Tuesday, 28 March 2017

Printing Process - Relief Print, Platen Press, Flatbed, Cylinder Rotary, Offset, Screen Printing

INTRODUCTION

The art and science of making a large number of duplicate reproductions of an original copy is termed as printing. It may also be defined as the art of preserving all other arts. Printing is the medium tor printed communication.
We start our days with newspapers, then we buy milk packets, every day we come across many books, currency notes and so many printed items. So, we can say that printing is a part and parcel of our life. Three major printing processes basically are used to print printed materials. These are Relief Printing Process, Planographic Printing Process, and Screen-Printing Process. In addition, there are many minor or highly specialized printing processes. These include Die Stamping, Thermo Printing, etc.
We shall discuss only the major printing processes in detail. In order to understand these processes, we must understand the following two terms clearly:
Image Area: It is that area on the printing surface that receives ink. For example: raised portions of a rubber stamp.
Non-Image Area: It is that area on the printing surface, which does not receive ink. For example: depressed portions of a rubber stamp.

PRESENTATION OF CONTENT

There are three major printing processes. In this article we shall discuss about these three processes in detail. The content of the lesson shall be presented as follows:
Relief Printing Press
Planographic Printing Process and
Screen-Printing Process

RELIEF PRINTING PROCESS

It is the oldest printing process and came into being with the invention of movable types in the fifteenth century by Johan Gutenberg. The matter, which is to be printed is a mirror image (reverse) or is backward reading (right to left).
The image to be printed is raised and the non-image area is depressed.
The basic principle behind this process is that there is a physical separation between the image areas and the non-image areas. Image areas are raised, and catch ink to produce impression on paper while non-image areas are lowered and do not catch ink.

STEPS INVOLVED IN RELIEF PRINTING

First of all, relief printing plates are prepared i.e. the master is prepared with a combination of metal or wooden types are assembled together. This is known as typesetting or composing. Also used are illustrations prepared by photomechanical methods called blocks. All the composed matter are combined and locked together in a frame. In this the image areas are raised while non-image areas are depressed on the master/printing surface.
Ink rollers on the master apply ink, image areas receive ink and non-image areas do not.
The printing surface or master is then pressed against the substrate to obtain the impression. Ink is transferred from image areas on the substrate. The non-image areas, which are depressed, don’t come in contact with the inking rollers or the paper and so give no impression. Examples for this process are:
  • Letterpress
  • Flexography

LETTER PRESS PRINTING PROCESS

Letterpress Printing Process- Platen Press Flatbed Cylinder Rotary
Platen Press Flatbed Cylinder Rotary
Printing originated with the letterpress. -The nomenclature "letterpress" brings to mind the images of raised letters pressing against a surface, on which their shape of ink is transferred. Actually letterpress printing is not just meant for printing only letters but also borders, rules, illustrations, etc. Letterpress is a relief printing process. On the basis of printing surface or master and the surface on which paper is placed, the letterpress printing machines can be classified in three main groups. These are:
  1. Platen Press
  2. Flatbed Cylinder
  3. Rotary

PLATEN PRESS:

The surface on which the paper (to‘ be printed) is placed for printing is flat and is called platen, and the forms or master or printing surface is also placed on a flat surface known as the flatbed. Since the paper is put on the platen (flat surface), therefore this group of machines is also known as platen press or platen machine or treadle machine.

Process Platen Press

First of all a forms or printing surface is prepared for printing each letter and image is cast separately using wood (engraving/carving) or alloys made up of tin, antimony, lead etc. Then these letters and images are arranged together and locked in a frame firmly. This is technically known as the forms or chase. It acts as the printing surface.
The master frame is fixed in a plane surface and inking is done by inking rollers which pick up the ink from a revolving ink disc fixed above the ‘machine and then pass it across the forme.
Paper is fed by inserting in between the two flat surfaces. The plane surface on which paper is placed for printing is known as platen.
Bringing the flat surfaces in contact with each other does printing. The whole composed matter comes under the pressure at the same time where a controlled pressure is required to transfer the ink from the matter to the paper clearly and correctly.

Types of Jobs Suitable for Platen Machines:
These types of machines are best suited for printing letter heads, cards, bill forms, leaflets, pamphlets, inserts, visiting cards, office files, serial numbering, etc. Platen presses can also do embossing, die cutting, creasing and foil stamping, numbering, etc. which other printing presses just cannot.

Advantages of Platen Press

  • Flat to flat type of machines are available in different sizes to suit different jobs. The printing work can be stopped in between and any correction can be carried out.
  • Small works in less numbers can be printed at very cheap rates.

