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Basics of offset printing and other printing techniques

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  • These products are different because they have different levels of printing quality and their circulation is also different : Ikea magazine is printed in …….so printed in gravure when customer magazine like Air France for example have a circulation of 300 000 ex or books have a circulation of a few thousands ….. Quality Low Medium High Ink-jet Electro photo- graphy Gravure Heatset offset Sheet-fed offset
  • So let’s have an overview of our program : Offset printing is the most common printing method applied to Sappi products so we’ll spend much more time on this process . Lithography Method was invented by Alois Senefelder in 1798 Then we’ll have a quick view on other technics
  • In this slide there is the print quality level and suitable job size for different printing methods. The first two bubbles are desktop office printing and colour copying, third is DIGITAL printing, and then the conventional printing methods of which offset is the major one.
  • PLAN A FAIRE SUR PAPERBOARD First of all, printing is applying colors on a substrate; so it is important to understand what’s make the color ?
  • Colour is not a physical reality; colour is in the brain and not in the material. Colour is an interpretation made by the brain of perceptions coming from the eye. Colour is different from one to the other. If there is no light or no electromagnetic radiation, no colour can be seen!
  • SLIDE A FAIRE SUR PAPERBOARD We only see material without colour. Material has the property to reflect more or less some rays which the human eye are sensible to. What do we need to see colours ? 3 elements : A light source, an object and a processor which is the eyes It is a complex mechanism to explain how electromagnetic radiations are turned into the coloured sensation for the human being :
  • All humans perceive color in a similar manner, but not all perceive them identically.􀂃Light enters the eye through the lens and is focused on to the retina. There, an array of photoreceptor cells respond to light and pass the signals on to the brain.􀂃The light receptors are called rods and cones.􀂃Rods are not sensitive to color and work at low levels of illumination.􀂃3 kinds of cones:long (RedRed), ), medium (GreenGreen))short (BlueBlue) wavelengths.) wavelengths. The eye is picking up the photons * The retina is coding the photons This code is sent to the brain which translates in coloured sensation Because of rods, we can see in the dark but only black and white. Because of the three cone cells, light stimulates the eye three dimensionnally. Furthermore this is the basis for multi colour printing using three primary colours. Multi colour reproduction does not imitate the spectral distribution of the original colour but it stimulates similar colour perception in the human eye. This is possible because physically different spectrums can cause the same colour perception.
  • We are exposed to lots of electromagnetic rays. Only a small part can be seen. Light can be from a physical perspective, small particles of energy called photons which move at 00000km/heure or electromagnetic waves who move at the same speed. When the wavelength is between 400 and 700, the ray can be seen as visible light; these rays mainly comes from the sun. Visible light is about 80% of the rays tranmsitted by the sun. UV is making a reaction on lignin which makes the paper becoming yellow. OBA turn the UV light into visible blue light (fluorescence). Every colour group represents third of the spectrum. White light is a mixture of all the colours. The speed with which waves are moving is called the frequency and the length of one wave is the wavelength Faire dessin au tableau
  • They are technically 2 ways to mix colours : substractive and additive. With the substractive mixing, white light is filtered by coloured filters. With the additive method, the required colour is generated by mixing lights of different colours (like television).
  • With the additive method, the required colour is generated by mixing lights of different colours (like television). Green + Blue = CYAN Green _+ Red = YELLOW Blue + Red = MAGENTA
  • In printing the colour mixing method is mainly substractive. The printing of colours is based on the selective absorption by the ink of a certain wave length area. If the blue spectrum is absorbed by the ink, the reflected light is seen as yellow; furthermore absorption of the green produces magenta and absorption of the red gives cyan. Colour mixing in printing is also partly additive when the half tone dots of different primary colours are near each other in the light tones. The three primary colours used in printing are Cyan, Magenta and Yellow.
  • In principle it would be possible to make black by mixing all the 3 primary colours Mixing the 3 primary colours theoretically results in black. ->In practice the result is brown ! => To remedy this, black is used as the fourth primary colour. It improves the quality and is also cheaper than coloured inks
  • There are 3 typical charcteristics defining offset : a flat plate, a fountain solution and a rubber blanket On a printing plate there are two areas: 1. Areas accepting water and due to that being repulsive to ink = Non-Image areas (= hydrophilic) 2. Areas accepting ink and at being repulsive to water = Image areas (= hydrophobic) The behavior of the image and non-image areas is caused by a complex chemistry. The distinction between image areas and non image areas is based on differences in surface energies and hydrophilicity. The image area is a friend of grease and repellent to water. The non image area is a friend of water but a repellent to grease. Grease means ink and water is fountain solution. The netire process is based on the property that grease and water do not mix.
