CASE HISTORIES
New Installation:
Custom Compounder Goes Gravimetric
Bayshore Industrial, LaPorte, Tex., supplies compounds and concentrates to North America's major resin producers. The company has been a pioneer in manufacturing polyolefin-based materials since 1980. It produces more than 350 products -- more than 200 million pounds of material annually. To keep up with production demands, in 2000 Bayshore added a production line for its MasterSeries custom compound products. These custom compound products are additive concentrates that include several ingredients.
The company's several custom compound lines handle various, ever-changing formulations. These can include such plastic resins as polyethylene, polypropylene, and polystyrene pellets or powder; such minerals as calcium carbonate, talc, and diatomaceous earth; and such major and minor additives as slip agents, antistatic aids, antioxidants, process aids, and others.
To create a custom compound, the company's pneumatic conveyors transfer resins, minerals, and additives -- each with its own characteristics and varying from free-flowing to non-free-flowing -- from railcars to storage silos. From the silos each ingredient is pneumatically conveyed to a receiver hopper above a feeder in a particular production line.
Each major ingredient is transferred to a feeder pneumatically. From each feeder the ingredient is metered into a common mixing screw conveyor. The feedrate differs from feeder to feeder depending on the ingredient, ranging from 20 lbs./hr. for a minor additive to 5,000 lbs./hr. for a base resin. The mixing screw conveyor transfers the ingredients to a twin-rotor continuous mixer. After the ingredients are mixed, the mixture is fed into a hot-melt extruder for pelletizing.
Finding the right gravimetric feeders
On previously built continuous feed production lines the company uses several different feeder types to feed the ingredients to the continuous mixer. The types vary depending on the kind of ingredient to be fed. Operating and maintaining multiple feeder brands, styles, and sizes hasn't been cost-effective for the company.
As Bayshore began plans for the new continuous production line, the company knew that multiple ingredients, constant changeovers, and feeder accuracy would be important focus points. The interchangeability and maintainability of gravimetric feeders would be ideal for future expansion.
Bayshore had three gravimetric feeder suppliers provide bids for its new compounding line. One of those companies was AccuRate, Whitewater, Wis. "I had gotten some literature, and I visited their booth at the Powder Show," Russell Foreman, Bayshore's project manager explained. "And that's when I started looking in detail at the company's gravimetric feeders." To investigate further, Bayshore sent the equipment supplier a list of ingredients and a sample of each. Bayshore also provided information on the ingredients' characteristics and the company's desired feedrates and accuracy levels.
Bayshore told the supplier that quick and complete ingredient cleanout was crucial for the line's frequent ingredient changeovers. The feeders would also have to be arranged in a specific format, with each feeder metering the ingredient to the mixing screw conveyor. "We had to have a feeder at each location that could handle a wide range of parameters." Foreman said.
The equipment supplier analyzed Bayshore's requirements and recommended their MECHATRON® gravimetric feeders because of their configurability and material handling versatility. Also, several of the compound ingredients are pressure-sensitive: Their bulk density changes with headload changes. The gravimetric feeders are able to sense these density changes and change the feeder speed output to maintain the ingredient setpoint, making the feeders more accurate than volumetric feeders. The gravimetric feeders also provide feedrate verification, allowing tracking of an individual feeder's accuracy, overall feeder accuracy, and ingredient usage. This allows the manufacturer to provide customers with a report confirming the custom compound manufacturing accuracy.
After learning about the feeders' interchangeability, maintainability, and overall construction quality and the supplier's performance guarantee, Bayshore was convinced. The company placed an order for seven of the gravimetric feeders.
Following the completion of the production line and prior to startup, the equipment supplier sent a representative to train the Bayshore employees to use and maintain the gravimetric feeders.
From the silos the ingredients are pneumatically conveyed
to a receiver hopper above each gravimetric feeder.
Feeder operation
The MECHATRON® feeder handles ingredients, environments, and application requirements common to the chemical, plastics, food, and pharmaceutical industries. Each of the feeders installed in the new Bayshore production line has a Coni-Flex™ flexible conical feed hopper with external agitating paddles, a Type 316 stainless steel 13-cubic-foot conical extension hopper, a single-screw feed head, and an AC motor drive package. Each feeder also has a weigh section with twin strain-gauge load cells, a locally mounted and pre-wired DISOCONT® gravimetric control system, and a Type 304 stainless steel base.
