BOOK TWO / Colour & Process Colour Management Colour management is a process that governs how colour is translated from one pieve of equipment in the printing process to another. Colour management is needed to ensure accurate and predictable colour reproduction because each device responds to or produces colour differently. Gamut & Colour space Gamuts and colour spaces areused to calculate the range of colours that can be produced with a given set of colourants on a particular device or system. Gamut In the printing industry the common gamuts are RGB, CMYK and Hexachrome, which has a 6 colour gamut of CMYKOG (orange and green). The range of spectral colours visible to the human eye can also be seen as a gamut. Colour space Each device used in the graphic design and printing industries produces or reproduces a certain array of colours called a colour space. RGB is an additive primary colour space that computer monitors use and CMYK is the subtractive primary colour space used in the 4 colour printing process. Colour profiles Euroscale Coated is a colour profile that uses specifications designed to produce quality separations using the CMYK process. It was created to define CMYK for offset-printing on glossy paper, with colours generated by mixing the four process colour inks under the following conditions; 350% total area of ink coverage, positive plate and bright white-coated stock. The Euroscale Uncoated profile is a CMYK working space created for use with uncoated stock under the following printed conditions: 260% total area of ink coverage, positive plate and uncoated white offset stock. SWOP (specifications for web offset publications) is a standard colour profile used to ensure the consistent quality of advertising in publications in North America. Adobe Photoshop uses the SWOP profile as its default for making CMYK colour separations. Common colour spaces RGB The RGB colour space can reproduce about 70% of the colours in the spectral gamut that can be perceived by the human eye. sRGB Standard RGB is a standardised, device-independent, calibrated colour space defined by HP and Microsoft in the 1990s to provide a consistent way to display colour images on computer screens (CRTs). ColourMatch RGB ColourMatch RGB has a wider colour space than sRGB and was developed to closely simulate CMYK press work. ColourMatch RGB features a low gamma than that of sRGB. Gamma affects how bright mid-tones appear and so switching to ColourMatch RGB can be a simple way to brighten a photo. Pantone & Spot Colours Spot colours are used to ensure a particular colour will be printed. This may be necessary if the colour is outside the gamut/possibilities of CMYK printing process, or if a specific colour needs to be printed for a corporate logo or such. Special colours have greater intensity and vibrancy as they print as a solid colour and not a combination of half tone dots. Spot colours and CMYK (show example) Show how CMYK process colour display differently to spot colours. Explain this is because CMYK is printed as half-tone dots where as spot colours are printed as a solid, continuous colour. Mixing a spot colour Spot colours are made from various base elements, mixed according to a specific recipe. Spot colour inks can be bought pre-mixed and ready to use or they can be created by mixing the constituent parts. Pantone systems The pantone PMS colour system has developed to include a wide range of different colours, including special solid, hexachrome, metallic and pastel colours.
The pantone system allocates a unique reference number to each hue and shade to facilitate communication between designers and printers. U = Uncoated C = Coated EC = Euro Coated M = Matte Pantone guides explained Pantone solid A range of solid metallic, pastel and process colours that can be used on different paper stocks and substrates. The flourescent opposite would be Pantone 806U, 806C or 806M depending on whether it is to print on matte, coated or uncoated stock. Pantone pastels A range of flat, solid, but very pale colours. These are different to tints as they print as a solid colour without visibl dots. They are available in both coated and uncoated swatches. Pantone hexachrome A range of 6 colour process colours used for hexachrome printing. In addition to the CMYK process colours, the system adds green and orage process colours allowing it to reproduce 90% of the Pantone PMS colours. Pantone metallics A range of over 300 special colours that give a metallic effect includign silver, gold and copper colours. Metallics are available in varnished and unvarnished coated swatches. Colour in Print Before sending a design to print there are a range of methods to ensure that a colour used will appear as intended. Preparing colour for print There must be a number of pre-press checks made to ensure clear communication between design, client and printer. This is vitally important if the client is to end up with the work that they have been expecting. Printed pages or panels Printed pages (or PP) refers to the actual number of pages printed on and not the number of sheets printed on. For example a booklet made from 4 sheets with print on every side will have eight printed pages once folded. The key is to remember that one sheet of printed double sided is equal to 2 printed pages. The same rule of thumb applies to the use of panels, which is simply another way of folding a printed sheet. Before sending a file to print. 