The Basics of Flexography
▼Basic Knowledge ▼Processing ▼Printing Presses ▼Print Accuracy
Basic knowledge
What is flexo printing suited for?
Flexo plates are made of a flexible resin material which makes it the preferred method for printing on corrugated cardboard due to its high risk of deformation. Additionally, the flexible nature of the plate allows for printing on a wide range of materials regardless of surface smoothness such as wood. Finally, flexo plates can be made without solvents and print with solventless ink which makes it well suited for products which require a high degree of safety such as food and beverage packaging, medicines, and baby products.
What kind of printing can you do with flexo printing?
Flexo printing is compatible with a wide variety of inks and can be used on any printing material, so you can print not only on cardboard and paper, but also film, metal, and cloth. Full color printing is possible through the use of process and special color inks. Through the use of a roll shaped photopolymer plate it is possible to print endless patterns (across a continuous reel of paper for example).
What kind of printing method is flexo printing?
This method uses a plate made from flexible resin to apply ink directly onto a medium such as paper or film. Flexography is often considered to the resin letterpress printing used in the Japanese label industry, though they are distinguished by their ink specifications, printing machines, and inclusion of an anilox roll. In flexography, ink is applied to the resin plate by means of an engraved roll fitted to the printing machine, called an anilox. Typically, low-viscosity water and alcohol-based inks are used, however UV inks are also compatible. Generally, flexo is characterized by its low ink consumption and fast drying times. Another characteristic is its ability to apply printing pressure even if the surface of the material is slightly rough. For these reasons flexo is widely used in the US and Europe for packages, flexible packaging material, and labels.
What are flexo printing plates?
Flexography is often considered the successor to letterpress printing and applies ink directly to a surface rather than the intaglio printing of gravure or the lithographic printing of offset. Flexo plates are made from a special photosensitive resin material that hardens when exposed to ultraviolet light. The flexible characteristic of these resin plates is a major feature of flexography which allows it to print on rough surfaces. A separate plate is created for a single color based on the finished design and the normal platemaking process is performed in order of: exposure, washing, drying, and finishing. In recent years a new plate making technology has emerged where the resin blank can be cut by directly irradiating it with a laser. Some usual overseas plate resin manufacturers are DuPont and Flint, while Japanese manufacturers are Asahi Kasei, Kodak, and Nippon Denshi Seiki.
How prevalent is flexo printing today?
In Japan, flexo is primarily used for corrugated sheet where it holds almost 100% market share.
Its second most common applications are in the fields of labelling and bag making.
At present, there hasn’t been much adoption of flexo for folding cartons or flexible packaging in Japan though attention has been increasing in recent years.
Even if the underlying technology is the same, the necessary machines and specifications can vary greatly between corrugated and other substrates.
In Europe and the US, 70% of flexible package printing is said to be done with flexo making it a major player in those markets.
Why is flexo printing not as popular in Japan?
The first factor is that the current quality of flexo printing is often not recognized.
There exist in fact, many products which ship to market that take advantage of flexo’s high number of lines and print volume, however they represent a small share of the overall market.
An additional hurdle has been the difficulty of making initial investments in printing equipment caused during times of economic stagnation.
When looking at US and European companies however, it is clear to see that flexo has attracted a lot of attention in terms of production efficiency.
Why is flexo printing currently getting so much attention now?
The primary reasons are the wide variety of usable substrates, potential for efficient production, and the wide range of applications which it is applicable to. In addition, flexo technology has improved dramatically over the years to the point that it now produces results comparable to offset or gravure.
At present, flexo is said to account for 70% of the flexible package printing in the US and Europe so there are high expectations for future growth in Japan as well.
What are the environmental benefits of flexo printing?
While offset and gravure often use oil-based inks, flexo most commonly uses more environmentally friendly water-based or UV inks. Flexo plates are also much more environmentally friendly with the advent of solventless plate production methods.
What is the flexo platemaking process like?
