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Printing and publishing-Maseno university

Posted on: November 7, 2008

PRINTING METHODS/PROCESSES

 

Definitions

 

Printing refers to the industrial process for reproducing texts and images with ink on paper or other suitable material using a printing press. The printing process involves: Prepress- phase which encompasses typesetting, design and layout, page proofs, colour separation and filming. Press process phase which involves plate making, printing, binding and finishing.

 

Page proof refers to the final sample of a page, reproduced by a commercial printer for a customer’s approval. Corrections or changes made after the page-proof’s approval are expensive and time consuming.

 

Printing press is a mechanical printing device for making copies of identical text on multiple sheets of paper. The printing press was invented by Johannes Guttenberg in 1445. Guttenberg also invented the first oil –based printing ink.

 

Printing methods

 

1. Lettepress

This was the major printing method until the 1970s when it was takeover by offset lithography and is essentially redundant. It is similar to the way a rubber stamp works. Ink is applied to a raised/relief surface and transferred to overprinting is done from a metal/plastic printing plate, usually a rotary press. Rotary press is a printing press in which the plate is wrapped around a cylinder. There are two types, direct and indirect. Direct presses print with a plate cylinder and an impression cylinder. Indirect rotary presses (sheet-fed offset presses) combine a plate cylinder, a blanket cylinder and an impression cylinder.

 

Rotary press is designed for high-speed work and for economy for long press runs. Both the printing paper and the printing surface are on cylinders. The printing press is curved so that it will fit the cylinder. Printing takes place as the two cylinders strike against each other. To reproduce the letterpress plate, the negative of the photographed image is laid on top of a sensitized plastic or copper plate and exposed to light. Since everything on negative is in reverse, the image areas on the negative are transparent. This allows light to pass through the negative to the plate which has been treated with a light sensitive emulsion.

 

The emulsion hardens in the areas exposed to the light and thereby forms an acid resistant protective covering over the image area of the plate. The plate is then placed in an acid bath; this etches away the no-design areas, leaving the desired printing image raised on the surface of the plate. The result is the line plate.

 

Advantages

Disadvantages

-Denseness of the ink (not diluted by water or spirits as in offset or gravure printing methods)

– Quality of impression; excellent characteristic for high quality private presswork

-Relatively slow

-High cost of metal type and blocks

-Superseded technology

-The fact that more expensive papers are required to achieve the kind of quality for offset lithography

 

 

2.  Rotogravure/gravure

 

Gravure is an “intaglio” process meaning that the printing image is recessed into the plate or cylinder rather than being flat as in letterpress or lithography. The image to be printed has to be etched or incised into the printing cylinder. 

 

Ink is applied by a roller and thin, flexible steel blade (doctor blade) is drawn across the cylinder, removing any excess ink from the non-printing areas. The paper is then positioned over the cylinder and pressed against it by a rubber-coated roller. The pressure forces the paper into the recesses of the cylinder so that it picks up the ink, thus forming the image. The finished print is then removed.

 

Most gravure is done using web-feed machines which use reels of paper and fold the printed paper. Web-feed gravure predominates where runs are long (300, 000 copies and above), e.g. weekly magazines, mail order catalogues and colour supplements. It is also used for some kinds of packaging, printing on cellophane, decorative laminates and wallpaper.

 

Advantages

Disadvantages

·         Simple printing method (after printing surface has been made)

·         Can maintain consistent colour

·         High speed

·         Straightforward drying by evaporation

·         No fixed cut-off as with web-offset

·         Good results obtainable on cheaper paper

·         High cost of cylinders

·         Viable only for long runs

·         Longer lead times than offset lithography

·         High cost of proofs; if press proofs are needed

·         High cost of corrections for reprints as cylinder must be replaced.

 

 

3. Offset lithography

Predominant process of printing but may be replaced by digital printing. Basic process of litho was invented by Senefelder in Bavaria in 1798, but offset principle was applied in the early 20th century when litho began to be used for commercial printing. It is used to print a wide range of items ranging from letterheads to packaging, books and magazines.

 

It is a planographic process as the printing surface is flat rather than raised as in letterpress or recessed as in gravure. The area to be printed is treated chemically so that it accepts grease (ink) and rejects water, while the non-image or background area is treated to accept water and reject grease. The whole surface has both water and ink (with addition of alcohol to aid dispersion) applied to it. When the plate is pressed against the surface of the paper, only the image area is printed.

