“Develop success from failures. Discouragement and failure are two of the surest stepping stones to success.”
Unit - 3
Types of flexo inks:
1. Dye based ink; these have been developed from the original aniline inks. It is making solutions of the dye and lacking agent in the solvent either by cold or hot blending on high speed stirring equipment. Dye based ink are usually used for printing on paper, general purpose bags, wrappings, waxed bread wrappers.
Dying rate of the dye based ink is slow. You can increase it by adding acetone getting higher speed of production or you can slow down by adding glycol either but you will get good result with bad odours.
2. Water based inks: In this type of ink water is used as a solvent, where resins are mixed well with water. For example shellac is used as a main resin. This is mainly dry by absorption. It used to print on paper, paperboard, kraft, newsprint and corrugation but now is widely used in newspaper because they do not rub off on readers hand and has less slow through. The wax compound is added in ink for rub resistance and anti-foaming agent. But it is poor level of ‘gloss’ and slow drying.
3. Solvent based ink:-it is mainly dry by evaporation and it suited print on plastic films, aluminum foil and gloss ink. In this many types of resins and solvent used.
Following are the solvent used in ink are: alcohol, esters, aliphatic hydro carbons, glycol ethers.
TYPES OF FLEXO INKING SYSTEMS
Flexography can be distinguished from other printing processes by its inking systems. The metering roller is known as the anilox roll, and it is the primary determiner of ink film thickness. It determines uniformity and consistency. Developments in anilox rolls continue to be at the heart of process improvement. The laser-engraved anilox permits use of doctor blades, which together provide consistent image uniformity over long runs and over long periods of time. All of this is essential for repeatability and predictability. Today flexography is competitive with all processes, largely due to the modern anilox roll technology.
INK METERING
In every printing process there must be a method to meter the quantity of ink. One must control the film thickness of ink in order to use the least amount of ink required for proper density/darkness and solid uniformity or coverage. The surface of the anilox roll is covered with tiny cells, all equally spaced and of the same depth and shape. The cells are specified by the number of cells to the linear inch and the depth of the cell, or its volume.
Anilox cells are often described as fine or coarse, depending on the cell count. A roll having 200 cells per inch is rather coarse, one with 400-500 cells per inch is average, and one having over 700-800 cells per inch is considered fine. Figure 3.1 shows that cell count alone does not reveal all one needs to know about the ink delivery capacity of the anilox roll.
Figure 3.1 - Cell count alone does not reveal all one needs to know about the ink delivery capacity of the anilox roll. A 360-line anilox roll with a deep cell (top) can carry more ink volume than a coarser 200- line roll (bottom) with very shallow cells.
A 360-line anilox with a deep cell can carry more ink volume than a coarser 200-line roll with very shallow cells. Therefore, when specifying an anilox one must always define the cells per inch or cell count, and either cell depth or volume. Cell dimensions are specified in microns. A micron is a millionth of a meter. To appreciate this, consider that there are 25.4 microns in 0.001 inch. Volume is measured in bcms, “billion cubic microns” per square inch (an interesting measurement combining a mixture of metric and English units).
A volume of 1.0-2.0 bcm is a low volume, probably used for fine screens/halftones on smooth substrates. A volume of 4.0 bcm is a middle-of-the-road anilox roller while a 7.0 bcm roll is found where bold solids are being printed on a very rough and absorbent surface. To increase or decrease the amount of ink in flexography, one changes to another anilox roll that carries the desired amount.
The anilox roll is at the heart of the flexo process. There are several common formsof ink metering systems found on flexographic presses.
