Mark J. Kirwan Paper and Paperboard Packaging Technology

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areas of the printing surface. The relief image area plate can then be affixed

around a printing cylinder and placed in the printing press. During the printing

cycle, the raised surface is rolled with a sticky ink and the inked image then transferred

to the label substrate under pressure.

Letterpress printing roll-label presses, primarily used for self-adhesive label

production, are narrow-web (200mm, 250mm, 360mm, 400 mm or 450 mm)

machines and are either web-fed semi-rotary, intermittent-feed, machines or web-

fed full rotary printing presses – both of which can include up to six, eight or more

printing units, together with all the necessary reel unwind, die-cutting, waste-stripping

and rewinding capabilities. Almost all are utilising UV-curing inks which have the

advantage of being fully dried on the press, thereby reducing lead times and allowing

subsequent in-line finishing operations, such as lamination and embossing, to be

carried out immediately. UV inks also have excellent resistance to rubbing and are

chemically resistant to a wide range of products.

In intermittent-feed roll-label presses the web of labelstock is progressively and

intermittently advanced to the printing position, printed and then moved on for die-

cutting and waste-stripping operations. The presses are slower than full rotary presses

but do offer advantages for short (10000 or so labels) to medium (25000–50 000)

label runs where they can provide press flexibility and quick changeover capabil-

ities and, due to the variable feed length that can be selected on an intermittent feed

system there are no costly changes involved in changing cylinders, gears, etc.

Rotary letterpress in-line, common impression drum (Fig. 4.5) or stack roll-label

presses (Fig. 4.6), started to be installed in label printing and converting plants in

the late 1970s, and by the mid-1980s made up almost 70% of all new roll-label

presses being installed in Europe. Even today, in spite of the rapid growth of

Substrate

Inking train

Plate

cylinder

Ink tray

Impress

cylinder

Figure 4.4 Letterpress printing process. Source: Labels & Labelling Consultancy.

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flexographic label printing, rotary letterpress still has around half of the total

installed presses in Europe; much less in North America.

Compared with intermittent-feed letterpress machines, rotary letterpress offers

significant market advantage in that they can produce up to three times or more

output – and all to a high quality, consistent result with, probably, less dependence

upon operator skills than is required by other printing processes. Rotary roll-label

presses may have an in-line printing head configuration, or the print heads may be

placed around a common impression drum or arranged in a satellite or stack pattern.

4.6.2 Flexography

Like letterpress, flexography is a relief printing process (Fig. 4.7) in which ink is

applied only to the raised surface of the image on the printing plate and, transferred

Figure 4.5 Diagram of a common impression drum label press. Source: Labels & Labelling

Consultancy.

Figure 4.6 Diagram of a stack label press. Source: Labels & Labelling Consultancy.

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from there, to the surface of the label substrate. The key differences between

letterpress and flexographic printing lie in the method of metering and applying

the ink to the raised surface, and to the plate itself.

Narrow-web flexographic printing is the most widely used label printing process

worldwide for the production of self-adhesive labels. In the USA, it makes up

more than 90% of the installed press base, and around 40–50% of the installed

base in Europe. It is used for the production of almost all types of labels available

today and has become increasingly accepted for the production of higher quality

prime labels, particularly with digital plate imaging and the latest UV-curing flex-

ographic press technology. Like rotary letterpress, there are three basic types of

flexographic press configuration: in-line, common impression drum and stack.

Any of these configurations may be used for printing a wide range of materials.

Printing plates for flexographic printing may be made from a softer, more resilient

rubber or flexible photopolymer plate material than letterpress, though similar expo-

sure and wash-out stages to that of letterpress plates are required. However, rather

than the thicker, sticky ink used for letterpress printing, the flexographic process uses

a very thin solvent (declining) or water-based (growing) ink. The thin ink requires

a special type of ink metering system which uses an engraved anilox roll containing

thousands of tiny recessed cells – which carry the ink – and a doctor roll or blade

which removes excess ink from the anilox roll surface. The degree of fineness and

the shape of the cells can be changed depending on the effect required.

The last print station on a flexographic label press is frequently used for applying

a varnish so as to provide additional scuff, rub and chemical resistance. For self-

adhesive-label printing flexographic presses also incorporate a die-cutting station

(with flexible dies on a magnetic carrier or a full rotary cylinder), matrix waste

removal, slitting units and a re-winder.

Quality of origination, pre-press, press maintenance and good press fingerprinting

are important factors in the production of good quality flexographic printed labels.

Ink pan

Plate

cylinder

Substrate

Impress

cylinder

Anilox

cylinder

Fount

roll

Figure 4.7 Flexographic printing process. Source: Labels & Labelling Consultancy.

