In 1890 the first flexo press was built in Liverpool by the English company Bibby Baron and Sons. Other presses which also used rubber plates and aniline inks were developed in Europe, with Germany manufacturing the majority of flexo/aniline presses.
Originally, the inks used for flexo were aniline dye inks and the name aniline printing persisted. In the United States the aniline process was used extensively in the food packaging sector, but in the 1940s the use of aniline inks was banned in food packaging. Aniline is a colorless, oily, partially water-soluble organic compound derived from nitrobenzene.
Safer coloring agents were developed, but the aniline processes still carried a bad reputation and as a result the print sales which used this process declined. Franklin Moses in 1951 started a campaign to change the name of the process from aniline to the flexographic process which was subsequently renamed ‘flexography.’
FLEXOGRAPHIC PRINTING IN LABEL MANUFACTURING
Flexographic printing was for many years considered an inferior printing process and serviced the lower end of the label market. Labels requiring higher quality were generally printed using the offset litho or letterpress processes.
In the last three decades considerable progress has been made in the manufacture of highly engineered flexo presses and developments in printing plates, anilox rollers and printing inks now makes flexo the leading process for the manufacture of self-adhesive labels.
The introduction of UV ink curing has had a big impact on the flexo process. The switch from aqueous and solvent based inks to UV cured ink in particular has improved print quality and made it easier to print filmic and metallic substrates and has reduced the usage of solvents.
TYPES OF FLEXO LABEL PRESSES
There are three configurations of flexo presses used in the label industry: the stack press; the common impression press; the in-line press.
All these presses can be equipped with solvented, water-based and UV drying systems dependent on the press specification. The press configuration most widely used for label manufacture is the in-line press.
THE PRINCIPLE OF THE FLEXOGRAPHIC PROCESS
Flexography is a relief or raised image printing process using the same principle as letterpress. A flexible rubber or photopolymer plate is mounted on to the plate cylinder using a filmic double-sided adhesive tape.
The plate image is inked with a liquid ink which is transferred from the anilox roller, direct to the surface of the image area and then printed onto the substrate, using a very light controlled pressure (impression).
The basic flexo printing unit comprises of an ink tray or duct, the ink applicator roller, the anilox roller, the plate cylinder and the impression roller.
Figure 5.1 illustrates the layout of a standard flexo unit. The applicator roller runs in the liquid ink held in the ink tray.
The ink can be manually poured into the ink tray or alternatively pumped into the tray using a circulatory system to ensure that the ink viscosity is maintained. The applicator roller applies ink to the anilox roller and the pressure between these two rollers can be adjusted to increase or decrease the ink film to the printing plate.
Figure 5.1 - Basic flexo unit. Source- 4impression
THE REVERSE ANGLE DOCTOR BLADE
The need for ink film accuracy and consistency has led to the development of a number of modifications to the flexo system which have improved the control and accuracy of the ink film delivered to the plate.
The removal of excess ink from the anilox roller was greatly improved by the use of a reverse angle doctor blade unit (See Figure 5.2) and similar in principle to the type used in the gravure process. This development gives greater control over the ink film offered to the printing plate regardless of the press running speed.
Figure 5.2 - Location of reverse angle doctor blade. Source- 4impression
THE CHAMBERED DOCTOR BLADE SYSTEM
The modern flexo press now uses a very accurate and efficient method of anilox doctoring; this is called a chambered doctor blade system. (See Figure 5.3 and 5.4).
Figure 5.3 - The chambered doctor blade system. Source- 4impression
Figure 5.4 - Actual chambered doctor blade system. Source Tresu
The applicator roller is not required for a chambered system and the doctor blade unit has two blades which are held in the ink chamber unit. The blades are positioned above and below the center point of the anilox roller and maintain a constant contact with the anilox face, thereby creating the enclosed ‘chamber’ which holds the ink. The unit operates under a low pressure to ensure an even contact with the anilox face. The ink levels within the chamber are maintained using a circulatory pumping system.
THE ANILOX ROLLER
Anilox rollers (See Figure 5.5) are one of the most important elements of the flexo process. The full surface of the anilox roller is engraved with recessed cells and each cell holds a specified volume of ink thus ensuring that a consistent ink film is delivered to the printing plate.
Figure 5.5 - Anilox roller. Source- Mark Andy
Each individual anilox roller is made up of a steel shaft with end bearings which hold a copper sleeve.