  • Two things are very important: 1.Image areas are hydrophobic and non-image areas are hydrophilic . Reason: Special chemistry of printing plates 2.Dampening water and ink do not mix. -> In the hydrophobic areas a ink layer will stay on the plate -> In the hydrophilic are a water film stay on the plate
  • Image areas are hydrophobic and non-image areas are hydrophilic … … here to be seen on a magnified picture of a of set plate.
  • The image areas are slightly raised as you can see.
  • Theoretically it is possible to transfer the ink directly to substrate. = Direct transfer. The direct ink transfer is not in practical use.
  • The ink layer on the image area of the plate is... ...transferred to the rubber blanket… … from the rubber blanket the ink is transferred to the substrate. Why using a rubber blanket? 2. Higher speed (Rubber allowed rotate printing) 1. Bigger printing runs (Hard plate and soft rubber) 3. Rubber allows printing on uneven surfaces as well
  • The plate is wetted by fountain solution before it is inked Water forms a film on the non image areas. When ink is fed to the plate it is unable to transfer to the damped areas and it will only be taken up by the water free image areas. The principle of offset is indirect : this is how offset got his name. Many ink rollers are needed to spread the tacky ink into a thin layer. – La couche d'encre sur le matériel est entre 0.7 et 1.1μm Now let’s go into more details about the different elements
  • How does a printing plate work ? It is an assembly of several chemical layers stacked on an aluminium basis. The thickness of the alu plate can vary from 0,15 to 0,4 mm . Then we have at least 2 layers One layer of aluminium oxide which attracts water on non image areas of the plate and another layer which is light sensitive and whose formula is kept secret which attracts ink on image areas. The action of light destroys the coating on non –image areas . Détruire à la lumière la couche imprimante Dans le cas d'une plaque traditionnelle positive, l'action de la lumière d'insolation détruit la couche imprimante sur les zones non masquées par le film, c'est-à-dire sur les zones ne devant pas être imprimées. Dans le cas d'une plaque négative, la lumière, au contraire, solidifie la couche imprimante sur les zones non masquées par le film, qui sont cette fois-ci, les zones à imprimer. Une image positive après insolation En CTP, le principe est le même : le laser de l'imageuse vient détruire la couche imprimante sur les zones vierges de la page ou solidifier la couche sur les zones à imprimer. C'est pourquoi il existe des plaques CTP positives ou négatives. Dans tous les cas, les plaques portent une image positive après insolation et développement. The printing plate has a light sensitive coating. We expose the plate to light through either a positive or negative film. Most plate making is now filmless with the image being directly exposed on the plate. CTP (computer to plate) can be used when the plates are on the press.
  • How does a printing plate work ? It is an assembly of several chemical layers stacked on an aluminium basis. The thickness of the alu plate can vary from 0,15 to 0,4 mm . Then we have at least 2 layers One layer of aluminium oxide which attracts water on non image areas of the plate and another layer which is light sensitive and whose formula is kept secret which attracts ink on image areas. The action of light destroys the coating on non –image areas . Détruire à la lumière la couche imprimante Dans le cas d'une plaque traditionnelle positive, l'action de la lumière d'insolation détruit la couche imprimante sur les zones non masquées par le film, c'est-à-dire sur les zones ne devant pas être imprimées. Dans le cas d'une plaque négative, la lumière, au contraire, solidifie la couche imprimante sur les zones non masquées par le film, qui sont cette fois-ci, les zones à imprimer. Une image positive après insolation En CTP, le principe est le même : le laser de l'imageuse vient détruire la couche imprimante sur les zones vierges de la page ou solidifier la couche sur les zones à imprimer. C'est pourquoi il existe des plaques CTP positives ou négatives. Dans tous les cas, les plaques portent une image positive après insolation et développement. The printing plate has a light sensitive coating. We expose the plate to light through either a positive or negative film. Most plate making is now filmless with the image being directly exposed on the plate. CTP (computer to plate) can be used when the plates are on the press.
  • The pigment gives the color to the ink; they are mostly organic ones but sometimes inorganic are used too (carbon black). The binders are hard resins and drying oils; the main function of the binder is to key pigments to the paper; Thin mineral oils are used as solvents in inks; their function is to solve the binders. Additive are used in the ink to adjust the performance at the press; we can add driers for example which are cobalt and manganese; waxes are usually polyethylene Lots of changes these last years due to speed increased and environment
  • Blankets are mainly characterized by the following properties : release, hardness, compressibility and surface Hardness Harder blankets have better release properties and they are more durable Softer blankets have better contact with paper/board and better ink transfer properties
  • Requirements of a fountain solution : Quick spreading over non-image area of the plate Desensitization of the non-image area of the plate Good transfer on rollers Stable pH Minimal corrosive effects No bacterial spoilage No foaming Lubrication of the blanket Requirements: Quick spreading over non-image areas of the plate Emulsion with the ink Stable pH Protection of the non-image areas of the plate Cooling of the inking unit Properties pH 4.5-5.5 If too low ink does not dry If too high emulsification increases Surface tension Low surface tension improves wetting of the plate Viscosity Higher viscosity improves the fountain solution transfer in the rollers Hardness Conductivity Temperature
  • Because one half tone image may contain thousands of different shades, it would be impossible to print the tones without screening. Screening means changing the colours into dots, the size and/or amount of which varies according to the shade. There are 2 types of screens : Conventional screen Stochastic screen
  • In stochastic screening, the image is reproduced from small, almost randomly distributed dots. Therefore there is no screen ruling at all. By stochastic screen it is possible to produce superior details and a continuous-tone-like appearance.