1 Delete all unused colours 2 Ensure all that you want to print in black is actually black, not in registration, as registration will print in all prints. 3 Ensure all that should be in registration, and not in black, as black will only print on the black plate 4 Ensure all spot colours are accounted for. If the job is printing with a special colour, all i well; if the job is printing CMYK only then convert the spot colours to CMYK. 5 Ensure all images are converted to CMYK and not RGB. This includes logos, maps, additional icons. 6 Ensure you are clear that your colour-fall matches the printer’s expectations. If the printer is expecting a 4-colour job then supplying a file with a spot colour will cause confusion. 7 Ensure your imported swatches are of the right value. If the job is being printed uncoated then set any spot colours as uncoated, not coated or unspecified. Tints and mixing colours Process and special colours can be combined using tints and overprinting to produce many different colour effects. Tints The 3 trichromatic process colours (cyan magenta and yellow) can be printed in increments of 10% to produce 1,300 tints. Tints of below 10% and above 90% may not print accurately due to spot gain.
Overprinting Overprinting is where one ink overprints another so they mix to create different colours. To overprint effectively the designer needs to take in to account the order in which the colours are printed. Multi-ink Tints of 2 or more process colours can be combined to create new colour combinations using the multiink function. A note about percentages As each CMYK ink can be applied with a value ranging from 0 to 100% a colour is therefore expressed as the percentages of each ink. The total value of these percentages should not be more than 240 as this results in a muddy colour. Tint charts (make chart showing the tint increments) Tint charts give a reasonable idea of how colours will print when combined together. However the accuracy of these colours depends on the colour control in the printing process, the press used and the stock being printed on. Tints allow for an increased range of colour possibilities available when the budget for a job is insufficient to cover the cost of 4 colour printing. The adds variation to colours if there is a limited colour palette. PRE-PRESS Pre-press emcompasses a range of different processes through which the raw materials for the visual elements of a print job are created and brought together in the final design and prepared for the printing process. This chapter will discuss aspects such as scanning images, resolution, file formats, page impostion, ink trapping and proofing, in addition to many other methods that are used to produce a printed product. The pre-press stage is where aspects that may cause printing problems later must be addressed. Resolution Resolution refer to the amount of information contained within a digital image. Images containing more information have a higher resolution and therefore higher print quality. Resolution and pixel depth Resolution is also determined by pixel depth, the number of bits available to generate a colour for each pixel. Greater bit depth means that more colours are available and more accurate colour reproduction is possible in the digital image. 1- bit (bitmap) 1 bit means that an image has either black or white pixels so no continuous tones are possible. 1-bit images are best used for line art. 8 - bit (greyscale) 8-bit pixel depth means it is possible to reproduce 256 shades of grey. 8-bit pixel depth allows for the continuos tones of photographs. 8-bit (indexed colour) 8-bit pixel depth can reproduce a palette of 256 colours. While it can produce continuous tones the imited colour palette makes for washed out looking images. 24-bit (RGB) Produces a wider palette of about 16 million colours, allowing a more realistic continuous tone. Using the RGB system wth each colour/channel containing 8 bits. 16-bit (RGB) 16-bit pixel depth means that each of the RGB colour channels has 16 bits. The result is a 48-bit image capable of containing billions of colours. 32-bit (CMYK) Converting a 24-bit RGB image into CMYK creates a 32-bit image. The final image will contain less information than the 16-bit original and will be duller than he 24-bit RGB used on screen graphic. The 32-bit CMYK should be used solely for printed applications. SPI, PPI, DPI & LPI Resolution is a measure of the number of pixels contained in a digital image.