For analog production:
- The color-separated design data is output to film using an imaging machine
- The film is fixed to a resin blank
- The resin blank and mask film are then exposed to UV light
- The resin plate is washed with solvent to remove the uncured material
- The plate is dried and finished
For digital production:
- In steps (1) and (2) the surface of the resin blank is treated with a laser rather than using a masking film.
- The rest of the process is the same from step (3) onwards
For more information, please refer to the page “How our flexo plates are made”.
Is it possible to create flexo plates without the use of solvents?
It is possible. There are currently three existing methods.
- Cutting the resin blank directly through the application of a laser. This method is currently available through a few companies including FUJIFILM (Japan), and seems likely to receive continued development in the future
- Wiping away the excess resin after exposure with the use of a specially designed cloth. This method is currently produced by DuPont (USA).
- Washing away the excess resin material similar to the traditional method but with the use of a water-based cleaning agent rather than solvent. This method is currently available from Asahi Kasei (Japan).
What kind of ink do you use in flexo printing?
Flexo printing can use almost any variety of ink such as water-based, solvent, UV, etc…
It is a general characteristic of flexo printing that compatible inks are low viscosity (except for UV), require lower ink usage, and are faster drying.
What do you mean by “ink and resin plate compatibility”?
There are many lineups of resin plates that look similar at first glance, depending on the ink. Each material manufacturer has a lineup of resin plates for UV, water-based, and solvent inks as well as coatings. If you accidentally use an ink other than the recommended variety, the resin can swell and become uneven causing print errors or even damage to the plate itself. This is all to say that in flexo printing, ink and plate selection is very important.
In addition, the type of resin plate changes based on the printing medium it will be used with. A thick resin plate is mainly used for materials without a smooth surface, while thinner plates are typically used for smooth mediums.
Where can I get flexo ink? Can it be imported from overseas?
They are available at all major domestic ink producers. We recommend that you consult with us about the right choice for your press and desired print media.
What materials can you print on with flexography?
Unlike offset, flexo can be used on an enormous range of materials such as paper, cardstock, cardboard, metal foil, and plastic.
Are there any places in Japan that can do color process printing on cardboard?
Although small in number, there are a few companies that have print equipment, such as those by BOBST, who can produce and direct ship fine cardboard with process color printing.
Is it possible to perform full color process printing directly on cardboard?
Yes, it is possible. In fact, this type of packaging is incredibly popular in Europe and the US. In these regions, typically in the case that a coated liner is used for the outer surface it will be E flute cardboard, BE double cardboard if considering print quality, and others such as EF double cardboard might show up.
Currently in Japan process color direct printing is almost unheard of but given its productivity and economic efficiency, it is highly likely to become widespread in the future.
What kind of software is used to edit pre-press design data before flexo printing?
Adobe’s Illustrator and Photoshop are commonly used software in the design industry to manage digital data however they are not specialized for flexo.
ESCO’s PackEdge is often used as a dedicated software for flexo prepress systems.
About processing
How is die cutting performed in flexo printing?
For flexo printing, a rotary die press is common and typically used for in-line punching directly after printing.
While rotary die cutting may be common, due to the higher cost of the dies, cheaper Thompson cutting may be performed in a separate process after the sheet is printed.
What kind of post-processing (in-line processing) is possible with flexo printing?
In addition to punching and pasting, embossing and hot printing are also available in-line for flexo printing.
Why can post-processing be done in in-line with flexo printing?
Most flexo compatible inks have very low drying times so there’s no need to wait after printing to apply additional processing.
About flexo presses
Which type of flexo press is most common, sheet-fed or web?
Sheet-fed presses are typically used for corrugated printing while web-fed is common for most other applications.
What types of flexo presses are there?
There are three general types determined by the layout of the unit:
• Inline; where each unit is arranged parallel to the floor
• Stack; where each unit is arranged vertically from the top
• CI (Central Impression); where each unit is arranged in a circle around a central drum
There is a further division around units which are sheet-fed (media is fed into the press one sheet at a time) or web-fed (media is fed continuously from a wound roll), however sheet-fed machines can simply be considered inline presses.
