“It’s like trying to feed a bunch of toothpicks into a machine at an even federate” says Charles Weber, president of Composite Products Inc. (CPI), Winona, Minn. While the material Weber is referring to here isn’t toothpicks, it’s something equally tenacious – fiberglass. This difficult material has a high aspect ratio and often bridges and clogs leading to unnecessary downtime.
CPI had tried several different methods to process the fiberglass. One of the process steps involves feeding the fiberglass (via gravity-discharge) directly into an extruder after a continuous fiberglass strand is cut into equal lengths. Although this method worked, the cutter often needed maintenance, creating massive amounts of downtime and decreasing efficiency. After a few years of utilizing this method, the company began ordering discretely chopped fiberglass from glass manufacturers and replaced the cutting machine with a pinwheel feeder. After using the feeder only a short time, the company realized that the pinwheel feeder wasn’t the solution to its feedrate problem either. Says Weber, “The pinwheel feeder had a mechanical pin that, as the rollers turned, held back and dispensed the material. The pin geometry was critical – if any inconsistent material came through, it easily plugged and clogged the feeder.” A screw feeder would theoretically supply fiberglass at an even feedrate but would be far too aggressive for the material and end up damaging the fibers. Obviously, another solution was needed.
With these difficulties in mind, Weber discussed CPI’s feeding problems with Shlomo Gicza (the president of ARBO) at a Midwest trade show. The ½”-1” long fiberglass fibers were tested in-house at the ARBO test lab in Toronto, Ontario. After several attempts and design iterations, we produced a feeder that fed the material consistently and accurately. The 3.5- cubic-foot hopper holds a maximum capacity of about 90 pounds of material. “In a traditional feeder, the hopper typically cones down like a funnel,” says Weber. “However, because of the nature of the long fibers, you can’t funnel this material because it will eventually just plug the hopper’s opening.” Instead, the hopper has an agitator mounted to its flat bottom and two baffles on the hopper’s sides that facilitate material flow. The agitator has a vertical axis and the agitator’s paddles spin in a horizontal plane. The agitator rotates very slowly and gently pushes the material into a large tube opening located above the vibratory feeder tray. It does not impart any downward force while breaking up bridges and stopping them from forming.
The material falls down the tube attached to the hopper onto a 9-inch-wide, 24-inch-long, horizontally mounted feed tray. The tray has 2-inch-tall sidewalls and is constructed of stainless steel coated with Plasmadize®. This rough ceramic material increases the coefficient of friction to give the tray more “bite” to move the fiberglass along. The tray feeder uses ARBO’s patented vibration technology, a variable-amplitude functioning mode that can be set to increase or decrease the vibrations to provide the specified feedrate. The vibrations’ amplitude is longitudinal and moves the material forward. The tray’s vibration operates at the natural (or resonant) frequency of the tray and its material, creating a parallel, even material discharge from the tray to the extruder’s feed throat. The feeder has two control loops — one calculates the resonant frequency and the other varies the amplitude of the vibration to match the feedrate set point. The vibratory tray feeder has no moving parts, bearings, or brushes that can wear or need replacement, it doesn’t require mechanical adjustments, and no lubrication is necessary. In addition, the resonant-frequency method allows the company to achieve very accurate feedrates.
Since installing the feeder, the company has experienced many benefits and is extremely happy with the new feeding system. Says Weber, “We’ve seen an increase in production capacity because we don’t have the maintenance issues that shut down the line in the past, and because the feedrate is more accurate and consistent, our product quality has increased. Also, we’re no longer spending money on replacement parts, so our maintenance costs have decreased.”
CPI currently has nine of the resonant-frequency tray feeders in its facility for handling fiberglass as well as other materials, and it incorporates these feeders when it supplies its licensed extrusion process to customers.
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