Screenprinting Presses

There are three types of screenprinting 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



While the public thinks of garments in conjunction with screenprinting, 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

Printing technique -IV 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 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.

Printing technique

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 floodbar) 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.

Printing Techniques-III

ELECTRONIC PRINTING PROCESSES

All the processes previously discussed employ a fixed printing surface that transfers the same pattern of ink during each cycle of the press. Simple physical ink-transfer mechanisms allow these processes to operate at high speed. Because of the high cost of making a set of plates, mounting them on the press, and running the press until the printing is in register (properly aligned) and colors are correct, these processes require fairly long press runs to be economically feasible. For short-run printing—especially of highly variable information, electronic processes are more economical. These processes do not use printing plates, and they produce good reproductions without wasting paper.

Electrophotographic Printing.

Modern electrostatic office copiers have a printing surface that can be instantaneously formed by photographing or scanning an original. The surface is coated with a photoconductive material such as selenium or cadmium sulfide. In the dark, a photoconductor acts as an insulator, retaining a charge of static electricity. Areas of the surface illuminated in a camera or by a laser beam become conductive and lose their charge. The remaining areas retain their charge, attracting oppositely charged particles of colorant called toner. The toner is then transferred to a piece of paper or plastic using electrostatic forces rather than pressure. This cycle is repeated for each copy, making the process far too slow and complicated for mass printing applications. For small quantities, however, some color electrophotographic printers can reproduce color originals with image quality that approaches that of offset lithography.

Ink-jet Printing.

A computer-controlled array of ink nozzles can produce images on a moving sheet or a web of paper. Simple ink-jet printers are used routinely to print variable information such as the expiration dates on food packages or address labels on direct mail pieces, and are sometimes installed on the end of a conventional printing press. Sophisticated color ink-jet printers are able to produce lithographic-quality reproductions in extremely short runs.

Microcapsule Printing.

This technology uses paper impregnated with billions of microscopic capsules of liquid photopolymer-based dye. The paper is exposed to light reflected from an original image, and the dyes inside the capsules harden in proportion to the amount of light they receive. The exposed paper is then pressed through steel rollers against a receiver paper, and varying amounts of unhardened dye are deposited on the receiver to form an image. The process can be used to make high-quality color reproductions in small quantities.

Printing Technique-III

Gravure

Gravure, also called rotogravure, is a high-volume printing process employing an ink transfer mechanism that is fundamentally different from that of relief printing. The printing surface is a polished metal cylinder covered with an array of tiny recesses, or cells (as many as 50,000 per sq in), that constitute the images to be printed. The cylinder, which can be 2.5 m (8 ft) or more in length, is partially immersed in a reservoir of solvent-based fluid ink. As the cylinder rotates, it is bathed in ink. A steel blade called a doctor blade running the entire length of the cylinder wipes the ink from the polished surface, leaving ink only in the cells. The ink is then transferred immediately to a moving web of paper forced against the cylinder under great pressure.

Gravure cylinders are constructed of steel with a thin surface layer of electroplated copper. The copper can be either chemically etched or electronically engraved to form the cells that will transfer ink. Once the cells have been created, the cylinder is electroplated with a thin layer of chromium to produce a hard surface for the doctor blade. Each cell transfers a tiny spot of ink to the paper. The cells can be made to vary in depth from one part of a cylinder to another, causing the darkness of the resulting ink spots to vary also. This enables gravure to print a wide range of gray tones and thus to render excellent reproductions of photographic originals.

Color printing is accomplished by using separate printing cylinders for the cyan, magenta, yellow, and black inks. Each cylinder is housed in a separate printing unit. The web is transported by rollers from unit to unit and can reach speeds of close to 900 m (3000 ft) per minute. After each color is printed, the web passes through a dryer, where the solvent base of the ink is evaporated. The solvent is either reclaimed or burned to produce energy. Some gravure printers have begun to use water-based inks. This trend is likely to continue because of health and environmental threats posed by the use of hydrocarbon-based solvents.