The cells are engraved onto the copper surface and the cells can be varied in both depth and size dependent on the specification required. The ‘cell volume’ is the measure of the ink capacity of the cell and this is determined by the width and depth of the cell.
The depth and shape of the cells is a key factor in the efficient delivery of a uniform ink film to the printing plate and allows the operator to vary the volume of the ink film by changing the anilox roller to the achieve the correct color.
The number of cells on an anilox roller are measured in cells per linear inch (CPI) or the cells per centimeter (CPC). (See Figure 5.6). As cell count increases, ink delivered to the plate decreases, but as the line screen resolution of the image increases, the CPI should also increase.
Figure 5.6 - Examples of anilox cell structures
After the roller has been engraved, a ceramic coating is applied to it giving it a very hard and durable surface finish and extending the life of the roller.
The screen angle of the anilox cell is very important. Print faults can occur that are caused by incorrect angles of the engraved cells. This can create a ‘moiré’ pattern which is a result of a screen clash between the anilox cell angle and the angle used for imaging the dot on the printing plate.
Anilox cells are normally engraved at an angle of 45% - 60% from the center of the anilox roller in the horizontal position. (See Figure 5.7).
Figure 5.7 - Anilox roll cells are engraved at one of three angles- 30°, 45° or 60°.
Careful monitoring of the ink densities delivered by each anilox roller is important and is strongly recommended. Each roller should be inspected on a regular basis for any damage or reduction in the cell depth and a record of the ink volume of each anilox should be kept to ensure that the anilox specification is correct for the next print run.
ENGRAVING THE ANILOX ROLLER
The engraving of the cells on a modern anilox roller is now predominantly carried out using a laser engraving system. Previously the majority of anilox rollers would be engraved using mechanical engraving and this method of engraving is still in use today. The cell formation for this type of engraving differs in shape from the laser engraved cell.
Mechanical engraving can produce a Pyramid cell (See Figure 5.8) which is a fully inverted pyramid shape. The other mechanical cell is the quadrangular cell, (See Figure 5.9) which is a truncated pyramid shaped cell offering a higher ink release factor and good uniformity over the face of the roller.
Figure 5.8 – 5.9
Anilox rollers are specified by the number of cells per linear inch (line screen) and this can range from 250 to upwards of 2000 line screen. The majority of anilox rollers are screened at 800 LPI, but there has been an increase in the demand for 800-1200 LPI.
When preparing the next print run the press operator will select an anilox roll with a higher or lower LPI depending on the content of the graphics to be printed. If a heavy volume of ink is required the operator would select a low LPI screen value. A higher LPI value would be used for finer detail, for instance in producing CMYK fine screen printing.
Graphics can often be a combination of both fine screen and solid content and therefore it is necessary to have a portfolio of anilox rollers to suit each specific color/screen requirement.
LASER ENGRAVED ANILOX ROLLERS
Modern anilox rollers are laser engraved.
This method of engraving gives the printer a considerable range of very accurate and consistent LPI screen options. The laser cut cell differs from the mechanically engraved cell and comprises of a circular ‘scoop’ style cell shape.
The size and depth of each cell can be controlled and reproduced to a very high degree of accuracy which means that a specific anilox value can be repeated as and when required. This is a big advantage when re-engraving worn anilox rollers.
WEAR ON THE ANILOX ROLLER
Anilox rollers are subjected to considerable abrasive wear. The abrasive action of some ink pigments and the impact and pressure of the doctor blade will reduced the cell depth and thereby affect the ink volume. See Figure 5.10.
ANILOX ROLLER MAINTENANCE
The maintenance of the anilox roller is most important. If any dried ink or varnish remains in the bottom of the anilox cells then the volume of ink delivered by the cell will be reduced and the color density will change. The cleaning, storage and identification of the anilox roller has to be methodically carried out and the operator needs to have easy access to each roller and know that it is suitable for reuse with a guarantee that it will deliver the same volume of ink as when previously used.
It is important that the anilox roller is thoroughly cleaned at the end of a print run, in particular any anilox rollers that have been used for solvent based or aqueous ink systems. These inks will dry rapidly if left to stand and cleaning must be done immediately to avoid ink drying into the anilox cells and creating a problem knows as plugging. This occurs when a blocked cell creates tiny pinholes in the printed image.
Cleaning can be more effective if a fine brush is used to remove the ink, but the use of a brass brush is not recommended as this can damage the anilox surface.
The most effective method of cleaning the anilox is by using an ultra-sonic cleaning unit. (See Figure 5.11) This is a very effective and environmentally safe cleaning method.