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It is essential to get everything correct at the beginning, as it is not easy to make

modifications on the run.

In recent years, UV-flexographic label printing has gained ground to successfully

compete with UV-letterpress label production for multi-colour and half-tone work

and, with the latest origination (which is closer to that used for letterpress and

offset litho) and direct-to-plate imaging technology, is now claimed by the best

flexographic printers to come close to offset litho quality. As such, the process has

become widely accepted for the printing of food, supermarket, retail and other

process colour labels.

Apart from narrow-web flexographic presses used for self-adhesive label

production, the process is used in wide-web formats (500 mm, 600 mm wide and

upwards) for the printing of some sleeve labels and wrap-around film labels, as

well as for cartons.

4.6.3 Lithography

Lithography, mostly referred to as offset lithography or simply as offset, is the

most commonly used printing process for the printing of glue-applied paper labels

using a wide variety of sheet-fed presses. More recently, offset is also being used

in roll-fed press formats for high quality self-adhesive label production and in both

sheet-fed and roll-fed versions for in-mould label production and for some new

developments in cut-and-stack and patch film labels.

Offset lithography is a high quality planographic process (Fig. 4.8) in which the

image and non-image areas of the printing plates are on the same plane (flat)

surface but are differentiated chemically in a way in which the image areas are

Substrate

Inking system

Blanket

cylinder

Damping system

Plate cylinder

Impression

cylinder

Figure 4.8 Offset lithographic printing process. (Reproduction, with permission, from Iggesund

Paperboard.)

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made ink receptive and water repellent, while the non-image areas are water

receptive and ink repellent.

Plate-making is relatively simple. Thin sheets of a grained and light-sensitive

coated (aluminium) plate are exposed through a negative or positive film – or by

computer imaging – to create the image and non-image areas, and then treated

chemically to aid ink or water receptivity as required.

Once mounted on the plate cylinder in the press (whether sheet-fed or roll-fed),

the plate is repeatedly contacted after each print with damping and inking rollers;

damping, which is mostly water, to the non-image areas, and the ink to the image

areas. UV-cured inks are widely used and many labels are UV varnished in-line

wet-on-wet on the printing press.

For self-adhesive label production, web-fed in-line offset presses will also

incorporate multiple print heads, die-cutting, waste stripping and rewinding in one

pass. More commonly in self-adhesive label production, offset litho print heads

are used in combination with another process, i.e. screen, hot stamping or UV

flexo varnish, for high added-value applications in, say, the cosmetics or wine-

labelling sectors.

In the last few years, a new generation of mid-web (400–600 mm wide)

roll-label offset presses have been developed to target the more efficient, higher

added-value in-line printing and converting of traditional sheet-fed glue-applied

labels for the wines and spirits markets.

4.6.4 Gravure

Gravure, or photogravure, is a true photographic process which is able to reproduce

high quality pictures, excellent colour densities and strong solid areas. It is primarily

used as a long-run process for wet-glue applied labels and for flexible packaging.

Now, gravure units are also being incorporated into narrow and mid-web presses

for labels and flexible packaging where the ability of gravure to apply varnishes,

lacquers, metallic inks and top coatings is a particular advantage.

The image carriers for gravure primarily consist of steel cylinders with an

outer shell of copper onto which images – consisting of millions of tiny cells of

varying depths and areas – are produced by chemical etching, laser etching or

electromechanical engraving. Although the most expensive of all the image

carriers used for printing labels, they have the advantage of being able to print

very long runs in millions. For additional run life, the cylinders can be chromium

plated.

Gravure printing presses are the simplest of all label-printing presses (Fig. 4.9).

The printing cylinder containing the cells rotates in a thin, fluid, solvent or

water-based ink which fills the recesses. Surplus ink is scrapped from the

cylinder surface by a flexible doctor blade, the ink in the cells (the image) then

transferring to the label substrate using pressure against a rubber-covered

impression cylinder.

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4.6.5 Screen process

One of the oldest methods of printing, screen process (silk-screen, Figure 4.10) is

mainly used for the production of self-adhesive labels for cosmetics and toiletries,

pharmaceutical and industrial and outdoor applications. Most frequently found in

combination in a press line with letterpress, UV flexo or offset, the screen process

has the ability to print a smooth, controllable lay-down of ink and to provide dura-

ble, high quality labels where good ink coverage is required for resistance to

weather, moisture, chemicals and abrasion. It can also print a good opaque white

image, something which other label printing processes find it difficult to achieve.

Substrate

Doctor blade

Impression cylinder

Printing cylinder

Figure 4.9 Gravure printing process. (Reproduction, with permission, from Iggesund Paperboard.)

