Screen printing is also a stencil method of print making in which a design is imposed on a screen of silk or other fine mesh, with blank areas coated with an impermeable substance, and ink is forced through the mesh onto the printing surface. It is also known as silkscreen, seriography, and serigraph.

Printing technique

A screen is made of a piece of porous, finely woven fabric called mesh stretched over a frame of aluminium 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 paper 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 proportional 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.

There are three common types of screenprinting presses. The ‘flat-bed’, ‘cylinder’, and the most widely used type, the ‘rotary’.[8]

Textile items printed with multi-colour designs often use a wet on wet technique, or colors dried while on the press, while graphic items are allowed to dry between colours that are then printed with another screen and often in a different color after the product is re-aligned on the press.

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 a stencil remover. 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 (the points where threads cross).[13]

While the public thinks of garments in conjunction with screenprinting, the technique is used on tens of thousands of items, including decals, clock and watch faces, balloons, and many other 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.

Stenciling techniques

There are several ways to create a stencil for screenprinting. An early method was to create it by hand in the desired shape, either by cutting the design from a non-porous material and attaching it to the bottom of the screen, or by painting a negative image directly on the screen with a filler material which became impermeable when it dried. For a more painterly technique, the artist would choose to paint the image with drawing fluid, wait for the image to dry, and then coat the entire screen with screen filler. After the filler had dried, water was used to spray out the screen, and only the areas that were painted by the drawing fluid would wash away, leaving a stencil around it. This process enabled the artist to incorporate their hand into the process, to stay true to their drawing.

A method that has increased in popularity over the past 70 years is the photo emulsion technique:

1. The original image is created on a transparent overlay such as acetate or tracing paper. The image may be drawn or painted directly on the overlay, photocopied, or printed with an inkjet or laser printer, as long as the areas to be inked are opaque. A black-and-white positive may also be used (projected on to the screen). However, unlike traditional platemaking, these screens are normally exposed by using film positives.

2. A screen must then be selected. There are several different mesh counts that can be used depending on the detail of the design being printed. Once a screen is selected, the screen must be coated with emulsion and let to dry in the dark. Once dry, the screen is ready to be burned/exposed.

3. The overlay is placed over the emulsion-coated screen, and then exposed with a light source containing ultraviolet light in the 350-420 nanometer spectrum. The UV light passes through the clear areas and create a polymerization (hardening) of the emulsion.

4. The screen is washed off thoroughly. The areas of emulsion that were not exposed to light dissolve and wash away, leaving a negative stencil of the image on the mesh.

Photographic screens can reproduce images with a high level of detail, and can be reused for tens of thousands of copies. The ease of producing transparent overlays from any black-and-white image makes this the most convenient method for artists who are not familiar with other printmaking techniques. Artists can obtain screens, frames, emulsion, and lights separately; there are also preassembled kits, which are especially popular for printing small items such as greeting cards.

Another advantage of screenprinting is that large quantities can be produced rapidly with new automatic presses, up to 1800 shirts in 1 hour.[14] The current speed loading record is 1805 shirts printed in one hour, documented on 18 February 2005. Maddie Sikorski of the New Buffalo Shirt Factory in Clarence, New York (USA) set this record at the Image Wear Expo in Orlando, Florida, USA, using a 12-colour M&R Formula Press and an M&R Passport Automatic Textile Unloader.with The world speed record represents a speed that is over four times the typical average speed for manual loading of shirts for automated screen printing.[12]

…

Details refer to wikipedia

Development of the offset press came in two versions: in 1875 by Robert Barclay of England for printing on tin, and in 1903 by Ira Washington Rubel of the United States for printing on paper.

Present day

Compared to other printing methods, offset printing is best suited for cost-effectively producing large volumes of high quality prints in an economically sound manner that requires little maintenance.[5]
[edit] Applications

Offset lithography is one of the most common ways of creating printed matter. A few of its common applications include: newspapers, magazines, brochures, stationery, and books.

Many modern offset presses use computer to plate systems as opposed to the older computer to film workflows, which further increases their quality.
[edit] Advantages

Advantages of offset printing compared to other printing methods include:

* Consistent high image quality. Offset printing produces sharp and clean images and type more easily than letterpress printing because the rubber blanket conforms to the texture of the printing surface.
* Quick and easy production of printing plates.
* Longer printing plate life than on direct litho presses because there is no direct contact between the plate and the printing surface. Properly developed plates running in conjunction with optimized inks and fountain solution may exceed run lengths of a million impressions.
* Cost. Offset printing is the cheapest method to produce high quality printing in commercial printing quantities.

