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.
Cross-tacking systems are complex; they require three charging bars, special insulating plates to hold the charging bars in place and other design features. They cannot be easily retrofitted to existing stackers and usually come fully installed with the stacker, requiring little or no intervention on the part of the user.
Compensating stackers which receive magazines singly and are not designed with cross tacking systems can be retrofitted with an electrostatic system in the incline feeder section.
Electrostatic stack tacking in the incline feeder
Since a number of different commercially available electrostatic systems can be used for incline tacking it is important for a printer to understand how the system functions, how to select the charging equipment and how to maintain it.
Incline tacking systems typically use a pair of charging bars, one placed above the magazine’s path into the stacker, the other placed below (see Figure 1). The ionizing electrodes in the bars are normally aligned against each other. The positive voltage is applied to one bar and the negative voltage to the other using either a pair of high voltage charging generators or a single dual-polarity generator. The distance between the bars should be two inches or less, since the smaller the gap between the bars, the lower the voltage required to generate an adequate tacking effect.
Figure 1. Electrostatic system in the incline feeder of the compensating stacker. 1 – book in the incline feeder; 2 – non-conductive belts; 3 – lower charging bar; 4 – book between the charging bars; 5 – book after passing between the charging bars; 6 – upper bar.
When the bars are energized with no magazines in the incline feeder, the opposite polarity air ions produced by the opposed bars will flow between the bars completing the electrical circuit. When magazines move up the feeder and between the bars, they interrupt this flow and ions of opposite polarity deposit on the front and back covers, leaving these surfaces oppositely charged. The moving magazines carry these charges away, as a “convection” electrical current, again completing the electrical circuit.
The magazines are compressed by the electrostatic force between the front and back cover pages with the air being squeezed out. While that certainly contributes to forming a neat integral stack, it is the secondary effect which is most important. When a charged magazine is dropped into the stacker, it lands with its back cover on top of the front cover of the previous magazine. Opposing charges on the front cover of one magazine and the back cover of the next magazine create an attraction between them, causing the books to adhere to each other, as shown in Figure 2. This attraction keeps the magazines from shifting when the stack is in motion.
The result can be quite dramatic. With certain books, production speeds on a Goss SP 2200 without incline tacking are typically only 175 to 200 per minute. When an electrostatic system is properly installed in the feeder, throughput can exceed 300 books per minute.
Selecting and using electrostatic systems in incline feeders
Figure 2. Surface charging in electrostatic incline stack tacking.
Step 1. Install nonconductive belts to avoid arcing from the bars to the belts.
Step 2. Install charging bars with effective lengths one inch shorter than the most common height of the magazines that you run.
Figure 3. Effective bar length vs. magazine width in electrostatic stack tacking. Effective bar length must be shorter than the magazine’s long dimension. Note: Ions from correctly-sized bars deposit on both sides of the book. If the bars extend beyond the edges of the magazine, a high portion of the charging current will flow through the air between the ends of the bars, creating an unequal distribution of current. When this happens, the ends of the bars will overheat causing the bars to burn out.
Step 3. Adjust the output of the charging generator to achieve optimal tacking, as confirmed by the production of square bundles coming out of the shrink wrap tunnel. Note that the degree of electrostatic pinning force is determined primarily by the value of the charging current, not by the voltage.
As the magazine passes between the bars, the total current drops to one third of that flowing between the unobstructed bars. This occurs because the magazine surface can only carry a limited amount of charge. For example, if the current between two charging bars when unobstructed is 1 milliampere, then the current will drop to approximately 0.3-0.4 milliampere when a magazine is passing between the bars (the actual value depends on the magazine’s size and thickness, and the type of the paper, coating and ink).
The current fluctuations repeat, with the current rising after the magazine passes the gap and falling as the next book moves between the bars. It is possible to see some fluctuations in the current meter; however the displays in charging generators are not capable of showing such fast changes, as the current rises and falls within 0.1 second in the fastest stackers.
The most effective tacking is obtained when using charging generators that feature a Constant Current Mode (CCM). In the constant current mode, the generator automatically adjusts the voltage to maintain a preset current. The system maintains stable and strong pinning power, lowering the output when there is open space between the magazines and adjusting the voltage to compensate for changes in the line speeds, ambient conditions or paper dust buildup on the ionizing electrodes. Note:Effective control of the pinning process is possible only when the bars do not extend beyond the edges of the books – see Step 2. With long bars, the current flowing directly between the bars is not contributing to the tacking process. Under such conditions the monitored current value is meaningless.
Step 4. Keep the ionizing electrodes sharp.
The electrodes in the positive charging bars wear out 3-4 times faster than those in the negative bars. There are three methods for keeping these electrodes sharp:
- Use tungsten pin electrodes.
- Use replaceable stainless steel pin electrodes.
- Increase the number of pin electrodes to spread the current over a larger number of electrodes and reduce the current density at each electrode.
Step 5. Keep the ionizing electrodes clean.
Paper dust should not be allowed to build up or cover the electrodes. Clean the charging bars often. Use a metal brush to thoroughly scrub the electrode channel during the makeready phase. Overly long bars are especially problematic as the ends of the bars will collect excessive amounts of paper dust from the sides of magazines.