Food Packaging Technology 3(2+1)

Lesson- 24 Printing

24.1 Introduction:

Printing is a process in which text and images are reproduced, typically with ink on paper using a printing press made from letters, photographs and drawing. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing. The basic systems of printing are: - 1) the original, (2) the plate, (3) printing ink, (4) a printing medium such as paper and (5) a printing machine. Printing can be classified into two parts:

1) Direct Printing: In this type of printing, printed material comes in direct contact with the plate so that the ink is directly applied to the printing medium.

2) Indirect Printing: In this printing, the ink is first applied to the blanket cylinder from the plate and then printing medium comes in contact with the blanket

24.2 Printing Technologies

Numbers of printing are in trend for printing purposes of packaging materials. Which are?

24.2.1 Lithography

Lithography is a method in which printing is applied on a smooth surface. Lithography is a printing process that uses chemical processes to create an image. Lithography is a form of planographic printing, meaning that the surface is flat, in contrast to relief printing  or intaglio printing. For instance, the positive part of an image would be a hydrophobic chemical, while the negative image would be water. Thus, when the plate come in contact with a compatible ink and water mixture, the ink will adhere to the positive image and the water will clean the negative image. This allows for a relatively flat print plate which allows for much longer runs than the older physical methods of imaging (e.g., embossing or engraving). High-volume lithography is used today to produce packaging materials, 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.

24.2.2 Colour printing

Chromolithography is a method for making multi-colour prints. This type of colour printing stemmed from the process of lithography, and it includes all types of lithography that are printed in colour. Lithographers sought to find a way to print on flat surfaces with the use of chemicals instead of relief or intaglio printing.

Chromolithographs are mainly used today as fine art instead of advertisements, and they are hard to find owing to poor methods of preservation and also because a cheaper form of printing replaced it. Many chromolithographs have deteriorated because of the acidic frames surrounding them. As stated earlier, production costs of chromolithographs were low, but efforts were still being made to find a cheaper way to mass produce colored prints. Although purchasing a chromolithograph may have been cheaper than purchasing a painting, it was still expensive in comparison to other color printing methods which were later developed.

24.2.3 Screen printing

Screen printing has its origins in simple stenciling, most notably of the Japanese form (katazome), used who cut banana leaves and inserted ink through the design holes on textiles, mostly for clothing. This was taken up in France.

24.2.4 Flexography

Flexographic printing is widely used in western countries. Flexography (also called "surface printing"), often abbreviated to "flexo", is a method of printing most commonly used for packaging (labels, tape, bags, boxes, banners, and so on).A flexo print is achieved by creating a mirrored master of the required image as a 3D relief in a rubber or polymer material. A measured amount of ink is deposited upon the surface of the printing plate. The print surface then rotates, contacting the print material which transfers the ink. Ink is picked up by a cavitated anilox roll and transferred to the printing plate. The ink is then transferred to the film. Because the costs of producing the plates are relatively low, flexographic printing is cost effective, especially for short runs. Its printing quality is inferior to that of modern printing techniques such as offset printing and gravure printing. Now a day this printing technology is utilized for printing on polyethylene bags, corrugated boxes and carton after using the photosensitive resins of improved printing quality.

24.2.5 Digital press

Digital printing is the reproduction of digital images on a physical surface, such as common or photographic paper or paperboard-cover stock, film, cloth, plastic, vinyl, magnets, labels etc. it is now possible to create artwork on a computer and transfer the image directly to the packaging film. A design is created on a computer; it may be an individual design or replicated to give several hundreds of impressions. The ink, usually in powder form, is attracted on to the film surface and cured in place. Special coatings are necessary to receive the ink. A standard heat-sealable coating on the reverse side allows the film to be made immediately into packages.

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

1. Every 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
2. The Ink or Toner does not absorb into the substrate, as does conventional ink, but forms a layer on the surface and may be fused to the substrate by using an inline fuser fluid with heat process(toner) or UV curing process(ink).
3. It generally requires less waste in terms of chemicals used and paper wasted in set up or make ready.
4. It is excellent for rapid prototyping, or small print runs which means that it is more accessible to a wider range of designers and more cost effective.

24.2.6 Frescography

Frescography is a method for reproduction/creation of murals using digital printing methods. The frescography is based on digitally cut-out motifs which are stored in a database. CAM software programs then allow entering the measurements of a wall or ceiling to create a mural design with low resolution motifs. Since architectural elements such as beams, windows or doors can be integrated, the design will result in an accurately and tailor-fit wall mural. Once a design is finished, the low resolution motifs are converted into the original high resolution images and are printed on canvas by Wide-format printers. The canvas then can be applied to the wall in a wall-paperhanging like procedure and will then look like on-site created mural.

24.2.7 3D printing

Three-dimensional printing is a method of converting a virtual 3D model into a physical object. 3D printing is a category of rapid prototyping technology. 3D printers typically work by 'printing' successive layers on top of the previous to build up a three dimensional object. 3D printers are generally faster, more affordable and easier to use than other additive fabrication technologies.

24.3 Modern printing technologies

24.3.1 Offset printing

Offset printing is widely used for printing on folding cartons in the packaging field. Offset printing is a widely used printing technique where the inked image is transferred (or "offset") from a plate to a rubber blanket, then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a film of water, keeping the non-printing areas ink-free.

Offset printing has the following advantages:

1)  Plate making time is short in this printing technology.

2) Plate making cost is less than that of gravure printing.

3) It is suitable for multicolor printing.

4) In offset printing large sized plates are easily made.

24.3.2 Gravure

Gravure printing gives good quality of printed matter, either in monocolor or in four colors. Gravure printing is an intaglio printing technique, where the image to be printed is made up of small depressions in the surface of the printing plate. The cells are filled with ink and the excess is scraped off the surface with a doctor blade, then a rubber-covered roller presses paper onto the surface of the plate and into contact with the ink in the cells. The printing plates are usually made from copper and may be produced by digital engraving or laser etching. Gravure printing also has a high printing speed and is suitable for high volume printing. In packaging, gravure printing is performed on most flexible packages using cellophane, plastic film or aluminum foil. Gravure printing is used for long, high-quality print runs such as magazines, mail-order catalogues, packaging, and printing onto fabric and wallpaper. It is also used for printing postage stamps and decorative plastic laminates, such as kitchen worktops.