[edit] Disadvantages

Disadvantages of offset printing compared to other printing methods include:

* Slightly inferior image to rotogravure or photogravure printing.
* Propensity for anodized aluminum printing plates to become sensitive (due to chemical oxidation)
* Time and cost associated with producing plates and printing press setup. As a result, very small quantity printing jobs

Types

Offset printing

Photo offset
Side view of the offset printing process. Multiple ink rollers are used to distribute and homogenize the ink.[6]

The most common kind of offset printing is derived from the photo offset process, which involves using light-sensitive chemicals and photographic techniques to transfer images and type from original materials to printing plates.

In current use, original materials may be an actual photographic print and typeset text. However, it is more common — with the prevalence of computers and digital images — that the source material exists only as data in a digital publishing system.

Offset litho printing on to a web (reel) of paper is commonly used for printing of newspapers and magazines for high speed production.

Ink is transferred from the ink duct to the paper in several steps :-

1. Ink duct roller delivers ink from the ink duct to the ink pyramid.
2. The drop roller transfers ink from duct roller to distribution roller. It is never in contact with both rollers at the same time.
3. The distribution rollers evenly distribute the ink. The first distribution roller picks up the ink from driving rollers, and the last distribution rollers transfer the ink to the form rollers.
4. The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers.
5. Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.
6. Ink form rollers transfer ink from the last distribution rollers on to the printing plate.
7. The printing plate transfers the ink to the offset cylinder(typically called blanket cylinder) usually covered with a rubber ‘blanket’.
8. The paper is then pressed against the blanket cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image.

Types of paper feed
Sheet-fed litho

“Sheet-fed” refers to individual sheets of paper or paperboard being fed into a press. A lithographic (“litho” for short) press uses principles of lithography to apply ink to a printing plate, as explained previously. Sheet-fed litho is commonly used for printing of short-run magazines, brochures, letter headings, and general commercial (jobbing) printing.
Web-fed litho

“Web-fed” refers to the use of rolls (or “webs”) of paper supplied to the printing press. Offset web printing is generally used for runs in excess of 5 or 10 thousand impressions. Typical examples of web printing include newspapers, newspaper inserts/ads, magazines, catalogs, and books. Web-fed presses are divided into two general classes: “Cold” or “Non-Heatset,” and “Heatset” offset web presses, the difference being how the inks that are used dry. Cold web offset printing dries through absorption into the paper, while heatset utilizes drying lamps or heaters to cure or “set” the inks. Heatset presses can print on both coated (slick) and uncoated papers, while coldset presses are restricted to uncoated paper stock, such as newsprint. Some coldset web presses can be fitted with heat dryers, or ultraviolet lamps (for use with uv-curing inks). It is also possible to add a drier to a cold-set press. This can enable a newspaper press to print color pages heatset and black & white pages coldset.
Web press v. sheet-fed (photo offset)

Sheet-fed presses offer several advantages, because individual sheets are fed though, a large number of sheet sizes and format sizes can be run through the same press. In addition, waste sheets can be used for make-ready which allows for lower cost makereadies, so that good paper is not wasted while setting up the press, for plates & inks (waste sheets do bring some disadvantages as often there are dust, offset powder particles that transfer on to the blankets and plate cylinders, thereby creating imperfections on the printed sheet in the form of “hickies”).

Web-fed presses, on the other hand, are much faster than sheet-fed presses, with speeds in excess of 20,000 cut-offs per hour. Their speed makes them ideal for large runs such as newspapers or magazines. However, web-fed presses have a fixed cut-off, unlike rotogravure or flexographic presses.
Types of commercial offset processes
Perfecting press

A perfecting press, also known as a duplex press, is one that can print on both sides of the paper at the same time (Bruno, Romano, and Riordan 137). Web and sheet-fed offset presses are similar in that many of them can also print on both sides of the paper in one pass, making it easier and faster to print duplex.
Offset duplicators

Small offset lithographic presses that are used for fast, good quality reproduction of 1- and 2-color copies in sizes up to 12”X18”. (Romano, & Riordan 139–141)

Offset duplicators are made for fast and quick printing jobs; therefore have faster make-readies and turn-around time, printing up to 12,000 impressions per hour.