 

Offset litho uses the principle of planographic printing. It is called offset because the image is printed first on a rubber blanket and the rubber blanket then prints (offsets) it onto the paper. Why the blanket is used?  It is used so as to prevent the delicate lithographic plate from coming into contact with more abrasive paper surface that could cause significant wear and tear on the plate during the run.

As with letterpress, the offset press must perform the operations of feeding, inking, printing and delivery of paper but in addition water (with alcohol) has to be applied to the plate by a dampening unit.

 

Sheet-fed offset presses range in size from small offset presses that can print on paper up to 8.5 by 11 inches to high machines that can print sheets 40 by 60 inches. They can print single colour on only one side of the sheet or multiple units enabling them to print up to six colours on both sides of the sheet in one pass through the press. (Printing both sides of the sheet is called “perfecting”).

 

Web-offset presses print onto reels (rolls) rather than sheets of paper. They are very fast, the fold paper as they print and are used to print magazines, brochure, mail order catalogues, books and long run promotional materials and newspapers.

 

Advantages

Disadvantages

·         Cheap printing surface

·         Fast make ready

·         Good reproduction of detail and photos

·         Rubber blanket enables use of a wide range of papers

·         Colour variation due to problems with ink/water balance

·         Dampening can cause paper scratch pr warping after binding

·         Dense ink coverage difficult to achieve

·         Fixed cut-off web-offset restricts available sizes

 

4. Screen printing/serigraphy

Screen printing, also known as serigraphy, is a method of creating an image on paper, fabric or some other object by pressing ink through a screen with areas blocked off by a stencil. The technique is used both for making fine art prints and for commercial applications, such as printing a company’s logo on coffee mugs or t-shirts.

Screen printing is a printing technique that uses a woven mesh to support an ink blocking stencil. The attached stencil forms open areas of mesh that transfer ink as a sharp-edged image onto a substrate. A roller or squeegee is moved across the screen stencil forcing or pumping ink past the threads of the woven mesh in the open areas.

 

A stencil cut by hand or made digitally or photographically is supported on a screen of synthetic fibre (e.g. mylum or polyester) metal. Originally screen was made of silk hence the term silk screen printing. The screen is stretched tightly over a frame of wood or metal and ink is spread across the screen by means of a rubber squeegee that squeezes the ink through the screen in the image areas.

 

The stencil prevents ink going through in the non-image areas. It is ideal for posters, plastic and metal signs, t-shirts, CDs, DVDs, simulated –wood car dashboards and door trims, bottles, electronic circuits etc.

A screen is made of a piece of porous, finely woven fabric called mesh stretched over a frame of aluminum or wood. Originally human hair then silk was woven into screen mesh, currently most mesh is made of man made materials such as steel, nylon, and polyester. Areas of the screen are blocked off with a non-permeable material to form a stencil, which is a negative of the image to be printed; that is, the open spaces are where the ink will appear.

The screen is placed atop a substrate such as papyrus or fabric. Ink is placed on top of the screen, and a fill bar (also known as a flood bar) is used to fill the mesh openings with ink. The operator begins with the fill bar at the rear of the screen and behind a reservoir of ink. The operator lifts the screen to prevent contact with the substrate and then using a slight amount of downward force pulls the fill bar to the front of the screen. This effectively fills the mesh openings with ink and moves the ink reservoir to the front of the screen. The operator then uses a squeegee (rubber blade) to move the mesh down to the substrate and pushes the squeegee to the rear of the screen. The ink that is in the mesh opening is pumped or squeezed by capillary action to the substrate in a controlled and prescribed amount, i.e. the wet ink deposit is equal to the thickness of the mesh and or stencil. As the squeegee moves toward the rear of the screen the tension of the mesh pulls the mesh up away from the substrate (called snap-off) leaving the ink upon the substrate surface.

There are three types of screen printing presses. The ‘flat-bed’ (probably the most widely used), ‘cylinder’, and ‘rotary’.

Textile items are printed in multi-color designs using a wet on wet technique, while graphic items are allowed to dry between colors that are then printed with another screen and often in a different color.

The screen can be re-used after cleaning. However if the design is no longer needed, then the screen can be “reclaimed”, that is cleared of all emulsion and used again. The reclaiming process involves removing the ink from the screen then spraying on stencil remover to remove all emulsion. Stencil removers come in the form of liquids, gels, or powders. The powdered types have to be mixed with water before use, and so can be considered to belong to the liquid category. After applying the stencil remover the emulsion must be washed out using a pressure washer.