1. Two-roll ink metering system
The old standard system is called the “two-roll system” (Figure 3.2 top). The anilox receives a flood of ink from the fountain roll which is suspended in a pan of liquid ink. The fountain roll is run in tight contact against the anilox roll. The fountain roll turns slower than the anilox, creating a wiping action. This causes most of the surface ink to fall back into the fountain, leaving only the ink inside the cells on the anilox roll. The ink in the cells is then transferred to the plate as they come in contact. In the two roll system, the efficiency of the wiping action is affected by the durometer of the rubber fountain roll. A harder, higher durometer like 80 wipes the surface (lands) of the anilox more efficiently than a soft roll with a durometer of 50. (The lands are the tops of the walls between cells which support the rubber roll or doctor blade and define the cells.)
Figure 3.2 - Two roll ink metering system.
2. Modified two-roll with a doctor blade ink metering system
The second metering system is a modified two-roll with a doctor blade. The rubber fountain roll is backed away so it floods the anilox with ink. The doctor blade is set at a reverse angle to the direction of rotation of the anilox. This reverse-angle doctor blade is engaged with just enough pressure to wipe the surface areas clean of all ink. This produces a much cleaner wipe than the two-roll system. Figure 3.3 bottom illustrates the two-roll with doctor blade ink metering system.
3. Reverse angle doctor blade ink metering system
A third configuration of the metering system is the simple doctor blade design (see Figure 3.4) where the anilox roll is suspended directly in the ink fountain (removing the need for a fountain roller) and the reverse angle doctor blade shears off the excess ink letting it fall back into the ink fountain. Here the ink metering is performed by a doctor blade (a strong strip of steel, plastic, or other material) that is placed between the fountain and the nip between the anilox roller and the plate cylinder. The angle and pressure of the doctor blade ensure a controlled and uniform ink metering.
Figure 3.4 - Reverse angle doctor blade ink metering system
4. Chambered doctor blade ink metering system
The last common print station design is the chambered doctor blade system. As shown in Figure 3.5 the ink fountain is replaced with an assembly mounted against the anilox roll. On one side of the chamber there is a reverse-angle doctor blade that performs the metering function. The other side of the chamber is sealed by a containment blade, which keeps the ink from escaping or leaking out of the chamber. The ends of the chamber are sealed with gasket-like materials. Ink is pumped into the chamber and usually returned by gravity to the ink sump. The chamber blade metering system keeps the ink enclosed at all times, reducing the loss of volatiles and maintaining the ink in a constant and clean condition.
Ink is pumped onto the surface of the anilox roller, where the top doctor blade is responsible for metering. This system is typically used on high-speed presses, and is popular due to the fact that, since the inking system is not exposed to the air, ink viscosity can be tightly controlled.
Figure 3.5 - Chambered doctor blade ink metering system
Conclusion
Flexography is a process where a precisely engraved anilox roll prints a thin film of ink onto the raised surface of the plate, which offsets the ink onto the substrate. It is imperative that the same amount of ink is delivered hour after hour and job after job if the process is to be predictable and profitable. Although there are many two-roll (sometimes called roll-to-roll) metering systems, the doctor blade is clearly the choice for repeatability. For some who prefer “art” over science, the two-roll system does allow the operator to vary the ink film. Of course, achieving the same variation on repeat orders poses a problem, complicated further when a different operator is at the controls.
The best of flexo printing requires precise doctor blade metering, and the chamber blade system is the system of choice, at least until something better is developed. The flexo press is easily retrofitted with the latest metering systems; many older machines still in sound mechanical condition are being retrofitted to bring them up to the print quality capacity of much newer presses.
3.2. TYPES OF ANILOX CELLS AND CLEANING SYSTEMS
THE ANILOX ROLL
The anilox roll is a uniformly imaged gravure cylinder. Figure 3.6 illustrates cells of two specifications, showing the depth and the opening. It also shows the land area critical to print quality, including solid uniformity and clean printing screens or halftones.
Figure 3.6 - Anilox rolls with cells of two different specifications. This shows the depth and the opening, and the land area, which is critical to print quality, including both solid uniformity and clean-printing screens and halftones.