Squeegee

Substrate

Ink

Figure 4.10 Silk screen printing process. (Reproduction, with permission, from The Institute of

Packaging.)

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The image carrier for screen printing is most often made from a nylon, polyester

or fine wire mesh onto which a photo-sensitive coating has been placed. Exposure

to light through a film positive of the label image causes the coating to harden in

the non-image areas enabling the unexposed (unhardened) areas to be washed

away. This leaves a negative image of the label remaining on the screen mesh.

For printing, the screen is mounted on a frame and placed in position on the

screen printing and die-cutting press. Ink is pressed through the open areas of the

mesh by a blade called a ‘squeegee’ to produce a print on the label substrate –

which in the screen process can be virtually any material. Ink drying today is

mainly undertaken by UV curing.

Screen printing lays down a very thick film of ink and is the only process that

allows a light colour to be printed over a dark material with satisfactory results.

Very high quality work is possible and fine line or tonal work can be produced

with equal success. This process is ideal for achieving visual impact with fluorescent

inks and for printing raised tactile characters (braille).

Screen presses used for self-adhesive label production either use a flat-screen in

a stop-and-go motion, or use rotary screens in which the squeegee is mounted

inside the cylinder. Flat screen printing is a relatively slow process as it requires

the web being printed to come to a stop for each print cycle. Although the rather

slow, flat style of printing unit gained partial acceptance with label printers, the

move towards building presses in which a combination of printing processes,

including screen, demanded something more – rotary screen, which is faster and

has a continuous motion.

The rotary method of screen printing has been used, for many years, for the

printing of fabrics and wallpaper and this technology was developed to create

narrow-web rotary screen units that could be interfaced with, say, letterpress,

flexographic, offset or, today, even gravure units for high quality and special

effect multi-process printed self-adhesive labels. In the rotary format, both the

mesh carrying the photographic image and the label substrate come together as

cylinders, which reduces the possible area of contact and sharpens up the image

being printed. The supply of ink is retained inside the rotary screen and the squeegee

is also located inside the cylinder.

Rotary screen or screen combination narrow-web presses make up around

10–15% of all narrow-web roll-label presses installed each year.

4.6.6 Hot foil blocking/stamping process

Foil blocking is being used more and more as either the sole printing process or as

a process incorporated alongside letterpress, flexographic or offset printing in a

combination process press, in order to add an extra added-value capability to the

printed label. It is a dry printing process, using no inks and involving no colour

mixing or matching. The process can print on a wide range of surfaces and produce

bright effects from metallics, or high opacity colours and uses relatively simple

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equipment. Hot-foil printing presses may be narrow-web, web-fed machines, such

as those used for self-adhesive label production, or larger sheet-fed presses used

for foil blocking of large sheets of glue-applied or in-mould labels.

Units designed for the hot-foil printing or decoration of labels come in a variety

of configurations and widths, and can be in the form of a stand-alone press or, in

the case of some narrow-web self-adhesive label presses in combination in-line

with other printing processes, provided the press is fitted with a suitable drive and

registration system.

The printing plate used for hot foil blocking needs to be of a hard material and

to have a raised image similar to that used by the letterpress process. The fact that

image transfer relies upon both heat and pressure restricts plate materials to either

a very hard thermoformed plastic plate for very short runs or plates produced from

brass, steel or zinc for longer runs. While rotary foil blocking has gained some

ground, the majority of hot foil blocking for label printing is performed in the flatbed

format.

The actual hot-foil printing process is achieved by transferring either a col-

oured pigment or a metallic coating from a ribbon of plastic material known as

the ‘carrier’ onto the surface of the label material to be printed. This transfer is

performed through the application of heat and pressure, and the length of time

the heated coating area is in contact with the substrate is known as the ‘dwell

time’. The balance and control of these elements is critical and must be indi-

vidually calculated for the surface to be printed, and the type of ribbon or foil

being used.

A more recent development of foil blocking is a cold-foil process in which a

print unit is used to print a special adhesive on the label web in the area where the

metallic effect is required. When foil is brought into contact with the adhesive, it

adheres to it to produce the printed foil design on the label. Cold foiling is a less

expensive means of achieving foiling than the hot process.

Once printed, the surface of hot or cold foil images may be varnished, over-

laminated or encapsulated in order to provide a hard-wearing, durable surface.

Foiling is used to provide a luxury (metallic) look on many cosmetics, toiletries,

health and beauty labels, on wines and spirits labels, and in other higher added-value

label applications.

4.6.7 Variable information printing, electronically originated

The one aspect that none of the conventional mechanical label printing presses can

handle economically is the printing of variable text or images that include bar

codes, sequential numbers, batch and date codes, price-weight information, lot

numbers, names, mailing addresses, etc. To perform these functions using letter-

press, flexography, litho, gravure or screen presses would involve stopping the

press after each print, changing all or part of the printing plate, then printing the

new image. An expensive and time-consuming operation.