They are able to print business forms, letterheads, labels, bulletins, postcards, envelopes, folders, reports, and sales literature.
Sheetfed offset

In sheet-fed offset, “the printing is carried out on single sheets of paper as they are fed to the press one at a time.” Sheet-fed presses use mechanical registration to relate each sheet to one another to ensure that they are reproduced with the same imagery in the same position on every sheet running through the press. (“What is Offset Printing”)
Process

The actual process of printing is quite involved. One of the most important functions in the process is Pre-press Production. This stage makes sure that all files are correctly processed in preparation for printing. This includes converting to the proper CMYK, finalizing the files, and creating plates for each color of the job to be run on the press. The sheet fed press consists of different systems that complete the actual process; feeder system, printing system, inking/dampening system, and the delivery system.
Feeder system

The feeder system is responsible for making sure paper runs through the press correctly. This is where you load the substrate and then correctly set up the system to the certain specifications of the substrate to the press. (DeJidas & Destree, 2005, p. [55-57])
Printing/inking system

The Printing Unit consists of many different systems. The dampening system is used to apply dampening solution to the plates with water rollers. The inking system uses rollers to deliver ink to the plate and blanket cylinders to be transferred to the substrate. The plate cylinder is where the plates containing all of the imaging are mounted. Finally the blanket and impression cylinders are used to transfer the image to the substrate running through the press. (DeJidas & Destree, 2005, p. [143])
Delivery system

The delivery system is the final destination in the printing process while the paper runs through the press. Once the paper reaches delivery, it is stacked for the ink to cure in a proper manner. This is also where you can check on sheets to make sure they have proper ink density and registration.
Web offset

A high run, speed printing press that uses rolls of paper rather than individual sheets (Spectrum Printers).

Web Offset Presses are beneficial in long run printing jobs, typically press runs that exceed 10 or 20 thousand impressions. Speed is a huge factor when considering turn around time for press production; some web presses print at speeds of 3,000 feet per minute or faster. In addition to the benefits of speed and possible faster turn around times, some web presses have the inline ability to print, but also cut, perforate, and fold.
Blanket-to-blanket

A printing method in which there are two blanket cylinders through which a sheet of paper is passed and printed on both sides. (Commercial color offset printing – a compendium of commercial printing terminology)

Blanket-to-blanket presses are considered a perfecting press because they print on both sides of the sheet at the same time. Since the blanket-to-blanket press has two blanket cylinders, making it possible to print on both sides of a sheet, there is no impression cylinder. The opposite blanket cylinders act as an impression cylinder to each other when print production occurs. There are also two plate cylinders on the press.
Blanket-to-steel

A printing method similar to a sheet offset press; except that the plate and cylinder gaps are very narrow. (Romano, & Riordan 139–141)

Blanket-to-steel presses are considered one-color presses. In order to print the reverse side, the web is turned over between printing units by means of turning bars. (Romano, & Riordan 139–141)

The method can be used to print business forms, computer letters, and direct mail advertising.
Variable-size printing

A printing process that uses removable printing units, inserts, or cassettes for one-sided and blanket-to-blanket two-sided printing. (Romano, & Riordan 139–141)
Keyless offset

Keyless offset is a printing process that is based on the concept of using fresh ink for each revolution by removing residual inks on the inking drum after each revolution. (Romano, & Riordan 139–141) It is suitable for printing newspapers.
Types of platemaking

Generally, “the plates used in offset printing are thin,” and are mostly made of aluminum, though sometimes can be made of multimetal, paper, or plastic (Kipphan 209).
Polyester plates

Polyester plates can be used in place of aluminum plates for smaller formats or medium quality jobs, “as their dimensional stability is lower” (Kipphan 209).
Computer to Plate (CTP) / Direct to Plate (DTP)

Computer to Plate (CTP) is a newer technology that allows the imaging of metal or polyester plates without the use of film. Eliminating the stripping, compositing, and traditional plate making processes, CTP revolutionized the printing industry and led to reduced prepress times, lower costs of labor, and improved print quality.

Most CTP systems used thermal CTP as opposed to violet CTP, though both systems are effective, depending on the needs of the printing job. Thermal CTP does have the advantage of extremely high quality, but Violet CTP does cost significantly less. Thermal plates are generally used for longer runs, while Violet CTP is employed for shorter runs, and popular with 2-up and 4-up applications (Bruno, Romano, and Riordan 126).

Thermal CTP has the added bonus of utilizing binary exposure, which limits the risk of under or overexposure, and makes it possible to work under yellow light (Bruno, Romano, and Riordan 126).