SPI Number of samples a scanner head takes as it passes over a source image. The higher the number the better resolution the image will be. PPI Describes the number of pixels both horizontally and vertically in an image. A low resolution image/one with less pixels will print out pixelated. DPI Measures the ink dots a printer can deposit on a substrate per inch. For offset litho printing purposes a resolution of 300dpi is standard. LPI Number of cells in a half-tone grid used to reproduce continuous images such as photographs. Scanning Scanning is a process through which an image or piece of artwork is converted into an electronic file. Flatbed scanning FThis method is cheap and easy to use and produces good reproduction of flat tone artwork. This method is not suitable for very high quality reproduction as they have lower resolution capacity than other scanners. They need accessories to be able to scan transparencies. Drum Scanning A drum scanner uses photomultiplier tubes rather than a charged couple device to obtain an image. The original is mounted on to the scanner drum, which is rotated before scanner optics that separate the light from the artwork into red, green and blue beams. Drum scanners produce very high resolution images but are very expensive to buy. Colour scales These scales are placed against the original artwork and the scanned result to assess colour reproduction quality and allow for adjustments to be made. Calculating image sizes Image size can be changed during the scanning process to maximise reproduction quality. Enlarging images A scanner takes a specific number of samples fromt he source image when it scans. The SPI is specified according to the quality and resolution required. A source image can be enlarged or reduced during the scanning process through scaling. Previously scanned material When an image is scanned from a book or magazine it may have a moire pattern because the printed image was made with screened half-tones, and scanners do not remove these. This can be used by using the noise filter in Photoshop. Creative techniques Overprinting Printing 2 inks over each other to create a new colour. Setting overprint By default text and objects knock out rather than overprint as the ink for one item is printed into gaps left for it. Objects can be set to overprint, but colours can only overprint in the order that they are laid down in the printing process. With CMYK systems, cyan can overprint all colours, while yellow can onlu overprint black. Backgrounds frames and text can all be set to overprint. The 4 process colours ‘overprint’ the stock they are printed on. Default settings preserve each colour as they knockout each other. Set to overprint, the colours overprint one another in the printing process order. All 4 colours can overprint to create a stronger black than printing black alone. Artwork This is an important stage of the print process.
Bleed The printing of a design over and beyond it’s trim marks. 3mm bleed is the norm. Trim The process of cutting away the waste stock around a design to form the final format once the job has been printed. Registration The alignment of 2 or more printed images with each other ont he same stock. Printing a 4-colour job To print a 4 colour print, the design needs to have bleed so that once it is trimmed it will not have a white-edge of unprinted stock. Normal design practice stipulate 3mm bleed. Registration black Registration black is a black made up of all 4 process colours (CMYK). Using registration black for type and image is a common mistake and is undesirable. This is because all type will appear of each plate, making registration a potential issue. Registration black can be used for crop marks if self drawing them. Registration problems Registration problems can occur when more than one colour is printed. A 4 colour image looks blurred or distorted due to mis-registration. A) B) C) D) E) colour
A greyscale image prints fine as it prints just with a black plate. Any single colour printed from a single plate will be fine. A misaligned 4 colour black causes problems A poorly aligned duotone image. Mis-registration of low type size can cause illegibility. Reversed out text should be done on 1 to reduce this problem.
Trapping The aim in the print process to get accurate registration. This is not always possible, gaps can appear when 2 inks printed as solid colours are placed next to each other. The is solveable through ink trapping. Spread & Choke The main trapping options used to prevent small gaps appearing between blocks of colour are spread choke and centred trapping. Most ink traps use spreading where lighter colours will spread to merge with darker colours. Choke can be used to reduce the size of the aperture that an object will print in, a centred trapping uses a combination of enlarging the object and reducing the aperture by the same aount. Knockout & overprint (untrapped options) There are 2 options when dealing with untrapped colours, knockout and overprinting. An overprint is when one colour prints over another, a knockout is where there is a gap left for a colour to print in to. Reverse out When istead of being printed a design is removed from the block of printed colour and left as an unprinted area (show example). Surprint 2 elements printed on top of each other where one is a tint of the same colour. (example 70% black printed onto 100% black) (show example). Types of black There are several types of black to choose from. 4-Colour black The 4-colour black si the darkest black to chose from and is made when all 3 primaries are overprinted on to each other. Warm and cool blacks Flat areas of black can be enhanced by applying a shiner of another colour underneath. To make a warm black add magenta to the black, for a cool black add cyan. Rich black Printing black with a 50% shiner of cyan, yellow and magenta will create a rich black. This will reduce any registration problems.
