a direct response to one of our customers’ needs,” Parsons said. As part of the company’s continuous improvement philosophy, Polydeck heavily invests in ongoing R&D, he explained, developing and improving products through prototyping, testing, and evaluation.
The injection molding that Polydeck uses exclusively to produce its rubber and polyurethane screen panels requires specific skills and equipment. The venting process for instance, according to Parsons, has to be perfect. “During the injection molding process, the raw polymer is heated and mechanically mixed in a chamber and then forced out under high pressure into a cooled mold,” he said. “When the polymer is introduced to an injection-molded dye, it’s critical that all of the air escapes, leaving 100% polyurethane or 100% vulcanized rubber. This produces a clear, crisp, sharp aperture achieving size tolerances that can’t be attained from cast molding. The poured casting process uses a liquid reducer that must evaporate, which creates distortion from shrinkage. As it dries, the liquid dissipates and the product deforms. If injection molding is done correctly, it produces a consistently sharp product. We have some very talented manufacturing engineers that have perfected this process.”
Injection molding has other advantages as well, such as the ability to mold in surface features. The company’s new Cutter panel for example, uses surface features and very small apertures specifically designed for heavy-media recovery. “The enhancements on this panel improve drain rates,” Parson said. “It has a cutting edge that is tilted into the flow similar to the cutting action on a triple-blade razor.” Polydeck introduced it to the market about two months ago after over a year of design and testing.
Their product testing usually involves several types of coal. The grind of the coal creates different shapes or fragments that vary by region, Parsons explained. The shapes can be flat slabs or pyramids, and each presents a different set of problems for the screen deck. “The flat particle will pass a long slender slot, while the pyramid shapes can get caught in the aperture like little pegs, so each has to be handled differently. The best way to identify these issues is through a good screening analysis,” Parsons said.
Another new screen media that the company has developed that also features molded surface features is the VST screen panel. This design incorporates slots at opposing 45? angles ,creating a zig-zag flow which should yield a higher retention time and allow more time to drain. “Sometimes the material flow is so rapid, the aperture is not recognized. Unless the energy can be slowed, water will carry through the process,” Parsons said. “The material washes off the end instead of falling through the holes. By enhancing the retention time with the VST panel, the material can find the holes, and the slower flow gives the liquid an opportunity to dissipate.”
An important aspect of the screening process that often gets ignored is the scalping stage. To work properly, a scalping screen has to be able to manage a product that’s had a fairly substantial amount of water added to it. The water and dust suppression additives applied through spray nozzles and misters underground raise the inherent moisture on the raw feed, creating a dilemma for the plant.
“The mine adds just the right amount of moisture to this mix of coal, rock and clay to create a mixture that is one of the most difficult to screen,” Parsons said. “Operators can literally grab handfuls of the raw feed and form mud balls out of it. When it dries, it can become as hard as concrete, and in cold weather, there may be enough water in the mixture to cause rapid freezing. Polydeck has developed a version of its Armadex bolt-down rubber product known as Armaflex to deal specifically with this issue. These screen sections use a flexible rubber and custom steel backing that provides a tremendous amount of secondary movement.”
￼The cutter panels have been designed specifically for heavy-media applications.
In addition to the vibratory motion created by the screening machine, the secondary flexing motion created by the panel releases raw material from the media. “The object is to keep this mixture from building up and hardening and blinding the screen,” Parsons said. “The Armaflex rubber is relatively soft but with high wear characteristics, similar to a racing slick.” Parsons likens the process to how a tire sheds mud from its sidewall as it changes shape during travel.
With the help of Buckingham Coal in Glouster, Ohio, Parsons was able to test several concepts on their scalping screen over the course of about three years. Observing the results they’re getting with Armaflex, Parson believes they have found the best way to keep the apertures open.
Armadex bolt-down rubber sections require a completely new production process, which the company refers to as binary injection molding. “Because these panels are so large, the cavities of the mold are partially filled at low pressure,” Parsons said. “The process then changes to one of low volume and extremely high pressure. Injection molding removes all of the air voids creating a finished product with rubber that is fully vulcanized and a homogeneous throughout.”
Polydeck is the only company doing this. “We patented this process,” Parsons said. “We have the largest injected molding facility per square inch.”
Armadex steel backed screen sections can range from 2 x 2 ft. up to as large as 4 x 8 ft. and can be as thick as four inches.
The VST panels increase retention time
Utilizing a huge library of aperture molds as well as water jetting, apertures can be placed anywhere on the screen section providing custom solutions for specific customer needs. “It’s so customizable, it’s limitless,” Parsons said. “For example, the plant could specify 16 inches of solid material to create an impact section to deflect the energy. Then maybe the next 12 ft would contain longer slotted panels so the material recognizes the opening at higher speed and energy. Then toward the end, the slots can be shortened to attain the proper cut.”
The secondary motion of Armaflex can be adjusted with the rigidity of the base steel. In one application, Parson said he removed two thirds of the support steel to give it more action. “In that case, the screen had a tremendous amount of secondary motion with zero blinding, because the panels were alive,” Parson said. As good of a solution as this is, its effectiveness still has a great deal to do with the vibratory screen’s operation. He cautions that operators need to avoid critical frequencies where the panel just starts jumping up and down. To avoid that, Parsons suggests performing a specific analysis of the machine and the media. Once the critical frequency has been determined, the system can be designed to operate either above it or below it gaining a secondary motion with no more, and no less energy than is required.
Durex Celebrates 50 Years
During 2015, Durex wire screen media product line will celebrate 50 years of excellence. The Durex product facility, based in Luck, Wisconsin, is proud of the reputation it has earned during the past half century of successfully meeting and exceeding customer needs, according to Weir Minerals North America.
Durex Products was founded in 1965 by R.D. Scarlett, a former Fortune 500 company executive and civil engineer. In an already highly competitive aggregate screening market, where woven wire screen cloth was viewed as merely a commodity, Scarlett saw an opportunity to successfully apply innovative product development with customer service.
The company initially offered woven wire screens out of its facility in Luck, Wisconsin, and quickly gained market share by aggressively serving the needs of sand and gravel and crushed stone customers in Wisconsin and Minnesota with prompt delivery, quality products and a willingness to help producers solve difficult screening challenges.
In the early 1970s, Durex Products expanded farther into the Midwest, North Atlantic and southeastern United States. During the mid-1970s, as new technology was developed in the manufacture of longer lasting screens, Durex Products saw the potential of synthetic screen media to increase wear life and reduce maintenance costs for its customers. The company expanded its production line to include polyurethane screen media.
Over the following decade, it became apparent to the engineers and application experts at Durex Products that not all screening challenges could be solved with wire cloth or polyurethane screen surfaces. In 1986, the company added a rubber screen manufacturing line. By the early 1990s, with its full line of woven wire, vulcanized rubber and molded polyure-thane screens, Durex Products was positioned to provide extremely cost-effective, production-enhancing solutions for its customers in North America.
During the past 50 years, and even as Durex products subsequently became a part of the Weir line-up, the brand has continued to maintain its reputation for product innovation. Rubber and molded polyurethane screens and wear products subsequently became marketed under the Linatex brand, which is transitioning to a new facility in Salt Lake City, Utah.