The ultrasonic cleaning system works through the turbulence that is created when air bubbles, which are generated by the system, implode on the surface of the roller and ultrasonic waves bombard the cells agitating the cleaning solution in the cell cavities and removing any dried ink.
The process can be improved if the cleaning solution is heated to a recommended temperature.
Figure 5.11 - Ultrasonic anilox cleaning
THE FLEXO PRINTING PLATE
There are two types of plate material used for flexographic printing; rubber and photopolymer. Each of these materials can offer various thicknesses of plate and ‘shore’ hardness. The majority of label work printed by the flexo process is produced using polymer plates and therefore this article will focus on the imaging and mounting of the polymer plate.
The structure of the polymer plate comprises of a filmic base giving the plate its stability. The plate is coated with light sensitive polymer, which varies in depth dependant on the thickness of the plate required. (See Figure 5.12).
Figure 5.12 - Flexo plate structure prior to exposure. Source- 4impression
IMAGING THE PLATE
There are two methods used for imaging the flexographic plate.
Contact imaging, where the imaged film is placed in direct contact with the plate being imaged and then exposed to ultra violet light which establishes the image on the plate.
CtP, computer-to-plate where the image is created directly on to the polymer plate using a laser controlled by a digital file.
CtP imaging is very much the dominant system used in the label industry, but film based contact imaging is still in use today.
The film, which can be negative or positive, is created from a digital file placed in direct contact with the polymer plate and then exposed to a UV light source. The three main processes involved in contact imaging are as follows:
Imaging the flexo plate - the plate is back-exposed (UV) to create the base of the plate and then front exposed (UV) through the negative to produce the image and also harden the polymer image.
The plate is washed out in a wash-out unit to remove the unexposed photopolymer
The plate is dried and then is post-exposed and finished.
This is the process of exposing the back of the polymer plate which is to be imaged, to a UV light source. This exposure creates a limited polymerisation at the base of the plate and thereby gives increased stability to the plate and aids the anchoring of the dot to be printed.
The main exposure is made to the face of the polymer material, through the film and establishes the image that will appear on the finished plate.
(See Figure 5.13). If using a negative film, the transparent areas of the film allows the UV light to pass through onto the polymer material and the polymerisation process hardens the image areas thereby creating the detailed solid areas and halftone dots.
Figure 5.13 - Exposure of flexo plate. Source- 4impression
The quality of the imaged plate is dependent on the quality of the film used, correct exposure times and the condition of the processing equipment.
The exposure time is influenced by the condition of the exposure unit. It is important that regular testing procedures are carried out to ensure that the UV light source, generated by the UV tubes are transmitting at the specified levels and that the vacuum system which secures the film and polymer material together is operating correctly, providing a 100% contact of film to plate.
Additional tests should be made to identify any variation in the polymer material that can occur from one batch of plate material to another.
The areas of the polymer which have not been exposed to the UV light and therefore not been polymerised can now be removed. This process is called the wash-out process.
The depth of the relief image area will vary with the thickness of polymer being used and also the amount of back exposure the plate received prior to the imaging process.
The wash-out of the plate is carried out in a washout unit. The plate is firstly positioned in the unit and made secure whilst a rotating brush is placed over the plate and set in motion to gently brush away the non-image areas. (See Figure 5.14).
At the same time a solution of water and soap (approx 40 degrees centigrade) flushes away the waste polymer. Filtering of the waste is also done to ensure that disposal of the waste meets any environmental requirements.
This wash-out process can also be done using solvent, but care must be taken to ensure that the wash-out time is restricted to a minimum, to avoid any swelling of the polymer material, as this will affect the dot formation (in particular the fine tones as the dots will become distorted).
Figure 5.14 - Wash out of flexo plate
After the imaging and wash-out process is done, the drying of the plate is completed in a heated air cabinet. The amount of drying time required to ensure that the plate is fully dried will vary dependent on the thickness of the plate material and the type of wash used.
Any solvent which remains in the plate has to be removed using the drying process. It is important that the specified drying time and temperatures used are maintained and that the plate is evenly dried across the entire plate surface. It is also advisable to allow the plate further drying time at room temperature to ensure the correct finish.
Post exposure is an additional exposure to a UV light source (without using the film) and allows the parts of the relief image area that are not fully polymerised to be fully cross-linked (hardened) thereby ensuring that the finished plate is even over the whole surface.