The specifications of the cells in the anilox roll determine its capability for specific applications. For example, an anilox roll with 200 cells per inch, having a cell depth of 30-35 microns, will carry a volume of 7.5 bcm (billion cubic microns per square inch). This is a lot of ink. It would be like a six-inch paint brush, only good for very heavy applications of ink. You could paint a barn or rough siding with a six-inch brush and you could cover a very rough, absorbent kraft paper with a 200-lpi (lines per inch) 7.5-bcm anilox roll. However, if you wanted to do fine work, like fine lines and 133line halftones on a smooth and coated paper you might want a 600lpi 1.6-bcm anilox roll. Determining the best anilox roll for a given production scenario is a MUST; first an explanation is required of the specifications and how they relate to the substrates to be printed and the variety of graphics required to be reproduced.
3.2.1 ANILOX ROLL SPECIFICATIONS
CELL COUNT
Cell count refers to the number of rows of cells per linear inch (specified to linear centimeters in the metric world—divide by 2.54 to convert). A cell count of 180 would be very coarse, found only in coating or low-end imaging applications where substrates are poor and quality is not a priority. A cell count of 360, once considered fine, is now a middle-of-the-road roll used in good work on absorbent paper and paperboard substrates. Today cell counts of 700 and above are commonly used for very high-quality imaging on smooth, high-holdout (not absorbent) substrates. This explanation places importance on the substrate in choosing an anilox roll. Images, however, are also very important in determining the cell count.
CELL DEPTH
Cell depth is the next specification and is just as important as cell count. These two specifications determine cell volume, which is the determiner of density in a given application. Figure 3.7 shows that three aniloxes of the same cell count may have very different volumes depending on the cell depth. It is volume that interests the printer. When specifying an anilox roll determine the cell count and volume to do the job and leave the depth to the anilox supplier.
Figure 3.7 - Three anilox rolls at the same cell count may have very different volumes, depending on cell depth. The top roll shown here would have a 5-bcm volume, the botom has a 2.5-bcm volume.
CELL VOLUME
Cell volume is the key to coverage and uniformity of solids. More volume results in more ink and, thus, better coverage. However, too much volume of ink also results in dirty print. If there is too much ink to sit on top of the relief image of the plate, it will flow over the shoulders and result in dirty print.
High-resolution images require high-line, low-volume anilox rolls. There are rules of thumb for determining anilox cell count from halftone lines per inch. It is common to demand at least 3½ to 4½ times more cells on-the anilox than the lines per inch in the halftone. This is to prevent anilox moire, an objectionable pattern caused by the screen of the graphics interacting with the anilox screen pattern.
Figure 3.8 shows the importance of the cell count the ability to produce clean printing. It can be seen that a high-line count (“line count” and “cell count” are terms used interchangeably) roll has enough cell walls to support very fine screened images. A coarse cell will allow small percentage dots to fall inside cell, without being supported by a cell wall, and thus permit ink to flow around the image onto the shoulder of the dot. This causes “dirty print” or dots to join wherever a dot is unsupported by a land area. A high-line-count fine anilox roll will produce clean printing of fine screens and type.
Figure 3.8 - The importance of cell count to the ability to print cleanly. The top shows a 900-line anilox roll with a 25-micron cell opening and 3-microncell wall; the bottomshows a 550-line anilox roll with a 42-micron cell opening and 4-micron cell wall.
The best anilox roll specifications yield just enough ink to deliver the required density and solid uniformity while not overinking the fine screens in the plate. This roll has enough cells to provide lands to support the finest image areas. Cell angle can also be controlled. While traditionally the cells are angled 45° from the axis of the roll, it is possible to fit more cells into an area when they are aligned at 60°. Since this provides more cell openings and less land area, or space between cells, 60° rolls achieve better uniformity with less ink. The 60° angle is also better in avoiding moire with traditional graphic screen angles since it no longer falls in line with the most desirable image angle of 45°. Today most new rolls are purchased with 60° cell angle. Sometimes a flexo printer concludes that the ideal roll is a very-high-line-count, even when printing on an absorbent substrate. The all that is needed is very deep cells to achieve the required volume. This introduces one last concept to be considered, depth-to-opening ratio. Figure 3.9 illustrates several depth-to-opening ratios. It shows that very high volumes might be engraved into an anilox of a high cell count. However, the bottom row illustrates that when the depth exceeds a certain point, no more ink is released to the plate. There is a range within which volume on the roll can be used to control in film on the printed substrate. Beyond that range, no additional ink can leave the roll and there will be no increase in density.
