Prepress 3 printing processes

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Printing processes

Printing processes Lithography The Lithographic printing process today continues to hold a tight grip on the overall market share, specifically in relation to general printing.

However the process has its own limitations. These limitations are mainly due to the range of substrates and inks it is able to handle.

Lithographic printing is often referred to as offset printing. A German, by the name of Aloys Senefelder, established this process of printing at the end of the 18th century. He experimented with the principle that oil (ink) and water do not mix, and found that a wet limestone surface would repel an oil-based printing ink. This process was called ‘chemical printing’ by Senefelder, and became very popular as an art medium because it enabled artists to reproduce multiple copies of their original freehand drawings.

Over the past 50 or so years, major improvements in the process have been developed. One of the differences is the material used to form the image carrier (plate). Today, instead of using limestone as the image carrier, the most common material is aluminium, however other materials are also used including plastic and paper.

The image carrier is referred to as ‘planographic’, in that the image and non-image areas are on the one plane (surface).

The non-image area of the plate is termed ‘hydrophilic’, meaning ‘waterloving’, and the image is termed ‘hydrophobic’, meaning ‘non-water receptive’. In this lithographic process, an oil-based ink and a waterbased fountain solution is applied to the printing plate.The fountain solution wets the non-image area of the plate, while the ink wets the image areas.

The transfer of ink to the paper is via the offset process. This process transfers the ink from the image carrier (plate cylinder) which is right reading, to a blanket cylinder (wrong reading), which has a rubber (or in most cases, a synthetic rubber) blanket material. The blanket then transfers the image to the substrate (right reading).

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Printing processes

The lithographic printing plate is the least expensive image carrier to produce, and this, combined with fast running speeds and quick makeready, has given rise to the popularity of this process.

The printed products produced by this process include: magazines, newspapers, bookwork, brochures, greeting cards, packaging products, catalogues, letterheads, posters, billboard signs, continuous stationery, plus other specialised products.

The substrates that are suitable for printing by this method include: paper, board, plastic, soft metals i.e. aluminium, tin plate etc..

Technology advancements in the past 20 years has seen an increase in computer controlled operations that have eliminated some of the manual skills in the operation of the press. Those advancements include automatic plate installation and removal, computer controlled inking and registration devices, as well as colour and density control devices.

There are two subprocesses in lithographic printing. They are: • Sheetfed offset • Web offset (heatset & coldset)

Lithographic sheetfed press This type of press is mainly used in the production of short-run and medium run, high quality and tight register work. It has a market in the production of commercial and packaging products. Other common products produced by this method include periodicals, business forms, books and pamphlets, and they can be produced at relatively fast speeds.

Sheetfed presses come in small (called small offset) and large formats.

The type of jobs to be produced determines which format press is required. Small offset presses can be cost efficient and easy to operate.

Large format offset machines produce higher quality products and at faster speeds.

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Graphic Prepress

Printing processes

Two colour litho sheetfed printing press photo courtesy of Heidelberg Australia

The sheetfed offset printing press configuration includes an image carrying cylinder (plate cylinder), blanket cylinder, and an impression cylinder. With the aid of an inking and dampening system, it forms a complete printing unit. When 2 or more units are added inline, they enable the production of multi-colour printing.

A constraint to sheetfed printing is that it is not naturally a perfecting (printing on both sides at one pass through the machine) press, however some presses have been modified to enable this process. With the aid of computerisation, the operation and makeready procedures have been greatly reduces in recent times, resulting in reduced operation costs.

Lithographic web offset press Different size web offset presses are used for different production requirements. As opposed to a sheetfed press, these press configurations print on a continuous stream of paper called the ‘web’.The printing requirements range from large magazine formats to small continuous stationery products. In recent years, technical advances have put the web offset press in line with the quality produced by sheetfed offset presses.

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Printing processes

There are 3 different press configurations utilised in this form of printing: • Blanket to blanket • Satellite • In-line The blanket to blanket configuration is the most common, and is used for most commercial products printed by web offset. Each printing unit on a blanket to blanket press has 2 blanket cylinders, and at the same time, printing on both sides of the substrate. Each blanket cylinder acts as the other’s impression cylinder.