Once the ingredient enters the feeder, the flexible conical hopper and external paddles begin the feeding process. The paddles, positioned outside the hopper on opposing sides, massage the hopper walls, fluidizing the ingredient in the hopper without compaction or aeration. The fluidized ingredient falls into the feed screw flights and is metered into the mixing screw conveyor.
Each feeder can have a single- or dual-motor configuration. Selecting this configuration depends on material headload, ingredient flowability, required feedrate turndown ratio, and feed screw length. The single-motor configuration was selected for Bayshore's application because of the ingredients' low bulk density and low volume. This configuration has a gearmotor drive that works in conjunction with sprockets and an automobile-grade toothed belt to drive the feed screw and agitation paddles.
Seven gravimetric feeders continuously meter the
custom compound ingredients.
Two stainless steel strain-gauge load cells are mounted on the feeder's sides. Each load cell has a rating in pounds. To configure the feeder's control system, the weigh section's required capacity is calculated by adding the feeder's empty weight to the weight of the ingredient that will be placed in the feeder.
The two load cells form a resistor network. Their values change as they're stressed or bent by the loaded feeder's weight. The control system sends a known voltage to each load cell, which passes through the resistor network, producing a voltage output that's calibrated to represent a known weight. During feeding, the weight stressing or bending the load cell is constantly changing, varying the load cell's resistance and the voltage output, which is sent to the gravimetric controller for processing.
The control system takes weight measurements continuously, 30 times a second, and processes them to form a feedrate value that's constantly compared to the feedrate setpoint value stored in the control system's memory. If there's a difference between the measured feedrate and the feedrate setpoint, the control system sends a signal to increase or decrease the feed screw's speed.
The control system comes mounted, pre-wired, and pre-configured and simply requires power at startup. "We have one control system for all seven feeders. However, the feeders are set up so we can control them individually if required," Foreman explained. "The control system's operation is fairly simple. We define a recipe and a production rate, and we establish feeder configurations based on the ingredients each feeder is metering. After starting the production process, parameters can be changed on the fly."
Each feeder can be disassembled, cleaned, serviced, and reconfigured from the non-process side without moving the feeder or removing its extension hopper. The drive assembly -- the motor, sprockets, belts, and mechanical linkage required to turn the feed screw and power the agitating paddles -- can be swiveled up on an axis perpendicular to the feed screw. In the swiveled-out state, the feed screw, its shaft seal, and the conical feed hopper can be removed from the feeder's back for cleaning, reconfiguration, or maintenance. This has proven to be a huge asset for Bayshore. Each time ingredients are changed -- which can be as often as three times a day -- the company must disassemble each feeder for cleanout.
Easier changeovers, interchangeable spare parts
"It's important that any ingredient left in the feeder is thoroughly removed during changeover from one recipe to the next," Foreman said. "Rotating the drive assembly out of position gives us access to the feed and extension hoppers for complete material cleanout."
Bayshore has increased profitability by decreasing operation requirements and decreasing downtime for ingredient changeover. Since installing the new feeders, the company has achieved over 95 percent online time and 98 percent on-spec time.
Bayshore hasn't experienced any problems with the feeders, but they're prepared if something should happen. Foreman explained, "We've ordered interchangeable spare parts that will fit all seven feeders. Most parts will satisfy the entire production line."
Bayshore is so pleased with the feeders' performance the company is setting up another new production line with the same equipment parameters, utilizing a similar feeder configuration.
This article was originally published in the June 2001 Issue of Powder and Bulk Engineering and has been reprinted with their permission.
For more information on Powder and Bulk Engineering, go to their web site at: www.powderbulk.com
746 E. Milwaukee Street, P.O. Box 208, Whitewater, WI 53190
(888) 742-1249 • (262) 473-2441 • Fax: (262) 473-2489
e-mail: mktg@accuratefeeders.com
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