Disadvantages of Platen Press

  • The speed of printing of flat-to-flat type of machines is very slow. The average speed is 1200 impressions per hour.
  • Since the paper used for printing by these machines is in the form of sheets, a lot of time is wasted in changing the sheets of papers repeatedly. So printing becomes a very time consuming process.
  • In flat-to-flat type of machines printing is possible only in one colour during one impression.

FLAT BED CYLINDER PRESS:

This group of letterpress machines is also known as flat bed cylinder presses. The surface of the printing surface remains flat while the surface carrying paper is cylindrical. Earlier these presses were operated by steam power, but now days they are operated by electrical power.

Process of Flat Bed Cylinder Press:

  • The average speed is 1200 impressions per hour.
  • At first a matter is prepared in the same way as in the platen press and is placed on a plane surface called flat bed.
  • This bed travels to and fro from one end to the other end of the machine. Inking is done by the ink rollers, which are rolled over the bed.
  • The impression cylinder is a part of these machines, which is used to grip the paper and to apply the pressure. It revolves about its own axis.
  • Due to the to and fro motion of the flat bed and the pressure applied by revolving impression cylinder, the image areas are printed on the substrates.
Examples of Fiat Bed Cylinder Press
  • Stop cylinder machines.
  • Swing cylinder machines
  • Single-revolution-cylinder machine
  • Two-revolution-cylinder machine
Types of Jobs Suitable for Flat Bed Cylinder Machines:
  • These machines are efficient enough to print considerably longer run jobs i.e. in larger numbers and for much bigger paper sizes.

Advantages of Flat Bed Cylinder Press

  • Flat to cylinder type of machines are cheaper and flexible in printing.
  • Since there is a revolving impression cylinder and is power driven the printing speed quite high. The average speed of these machines is 4000 impressions per hour.

Disadvantages of Flat Bed Cylinder Press

  • Although speed of flat bed to cylinder type of machines is higher than platen ones, yet this speed is not optimum for larger printing jobs. These machines are comparatively very slow with respect to modern printing processes (offset, gravure, rotary letter presses etc.)
  • Like platen machines, these machines also use individual sheets for printing, so they are also time-consuming.

CYLINDER-TO-CYLINDER (ROTARY):

In this group of machines, the printing surface as well as the platen is Cylindrical. The printing surface is prepared by duplicating process in round Shape/curved/flexible to wrap around the cylinder. The printing surface or the master has relief images. It is known as stereotypes and electrotypes. These are prepared by electronic and mechanical techniques. The printing surface or cylinder and the impression cylinder maintain consistency and proportion with each other.

Process of Rotary Press

  • The master, which is first fixed on to the plate cylinder (Stereotype and electrotype);
  • Then one end of the paper web is fixed in between these two cylinders. When both the cylinders start revolving on their axis, the reel of paper is pulled according to the speed of the cylinder.

Types of Jobs Suitable for Rotary Press

The paper printed is in rolling form. After printing it is cut in to sheets as per the requirement. This is a fast method of printing. These machines are suitable for printing of newspapers, magazines, books, etc. in a large quantity. The speed is about 20000 to 30000 impression per hour.

Advantages of Rotary Press

  • Cylinder to cylinder type of machines has higher speed in comparison to other letterpress machines.
  • The Use of paper in the web form allows continuous printing. 
  • Rotary movement of cylinders allows faster printing.
  • Inline operations can be incorporated in these machines; these operations include cutting, folding of paper trimming of paper, packaging etc.
  • Two three or four colours are possible in these machines. The number of colours that can be printed depends on the number of units through which the paper passes during printing.
  • Since the cylinders are in continuous motion, energy is not wasted in accelerating them again and again.

Disadvantages of Rotary Press

  • It requires more time for make ready procedure before printing, labour and technical
  • Skilled men are required for the preparation of master (stereo and electroplates)
  • Initial cost of setting up these machines is very high.
  • These machines are not suitable for small size jobs i.e., less numbers of copies.

PLANOGRAPHIC PRINTING PROCESS

It is based on the principle that water and-oil do not mix and repel each other. The term planography means that the image and the non-image areas are on the same plane unlike in relief process (letter press) where the image areas are raised. In this process both image and non-image areas are chemically separated but both lie on the same plane. Image areas are prepared with certain greasy or oily materials. Non-image areas are prepared with some water absorbing materials.
Examples:
Offset Press
Lithography Press