Figure 3.9 - Illustration of several different depth-to-opening ratios. Very high volumes can be engraved into anilox rolls. However, the bottom row illustrates that when depth exceeds a certain point, no more ink is released to the plate. Higher volume results in higher ink transfer, up to about 35% depth-to-opening ratio
Until now the discussion might suggest that the anilox is the so determiner of ink film thickness. But this is far from the case. The ink itself is a major player. It has been assumed that the amount of liquid that is printed controls the dry ink on the product. Actually it is the amount of solids, particularly the colorant, or pigment. Figure 3.10 illustrates that one ink may require 40% more liquid be printed to result in the same density and solid uniformity. This, of course, would require a 40% higher volume anilox roll. Such an anilox would not print as clean. Therefore, when people talk of high-line-count low-volume aniloxes, you must realize they are also talking about inks with the maximum amount of pigment and the least amount of liquid necessary for transfer and adhesion.
Figure 3.10 - One ink may require 40 % more liquid be printed to result in the same density and solid uniformity. This, of course, would require a 40 % higher-volume anilox roll, and would not print cleanly. In the illustration, the top anilox prints excellent density using a highly pigmented ink. The bottom anilox requires 40 % more volume to print the same density because the ink contains less pigment and more liquid.
3.2.2. TYPES OF ANILOX ROLL BASED ON CELL SHAPES:
Anilox Roll is engraved with tiny cells. They normally have an inverted pyramid shape. These cells or pockets when filled with ink from fountain roll carry up an exact quantity of ink to the printing plate. Choosing a proper anilox for the job is important for successful flexographic printing. If the cell count is more, the ink carrying is also less. The Anilox rolls come in various sizes with various shapes of cells. Three basic shapes of Anilox roll cells are
i. Inverted pyramid shape cells
ii. Quadrangular shape cells
iii. Trihelical shape cells
Figure 3.11 - The two most common cell shapes used in flexo printing, the quad and pyramid
i. Inverted pyramid:
Anilox roll with inverted pyramid shaped cells are recommended for all types of flexo inks as well as varnishes and coating.
Quadrangular Cell:
Anilox roll with quadrangular shaped cells carry more volume of ink in comparison with inverted pyramid cells. These cells are oftenly used with reverse angle blade.
Trihelical Cell:
Used to apply heavy viscous coating. This type of Anilox roll can be used with or without reverse –angle doctor blade.
Anilox rollers are normally engraved. After engraving they are copper finished then hard chrome plating is applied to increase their life.
3.2.3. TYPES OF ANILOX ROLLS BASED ON ROLLER SURFACES
i. LASER-ENGRAVED CERAMIC ANILOX ROLLS
Laser-engraved ceramic anilox rolls are the dominant type of roll being used today. This is a steel roll that has been machined to very precise dimensions and tolerances. It has a plasma sprayed chromium oxide surface built up to a thickness of 0.00~0.010 in. The cells are burned into the ceramic with a CO2 laser that literally vaporizes the coating, leaving a precise cell. The cell count and depth are computer-controlled, meaning that theoretically any specification can be set.