COMPUTER TO PLATE - DIRECT IMAGING
Flexo plate imaging can also be done without the need for film originals, using a process called CtP or (Computer to Plate).
Digitally driven ‘direct imaging’ systems are now used extensively giving the label printer fast, accurate and consistent screening and plate imaging.
The file which contains the image to be printed is transferred to a device called an imager and the image is created using laser imaging. This eliminates the need for film. (See Figure 5.15).
Direct imaging of the flexographic plate uses a high powered laser to burn the image ‘directly’ on to the polymer plate, in a single operation. After the imaging process the plate is washed out to remove the surplus ‘non-image’ area of the plate which is then dried and finished.
This method of imaging the plate reduces the costs which are associated with the multiple processes needed when using the film contact system. In can be seen in Figure 5.16 that the workflow for contact imaging has eight stages compared to direct imaging which has only four.
Figure 5.15 - Plate imaging - convertional versus CtP
Figure 5.16 - Comparision sequence - film, mask abation, direct laser imaging processes
An alternative laser imaging system is the laser ablation system in which an infrared-ray ablation layer (black mask) is coated onto the surface of the polymer plate and covers the entire plate. The plate is then imaged by removing the areas of the mask which form the image. This removal uses a high-powered digitally driven infrared laser to remove the masked areas that will form the image, revealing the polymer underneath which is unaffected by the laser ablation. A UV light source is then applied which polymerizes the image area where the mask has been removed.
A second exposure from the back of the plate generates the base of the plate. The plate is then washed out to remove the non-polymerised area - including the mask. The plate is then dried and given a final UV expose to fully harden the polymer.
Preparation is the key word when mounting flexo plates. The print cylinder surfaces should be thoroughly cleaned to ensure that the surface of the cylinder is not contaminated with blade cuts, grease or debris. Any contamination of the cylinder surface will cause problems with the print quality.
Any foreign particles trapped between the surfaces of the print cylinder, the mounting tape and the back of the printing plate or any greasy residue will affect the adhesion power of the mounting tape causing plate lift during the print run.
Double sided filmic adhesive tape is used to fix the printing plate to the printing cylinder. These tapes can be varied in thickness to allow for any variations in the diameter of the print cylinder and can be used to compensate for any under or oversized print cylinders. It is critical that the outside peripheral of each printing plate is exactly the same. If this is not done, accurate print to print register will be impossible.
It is strongly recommended that the same brand of tape should be used on each set of print cylinders. Some tape manufacturers incorporate a thin layer of foam within the mounting tape which assists in smoothing out any small deviations in the plate cylinder.
MOUNTING FLEXIBLE PLATES
Even though plate mounting is usually done using a plate mounting system equipped with mechanical and optical aids, the mounting of a ‘flexible’ printing plate (See Figure 5.17) is dependent on manual skills.
Figure 5.17 - Plate mounting unit. Source- Nuova Gidue
The one rule that must be followed is to ensure that the plate will be in the correct position at the first attempt of mounting.
This involves a careful check to ensure that the registration lines on the leading edge of the plate are correctly in-line with either the optical system on the mounting system or alternatively the engraved grid on the print cylinder.
These engraved lines run both horizontally and circumferentially round the print cylinder. (See Figure 5.18).
Figure 5.18 - Mounting plates by hand using guidelines
Generally speaking the plate is usually mounted in the center of the cylinder and it is recommended that the center of the plate is marked and lined up with the mark located with the center of the cylinder.
It is important that a little extra time is spent in ensuring that the leading edge of the plate is in the correct position before completing the full mounting as any removal of the flexible plate to ‘try again’, can distort and stretch the plate making any attempts at re-mounting very difficult.
The bearer bars are strips of plate material which are positioned on the outside area of the printing plate (See Figure 5.19) creating an even impression over the plate cylinder. This reduces the plate bounce which can occur between the gaps in the image.
Figure 5.19 - Flexo printing unit showing position of bearer bars. Source- MPS
THE FLEXO PRINT CYLINDER
Regular maintenance of the press is very important. Bearings, print cylinders, gears etc. are subject to wear and it is advisable that replaceable parts are held available to avoid any delays in the event of a breakdown. Presses are subject to stresses and strains caused by the use of heavy tooling units, continuous running and poor maintenance.
Cylinder bearings have a limited life and a procedure for the regular checking of the print cylinders should be carried out including cleaning, checks for wear and adequate lubrication.