Imposition Imposition is the order in which the pages are printed. Planning Information on stock, spot colours and more things can affect the order that a printer decides to print. Pages to be printed with similar colours and stocks can be grouped together to reduce costs. Pages to view Refers to the number of pages printed on one side of the sheet of stock. Throw-outs and gatefolds These are methods of inserting oversized pages into a publication. Gatefolds A gatefold is a folded sheet with 4 panels. These panels fold in-wards. They are used to provide extra space in smaller publications. Throw-outs A throw-out is half a gatefold that is bound in to the spine and only opens out on one side. The throw-out side must be smaller than the publications to allow for easy folding. Tip-ins A tip-in is the attachment of a single page into a publication by wrapping it around the central fold of a section and glueing along the binding edge. If the tip-in is shorter that the publication it must be aligned to either the top or bottom edge. Tip-ons A tip-on is when a page or other element is pasted in to a publication. Proofing Refers to steps being taken to ensure an accurate print reproduction. Types of proof Soft or screen proof A proof used to check layout and colour information and to check the screen structures of a print. Advantages Intended to eliminate moire, rosette and other undesired effects. Disadvantages Screening must be erformed before a screen proof is printed and printing data contains no screen information. Laser proof A black and white computer print. Advantages Shows photos, text and position. Cheaper than a blueline. Disadvantages Low resolution and may not reprduce at actual size. Pre-press proof An analogue or digital proof that gives an approximation of what the finished piece will look like. Advantages Inexpensive, particularly digital proofs. Disadvantages Colour not as accurate as press proof as does not use actual printing inks. Blueline, Dylux or salf proof A contact print produced from film. Shows imposition, photos and text as will appear when printed, together with trim and binding edges. Advantages Rapid as no processing is involved and pages can be folded, trimmed and stitched to approximate the finished job. Disadvantages one colour and does not reflect paper stock or true colour. Proof has a blue colour and the image fades with time.
Scatter proof A proof of an individual photo or group of photos not included as part of the individual page layout. Advantages For checking colour before the final proof. Many photos can be proofed at once to save time and materials Disadvantages Images not seen in the layout. Composite integral colour proof High quality proofs (such as match print of chromalin) produced using 4 sheets (one for each colour) laminated together in register. Advantages very accurate colour proof produced from the colour separation film used to make printing plates. Disadvantages Time consuming and labour intensive as an additive proof takes about 30 minutes to produce. Press or machine proofs A proof produced using the actual plates, inks and paper Advantages Realistic impression of the final print. Can be produced on actual print stock. Disadvantages Costly as the plates have to be set up, particularly if another proof is required following changes. Contract proof A colour proof used to form a contract between the printer and the client, final proof before going to press. Advantages Accurate representation of the final print job. Disadvantages N/A Printing Understanding print order Print order refers to the order in which the colours are printed. For 4-colour the process is print, cyan, magenta, yellow and then black. Black is labelled K for key, as black is the colour the other colours key to when registering. The acronym CMYK implies a sense of order: cyan, magenta, yellow, black. While work is frequently printed in this order the printer can change the order if overprints are included or area of single flat colours are used. Printing imposition Imposition is the arrangement of pages in a sequence and the position in which they will be printed before being cut, folded and trimmed. Printing plate Each time the sheet goes through the printing press to receive an image is called a pass, and so doublesided printing usually requires 2 passes. Gripper edge Grabs a piece of paper and draws it into the printing press. On the printed imposed sheet space needs to be allocated for the gripper edge. Types of sheet work Sheet work Printing one side of a sheet of paper, turning it over and printing the other side with a separate plate. Work and turn Printing one side of a sheet, turning it from front to back and printing the second side with same sheetedge alignment on the press. Work and tumble Both sides of the sheet are set on one plate. The sheet is printed and turned over side to side to be printed again (backed up). Work and twist Printing one half of the sheet, turning it 180ยบ and then going back through to print the other side.
