Challenges of folding and cutting ribbons on press
After a printed web has sped through the chill roll stand, it is slit into ribbons. As the ribbons flow over a former board, they are folded together and firmly “nipped” downstream. The folded ribbons then pass a rotary knife, which cuts them into signatures. Achieving an accurate and precise cut can be a challenge, as web or ribbon tensions can vary from layer to layer and this will move the ribbon out of cutoff registration, occasionally. After the cut, the signatures are no longer in tension and are subject to new forces of momentum and air drag along the leading, side and trailing edges that can cause “dog-ears,” or areas where the edges of the signature have folded back.
Electrostatic ribbon tacking
Electrostatic ribbon tacking is the preferred method for helping meet the challenges of folding and cutting processes. The technology uses an electrostatic charge to hold multiple ribbons together, making them behave like a single web and preventing the leading, side or trailing edges of the signature from “peeling away” from the signature package. This allows the electrostatically bonded ribbons to be cut with the required precision, as the individual ribbon tensions are equalized. Electrostatic ribbon tacking enables the pressroom to deliver crisply folded signatures to the bindery without “dog-eared” edges at speeds of up to 3,000 fpm.
Electrostatic charging is achieved using two charging bars facing each other, one on each side of the multi-ribbon web. A dual polarity high-voltage charging generator applies a positive voltage to one bar and negative voltage to the other. The opposing bars produce airborne ions of opposite polarity. These ions stream toward the web charging its surfaces and causing all the ribbons to hold tightly together.
The bindery dilemma
Figure 1 shows two possible locations for pinning the ribbons together. Electrostatic tacking can be done after folding, downstream of the nips (“downstream charging”) or before folding, near the roll at the top of the former, or RTF (“upstream charging”). The downstream ribbon tacking location is more common than the upstream location.
The fundamental difference between upstream and downstream charging is the charge polarity distribution on the signature’s outside surfaces. After downstream charging, each signature will have opposite polarity charge on its two exterior sides. In the case of upstream charging, the same polarity charge will exist on the two exterior sides of the signature. The charge distribution factor has important ramifications for the subsequent bindery processes.
While downstream charging securely holds the signature together through cutting and conveying, there is a potentially serious drawback. Because the signature retains a charge of opposite polarity on the two exterior surfaces, they can attract to each other as they come together in a shingled stream, sometimes strongly enough to interrupt a flow or bump turn. These signatures also can be troublesome in the bindery department, where they might be held together so strongly that they cannot be opened reliably, causing jams on saddle binding equipment.
Upstream charging avoids these problems. The same polarity charge on both exterior surfaces of the signatures eliminates static attraction between one signature and the next as they are shingled on a delivery belt. Because upstream charging also leaves the “centerfold” of the signature charged to one polarity, signatures open easily in downstream saddle binding processes.
While many printers continue to use downstream charging, those who understand the benefits of upstream charging employ it exclusively, especially for high-volume production. Printers who utilize the downstream location for ribbon tacking must avoid excessive charging, which could cause problems in transporting signatures and in finishing.
Five rules of electrostatic ribbon tacking
Rule 1 The ribbons must be in intimate contact with each other, no air gaps between them at the location where the charging bars are installed.
Ensuring contact between the signatures is especially important for upstream charging installations, where the ribbons usually come together at the RTF with gaps between them. Depending on the folder design, an idler roller might need to be added to force all the ribbons together ahead of the bars’ location.
Rule 2 The charging current values displayed on the charging generator are far more important than the charging voltage.
The pinning force is determined primarily by the value of the charging current, not the voltage. A simple example will demonstrate that point.
In a first test, two charging bars are placed at one-inch distance on each side of the web. The operating voltage is set to 20 kV and the bars deliver 2 mA of charging current to the web. In a second test, the same bars are moved to 1.5 inches from the web, the operating voltage is still the same 20 kV voltage. Now, from farther away, the bars deliver only 0.5 mA charging current, and half the pinning force. Clearly, the voltage is a wrong measure of performance. This can also be observed when the ionizing electrodes get dirty, causing the current to go down and diminishing the pinning force, even though the voltage stays constant.
Also, bringing the bars closer will produce the required charging current at lower operating voltage helping extend the equipment’s life. For the bars installed on opposite sides of the web, the distance from each bar to the web should not be more than one inch.
Rule 3 The effective lengths of the charging bars must be shorter than the width of the ribbons by about one inch.
The ions from the correctly sized bars deposit on both sides of the ribbon, charging its surfaces, as shown in Figure 2a. The moving ribbon, as a “convection” electrical current, carries these charges away. That current is doing the work of tacking ribbons together.
If the bars are too long, extending beyond the edges of the ribbon, as shown in Figure 2b, a high portion of the charging current is flowing through the air between the bars at both ends. While the cost of wasted energy is certainly a consideration, two other factors are more important.
The density of the ion current flowing between the bars outside the edge of the ribbon is much higher than in the sections of the bars separated by the moving ribbon. As much as 75 percent of all energy could dissipate in one or two inches of the bar length. The ends of the bar would overheat causing the bars to burn out. This condition can be aggravated by the paper dust collecting around the ionizing electrodes of the bars in the location of the ribbon edges.
Effective control of the pinning process is impossible when the bars extend beyond the edges of the ribbon. Because the high portion of current is flowing directly between the bars and not contributing to the ribbon tacking, the monitored current value is only marginally helpful.
Using “charging bar covers” to block unused electrodes is impractical for the overly long bars. Damage to the bars at the boundary between the covered and uncovered electrodes could result.
Rule 4 Don’t apply excessive voltage to the bars, especially if they are longer than the width of the ribbon.
If you observe a distinct purple or bluish glow in between the bars at the ends which extend beyond the ribbon’s edges, then the voltage is too high. This condition eventually will lead to bar failure, as we explained in Rule 3. Turn the voltage down until the glow disappears, and when the time comes to replace the bars, install shorter ones.
Rule 5 Clean your charging bars often.
Don’t allow paper dust to build up or cover the electrodes. Use a metal bristle brush to scrub the electrode channel weekly.
Following these five rules will help printers achieve the full benefits of electrostatic ribbon tacking while avoiding downstream problems in the bindery.