PRINTING BY THE FLEXOGRAPHY METHOD Flexography is a printing process whereby printing is achieved through ink transfer from engraved printing plates (elastic or photopolymer masters). Wet ink can be alcohol - soluble, water - soluble or UV stabilised (state - of - the - art technology). Low printing pressure is applied to the substrate due to ink fluidity as well as the pliant behavior of masters. Inks being in a wet state provide quick drying due to solvent evaporation and allow attainment of high production speeds, even on non - absorbent materials such as films. Soft and pliant masters can be mounted and aligned off the printing press. They can be quickly replaced in case they become deteriorated. Ink transfer and dosing is performed using cylinders with engraved microcells (anilox). These operate in conjunction with flexible cylinders or doctor blade chambers. Printing unit comprises of the following main parts: 1. ink transfer station 2. plate cylinder 3. impression cylinder The purpose of the ink transfer station is to receive a constant, regulated wet ink film and to apply the same on the surface of the master. This station comprises of the anilox cylinder and a container housing the flexible cylinder or the closed inking system with doctor blades. The closed system fitted with two doctor blades is benefiting from constant ink dosing irrespective of linear production speed. Blades are available in thickness ranging between 0,004-0,012"; they are produced from a steel alloy base or plastic materials e.g. polyamide, polyester or very high molecular weight polyethylene. These benefit from reduced wear caused to anilox cylinders. Plate cylinder comprises of a steel core and is fitted with elastic sleeves that carry the engraved printing plate. The latter is mounted thereupon by double sided self - adhesive tape (compressible or rigid) and its thickness ranges between 1,14 and 5 mm. Impression cylinder is made from high quality steel and is used to carry the substrate intended for printing. Ink transfer is determined according to anilox cylinder production material as well as shape and angle of engraving and number of cells per sq. centimeter. Furthermore, when an elastic feed cylinder is used, increase to production speed and reduction of hardness of this cylinder subsequently increased the quantity of ink transferred. It is worth noting that an assumed anilox wear by 15 % entails a reduction of the ink transferred by 33 %. The printed image dries at the drying stations, installed in series with the printing stations. Then, the printed surface passes through the main drying tunnel and is transferred to the wrapping station. The distance travelled by the substrate is controlled by electronic, mechanical and pneumatic process systems, to ensure correct tensioning, allocation and cooling of the output reels. The following machine types are currently in business: A. CI - common impression press Β. Stack press C. In line D. Sheet-fed The first of the above types is the most common. It uses a main cylinder (controlled temperature type) and 4 to 8 printing stations organised around it. Substrate is introduced from the top of one side and output from the top of the opposite side. Excellent matching is achieved, even on thin films. The second type is used for paper and film printing. It comprises 2 to 6 stand-alone printing units allowing printing of one or even both sides of the substrate. Series - connected machines are common in the paper printing sector. Substrate is unwound from the roll and printer laid out horizontally, while passing through the aforesaid stand-alone
printing stations. Sheet - fed machines use paper sheets, not rolls. In flexography, ink must exhibit appropriate flow characteristics, allowing transfer by a series of cylinder and immediate drying on the substrate. When ink is circulating in the system comprising of inking system pipework - ink transfer station, part of the solvent evaporates resulting in increasing the quantity and density of the deposited ink. Viscosity regulation can be applied either manually or automatically, adding the appropriate solvent and / or solvent mixture (closed viscosity control and correction system). Contrary to solvent based inks, water based inks are slow in losing volatile ingredients. Nevertheless, attention is required to continuous pη (degree of acidity) monitoring and correction. The following factors influence the selection of printing rate: 1. machine speed 2. substrate used for printing 3. ink transfer system (anilox cell size, use of knives etc) 4. drying temperature 5. applied solvent mix 6. coating thickness A common error observed in the course of the above printing process is hue intensity regulation by adding solvent. Should flexography ink get thinner than a specific limit (e.g. 17 sec Zahn cup NO 2) there is a risk of failure of the printed product (due to issues related fo adhesion, resistance to friction, flexibility, partial solids sedimentation: pigment + resins). Increasing of linear production speed ntails an increase to shearing stresses applied to wet ink; thus, the thickness of ink deposited on the substrate also increases. Technical innovations introduced to the printing sector during the late 10 years intended to: Α. reduce machine preparation and change of printing production times, therefore a reduction to production cost (research has demonstrated that these times may consume up to 50 % of total machine utilization time) B. achieve improved quality as well as stability and uniformity of the production procedure and process C. allow business to penetrate areas traditionally dominated by rotogravure. Some of the innovations introduced are the following: A. Using robot automation systems for changing plate cylinders and anilox cylinders B. Systems for automatic cleaning of inking systems, doctor blade chambers & anilox C. manufacturing of machines equipped with 12 printing stations on two main rollers D. Systems allowing automatic changeover of printing process in 15 minutes on 12 stations (NOTE: changeovers per station are estimated at 30 minutes on older machines; thus for fourcolour process a two-hour machine stop is required) E. Manufacturing of gearwheel-less machines resulting in quicker production changeover and setup, improved thin film printing, less waste rates etc. F. Use of filtered, heated and compressed air in the drying system resulting in speed gains between 25 to 40 % as well as reduction in solvent retention and production noise levels G. Use of six and eight colour process inks