Screen angles Screen angle refers to the inclination or angle of the rows of half-tone dots that are used to form colour images in the 4-colour printing process. Why angles? Rows of half tone dots are set at different angles to prevent them from interfering with each other. If the dots were set at the same angle moire patterns would form. (show an example fo a moire pattern) Stochastic printing Stochastic or frequency modulation printing is a method that uses different dot sizes and placements as alternative method to help prevent the appearance of moire patterns. The overall effect is similar to the grain in photographic film. It can give a very good continuous tone. This is because half tone dots in prints have very little visibility and produce a high quality, detailed reproduction, As stochastic printing removes the barriers of screen angles you can print with more than the 4 process colours. Such as the hexachrome system. Stochastic printing also allows for more accurate reproduction of pastel colours and light tints can be achieved. Gradients and tints Gradients A graident uses one or more colours that combine to create the colour effect. One of the colours typically gets stronger or darker whilst the other one gets lighter. Linear gradient A linear gradient blends from one colour to another in a series of vertical steps. Multiple colours The gradient features multiple colours. The gradient can be adjusted to make the changes in colour sharper or more subtle. Radial gradient A radial gradient creates a blend in a circle pattern so that it issues from a central point. Tints A tint is a colour printed at a percentage of 10 to 90% of a full solid colour and is created using half tone dots of different sizes so that there is colour dilution from the substrate. Tint books Many tints are achievable with the standard process colours, either alone or in combination. Tint books show tints on a variety of stocks so you can see how the tints appears on coated and uncoated etc. On-Press Colours can be adjusted on a printing press whilst the job is being printed. Adjusting colour Colour adjustments may be made on press to account for colour variation caused by changes in ink density or plate pressure. A printer will make adjustments to make sure the colours are printing as they are on the proof. Proof marking A designer needs to review a wet proof of the job and mark up where changes to the colour are necessary. An eye glass is needed to check colour production against control strips and uses symbols to describe/ signal the changes that need to be made to the print colour. Colour checking, basic tools To check the colour of a job or printed sheet is pulled from the press and checked using a colour densitometer, a device that uses a light source and a photoelectric cell to measure optical density, or spectrophotometer. A printer also uses a loupe or eyeglass to check colour registration. Striker bar A printed sheet has a striker bar containing a series of predefine colours printed along it’s edge for colour checking. The bar includes additive primaries, subtractive primaries and overprints.
The press A modern lithographic printing press can control the colour density/plate pressure on the stock. This allows the printer to adjust in increments the balance of each colour/plate being printed. The imposed sheet The way a sheet is imposed affects how much colour can be altered while printing. When all the colours are similar there isn’t usually a problem. However when there are isolated patches of solid colour, such asa black square, are more difficult to alter, STANDARD PRINTING TECHNIQUES Digital Digital printing refers to methods of printing from a digital based image directly to a variety of media.[1] It usually refers to professional printing where small run jobs from desktop publishing and other digital sources are printed using large format and/or high volume laser or inkjet printers. Digital printing has a higher cost per page than more traditional offset printing methods but this price is usually offset by the cost saving in avoiding all the technical steps in between needed to make printing plates. It also allows for on demand printing, short turn around, and even a modification of the image (variable data) with each impression.[2] The savings in labor and ever increasing capability of digital presses means digital printing is reaching a point where it could match or supersede offset printing technology’s ability to produce larger print runs of several thousand sheets at a low price Process The main differences between digital printing and traditional methods such as lithography, flexography, gravure, or letterpress are that no need to replace printing plates in digital whereas in analog printing plates are continuously replaced, resulting in a quicker and less expensive turn around time, and typically a loss of some fine-image detail by most commercial digital printing processes. The most popular methods include inkjet or laser printers that deposit pigment or toner onto a wide variety of substrates including paper, photo paper, canvas, glass, metal, marble and other substances. In many of the processes the ink or toner does not permeate the substrate, as does conventional ink, but forms a thin layer on the surface that may be additionally adhered to the substrate by using a fuser fluid with heat process (toner) or UV curing process (ink). Applications - Digital printing has many advantages over traditional methods. Some applications of note include: - Desktop publishing – inexpensive home and office printing is only possible because of digital processes that bypass the need for printing plates - Variable data printing – uses database-driven print files for the mass personalization of printed materials - Substrate Printing - printing to surfaces including, but not limited to: wood, aluminum, acrylic, plastic, PVC, ABS, polycarbonate, glass, crystal, stainless steel and ceramic tile.