  • Because the dot size in the stochastic screening is very small (15 - 40 µm) it is possible to print sharper images.
  • In conventional screening the size of the dots varies according to the tone. Light tones need small dots and dark tones need bigger dots. The dots are so small that the human eye sees them as a single colour. In conventional screening the dot size varies, but their distance from dot center to dot center is constant. The distance is determine by screen ruling. It shows how many lines of dots there are per centimeter or per inch.
  • The eye has limits ...
  • Speeds ~ 5 000 - 20 000 copies/h One side printed in one unit (both sides 4+4 units) Extra colour units (PMS-colours) or varnishing units Sizes A3 (2/4 pages) A2 (4/8 pages) A1 (8/16 pages) A0 (16/32 pages) XXL (64 A4) Sheet offset printing: Sizes from DIN A4 to 120 x 162 cm 1 - 12 printing units max. 18,000 sheets / h One or two-sided printing Basis weight from 70 - 400 g/m² usual
  • Why more than 4 colours ? Spot Colours Brand Colours Metallic printing inks Heavy ink layers Solid areas
  • A SRA = Supplement Raw format A (refers to untrimmed paper formats)
  • The sheets are taken one by one by suction
  • This is a picture taken from above the press
  • This picture shows how the sheet is transferred from one printing group to another one
  • Heat Set Web Offset (HSWO) Paper in reels normally from 28gsm to 150gsm both coated and uncoated A major difference Is that Both sides are printed in one pass in four or more colours. The web runs between two blanket cylinders (no impression cylinder) on each unit. The ink dries through absorption in to the paper and through evaporation as the web travels through an oven. Usually the web is cut and folded in to signatures. Sometimes it is sheeted or is reel to reel. Reels in various widths Blanket- to-blanket Heatset ink: solvents evaporate in dryer Inline folding / finishing Cut-off length depends on press Limitations in colours/ grammages Fluting / more printing gloss 8-16-24-32-48-64-72-80 pages Delivery in signatures / reels / sheets
  • 1970 : Single web press 100 m/min ou 2m/sec Laize 1 meter 20% wate Today Double web presees 1000 m/min ou 18m/sec 2,86 m de large 2 a 5 % waste automation
  • As many other industries, HSWO is more and more automated… Press console From this place they can adjust ink levels, register, pressure between cylinders, etc etc …. The press console provides electronic control for register, image control, ink and dampening easily. The press operator monitors the console and, if neceseasily. The press operator monitors the console and, if necessary, makes press adjustment by adjusting switches on the console, which cause the appropriate adjustments to be made on the press. Today, computer controlled consoles are the norm for process control and providing a consistent printed product. The color control console is where the printer will add or subtract color across the entire sheet or in localized areas. On the console are keys which correspond to "zones" in the inking system. Electronically the operator can select the color unit they want to change at the console. This will correspond at the selected print unit. If the printer wants more ink across the whole sheet, the ductor roll will contact the "feed" or "ball" roller longer. If the printer wants to only add color to a localized area, small plates at the ink fountain will open allowing more ink to pass at that spot only. This gives the printer great flexibility!
  • Relation colour / water / paper : The paper quality plays an important roll in relation to the water and the colour absorption capability in the printing result and has to meet several requirements, among which you can find: A good absorption to support drying and adhesion of the colour. The paper has to absorb the water quickly and uniformly to avoid that the following colour is badly printed after printing the first colour as the water film coming onto the paper when printing the single colours is not completely absorbed by the paper (repelling of the colour). If insufficient water is absorbed by the paper, the colour may start to emulsify (= to mix with water), influencing the printing quality and the drying properties extremely in a negative way. The paper has to be resistant against fountain solution with a pH of approx. 5 (the coating layer should not detach). In case of a contact with moisture the paper should practically not deform . The paper surface has to be resistant against the forces being released during cracking of the colour if the paper loosens from the rubber blanket.
