Wednesday, 10 October 2012

Print Production Seminar//notes

Printing Processes

LITHOGRAPHY (LITHO)
aluminium plates (etched) reversed onto a roller then reversed onto paper
the image can be printed directly from the plate or it can be offset
high-volume lithography is used to produce posters, maps, books, newspapers, and packaging—just about any smooth, mass-produced item with print and graphics on it 
most books, indeed all types of high-volume text, are now printed using offset lithography




The modern lithographic process


For offset lithography, which depends on photographic processes, flexible aluminum, polyester, mylar or paper printing plates are used instead of stone tablets. Modern printing plates have a brushed or roughened texture and are covered with a photosensitive emulsion. A photographic negative of the desired image is placed in contact with the emulsion and the plate is exposed to ultraviolet light. After development, the emulsion shows a reverse of the negative image, which is thus a duplicate of the original (positive) image. The image on the plate emulsion can also be created by direct laser imaging in a CTP (Computer-To-Plate) device known as a platesetter. The positive image is the emulsion that remains after imaging. Non-image portions of the emulsion have traditionally been removed by a chemical process, though in recent times plates have come available which do not require such processing.High-volume lithography is used presently to produce posters, maps, books, newspapers, and packaging—just about any smooth, mass-produced item with print and graphics on it. Most books, indeed all types of high-volume text, are now printed using offset lithography.

If this image were transferred directly to paper, it would create a mirror-type image and the paper would become too wet. Instead, the plate rolls against a cylinder covered with a rubber 
blanket, which squeezes away the water, picks up the ink and transfers it to the paper with uniform pressure. The paper passes between the blanket cylinder and a counter-pressure or impression cylinder and the image is transferred to the paper. Because the image is first transferred, oroffset to the rubber blanket cylinder, this reproduction method is known as offset lithography or offset printing. The plate is affixed to a cylinder on a printing press. Dampening rollers apply water, which covers the blank portions of the plate but is repelled by the emulsion of the image area. Hydrophobic ink, which is repelled by the water and only adheres to the emulsion of the image area, is then applied by the inking rollers.



ROTOGRAVURE (ROTO)
copper plates (mirror image) ink is transferred directly onto the print surface
involves engraving the image onto an image carrier
good for longer print runs
used for high volume, durable products and commercial printing such as magazines, postcards and corrugated product packaging
this process can also used for bank notes



Rotogravure (Roto or Gravure for short) is a type of intaglio printing process; that is, it involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a cylinder because, like offset printing and flexography, it uses a rotary printing press. Once a staple of newspaper photo features, the rotogravure process is still used for commercial printing of magazines, postcards, and corrugated (cardboard) product packaging.

PROCESS:

In direct image carriers such as gravure cylinders, the ink is applied directly to the cylinder and from the cylinder it is transferred to the substrate
While the press is in operation, the engraved cylinder is partially immersed in the ink fountain, filling the recessed cells. As the cylinder rotates, it draws ink out of the fountain with it. Acting as a squeegee, the doctor blade scrapes the cylinder before it makes contact with the paper, removing excess ink from the non-printing (non-recessed) areas and leaving in the cells the right amount of ink required: this tool is located quite close to the paper so that the ink left in the cells does not have enough time to dry. Next, the paper gets sandwiched between the impression roller and the gravure cylinder: this is where the ink gets transferred from the recessed cells to the paper. The purpose of the impression roller is to apply force, pressing the paper onto the gravure cylinder, ensuring even and maximum coverage of the ink. The capillary action of the substrate and the pressure from impression rollers force the ink out of the cell cavity and transfer it to the substrate (Figure 1). Then the paper goes through a dryer because it must be completely dry before going through the next color unit and absorbing another coat of ink.

Advantages

The rotogravure printing process is the most popular printing process used in flexible-packaging manufacturing, because of its ability to print on thin film such as polyester, OPP, nylon, and PE, which come in thicknesses of 10 to 30 micrometers.
Other appreciated features include:
  • printing cylinders that can last through large-volume runs without getting worn out
  • good quality image reproduction
  • low per-unit costs running high volume production

[edit]Disadvantages

Shortcomings of the gravure printing process include:
  • high start-up costs: hundreds of thousands of copies needed to make it profitable
  • rasterized lines and texts
  • use of chemicals in the ink.



FLEXOGRAPHY (FLEXO)
similar to litho, but can enable more mirror detail
uses a flexible relief plate 
high volume, labels, carrier bags...
good quality print but not stick
can fold and glue as well





Flexography is a form of printing process which utilizes a flexible relief plate. It is essentially a modern version of letterpress which can be used for printing on almost any type of substrate, including plastic, metallic films, cellophane, and paper. It is widely used for printing on the non-porous substrates required for various types of food packaging (it is also well suited for printing large areas of solid colour).




PAD PRINTING
print process that can transfer a 2D image onto a 3D object
e.g. a printed golf ball
used for printing on otherwise impossible products including medical, automotive, promotional, apparel, and electronic objects, as well as appliances, sports equipment and toys 




Pad printing is a printing process that can transfer a 2-D image onto a 3-D object. This is accomplished using an indirect offset (gravure) printing process that involves an image being transferred from the cliché via a silicone pad onto a substrate. Pad printing is used for printing on otherwise impossible products in many industries including medical, automotive, promotional, apparel, and electronic objects, as well as appliances, sports equipment and toys. It can also be used to deposit functional materials such asconductive inksadhesivesdyes and lubricants.


SCREEN PRINTING
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 or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate. A fill blade or squeegee is moved across the screen stencil, forcing or pumping ink into the mesh openings for transfer by capillary action during the squeegee stroke.
Screen printing is also a stencil method of print making in which a design is imposed on a screen of polyester or other fine mesh, with blank areas coated with an impermeable substance. Ink is forced into the mesh openings by the fill blade or squeegee and onto the printing surface during the squeegee stroke. It is also known as silkscreenserigraphy, and serigraph printing. A number of screens can be used to produce a multicoloured image.




Screenprinting is more versatile than traditional printing techniques. The surface does not have to be printed under pressure, unlike etching or lithography, and it does not have to be planar. Different inks can be used to work with a variety of materials, such as textiles, ceramics, wood, paper, glass, metal, and plastic. As a result, screenprinting is used in many different industries, including:
balloons
clothing
decals
medical devices
printed electronics
product labels
signs and displays
snowboard graphics
textile fabric
thick film technology

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