[5] - Fine art – archival digital printing methods include real photo paper exposure prints and giclée prints on watercolor paper using pigment based inks. - Print on Demand – digital printing is used for personalized printing for example, children’s books customized with a child’s name, photo books (such as wedding photo books), or any other short run books of varying page quantities and binding techniques. - Advertising – often used for outdoor banner advertising and event signage, in trade shows, in the retail sector at point of sale or point of purchase, and in personalized direct mail campaigns. - Photos – digital printing has revolutionized photo printing in terms of the ability to retouch and colour correct a photograph before printing. Flexography Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. It is essentially a modern version of letterpress which can be used for printing on almost any type of substrate, including plastic, metallic films, cellophane, and paper. It is widely used for printing on the nonporous substrates required for various types of food packaging (it is also well suited for printing large areas of solid color). Process overview 1. Platemaking 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. 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. 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. 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 metal 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] The sheet is then fed through a dryer, which allows the inks to dry before moving on.In the case a UV ink is used, the sheet does not have to be dried, but is dried from UV rays. Basic Parts of the Press Unwind and Infeed Section�- The roll of stock must be held under control so the web can unwind as needed Printing Section- Single color station including the fountain, anilox, plate and impression rolls Drying Station- High velocity heated air, specially formulated inks and an after-dryer can be used Outfeed and Rewind Section- Similar to the unwind segment, keeps web tension controlled. How Does it Work? Brief Overview of the Printing Proc Ink controls 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. 1. Fountain Roller The Fountain Roller transfers the ink that is located in the ink pan to the second roller, which is the Anilox Roller. 2. Anilox Roller This is what makes Flexography unique. The Anilox Roller meters the predetermined ink that is transferred for uniform thickness. It has engraved cells that carry a certain capacity of inks that can only be seen with a microscope. 3. Plate Cylinder The Plate Cylinder holds the printing plate, which is soft flexible rubber. Sticky 2-way tape is used to mount the plate to the Plate Cylinder. 4. Impression Cylinder The Impression Cylinder applies pressure to the Plate Cylinder, where the image is transferred to the substrate.
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 Âľm may cause problems when deinking recycled paper. Presses Stack Press Color stations stack up vertically, which makes it easy to access. This press is able to print on both sides of the substrate. Common Impression Press All color stations are located in a circle around the impression cylinder. This press can only print on one side. Advantage: Excellent Registry In-Line Press Color stations are placed horizontally. This press prints only on one side. Advantages: Can print on heavier substrates, such as corrugated boards. Pad printing Pad printing is a printing process that can transfer a 2-D image onto a 3-D object. This is accomplished using an indirect offset (gravure) printing process that involves an image being transferred from the clichĂŠ via a silicone pad onto a substrate. Pad printing is used for printing on otherwise impossible products in many industries including medical, automotive, promotional, apparel, and electronic objects, as well as appliances, sports equipment and toys. It can also be used to deposit functional materials such as conductive inks, adhesives, dyes and lubricants. Physical changes within the ink film both on the clichĂŠ and on the pad allow it to leave the etched image area in favor of adhering to the pad, and to subsequently release from the pad in favor of adhering to the substrate. The unique properties of the silicone pad enable it to pick the image up from a flat plane and transfer it to a variety of surfaces, such as flat, cylindrical, spherical, compound angles, textures, concave, or convex surfaces. History While crude forms of pad printing have existed for centuries, it was not until the twentieth century that the technology became suitable for widespread use. First gaining a foothold in the watch-making industry following World War II, developments in the late 60s and early 70s, such as silicone pads and more advanced equipment, made the printing method far more practical. The ability to print on formerly unprintable surfaces caught the imaginations of engineers and designers, and as a result pad printing exploded into the mass production marketplace. Today, pad printing is a well established technology covering a wide spectrum of industries and applications. Process Pad printing cycle From the home position, the sealed ink cup (an inverted cup containing ink) sits over the etched artwork area of the printing plate, covering the image and filling it with ink. The