  • Hot air Inlet temperature > 200°C Web temperature approx. 120°C The length of the oven is very important
  • Prevent cracking at the fold The application of a thin film of water/silicone is used mostly to reduce "marking" in folder units with coated papers. Coated papers, due to the latex coating, can acquire "marks" when passing through the folder unit. Uncoated papers benefit during the dry winter months if the printer adds a fabric softener to the water which, in turn, reduces web static.
  • Signature = group of pages
  • There are three basic types of folders: former folders, jaw (= machoire) folders, and chopper, or quater folders. Often, all three folding devices are incorporated into a combination folder. A former folder folds the web by passing it over a triangular-shaped former board. This action makes a "with the grain" fold by folding the web along it length. Additional folds after the former fold are made with jaw and chopper folders. A jaw folder folds the web across its width (cross grain) by allowing it to travel around a cylinder equipped with a tucker blade that forces the paper into a jaw (opening) on an opposing cylinder. After passing through the folding jaw, the web is automatically cut into individual signatures and, if necessary, passes to a chopper folder. In the chopper folder each signature is forced between two rotating fold rollers that make the final fold, against with the grain.
  • Gravure- Widely used printing process- suited for long runs, where a high quality reproduction is required. Cells of varying shape, depth, and area are chemically etched, electromechanically engraved, and/or laser beam engraved into copper image carrier. (Laser engraved into the zinc layer). The engraved image is later chromium coated to increase the durability of image carrier. • Variation in depth enables different volumes of ink to be delivered. Compared with other printing processes (except screen), the ink film thickness delivered by the cells is high, up to 6 micron for most work on paper The cylinder is engraved. When the print job is finished or the cylinder is worn out the copper bracket is stripped off. The copper cylinder can be chrome plated to make it longer lasting. Gravure - Main strengths Very high quality colour reproduction is possible Waste levels are relatively low, Print quality is very consistent, Inks are relatively volatile liquids and key to a wide range of substrates without requiring a lot of energy, It is possible to print continuous designs, Very high production speeds are achieved, Running costs are relatively low Gravure - Main weaknesses High cost of producing cylinders compared to plates for other processes.Lead times for producing cylinders are comparatively long. Corrections are more difficult.Only really cost efficient for longer runs. Print quality can be very poor if the substrate roughness is not within the minimum specification The ink is picked up in the engraved cylinder cells, the excess is removed by the doctor blade; the paper passes through the nip between the printing cylinder and the impression cylinder; the ink is transferred to the paper as a series of dots; electrostatic assistance helps the transfer of ink from cylinder to paper (YMCK = ink sequuence).
  • 4, 32 metres 18 ribbons
  • PACKAGING AND CONVERTING • Packages- 6% of total value of all manufactured goods • Beverage packaging- food products- spent approx. • $1 billion/year on packaging • Folding cartons, flexible packaging, labels, wrappers, composite cans, metal, glass GRAVURE FOLDING CARTONS • Wet, dry foods, beverage carriers, bakery, candy boxes; • Gravure- used by 65 North American folding carton producers • 29% share on the market- gravure FLEXIBLE PACKAGING • Flexible packaging- 5% annual growth (as a whole) • About 1/3 market share- gravure • Flexible packaging – package products weighting less than 25 lb. • Paper, plastic film, foil laminates, different combinations • Fully printed packaging materials, combination materials, (converted wrap) • Single- serving food packages4 GRAVURE LABELS AND WRAPPERS • Roll, sheet labels applied to cans, cartons, bottles, tags, self-adhesive products, • Most of them printed on paper substrate PRODUCT GRAVURE • Narrow width plastic films- to vinyl shower curtains, wall covering, decorative laminates, floor coverings, tissue products GIFT WRAPS • 90% of all gift wraps- printed; Gravure- dominant 70% of all production • Christmas wrapping paper • Decorative laminates- general purpose- horizontal surfaces .050in • Vertical surfaces- as thin as .028 in FLOOR COVERINGS Sheet vinyl coverings 18 in. to 6,12,15-ft width • Inventories of engraved cylinders – hundreds of patterns • 84% vinyl floor coverings printed gravure TISSUE PRODUCTS • Paper towels, bathroom tissue • ¾ of all tissue product are printed with patterns, • ¼ of bathroom tissue printed; 1/8 of all napkins- printed • Sanitary tissue- flexo, gravure declines OTHER PRODUCTS • Cigarette filter tips, heat transfer paper, marbled book end papers, pill and candy trademarking, auto windshield tinting GRAVURE WALLCOVERINGS • Traditional gravure • Gravure- 40-45% of all wall coverings- almost entirely on vinyl GRAVURE VINYL PRINTING • Curtains, shower curtains, tablecloth, ceiling tiles, automobile upholstery, vinyl fabrics for outdoor furniture (umbrellas, chairs) • Unsupported vinyl, vinyl backed with paper, fabrics. • Some- printed on paper and vinyl coated. DECORATIVE LAMINATES • Thermoset laminates in furniture, construction – wood grain – used in furniture. High pressure laminates- formed by high pressure- 1,000-1500 psi • Contains up to 18 layers of polymer saturated paper • Top sheet-translucent- melamine impregnated Magazines and catalogues (45%); Flexible packaging (50%) and confectionery wrappings Folding box cartons (34%), liquid cartons and cigarette packets (75%);
  • Where gravure has the dots engraved in the cylinder, with Flexography the dots are raised above the surface. A wide range of substrates can be used from foils to paper. It has many applications such as the printing of – Newspapers Wallpaper Food and hygiene packaging Self adhesive labels Corrugated boxes Très présente dans le domaine de l'emballage, la flexographie est utilisée pour l'impression de supports aussi variés que le film polyéthylène, le papier , le carton ondulé et le carton plat. Longtemps, les clichés étaient composés de caoutchouc, avant l'apparition des plaques photopolymères dans les années 70. A flexo print unit consist of a plate cylinder carrying the flexible plate. A C ounter I mpression cylinder is used to back the substrate for the printing. The liquid flexo ink is transported out of the ink pan by the ANILOX ROLLER; The excess of ink is wiped off the surface of the anilox roller by means of a DOCTOR BLADE
  • The application ranges for Flexo are various. Flexo is the printing process with the highest rates of increase.