The ceramic surface is extremely hard, which is very important to print quality. Since high-quality flexo printing is achieved with doctor-bladed ink metering systems, the rolls must not wear or repeatability would be impossible. While ceramic rolls do wear, it occurs over an extended period of production.
ii. CONVENTIONAL (OR) MECHANICALLY ENGRAVED CHROME ANILOX ROLLS
While today the vast majority of new rolls being purchased are laser engraved ceramic, there are still many rolls in the industry of the engraved chrome technology. These rolls, also called mechanically engraved, or simply “chrome,” are manufactured by a displacement process, the same as knurling. A hard, precise tool called a mill contains a male pattern of the cells (Figure 5-9). The mill is forced under tremendous pressure into the steel- or copper-covered steel roll. During several passes over the roll the cells are made deeper and deeper until the roll has reached full engraved depth. Just as ice dropped into a glass of water raises the level of water in the glass, this process displaces the metal up into the mill while the mill is pressing deeper into the surface. Since every cell is produced from the same “master” the conventional engraved chrome roll is a very uniform “gravure cylinder.” The roll is electroplated with a hard chrome to provide protection from wear, hence the name engraved chrome. Figure 3.11 illustrates the two most common cell shapes used in flexo printing, quad and pyramid.
Figure 3.11 - The two most common cell shapes used in flexo printing,the quad and pyramid
Engraved chrome has limitations that helped to move the market to laser-engraved ceramic, the greatest being its lack of resistance to the wear caused by doctor blades. Since new cell specifications require a lengthy process, demanding very high craft skills, to make the engraving tool, it was not possible to perform quality improvement experiments in a timely and economical fashion. These two factors were major contributors to the early acceptance of laser engraving as an alternative approach to anilox roll production. In little over a decade the dominant roll of choice changed from engraved chrome to laser-engraved ceramic.
There are other types of anilox rolls. Conventionally engraved rolls can be plasma sprayed with ceramic instead of chrome and yield better life. These rolls are called engraved ceramic. This approach, however, has never been widely adopted. Another approach to anilox cell production is electromechanical engraving. This method uses the same machines employed in the production of gravure cylinders. The Ohio Engraver and the Helioklischograph are the two most common tools to employ a diamond stylus in cutting precise cells into a copper surface. The copper is then electroplated for wear resistance.
One last technique known as random ceramic has been employed. This is a roll which is simply plasma-sprayed with chromium oxide particles. The coarser the particle the more ink carrying capacity. Like sandpaper, the rougher the surface, the more ink, and the finer the particles, the less ink. This is a simple system, not as uniform in its ink delivery, and is used relatively little compared to other types.
DEVELOPMENTS
Lightweight cylinders are now being used to replace the standard steel construction. Modern materials such as carbon fiber can be used to build the base roll without the weight of steel. These are much easier to handle, and shipping issues are reduced. The same is true of the use of sleeves, similar to those being used for plate cylinders. It is important to note that many new ideas continue to develop in this and other aspects of flexo printing, which is a sign of the atmosphere change and development that characterizes flexo technology.
3.2.4. TYPES OF ANILOX ROLL CLEANING SYSTEMS
i. Roll Cleaning System:
The level of cleanliness of anilox roll and indeed the ease of achieving, is one of the most important problems facing the flexo printer today. This is due to the rise in screen counts and the ever increasing requirement for quality improvements.
ii. Jet wash type system:
These are very simple mechanical device that utilize specific heated chemicals fired at the roll under high pressure. They are not generally not screen counts sensitive and will work over a range of screen counts. On the downside, their success is heavily reliant on the type and condition of the chemicals employed, which can be expensive, being applied at by volume with water. Performance can drop dramatically as chemical becomes contaminated.
iii. Powder blasting system:
Generally single roll system, they use the impinging force of a particle to knock out the contamination and are supplied as either wet or dry systems. Work well up to moderate screen counts when the operator is fully lined and interested. Gaps between roll and nozzle, air pressure, feed rate and the feed itself all need to be monitored and controlled. Also these system can have an issue with ink. Powder is a total loss and can be used for on press cleaning, however this can be a messy process.
