An introduction to label printing and converting

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Dies and tooling

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Many of the finishing operations mentioned above may be carried out in different ways. For example, die-cutting may take place using flatbed cutting dies, solid rotary dies or flexible wrap-around dies . even laser cutting today. Perforating may also be undertaken from a flat die or a rotary die. The same applies to embossing or hot foiling where there may also be segmented dies.

The challenges involved in manufacturing and using cutting, perforating, foiling, embossing or punching dies is that labels . and unsupported film . may frequently be required to be produced using different types of paper, film, foil, metallic, etc., substrates, with different types of liners to cut to, as well as different types of adhesive. Some substrates and adhesives may well be more abrasive or challenging to cut than others, perhaps requiring different cutting angles or bevels.

Understandably therefore, the manufacture of the different types of dies and tooling is a highly specialized and quite complex process, or sequence of processes, using hardened metals and coatings or surface treatments to give different types of quality, performance and cutting profiles.

Many or most of the different types of cutting, punching or perforating dies need an accurate solid surface to cut against, . called an anvil or base roll . or additionally, in the case of flexible dies, a magnetic cylinder on which they are held firmly in position. In some printing processes, flexible printing plates may be used, again held in place on a magnetic print cylinder, or perhaps they are in the form of a sleeve fitted onto a sleeve cylinder. Gear wheels and bearers may need to form part of these various cylinder constructions.

The flexographic printing process has an additional requirement for an anilox roll that carries the fluid ink from the ink tray or pan to the printing cylinder. Again, precision made, anilox rolls are quite complex in their manufacture and requirements and have a significant impact on the quality of printing that can be produced.

The manufacture and production of many of these ancillary manufactured products used on a pressure-sensitive roll-label and some other narrow-web presses is commonly known as tooling and the items themselves as tools. Put together and depending on the specific supplier or sector, tooling can generally be categorized under one or more of the headings shown in the following flow chart (Figure.1.1):

The nature, role and function of each of these types of tooling is outlined below.

THE NATURE, ROLE AND FUNCTION OF TOOLING

Before moving on to understand the materials, technology and production of the different types of tooling it would be of value to a know a little more about each type of tool and to be able to recognize what they look like and how, when and where they are used in label printing and converting operations, starting with the various types of label die-cutting tools.

Die-cutting tools are used in the process of cutting a label to size and shape with a die. Most self-adhesive labels have to be die-cut to shape as part of their manufacturing and finishing procedure. Depending on the type of label and the printing and/or die-cutting requirement, this operation may be performed using flexible dies, solid rotary dies or flat dies or, most recently, with digital laser die-cutting technology.

Historically, flat die-cutting tools were the dominant technology used on the intermittent feed and semi-rotary presses of the 1970s and 1980s. As rotary printing started to become the major label printing process so the industry moved to solid rotary dies. More recently, flexible dies have become the die-cutting process of choice, overtaking solid tools.

Today, flexible dies are said to account for more than 80 percent of all label dies used in Europe and around 70 percent in North America, largely due to their much easier transportation, storage and handling, as well as not insignificant cost savings. An increasing trend towards shorter run lengths has also influenced the trend towards flexible dies.

Hot-foil blocking/stamping is used on both short and long runs today. Further, traditionally hot foil stamping was used on rotary presses with cylinders; however, today flexible dies are used on rotary presses and flat dies are used on other narrow web presses.

The punches are held in position with a set of screws, tightened into a groove. Rings can be adjusted across the web.

There are also microhole punching tools which are designed to create tear-off holes in postage stamps. The punched waste is removed through the hollow die cylinder with working widths up to 500 mm. The modules are connected mechanically or electronically to the converting machine.

Printing cylinders. Standard printing cylinders form the basis of every label printing machine. These cylinders, together with hot foiling and embossing cylinders, are all manufactured with the greatest care and finite precision in order to guarantee optimum fit and run-out accuracy.

