Print processes: letterpress printing

The mechanisation of letterpress printing began with wooden printing presses based on a platen type configuration with the ‘forme’ being laid on the bottom platen. Ink was hand applied to the surface of the type with a leather covered ink ball and paper was then carefully laid on top of the inked type before pressure was applied by the top platen using a large threaded screw.

This revolutionised printing as it allowed the mass production of books and documents. One of the most famous printed books was the ‘Gutenberg Bible,’ of which the first 200 copies were printed in 1455.

By the very end of the 18th century steel-made presses were being developed that used a knuckle and lever arrangement instead of the screw and the manufacture and use of inking rollers speeded up the process, paving the way for further automation.

MARKETS

The use of letterpress in the commercial print market is now small and shows a declining market usage, however letterpress is still of importance in the self-adhesive label sector. The label industry trend towards shorter runs has created a market for intermittent feed letterpress technology on shorter run jobs which require complex in-line multi-process printing, embellishing and conversion.

Typical applications have included short runs of high added value wine, spirit and cosmetics labels.

A lower capital investment makes it a popular process in the developing countries and whilst the quality of letterpress printing has never been in question, over the last two decades it has been displaced by other printing techniques. Preparation time for letterpress can be quite lengthy and its use is being overtaken with faster and more efficient printing methods such as the offset lithographic and flexographic processes.

In the more mature European labels markets, surveys show UV flexography has overtaken letterpress as the leading narrow web process in the global narrow web market. But it would be an error to think that this is an obsolete technology and there is still a large base of both rotary and intermittent fed letterpress machines running label work on a daily basis.

The development of semi-rotary, servo-driven systems makes the process very competitive on short runs that require complex embellishing and converting, particularly when matched with flatbed die-cutting.

Converters are often reluctant to make the shift from letterpress to flexography because of the need to change technologies and the more expensive ink and plate systems associated with other printing methods.

PRINCIPLES OF PROCESS

Printing is by a relief image plate.The relief (print) areas receive a film of high viscosity ink and under a controlled pressure known as the ‘impression,’ the image is transferred direct from the plate to the substrate. (See Figure 3.1).

Figure 3.2 shows the layout of a rotary letterpress unit of the type used in the label industry. At the top of the unit you can see the ink reservoir known as the ink duct, which distributes a controlled amount of ink to the inking distribution rollers. A consistent film of ink is applied to the printing plate by the forme rollers/ink train.

The substrate being printed travels between the rotating plate cylinder and the impression cylinder and the printing pressure is adjusted by increasing or decreasing the plate cylinder pressure to the substrate. There is very little ‘give’ in the printing plate, so in order to achieve a good ink film transfer and a sharp image, the pressure between the printing plates and the impression cylinder or flatbed requires a very careful setting.

A skilled operator will vary the hardness of the impression roll ensuring that the plate to substrate contact is a ‘kiss’ touch. Careful balance of pressure between plate and impression roll is crucial as it determines the print quality. Too much impression creates a squashed or halo effect and too little impression creates missing dots and poor print.

Accurate plate inking roller settings are very important and impression adjustments should be made throughout the press run to make sure the correct printing pressure is maintained.

Letterpress printing uses a paste ink held in an ink duct and this is transferred to the surface of the printing plate through a series of ink distribution rollers. These distribution rollers rotate and oscillate from side to side, breaking down the ink film and ensuring that correct ink film weight ink is deposited on the printing plates with each revolution of the press.

LABEL PRESS CONFIGURATIONS USED FOR LETTERPRESS PRINTING

Letterpress label printing presses come in a number of different configurations - platen, flatbed, semi-rotary and full rotary. All of these press configurations are used today for the manufacture of self-adhesive labels (See Figure 3.3).

LETTERPRESS - PLATEN-TYPE

A platen press is made up of two flat surfaces called the top bed and the bottom bed (See Figure 3.4). The substrate to be printed is positioned on the top bed and  the forme/plate is locked onto the bottom bed. The inking rollers traverse down the face of the plate and the plate or type is inked.

The top and bottom bed come together pressing the substrate against the inked plate thereby producing the printed substrate.

LETTERPRESS SHEET-FED - FLATBED

The flatbed letterpress ‘sheet-fed’ press is a single color press. 

The flatbed configuration is constructed with a bed which holds the forme. This bed lies in a horizontal position whilst the rotating impression cylinder is in a fixed position and located over the flatbed. (See Figure 3.5). The bed moves back and forth in a horizontal direction passing under the revolving impression cylinder which carries the substrate being printed.