  • La sérigraphie permet de réaliser des impressions sur papier, carton, plastique souple ou rigide, métal, textile, verre, bois et sur les auto-collants. Called screen printing because the ink is forced through a screen. More suitable for solid colours and thought to have been with us for about 4,000 years and now used for many applications such as – Clothing & Textiles Product labels Printed electronics , including circuit boards Large formats, i.e. posters and balloons Medical devices Signs and Displays
  • There are sometimes problems to understand what people mean by digital printing. The "real" digital printing is printing without films and plates ; computer-to-press printing occurs where the data is in digital form but plates are made at the press.
  • Here are the main pros and cons today - tomorrow they might be different. PROS: The possibility of digital printing is in small runs that can mean either short run lengths or personalisation. Personalisation is the real benefit. It does not only mean some text such as names, addresses etc, but personalising the content. Versioning means e.g. different language versions. On-demand printing happens when e.g. 10 copies are printed today, 20 tomorrow etc. Decentralised printing is delivering digital data to different printing plants to be printed near the distribution area. A digital printer needs different skills compared to a conventional printer. He works mainly by a computer. Run length in conventional printing is defined by the circumference of the print cylinder. In digital printing there is a possibility to print several metres long images. This is useful e.g. in wallpaper borders. CONS: Even though digital print quality has improved a lot, it is still not as good as offset in general. Typical faults are stripes and mottling especially in solid areas. Printing speed in sheetfed offset is, calculated in m/min, around 150 m/min as in digital printing it is e.g. in Xeikon 7,35 m/min (or 15 m/min). Small reels (width 32 or 50 cm) /sheets (SRA3) mean limitations to the printed products and also difficulties to board mills in converting. In Xeikon-based machines there are only four primary colours available, but in Indigo also two additional colours are possible. However, in digital toners the colour space is wider than with offset inks. The further converting, diecutting etc is not digitised at all. After the latest printing technology, very conventional converting methods come in the picture. There is, however, work going on in laser diecutting where cutting is not a problem, but creasing is! Today e.g. the toners are expensive and so is the unit price of a copy. Only one series of toners is available per press. Of course for short runs the price is also high in offset and personalised products in offset are impossible in practice.
  • Indigo : printing cycle similar to dry toner Ink transfer similar to offset
  • Electrophotographic method works in the following way: 1. The drum is covered by a photoconductor material which maintains charges in darkness but discharges in light. The photoconductor drum is first charged with even charge. 2. A latent image is formed by discharging the drum by laser or LED (light emitting diode). 3. The latent image is developed by toner (ink). It sticks to the opposite charges. 4. Then the toner image is transferred on the board in an electric field, again opposite charge attracts. 5. The last step is fusing where toner is fixed to the board by heat and/or pressure. The same principle as in laser printers and copying machines, but in printers the speed and resolution are lower in copying machines the original is a hardcopy possibility to print on reels Negatively charged toner particles stick on the positively charged image areas until transfereed to paper. A strong electrical dischare on reverse side of the paper The electric field (corona) and toner particles have opposite charges Important chraacteristic of pare is Resistivity and dielectricity IR fusing without pressure for xeikon
  • There are two principle ink jet methods: continuous stream and drop-on-demand. In CIJ (continuous ink jet) the droplets are formed continuously and the printing droplets are selected from the stream. The non-printing droplets are returned to the ink circulation. All the droplets have similar size. By this method very high speeds are possible. CIJ is largely used in b&w marking. The new 4-colour press of Scitex belongs to the VersaMark family (in the picture).