iv. Polymer bead blasting system:
The same rules apply as with powder blasting. The units work well with the correct beads up to moderate screen counts when the operator is fully trained but the units suffer from the same limitation as powder blasting. These cannot generally be used as on press system is that there is no waste to consider. There are couples of fringe systems that have entered the market over the last few years that deserve mention.
v. Dry ice system:
These as the name suggest utilize the dry ice to blast out the contamination. The equipment is also used as a general press cleaning system and although when properly used and controlled will clean the roll well but not enough is known about the longer term effects on the ceramic and the units are very noisy in operation. Again, there is no waste to consider with these systems.
vi. Laser Cleaning System:
These systems utilize generally the same laser used to cut the cells as to clean them. This vaporization of the contamination will render the roll clean but the systems are very expensive in comparison with other devices available and as it is generally the lower skilled operative that are left to clean the rolls, the question of skill level should be raised.
vii. Ultrasonics:
This system work by the flexing of the base of a filled tank at very fast rate. So fast, that on the downward stroke a vacuum is created under the water microns. On the upward stroke the vacuum is closed and pushed up into the fluid in the form of microscopic vacuum bubbles that collapse on contact with the roll surface, sucking out the contamination. When correctly controlled and combined with a suitable cleaning chemical (usually at 10%with water), this method will give excellent results. The system generally do not require a skill to operate and when used regularly are very quick and effective, however waste is a consideration, although this can be neutralized in certain circumstances and the volumes are low in comparison with jet wash system.
viii. Alpha sound:
This equipment utilizes ultrasound but, there are differences compared with ultrasonic cleaning systems generally available today for this purpose. This technology embraces and manipulates various frequencies and power levels to specifically target screen count ranges. Tight control over the base technology is the key and other various system features stop operator error and protect the roll. There is enough room in the marketplace for all the above roll cleaning methods but what is the best? That is the operator to decide.
3.3. SELECTION OF SUITABLE ANILOX ROLLER
CHOOSING THE ANILOX ROLL
There are always several, if not many, considerations to be made in the choice of anilox rolls.
1. Substrate. If only one substrate is to be printed then the choice is easy. Many times one anilox roll must be used for a range of substrates. This calls for the anilox which delivers the least ink required to achieve density and solid uniformity on the most absorbent of these substrates.
2. Anilox cost. If one roll costs $15,000-20,000 for a five-color press, one may have to settle on just one or two sets of rolls for all one’s needs. This is especially true in the corrugated industry. Therefore, even to print a variety of substrates and types of graphics, a compromise must be found or economic reasons.
3. Time. Modern flexo presses generally provide for quick changes of anilox rolls; however, most presses in use today are not so equipped. This means that to optimize the anilox to the job at hand, the changeover times may be prohibitive, again, a compromise is necessary.
4. Graphics. It is common for customers or designers to specify graphics with fine screens on substrates of less than ideal surface. Since most jobs mix screens with solids this scenario presents problems. In this case the anilox will probably be chosen to achieve adequate solid density and uniformity while delivering more ink than necessary to the screens; another common compromise.
5. Productivity. While there are many influences on productivity one common example is the availability or lack of dryers. The classic case is in the envelope world where very high speeds are expected without availability of dryers. Drying relies more on absorbance into the paper. The substrate is also generally rougher than ideal so that viscosities (fluidity of ink) must b lower, more liquid, and thus anilox volumes higher in order to deposit sufficient pigment for density and uniformity. Bottom line: fine screens are likely to suffer.
These realities point to the value of planning all jobs with input from the entire production team. It is unrealistic to expect a customer and designer to understand so much. Since everyone seeks total success in any project, working together from the beginning will result in the best achievable results given the specific realities hand.
Types of flexographic printing presses: flexographic presses are usually rotary web presses equipped for the printing of one or more colors. To meet high speeds with consistent printing quality.
