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Article | April 30, 2001
Downtime shrinks with new wrappers
When Georgia-Pacific needed to speed up wrapping rates for its 9” Dixie paper plates, management picked servo-driven, side-seal shrink wrappers that maintain high speed and tight registration.
The automatic Arpac wrappers run 24 hours seven days a week. As much as GP would enjoy running the wrappers nonstop there are two reasons for stoppage: changing film graphics and changing heights of stack plates. “We routinely change SKUs but the number of plates in the stack may not change” says Markwell. “We often stay on the same count and only change to a different customer’s film.”
The recent addition of seven new paper plate presses at Georgia-Pacific’s Bowling Green KY plant meant more shrink wrappers had to be installed as well. “The existing wrappers couldn’t handle the increased throughput” says GP’s project engineer Dana Markwell. “We needed machines that could maintain high speed with tight register.”Though GP produces a variety of plate sizes 9” plates are its bread and butter and the new shrink wrappers are dedicated to this size. Markwell began looking at machinery capable of handling these plates at 100 packs/min. “Most manufacturers told us it wasn’t possible for a side-seal machine” he says.Why was side-sealing essential? Because the low-density polyethylene shrink film used for the 9” plates is flexo-printed in such a way that finished wrapped packages have graphics on the top and bottom. A bottom sealer would spoil bottom-side graphics.After packaging equipment distributor Xpedx (Nashville TN) introduced GP to the Model 120SS thermal side-seal wrapper made by Arpac (Schiller Park IL) two machines were quickly purchased and installed side-by-side in fall ‘99. Stacks of plates exit the plate-making presses and move down a conveyor that has two spurs one for each shrink wrapper. One size stack is guided down one spur and another size goes down the other.The Model 120SS is capable of wrapping 120 packs/min. GP normally runs a 24-count stack at 70 packs/min and 48- and 125-count stacks at 66 packs/min. Servo-driven seal heads play a key role in permitting the Arpac equipment to run reliably at high speeds. According to Markwell the inclusion of servo technology was an important factor in selecting the Arpac wrappers.Also servo-driven on each machine are the infeed conveyor and main drive. All three servo drive motors are from Compumotor (Rohnert Park CA). Their motions are tightly synchronized by an Allen Bradley programmable controller from Rockwell Automation (Milwaukee WI). Up to 16 product recipes can be stored in the system and operators make their selections on a PLC touchscreen operator interface.Production is 24/7
Making a film-only changeover takes 5 min which is comparable to how long it takes on other shrink-wrap systems in the plant. But when the number of plates in a stack changes it takes one operator 15 min and requires no tools. On the other wrapping machines in the plant it takes one operator 25 min to perform the same task.
“For a count change you have to adjust the film-forming rods for the taller stack count and make a change to the opening on the seal head for the taller stack” Markwell explains. “Through the touchscreen control panel adjustments for speed are made. When you input the changes into the PLC it automatically calculates the motion for the servo drive.”
Eye marks printed on the film are constantly read by photo-eyes to keep registration tight. Markwell adds “The PLC controls the film placement with the film infeed motor and it also controls the timing of the film infeed motor with the seal-head motion.”
The Arpac equipment features a pin-perf roller that perforates the film as it unwinds. “As a stack of plates enters the shrink tunnel wrapped in film an air pocket is created at the top of the stack” Markwell says. “Perforating the film allows the expanded air to escape.”
To reduce distortion of the printed film GP requested a top hold-down belt for the shrink tunnel. According to Markwell when film goes through the shrink tunnel the film makes contact with the rim of the paper plate whereas the middle of the plate does not make contact. This causes the shrink characteristics to be different which causes distortion in the graphics.
“Rather than printing graphics to shrink uniformly the top hold-down belt actually stabilizes the top of the film so that the graphics do not distort” he says. It runs parallel to the bottom belt and is lowered to the height of the stack going through. GP has added the top hold-down belt to all its shrink-wrapping equipment.
Markwell is pleased with the performance of the Arpac equipment. “It’s well built with a robust design. That was a big benefit for the type of throughput required” he says.
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