  • In DOD ink jet drops are formed only in need, so every drop is printing. Resolution in principle is higher, but speed lower than in continuous stream. E.g. Aprion (separated from Scitex) is concentrating in this technology. Their DPS-65 in seen in the picture. A wide range of applications including codings (addressing, use-by-date), posters, billboards, office and home printing, proofing, textile printing,... In the future... wallcovering (e.g. borders) corrugated board
  • Transcript

    • 1. Basics of offset printing & other printing methods1
    • 2. Products Offset Sheet- Heatset Coldset Gravure Flexo Digi fed Magazines, catalogues X X X Brochures, annual reports etc. X X Direct marketing X X X X X Books X X X Newspapers X X Packages, wrappers X X X2
    • 3. Why ? A better understanding of the first end use of our products (to be printed) will allow you to better advise your customers and thus give added value to your sales3
    • 4. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion4
    • 5. High Placement of printing methods Ink-jet Sheet-fed offset Heatset offset GravureQuality Medium Electro photo- graphy Low 100 1 000 10 000 100 000 1 000 000 10 000 000 Run length 5
    • 6. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion6
    • 7. What is colour ? Colour Without light, no object has a colour7
    • 8. What do we need to see colours ? A light source (Sunlight, Light Bulb, Candle) A receiver and processor (eye + brain) An object to interact with the light source8
    • 9. Human eye • light enters the eye through the lens and is focused on the retina There, an array of photoreceptor cells respond to light and pass the signals on to the brain which translates into colour sensation There are 2 types of cells : -> rods = black & white -> cones = colours 3 different types of cone cells sensitive to the wavelength of => red light => green light => blue light Cells light stimulates the eye three dimensionally!!9
    • 10. Spectrum of electromagnetic radiation waves Gamma X-ray UV IR Radio Radar Wavelength 0,001 0,01 0,1 1,0 10 100 10³ 10 4 10 5 10 6 1010 1011 10 12 10 13 nm Spectrum of the visible light photon violet blue green yellow orange red nm 400 500 600 700 different wavelengths = different colours400 - 500 nm = blue 500 - 600 nm = green 600 - 700 nm = red10
    • 11. How to create colours ? : 2 methods Additive Subtractive Yellow Cyan Magenta11
    • 12. How to create colours : additive method Additive Lights : Red + Blue + Green = White For example a Television or a Computer Monitor12
    • 13. How to create colours? Based on this (Green + Blue = CYAN, Green + Red = YELLOW, Blue + Red = MAGENTA), we can reproduce every colour based on 3 selected colorants Each colorant will have to act on a primary colour of the spectrum (Red, Green or Blue) For example to act on Red, we need a colorant which absorbs only Red and not Green and not Blue. This is the CYAN Ink which absorbs red light reflects blue light blue + green = cyan ink reflects green light13
    • 14. How to create colours : subtractive method Selective absorption of certain wavelength by the ink layer. Magenta ink Cyan ink Yellow ink reflects red absorbs red reflects red reflects blue reflects blue absorbs blue absorbs green reflects green reflects green14
    • 15. Primary colours in printing Mixing the 3 primary colours theoretically results in black. => In practice the result is brown! => To remedy this, blacK is used as the fourth primary colour. CMY blacK CMYK CMY Black CMYK + =15
    • 16. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion16
    • 17. Principle of offset printing On a printing plate there are two areas: • Areas accepting ink and being repellent to water = Image areas (= hydrophobic) • Areas accepting water = Non-Image areas (= hydrophilic). The areas are occupied by water. Due to the different surface tension water and ink do not mix. The behaviour of the image and non-image areas is caused by a complex chemistry.17
    • 18. How do we get printed and unprinted areas? Ink = hydrophobic Water = hydrophilic IMAGE NON-IMAGE Function of the offset plate18
    • 19. How do we get printed and unprinted areas?Image Non-Image Magnified (15x) of an offset plate19
    • 20. How do we get printed and unprinted areas? Non- ImageImage Magnified (1000X) picture of an offset plate20
    • 21. How do we get printed and unprinted areas? Substrate = direct transfer IMAGE NON-IMAGE Function of the offset plate21
    • 22. How do we get printed and unprinted areas? Rubber Rubber 1st transfer 2nd transfer Substrate IMAGE IMAGE NON-IMAGE NON- IMAGE Function of Rubber blanket = indirect transfer22
    • 23. Offset printing unit23
    • 24. The printing plate Light sensitive coating Aluminium oxide coating Grained aluminium base24
    • 25. The printing plate25
    • 26. Basic composition of offset ink Pigments ~10-20 % organic and inorganic Binders ~30-50 % Hard resins Alkyds Vegetable oils Solvents ~20-30 % Mineral oils Vegetable oils Additives ~0-5 % waxes, rheology modifiers, antioxidants, fillers...26
