Creative Drying Pulls Nonwoven Line Into Shape
Rogers Corp., headquartered in Rogers, CT, develops, manufactures and markets specialty materials for a range of applications in the imaging, communications, computer, transportation and consumer markets. Following an acquisition, the company added a new manufacturing operation to its composite materials manufacturing plant located in Killingly, CT. The Killingly plant historically has manufactured high-loft air laid needled nonwoven material for medical padding, prefiltration, and electrical and thermal insulation applications. Now, Rogers also produces a line of nonwoven roller covers used to carry fountain solution to the printing plate in conventionally dampened lithographic offset printing presses at the Kilingly site.
Rogers selected National Drying Machinery Co. to supply the dryer required for this new manufacturing operation after a search process within the thermal processing industry.
"We looked at a number of dryers," said Dennis Austin, manufacturing engineer at Rogers. "Some were expensive and offered features that we did not need for this particular manufacturing operation. National's capabilities, plus their costs, were a good match. They were the best value we found, and they offered a quick delivery."
Solving Shrinkage and Product Width IssuesThe roller cover produced by Rogers has a pore system that allows a precise amount of water to be released and absorbed, ensuring the proper ink-to-water balance imperative for consistent printing. To create this precise pore system, the high-loft air laid needled nonwoven web material goes through a multistep manufacturing process that includes saturating it with a proprietary solution to enhance its performance capabilities. During the saturation process, the web doubles its weight. Aggressive yet consistent and gentle drying is required to reduce the water content to 1 to 2% to create the paper-like material utilized in the manufacture of the final roller covers.
A natural side effect of drying and heat treating the web is overall product shrinkage. According to Austin, without adequate controls, this shrinkage can be as high as 40%, and at that level can dramatically affect product line economies. For manufacturing management at Rogers, minimizing the shrinkage factor while finding a dryer that was versatile enough to handle current and future products with varying product widths was paramount to the decision process. The selected dryer also needed to be installed within their six-month timetable yet meet the budget objectives.
As part of the development process, National initially performed in-house laboratory testing on Rogers nonwoven web material to understand key web properties such as shrinkage and pressure drop and to determine temperature and pressure effects on the web.
"Based on this research and the company's background in drying and heat and mass transfer, National was able to customize a high efficiency dryer design for Rogers," Aidan Niggel, product manager for synthetic fiber machinery at National, said. "This approach keeps the material affixed to the belt and restricts machine- and cross-directional product shrinkage."
In addition, the fans, engineered plenum and conveyor were designed for proper airflow management, creating a balance of positive pressure above the conveyor bed and negative pressure below.
Rogers and National engineers met to discuss the options and benefits of selecting a drum dryer vs. a conveyor dryer. Although Rogers initially considered drum dryer technology, the company opted for a conveyor dryer because its design provided a continuous suction pressure across the material -- a feature that a drum dryer, with its split second transfers, could not provide. This continuous suction holds the web in place and prevents unnecessary shrinkage, thereby increasing overall efficiency.
All pertinent variables -- heating and cooling retention times, product loading, differing product widths, operating temperatures and air velocities -- were factored into the final design. The 36" unit installed at Rogers includes the dryer housing, air locks, and feed and discharge ends. It is gas fired for quick heatup and clean operation.
The fully enclosed, stainless steel dryer housing is built with 4" insulated panels for thermal efficiency and operator safety. Nonconductive seals, internal framework and positive door latches minimize heat loss and improve processing performance. Polished product-contact areas prevent product snagging and tearing of the web. Full-height doors along the length of the machine ease access for Rogers' maintenance and housekeeping crews, and a walk-on roof allows workers to easily work on overhead fans, ductwork, piping and conduit.
The continuous suction technology utilized in the Rogers dryer allowed the company to effectively process nonwoven products with varying widths. To prevent shrinkage and maximize machine throughput, a consistent and strong suction force is required. National developed an adjustable deckle system that accommodates material widths from 60 to 96".
"The deckle system consists of stainless steel side pieces along the entire length of the dryer interior," Niggel explained. "The pieces do not contact the conveyor belt or hold the material in place. Instead, they channel airflow through the fabric by blocking its passage through the conveyor belt area uncovered by the material."
At material widths of 60", the deckles are fully extended while at widths of 96", they are fully contracted. Intermediate adjustments are made as needed.
Conveyor belt design also required special consideration as the belt had to be lightweight, structurally able to support itself and the product, and able to run at speeds as great as 60 ft/min with no slippage from the truss. In addition, it had to impose as little resistance as possible to airflow, leave the product relatively unmarked and be economical. The final belt design uses a 304 gauge stainless steel mesh with a tight weave that minimizes marking of the material but still allows air to pass with minimal restriction.
Heating Zones Maximize Drying BenefitsThe science and scientific principles of drying have remained unchanged, yet each new generation of manufacturing improvements impacts a dryer's handling and processing characteristics. Construction improvements allow greater process control, so the dryer can mimic the natural drying curve of a given product. State-of-the-art dryers are zoned and sealed more effectively to allow precise air and heat management throughout the evaporation process. As the number of dryer zones increases, the ability to control the drying process increases. However, as the number of zone partitions increases, dryer cost also increases. Maximizing drying benefits while minimizing costs is where art meets science.
Rogers' dryer is divided into two 10' heated zones. The first zone runs at a high temperature to remove as much moisture from the web material as possible. The second zone runs at a slightly lower temperature to complete the drying process without burning the material. Because of the level of air movement within the dryer, National added two 20" air locks on the dryer's feed and discharge ends. The air locks help prevent air from entering or escaping the dryer, thereby maintaining stable and consistent pressure and temperature conditions.
But, the design of the dryer's heated zones is not the only critical consideration. For Rogers to get the best overall product, the material must be fed onto the conveyor belt properly. To do this, Rogers synchronizes the speed and rotation of the unwind and rewind stands feeding and retrieving material from the dryer. The more quickly and evenly the material is pulled down to the belt, the better the overall product characteristics. To assist in the proper transfer from the unwind stand, National fitted the feeder mechanism with a suction box that pulls the fabric down and holds it in place on the belt. This eliminates much of the shrinkage problem before it even begins.
Assembly Saves on Installation CostsTo eliminate unnecessary construction costs during installation, National shipped the conveyor dryer to Rogers' site in pieces, except for preassembled parts such as doors, fans, heating units, etc. Once the machine arrived on-site, National dispatched a team to install it. Within four weeks of delivery, National's field technicians, working with utility vendors, had the Rogers dryer fully operational.
The entire delivery time, from sales order to operation, was a mere six months. Rogers had the machine they needed in place in time to meet their production timetable, and within weeks of installation, Rogers was supplying its distribution network with dampening and ductor sleeves made from material processed in a National dryer.