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Controlling ribbons and signatures

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.

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