    • 27. Offset blankets Oil-resistant synthetic elastomers attached to textile fabrics Reproduction properties Release properties27
    • 28. The fountain solution Surface tension of liquid has to be lower than surface energy of solid to ensure quick spreading •Water 85-98 % •IPA (Isopropanol) 0-20 % •Additives 2-4 % weak organic acid(s), buffers, film formers, surfactants, chelating (sequestering) agents, biocides, anti-foams, humectants, glycols, corrosion inhibitors pure water, 72 mN/m water + 6% IPA, 45 mN/m surface energy of non-image area, 50 mJ/m 228
    • 29. Screening One half-tone image may contain thousands of different shades………29
    • 30. Different screening Conventional screening Stochastic screening  distance between the centers of  tone is formed by altering dot position the dots is constant and spacing, number of dots30
    • 31. Stochastic screen Standard dot size, Dot size and but the amount of amount of dots them vary. vary.31
    • 32. Screen ruling – conventional screening Light tones ~ small dots The dots are so small that the human eye Dark tones ~ bigger dots sees them as a single colour. Tonal value 75% Tonal value 25% 40l/cm Tonal value 50% 60l/cm 80l/cm Tonal value 100% Screening ruling (L/cm or L/inch)32
    • 33. Human eye In case of the 4-colour printing the colours are mixed in the eye when looking at them, as the eye cannot differ between the small, close picture elements. Normal screen surface Strongly magnified screen surface33
    • 34. Print density  Print density (darkness) is created by increasing the inked area (halftone percentage) Black 20% Black 40% Black 60% Black 80%34
    • 35. Print colour  Print colour (tone) is created by mixing halftone percentages of process colours C80%/M20% C60%/M40% C40%/M60% C20%/M80%35
    • 36. Examples of screening methods Staccato 25µm Conventional 60 l/cm36
    • 37. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion37
    • 38. Sheet fed offset press Margin - Ink rollers & Damping rolls Feeder Anti set-off powder systemDelivery ofthe printed sheets Settings of pressure and register 38
    • 39. After the 4 printing units39
    • 40. Sheet fed presses 5 colour units + 1 varnish unit Delivery Varnish Ink units Feeder  4/4 colour units, change of the printing side of the sheet in the perfecting system40
    • 41. Different formats41
    • 42. Feeder42
    • 43. Ink Rollers43
    • 44. Sheet Transfer Transfer cylinder44
    • 45. Spray powder to avoid ink set-off distance holder between the sheets. increases the amount of oxygen to improve the oxidative drying process. Spray 10 to 80 microns calcium carbonate organic, vegetable based powder of natural starch 300x - CaCo345
    • 46. Varnish Why ? More gloss Protect Less drying time before converting Sometimes two varnish units High gloss and matt varnish at the same run Premier and UV- varnish when using conventional inks Better gloss than one unit applying46
    • 47. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion47
    • 48. Basic design of heatset press Web widths: ~50- (over) 200 cm Web speed: ~4-15 m/s Web tension: ~600-800 N/m Drying (web temp): ~110-140 °C Folding: usually on-line siliconization unit reelstand dryer printing units infeed chill rolls folder48
    • 49. Heatset presses - different designs additional colour varnishing unit (offset varnish)49
    • 50. Operating press Driving the press50
    • 51. Infeed51
    • 52. Heatset press – an example52
    • 53. Paper web in one unit Plate cylinder Blanket cylinder Paper web Blanket cylinder Plate cylinder53
    • 54. Oven54
    • 55. Oven55
    • 56. Heatset dryer Web delay in the dryer is ~1 s dryer length is determined by the press speed (10 m/s  10 m) Dryer has multiple modules Hot air from the nozzles carries the web through the dryer Exhaust air is either circulated back or is let to atmosphere through treatment (e.g. afterburner)56
    • 57. Temperature profiles in the dryer Web exit temp. usually 100-140 C (90-180) Usually declining temperature profile web temperature rises faster and solvent evaporation starts earlier °C 300 255°C 195°C 200 165°C 130°C 100 drying air temp web temp 1st 2nd 3rd 4th57
    • 58. Chill rolls Thermoplastic ink binder is in soft form after the dryer chill rolls cools down the web and solidifies the binder (better rub resistance) Steel cylinders (3 - 6) with cold water pumped through Web tension has to be high enough to ensure good contact58
    • 59. Silicone application59
    • 60. Center Cut60
    • 61. Aligning ribbons61
    • 62. Different kind of folders Cylinder/Jaw folder Former fold62 Chopper folder
    • 63. Example of fold 16 pages Former fold Jaw fold880 mm Chopper/ jaw fold 630 mm63
    • 64. Folding possibilities 310 x 440 mm 310 x 220 mm 155 x 110 mm64
