Tansferring an image from one material to another is called printing. This process plays an essential role in visual communications.
Printed matter is produced by eight basice processes.
1, Relief or letterpress printing.
2, Gravure or intaglio printing.
3, Planographic or lithographic printing.
4, Screen-process printing.
5, Photographic printing.
6, Heat transfer printing.
7, Xerographic printing.
8, Ink Jet printing.
Each process will be described in more detail later. They all can produce printed matter in single colors and in full color. The first three methods account for most printed matter produced each year.
Relief or letterpress printing
When ink is applied to a raised surface and paper is pressed against the inked surface, the ink is transferred to the paper, Figure 1-2. Relief and letterpress are terms that describe printing methods that use this principle. The rubber stamp is a device that prints by the relief or letterpress method. It transfers ink from a raised surface to paper. Typewriter elements, also print in this way.
Figure 1-2. Relief or letterpress printing. The lowered sruface does not print because it doesn’t come in contact with the ink or the paper.
Foundry type, Monotype, Linotype, and Ludlow are used by the letterpress printer to trasfer letters and numbers to paper, Figure 1-3. The plates of wood cut, wood engraving, linoleum block, and photoengraving can also be used in letterpress printing. Their raised surfaces can print photoengraving and illustrations as well as letters and numbers.
Relief or letterpress printing is the oldest printing method. Today a wide variety of letterpress plates are used on several types of presses to print a rich and varied assortment of products including nespapers, books, and magazines.
IMAGE REVERSAL. As you can see, the principle of printing from a raised surface is not difficult to understand. There is one problem to consider, however. Look at Figure 1-2 again. Note that the shape of the raised surface that printed the letter P on paper is “wrong reading” or backwards. The raised surface was purposely prepared this way so that it would print a “right reading” image on the paper.
Gravure or intaglio printing
Relief printing transfers ink from araised surface to paper. The gravure process is just the opposite. In gravure printing, ink is transferred from a lowered surface to paper. The image area of a gravure plate is cut below or into the surface of the plate
Intaglio(pronounced in-tal-yo) is another name for gravure. Figure 1-4 also illustrates the gravure or intaglio process. The entire surface of the plate is inked and then wiped clean. This leaves ink in the lowered areas of the plate. Paper is then pressed against the plate and ink transfers to it.
Paper is flexible. It can bend and stretch to get into the lowered areas of a gravure plate. Figure 1-5 shows how ink is transferred to the paper.
Karl Kleitsch is generally credited with inventing the gravure process in 1879. In 1894, he developed a press that could print from etched copper cylinders instead of flat plates. Within twenty years gravure cylinders were being used to print a variety of products including a portion of New York Times.
Today both flat and cylindrical gravure plate are used on a variety of presses to print Sunday newspaper supplements, magazines, major mail order catalogs, stamps, and even paper money. Much of our printed fabric is also produced by gravure.
IMAGE REVERSAL. Like the raised image on a relief plate, the lowered image on a gravure plate must also be prepared in reverse. Look at Figure1-4 Note the shape of the lowered surface that printed the letter P. It is”wrong reading” or backwards. Thelowered surface was purposely prepared this way so that it would print a “right reading” image on the paper.
Planographic or lithographic printing
To print from a flat surface is also possible. Planographic and lithographic are both terms that describe methods of printing from a flat surface.
Planographic printing is based on the principle that grease and water do not mix. The process works this way. First, a greasy image is placed on a flat plate. Timage may be drawn directly on the plate with a grease pencil. It may also be placed on the plate photographically instead.
next, water is applied to the plate. The water will cover the non-image area of the plate. water will be repelled from the image area because WATER AND GREASE DO NOT MIX!
The entire plate is then coated with ink. Ink is a greasy substance and adheres to the greasy image. Ink is repelled from the wet areas of the plate because WAER AND GREASE DO NOT MIX!
Paper is then pressed against the surface of the plate and the inked image is transferred to the paper. The process of printing from a flat surface is shown in Figure 1-6.
Planographic or lithographic printing prints from a plane or flat surface, one neither raised nor depressed. Tprinting image is formed chemically by making some areas of the plate grease receptive and water repellent, while others remain water receptive and grease repellent.