On the impression stoke, the traveling bed and rotating impression cylinder are synchronised and the forme and cylinder travel together to transfer the image to the substrate. The impression cylinder then lifts to allow the bed to return to its first position; and the cycle is repeated.

Flatbed cylinder presses are very slow running, with a maximum speed rate of 5,000 impressions per hour.

SEMI-ROTARY - LETTERPRESS

This type of semi-rotary press configuration uses a stop–start web feed system and can be fitted with multiple letterpress printing units, it can also be fitted with flatbed hot foil stamping units, flatbed embossing and flatbed die-cutting units and waste rewind.

The semi-rotary print head differs from the full rotary press in two important areas. Firstly the printing head is positioned to print ‘across’ a stationary web (See Figure 3.6) and not ‘down’ the web as in a full rotary printing head.

The printing plate is located onto the plate cylinder and receives the ink film from a ‘fixed’ position inking unit. This cylinder rolls across the stationary web and transfers the image to the substrate. The cylinder then traverses back to the start position and fresh ink is applied to the plate as the web moves forward in readiness for the next printing cycle.

ROTARY LETTERPRESS - TRANSLATORY FEED

The letterpress technique is also used on intermittent (‘translatory’) fed presses. The press has full rotary printing units linked to a translator or reciprocating web feed (See Figure 3.7) and is a completely different configuration to the semi-rotary press.

The press layout is the same as a full rotary web-fed press, but the difference lies with the web transportation system. The web travels through the press with a forward/backward reciprocating action and the printing cycle takes place as the web travels forward. The print unit has a ‘fixed’ cylinder repeat length meaning there are no variable size print cylinder changes required.

ROTARY LETTERPRESS - STACK PRESS

The term ‘Stack’ describes the way in which the print units are positioned on the press (See Figure 3.8). The letterpress print units are ‘stacked’ on top of each other with sufficient space between each unit to allow for a drying unit to be located. The stack can be configured to provide twin stacks allowing 6 print units to be used.

This type of configuration played an important part in the development and manufacturing of HAV (high added value) self-adhesive labels.

Typical of this type of press are the Gallus R160 and R200. These ‘stack presses’ allow for the production of high quality print, on both filmic and metallised substrates, using combinations of the letterpress, flexographic and screen processes, linked to a UV curing/drying system. A short web path and easy makeready makes this type of press suitable for short or long run work.

ROTARY LETTERPRESS - CENTRAL IMPRESSION (CI)

This configuration of label press features a single large diameter central impression cylinder with letterpress printing heads located around the CI cylinder (See Figure 3.9) and the drying units positioned between each print head.

These presses can be configured with up to six print units positioned around the CI cylinder. The CI cylinder has a synthetic rubber surface which cushions the impression between the printing plate, the substrate and the impression cylinder, ensuring there is no metal to plate contact between the CI cylinder surface and the printing plate, which could lead to damage to both the printing plate and the CI cylinder.

TYPES OF LETTERPRESS PLATES

The letterpress plates used in the label industry comprise of three types; zinc/magnesium plates, polymer plates, and steel backed plates.

The zinc/magnesium plate is a ridged flat plate and is used in presses with a flatbed configuration. A chemical etching process is used to create the relief image.

The polymer plate is the plate most widely used in the label industry and the most suitable for full rotary presses. The plate is a film backed flexible plate, which is mounted on to the plate cylinder using accurately calibrated double sided adhesive film tape.

The steel backed plate is a polymer structure fused onto a thin steel shim. This type of plate can be supplied flat for mounting onto a magnetic base or in a semi-rotary format, which has punched locating holes positioned at the leading and back edges of the plate. These locating holes correspond with pins on the print cylinder configuration used on semi-rotary intermittent feed presses.

IMAGING THE LETTERPRESS PLATE

The imaging of photopolymer plates is done by exposing the polymer to a UV light source through a film negative. The film and plate are positioned in contact with one another and placed in a vacuum sealed unit to ensure full and even contact.

Exposure units may be flat or rotary. If the plate is for flatbed application then the exposure is done flat, but if the plate is for semi-rotary use then it is done in a rotary unit. This is necessary because allowance has to be made for the distortion factor that is created by the curvature of the printing cylinder.

UV light is then applied and the image area is polymerised and hardens leaving the unexposed area still soft, which can then be removed.

The washing/brushing process is done in a washing unit and the removal of the non-image area can be achieved with direct sprays, pads, or a rotating brush and sprayed with clean warm water. The brush must be soft to avoid damaging the plate surface.

The plate is then dried using warm air and a second post cure exposure is given to the plate to ensure the polymer is fully hardened. (See Figure 3.10).

CTP (COMPUTER TO PLATE) IMAGING

Letterpress plate imaging is also done using CtP (Computer to Plate) direct ablation.

