From the hopper, the pigment enters a separate 32 ft (975cm) long flexible screw conveyor, which employs a flexible stainless steel screw having specialized geometry to move non-free-flowing materials. The conveyor transports the pigment at a 45-degree incline, discharging it into one of four loss-of-weight feeders, each dedicated to a 21 ft (640cm) high premix tank located approximately 23.5 ft (716 cm) from the hopper, where it is blended with varnish to form an approximately 30 percent solids slurry.
The flexible screw conveyor discharges according to the amount of weight lost, whereby load cells on which the discharger is mounted, send signals to a PLC to activate the screw when 20 lb (9.1kg) of pigment have been released into the hopper, and cycle five times to feed 100 lb (45.4kg) into the premix tank.
“Individual slurries are then fed into a holding tank with separate compartments that can hold up to 4 premixes,” said Buckett. “From the holding tank, the different color slurries are fed separately into dedicated horizontal bead mills, each with a capacity of approximately 53 gal (200 l), for particle size reduction and encapsulation of the pigment to create the finished ink base for each color. Various proprietary ingredients are then added to each base to formulate the finished inks.”
Gravure Inks Require Special Handling
The same pigments that produce heat set inks at the Hartford, WI, facility are used at the Martinsburg, WV, plant to produce solvent-based gravure inks. “Here four bulk bag dischargers, each with an auxiliary manual bag dumping station, are installed on the second floor,” said Glenn Gogin, CR/T Process Specialist Manager. The operator uses the dumping station to introduce a 50 lb (23kg) bag of special additives which are conveyed through a flexible screw conveyor along with the pigment to a hopper above a mixing tank dedicated to each color. The pigment and additives gravity feed into each mixing tank when called for by the process.
The contents of each mixing tank are then fed into a holding tank similar to the holding tank used at the Hartford facility. Unlike Hartford, however, the Martinsburg plant uses a novel concept, painting the bulk bag dischargers, hoppers, mixing tanks and other plant equipment to match the color of the pigment each is handling.
“Because the pigments are mixed with toluene, a volatile and flammable organic solvent, the finished inks are blended in explosion-proof mixing tanks located downstream of the holding tank,” Gogin noted. “To keep Volatile Organic Compound (VOC) emissions to a minimum, a gravure pressroom engineered as a permanent total enclosure is located within the Martinsburg facility. Large diameter piping allows nearly all of the solvent-laden air within the pressroom to be captured for reuse. The system routinely achieves efficiencies, which are considerably above the standard of 92 percent required by the EPA and the 96.86 percent industry average.”
For both bulk bag unloading systems, Buckett says “We sent samples of each color pigment to Flexicon for analysis and specified the feed rates we needed. They took it from there and designed the system to meet these criteria. We worked closely with local representative Allen Davis Associates throughout the process and they were extremely helpful. The system works so well that in the Hartford plant we routinely process about 160,000 lb (72,575 kg) of dry pigment each week through the four bulk bag dischargers with no problems at all.”
David Boger is vice president, sales & marketing for the Flexicon Corporation (website: www.myPRINTResource.com/10830816; phone: 888-FLEXICON), with the world headquarters located in Bethlehem, PA. During his tenure at Flexicon, the author has previously held the positions of service engineer, applications engineer, and sales manager. He holds a B.S. in Chemical Engineering from Rensselaer Polytechnic Institute, Troy, NY.