Thermal CTP involves the use of thermal lasers to expose and/or remove areas of coating while the plate is being imaged. This depends on whether the plate is negative, or positive working. These lasers are generally at a wavelength of 830 nanometers, but vary in their energy usage depending on whether they are used to expose or ablate material. Violet CTP lasers have a much lower wavelength, 405–410 nanometers. Violet CTP is “based on emulsion tuned to visible light exposure,” (Bruno, Romano and Riordan 126). The general trend of platesetters has been to move toward coatings whose success on press is independent of post imaging chemical bath processing.
Types of chemicals used
Paste inks for offset litho

Offset printing uses inks that, compared to other printing methods, are highly viscous. Typical inks have a dynamic viscosity of 40–100 Pa·s.[7]

There are many types of paste inks available for employment in offset lithographic printing and each have their own advantages and disadvantages. These include heat-set, cold-set, and energy-curable (or EC), such as ultraviolet- (or UV-) curable, and electron beam- (or EB-) curable. Heat-set inks are the most common variety and are “set” by applying heat and then rapid cooling to catalyze the curing process. They are used in magazines, catalogs, and inserts. Cold-set inks are set simply by absorption into non-coated stocks and are generally used for newspapers and books but are also found in insert printing and are the most cost-conscious option. Energy-curable inks are the highest-quality offset litho inks and are set by application of light energy. They require specialized equipment such as inter-station curing lamps, and are usually the most expensive type of offset litho ink.
Inks
Letterset

Letterset inks are mainly used with offset presses that do not have dampening systems and uses imaging plates that have a raised image. (Romano, & Riordan 160)
Waterless

Waterless inks are heat-resistant and are used to keep silicone-based plates from showing toning in non-image areas. These inks are typically used on waterless Direct Imaging presses. (Romano, & Riordan 160)
Single fluid

Single Fluid Inks are newer ink that uses a process allowing lithographic plates on a lithographic press without using a dampening system during the process. (Romano, & Riordan 160)
Ink/water balance

Ink and water balance is an extremely important part of offset printing. If ink and water are not properly balanced, you can end up with many different problems such as scumming, trapping, and ink density issues. With the proper balance, the job will have the proper ink density and should need little further adjustment.
Fountain solution

Fountain solution is the water-based (or “aqueous”) component in the lithographic process that cleans the background area of the plate in order to keep ink from depositing (and thus printing) in the non-image (or “white”) areas of the paper. Historically, fountain solutions were acid-based and made of gum arabic, chromates and/or phosphates, and magnesium nitrate.

While the acid fountain solution has come a long way in the last several decades, neutral and alkaline fountain solutions have also been developed. Both of these chemistries rely heavily on surfactants/emulsifiers and phosphates and/or silicates to provide adequate cleaning and desensitizing, respectively. Since about 2000, alkaline-based fountain solutions have started becoming less common due to the inherent health hazards of high pH and the objectionable odor of the necessary microbiogical additives.

Acid-based fountain solutions are still the most common variety and yield the best quality results by means of superior protection of the printing plate, lower dot gains, and longer plate life. Acids are also the most versatile, capable of running with all types of offset litho inks. However, because these products require more active ingredients to run well than do neutrals and alkalines, they are also the most expensive to produce. That said, neutrals and, to a lesser degree, alkalines are still an industry staple and will continue to be used for most newspapers and many lower-quality inserts.

In recent years alternatives have been developed which do not use f solutions at all (waterless printing).
Identification

Every printing technology has its own identifying marks, as does offset printing. In text reproduction the type edges are sharp and have clear outlines (see following picture). The paper surrounding the ink dots is usually unprinted. The halftone dots are always irregular and blurry though there are different screening methods (AM andFM). [8] [9]
In industry

Offset lithography became the most popular form of commercial printing in the 1950s (“Offset Printing”). Subsequent improvements in plates, inks, and paper enhanced the technology and maximized its superior production speed and plate durability. Today, lithography is the dominant printing technology in the US, and most lithography is printed as offset lithography.

Today, offset lithography is “responsible for over half of all printing using printing plates” (Bruno, Romano and Riordan 137). The quality of the prints made is consistently high, and the volume of prints created for their respective cost makes commercial offset lithography very efficient for businesses, especially when many prints must be created quickly.

Process overview

1. Platemaking[3]
The first method of plate development uses light-sensitive polymer. A film negative is placed over the plate, which is exposed to ultra-violet light. The polymer hardens where light passes through the film. The remaining polymer has the consistency of chewed gum. It is washed away in a tank of either water or solvent. Brushes scrub the plate to facilitate the “washout” process. The process can differ depending on whether solid sheets of photopolymer or liquid photopolymer are used, but the principle is still the same. The plate to be washed out is fixed in the orbital washout unit on a sticky base plate. The plate is washed out in a mixture of water and 1% dishwasher soap, at a temperature of approximately 40°C. The unit is equipped with a dual membrane filter. With this the environmental burdening is kept to an absolute minimum. The membrane unit separates photopolymer from the washout water. After addition of absorb gelatine for example, the photopolymer residue can be disposed of as standard solid waste together with household refuse. The recycled water is re-used without adding any detergent [4]. The second method used a computer-guided laser to etch the image onto the printing plate. Such a direct laser engraving process is called digital platemaking. Companies such as AV Flexologic, Polymount and Screen from The Netherlands are market leaders in manufacturing this type of equipment.

