Print processes: lithographic printing

In 1796 the Bavarian author Alois Senefelder invented lithography, a printing process that used chemicals to create the image.

By the 1850s the process of chromo-lithography enabled the printing of up to twelve different colors, using a combination of dots and solid areas to create the printed image. This method of printing with stones dominated for the next sixty years, especially in the mass market. Thereafter, developments in the printing process concentrated on the speed of output and keeping manufacturing costs down, whilst maintaining quality.

In 1875 Robert Barclay patented the first rotary offset lithographic printing press. This press used transfer printing technologies that employed a metal cylinder instead of a flat stone.

The offset cylinder was covered with specially treated cardboard that transferred the printed image from the stone to the surface of the metal. Later, the cardboard covering of the offset cylinder was changed to rubber, which is still the most commonly used material today.

Lithography is commonly referred to as offset lithography or simply offset and is the most popular printing process for the printing of glue applied paper labels.

The use of the offset litho process on roll-fed self adhesive label presses increased in the early 1990s with the introduction of the Gallus T250, which is an intermittent feed, multi-process label press.

This was followed shortly after by the Nilpeter M300 full rotary, multi-process label press. One of the drivers for these developments was the need, within the label industry, for a print process that would produce high quality tonal graphics mainly for the personal care, wines and spirits and food markets.

Driving this development was the need to produce self-adhesive labels that fully matched litho printed body, back and neck wet glue labels already used in the wines and spirit industry.  

LITHOGRAPHIC PRESS CONFIGURATIONS USED IN LABEL MANUFACTURING

There are two formats of lithographic printing press used for the printing of labels: sheet-fed and reel-fed.

With sheet-fed litho the substrate to be printed is guillotine cut into sheets, creating a stack which is then loaded into the press and printed as a single sheet. This compares to a web-fed press where the substrate is fed from a master roll and then printed as a continuous web.

With litho printing each printing unit prints a single color and a press can be configured with the desired number of print heads as required. For example a 6 color press will print up to 6 individual colors or less if required.

Reel-fed presses are able to process a reel-fed substrate, which can then be cut into single sheets after the printing and converting processes.

The ability to print substrates from the reel is of great benefit, allowing the web press to process lightweight substrates (both filmic and paper).

This advantage derives from the principle of the process which means that the substrate face is only exposed to a flat (planographic ) surface during the printing operation and is not distorted by contact with a printing plate which has a ‘relief’ image surface, as is the case with the letterpress and flexographic printing processes.

 
THE SHEET-FED LITHO PRESS

Within the label industry the sheet-fed press is used predominantly for the printing of wet glue applied labels. Individual sheets are held in a stack or pile and fed one at a time into the press. Delivery of the printed sheets is through the press ‘delivery’ system, where the printed sheets are placed into a stack or pile at the end of the press. The sheets can be then reprocessed or cut into single labels for product application.

The commercial print market is the largest user of the sheet-fed offset litho process with presses usually comprising of 6 print units plus a coating unit for overall or spot varnish requirements.

Larger volume book and magazine production is generally printed on an 8-color press which is configured to print 4 colors on the front side of the sheet and 4 colors on the reverse of the sheet; this process is known as ‘perfecting’.

Sheet-fed litho usage within the label industry falls into three categories:

1. Wet glue labels.
2. In-mold labels
3. Self-adhesive labels printed in sheet form

REEL-FED LITHO PRESS

The printing, embellishing and converting of self-adhesive labels is carried out on a web-fed press, also known as a reel-fed press. Figure 4.1 shows a web-fed 4 color press commonly used in the label industry.

This type of press prints onto a continuous web and not in a single sheet format, which means that the reel is placed on the in-feed section of the press, known as the unwind unit, and the printed and converted reel exits after unit 4 onto the rewind unit.

Figure 4.1 shows a 4-unit web-fed press printing in the process colors of CMYK, Cyan unit 1 – Magenta unit 2 - Yellow unit 3 and Black unit 4. The printing head is mounted on the top of each unit and the die-cutting station is typically positioned between unit four and the rewind unit. 

One of the major differences between the sheet-fed press and the web-fed press is the control of web tension during the printing operation. Sheet-fed equipment does not require web tension control as each sheet is individually controlled by a system of gripper cylinders which ensure very accurate print to print registration between each color.

