Inline finishing enhanced through the use of electrostatics

January 25, 2008

Inline finishing: A complete product leaves the pressroom

Everybody is familiar with promotional inserts in newspapers, magazine foldouts with perfume-scented envelopes, and direct mailers with coupons and advertisements. These products are commonly produced "inline" by converting the web of printed paper into a finished product for delivery to consumers. The process of making these products is known as “inline finishing.”

The inline finishing process is done on specialized equipment that allows the printer to move the printed web through some or all of the following processes: slitting, folding, diecutting, gluing, coating, perforating, perfume scenting, rotary cutting, shingling, sheeting and/or stacking. Using this approach, the printer produces whatever product a customer’s marketing department has invented. Electrostatics are utilized in inline finishing to assist in the production of unique products.

Electrostatic pinning of folded webs

One of the most common inline finishing processes is the folding of a web in preparation for making an envelope-like construction. This process most often uses a folding section called a plow tower or plow folder. When successive plow folds are required, air can be trapped within the layers and eventually “inflate” the folded web into a “football.”

Nip rollers can be used to avoid air entrapment, but this does not keep the web halves closed. Electrostatic charging solves this problem by squeezing the air out of the folded web and keeping the layers together with electrostatic force as the web proceeds downstream.

Electrostatic charge is best applied to the plow-folded web at a point where the folding process is complete, as shown in Figure 1. Immediately upon charging, the web tightens up and is held closed by the electrostatic force between the front and back web surfaces. Any remaining air in the fold is forced out from between the layers. This charging process can be repeated as more webs are folded.

A single charging bar, in combination with a grounded idler roller (Figure 1), or a pair of opposite-polarity charging bars can be used for either a folded web or several ribbons of printed stock. In the case of a single charging bar, it is connected to a charging generator of negative polarity output. The distance between the single bar and the metal roller should not exceed one inch. When two charging bars are used, one is placed above the web, the other below, and the ionizing pin electrodes of the two bars are aligned against each other. The distance between the bars should be two inches or less. Generally, the smaller the gap between the bars, the lower the voltage required for a sufficient tacking effect.

A single web can be "married" into an existing plow folded web to add an extra unbound page. Static charging is an excellent way to make sure the single web does not slip out of register after it joins the existing web.

Electrostatic tacking of diecut 'coupons'

Inline finishing processes also are used to insert single or multiple “coupons” into newspapers or direct mail pieces. The coupons can be configured as either precut pieces loaded and inserted into a magazine or as diecut pieces placed into a moving web. In either case, once the coupon is completely registered on the web, it needs to be held in place to prevent it from shifting or slipping off the web. Conventionally, this is done using a combination of pinch rollers and belts, or with glue. As an alternative approach, electrostatic tacking has the ability to tack a coupon without any contact. Because electrostatic force acts past the point of charge application, its use replaces glue dispensing equipment and consumable materials, as well as simplifying equipment design by eliminating rollers and belts.

Figure 2 shows an arrangement suitable for placing a diecut coupon on a “carrier” web using electrostatic charging. The die roller, the anvil roller and the “carrier” web are a very tight fit in this configuration, but there is adequate room for a small charging bar to be installed directly above the grounded metal idler roller, which guides the carrier web. The coupons exit the nip and cross a short air gap to land on the carrier web. As soon as its leading edge enters the space under the charging bar, the electrostatic force pins the coupon to the carrier web. Static charge keeps the coupon from slipping as it proceeds downstream.

Guidelines for using electrostatic systems for inline finishing

Rule 1: For web pinning, use charging bars that have an effective length, one inch shorter than the most common web width. If the bars are too long and extend beyond the edges of the web, an unacceptably high portion of the charging current flows uselessly through the air between the bar and the grounded metal roller or between a pair of bars. (Refer to Parts 2 and 3 of this series for a full explanation.)

A single common length of charging bar can be used and angled when bar length exceeds web width. Holders that normally are used with inline accessories, such as a fold helper or a brush, can be used to hold the angled charging bars.

Rule 2: Adjust the output of the charging generator to achieve optimal tacking as confirmed by the web staying together or by the “onserts” remaining in the registered position. Note the charging current value rather than the voltage, as electrostatic pinning force is determined primarily by the value of the charging current.

The charging generators are most effective when operated in constant current (CC) mode. In this mode, the generator automatically adjusts the voltage to maintain a preset current, maintaining a stable and strong pinning power despite changes in the line speeds, ambient conditions or paper dust buildup on the ionizing electrodes.

Rule 3: Do not apply excessive voltage to the bars.

If you observe a distinct purple or bluish glow at the end of the bars, then the voltage is too high. Turn the voltage down until the glow disappears. A voltage below 15 kV will be sufficient for most inline applications with properly installed bars.

Rule 4. Keep the ionizing electrodes clean.

Do not allow paper dust to build up or cover the electrodes. Clean the charging bars often using a metal brush to scrub through the electrode channel during makeready. This will maintain the strongest pinning power over longer press runs.

Note: Charging could leave some residual charge on the finished products; it is recommended to neutralize the exterior of the product before shingling or stacking. Doing so will allow the completed products to shingle or stack without “clinging” together. Neutralizing the charged web before and, in some cases, after a rotary diecutter also could be helpful.

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