Incline stack tacking challenges
Even though the latest perfect binders and saddle stitchers, such as the Goss SP 2200 or Muller Martini Supra, can process up to 360 books per minute, it is not always easy to achieve their rated speed.
Bottlenecks can occur in compensating stackers where belts convey the magazines up the stacker to be dropped into the compensator where they are stacked to varying heights that meet postal routing specifications. Magazine stacks must move quickly through the compensator in order to keep up with the upstream equipment. When the stacks are pushed out on to the conveyor or rollers leading to the shrink wrap tunnel or other packaging equipment, the mechanical forces which kept the stack straight are no longer present. As a result, some of the magazines and catalogs may shift resulting in uneven bundles. The USPS can reject such bundles, forcing the bindery to separate and re-run the magazines off-line.
In particular, magazines with UV-coated covers, either perfect bound or saddle stitched, have slippery surfaces that make them prone to shifting. In addition, high page count saddle-stitched magazines are challenging since the spine side is thicker than the open side and this can cause books to slide over toward the open side and “shingle over” as they exit the compensator.
The entire line must slow down if the stacker does not produce neat, true stacks and additional personnel may be needed to manually straighten the stacks. While oval strappers can sometimes be used, this is not altogether desirable since the strapping material can cause damage to the books. Additionally, the post office may need to route individual magazines to their destinations. Strappers are also an extra piece of equipment that needs maintenance and represents a potential source for production downtime.
Electrostatic stack tacking
What is difficult to accomplish by mechanical means can be done with electrostatics. Electrostatic force of attraction can hold magazines in a neat stack and preserve the shape achieved in the compensating stackers.
The method of holding stacks together electrostatically is known as stack-tacking. There are two practical electrostatic stack-tacking arrangements that are used in compensating stackers. One type is known as cross-tacking and it employs three charging bars inside the stacker. The second type is incline-tacking with two charging bars installed in the incline feeder.
In the cross-tacking system two plates hold the assembled stack from each side. After the stack has been completed, a third plate comes down and squeezes the stack. The charging bars installed in each of the three plates are energized to create an electrostatic field to tack the magazines together and keep them from shifting. Cross tacking is the only electrostatic option for magazines conveyed by the belts up the stacker in a shingled stream.
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