This is a process which uses specially formulated letterpress plates coated with a black ablation layer which means that the letterpress plate can be imaged directly via a digitally driven laser which is controlled from a computer file. The same laser systems are used for the imaging of a digital flexo plate.

The hard, metal-reinforced letterpress plates are coated with a black ablation (LAMS) layer that is evaporated by laser and removed by suction from the parts to be printed. The plates are then cured with UV light,  washed out, dried and post-exposed. (See Figure 3.10).

If the press dot gain is established then the gain can be calibrated into the file and the plate image can be adjusted accordingly. This facility is of considerable benefit when linked to good impression control, allowing the printing of high quality process color reproduction.

MAINTENANCE PROGRAM

A press that is not regularly maintained will not give the optimum printed image. A regular maintenance program therefore is very important.

Bearings, print cylinders, gears etc. do wear and it is advisable that replaceable parts are held available so they can be changed when required without any delays being experienced. Presses are subject to stresses and strains caused by the use of heavy tooling units, continuous running and poor maintenance.

Cylinder bearings in particular have a limited life and a program of planned maintenance for the cylinder bearings should carried out, including cleaning, checks for wear and adequate lubrication.

LETTERPRESS PRINT CYLINDERS

In the flexographic, letterpress and litho processes the printing plate is located on the print cylinders. The cylinder needs to have accurate and even contact with the inking rollers and the surface of the substrate.

The print cylinders should run perfectly true with an accuracy of ±0.025 mm ensuring that the pressure on the adjacent rollers is constant. Accurate measurement should be made when the press has been running for a short period to allow the running parts to warm up.

CYLINDER GEARS

The care and condition of the print cylinder gears is very important. When the gear is manufactured it is intended to be meshed at a certain depth with the gear it is driving. This is called the pitch diameter. When meshing the print cylinder gear with the impression roll gear, this pitch diameter is critical to accurate register.

Too deep or too shallow a mesh will cause loss of register between one color and the next. Regular lubrication is preferred to intermittent applications, as this will maintain a constant film of oil and even out temperature fluctuations.

The print cylinder circumference is based on the print repeat length in inches, millimeters or the number and size of gear teeth. Allowance has to be made for the thickness of printing plate plus the mounting tape and also for the effect of pressure and any expansion in the print cylinder.

The calculation would be as follows;

A print repeat length of 12 inches (or 96 1/8’ gear teeth) equals a circumference of 300.8 mm.The print cylinder must have a circumference that is smaller by 3.14 (Pi), to allow for two thicknesses of plate and mounting tape, 3.14 x 2 (1.7 + 0.3) mm and a small allowance for the effects of the printing pressure and thermal expansion, say 0.01 to 0.03 mm.

PLATE MOUNTING

Before mounting the printing plate the print cylinder surfaces should be thoroughly cleaned to ensure a clean, uncontaminated surface.
A contaminated cylinder surface will cause problems with the print quality.

Foreign particles trapped between the surfaces of the print cylinder, the mounting tape and the back of the printing plate and any grease or oily residue will affect the adhesion power of the mounting tape causing plate lift during the print run.

Double sided filmic adhesive tape is used to fix the printing plate to the printing cylinder. (See Figure 3.11).

These tapes can vary in thickness to allow for any slight variations in the diameter of the print cylinder and will compensate for any under-or oversized print cylinder. It is critical that the outside peripheral of each printing plate is exactly the same. If this is not achieved then accurate print to print register will be impossible.

It is strongly recommended that the same brand of tape should be used on each of a set of print cylinders.

Some tape manufacturers incorporate a thin layer of foam within the mounting tape which assists in smoothing out any small deviations in the plate cylinder.

MOUNTING FLEXIBLE PLATES

There are some basic principles that need to be followed when mounting plates. The mounting of flexible plates relies on manual skills, even though it is usually carried out using a plate mounting system equipped with mechanical and optical aids. The one rule that must be followed is to ensure that the plate is in the correct position at the first attempt of mounting.

This involves a careful check to ensure that the registration lines on the leading edge of the plate are correctly in-line with either the optical system on the mounting system or alternatively the engraved grid on the print cylinder. These engraved lines run both horizontally and circumferentially round the print cylinder. Generally speaking the plate is usually mounted in the center of the cylinder and it is recommended the center of the plate is marked and the mark located with the center of the cylinder.

It is important that a little extra time is spent in ensuring that the leading edge of the plate is in the correct position before completing the full mounting, as any removal of the flexible plate to ‘try again’, can distort and stretch the plate, making it difficult to remount  and correctly position it over the whole of its area.

