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The science of offset printing plates

13 September 2017

The principal operational functioning of all offset presses is the same, whether single colour or multi-colour, sheetfed or web-fed, domestically manufactured or imported. The presses make one impression with each rotation of the cylinders. All offset presses have three printing cylinders (plate, blanket and impression), besides inking and dampening systems. As the plate cylinder rotates, the plate comes in contact with the dampening rollers first and then the inking rollers. The dampeners wet the plate so that the non-printing areas repel ink. The inked image is then transferred to the rubber blanket and paper is printed as it passes between the blanket and impression cylinders.

som-sapru3810-699x380 Som Nath Sapru

Besides the printing press, the most important aspect of offset printing is the plate – which metal or substance it is manufactured with and what chemical processes it has to go through before it is mounted on the plate cylinder – all these factors matter a lot for the end product — the reproduction.

Printing plates must have the ability to transfer an image to paper, cardboard or any other substrates. Printing plates are usually made from metal, plastic, rubber, paper, and other materials. The image is put on the printing plates with photochemical, photomechanical or laser-engraving processes.

It is well understood that metal printing plates are more expensive but these last longer and have greater accuracy and can be used for larger print runs.

Offset printing plates are thin (up to about 0.3mm), and easy to mount on plate cylinder, and they mostly have a mono-metal (aluminium) or, less often, multi-metal, plastic or paper construction.

Aluminium has been gaining ground for a long time among the metal-based plates over zinc and steel. The necessary graining of the aluminium surface is done mechanically either by sand-blasting, ball graining, or by wet or dry brushing. Nowadays, practically all printing plates are grained in an electrolytic process (anodising), that is, electrochemical graining with subsequent oxidation.

The imaging, ink-accepting coating (light-sensitive coating is applied to the base material. This material is usually a polymer, or copper in the case of multi-metal plates (bi-metal plates). Light sensitive, diazo (photo-polymer) pre-coated aluminium printing plates are now the predominant plates in print shops. The image transfer is produced via different properties on the surface of such plates after they have been exposed and developed. The remains of the original light-sensitive coating or the light-sensitive coating changed by the effect of light are the ink-accepting (oleophilic) elements that create the image.

Chemical changes occur as a result of the penetration of photo-effective (actinic) light (light containing UV rays), causing the light-sensitive coating to react differently depending on its type and structure. There are two types of photochemical reactions when developing the printing plate:

1. Hardening of the light-sensitive layer by light (negative plate-making)

2. Decomposition of the light-sensitive layer by light (positive plate-making)

With positive platemaking and conventional printing plate production, a positive film is used as the original, that is, the non-translucent, blackened sections of the film correspond to the ink-accepting surface elements on the plate.

In the case of negative platemaking with ‘negative plates’, a negative film is used as an original, that is, the ink-accepting image areas of the printing plate correspond to the translucent, light areas on the film. For quality assurance and monitoring during plate-making, control elements are copied onto the plates.

There are several types of lithographic plates in use today.

Diazo plates: These utilize diazo (an organic compound) coating, are pre-sensitised, and can be made either from photographic negatives or positives (but are most commonly made from negatives). After exposure to high-intensity light, an emulsion developer (comprising an acid solution containing a lacquer and a gum-etch) is added to the surface. The unexposed portion of the coating (which, when made from photographic negatives, comprises the non-image areas of the plate) is dissolved by the solution, and a treatment of gum makes these areas water-receptive. When diazo plates are made from photographic positives (in which case it is the image areas that are unexposed) special solvents are required to protect the image areas. Diazo plates can be used for print runs as large as 1,50,000. Special pre-lacquered diazo plates can be used for runs as large as 2,50,000.

Photopolymer plates: These are produced in a manner very similar to diazo plates, but the coatings used are more inert and abrasion resistant, and can consequently be used for longer print runs than is possible with diazo plates. Special thermal curing processes can give photopolymers an increased strength, allowing them to be used for print runs as high as 1,000,000.

Silver halide plates: These utilise light-sensitive coatings similar to, although slower than, those used on photographic films. Silver halide plates can be exposed either photographically or by lasers guided by computer data. They are commonly used to print single-colour documents from digital artwork. Silver halide emulsions can be used to coat anodised aluminium and these are often used for colour printing produced from digital artwork.