With web-fed presses the control of the web tension is critical and the web control system must ensure that there is no movement of the substrate as a result of substrate shrinkage or stretch, which in turn can create mis-register of the colors being printed. Modern web-fed presses use very sophisticated digital web control and print register systems, using web sensors linked to step motor technology and the controlled use of ink drying and curing systems that minimise the amount of heat exposure to the substrate during the printing process.

This limits the shrinkage and stretch factors, particularly when filmic substrates are being printed. 

OFFSET - CYLINDER REPEAT LENGTH

Litho printing in the label industry has experienced some difficulty in adapting to the varying size requirements of printing labels in a reel-to-reel format.

Offset lithography is ideal for sheet printing and reel printing with a fixed repeat length. However, because of the cylinder configuration of an offset unit, the plate cylinder sizes are restricted. A number of successful attempts have been made to modify the system to make it more adaptable to the wide variations required when printing labels in all sizes.

Consecutive jobs are rarely of the same repeat length and therefore require a time consuming changeover of the cylinders holding the printing plates.

Some press manufacturers have introduced systems which appear to effectively eliminate the need for extensive changeover of rollers and gears in order to accommodate the wide range of printing lengths required.

The semi-rotary systems are one way of eliminating the changeover of print cylinders and gears. However, these systems are much slower running than the modern full rotary systems.

The semi-rotary press design uses a forward and backward movement of the web rather than a continuous forward movement, thus taking material through and then bringing it back to compensate for the various repeat lengths required by each job. The web is automatically brought back after each printing cycle, controlling the gap between images and ensuring that the printed image remains in register between one print station and the next.

THE PRINCIPLES OF THE OFFSET LITHO PROCESS

Modern offset litho printing uses a printing plate made from thin grained metal sheet that carries the photographically developed image.

The process works on the principle that oil and water do not mix. If an imaged area on a plate Figure 4.2 is damped with water then the image area will reject the damping, but the non-image area will accept the damping (See Figure 4.3).

If ink is passed over the plate, the image area will accept ink, but the damped non-image area will reject ink. (See Figure 4.4). The inked image on the plate is now ready for transferring onto the offset blanket and then to the substrate.

You can see the non-printing area in light grey, the print area in dark grey and the damper roller in blue.

The inking roller, in red, passes over the plate. The oil based ink is rejected in the non-print image area by the film of damp, but is accepted by the damp free print image area.

The print area is inked and the image transferred onto the offset blanket and then ‘offset’ printed onto the substrate.

THE PRINTING CYCLE

Figure 4.5 shows the cycle of damping and inking on a litho printing unit, with the plate cylinder, damper rollers and inking rollers in position.

This graphic illustrates the rotation of the plate, blanket and impression cylinders and shows the roller trains used for the damping and inking of the printing plate.

The inked image (purple) is then transferred onto the rubber offset blanket and then under pressure onto the substrate (yellow). The ink is then dried or cured.

The volume of the ink film to the plate is controlled via the ink reservoir (called the ink duct) and is regulated through a system of rollers (called the roller train). The roller train ensures that the ink film is correctly regulated and consistent on every revolution of the plate cylinder.

THE OFFSET UNIT

Figure 4.6 shows the location of the three cylinders and the inking and damping roller systems within the printing unit.

In printing mode the inked image is transferred from the plate cylinder onto the offset blanket (positioned on the blanket cylinder) and the pressure between the blanket and the impression cylinder transfers the printed image to the substrate.

The control of the damping process is critical in achieving the correct print quality. Any imbalance associated with insufficient damping will allow the ink film to contaminate the non-image area and this will result in ink being deposited in the non-image area thereby creating a scumming or catch-up.

Scumming can be adversely affected by the ambient heat around the press and also the heat generated when running the press at speed.

Modern damping units use a very controlled system of alcohol damping, including the use of chemical additives to produce the optimum damping control. This solution is called the ‘fount solution’.

The arrows identify the three cylinders on a reel-fed self-adhesive label press, showing the plate cylinder, the blanket cylinder and the impression cylinder positioned below the blanket cylinder.

The litho process produces an evenly inked image and although the process applies only a thin film of ink, the image is sharp with no edge blemishes and thereby delivers an excellent printed result.