THE ROTOGRAVURE-TRUE INTAGLIO-STEEL DIE PROCESS This process comprises in image engraving onto a cylinder (the gravure cylinder) in the form of cells (dots) acting as ink wells. Subsequently the cylinder is etched by knife and retains only ink entrapped in the cells on its surface. Substrate is rolled between the gravure cylinder and the impression cylinder. Applying suitable pressure, ink is transferred from cylinder cells to the substrate. Printing unit includes the following: - the inking system, whereby printing cylinder rotates and its cells are filled with alcohol - soluble ink - doctor blades used to remove excess ink from the gravure cylinder - ink fountain and ink feeding system - rubber-coated impression cylinder - print drying / volatile solvent discharge station (each colour must dry before introduced to the next station in line ) Initial rotogravure machines applied inking of cylinders by immersion in an open inking system, resulting in loss of solvents and consequent quality problems. Current technology allows ink injection on the gravure cylinder in closed inking systems with automatic ink viscosity control and correction. Thickness of doctor blades varies between 0,15 to 0,25 mm. They are made from flexible steel alloys. Application angle, blade sharpness as well as impression pressure are factors influencing the quantity of ink transferred to the substrate. Impression pressure can be mechanically or pneumatically adjusted, while pre-doctoring may often lead to progressive reduction of the pressure applied to the gravure cylinder, thus improved resistance to operating wear. Impression cylinder comprises a steel core and a rubber coating of 12-20 mm, 60 100 shore Α hardness. The harder and more compressible the substrate, the harder the rubber cylinder coating must be. Drying systems generate high-speed, adjustable-temperature air. It is further possible to introduce a solvent neutralisation or reclamation system. It is also common to introduce substrate heating and cooling rollers. Electronic substrate matching control, alignment, tensioning and printing systems are nowadays employed to achieve high quality and velocity. Methods used for printing cylinder manufacturing are the following: 1. conventional etching (now phased out) 40 µm wide cells allow a wet ink film thickness of 10 µm cell width is uniform, however their depth varies in order to achieve different colour tones 2. chemical treatment (direct engraving) (now phased out) 40 µm deep cells allow a 10 µm wet ink film cell depth is uniform; their width varies 3. electronic engraving (widely used) in this case engraving is performed by diamond tipped device and cells are pyramid shaped, with depths ranging between 2 to 50 µm. This method allows production standardization, reduced setup time and reduced ink consumption 4. laser engraving (state-of-the-art technology)
Rotogravure inks present a lower printing viscosity as compared to respective flexography inks (15-25 Zahn cup NO 2/25 'C). Their drying is achieved through evaporation of the solvent combination they contain; commonly no oxidation or other chemical modification is caused to dry ink film. Printing viscosity adjustment is achieved as a function of the following parameters: Α. ink rheological characteristics B. production factors (linear velocity, drying tunnel temperature, air velocity etc) C. solvent system evaporation rate D. engraving method and pattern; depth of cells E. ink scraping by the doctor blade system F. substrate used for printing High speed printing requires quick - drying inks, therefore their viscosity must be lower so that ink may be released from engraved cylinder cells at the point of contact to the substrate to be printed. Rotogravure ink changes state from low viscosity liquid to solid (tack free state) within a few seconds, due to quick drying of solvents. Solvent and resin jointly constitute the ink carrier that transfers the pigment to substrate surface (resin serves to bond the pigment on the substrate). Dyes are rarely used as these present less resistance to light as well as increased potential for generating adverse reactions. Certain special design, high cost dyes with increased resistance to light are used in aluminum foil printing, as their transparency ideally matches high reflectivity of the substrate. Additives are used to upgrade ink characteristics and functionality on printing process. The following are some of the additives used in rotogravure ink production: 1. anti-foaming 2. pigment wetting agents 3. fungicides (for inks and water based varnishes) 4. plasticizers 5. catalysts - adhesion improvers 6. surfactants (for film wetting from hydrous systems) 7. wax, lubricants etc. Figure 1 shows the typical composition of a rotogravure ink containing organic pigment. Nevertheless, in cases of white pigments where titanium dioxide pigment is used, pigmentation rate increases in the order of 25-35 %. Particular attention is required for pigment selection in rotogravure inks. Pigment must be able to function in conjunction with the resin system used in the ink; furthermore it must be resistance to solvents employed and provide satisfactory rheological characteristics to the end product / rotogravure ink.