    • 65. Imposition and plate making Imposition Done usually at the printer Depends on what press is used 16/32/48/64 pages Special software used Plate making From imposition layout65
    • 66. Press layouts 72/80-page 48/64-pagePagination 32-page Double perimeter Single perimeter 24-page 16-page 8-page66 Run length
    • 67. Manual delivery67
    • 68. Post-press68
    • 69. Perfect binding Operations  signatures are collated together in conveyor belt  signatures travels down the belt to the saw  saw trims off of the bind edge of so the binding glue can be applied between the individual pages  magazine travels to the covering station  magazine is trimmed with 3-knife cutting Separate cover is needed Long grain =MD69
    • 70. Saddle stitcher Operations  signatures are pulled open and dropped in proper order onto a moving "saddle".  assembled signatures receive a cover in the same manner and then pass under the stitching heads where wire staples are applied  magazine is trimmed with 3-knife cutting Usually used only product with pagination under 96 pages70
    • 71. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion71
    • 72. Gravure72
    • 73. Gravure73
    • 74. Gravure - applications Packaging & converting Flexible packaging Labels & wrappers Gift wraps Wall coverings Vinyl Decorative laminates Floor coverings Tissue products Stamps74
    • 75. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion75
    • 76. Flexography paper plate Double face tapeCounter pressure cylinder ink sleeve blade 76
    • 77. Applications • Paper sacks • Milk carton • Pocket Books • Plastic bags • Labels • Aluminium • Corrugated Board • Carton cups • Napkins • Tissue • Envelope • Bussinessforms • News Papers • Flexible packaging • Pocket Books77
    • 78. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion78
    • 79. Silk screen79
    • 80. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion80
    • 81. What is digital printing ? DIGITAL printing means printing directly from digital data original is in digital form no films are needed no plates are neededSlide 8181
    • 82. Pro or Contra digital printing ? Pro easily changeable info design and colours text no make-ready -> small runs with reasonable costs personalisation versioning on-demand printing decentralised printing easy to operate vs. offset ”unlimited” repeat length Contra  Print quality still lower than in offset  Low printing speed compared to conventional presses  Format limitations (small reels/sheets)  Limited or non-existing spot colours and metallic  Conventional further converting  Costs (toners etc)82
    • 83. Applications Production variable data  direct marketing  individualized statements  customized catalogues, brochures etc Commercial printing  on-demand and short run books  short run brochures83
    • 84. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion84
    • 85. Electro photographic principle 1. Photoconductor is charged 2. Latent image is formed by discharging - - - - - - -- - - - - the drum by laser or -- -- LED + ++ 6. Remaining toner -- - + - -+ is wiped off -- - 3. Latent image is developed by toner 5. Toner is fixed to the paper by heat = fusing + ++ + 4. Toner (image) is transferred to the paper in electric field, opposite charge attracts85
    • 86. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion86
    • 87. Ink jet : continuous 1. Continuous stream, CS, CIJ printing droplets are selected from the continuous stream by charging them selectively droplets have similar size high speed e.g. Scitex VersaMark87
    • 88. Ink jet : drop on demand 2. Drop-on-demand, DOD every drop is printing high resolution lower speed e.g. Aprion88
    • 89. Summary Offset printing What is colour ? Principle of offset printing Sheet-fed offset printing Heat-set web offset printing Other printing methods Conventional methods Gravure Flexography Silk screen Digital printing methods Laser (electro photography) Ink jet Conclusion89
    • 90. Print definition depends on paper quality LWC, Newsprint, 60l/cm 48l/cm Coated Fine, SC, 70l/cm 54l/cm90
    • 91. Different printing process, different results Offset Flexography Gravure (20%, 75 l/cm) (20%, 60 l/cm) (~20%, 100 l/cm) • Dots are uniformly • Ink is pressed to • Broken shape of dots and covered with ink edges, print density missing dots are typical for • Edges are ragged inside dot varies light tones (uncoated papers) • Edges are smooth • Doughnut shaped dots • Uniform text • Shadow in the edge • Text is screened, serrated of text edge91
    • 92. Examples of halftone dots and text Inkjet (20%) Electrophotography • Dots are quite uniformly (~20%) • Dots are quite uniformly covered with ink • Dot is formed by using several covered with toner • Edges are ragged droplets • Edges are ragged or even • Separate toner particles depending on paper (dry toner methods) • Satellites, escaped droplets • Uniform text92
    • 93. ? Will be happy to answer your technical questions : Questions ? Françoise Accou, +32 492.582.287, francoise.accou@sappi.com93