The reproduction of fine detail, including tonal images is excellent with only the digital process and the gravure process equal to it in print definition.

THE OFFSET BLANKET

To reduce wear (abrasion) of the plate, the image is transferred to the printing substrate via the offset blanket which is positioned on the blanket cylinder.

The blanket has a self-adhesive backing and the leading edge of the blanket is positioned and wrapped around the cylinder, the surplus tailpiece is then cut off to ensure that there is a smooth area between the two edges of the blanket.

Offset blankets are made of synthetic rubber and are available with a variable shore hardness allowing the press operator to choose the correct blanket hardness. It is important that the release factor of the blanket is correct to suit the substrate being printed, so that the inked image is fully transferred to the substrate on every revolution of the printing cycle.

Washing/cleaning of the offset blanket is very important as this ensures that the image is cleanly transferred on every print cycle. Ink residue and fibres from the substrate surface can contaminate the blanket and regular cleaning throughout the print run is recommended. Blanket cleaning is a manual operation but some litho presses are fitted with automatic blanket wash systems.

THE LITHO PRINTING PLATE

The image and non-printing areas of the printing plate are in the same plane i.e. a flat surface hence the term ‘planographic’ printing process.

The illustration in Figure 4.9 shows the structure of a lithographic printing plate, which comprises of a aluminium base, a primer layer and finally a photo sensitive coating.

PLATE IMAGING

There are two types of method for imaging the printing plate;

1. Contact imaging where an imaged film is in direct contact with the print plate and given a timed exposure to establish the image.
2. CtP (Computer to plate) where a digital file is imaged directly onto the plate without the need for film.

Although most imaging techniques use CtP (computer to plate) film based imaging is still in use. (Figure 4.10). The film negative or positive which is created from the digital file is placed in direct contact with the plate and exposed to a UV light source.

The image from the film is transferred to the printing plates using a photographic process. A measured amount of light is allowed to pass through the film negative thereby exposing the printing plate. On exposure a chemical reaction occurs that activates the ink receptive imaged area.

The plate is then developed and the image is chemically fixed. The plate is then ready for positioning into the press negative separated after exposure. The image is clearly visible and the plate is now ready for developing. 

COMPUTER TO PLATE - IMAGING (CTP)

Offset plate imaging can also be done without the need for film originals using a process called CtP or ‘Computer to plate’. (See Figure 4.12).
 

The digital file to be printed is transferred to a CtP device called a plate setter and the image is created using direct laser imaging.

After laser imaging, the emulsion that remains in the imaged area is removed leaving it ink receptive. CtP plates do not require chemical processing.

CtP plate imaging eliminates the need for film-plate exposure and chemical processing.

Once the litho plate has been imaged and checked, it is now ready for locating into the press prior to the printing operation taking place.

ON THE PRESS: PLATE CHANGE

The plate is first located into the plate cylinder with the leading edges of the plate positioned first. The plate is then wrapped around the plate cylinder and the trailing edge located, positioned and clamped. The operator then applies the correct tension to the plate to ensure that the position is correct and the image is in register.

PRINTING INKS

There are two types of litho printing ink systems:

Conventional oil based and UV (ultra violet) oil based. The UV system offers some considerable advantages over the conventional system and these advantages will be explored later in the article.

In Figure 4.13 you can see the consistency of the litho inks and press operators can easily handle the inks with a standard palette knife making color mixing /matching much easier.

INK DISTRIBUTION CONTROL VIA THE INK DUCT

The duct roller delivers ink to a drop roller which moves between the duct roller and the distribution rollers. On some litho presses certain distribution rollers are chilled/cooled to assist in controlling the temperature of the inking rollers which can generate a lot of heat within the roller train, as a result of the high speeds at which the rollers turns. The combination of heat and speed can also create ink ‘flying’ which produces a spay mist of ink. 

The purpose of the distribution rollers is to evenly distribute the ink film which is transferred to the plate via the plate inking rollers also known as ‘forme’ rollers.

Over a number of years the efficiency of the ink roller train has improved and press manufacturers have done considerable development work in providing an inking system that ensures that on every print cycle the plate receives the exact ink film required. These developments include varying the roller diameters and adjustable roller oscillation.