Figure 1. Exemplary composition of a colour ink. Solvent also acts as a lubricant between gravure cylinder and doctor blades. Blade application pressure can be adjusted in proportion to process speed and ink viscosity. The following rule of thumb applies: the lower the viscosity, the less the force applied from doctor blades to the impression cylinder. Blade wear will incur a more modest replacement cost compared to a potential costly gravure cylinder repair or refurbishment. Nevertheless ink viscosity may not drop less than a practical threshold; below this threshold, wear starts to increase again. As ink viscosity approaches solvent viscosity, the quantity of resin becomes insufficient and lubricating film is eliminated. The resin system employed must comply with the following ink characteristics: 1. complete adhesion to the substrate 2. full solubility in the resin system 3. rapid solvent release 4. high sheen levels 5. appropriate pigment wetting 6. satisfactory wear - resistance 7. flexibility and elimination of micro-crackinh on the final print Evaporation rate of an ink also constitutes an important factor of uniform, high quality printing. Too slow drying results in solvent retention, reprinting, ink transfer to paper tensioning and advance cylinders, etc. Too rapid drying will result in screening effects as well as solvent retention (on some ink systems, abrupt drying will cause the occurrence of a crust on the film, under which solvents are trapped). Ink evaporation rate depends from the selected solvent system, the substrate employed as well as ink resins. Furthermore, resin solubility as well as solvent release are key determinants of the evaporation rate of the specific ink. A resin diluted in a solvent mix will provide easier release of these solvents compared to a resin diluted in a single solvent. Surface finish and the presence of pores on substrate surface will contribute in obtaining evaporation rate. Solvent reclamation systems and the use of single solvent inks are widely common in Europe. These help to eliminate any adverse environmental impact as well as safe conditions for packaging intended from the food industry, due to potential solvent retention. The use of UV inks as well as water soluble inks is the current trend intended for reduction of volatile substances released to the environment, as well as the production of hygienically safer packages. ROTOGRAVURE PRINTING DEFECTS 1. Streaking When rotogravure doctor fails to provide effective wiping of the gravure cylinder, a narrow streak of ink is transferred to a blank surface on the substrate. It is common that paper fibers are introduced in ink, during printing. These develop agglomerated in the presence of alcohol, thus aggravating this effect. Solutions to this effect are ink filtering and modification of its composition. Another cause for this is the presence of non - dissolver particles (wax, hard pigment etc). 2. Scumming This is a fine, blurred ink deposit on inappropriate parts of the impression cylinder. The latter transfers such deposits to the substrate. The following are some of the causes of this phenomenon: poor knife regulation
ink drying on the impression cylinder micropores on cylinder surface excessive pigment concentration insufficient pigment dispersion etc. 3. Screening Poor colour separation is often attributed to erratic ink flow regulation. The solution to this is to reduce viscosity and / or retard drying. 4. Drying in During re-immersion in the inking system, effective wetting of the ink (resin in essence) remaining in cylinder cells by solvents is impossible. The result of this is the gradual deposit of an ink crust (inactive layer) in the cells. Deterioration of colour intensity as well as of printing quality can be restored by using appropriate solvent system (a mix of slow and quick - evaporating solvents). 5. Packing in 'Packing in' is the inability of the ink to re-dilute (as described in the above paragraph). This condition may be caused by a resin that cannot be re-diluted. In this case, a modification of the ink composition is necessary. In case the ink has been washed out by pigment, the latter will block cylinder cells; immediately necessitating halting of the machine and cleaning of the inking system, the cylinder and the ink distribution system. 6. Mottle (crawling) The occurrence of mottling - crawling during film printing is caused by excessive thinning of the ink used. Actions contributing to the elimination of these effects are appropriate viscosity regulation as well as utilization of appropriate varnish when a reduction to colour intensity is required. 7. Pinholing The inability to provide proper substrate wetting is often manifested as voids or holes in the print. This can be resolved by introducing special varnishes - anti pinholing compounds. High ink shearing often causes significant foaming and consequently bubble encapsulation in cylinder cells. The use of special anti-foaming additives is recommended. 8. Skipping Large skipped (non-printed) surfaces are the result of : - insufficient ink quantities in the inking system - the entrapment of foaming material under the knives - ink pollution from water (alcohom substances are often deliquescent) 9. Moiré pattern In case of Moire pattern occurrence, caused by using identical angle and size (screen size: lines / cm) in engraving cells, the only solution is re - engraving of at least one cylinder. 10. Rewetting Overlapping of two or more inks when these are applied on non - absorbent substrates often leads to re - dilution of the original ink in the subsequent one. A frequent solution to this problem is to regulate drying speed (first ink must be quick-drying while the second must be slower).
Another remedy is to modify the resin - solvents system in consecutive inks. 11. Blocking and set-off Poor drying as well as using high plasticizers concentration leads to the so-called blocking on packages. Particular attention is required as to appropriately select an ink for printing of overlapping substrates, as well as overlaps sensitive to solvents.