Wide Web In-Line Converting The Stack Press Makes it Reality

Source FLEXO Magazine August 2011

T-shirt bags, paper bags, cement bags, pet food bags—when it comes to postprint converting of large packaging in a flexo pressroom; one commonly finds a sometimes simple, sometimes sophisticated stack press driving the operation.

Such machines, available in 1-8 color configurations, take their rightful place in the converting line, where they’ve been deployed for decades and decades. They are most often installed “upstream” of bag machines that pump out the aforementioned product lines; and “downstream” of extruders, in cases where a 1 or 2-color press is dedicated to a single product—say a trash bag displaying nothing more than a printed logo.

That observation, as well as others, is shared by Marco Verini, technical services manager for Uteco North America. He maintains that, “The stack gives great flexibility to print front/back combinations, which is very useful when printing on tubing (mulch, insulated bags, etc.). Other benefits offered: up to a 50-in. repeat and 94-in web width.

Also of note, according to Verini, “In-line stack presses can be equipped with high capacity dryers, similar to gravure. “ Furthermore, “Stack presses are a smaller investment than the higher-end central impression (CI) printing presses, and provide reliable quality at moderate speeds.”

FORMAT & APPLICATIONS

Flexographic Technical Association’s Flexography: Principles & Practices, 6th Edition, describes the format and its technical applications. “In the stack press individual color stations (sometimes called sections or decks) are stacked one over the other, on one or both sides of a main press frame. Each color station is driven through either servo motors or traditional gear trains supported by the main press frame. Stack presses have three main advantages”, as FTA explains:

  • First, the operator can usually reverse the web to allow both sides to be printed during one pass through the press. Various web threading arrangements allow complete ink drying before the reverse side is printed, provided enough drying capacity is designed into the area where the web passes between the two series of stations.
  • Second, color-station accessibility facilitates changeover, wash-up, etc.
  • Third, the press can print large repeats.

Technical experts working on behalf of the association also write, “The stack press has proved useful in many applications and has been used to print on almost every type of substrate.” They admit, “It does have limitations when printing substrates that are extensible or of extremely thin gauges, generally restricted to color registrations.”

That said, they also note, “When heavier gauge materials are being printed, such as papers, laminated film structures and others that can tolerate fairly high web tension values, the stack press can profitably produce commercially acceptable registered products. The stack press lends itself well to applications such as printing in-line with other types of machinery. These add-ons may include extruders, bag machines, sheeters, laminators or other equipment.”

On some models, equipped with special color-station designs, those in-the-know say, “It’s possible to have 360-degree register on each station and independent engaging and disengaging to allow the remaining part of the machine to operate.” They also observe, “Since each color station is
independent from the others, it is easy to mechanically arrange various rotations of the inking parts. It is also possible to change the web lead to flood coat a sheet, or print coatings in a standard fashion. These techniques have been applied in the pre-printing of corrugated liner material.”

BROAD MODEL RANGE

Stack presses on the market today are designed to meet a broad range of customer expectations. Some are manufactured specifically to be placed in-line, beside extruders, welding machines and roll-to-roll applications. Others can accommodate very wide web widts of up to 2.5m / 98.4-in. Roll-to-roll versions handle both paper and paperboard applications. And, recently designed sleeve models, in both geared and gearless executions, facilitate fast changeover.

Even the simplest machines can be upgraded with features that offer: motorized decks and register racks, closed chamber doctor blades, print sleeve technology, automated registration systems and video camera-based print fault detection systems.

In general, stack presses are delivered with winders suitable for both paper and paperboard in diameters of 1000mm / 40-in or larger in both single or double shaft executions. A turret winder is suitable for flying splice roll changes at high production speeds. Other common features on traditional models are robotic, electric hoist systems for position print cylinders and anilox rolls in wide web dimensions.

In-line versions can be equipped with as many in-line stations as required by the customer. Generally, such models are completely gearless and equipped with single motors on counter pressure rollers, connected via an electric line shaft. Direct-drive servomotors are installed on plate cylinders and anilox sleeve cylinders. Both the printing plate and the anilox use sleeve technologies to facilitate fast changeover. Alternative
drying systems, to include UV Lamps or EB technology are counted among options. As are: In-line viscosity control, automatic washup systems, automatic register control, print inspection devices, computerized set-up, in-line slitters, in-line rewinders, motorized air recirculation systems and front/back printing capabilities.

Succinctly put, wide web, in-line converting offers targeted economies and efficiencies and the stack press remains positioned among a flexographic printer’s most valued tools of the trade. That’s a reality!

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