What are the characteristics of an inline press?
It is characterized as being easy to work with because it is divided into multiple segments linked in parallel with each color mounted horizontally.
What are the characteristics of a stack press?
A stack press is built by stacking units on top of one another. It is characterized by its space saving and high productivity qualities.
What are the characteristics of a central impression (CI) press?
Are there any flexo presses made in Japan?
Absolutely!
For corrugated applications:
Mitsubishi Heavy Industries, Isowa, Shinko Machine, and Ishikawa Seisakusho, etc…
For labels, folding cartons, and flexible packaging:
Orient Sogyo, Taiyo Kikai, Newlong Industrial, etc…
What is a “doctor blade”?
In flexo printing, ink is applied to the resin plate by means of an engraved roll mounted on the printing press called an anilox roll. The engraved cell structure of the anilox collects ink from an ink roller, however in order to maintain consistent quantity of ink, a metal blade is employed to scrape away the excess.
This so called “doctor blade” is a thin metal plate made of wear-resistant steel and designed to remove ink stably and with high accuracy.
What types of anilox rolls are there?
Anilox rolls with ceramic or chrome-plated surfaces are common and in recent years some synthetic materials have also begun to appear.
The surface cells (screen) can be shaped in diamond (rhombuses) and honeycomb (hexagons) patterns, with the cells evenly arranged at certain angles.
The amount of ink held by the anilox depends on the cell shape and number of lines.
About print accuracy
What determines the accuracy of flexo printing?
In flexo printing, the print accuracy varies greatly depending not only on the printing press as well as of course the ink, printing plate, and substrate; but even the anilox roll, cushion tape, and sleeve used in setting up the press can have an influence.
Does flexo printing require barcode verification?
Yes, flexo printing does require barcode verification at the time of printing.
This is due to some of the major differences in platemaking process compared to offset and gravure.
In flexo printing, various processes are performed to improve print reproduction during plate making. Primarily this includes barcode dot gain processing (preliminary thinning in anticipation of thickening during print reproduction) and expansion/contraction processing (applying a reduction rate in the printing direction).
If even some of these processes are incorrect, reading errors may occur even if there are numbers in the barcode.
Many companies perform barcode verification in the platemaking process, but we recommend that you also perform barcode verification at the printing site.
How many screen lines can be used in flexo printing?
Theoretically, up to 200 LPI should be possible with the latest flexo equipment and materials, however 175 LPI is a more realistic estimate.
There are in fact already some companies within Japan printing at 175 LPI. However, the number of printable lines can very considerably depending on the plate, anilox, press, substrate, etc…
What kind of print environment is required for direct color process printing on cardboard?
You’ll want to meet the following criteria:
- Have a flexo press with more than 4 colors and an anilox roll of over 350 LPI for each color.
- The printed surface must be coated with a liner and the surface smoothness of the corrugated cardboard must be extremely high.
- You must have color process plates such as digital plates available
Do you have to make any considerations around screen angle when flexo printing?
There are a few things to note.
To avoid moiré, shift the angles of C, M, and K by 30 degrees in the same way as offset and gravure.
It is common to put Y at 15 degrees between any two colors.
In the case of flexo, in addition to these, the anilox roll angle must be taken into account.
Problems often occur when the angle of the anilox roll is the same as the screen angle of the plate. In such a case the whole screen of the plate should be rotated at a constant angle.
Technical Details
▼Technical questions ▼Corrugated printing ▼Maintenance ▼Print media
Technical questions
How is pre-proofing performed in flexo printing?
Recently, simple proofing equipment for flexography has been developed. This is a system that can output production proofs using the same anilox, ink, and substrate as your actual press. (Manufactured by JM Heaford, UK)
As this equipment is not easily available in Japan however, we often see the use of digital proofs such as DDCP.
Does the print accuracy of flexo printing change depending on the cushion tape?
Yes, print accuracy changes depending not only on the plate finishing but also the cushion tape. If you find that your current cushion tape is not suitable for your printing machine, resin plate, or print contents; try switching to a different cushion tape with similar specifications.