The satellite Blanket to blanket web offset printing unit presses have a common impression cylinder (CIC) for a series of printing units. These units, comprised of a plate cylinder, blanket cylinder, inking and dampening systems, are grouped around the impression cylinder, which ensures that tight register can be maintained.

The in-line configuration encompasses a similar cylinder configuration to the common sheetfed press, however this form of printing prints on a continuous web. The main types of product to come off an in-line press are business forms or continuous stationery. Its market is rather narrow, and it has seen a gradual decline in recent years due to the amount of stationery and invoices produced in-house by businesses..

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Printing processes

Letterpress Letterpress printing is one of the oldest form of printing. Its origin dates back many hundreds of years and further developed by a German named Gutenberg. This process of printing has experienced a major decline in use, from being the dominant process, to now specialising in products such as business cards, wedding stationery etc. However in recent times, letterpress has gained popularity in the area of narrow web label printing.

This process of printing is known as ‘relief’, in that on the image carrier, the image is raised above the non-image area.

Letterpress prints from a relief or raised image plate

The three types of letterpress presses in use today are: • Platen • Flat-bed • Rotary

The platen press The platen press has been in operation since Gutenberg’s time. It is made up of 2 flat surfaces called ‘the bed’ and ‘platen’. The components that make up the image carrier are locked up in a steel frame called a chase.

This is placed on the bed and locked into place. The platen provides a smooth backing for the paper. Once the image carrier is inked, the platen gives impression to the substrate. These presses are commonly used for the production of letterheads, forms, flyers, as well as providing a means of embossing, imprinting, cutting and creasing and hot-foil stamping.

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The flat-bed press This type of letterpress printing uses either vertical or horizontal beds.They operate in a manner similar to the platen press, however this press enables the printing of a larger size substrate. This press incorporates the use of an impression cylinder rather than a platen. The substrate passes around the impression cylinder from the feeder on its way to the delivery. Cutting and creasing has become a popular operation on this style of press.

The rotary press Rotary letterpress presses are increasing in use. They can either be webfed or sheet-fed configurations. As is the standard in all rotary presses, they have a curved image carrier, and the most common type of image carriers are wrap around photopolymer plates.

The rotary web-fed letterpress presses are still used for the production of some newspapers, however in recent times, newspaper production has been dominated by the lithographic process. The rotary presses can print on both sides of the substrate as it passes through the machine.

The market place for letterpress printing includes long-run commercial printing, packaging, labelling, as well as book and magazine work.

A growth area for letterpress is narrow web label work. It is very popular for this classification of work and is capable of producing quality labels at high speed, with less wastage than lithography.

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Printing processes

Flexography This process of printing encompasses the same method of producing an image onto the substrate as does letterpress printing, in that both processes utilise the ‘relief’ method of printing. They both use an image carrier with a raised image, however the similarities end there.

Flexography uses solvent and water-based inks which have virtually no rub-off, set-off, or show through (being able to see a printed image on the underside of the substrate). It also has similarities to gravure printing, in that it uses low viscosity and fast drying inks.

Flexography has a reputation for being a low-cost, environmentally friendly process. It has in recent years become an alternative to other more popular processes of printing and has reached into newer markets.

A major advantage of this process is its ability to print on different substrates with acceptable quality at low cost. Relative short runs in Australia make this process more attractive than gravure.

The ever increasing market for flexography is evident in the wide range of products it produces: • newspapers and inserts • catalogues • flexible packaging • pressure sensitive labels • corrugated boxes • folding cartons • other specialised products.

Flexography in general uses an anilox inking system. This system uses a metallic or ceramic roller, and this roller is engraved with cells of a fixed volume. Only the required volume of ink is transferred to the image carrier. The anilox cylinder has a scraper (doctor) blade that removes any excess ink from the anilox cylinder and returns it to the ink fountain.

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Printing processes

The image is then transferred directly from the image carrier (plate) onto the surface of the substrate.

The printing units in use today consist of three basic types: the two roll unit, the two roll unit with a doctor blade (as mentioned in the previous paragraph), and the dual doctor ink chamber system. The two roll units are usually found on older flexographic presses, and on narrow web presses. Narrow web presses equipped for process colours often use the two roll unit with a doctor blade, and more modern wide web presses use the dual doctor ink chamber system.