One of the most common faults experienced by the pressman is called ‘ghosting’. This problem is most evident when printing solids which incorporate smaller reversed-out image areas. The problem manifests itself by printing a ghost image onto the solid areas which is very difficult to eliminate. This issue is usually attributed to inadequate rolling power and can be further aggravated by incorrect ink viscosity.     

CONTROL OF THE INK FLOW

Control and adjustment of the volume of ink which is applied to the printing plate is done via the ink duct adjustment.

The flow of ink to the inking rollers can be increased or decreased by adjusting the segmented plates via the duct keys which form part of the ink duct and are positioned against the duct roller. This allows the ink film density to be varied across the width of the web being printed. The image being printed may for example comprise of solid content in some areas which require more ink, and other areas with text only that require less ink.

Adjustment of the individual keys which control the flow can be done manually or through a remote keyboard.

ON THE PRESS: REMOTE INKING CONTROL

Modern offset litho presses are fitted with remote control inking systems.

These systems allow the press operator to make very fine adjustments to the ink flow during the print run. These color corrections are controlled from a master station and eliminate the need for the operator to move from print unit to print unit to make manual adjustments to each individual color.

Some litho presses, both sheet-fed and web-fed, use a closed loop color monitoring system for the control of the ink film being printed. The consistency of a specific color throughout the print run relies on maintaining a very accurate ink film weight.

These inking control systems offer major benefits by giving remote inking control through digital data, automated color adjustment during the press run and also quality control via on-press scanners that check both label content and color.

On-press scanners are used to take spectrophotometry and densitometry readings of the printed web or sheet. These readings relate to color density and dot gain* and the data is then cross referenced to the correct color specification. Any variation in color or dot gain is automatically adjusted on the press to ensure that the color specification and print sharpness is correctly matched with the print specification.

*DOT GAIN

Dot gain is the term used when the dot size prints larger than the correct specification, it is a print characteristic in which the size of the half-tone dot changes as a result of platemaking and also the printing process used.

When dots are transferred from a film to a plate, they will tend to grow in size during light exposure and the ink film which is transferred from the plate to the label substrate can increase in size. Fluid inks and compressible plates tend to increase the dot gain but this can vary according to press and substrate being used.

All printing presses will produce a certain amount of gain as a result of the engineering tolerance which occurs in the press manufacturing.  Dot gain can also stem from ink viscosity variances and from incorrect pressure between the plate and impression cylinder (flexo and letterpress) or between the plate and blanket cylinder or the offset blanket and the substrate (litho) and it is important that these pressures are set correctly in order to reduce dot gain to a controlled minimum.

Dot gain is largely predictable and allowance for dot gain can be accommodated at the repro stage provided specific plate, substrate and press dot gain information is known.

Typically, the printer will undertake a press fingerprinting analysis to obtain dot gain information, It is very important that the press dot gain characteristics are identified by carrying out a dot gain test to establish the gain for each individual press, this reading can then be applied to the digital data used in the preparation of the image file and the dot size is reduced on the film to compensate for the gain that will take place during platemaking and the printing process.

DRYING SYSTEMS

There are three types of drying systems used within the commercial and self-adhesive printing industry – oxidisation, Infra Red and Ultra Violet, known as UV.

The modern self-adhesive press is fitted with inter-deck drying units.This allows for each individual color to be cured/dried before printing the next color. This is called printing ‘wet on dry’ and eliminates the problems which can be encountered when printing conventional litho inks ‘wet on wet’.

DRYING SYSTEMS: OXIDISATION - NATURAL EVAPORATION OF OIL/SOLVENT

This is the method of drying the oil and solvent component used in these oil based ink systems by natural evaporation and where the drying process of these inks is not accelerated by using infra-red or hot air.

Oxidisation is a slow process because it relies on the absorbency of the substrate and exposure to the ambient air conditions
This method is not suitable for the web-fed presses used in the self-adhesive industry.

DRYING SYSTEMS: INFRA-RED/HOT AIR

Infra-red is the method of accelerated drying which uses either hot air or direct heat or a combination of both hot air and heat. (See Figure 4.14).

Drying by infra-red is only suitable for oil, aqueous (water-based) and solvented inks. This type of drying system is compatible with many of the web-fed presses used in the self-adhesive industry.
Infra-red drying is not suitable for filmic materials as the heat that is generated creates problems with shrinkage and stretching of the filmic substrate, in particular lightweight unsupported film.
INTERDECK UV

The drying or curing of UV printing inks takes place through the reaction of the ink chemistry to a strong ultra violet light source.