File:Aquasupreme2.gif

The third method is to go through a molding process. The first step is to create a metal plate out of the negative of our initial image through an exposition process (followed by an acid bath). This metal plate in relief is then used in the second step to create the mold that could be in bakelite board or even glass or plastic, through a first molding process. Once cooled, this master mold will press the rubber or plastic compound (under both controlled temperature and pressure) through a second molding process to create the printing plate.

2. Mounting
For every colour to be printed, a plate is made and eventually put on a cylinder which is placed in the printing press. To ensure an accurate picture is made, mounting marks are made on the flexographic plates. These mounting marks can be microdots (down to 0.3 mm) and/or mounting crosses. To make a complete picture, regardless of printing on flexible film or corrugated paper, the image transferred from each plate has to fit exactly in the images transferred from the other colors. Highly accurate and specific machinery is made for mounting these plates on the printing cylinders. One of the latest advances in this field is Fully Automatic Mounting Machine (FAMM), for which AV Flexologic won the FTA Technical Innovation Award in 2007.

File:Famm.gif

3. Printing
A flexographic print is made by creating a positive mirrored master of the required image as a 3D relief in a rubber or polymer material. Flexographic plates can be created with analog and digital platemaking processes. The image areas are raised above the non image areas on the rubber or polymer plate. The ink is transferred from the ink roll which is partially immersed in the ink tank. Then it transfers to the anilox roll (or meter roll) whose texture holds a specific amount of ink since it is covered with thousands of small wells or cups that enable it to meter ink to the printing plate in a uniform thickness evenly and quickly (the number of cells per linear inch can vary according to the type of print job and the quality required).[5] To avoid getting a final product with a smudgy or lumpy look, it must be ensured that the amount of ink on the printing plate is not excessive. This is achieved by using a scraper, called a doctor blade. The doctor blade removes excess ink from the anilox roller before inking the printing plate. The substrate is finally sandwiched between the plate and the impression cylinder to transfer the image.[6]
Flexographic printing inks

The nature and demands of the printing process and the application of the printed product determine the fundamental properties required of flexographic inks. Measuring the physical properties of inks and understanding how these are affected by the choice of ingredients is a large part of ink technology. Formulation of inks requires a detailed knowledge of the physical and chemical properties of the raw materials composing the inks, and how these ingredients affect or react with each other as well as with the environment. Flexographic printing inks are primarily formulated to remain compatible with the wide variety of substrates used in the process. Each formulation component individually fulfills a special function and the proportion and composition will vary according to the substrate.

There are five types of inks that can be used in flexography: Solvent-based Inks, Water-based Inks, EB (Electron Beam) curing inks, UV(ultraviolet) Curing Inks and two-part chemically-curing inks (usually based on polyurethane isocyanate reactions), although these are uncommon at the moment.[7] Water based flexo inks with particle sizes below 5 µm may cause problems when deinking recycled paper.
Ink control

The ink is controlled in the flexographic printing process by the inking unit. The inking unit can be either of Fountain Roll system or Doctor Blade System. The Fountain roll system is a simple old system yet if there is too much or too little ink this system would likely not control in a good way. The doctor blade inside the Anilox roller uses cell geometry and distribution. These blades insure that the cells are filled with enough ink.[2]
Applications

Flexo has an advantage over lithography in that it can use a wider range of inks, water based rather than oil based inks, and is good at printing on a variety of different materials like plastic, foil, acetate film, brown paper, and other materials used in packaging. Typical products printed using flexography include brown corrugated boxes, flexible packaging including retail and shopping bags, food and hygiene bags and sacks, milk and beverage cartons, flexible plastics, self adhesive labels, disposable cups and containers, envelopes and wallpaper. A number of newspapers now eschew the more common offset lithography process in favour of flexo. Flexographic inks, like those used in gravure and unlike those used in lithography, generally have a low viscosity. This enables faster drying and, as a result, faster production, which results in lower costs.

Printing press speeds of up to 600 meters per minute (2000 feet per minute) are achieveable now with modern technology high-end printers, like Flexotecnica [1], which introduced the world’s first 12-color central impression (CI) drum press at Drupa 2008. This groundbreaking technology won the prestigious FlexoTech (UK) Innovation Award in 2008 [2]. Two 12-color presses have been installed in Europe.

Other press formats, such as in-line and stack presses, are available from Tresu and other suppliers.

http://en.wikipedia.org/wiki/Flexography