The expense of manufacturing a set of gravure cylinders has restricted its use to long-run jobs (millions of reproductions). Mass-circulation monthly magazines, mail-order catalogs, and packaging are natural markets for the process. Gravure is also used to reproduce a variety of textures and patterns on decorative materials. Most of the simulated wood grains on inexpensive furniture, for example, are printed by gravure. New methods of manufacturing gravure cylinders using computer-controlled electronic engraving machines have reduced the time required to prepare a set of cylinders, but they are still far more expensive than lithographic printing surfaces

Printing Techniques-II

Relief Printing

Relief printing processes work on the same principle as a rubber stamp. Ink is applied to the raised portions of the printing surface, and is then transferred by pressure to paper or some other substrate. Two forms of relief printing—letterpress and flexography—are currently in use, distinguished by the physical characteristics of their printing surfaces and inks. Letterpress printing is accomplished using a hard metal or plastic printing surface and a highly viscous ink. Flexography employs a soft rubber or plastic printing surface and a fluid ink.

A

Letterpress Printing

Letterpress, the oldest form of printing, originated with the invention of movable metal type in the middle of the 15th century and was for five centuries the only viable mass printing process. In the mid-20th century, letterpress printing, despite its superiority in the clarity of impression and in the density of ink, lost its predominance to lithography, a much faster process.

Originally, letterpress printing surfaces were prepared by assembling thousands of pieces of metal type on which individual letters or letter combinations were cast in relief to create pages of text called type forms. Ink was applied to the raised areas of the form and then transferred under pressure to paper or vellum. Woodcuts and engravings could be combined with type to produce composite pages containing both text and graphics.

A

1

Duplicate Plates

The first letterpress printing plate was created by making a plaster mold of a type form and then casting a metal duplicate of the original, called a stereotype. Stereotyping became an extremely important technology during the Industrial Revolution because it yielded a one-piece printing surface that could be used in place of the original type form on a variety of automated printing presses. Curved stereotypes cast from papier-mâché molds were used on rotary letterpresses for printing daily newspapers until the early 1970s, when hot-metal machine typesetting was largely replaced by computer typesetting.

Another important duplicate plate, called an electrotype, was made by electroplating a thin layer of copper onto a wax impression of the original type form and then filling the resulting copper shell with type metal. Electrotypes retained more detail from the original relief surface than stereotypes and were therefore preferred to stereotypes for higher-quality letterpress printing.

A

2

Photopolymer Plates

In the late 1950s a radical new way of making relief printing surfaces was introduced; it employed a soluble plastic that hardened upon exposure to ultraviolet radiation. Since then a large number of photopolymer plate materials have been created. A thick coating of photopolymer on a metal or plastic support can be exposed to ultraviolet light through a piece of film that allows the light to pass through only those areas that will transfer ink. The photopolymer hardens, or polymerizes, in these areas, and the remaining unexposed coating is washed away with water or some other solvent. The result is a relief printing surface than can be mounted directly on a printing press.

In a variation of this process, a liquid photopolymer that solidifies when exposed to ultraviolet radiation is spread on a paper or plastic support. After exposure the unexposed liquid is blown away with air. These plates can be made rapidly and are therefore most suitable for newspaper printing, where deadlines are critical.

High-speed rotary web presses and photopolymer plates have allowed letterpress to remain competitive in some areas, such as in newspaper printing, despite the fact that lithography is now the uncontested leader among printing processes.

B

Flexographic Printing

The soft plates and highly fluid inks used in flexography make the process ideal for printing on nonporous materials such as foil laminates and polyethylene. Originally, all flexographic plates were made of molded rubber, which is still the preferred material when multiple copies of the same image are needed on a single printing cylinder. Rubber plate molds are impressions of original relief surfaces, such as type forms or engravings, and are normally used to make several duplicate rubber plates. The preparation of a printing cylinder using molded rubber plates is a time-consuming process because many rubber plates are mounted on a single cylinder and each plate must be carefully positioned in relation to the others.

In the 1970s photopolymer plate materials were introduced, and the time required to manufacture and mount a set of plates was reduced significantly. This has allowed the process to enter new markets, most notably newspaper printing. In addition, water-based inks can be used in flexography, eliminating the need for toxic solvents.

Flexographic printing presses are simple in design because the fluid ink is easily distributed to the printing surface without an elaborate inking system. Printing is usually done on rolls or webs of substrate rather than on cut sheets, and the printed rolls are then converted into finished products in a separate manufacturing process.

Digital Printing

Digital printing accounts for approximately 9% of the 45 trillion pages printed (2005 figure) around the world.

Printing at home or in an office or engineering environment is subdivided into:

* small format (up to ledger size paper sheets), as used in business offices and libraries

* wide format (up to 3' or 914mm wide rolls of paper), as used in drafting and design establishments.

Some of the more common printing technologies are:

* blueprint—and related chemical technologies.

* daisy wheel—where pre-formed characters are applied individually.

