Increase Stacking Capability without Compromise
by Ken Troemel and Kevin Coldren
Two charging bars installed on opposing sides of an incline stacker
Special bracket allows bar to pivot up for easy pin cleaning
For years, electrostatic pinning (or tacking) has been an important tool for the printer and an integral part of efficient bindery operation. Effective and consistent pinning reduces run times and labor costs. Until now, the tools and strategies employed for electrostatic pinning have remained largely unchanged while the operations around them change continuously. Now, by remotely managing and monitoring output current and choosing, locating and maintaining the correct electrostatic charge bar, pinning systems can keep up with the changes to deliver consistent and cost-effective results.
The theory of electrostatic pinning is quite simple – current is pushed through a charge bar and then the resulting high voltage is used to create a surface charge on various materials. This surface charge then acts as a temporary binding agent. In real world printing applications, however, it is not so simple. Many interrelated factors will affect the charging application. This article provides an overview of the charging process as it relates to the bindery, and it also identifies practices and products designed to increase the effectiveness of electrostatic pinning systems.
Incline stack tacking challenges
Despite the fact that the latest perfect binders and saddlestitchers can process up to 360 books per minute, it is not always easy to achieve that rated speed. Bottlenecks can occur in compensating stackers where publications are conveyed up the stacker, dropped into the compensator and stacked to varying heights. Magazine stacks must move quickly in order to keep up with upstream equipment. When stacks are pushed out onto the conveyor or rollers leading to the packaging equipment, the mechanical forces that kept the stack straight no longer are present. As a result, the magazines and catalogs may shift, creating uneven bundles. The USPS can reject such bundles, forcing the bindery to separate and re-run the magazines offline.
Making the process more of a challenge, perfect bound or saddlestitched publications with UV-coated covers have slippery surfaces that make them prone to shifting. High page count saddlestitched magazines also are difficult since the spine edge is thicker, causing books to slide toward the open edge and “shingle over” as they exit the compensator.
Unfortunately, the entire line must slow down if the stacker does not produce neat, true stacks, and additional personnel may be required to manually straighten the stacks. While oval strappers can be used, the strapping material can damage the books and the post office may need to route individual magazines to their destinations.
Use of electrostatics in the incline feeder
Incline Stacking Tacking systems typically use a pair of charging bars, with one placed above and one placed below the publications path into the stacker. The bars normally are aligned with each other. Positive voltage is applied to one bar and negative voltage to the other bar using a high voltage bi-polar charging generator. The distance between the bars should be three inches for 30kV generators operating in voltage mode (see Figure 1 on previous page). However, this distance can be less at lower voltages or in current mode.
When publications pass between the bars, the flow of negative and positive ions is interrupted, leaving the front and back covers oppositely charged. The bound product is compressed by the electrostatic force and when the charged book 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 magazines cause them to adhere together and keep the magazines from shifting before they enter the shrinkwrap tunnel.
The result of static tacking can be quite dramatic. With certain books, production speeds on a typical finishing machine without incline tacking typically are only 175 to 200 per minute. When an electrostatic system is properly installed on the same machine, throughput can exceed 300 books per minute.
Maintenance of charging bars is an important component of any charging system. Dust or dirt around the emitter pins will reduce the effectiveness of a charging bar; therefore, periodic cleaning is recommended to prevent deposits from accumulating on the bar. A clean nylon brush with isopropyl alcohol applied is preferred. However, check with the manufacturer for specifics. Figure 2 (see page 26) shows a special bracket designed for use on the stacker application. The bracket allows access to Superbars installed on the stacker for easy cleaning and maintenance. It also provides a safe mounting structure to prevent operator shocks or voltage leakage and arcing conditions that can occur with conductive mounting materials.
Selecting and using electrostatic systems in incline feeders
Since there are many electrostatic systems available for incline tacking, it is important for a printer or binder to understand how the system functions and how to select the most efficient charging equipment.
The most effective tacking is obtained when using charging generators that feature current control operation. With current control, 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 extending the life of the charging bar. Charging generators with current control also adjust the voltage to compensate for changes in the line speeds, ambient conditions or paper dust buildup on the ionizing electrodes. Charging generators with current control operation eliminate the need for operators to constantly adjust the charging system to maintain the required pinning performance.
However, not all current control power supplies are created equal. In addition to remote control and monitoring, it is important that the power supply has enough available current to handle whatever job is being run. Some jobs can be run with a minimal amount of current, while larger stacks or thick UV-coated books often will require more than 1mA of current. Selecting the correct power supply results in a more cost-effective and efficient bindery operation.
Similarly, there are several choices available when it comes to electrostatic charging bars. Bar construction (the resistor, number of pins and pin material), sizing and placement are essential for successful pinning. In the bindery, bars with resistors are preferred. The resistor ensures that the bar operates steadily and reliably without arcing.
While a tried and true technology, electrostatic tacking continues to evolve to meet the challenges imposed by todays printer. Many electrostatic charging systems are designed for simplified integration into a printing or bindery line. Additionally, charging generators are becoming more compact in size, with features like low 24-volt input power. These newer advances are becoming a standard when selecting charging systems, as this provides for quick installation without the need to run high-voltage cables. Charging bars also are being fitted with tungsten emitter pins, which provide continuous performance and can significantly extend the life of the charging bar without the maintenance of replaceable pins. The latest innovations include a serviceable integrated static charging generator and bar in one system, which makes the system ideal for use in a variety of pinning and roll-to roll applications.
Electrostatic charging can significantly enhance printing and bindery operations. Successful applications require a thorough understanding of charging technology and all the interrelated details, ranging from the charging equipment to the printing and bindery process itself.