Offset printing. Lithographic printing is often called offset printing. Unlike plates for letterpress and gravure(which have their printing surfaces shaped to form the desired image), the printing image on a lithographic plate simply rests upon the plate’s surface. The image can rapidly wear away when paper rubs against it during the printing process. This is especially true when the plates are used on high-speed printing presses. To minimize wear the image on the plate is first offset(transferred) to a rubber blanket, Figure 1-7A. Note that the right-reading image is reversed on the blanket. Figure 1-7B shows how the paper receives the image from the blanket. The paper does not make contact with the printing plate. The entire offset lithographic process is diagrammed in Figure 1-7C.
Alois Senefelder of Germany discovered the lithographic printing method in 1798. He printed directly on paper from a grease image applied to a heavy piece of limestone. Stone lithoraphy is slow and tedious.
Today a wide variety of lithographic plates is used on high-speed offset presses and duplicators to print more products than any other printint method.
Another printing method is to print by forcing ink through openings or holes in a stencil, Figure 1-8.
One common type of stencil printing is screen-process printing. This method is also called stencil printing, screen printing and silk screen printing.
STENCILS. The stencil controls what is to be printed. Basically a stencil is nothing more than a thin sheet of paper, film, or other nonporous material with lettering or a design cutthrough. The lettering or design is right reading not reversed. Stencils for screen-process printing may be hand cut from paper or film, prepared photographically, or painted directly on the screen.
PRINTING SCREENS. Astencil with open areas representing the desired image must be adhered to a screen. A screen is a piece of woven material, such as silk, nylon, dacron or stainless steel mesh, stretched tightly over a wooden or metal frame. The frame serves as the printing press in screen-process printing. A simple printing frame is shown in Figure 1-9.
THE PRINTING PROCESS. Screen-process printing is easy to understand. After paper is placed under the printing screen, ink with a paint-like consistency is applied to the top of the screen. Finally, the ink is spread and forced through stencil openings onto the paper below the screen. This is done by pulling a rubber squeegee over the screen. The screen process of printing is illustrated in Figure 1-10.
The idea of applying decorations to objects by stenciling is very old. Evidence shows that stenciling techniques were wedely used in china to decorate pottery and other objects.
Screen-process printing developed rapidly duing the early twentieth century. Awide variety of stencil materials materials has been developed over the years. Today, just about any surface of any shape or size can beprinted using screen-process methods.
Heat transfer printing
heat transfer or sublimation printing is not a new process. However, not until recently has the technigue grown rapidly. Today, heat transfer printing is used to print fabrics.
Have you ever used a pressing iron to print a novelty design on your T-shirt? If so, you have used heat transfer printing.
Modem heat transfer printing starts with a special ink and, in some cases, special paper. The design or pattem is printed on the paper usually by a gravure process. The paper can be a special themoprinting type, film coated paper, or a ground wood, clay coated type. Use either sheet or roll stock.
The dried image is placed over the cloth and heat is applied. At about 199℃ the ink goes from a solid to vapor (sublimates). This causes the dyes to go into the fibers of the cloth and color them.
The modem heat transfer printing machine is capable of printing 1000 meters of cloth per hor. A skilled operator is needed to supervise and adjust the machine.
Ink jet printing
Ink jet printing is a non-contact printing process. No printing surface such as type, plates made by photographic means, or film are needed.
Copy to be printed is prepared by computer. A minicomputer in the ink jet printer controls ink jets in the printing head. On command, microscopic droplets of ink emerge from the jets to print characters and images.
The printing head is a series of single jets. The most common used printing head has up to 1280 jets spaced at 0.0083 in.
The ink droplets are created in the system. They are either permitted to impact on the moving web of paper, or are deflected into a reservoir when no printing is desired. Deflected ink is reused. Printing is done in one pass of the paper under the ink jets.
The process, as developed by one company, is known as DIJIT (Direct image by Jet ink Transfer). However, the most common name for it appears to be ink jet printing. Its greatest use is in direct mail printing, personalized book publishing, addressing system and proofing.