Whirlpool upgrades thermoforming lines with servo motor/drive retrofit

“The important thing is how fast we started this up,” notes Michael J. Smith, engineering supervisor of Whirlpool Corp. ’s Plastics Operations in Evansville, IN, commenting on a retrofit project the division did, early winter, 2003. Facing obsolescence, the Evansville division’s three thermoforming lines were in need of electrical and mechanical power transmission work, including shaft and motor adapters, gear boxes, servo motors and drives, wiring changes, PLCs, and other modifications. “We had two other large shutdown projects over Christmas, and we didn’t have the resources to do it then, so we did it in two weekends in November to avoid a Christmas shutdown,” recalls Smith. “The small risk we took paid off from a business stance.”

Bosch Rexroth retrofit kit for servo motors and drives operates in this Whirlpool appliance manufacturing plant in Evansville, IN (left). Drivetop, the graphical start-up software, guides setup on the control terminal or PC step-by-step.

With the help of distributor Doug Rodgers of Morrell Inc. and Brian Van Laar, senior applications engineer with Bosch Rexroth Corp. ’s Electric Drives and Controls business unit, Whirlpool Corp. upgraded its thermoforming lines, which produce the plastic inner door panels for more than 5,000 Whirlpool refrigerators daily.

Work included new servo motors and drives on the de-stacker portion of the line, using Rexroth 40 Amp DKC01.3 Ecodrive servo drives and MKD servo motors. The de-stacker portion of a thermoforming line picks plastic sheets from a stack and indexes them into preheat ovens for eventual forming in the thermoforming press. From start to finish, the process comprises the de-stacker, preheat oven, final heat oven, vacuum form press, part exit conveyor, trim press, and boxing station. The de-stacker’s drive makes eight distinct moves in this process, all within 20 seconds. In this application, a stack has as many as 225 thin plastic sheets.

“The catch,” says Smith, “is the stack height always varies after the previous sheet is picked, and the de-stacker loads from two opposing sides for quick changeover.” The de-stacker rack descends at high speed until it trips a “whisker” switch and decelerates, moving at a slower speed until vacuum cups contact the sheet’s surface. “The difference in stack height is overcome by stopping the drive in a torque mode,” explains Smith, who also noted the retrofit has removed five seconds from the original de-stacker cycle. According to Smith, the pin chain transfer drives were also retrofit with servos on three machines. The pin chain resembles the chain on a chain saw; teeth are embedded into the sheet to enable transfer. Photo eyes in the ovens help disable and retract oven beds should a sheet fall from the chain drive, to avoid plastic fires.

The team upgraded the servo motors and drives on the de-stacker portion of one thermoforming line and on the transfer drives of all three lines. Whirlpool owns six similar thermoformers. Each door liner machine produces approximately 1,100 parts per shift during three shifts per day, five days per week. Smith said the machine is now running approximately two seconds faster, but the forming and trimming stations still limit the total product cycle. Future engineering is being considered to retrofit trim presses with alternate technology since the de-stacker now runs faster. The Ecodrive used in the Whirlpool retrofit is said to be a compact, cost-effective servo drive system for applications of 1 kW up to 27 kW.

During servicing, all drive specific data can be transferred to the replacement unit by exchanging the programming module. The holding brake responds directly via the drive, which saves programming and wiring requirements of the PLC. The drive can be connected directly, without transformer, to 3-phase voltages in the 22-480 V range. Rexroth MKD servo motors are said to be a cost-effective solution for general automation applications up to 637 lb-in. (72 Nm). Motors are IP65-rated and come standard with an incremental encoder. Future plans for Whirlpool’s Evansville division include the retrofit of two existing variable frequency drives on two additional de-stacker drives.

—Edited by Mark T. Hoske, editor-in-chief, Control Engineering, MHoske@cfemedia.com

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