In the flexographic, letterpress and litho processes the printing plates are located on the print cylinders. Each cylinder needs to have accurate and even contact with the inking rollers and the surface of the substrate, or in the case of the litho process, the offset blanket.

Printing cylinders (see Figure 1.11) used in the roll-label industry include plate cylinders, blanket cylinders and impression cylinders and these are made from solid aluminium or steel, or produced as a tube with end rings fitted, and with spur or helical gears.

There are also a number of suppliers offering unique coatings applied directly onto existing or newly manufactured printing cylinders. These coatings can provide additional surface protection thereby potentially increasing the life of the cylinder.

Magnetic cylinders and bases. Magnetic cylinders used with flexible dies provide an economic alternative to standard rotary die cutting tools. They are manufactured on CNC machines from high tensile and high alloyed stainless tool steel with fully hardened bearers. Hard ferrite or ceramic and rare earth permanent magnets, hardened bearings seats as well as bearing necks (journals) with fully hardened centering sleeves are usually standard. Higher-strength magnet configurations are available based on the application.

Magnetic cylinders are available for a full range of label presses and converting machinery, allowing converters to use flexible dies in many different applications.

Embossing tools. Embossing tool dies are used to shape/set the surface of a substrate to create a raised (embossed) or recessed (debossed) design. Embossing can be achieved with a matched male die and female counterforce (flat or rotary). The female counterforce has the required image incised into the surface; the male die has a matched raised image. An offset is applied to one or both images to enable the dies to accommodate the thickness of the material being embossed.

During the embossing process, the two dies are pressed together through a substrate to create a raised image.

In the process of debossing - which is the reverse of embossing - the positions of the relief die and the counter die are reversed.

Male/female embossing dies have their widest application in rotary embossing, although other approaches may be used for certain applications.

For less detailed images, more economic single male or female embossing dies used in conjunction with a hard rubber coated anvil may well achieve the desired result.

Embossing dies are produced to run in both dedicated embossing units or in an available cutter station.

The repeat size of the cutting cylinder will vary from job to job. A cutting unit is generally constructed so that the cutting cylinders are relatively easy to replace.

Put together, it can be readily seen that a variety of highly accurate and specialized tooling is needed to produce pressure-sensitive labels on a roll label press.

A typical press construction and location of some of the more commonly used tooling is shown in the Figure 1.18.

This series of articles are primarily concerned with tooling that is used at the finishing end of a label press rather than the manufacture and use of print cylinders and anilox rolls that are used in the analog or digital press section.

The nature and use of cutting, embossing and foiling dies, speciality dies and tooling, slitting wheels or blades, as well as magnetic cylinders and anvil rollers, are all set out in the following pages, together with guidelines for optimizing the cutting processes and for handling, storage and health and safety in the use of tooling.

Perforating and sheeting tools. In rotary form, perforation tools are cylinders that contain removable or floating blades that can be used to produce perforated or cut lines (sheeting or scoring). They are designed for cutting or perforating across a web to produce items such as A4 sheets, or to provide perforation lines that are punched into a label surface for, say, fan-folding.

The perforating and cutting blades themselves are able to be positioned equally stepped, or freely positioned around the circumference of the cylinder to provide for production flexibility. The blades are held in place by clamping or counter bars and set screws.

Magnetic cylinders are available for a full range of label presses and converting machinery, allowing converters to use flexible dies in many different applications.

Intro section

The production of pressure-sensitive labels involves a quite complex process of printing with up to 10 or more printing units, sometimes using two or more different printing processes. Ancillary operations typically follow as needed to die-cut the labels to shape, remove the waste material, trim the edges of the web and slit it into single roll widths – or sometimes into single sheets – prior to a final inspection, re-winding and then packing before shipping to the customer.

Depending on the specific label application, the types of labels to be produced and their added-value requirements or complexity, it may also be necessary to undertake a variety of other types of finishing or embellishing operations, such as varnishing, embossing, hot or cold foiling, perforating, scoring, sheeting or hole punching.

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