With the flexographic, letterpress and litho processes the printing plate is located on the print cylinders. The cylinder needs to have accurate and even contact with the inking rollers and the surface of the substrate or in the case of the litho process the offset blanket.
The plate/print cylinders should run perfectly true with an accuracy of ±0.025 mm ensuring that the pressure on the adjacent rollers is consistent. Accurate measurement should be made when the press has been running for a short period to allow the running parts to warm up.
PRINT CYLINDER GEARS
The print cylinder gear fits onto the end of the plate cylinder. (See Figure 5.20). These gears are manufactured to be meshed at a certain depth with the gear it is driving. This is called the pitch diameter.
When meshing the print cylinder gear with the impression roll gear this pitch diameter is critical to accurate register. Too deep or too shallow mesh will cause print registration problems between one color and the next. Regular lubrication is preferred to intermittent applications as this will maintain a constant film of oil and even out temperature fluctuations.
Figure 5.20 - Location of drive gear on the print cylinder
The circumference of the print cylinder is based on the repeat length of the printed image and is calculated in inches, millimeters, or the number and size of gear teeth; allowance has to be made for the ‘thickness’ of the printing plate and mounting tape.
The formula for this calculation is as follows;
A print repeat length of 12 inches (or 96 1/8’ gear teeth) equals a circumference of 300.8 mm, the print cylinder must have a circumference that is smaller by 3.14 (Pi), to allow for two thicknesses of plate and mounting tape, 3.14 x 2 (1.7 + 0.3) mm and a small allowance for the effects of the printing pressure and thermal expansion, say 0.01 to 0.03 mm.
SERVO DRIVE TECHNOLOGY
Servo driven motor technology is now widely used in the commercial, packaging and label industries. Each print and impression cylinder and anilox roller has its own servo motor. (See Figure 5.21), including the web transportation and web tension rollers and if required, the chilled rollers as well.
Figure 5.21 - Direct drive servo system. Source- Gallus Ferd, Rüesch
The servo motors are connected to a motion control unit via fibre optic cables. This unit controls the synchronising and function of the motors used on the press and commands each drive independently or as a whole system.
Servo or shaftless drive systems have re-invented the mechanical drive, removing the need for any mechanical drives or gear train and therefore many of the print problems associated with gearing.
Servo systems have substantially reduced substrate waste, speeded the makeready times and improved print registration and print quality.
Changeover from the finished job to the next job (called the makeready time) can be considerably reduced using servo systems, as it gives the facility to save job settings (job storage) and to recall the data for pre-register, auto-registration and print length when the job is repeated.
SERVO DRIVES - ADVANTAGES
No motors, gearboxes, clutch or gear trains required
No requirement for special gearing
Infinitely variable print length
No gearing issues between the plate and impression cylinders
Quicker job changeovers with faster make-readies and less waste
Computer-controlled simplified operation
Lower maintenance costs, greater press life and fewer moving parts
PLATE CYLINDERS AND REPEAT LENGTH
Flexographic presses used in the label industry are able to operate with a ‘variable’ repeat length facility. As the plate cylinders are removable and not in a fixed position, printing from cylinders of different diameters can be used, thereby allowing the printer to minimise the amount of material between each label gap and accommodate the varying print lengths required when there is a change in the label dimensions..
Step motors provide an independent drive to each roller and this allows the speed of the plate cylinder to be varied without the need for a plate gear change.
INKS AND DRYING SYSTEMS
There are three main flexographic ink systems used in the label industry. These are water-based, solvent-based and ultraviolet (UV). There are two lesser used ink systems known as electron beam (EB) cured inks and two-part chemically-cured epoxy inks which are used to obtain very high product resistant coatings.
In this article we deal with the three main ink systems used for label printing and the types of drying/curing used.
All the inks used in the flexo, gravure and screen processes are liquid inks which are dried or cured after each individual color is printed (ie multiple colors are printed wet ink onto dry ink).
Solvent based inks are not widely used in the label industry. Previously water-based inks provided the bulk of label work produce by the flexo process, but this has now changed and UV is the dominant ink system used in label manufacturing.
Printing inks consist of four component parts:
Colorants (pigment or dye)
Additives (reducers, waxes, driers, flow agents)
Carrier (aqueous or solvented).
The colorant provides the color to be printed and can be pigmented or dye based. The density of color is dependent on the volume of pigment immersed in the vehicle and the amount of pigment is known as the solid content.
Generally the more solids there are in the ink the more difficult it is to transfer the ink to the substrate. As flexo inks are ‘thin’ with a low viscosity it can sometimes be difficult to achieve the correct density of color required. This however does not apply to UV based inks which are 100 % solids.