Most screens are ready for recoating at this stage, but sometimes screens will have to undergo a further step in the reclaiming process called dehazing. This additional step removes haze or “ghost images” left behind in the screen once the emulsion has been removed. Ghost images tend to faintly outline the open areas of previous stencils, hence the name. They are the result of ink residue trapped in the mesh, often in the knuckles of the mesh, those points where threads overlap. [15]

While the public thinks of garments in conjunction with screen printing, the technique is used on tens of thousands of items, decals, clock and watch faces, and many more products. The technique has even been adapted for more advanced uses, such as laying down conductors and resistors in multi-layer circuits using thin ceramic layers as the substrate.

There are several ways to create a stencil for screen printing. An early method was to create it by hand in the desired shape, either by cutting the design from a non-porous material and attaching it to the bottom of the screen, or by painting a negative image directly on the screen with a filler material which became impermeable when it dried. For a more painterly technique, the artist would choose to paint the image with drawing fluid, wait for the image to dry, and then coat the entire screen with screen filler. After the filler had dried, water was used to spray out the screen, and only the areas that were painted by the drawing fluid would wash away, leaving a stencil around it. This process enabled the artist to incorporate their hand into the process, to stay true to their drawing.

 

A method that has increased in popularity over the past 70 years and is tremendously popular is the photo emulsion technique:

  1. The original image is created on a transparent overlay such as acetate or tracing paper. The image may be drawn or painted directly on the overlay, photocopied, or printed with a laser printer, as long as the areas to be inked are opaque. A black-and-white negative may also be used (projected on to the screen). However, unlike traditional plate making, these screens are normally exposed by using film positives.
  2. The overlay is placed over the emulsion-coated screen, and then exposed with an ultraviolet light source in the 350-420 Nanometer spectrums. Other light sources do not work well. The UV light passes through the clear areas and creates a polymerization (hardening) of the emulsion.
  3. The screen is washed off thoroughly. The areas of emulsion that were not exposed to light dissolve and wash away, leaving a negative stencil of the image on the mesh.

Photographic screens can reproduce images with a high level of detail, and can be reused for tens of thousands of copies. The ease of producing transparent overlays from any black-and-white image makes this the most convenient method for artists who are not familiar with other printmaking techniques. Artists can obtain screens, frames, emulsion, and lights separately; there are also preassembled kits, which are especially popular for printing small items such as greeting cards.

 

Advantages

Disadvantages

·         Can print a heavy film of ink

·         Economical for short runs such as 100 copies

·         Can  print on virtually any material

·         Very long screen halftones

·         Low output quality

·         Drying requirements

·         Difficult to achieve fine detail

 

 

5. Digital printing

This method is ideal for shorter runs of colour and b & w work. Unlike most other printing processes, the method does not require film or a plate to be made. Instead, it takes a file (PDF, or other suitable file format) and transfers the image digitally to the printing device.

 

Digital printing is the reproduction of digital images on a physical surface. It is generally used for low quantity print runs, and for the customization of print media. When used correctly digital printing can greatly impact an overall communication campaign.

It can be differentiated from litho, flexography, gravure or letterpress printing in many ways, some of which are;

  • Every printed impression made onto the paper can be different, as opposed to making several hundred or thousand impressions of the same image from one set of printing plates, as in traditional methods.
  • It requires less waste in terms of chemicals used and paper wasted in set up (bringing the image “up to colour” and checking registration or position).
  • The Ink or Toner does not absorb into the substrate, as does conventional ink, but forms a thin layer on the surface and may in some systems be additionally adhered to the substrate by using a fuser fluid with heat process (toner) or UV curing process (ink).

Some of the methods for digital printing include: Laser printing, Ink-jet printing and Magnetography. The first two techniques have already been discussed in your IT subjects. Magnetography: In this method, the printing drum has a magnetic coating. The image is created on the drum by energizing tiny electromagnets and is then developed by exposure to magnetic toner particles before being transferred to the paper.

 

Advantages

Disadvantages

·         Economical for short runs (500 or less)

·         Enables personalization of data

·         No film or plate

·         Shorter lead time

·         Large formats possible with ink-jet printing

·         Quality on earlier machines was inferior to offset but latest presses can match offset quality

·         Consumables (toner/paper) more costly

·         Slower press speed

·         Most presses wont print special colours

·         Currently only available for small sheet and reel sizes

 

Other printing methods include: Flexography, Collotype, Duplicating, Die stamping and copper engraving, Thermography and Lenticular printing

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