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To overcome these limitations, a number of methods of electronically producing

VIP have been developed. These methods include ion deposition, laser printing,

direct thermal, thermal transfer, dot matrix and ink jet. Some of these operate as

stand-alone printers or presses, some run in-line with fixed image printing presses,

or they are added on to some types of label application or print-and-apply systems.

Brief descriptions of these VIP methods are described below.

4.6.7.1 Ion deposition

Ion deposition is an electronic printing system that uses an electrographic, non-

impact imaging process in which a latent image is created on a sensitive drum by

bombarding it with ions. Powder toner is applied to the latent image – the powder

printed from the drum to the substrate using pressure, and then the drum cleaned

ready to receive the next image. Ion deposition printing is capable of printing bar

codes and other information onto substrates such as paper, boards, vinyl, polyester

and other filmic materials at reasonably high speeds. It may need to be over-

varnished or laminated to improve durability, rub or scuff resistance.

4.6.7.2 Laser printing

Like ion deposition, laser printing requires a latent image to be created on the

surface of a sensitised drum (this time by light). Electronically charged particles of

a powder toner are deposited on the image and the toner transferred to the

substrate using heat and some pressure. Because of the heat involved in laser

printing, there may be limitations on the label substrates that can be used. There

are also the so-called ‘cold’ lasers or cool lasers which require less heat and can

print on a wider range of materials.

4.6.7.3 Direct thermal printing

Direct thermal printing of labels is the main process used for adding price-weight

information, product description and bar codes to supermarket fresh-produce labels –

meat, fish, cheese, fruit, vegetables, etc. – which are weighed and priced on the

pre-packaging and labelling line or, in some cases, printed and labelled in-store.

The print head for direct thermal printing consists of numerous styli (needles)

in the form of a grid or matrix that are heated and cooled selectively by a micro-

processor controller. A special heat-sensitive coated paper is required which,

when heated by the styli, changes colour within the areas of contact to form the

required letters, words, numbers or codes.

As the special thermally printable coating is heat-sensitive, direct thermal printing

is primarily used for fresh and chill cabinet products that have a short shelf life in

store of several days. It is not normally used for long shelf-life labelled products or

for labelling for use in warm or hot conditions.

4.6.7.4 Thermal transfer printing

Thermal transfer printing also makes use of styli which are heated and cooled select-

ively. However, this time, rather than using a special thermally sensitive coated

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paper, the styli come into contact with a thin film one-pass ribbon which carries a

heat-activateable ink or coating on the underside. The required image is therefore

created by transferring the heat-activated ink coating from the film carrier to the

substrate, according to the pattern or shape of the heated styli.

Thermal transfer printing is used for printing variable information, such as

batch codes, date codes, sequential numbers, text, diagrams and bar codes onto

pallet, carton or box end labels, for warehousing and distribution requirements, for

bakery labels and for DIY and industrial labelling. Printers may be incorporated

into packaging and/or weighing lines or be stand-alone. Some are also print-

and-apply systems.

4.6.7.5 Dot matrix printers

Dot matrix label printing involves the firing of pins or hammers against an

inked ribbon and thus transferring ink onto a label face material, rather like the

early typewriter techniques. Each pin or hammer produces a fine dot and these

can be positioned in a grid to produce letters, words, numbers, codes or simple

graphics.

Impact printing systems such as dot matrix printers have applications for printing

onto card or for form/label combinations where an image may need to be created

through more than one layer of material. They are far less common today than in

the past.

4.6.7.6 Ink jet printers

Ink jet printing uses minute jets of ink which are activated and fired at a label

surface by means of electrical charges to form the desired image. Ink jet is a non-

contact method of VIP.

The various ink jet-printer manufacturers use a variety of techniques to form

the image. These include a single continuous jet, a multi-head jet, impulse jet and

a system using solid ink which is converted to liquid at the moment of transfer.

Ink jet printing can operate at speeds which are compatible with presses, label

applicator line or packaging line speeds and are therefore often used in in-line

operations to produce batch or date codes, sequential codes, batch codes and product

or delivery details.

4.6.8 Digital printing

The means to print labels digitally in four or more colours direct to paper or

synthetic materials began to emerge in the early 1990s and has now become a

label printing technology in its own right with around 200 digital label presses

installed worldwide by the early part of 2003. These presses are predominantly dry

toner (Xeikon) or liquid toner (HP Indigo) technologies. The first of a new generation

of digital colour ink jet machines also began to be installed in the label industry

during 2002/2003.