The vehicle consists of resins in which the pigments or dyes are mixed along with the additives and the carriers (i.e solvent or water) and it is this mixture of resins, pigments and additives that determine the characteristics and performance of the ink.
SOLVENT BASED INKS
Solvent based inks use an organic volatile solvent, mainly ethyl acetate and alcohol, used as a blend and not a single component. This ink system is also used for the printing of self-adhesive labels by the flexo, gravure, screen and inkjet printing processes.
The solvent controls the viscosity of the ink and is excellent when rapid drying is required. The viscosity of the ink is controlled by the amount of solvent in the ink. If solvent is introduced into the ink the viscosity reading will be lower because the ink will be thinner. If however the ink is allowed to thicken through the lack of solvent, then the viscosity reading will be higher.
Ink viscosities are very important as the color being printed will be adversely affected if the readings fluctuate. Viscosity readings can be done manually using a ‘zhan cup’* or if the press is using a circulatory ink pumping system then these readings can be automatically carried out and are usually linked to a system that automatically introduces the solvent as required. Drying is done through hot air driers which drive off the solvents as the substrate passes through the drying hood. The extracted air can then be passed through a solvent recovery system.
Press wash-up and clean down between each run is done using solvents that match the ink solvent system being used.
*Zhan cup - definition
Dip calibrated viscosity measuring device.
A stainless steel cup with a tiny hole drilled in the center of the bottom of the cup. After lifting the cup out of the ink, the user measures the time until it stops flowing to assess its viscosity.
DRYING SYSTEMS: INFRA-RED/HOT AIR
Infra-red is the method of accelerated drying which uses either hot air or direct heat or a combination of both. (See Figure 5.23).
Figure 5.22 - A summary comparison between mechanical and servo systems.
Figure 5.23 - Drying system for water-based/solvented flexo
For flexo presses which are printing with water-based or solvented inks the drying system is infra-red or hot air or a combination of both. This type of drying system is compatible with many of the web-fed presses used in the self-adhesive industry.
Infra-red drying is not suitable for filmic materials as the heat that is generated creates problems with shrinkage and stretching of the filmic substrate, in particular lightweight unsupported film.
WATER-BASED (AQUEOUS) INKS
Water-based inks, also known as aqueous ink, were the original inks used on the very earliest flexo presses. These early inks were dye based aniline inks.
The modern water-based ink system is capable of producing very high quality printing and is used extensively within the label industry.
This type of inking system is more environmentally friendly than solvent based inks, particularly as they do not present the fire risk posed by solvented systems.
The range of colors that are available are the same as both solvent and UV systems and these inks will print on paper, metallised substrates and give excellent results on non-absorbent plastics/film i.e. polyethylene, polypropylene and PVC. It is recommended that an in-line corona discharge pre-treatment is given to non-absorbent substrates to assist the ink key to the substrate and to improve the ink lay down.
Water-based inks do contain a small percentage of solvents and chemicals, but this is kept to a minimum. These solvents can affect the viscosity of the ink and color should therefore be carefully checked during the print run.
ULTRA VIOLET - UV FLEXO
UV flexo is a print process in its own right and not just a variation on the conventional flexo process. The main principles remain the same, but the facility to print a much finer dot, with a thicker ink consistency and 100% solids content is similar to the inks used for letterpress and the offset process, but with a lower viscosity.
The introduction of UV flexo has led to the development of a range of anilox rollers that give the printer a number of options, particularly when printing very fine tone work. It has also allowed the use of file origination that is the equivalent to that used in the litho process.
This has made a big impact on the manufacture of self-adhesive labels and allows the printing and converting of filmic and metallic substrates, using an oil based UV system. The drying or curing of UV ink is through the reaction of the ink chemistry to an ultraviolet light source. The curing process is a photochemical reaction that occurs when monomers and oligomers are mixed with photo initiators and then exposed to a UV light source. The curing process takes 1 - 2 seconds.
UV inks are a 100% solids system and do not contain solvents as the main carrier. This means that they are not subject to fluctuations in the ink viscosity and can overcome any color variation during the print run.
Because UV curing is a rapid process the press can be run at high production speeds, but curing can be affected by the color and density of the ink film and the intensity of the UV light source.
Opaque inks are the most likely to have a curing problem as the UV light source must penetrate fully through the ink layer to ensure a full cure.