It presses are used for packaging printing, they print only one side of the web but they can also be made to print both sides of the web. Web width vary form 4” to 99”. It is first printed and may therefore be sheet-fed, slitted, die-cut etc.
In addition to the variety of accessories available, presses vary in their die-cut etc.
1. In line type: in this type of presses, the printing unit consist of an impression cylinder and a plate cylinder is separate and remaining printing units also have similar structure. The web travels from one unit to another unit in a straight line. It requires elaborate register control system for multicolor printing.
2. Stack type press: the stack type press has two or more printing and inking units vertically arranged on one or both ends of the main press structure. The inking units and impression units are mounted on each side of the central side frames, so that a four color machine the first and second color units are on one side of the frames and the third and fourth units are on the other side of the frames. The closeness of the printing units in this press design provides satisfactory register and will produce excellent result on highly absorbent surface which old not require special drying.
It are only used for simple print jobs because of their poor register accuracy around +/- 0.2mm in longitudinal register.
3. Satellite type presses: the common impression or central impression press has one central impression cylinder with the plate and inking units groups around it. The central impression cylinder may be larger as five or six feet diameter. It give better registration than the stack type press. But it is more difficult of ink between the units, where over printing is necessary.
CIC diameter greater than 2 meter and web width of 300 to 300nm.
Flexographic printing units are interchangeable and can also be combined with offset and gravure printing units for the construction of hydrid system.
Flexographic printing presses machine:
1. 8 color CIC with state of the art control technology equipping of the press with plate cylinder by means of a robot system from a loading trolley, a change of plate cylinder sleeve through the side frame, it takes around 12 minutes to change 8 cycles, cylinder diameter is 2275mm web width up to 1770mm, printing speed 6m/s 34DS/8-CNC, fischer & krecke.
a. Astraflex, W& H ----------------------------- an 8 color press with CIC.
b. Lemanic 82F, bobst----------------------- flexographic press with in-line unit stand with intergrated, die-cutting unit for folding box production, speed -3.5 m/s.
c. Arsome EM 510, Heidelberg (gallus------- flexographic label printing press with U.V drying and rotary die-cutting, 2.5m/s.
d. Flexicourier, KBA ------------------ newspaper press for multicolor printing with 144 printing units.
e. Flexogold, Aurelia --------------------- sheet fed flexographic printing press, two color for 70 cm X 100 cm (28 inch X 40 inch) format, 12000 iph.
Flexo substrate:-
1. Paper and paperboard stocks.
2. Corruagated stock.
3. Films.
4. Foils.
5. Laminates.
Properties of substrate used for flexo printing:-
1. Printability.
2. Smoothness.
3. Gloss.
4. Whiteness and brightness.
5. Pick resistance.
6. Thickness.
Corrugated Printing Considerations:
Today's advanced technologies and materials have revolutionized printing on corrugated packaging materials. With modern printing technologies, and the ever-increasing capabilities of flexo, brands can simultaneously use corrugated product boxes to increase product visibility and promote a brand’s awareness, in addition to benefiting from the material's lightweight durability which is ideal for shipping.
The variety of available processes and materials have created several methods for printing on corrugated material, including flexography (flexo), offset lithography (litho), screen printing, and digital printing. Due to its versatility, flexographic printing has become one of the most commonly utilized.
We have a complete guide to Flexo Printing on Corrugated!
Not actually a direct printing method, lithography pre-prints images onto linerboard, which is then attached to the corrugated substrate. While this is an option for all printing methods listed, this is the only way to print on corrugated with litho, which guarantees an additional step in production.
This method easily accommodates foil stamping, spot gloss, embossing, and other embellishments and renders high-resolution images.
Lithography's nearly limitless options come at a premium. The set up, added embellishments, and pre-printing method all increase costs.
During the screen printing process, a squeegee forces ink through a screen, depositing color onto the substrate. Stencils (masks) shield negative space from ink and allow ink to pass through onto colored areas.