OFFSET PRINTING: Offset comes under a planographic printing process. It is based on the principle that water and oil do not mix with each other. The image areas are oily greasy in nature and readily accept Oil or grease based inks. On the other hand non-images areas accept water and hence repel away oily or greasy inks. Thus image and non-image areas are chemically separated on the printing surface.
Offset machines make use of planographic printing process. The offset machines use a thin plate on which image and non-image areas are photo mechanically prepared on the thin plate. It is wrapped around the image cylinder. There are the three cylinders, which rotate around each other. These cylinders are the plate cylinder, the blanket cylinder and the impression cylinder. The metal plate - on which the image and non-image areas are separated - is fixed on the plate cylinder which is fixed at the top, where the inking rollers and dampening rollers (which supplies water) apply or supply, ink to the image areas and water to the non image areas.
The inking system and dampening system are provided at the top of the plate cylinder. After that the ink is transferred from the plate cylinder to the blanket cylinder, which is arranged below the plate cylinder. From the plate cylinder to blanket cylinder the image is transferred in reverse i.e., it is the mirror image and backward reading. The image is readable on the plate cylinder but not on the blanket.
The paper then passes between the blanket cylinder and the impression cylinder. The impression cylinder exerts a little pressure, sufficient enough to transfer the ink from the blanket on to the paper. Offset machines are of two types:
  1. Web Offset
  2. Sheet fed offset

In web offset the paper is in the roll form or web form. In sheet fed machines the paper is fed in the form of sheets. Both these machines are available in different sizes.

The main units of offset machines are:
  • Feeding unit that takes care of the paper to be fed into the printing machines
  • Dampening unit, which contains cloth, covered rollers, installed at the plate cylinder. This unit supplies water to the non-image areas of the plate.
  • Inking unit: Which has inking rollers that apply the greasy ink onto the image areas of the plate. These rollers are in contact with the plate cylinder
  • Printing unit, which contains three cylinders called plate, blanket and impression cylinders, and
  • Delivery unit, which collects all the sheets after printing.

Advantages of Offset Press

  • The use of rubber blanket facilitates printing on less expensive papers is possible and also allows perfect transfer of ink.
  • The process is fast and can print more numbers of copies.
  • Offset can also print on large size papers and on the other materials like tin, plastic foil etc.
  • The amount of ink and the thickness of the ink can be controlled.
  • Master plate/printing surface is prepared at a very fast speed Using computers, photographic, electronic and mechanical techniques which go well with modern reproduction methods.
  • Good quality of pictures, multi colours can be easily printed.

Disadvantages of Offset Press

  • Technical skill is required to operate on offset machines.
  • Last minute corrections which is some times very necessary, the master/plate has to be re-prepared
  • The machine maintenance of an offset press is expensive, because it has large number of moving parts.
  • More space is required to set up an offset machine.
  • The initial investment is more

Applications of Offset Press

Offset machines are used in almost all national daily newspapers.
Used for printing of textbooks and other books and for general commercial printing.
Can print very good quality multi coloured calendars.
Can be used for printing of magazines posters catalogues
Fine line scripts as in Urdu languages can be printed easily by this method.
Large size maps, plans and packaging materials can be printed properly.

SCREEN PRINTING PROCESS

It is one of the major printing processes used these days for a wide range of printing jobs like the other printing processes. Artists for their creative works used silkscreen printing in the earlier times. It is also known as porous printing. Now days silk is not only the fabric used Nylon, Dacron, Polyester, wire screens are also being used. This process came into full-scale commercial use only' in the early part of this century.
This process is based on the fundamental face that by forcing ink through the pores of selected areas of a silk screen mesh images can be formed on the substrate placed below the screen. The selected porous areas on the printing surface are the image areas while the blocked areas on it are the non-image areas.

Steps involved in Silk Screen Printing

  • The image areas are drawn or tracked on the screen and then the non image area is blocked out on the fabric screen by using various methods. Thus at the first step, the master is prepared
  • The master or the screen mesh is fixed with the help of hinges on a tabletop. The ink or colour is forced through the porous image areas on the fabric screen and get deposited on the substrate. This force is applied using a rubber squeeze.

Suitability of jobs of Silk Screen Printing

  • By using this process, printing can be done on rubber, plastic paper, glass etc. The image can be transferred to almost any surface, whether flat or odd shaped.
  • The process is very simple and cost effective for small scale printing jobs.
  • This process is best suited for package, display designs, stickers, containers etc.
  • Wedding cards, visiting cards, letterheads etc. are printed with a good 'quality better than letterpress.
  • Pictures can also be printed up to the certain extent.
  • The printed image has a thick layer of ink and hence there is a little raised effect after printing that gives a good appearance
  • All the materials required for printing by the process are simple inexpensive and easy to handle. So very little capital is needed to start screen-printing units.
  • There are many new uses for screen process printing such as printing of electronic circuits.

Disadvantages of Silk Screen Printing

  • Drying of printing images takes time because of thick ink
  • Fine resolution colour pictures are difficult to print.
  • Most of the screen-printing work ‘is done manually and hence the speed of printing is very slow.
  • The amount of ink used is more in this process when compared to the other printing process.