UV ink does not dry unless exposed to ultra violet light and this means that any press stoppage does not create the problem of rapid ink dying as experienced with water-based and solvented inks. Washups are quicker and end of shift washups are not necessary. Also there is not the problem of dried ink in the anilox cells known as plugging.
Advances in polymer plate and imaging technology have further advanced the quality of UV flexo printing and the process can now hold a dot formation of 1%, allowing very fine tone work to be produced.
The polymer used for UV flexo plates is thinner and harder, similar to that used for letterpress polymer plates. This allows the dot produced to be more stable and therefore not subject to flexing during the printing operation. It also facilitates the accurate transfer of ink film which in turn means that dot gain is reduced and the ‘squashed halo’ effect which is inherent in conventional water-based and solvented flexo is eliminated.
UV CURING SYSTEMS
The modern UV system is far easier to operate and maintain than the earlier systems. The power of the ultra violet intensity can be adjusted to suit a particular ink curing requirement and the controlled removal of the unwanted heat generated by the infra-red wavelength ensures that the web is only exposed to a minimum amount of heat.
Heat is removed by the use of cooled air which is blown across the lamps and then extracted by the use of water cooled plates which also reduces the amount of heat generated by the lamps. These methods of heat removal allow a UV ink cure that is ‘cold cure’ and makes the printing of light filmic substrates much easier.
Figure 5.24 shows the position of the UV drying /curing units in the printing press. The curing systems used can be run at very high production rates as the curing of the UV ink takes place rapidly. However drying speed can be affected by the color and density of the ink film and the intensity of the UV light source.
Figure 5.24 - Curing system for UV flexo
LED UV CURING SYSTEMS
The term LED stands for a light emitting diode.
The UV light source is generated by passing an electric current across a series of diodes which then generate high intensity UV energy. This technology is incorporated into a unit suitable for UV curing on the press. LED lamps are smaller than mercury units (See Figure 5.25) which makes them ideal for the narrow-web presses used in the label industry.
The lamps come to full brightness without any warm-up time and have a much longer lifespan than the standard air-cooled mercury UV arc-lamp system used on label presses.
LED systems can reduce production downtime and remove the problems that can occur when the lamp shutter mechanisms fail. LED systems do not require the same routine checks to monitor the energy densities of individual lamps as are required for mercury lamps to ensure there is no degradation in output.
Figure 5.25 - LED Curing
Some of the advantages of LED systems are summarised below;
Low power lamps are cooler with less heat radiated onto the substrate
Approximately 50% reduction in energy costs
More effective curing of opaque inks
Reduced maintenance on shutter system
No energy degradation – longer lamp life
No generated ozone issues.
TONAL RANGE (AQUEOUS/SOLVENTED)
What are tonal values? All printed graphics are made up of a printed dot formation, the lighter the dot area, the lighter the color, the more dense the dot formation the darker the color. (See Figure 5.26). This variation of dot density and size gives the various tonal values required to form the printed tones.
Figure 5.26 - UV flexo dot break up at 1-2%
IDENTIFYING THE FLEXO PROCESS PRINTED WITH SOLVENTED OR WATER-BASED INKS
It is necessary to use a magnifying glass to identify the method of printing which has been used on a particular graphic reproduction.
Each different printing process has a particular characteristic which can be easily identified. As explained the printed image is made up of differing densities of dot and there are limitations on the size of the dot which can be printed by some of the print processes. Generally speaking the size of the dot which can be produced by the traditional solvented and water-based press is between 3-4%.
The modern polymer plate used for the flexo process can hold a one to two percent highlight dot. By looking at the highlight areas the printer can identify the size of the dot and also the quality of the print.
Flexo plates are a relatively soft plate compared to the much harder letterpress plate and this soft construction can affect the print quality. The softer the plate the more likelihood of a squashed effect on the printed dot. However, softer plates will transfer the ink film more smoothly than harder plates. This squashed dot can easily be identified by a halo effect on the printed dot.
This poor quality printing is the result of a combination of a soft printing plate, too much impression and a low viscosity ink.
One of the biggest problems confronting the printer is the problem of dot gain. This effect is created by an ‘increase’ in the specified printed dot size which affects both the tonal values and therefore the color being printed.
Dot gain can be a result of incorrect plate imaging but is generally a result of incorrect impression settings or poor engineering or wear on the press and the tooling being used. Dot gain can also increase if a harder ‘non- cushioned’ plate mounting tape is used. Alternatively a softer ‘cushioned’ tape can reduce the gain but does not always provide for a good ink transfer for the solid printing areas and can also create pinholes in the print.