UV inks are 100% solid system and do not contain solvents that must be evaporated during the curing phase.

Figure 4.15 shows the position of the UV units within the printing press. The curing systems used can be run at very high production rates, as the curing of the UV ink takes place rapidly. Drying speed however, can be affected by the color and density of the ink film and the intensity of the UV light source.

The increasing use of UV inks in the self-adhesive industry has made a considerable difference to the types of substrates which can be printed and converted. As the ink cure operates at low temperatures (known as ‘cold cure’), one of the major benefits to the self-adhesive manufacturer is the facility to print and convert filmic and metallic substrates. 

UV CASSETTE SYSTEM

The modern UV system is far easier to operate and maintain than the early systems (See Figure 4.16). The power of the ultra violet intensity can be adjusted to suit a particular ink curing requirement and the controlled removal of the unwanted heat generated by the infra-red ensures that the web is only exposed to a minimum amount of heat. This ‘cold cure’ UV makes the printing of light filmic substrates much easier.

TONAL VALUES

One of the key advantages offered by the litho process is unrestricted tonal values… but what is meant by the term tonal values?

All graphic content is printed using a dot formation. The lighter the dot area, the lighter the color. The more dense the dot formation the darker the color. (See Figure 4.17).

This variation of dot density and size gives the various tonal values required to form the printed tones.

IDENTIFYING THE LITHO PROCESS

It is necessary to use a magnifying glass to identify what type of printing process has been used on a particular graphic reproduction.

Each process has a particular characteristic which can be easily identified. As explained earlier the printed image is made up of differing densities of dot. The closer the dot formation the denser the color and the lighter the dot formation the lighter the color.

The offset litho process does not have the problem of limited fine dot reproduction. This means that the reproduction of very fine tonal values such as subtle tones and vignettes can be produced without difficulty, as the litho process does not have the same dot break-up experienced by other printing processes.

ADVANTAGES AND DISADVANTAGES OF THE OFFSET LITHO PROCESS

Summary of the advantages and the disadvantages offered by the litho process:

Advantages

• There are no tonal restrictions. The litho process has very clean edges on very fine line-work and printed tones that do not break up, allowing an extremely fine dot to be printed.

• Offset printing delivers excellent graphic and photographic reproduction.

• CtP imaging of the printing plate gives a fast and consistent plate output.

• Offset printing is very cost effective particularly when producing high quality work.

• Very high production speeds and fast makeready can be achieved.

• The litho process is ideal for multi-process combination presses. This is a major advantage to the self-adhesive industry as it allows the printer/graphic designer to choose the print process best suited to a particular graphic result.

Disadvantages

• Damping and inking imbalance on the press can be experienced, resulting in print problems in both the non-image and image areas

• Offset litho printing equipment can be expensive compared to other print processes used in the self-adhesive industry

• Dependant on the types of inks being used, litho plates have a shorter plate life compared to other print processes and the plate life can be affected by the use of metallic inks which are very abrasive.

• The litho process cannot print the heavy film weights which can be achieved by some of the alternative printing processes. An example of this would be the very heavy ink film weights that can be printed by the silk screen process, compared to the litho process.

WATERLESS OFFSET 

THE WATERLESS PROCESS

Waterless offset, as the term implies, is an offset litho process that does not use a damping system. The pre-sensitised printing plate used does not require a dampening process and therefore can overcome some of the ink and water imbalance problems that can occur with conventional offset litho printing.(See Figure 4.18).

The rejection of the oil-based ink in the ‘non-print’ area is provided by a pre-sensitised printing plate that is coated with an ink repellent silicone rubber layer. The ‘print image’ area is slightly recessed and formed from an ink receptive polymer surface.

THE WATERLESS LITHO - PRESS CONFIGURATION

Let’s take another look at the layout of a ‘conventional’ offset printing unit. (See Figure 4.19). As can be seen this unit has a standard damping system and an inking and distribution roller train. The temperatures of the surface area of the printing plate are not critical to achieve a perfect printing result and therefore the unit does not require a sophisticated cooling facility for the inking system or the printing plate.