sealed ink cup moves away from the etched artwork area, taking all excess ink and exposing the etched image, which is filled with ink. The top layer of ink becomes tacky as soon as it is exposed to the air; that is how the ink adheres to the transfer pad and later to the substrate. The transfer pad presses down onto the printing plate momentarily. As the pad is compressed, it pushes air outward and causes the ink to lift (transfer) from the etched artwork area onto the pad. As the transfer pad lifts away, the tacky ink film inside the etched artwork area is picked up on the pad. A small amount of ink remains in the printing plate. As the transfer pad moves forward, the ink cup also moves to cover the etched artwork area on the printing plate. The ink cup again fills the etched artwork image on the plate with ink in preparation for the next cycle.
The transfer pad compresses down onto the substrate, transferring the ink layer picked up from the printing plate to the substrate surface. Then, it lifts off the substrate and returns to the home position, thus completing one print cycle. Plate and ink interface technologies Open inkwell system Open ink well systems, the older method of pad printing, used an ink trough for the ink supply, which was located behind the printing plate. A flood bar pushed a pool of ink over the plate, and a doctor blade removes the ink from the plate surface, leaving ink on the etched artwork area ready for the pad to pick up. Sealed ink cup system Sealed ink cup systems employ a sealed container which acts as the ink supply, flood bar and doctor blade all at the same time. A ceramic ring with a highly polished working edge provides the seal against the printing plate. Printing pad Pads are three dimensional objects typically moulded of silicone rubber. They function as a transfer vehicle, picking up ink from the printing plate, and transferring it to the part (substrate). They vary in shape and diameter depending on the application. There are two main shape groups: “round pads” and long narrow pads called “bar pads”. Pads are also made in other shapes, called “loaf pads”. Within each group there are three size categories: small, medium, and large size pads. It is also possible to engineer custom-shaped pads to meet special application requirements. Image plate Image plates are used to contain the desired artwork “image” etched in its surface. Their function is to hold ink in this etched cavity, allowing the pad to pick up this ink as a film in the shape of the artwork, which is then transferred to the substrate. There are two main types of printing plate materials: photopolymer and steel. Photopolymer plates are the most popular, as they are easy to use. These are typically used in short to medium production runs. Steel plates come in two forms: thin steel for medium to long runs, and thick steel for very long runs. Both steel plate types are generally processed by the plate supplier as it involves the use of specialized equipment. Printing ink Ink is used to mark or decorate parts. It comes in different chemical families to match the type of material to be printed (please consult the substrate compatibility chart for selection). Pad printing inks are “solvent-based” and require mixing with additives before use. They typically seem dry to the touch within seconds although complete drying (cure) might take a substantially longer period of time. There are many more options. Inks that cure via the use of Ultra Violet light are convenient for certain applications. UV inks will not fully cure until UV light hits the ink. UV curable ink can be wiped off many substrates when mistakes are made. They can be cured with UV light in as fast as 1 second of light exposure. This depends on the ink, substrate and the light power and spectrum. UV inks can be reused as the pot life can be high as long as the ink stays clean, blocked from UV light and hasn’t broken down from sitting. This same feature makes it easier to clean than some solvent and epoxy like two part component inks. Also there are heat curable inks, which work in much the same way as UV in the sense that there is a “trigger” that cures the ink when pulled. Two part inks usually have a pot life of only a few hours or so. They must be disposed of when they cannot be revived (by means of retarders etc.) Climatic conditions will significantly affect the performance of any pad printing ink, especially the open ink well style printers. Too dry conditions can lead to faster evaporation of solvents causing the ink to thicken prematurely and too much moisture can lead to ink issues of “clumping” or something alike. Also the climate can affect other aspects of the printing process such as ink pick up and release from the plate to the pad to the substrate, as well as polymer plate to blade chattering or binding due to humidity. Making of printing plates There are two main techniques used to create a printing plate. The traditional technique requires a UV exposure unit and involves photo exposure with film positives and chemical etching of a photopolymer plate. A second technique known as “computer to plate” requires a laser engraver and involves laser etching of a specialized polymer plate. Although the latter technique is convenient for short run printing it does have several disadvantages over the former. Laser plate making is a process that requires the use of a very soft, low quality polymer coated plate. Thus, the standard cycle life that can be expected out of a laser etched plate is quite low (10,000 impressions on the high end). By comparison, a hardened steel plate can easily last for over 1 million impressions.