A stencil is made for each separate color, and each color is applied separately. It is a time consuming process that is often done by hand.
It produces richly colored images, but it is most suitable for small volume prints.
Digital printing presses function similarly to the printers we use at home. Unlike the other printing methods, digital printing does not require the creation of printing plates because a digital press uses electronic files to print images.
Digital printing is capable of rendering high resolution images. This method costs more per print than the other methods and moves more slowly, but without the need for printing plates, set up is less expensive.
This makes digital printing a great option for small volume printing, samples, or test marketing.
By far the most popular choice for printing on corrugated substrates, flexography uses an inked image carrier to transfer graphics onto a substrate. The process is gentle enough for corrugated substrates and can still achieve vibrant graphics on the final product.
Since flexography requires printing image carriers (one per color), there is some up-front cost associated with set up. Once the printing image carriers are ready, however, flexography is a cost-effective and high-speed method for printing large volume orders with attractive graphics.
The commercial printing industry has recently seen the popularity of hybrid presses grow. Consumer products are requiring more intricate prints and durable final products, which in turn has required printers to adjust their capabilities to meet demands.
This has resulted in many printers opting for hybrid presses which delivers more of the “pros” of each of the above printing methods, without sacrificing quality or efficiency.
One of the main reasons that printers are opting for digital flexo hybrid preses is it allows the flexographic printing process to better accomplish shorter runs, without sacrificing the quality of a final flexo print.
Hybrid presses enable full color digital inkjet technology to be utilized with an existing flexo press. Brand new digital presses that feature flexo printing stations are also available.
'Hybrid Flexographic Presses'. There are several varieties of "hybrid" printing processes that combine aspects of flexography with other methods.
Flexo Gravure. Flexo Gravure is a form of offset gravure. Offset gravure printing essentially replaces the flat offset plate with a longer-lasting gravure cylinder, transferring the image to a rubber blanket which, in turn, transfers the image to the substrate. In flexo gravure, offset gravure is performed on a flexographic press, with the gravure cylinder replacing the anilox roller. A rubber blanket (such as that used in offset lithography) is mounted on the flexo plate cylinder. The ink is transferred to the engraved cells of the gravure cylinder (which, unlike conventional gravure, need to be engraved so that the image is right-reading); the image is then offset onto the rubber blanket (where the image becomes wrong-reading), and is finally transferred onto the substrate. Flexo and offset gravure are utilized when the desire for the high-quality gravure image carrier and long life of the gravure cylinder are needed for substrates that are not easily printed by traditional gravure. The flexible rubber blanket ensures high-fidelity image transfer on a wide variety of surfaces.
Offset Flexo. Offset flexo is a hybrid of flexography and offset lithography in which the anilox roller transfers the ink to a flexo plate (which needs to have its image in positive-reading form) which then offsets the image to an offset blanket cylinder mounted between the plate and the impression cylinder. Cylindrical plastic containers need to be printed in this manner. In some presses, all the color stations are positioned around a single blanket cylinder, and a multi-colored image is registered on the blanket, a single multi-color image being transferred to the substrate in essentially one pass.
The main advantage of flexographic printing, as was mentioned earlier, is its ability to print on many different types of substrates. There are far too many flexo substrates used to provide a comprehensive list here; flexo presses print everything from breath-mint wrappers to plastic packages that hold king-size mattresses. In the past, different types of polymers (i.e., plastics) mixed together tended to yield poor substrates with low print characteristics, but new advances in chemistry and manufacturing are producing new blends of plastics—known as "plastic alloys"—which can impart different qualities to the final product, such as increased strength, chemical resistance, resistance to the penetration of oxygen or other gases, etc. As the substrates change, so must the ink; cooperative efforts between ink manufacturers and the manufacturers of substrates ensure that for each new substrate that can be printed a compatible ink will enable printers to utilize it effectively, efficiently, and economically.