The substrates that are suitable for printing by this method are: • paper • board • plastic • foils The image carrier is comparatively inexpensive and durable compared to other processes, and coupled with a simple printing technique of only utilising a two roller system to distribute the ink onto the image carrying cylinder, which allows for a cost effective and easy makeready and washup.

There are numerous types of presses designed for this process of printing, however there are three that seem to dominate: • Stack press • Common impression press • In-line press The Stack Press The stack press is mostly used for printing on paper of all grades and is less suitable to print on film. It is characterised by having one or more stacks (printing stations) arranged vertically on either side of the press frame. Each stack has its own image carrying (plate) cylinder which prints one colour of a multicolour impression. They are easy to setup and can print on both sides of the web at once.

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One disadvantage of this configuration of press is its poor control of registration.

Running materials such as paper, laminated film structures and others, which can tolerate fairly heavy tension values, the stack press can profitably produce commercially acceptable registered products.

The Central Impression Press (CI) The common impression press consists of a large cylinder around which are arranged either four, six or eight printing units. The cylinder is made of steel The substrate is supported by the cylinder, which greatly improves colour register, especially with materials that stretch easily, like thin plastic substrates. The substrate is held in place against the impression cylinder, and the colour is applied from printing stations, or decks around the cylinder.

Some advantages in this configuration is its high speed as well as a shorter makeready time compared to other flexo press configurations.

The In-line press On an in-line press configuration the colour stations are separate. The complete units are horizontally mounted next to one another, driven by common line shaft. These presses are used for the printing of bags, corrugated board, folding boxes.

In-line presses are also common in narrow web width for printing pressure sensitive labels, tickets etc. This design is compatible for specialised areas where the product design requires a short-run.

In summary, there are two main reasons why the process of flexography is fast growing in its acceptance as a major process in the printing industry. Firstly it is a relatively simple operation, and secondly it is easily adapted to the use of water-based inks, which is environmentally welcome.

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Printing processes

Gravure Gravure has a large market share in Europe and North America due to its long print run capabilities. In Australia, gravure is more restricted to large run packaging and specialised print products. Gravure presses have the ability to produce products such as: • magazines • catalogues • labels • packaging • folding cartons i.e. cigarette packets • wallpaper.

It uses an ‘intaglio’ process, where the image carrier (which is usually a chrome plated, copper cylinder) has the image etched below the surface of the cylinder. These image areas are usually ‘screened’ forming thousands of tiny cells in the surface of the cylinder.

During the printing process, the image carrier (cylinder) is immersed in an extremely fluid ink, and as it rotates the ink is picked up by those tiny cells, however the surface of the cylinder is also completely covered. By the use of a doctor blade, the unwanted ink on the surface of the cylinder is wiped away, leaving the recessed cells full of ink.

The transfer of ink to the printing substrate happens when the image carrier comes in contact with the substrate with the aid of an impression cylinder. At the point of contact, the ink is drawn out of the cells by capillary action. The deeper the cell, the more ink that will be held in that particular cell and the stronger the colour will be in that spot.

Gravure ink cells may vary in size, shape and depth

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Printing processes

The substrates that are suitable for printing by this method include: • paper • boards • foils • plastic Extremely long print runs are achievable as chrome-plated copper cylinders can last for millions of impressions. At the high quality end of the print market, it can be capable of fine colour reproductions with a near continuous tone effect.

Compared to web offset printing, gravure printing cylinders are capable of carrying more ink to the paper. They can successfully print on lightweight paper because less tension is required on the substrate. The gravure process results in less ink tack and less moisture than lithography.

Today, most of the gravure cylinders are engraved directly from digital files. In the past, chemical etching was the technology used for the gravure cylinder imaging, however it represents a very small percentage of the total engravings done today.

Gravure printed products are produced on the widest and fastest presses available for any printing process. Newer type presses can be as wide as three and one half metres.

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Printing processes

Screen printing The basic concept of screen printing as a stencilling process may go back several thousand years. As a commercial printing process, the main developments have occurred since 1930.