In flexo printing, a resin plate is attached to a sleeve (plastic cylinder) or directly to the plate cylinder using cushion tape.
Generally, solid printing uses a soft resin plate with hard cushion tape, while fine quality screen printing uses a hard resin plate with soft cushion tape for clearer print reproduction.
The typical criteria for good cushion tap are that it is easy to apply, leaves little residue, and does not change shape even after a long print run.
It also of course goes without saying that the precise thickness and adhesiveness of the product are also critical.
Recently there has been a dizzying pace of technological innovation for cushion tapes in the US and Europe. Some examples of this are tapes with an embossed finish on the surface of the adhesive layer to make it easier to apply, a porous structure to reduce deformation, etc…
How should I think about the influence of the anilox roll as it relates to flexo print resolution?
In flexo printing, screen reproduction is generally not determined by the finished state of the resin plate or printing medium alone.
The anilox roll (roll engraved with cells that transfers ink to the resin plate) installed in the flexo press also has a large impact on the screen ruling.
Print reproduction is improved by using an anilox with a lower line count for solid printing and a higher line count for intricate fine details.
As a general rule your anilox should have 5 – 6 times the lines as the screen ruling you intend to reproduce.
So for example, if you wanted to reproduce an image at 100 screen lines, then your anilox should have 500 – 600 lines of cells.
How should I properly store my anilox rolls?
After cleaning it is recommended to store it on a special stand with a cover to protect it from scratches in a place with low humidity, high temperature, and little dust.
It’s also good practice to keep your anilox somewhere where it will be easily available for use.
How should I store plates and sleeves?
Flexo plates and sleeves are typically under constant use.
After printing, it must be washed and stored in a place with little humidity, no excessive temperatures, little dust, and no exposure to UV rays (including sunlight).
In particular, long-term exposure to UV rays will greatly degrade the resin plate.
It is also essential to the product name, color, etc. when storing a resin plate, in preparation for the next printing.
What is the difference between letterpress and flexo printing?
Flexographics uses the same printing methodology as letterpress but with a flexible photopolymer resin plate, which has some similarities and differences with traditional letterpress plates.
Another characteristic difference is that letterpress uses thicker oil-based inks while flexo uses thinner, typically water-based or UV reactive inks.
In addition, flexo uses a single anilox roll to transfer ink to the plate while letterpress requires you to adjust the ink between several ink rolls.
Is endless printing possible with flexo printing?
Flexo printing generally uses a resin plate on a sleeve, but in recent years it has become possible to produce cylindrical shaped resin plates for endless printing.
This is a groundbreaking platemaking method where resin is wrapped around a sleeve and the platemaking and surface treating process is then carried out while the resin is cylindrical. There are only a few companies in the world, such as Seikodo in Japan and Köstlin in Germany, who are able to produce these advanced plates.
What is flexo CTP (computer to plate)?
For most CTP produced resin plates (for digital use) have a black surface layer which is first cut according to the design using a laser. After that it is finished through the typical procedure of exposure, post-processing, washing, and drying.
This style is produced by companies such as ESCO (Belgium), SCREEN (Japan), and others.
There is also another style where the plate is cut directly by the laser, and then the plate is finished in the typical fashion afterwards. This equipment is produced by FUJIFILM (Japan) and others.
Is there an optimal relief value for resin plates?
The relief (plate depth) is usually determined by the back exposure. The longer the back exposure, the shallower the depth, and the higher the reproducibility of fine characters and fine lines on convex portions. However, if the relief (plate depth) is shallow then haloing can appear around thin characters and fine lines in the punched part, it may not be possible to cleanly reproduce the desired image.
The relief will vary depending on the type of resin plate, printing medium, printing machine and ink.
Is it possible to manufacture flexo plates with ultrafine details?
At present, our company is able to reproduce 1PT (250 micron) fine characters and 20 micron lines and spaces with a resin plate, which is widely used in the field of microfabrication technology, mainly for the industrial systems.