Printing application examples - Medical devices (surgical instruments, etc.) - Implantable & in body medical items (catheter tubes, contact lenses, etc.) - Golf ball logos/graphics - Decorative designs/graphics appearing on Hot Wheels or Matchbox toy cars - Automotive parts (turn signal indicators, panel controls, etc.) - Letters on computer keyboards and calculator keys - TV and computer monitors - Identification labels and serial numbers for many applications Screen printing Screen printing offers a lot of versatility for the designer. It gives scope to experiment with a lot of different inks and materials. Screen printing can be an automated process, or hand done. The screen is made of a piece of porous and finely woven fabric stretched over a rectangular frame. Areas of the screen are blocked off allowing a stencil of the image to be printed through. The screen is placed on top of the substrate to be printed, a squeegee or rubber blade is used to press the ink through the stencil and on to the substrate. The screen can be used many times. The screen printing process has an immediacy that other print processes don’t. Offset Lithography A printing process where an inked image from a printing plate is transfered or offset on to a rubber blanket roller, which is then pressed against the substrate. Litho uses a smooth printing plate and works on the basis that oil and water will repel each other. When the plate passes under the ink roller, non image areas that have a water film repel the oily inks that stick to the image area. Litho produces good photographic reproduction and fine linework on a veriety of stocks. The printing plates are easy to prepare and high speeds are available,. These reasons make it a cost effective way to print. Offset litho is available in sheet and roll fed form. Sheet fed is more appropriate for smaller print runs such as flyers, brochures and magazines while web printing is more appropriate for higher run jobs like newspapers, magazines and reports. Web print Web printing prints from a roll of paper rather than separate sheets. This allows for higher printing volume and speed with a lower production cost. Web can be used with litho but most commonly with relief printing methods such a rotogravure and flexography as the plates are more durable. Due to the scale and cost of this production method, it is not suitable for low volume printing. Problems with litho and web printing Cost benefits are only achieved with medium to high print runs. For very high print runs the printing plates could suffer, thereby increasing the cost (for these jobs rotogravure is used). Colour problems may also emerge due to problems with ink/water balance on the plate and the water can cause more absorbant subrates to distort. A dense ink film is also difficult to achive. Rotogravure Rotogravure (Roto or Gravure for short) is a type of intaglio printing process; that is, it involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a cylinder because, like offset printing and flexography, it uses a rotary printing press. Once a staple of newspaper photo features, the rotogravure process is still used for commercial printing of magazines, postcards, and corrugated (cardboard) product packaging. Advantages The rotogravure printing process is the most popular printing process used in flexible-packaging manufacturing, because of its ability to print on thin film such as polyester, OPP, nylon, and PE, which come in thicknesses of 10 to 30 micrometers. - Other appreciated features include: - printing cylinders that can last through large-volume runs without getting worn out - good quality image reproduction - low per-unit costs running high volume production Disadvantages Shortcomings of the gravure printing process include: - high start-up costs: hundreds of thousands of copies needed to make it profitable - rasterized lines and texts - use of chemicals in the ink.