LETTERPRESS INKS AND DRYING SYSTEMS

Paste inks are used for both flatbed and rotary letterpress systems. The ink has a high viscosity and is therefore very similar to offset litho inks (often referred to as a short ink), (See Figure 3.12). The letterpress plate has a hard surface and this ensures a good ink film transfer to the substrate being printed.

The letterpress inking system uses an extended train of rollers which both rotate and oscillate from side to side in order to ensure that the thick paste ink film is evenly distributed prior to being applied to the printing plate.

The majority of letterpress label presses are equipped with ultraviolet (UV) curing systems and the UV ink is especially formulated to cure rapidly.

IDENTIFYING THE LETTERPRESS PROCESS

All printed graphics are formed using a dot formation. The smaller and less dense the dots, the lighter the color. The larger and more dense the dot, the darker the color. This effect is called the tonal value. (See Figure 3.13).

Each of the printing processes has a particular characteristic which can be easily identified. A printed image produced by the letterpress process can be identified by three characteristics:

1. The indentation on the reverse of the substrate that is created as a result of the pressure applied (impression) during the printing cycle.
2. The halo effect that can be created when there has been incorrect plate roller settings and too much printing pressure applied.
3. The ‘hard edge’ that is formed in the printed image as a result of dot break up, This is created by the inability of the letterpress process to print a dot  below 3%.

ADVANTAGES AND DISADVANTAGES OF LETTERPRESS PRINTING

Advantages

• Good color density

• Extended plate life

• Good printing of type and solids and sharp edge definition

• No ink and water balance problems

• Suitable for sheet-fed and web-fed printing

• Good legibility of text even on uncoated papers;

Disadvantages

• Limited tonal reproduction

• Minimum printable dot (3-5%)

• Not suitable for thin filmic ie shrink sleeves

• Relatively high printing plate costs

• Poor tonal reproduction when compared with offset litho

• Relatively coarse screen rulings have to be used for tonal reproductions

LETTERPRESS PRINTING- SUMMARY

Although surveys into print process trends show flexography has overtaken letterpress as the leading narrow web process in the global market, letterpress is far from defunct. Indeed, letterpress is making a comeback as an option for low run, high complexity jobs, driven by new technology innovations. The cost efficiency of the letterpress process in particular is a key factor in stimulating its revival.

As has already been highlighted, letterpress has certain intrinsic advantages as a printing system compared to offset and flexo. 

To achieve exact color adjustment in flexo, repro has to be corrected, or colors need to be specially mixed. With letterpress, however, color can be adjusted during makeready.

Letterpress also has advantages versus the offset process in that running speeds can be increased from 10 m/min up to 60 m/min with no color change. This is difficult to achieve with offset because the ink/water balance is changing and this therefore creates waste.

Semi-rotary letterpress also has particular benefits with low tooling costs, allowing it to combine processes such as UV and solvent flatbed or semi-rotary screen, hot and cold foil stamping/embossing, lamination, reverse printing and flexo UV varnish.

Although digital presses are selling fast, there are still a lot of converters, even in mature markets, who still prefer the letterpress route. Semi-rotary technology in particular is ideal for small and medium jobs, because of its very fast changeover and the print quality is almost comparable to a digital press. Digital printing equipment, of course, is also very expensive.

TECHNOLOGY ADVANCEMENTS

Letterpress continues to advance technologically. There are still developments in the press sector – with semi-rotary, servo-driven systems, competing with digital on short runs of products, particularly those with complex converting requirements.

In pre-press digital letterpress platemaking is also undergoing major developments.

One of the most important developments has been ink trays with segmented blades allowing ink to be delivered to the plate in the exact quantities determined at pre-press. Exact inking profiles of jobs can be saved and recalled, allowing repeat jobs to be set up quickly and consistently.

In developing markets, letterpress, particularly when matched with flatbed die-cutting, remains a powerful force, with converters often nervous about a shift to flexography because of the need to change to more expensive ink and plate systems.

Other technology developments have included new designs for printing sets and inking rollers, which help eliminate the effects of double imaging and ghosting, while new printing cylinder designs have delivered greater strike precision and increased uniformity of printed dots.

The development of magnetic sleeves also makes it possible to change between jobs very quickly and to carry out preparation away from the machine.

Other recent developments are occurring in letterpress platemaking with the introduction of water-washable plates. Digital letterpress plates are also emerging with digital stencil plates allowing conventional film processing to be replaced by digital mask ablation.

Specially formulated letterpress plates coated with a black ablation layer can be imaged on the same system as a digital flexo plate.

The letterpress process is alive and well and backed by technology advancements. It still has a significant role to play in the label sector.