Electrophotographic plates: These, like the principles of electrostatic printing found in many photocopiers, use a photoconductor (either mounted on a drum or on the substrate) that is sensitised to light utilising a corona discharge which imparts a charge to the photoconductor. Upon exposure to light in the non-image areas, the charge is dissipated, it remaining in the image areas, which then attracts an oppositely-charged toner. A drum-type photoconductor uses a selenium or cadmium sulphide cylinder which receives the image, which is then transferred to paper or other substrate, which can then be used as a printing plate. In another type, the photoconductor is a coating on the substrate itself, the toned image being formed directly on the substrate. Such plates then need to be desensitised in a manner akin to other photolithographic plates.

Bimetal plates: These are plates that are useful for long print runs. Copper, which is highly oleophilic, is electroplated on a hydrophilic metal, such as aluminium or stainless steel. Negative-working platemaking exposes the image areas to light, hardening the coating on top of the copper. The unexposed coating is then dissolved away, baring the copper, which is in turn removed chemically, baring the second metal beneath it. Some plates use a tertiary metal, which serves to merely support the other two and plays little role in the actual printing surface of the plate. Of primary advantage to these plates is their virtual indestructibility.

Ablation plates: These are primarily used in computer-to-plate systems, and consist of digital information controlling laser diodes which burn small holes in a coating applied to the surface of a polyester or metallic plate. Of primary advantage is the lack of chemical processing, which allows them to be mounted and imaged directly on the press.

Heat-sensitive plates: These plates utilise heat (in the form of infrared radiation) rather than light as the means of exposing the image vs. non-image areas. Their coatings typically consist of heat-sensitive polymers, etched by infrared laser diodes. With post-exposure curing processes, these plates can be made capable of print runs in excess of 1,000,000.

Hybrid plates: These utilise two separate coatings, a conventional photopolymer as the bottom layer, a silver halide coating providing the top layer. As the top layer is exposed — either with lasers or conventional, high-intensity light — the bottom layer is exposed by ultraviolet light. Removal of the upper layer and chemical processing of the bottom layer produces the image and non-image areas of the plate, printing being done from the bottom layer.

Of increasing importance and use today is the use of computer-to-plate systems. Normally commercial printers go for thermal plates and the printers who are involved with newspaper printing or large print-runs of textbook printing do use violet CTP plates. Undoubtedly, diode developments are definitely going to cut down on usage of thermal CTP plates, reason being the application and output of quality of reproduction difference in both the thermal and violet will be very negligible.

Talking to a cross-section of plate vendors, I gathered that until FY 2015-16, sales of platesetters was in the range of 1,100 to 1,500 units which is quite encouraging. After all what convinced printers to installations – it is the speed, enhancement of reproduction quality and remarkable saving on production time. All these elements are a must for newspaper printer, textbook printer and the printer of packaging products, yet another factor is the reduction in price range by about 5.7% by the end of 2015. This sharp dip in prices encouraged most of the newspaper establishments to go for CTP installations. In a nutshell, use of CTP technology and plates has reduced the cost of plate preparation with enhanced speed of plate processing.

There are number of plate manufacturers and suppliers in India who are manufacturing plates of international standard for all varieties of offset printing requirements. Few of them are Asian Reprographics, Ronsein Printing Plates, Ronak Printers, Technova, Superlucky Union, Orion Photosensitive Systems, Image Solutions, Innovative Flexotech, AM Graphics, Superprint, Independent Business Machines, etc. Out of all these, Technova is the world’s only manufacturer to manufacture full range of offset plates — violet, thermal, analog PS, UV CTP, inkjet and laser. Besides, it also offers the world’s widest range of pre-sensitised plates with total performance guarantee and lowest cost in-use.

While talking to end-users of Technova products, I gather that plate performance, cost-effectiveness and unsurpassed technical support and advice have enabled Technova to enjoy a market share of over 70% in the Indian sub-continent besides profitable shares in several export markets.

I understand from confirmed sources that Innovative Flexotech has sold 32.5 lakh units of PS plates until 2015 as against the actual usage 24.5 lakh units of positive thermal plates during that period.

All these technical innovations have contributed to superior technical performance of the reproductions which has attracted several overseas publishers to execute their mass publication activities by outsourcing with several Indian printers across the country.   

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