* dot-matrix—which produces arbitrary patterns of dots with an array of printing studs.

* inkjet—including bubble-jet—where ink is sprayed onto the paper to create the desired image.

* laser—where toner consisting primarily of polymer with pigment of the desired colours is melted and applied directly to the paper to create the desired image.

* line printing—where pre-formed characters are applied to the paper by lines.

* heat transfer—like early fax machines or modern receipt printers that apply heat to special paper, which turns black to form the printed image.

Vendors typically stress the total cost to operate the equipment, involving complex calculations that include all cost factors involved in the operation as well as the capital equipment costs, amortization, etc. For the most part, toner systems beat inkjet in the long run, whereas inkjets are less expensive in the initial purchase price.

Professional digital printing (using toner) primarily uses an electrical charge to transfer toner or liquid ink to the substrate it is printed on. Digital print quality has steadily improved from early color and black & white copiers to sophisticated colour digital presses like the Xerox iGen3, the Kodak Nexpress and the HP Indigo Digital Press series. The iGen3 and Nexpress use toner particles and the Indigo uses liquid ink. All three are made for small runs and variable data, and rival offset in quality. Digital offset presses are called direct imaging presses; although these receive computer files and automatically turn them into print-ready plates, they cannot insert variable data.

Small press and fanzines generally use digital printing or more rarely xerography. Prior to the introduction of cheap photocopying the use of machines such as the spirit duplicator, hectograph, and mimeograph was common.

For every newspaper, book, or other printed product, there is a production crew laboring behind the scenes, from printing press operators to bindery workers. As a printing technology major, you’ll learn the skills necessary to plan, prepare, and complete print jobs, from assembling film to operating printing equipment to cutting and collating the finished product.

Most programs offer both old-school and new-school techniques, so by the time you graduate, you’ll be prepared for hands-on production work as well as cutting-edge desktop publishing.

source:internet only

Lithography Printing Technology

II

Lithography

By far the most important and versatile printing process today is offset lithography. The underlying principles were established at the end of the 18th century by a German map inspector, Aloys Senefelder, who was experimenting with methods of producing limestone relief printing surfaces using an acid etching process. Senefelder found that a wet limestone surface would repel an oil-based printing ink, and that an image drawn on the surface with a grease pencil would repel water and attract ink. Any drawing on the stone surface could be reproduced by bringing a damp sheet of paper into contact with the freshly inked image. This cycle could be repeated several hundred times before the drawing could no longer be faithfully reproduced.

The process, called chemical printing by Senefelder, quickly became a popular art medium because it enabled artists to produce multiple copies of freehand drawings. By the late 19th century, multiple stones were being used to transfer as many as 30 separate colors to a single sheet of paper to produce exquisite color lithographs that resembled fine watercolor paintings. Modern color lithography uses only four inks for a wide range of natural colors.

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The Offset Principle

In the early part of the 20th century, it was discovered that ink could be transferred from the lithographic surface to an intermediate rubber surface and then to paper. The rubber intermediate, called a blanket, can transfer ink to paper and to a wide variety of materials that cannot be printed directly, including plastics and metals. Because the soft blanket conforms to the texture of the surface to be printed, lithographic image quality is unrivaled.

B

Offset Lithography Today

The function of the original stone printing surface is now served by thin aluminum plates, although other materials, such as stainless steel and plastic, can also be used. The plates are wrapped around the circumference of the printing cylinder and make direct contact with the rubber blanket cylinder. Rubber rollers carry ink and water to the plate surface. The ink is transferred first to the blanket cylinder and then to the paper.

Lithographic plates are the least expensive printing surfaces available today, and this fact has contributed greatly to the success of the process. Aluminum plate materials have a thin surface coating of light-sensitive material, such as a photopolymer, that undergoes a solubility change when exposed to an intense source of blue and ultraviolet light. Images are transferred to the surface by exposing the plate through a film positive or negative Some materials can be exposed directly, as in a graphic-arts camera or by a computer-controlled laser beam, thereby eliminating the expense of film and speeding up the platemaking process.

Modern offset lithographic presses range in size from small sheet-fed duplicators—used for small, single-color jobs such as brochures and newsletters—to massive web presses capable of printing millions of copies of magazines, catalogs, mailing pieces, and packaging materials in full color. No other process has such a broad range of applications.

soruce:internet

Introduction to Printing Techniques

Printing Techniques, several different ways in which printing may be accomplished, such as lithography, letterpress, flexography, gravure, and screen printing. All of these printing techniques use simple mechanisms for rapidly applying colorants to substrates such as paper or plastic to form multiple reproductions of original images for mass distribution.