We mold all of our plates using only our company’s domestically available materials and plate making equipment.
Corrugated Printing
What print resolution do you typically use for corrugated?
25-30 lines/inch is common for general corrugated board printing.
However, printing presses with high-definition anilox can produce much finer details than this.
When you say “pre-printing” on corrugated, what do you mean?
Most corrugated printing is performed on the outer face of the cardboard after it has already been formed into a layered sheet. When pre-printing on cardboard however, you first print on the front- liner paper and then glue that to the other layers to make the finished cardboard sheet.
Unlike printing on the completed cardboard sheet, the liner has good surface smoothness so it can get better print reproducibility and less color deviation.
It is common to print multiple liners together, especially for large lots.
What kind of backing material is used for corrugated printing?
It’s typical to use a 4mm thick urethane material with a hardness of 15 to 20 degress (Shore A). On top of that backing material, you should use a resin plate of about 3mm thickness pasted up on film.
What are the advantages and disadvantages of printing on corrugated with a 7mm vs a 3mm plate?
【Primary Advantages】
• Reduction of the plate weight
• Better reproduction of fine print
【Primary Disadvantages】
• Using both 3mm and 7mm plates at the same job site makes it harder to manage backing materials.
• Print reproduction is poor for solids
Maintenance
For flexo, how do you maintain cylinders and sleeves after removing resin plates and tapes?
Resin plates are usually used with cushion tape to attached them to sleeves and cylinders. These sleeves and cylinders are often shared between multiple products, and when the resin plate is removed, a small amount of the cushion tape’s adhesive layer remains.
This can adversely affect printing. To completely remove the adhesive layer, we recommend using special cleaning agents.
After applying cleaning agent to the dirty area, wipe it off with a cloth, gently wash the spot with water, and then dry it to return it to its original state.
How do you clean the printing plate?
After printing, the plate is usually smeared with residual ink.
Water (for water-based inks) or IPA (inks other than water-based) are common cleaning solutions, however we recommend using a cleaner specifically formulated for flexo plates.
Dirt can be removed more effectively by simply wiping the printing plate directly with a soft cloth dipped in detergent.
Please note that using ink medium for cleaning may cause damage the plate or sleeve.
Is there a specific cleaning agent for removing UV ink from resin plates?
Yes. Most ink manufacturers should have a purpose made cleaner for UV inks.
We also handle UV cleaning agents so please feel free to contact us.
Print Media
What print resolution is recommended for flexo printing on paper cups?
It depends on the printing machine but 100 LPI is typically considered a lot for flexo printing.
Is it possible to flexographically print on plywood? Is there anything I should be aware of?
It is certainly possible to flexo print on plywood, however there are a few things to keep in mind.
• If the surface is rough, it will cause rapid deterioration of the plate
• If the thickness of the board is not consistent, then the image will end up blurred or crushed.
• If the ink contains too much solvent, it will cause deterioration of the plate
I want to print on sheet steel, but is that possible with flexo?
I want to print on sheet steel, but is that possible with flexo?
It might be necessary to modify part of the printing machine’s system or consider the transferability of the ink, but it is possible.
While few, there are actual places that apply flexo technology to their production.
I want to apply a special ink onto film, can flexo printing handle that?
If it does not affect the resin plate or anilox roll, then it can be applied with a flexo printing machine.
Troubleshooting
When I adhere the plate to the cylinder, the edges continue to come off. Is there any way to prevent that?
When a plate is attached to a cylinder or sleeve, the edge of the plate rises especially if the cylinder diameter is small. In that case, the most effective way to suppress it with a special tape.
Please contact us for details regarding tape is right for your situation.
Other methods involve using double-sided tape with extremely strong adhesiveness to hold down the plate, or fixing the edges down with something like a sealant.
Can I use the same cleaning agent for resin letterpress plates (oil-based ink) on flexo plates?
No. Cleaning agents for resin letterpress plates have a very high concentration of solvents that are unsuitable for flexo plates.
How can I increase solid density in flexo printing?