The waterless printing unit, (See Figure 4.19)however, does not have a damping system and is fitted with a cooling system which supplies cooled fluid via the inner section of the individual rollers used in the inking system.

Chilled air is blown onto the surface of the printing plate to ensure that the ink rollers and the surface temperature of the printing plate are controlled. If the surface temperature of the printing plate exceeds 25 degrees centigrade the siliconised rubber layer will breakdown.

This will produce a similar imperfect image to that associated with insufficient damping, which results in the ink contaminating the non-image area (referred to as scumming or catch-up).

It is critical that the cooling system for the inking rollers, which can be chilled water or chilled oil, is capable of maintaining a consistent temperature below 25 centigrade.

WATERLESS PLATE IMAGING

The waterless lithographic plate is made up of an aluminium base which is covered with a primer and a photosensitive polymer layer and finished with a siliconised rubber layer and finally a protective layer of transparent film.

The plates can be imaged with positive or negative film. The film is brought into direct contact with the printing plate and exposed to ultraviolet light, in the same way as ‘contact Imaging’ in conventional litho plate imaging.

The UV exposure fuses the silicone to the photopolymer in the non-image areas creating a siliconised non-print area that will reject the ink. The next stage is the developing process when the silicone layer is washed away from the plate in the ‘image areas’, thereby allowing the ink to adhere to the image.

Plates can achieve screen resolutions of up to 800 lines per square inch.

WATERLESS AND CONVENTIONAL LITHO - THE PROCESS DIFFERENCES 

Below is a summary of the key differences between waterless versus conventional litho:

• No damping is required with the waterless process

• The waterless plates use water-repellent silicone rubber coating, eliminating the need for a damping system.

• The silicone rubber layer in the image area is removed in the developing process.

• Special inks are required when printing with the waterless process and these inks are suitable for UV curing systems.

• The press requires a suitable cooling system to maintain a low temperature for the inking /distribution rollers and printing plate surface

ADVANTAGES AND DISADVANTAGES OF WATERLESS LITHO

Advantages

• It gives better tonal values and shadow contrasts

• There is improved color density and color consistency as a result of the absence of damping and therefore no water and ink imbalance issues

• Sharper dot reproduction

• Waterless litho gives an excellent printing result when using metallic inks

Disadvantages 

• The critical requirement for accurate temperature control of the plate surface and the roller inking system,

• The surface temperature of the printing plate must not exceed 25 centigrade.

• Requires special inks

OFFSET LITHO PRINTING - SUMMARY

One of the key advantages offered by the litho process is the unrestricted tonal values it can achieve.

It is this ability to deliver a consistently high image quality that has seen the process dominate the wet glue sector for many years. Large volumes of litho labels produced on sheet-fed presses are used to decorate containers in high-value sectors such as wines, spirits and premium beers.

Litho has developed as a significant process in the self-adhesive label sector where the benefits of the process have been used for designs that require the reproduction of subtle tones and vignettes.

The litho process is ideal for multi-process combination presses where its advantages are often used in conjunction with other printing processes.

Offset change cassettes systems allow the process to be inserted into the press with other processes. An option for converters with platform flexo presses is to use interchangeable offset cassettes in the flexo press line.

Web offset has proven particularly popular with converters making the transition from sheet-fed to roll-fed labels. Reel-fed printing offers many cost benefits over sheet-fed production, where a sheet may need to be reprocessed over and over again to obtain a result that can be achieved in a single pass with reel-fed.

TECHNOLOGY DEVELOPMENTS

The latest developments in web offset technology are making the process more attractive for shorter runs of labels and increasing in-line options available to converters.

A key development that is encouraging this trend is the use of sleeves for plate and blanket cylinders. The cost of the aluminum-based sleeve system is believed to be around a fifth of the price of an offset cassette. The sleeves are mounted on hydraulic expansion shafts, allowing fast changes between jobs. This standardised system allows print and converting technologies to be switched quickly on the offset platform.

Known as variable-size offset printing (VSOP) it is very easy to change the printing length without changing the complete offset insert. The biggest advantage of this fast and uncomplicated changeover is that it is very cost effective, especially when many different printing repeat lengths are required.

VSOP is a cost-effective alternative to conventional flexo and rotogravure printing on small-to mid-size label runs where the quality and the pre-press costs are tipping the scale in favor of offset printing.