IDENTIFYING DOT GAIN
To effectively measure dot gain it is strongly recommended that the dot gain characteristics of each individual press is first established. This is not a difficult operation and requires a printed sample to be taken on the press being used and using a printing plate with a known dot specification.
Press settings must be carried out to get the optimum print quality. After the print sample has been produced the gain can be accurately measured to establish the dot gain of the press.
IDENTIFYING UV FLEXO
The modern flexo plate used for UV flexo tends to be harder and when used in combination with a much higher viscosity UV ink, gives the high quality flexo printing now produced in the label industry.
The UV process does not experience the problem of thin inks, and dot/print squash is very much reduced .The modern UV flexo press will print a very crisp dot of 1-2%.
FLEXO ON THE PLATFORM PRESS
The use of the flexo process on combination presses is now commonplace. The process is compatible with all the major printing processes and often forms the main process of the combination press.
ADVANTAGES AND DISADVANTAGES OF THE FLEXO PRINT PROCESS
Suitable for high speed web-fed printing.
Suitable for most materials.
Ideal for food packaging/newspapers.
Ideal for non-absorbent stocks.
Suitable for combination printing.
The pre-press costs are significantly less than for gravure.
The printing unit design allows for the repeat length to be easily changed.
Limited halftone reproduction
Process is subject to bar marking and ghosting.
Rapid drying ink creates press difficulties
The ink system can use relatively volatile liquids
FLEXO – SUMMARY
Flexo is far and away the predominant label converting method both in North America and also in Europe.
Advances in all of the components of the process inks, plates, anilox rolls, and the presses themselves have all played a significant role in the evolution of the flexo process, and they continue to do so today.
Developments in inks formulations with increased color strength, able to deliver the required film thickness and deliver high resolution images, have had a significant impact on the development of the process.
A significant trend has been the transition from solvent based inks to water and UV based inks.
Environmentally friendly water-based ink, many formulated with renewable plant based resins are on the increase, as are low migration UV flexo inks for use on primary food packaging.
The key benefits with UV flexo include more brilliant colors, zero or very low VOCs and the reduction of clean ups between shifts.
Directly related to the trends and advances in flexo inks is the method used to deliver them onto the substrate – the anilox roll.
The introduction of ceramic laser engraved anilox rolls has enhanced ink distribution and enabled an increase in cell concentration. The practice of engraving deeper and at higher line counts has contributed to both color and image quality.
The innovation of fibre optic laser technology that is able to create a cell profile with much smoother cell wall linings, thereby allowing a better ink release, has been a key factor contributing to image quality.
FLEXO PLATES AND WORKFLOW AUTOMATION
Flexo plate improvements continue to play a major role in print quality.
The following factors have been important:
CDI (Computer Direct Imaging). CDI imaging heightens results to levels that previously were not available. The use of new screening algorithms achieves a very wide tonal range with smooth and detailed highlights and graduations going to zero.
CtP in flexo platemaking computer to plate (CtP) and the use of laser ablation continues to move forward
Automating the workflow is a major trend in flexo. Software workflows have eliminated several steps from the process resulting in huge timesaving and in error reduction.
Servo technology, which eliminates the traditional line shaft/gear box drive systems, has lifted the bar in press construction and created new opportunities for in-line flexo.
Each plate, anilox and impression cylinder is able to receive its own servomotor along with web transport rolls such as tension rollers and chill stands, thus helping to eliminate inconsistency, whilst increasing flexibility and quality.
Registration control is vastly improved through the use of servos, thereby leading to a significant reduction in waste.
Today, flexo has carved out its own niche in the packaging and label printing sectors enabling cost effective, high quality in-line printing and converting for everything from short-runs to high-speed long runs on paper, film or board.
Flexo is perhaps the most versatile printing process in the world and the process is perfectly aligned with the future of a dynamic label and packaging industry.
Gears: Gears have inherent errors from the machining process. Mechanics wear out with use.
Cylinders driven from servomotor provide better synchronisation and eliminates mechanical wear
Mechanical device or sensor requires the press to move and create waste before timing is accomplished.
Servomotors can be electronically adjusted prior to machine movement.
Repeat size change
Limited to gear pitch of cylinders. This creates waste or design limitations.
Electronic gearing provides infinitely variable repeats.
Gears limit speed and cause vibration from resonant frequencies.
Gears and associated limitations are removed.