Multiple colors can be printed in one pass through the press. Spot color printing uses custom mixed inks to reproduce specific colors and is widely used in package printing, where large areas of uniform color are common. Process color printing uses four transparent inks—cyan (blue-green), magenta (red), yellow, and black—printed one on top of another in varying amounts. Color photographs and other artwork can be faithfully reproduced by this method.

Most modern printing presses transfer ink from a cylindrical printing surface to moving sheets or rolls of substrate. Presses that print on rolls, or webs, can achieve speeds of 600-900 m (2000-3000 ft) per minute. Presses that print on sheets are generally slower than web presses but can print on thicker substrates, such as bristol board and sheet metal.

Since the 1960s, advancements in photography and electronics have had a profound effect on the manufacture of printing surfaces. Light-sensitive materials such as diazonium resins and photopolymers make it possible to produce durable printing surfaces photographically rather than mechanically. Computer-based systems allow the rapid production of the films used to transfer images to printing surfaces. Some printing surfaces can even be prepared directly by machines employing computer-controlled laser beams or diamond styluses. Images generated on computer systems and stored in databases can now be transferred directly to printing surfaces without any intermediate steps. Taken as a whole, these changes have been called the prepress revolution

Printing Techniques

Preprinted bar code labels can be produced using any printing process. In fact, some preprinted labels are produced using the same printing systems found in on-site printing systems. However, most preprinted label vendors use one or more of the following preferred printing systems because of their speed and accuracy: Film Master/Printing Plate, Ion-Deposition, or Photocomposition.

The film master/printing plate approach to preprinted labels use a very accurate photographically produced film master of the bar code. This film master is used to produce a printing plate, and the plate is used in a variety of commercial printing presses to produce the preprinted label. Since the data coded in the film master bar code is fixed, This system cannot produce sequences of bar coded serial numbers, or bar coded variable, customer supplied data.

Ion deposition is an electrographic, non-impact imaging process capable of printing bar codes and other information on substrates such as paper, vinyl, polyester, and tag stock at very high speeds. The ion deposition printing station operates similar to a xerographic photocopier, except the image of the bar code is electrostatically placed on a dielectric drum rather than optically imaged on a photosensitive drum. Sequential labels and variable data labels can be printed by ion deposition.

The "best" preprinted bar code symbols are produced by photocomposition. The use of the photographic process to produce consecutively numbered bar code labels has some advantages over other commercial production methods. If the primary considerations are: (1) overall quality of the bar code image; (2) high density messages; or (3) flexibility in label size and construction to meet special application requirements, then the photographic process may be particularly suitable.

Because of the cost and complexity of the technology, photographically produced bar code labels can only be produced by preprinted label companies specializing in this printing process.

Photographically printed bar code symbols are produced by special computer controlled photographic printing machines that produce original images of each bar code symbol, not copies. The hardware and software that make up the photographic process are complex. The actual process of creating the image, however, is relatively straightforward.

A moving beam of intense light "strokes" the bar code image through a lens system onto photosensitive material. This is done in a raster fashion, similar to the way a television image is produced. Instructions for creating the size, shape and placement of each individual bar and character are stored electronically in the computer's memory.

After the image is created, the photosensitive material is processed to develop and "fix" the image, much as photographic film is developed into slides or prints. Next, one of a variety of pressure-sensitive adhesives is applied to the back of the image-bearing material.

After the adhesive with its accompanying release liner is applied, die-cutting is done to create individual labels or label sets of the size and shape required. Die making, previously an inexact art, is now a science, and is being aided by lasers and other computer-controlled processes to produce labels to very exacting tolerances.

In most cases, the photosensitive material that bears the bar code image is photographic paper. But unlike a conventional label where ink rests on the surface of the paper, the photographic process allows the image to be formed within the paper. In this way the bar code image is naturally protected from excessive abrasion, smudging, or smearing. Another important advantage in the photographic process is the nature of the paper and the way the image is developed. This means that it will not fade when exposed to sunlight or ultraviolet radiation, important for bar code labels that depend so heavily on contrast between the background and the bars.

Photographically produced bar code image quality remains constant regardless of the material chosen for the photographic process. Typical resolution is 3,000 lines per inch with a bar dimension tolerance of less than 1 mil X-dimension This extremely high resolution makes very high density bar codes possible using the photographic process.