The most common method of screen printing is to use a flat, stretched, woven mesh which is held on a frame. The printing area remains open mesh, while the non-printing areas are covered with either a stencil or a form of screen filler.

The ink or other variable printing substance is Screen printing process pushed through the mesh openings with a squeegee. The image or printed substrate is then dried.

Ink, stencil, squeegee and mesh components may all vary, according to the specific printing substrate. These substrates may include: • paper, board • textiles • pressure sensitive stickers • ceramics and glass • plastics, both flat and cylindrical • wood, metal • specialty items i.e. circuit boards, membrane switches.

Screen printing can vary the ink deposit and provide light fastness and acid and chemical resistance.

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Graphic Prepress

Printing processes

Pad printing Pad printing was developed over 500 years ago, but was not widely used until technological breakthroughs were made in the 1960s.

Pad printing is an indirect gravure printing process. The copy or image is etched into a steel plate called a cliche. The cliche is inked and then cleaned with a doctor blade. This leaves the ink in the etched areas only.

A flexible silicone pad picks up the image from the cliche and then transfers the image to the product.

Pad Printing can be applied to virtually any surface including flat, cylindrical or odd shaped objects, using either one or more printed colours.This system is adaptable to a variety of shapes and contours on parts with a variation in the surface, yet fine detail and precise registration can be achieved.

Pad printing is ideal for decorating toys, plastic housewares, and injected mould components. The technology also lends itself to marking electronic components such as resistors, canisters, and connectors. Any part, plastic, ceramic, or metal can be production marked by this process. The equipment can be semi or fully automated, yet remains a low capital expenditure.

The process of pad printing is used most often to print unevenly shaped objects. The pad printer utilises a rubber-like pad that picks up the ink image, comes forward and down, pressing firmly and conforming to the shape of the object and leaving the ink behind. An example of an object that can only be printed by this method is a golfball due to the dimpled surface. It can also be used to print personalised logos for Promotional Items, Premiums & Incentives, Business Gifts, Corporate Gifts, Athletic Equipment, Glassware, Industrial Parts and Manufactured Goods.

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Printing processes

Digital printing Digital printing systems can be identified in a number of different ways but they all have one thing in common — they are either directly connected to an external computer or computer network through a RIP (Raster Image Processor) or they have a computer and RIP built in as an internal integral part of their production system. The systems then divide up into processes such as electrostatic or other image creation means; involving sheet fed or reel fed substrates; mono colour or CMYK colour; toner based or liquid ink or printing ink based; and continuous tone or half tone output.

True digital printing systems The ‘true’ digital printing system relies on the definition that each image it creates and prints in sequence can be totally different to the one before in pictorial and text form (i.e. it is personalised) at production speeds that exceed 1000 images per hour. The same equipment is also capable of producing multiple copies of the same pictorial and text images at the same production speeds as above.

Hybrid digital printing systems A ‘hybrid’ digital printing system relies on a plate being produced as an integral internal function of the equipment, using specific digital computer to plate technology. The platemaking process is not external to the printing system. A new plate or set of plates is produced each time a job is required, from the plate material stored in the equipment printing function. The hybrid system can only produce multiple copies of the same pictorial and text images but at production speeds close to those obtained from a similar size offset press. It cannot produce individual personalised images. Thermal ablation plates, used for on-press imaging, require no chemical treatment after exposing.

Multicolour hybrid digital sheetfed printing press photo courtesy of Heidelberg Australia

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Printing processes

Review questions for printing processes Q1 Describe and compare the image and non image areas on the plate or cylinder for lithography, flexography and gravure. ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................ ............................................................................................................................

Q2 State an advantage lithography has over all the other printing processes. ............................................................................................................................ ............................................................................................................................

Q3 State an advantage flexography has over all the other printing processes. ............................................................................................................................ ............................................................................................................................

Q4 State an advantage gravure has over all the other printing processes. ............................................................................................................................ ............................................................................................................................

Q5 State an advantage screen printing has over all the other printing processes. ............................................................................................................................ ............................................................................................................................

Q6 State an advantage digital printing has over all the other printing processes. ............................................................................................................................ ............................................................................................................................

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