It is possible to select a higher density ink or apply thicker ink, but it is better to replace the anilox roll with one that has a large amount of cells (generally, a roll with coarse lines has a large amount of cells). It is most effective in increasing concentration.
What kind of planning is necessary to get clean solids?
For solid printing, use a plate material with a lower hardness (softer) than usual and a high ink receptivity.
On the other hand, if you use cushion tape that is harder than usual, the solid print will look better.
What kind of planning is necessary to print small letters clearly?
For flexo plates, we recommend using digital plates, which are more accurate and sharper.
If you use a slightly harder plate material and a cushion tape that is softer than usual, fine print will be fine.
Horizontal dark streaks keep appearing. What is the cause?
There are various causes, but the high viscosity of the ink, the excessive hardness of both the plate and the cushion tape, and the vibration of the flexo printing machine are thought to be factors.
Solutions to these problems include lowering the viscosity of the ink, lowering the hardness of the plate and cushion tape, and printing at a printing speed in the area where the vibration of the printing press is low.
Vertical dark streaks keep appearing. What is the cause?
As with the horizontal direction, there are many possible causes, but it could be a scratch on the anilox roll or doctor blade, or something on the machine that is in contact with the print surface.
What sorts of things can cause misregistration in flexo printing?
The printing registration will not match unless printing plate finish, the plate element positioning, sleeve dimensions, and cushion tape thickness are all accurately aligned.
Of course, problems on the machine side such as gear accuracy and problems with printing media are also possible.
What should I do if I want to reduce fringing in my flexo printing?
It’s important the reduce the anilox pressure (the pressure between the printing plate and anilox).
Increasing the anilox pressure not only causes haloing but also reduces the durability of the resin plate. Minimizing the impression pressure is also important. In the industry this is referred to as a kiss touch. Compatibility between the resin plate, media, and ink is also a major marginal factor. It is important to select the most suitable resin plate (material) for the printing.
Also, the ink supplied from anilox is received by the resin plate, and after that, the ink cannot be supplied sufficiently to the printing medium, and the ink remains on the surface of the resin plate, and the next ink is supplied from the anilox, resulting in a marginal. sometimes. As a solution in this case, it may be possible to solve by processing the surface of the resin plate so that the ink release from the resin plate is improved.
How can I improve the durability of my resin plates?
First of all, do not apply too much printing pressure or ani pressure when printing. When pressure is applied to the resin plate continuously, the stress causes the resin plate to wear out. In addition, compatibility with ink also affects durability, so it is important to select the most suitable resin plate material for the ink.
The compatibility of the cleaning agent when cleaning the resin plate after printing is also important. Some cleaning agents may cause a chemical reaction with the plate material and cause stress to the resin plate. For storage, it is necessary to avoid ultraviolet rays and store in a place that is not hot or humid to prolong its life.
Other
What kind of organizations are involved in flexo printing?
In Japan there are organizations such as the Flexo Technology Association (FTAJ) and the All-Japan Flexo Plate Manufacturers Association.
What flexo focused publications exist?
Flexo printing technology handbook (Flexo Technical Association), Introductory flexo printing (Printing no Sekaisha), etc. are published by industry publishers.
What is the PRTR system?
PRTR is an abbreviation of “Pollutant Release and Transfer Register”.
Regarding chemical substances that may be harmful to human health or the ecosystem, the business operators themselves ascertain the amount of each chemical substance released from the business site and the amount of waste transferred outside the business site, and report it to the government. The government then collects and publishes released and transferred amounts based on reported data and estimates.
https://www.meti.go.jp/policy/chemical_management/law/
What is the SDS system?
SDS is an abbreviation for “Safety Data Sheet”.
In order to promote the improvement of chemical substance management by businesses, when transferring or providing “chemical substances or products containing them” designated by the PRTR Law to other businesses, this system obligates the provision of information on characteristics and handling in advance, and requires efforts to display it on the label.
Details can be found on the website of the Japanese Ministry of Economy, Trade and Industry.
https://www.meti.go.jp/policy/chemical_management/law/