First published in Coating International, 2015 issue.

Written by Harper Product Development Engineer, Tony Donato

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There have been a number of articles written on the upcoming revolution of printed functional devices and intelligent packaging. From working with a number of Universities as well as internal testing over the past 5 years I would like to address some of the challenges and possible solutions when trying to print these products roll to roll. I am not going to describe how the print transistors and other multilayer products work but I will state where we are today and what needs to be understood when attempting printing in a roll-to-roll process.

Around 10 years ago, Printed Electronics became the hot topic for futurists, innovators and venture capitalists. There are many examples of successes and failures but, as with motor racing, the old saying «the best way to end up with a small fortune is to start off with a large fortune» has been truthful in both fields. All of the money spent has not necessarily made the investors more money. But, it has helped to move the industry toward the dreams of the futurist.

Printing Electronics

Defining printed electronics has almost become an industry in itself. I will use a definition of «conductor traces, semi-conductors, insulators and or compounds applied in single or multiple layers applied to a substrate flexible or rigid that react or are stimulated with electrical current, gases, chemicals or temperature.» With that said, we need to look at the early days of electronics when the circuit boards were chemically etched and not start off by looking at nanometre dimensioned components of today that are made in cleanrooms in a very efficient method and which have proven to be extremely reliable.

Print Applications

Over the past 20 years, circuits and simple devices have moved into the flat screen-printing process. This process has been used and will continue to be very successful for designs where the trace line widths and thicknesses provide an economic way to produce reliable printed components and devices. All current trends indicate that flat screen-printing will continue to fill the need for my definition. But, as inks and applied materials keep improving to the point that they can provide the same functional performance with a narrower and thinner deposit, roll-to-roll processes like Flexography and Rotogravure are being seen as the processes that can be used to produce products in an economical manner.

When it comes to ink film thickness produced from the various processes used in graphic printing, digital printing typically yields the thinnest film followed, in order, by offset lithography, flexography, gravure, and screen. I believe, as do many, that Flexo and Gravure can be investigated as the replacement for flat screen printing when the demand for products increases and for products being designed where the printed circuits and layers can be combined into what is being call hybrid assemblies. A traditional nano dimensioned switching device (chip) can be assembled into printed portion to complete the device.

Printers need to think about the basics of functional printing. In graphic printing, Flexo & Gravure can place colored dots in a way to trick the human brain into thinking the eye is seeing continuous images as seen in nature. This type of printing can be done very fast, anywhere from 500 to 2000 fpm (150 to 600 mpm). In printing functional devices, we have to first understand that printed inks have to perform under the laws of electrical physics so currents can flow. Overly simplified, «electrons cannot jump» (at the required low voltages) from dot to dot. So, an incomplete printed trace line of ink will not be acceptable.

In moving from flat screen-printing to a R2R (roll-to-roll) process, R2R can be accomplished at very fast speeds. The issue with press speeds today is that we can print graphics extremely fast (500 fpm (150 mpm) narrow web and 2000 fpm (600 mpm) wide web.) But, that also means scrap can be printed extremely fast. For all practical purposes, with exceptions like glucose strips, the number of needed electronics related printed products required for a given batch size, can be accomplished at press speeds in the 50 to 150 fpm (15 to 46 mpm) range to help control print consistency and quality. Currently there is no need for speed at 500 fpm (150 mpm). The functional printed components need to meet a reliability and reject standard far beyond what is required for packaging. Again, we are dealing with electrical physics and not just tricking the human brain with information. That means one of the important issues is how to measure and inspect the printed products. This is one of the many challenges we face in the PE industry.

Ink Selection

The cost of the inks involved can be very expensive and in many cases hazardous. So the obstacles start with the amount of ink needed to start the process. A 5-gallon (20 litre) sump with nano silvers inks costing as much as $1,000/litre is out of the question. The innovative experimenters have been creating micro chambers to hold just enough ink to wet the anilox roll or gravure cylinder. In handling these inks, we also have to explore the reality of handling inks that have particles so small they can be absorbed through the skin into the human body. So, the inks create the challenge of controlling the ink usage and the handling of additional hazardous waste from all clean-up steps.Since more inks are moving to water based, we can look at the electroplating industry where, for years, they have had to develop metal recovery systems to pull all the precious metals from the liquid waste streams. Maybe laundry equipment needs to be made to pull the metals from towels. From the beginning, ink application and handling, waste clean-up, and recovery of materials from rejected printed batches need to be considered.

The ink developers are challenged to find materials that will give the electro and chemical properties and keep the costs down. This is not the only challenge for the ink companies. They also need an ink that allows enough levelling to complete the circuits while, at the same time, keeping the deposit thickness where the ink functional properties are acceptable without allowing the ink deposit to spread. The ink deposits need to stay in place during drying since, in most cases, the gaps and intentional spacing may be very close.

Drying or sintering the inks is a science in itself. Again, ink products must be designed such that, when the carrier is evaporated, the deposited materials can properly function. Many materials need curing at 160°C for 2 to 3 minutes. This may require a sheeting process with the printed products cycled through an oven. There are very creative curing devices that use near infrared in line dryers (curers) and others that have pulsing light sources that are intense enough to sinter (fuse together) the inks into a basically continuous printed wire. The challenge is to match the potential project with the ink and substrate for the equipment you have or want to add.

Substrate Selection

Substrate selection for a given conductive project is very important. Surface flatness and absorption is very critical in order to print a continuous trace line that will have the consistent electrical property (conductivity or resistivity) per liner inch. Films like PET have been used but calendared and coated papers improve the three-dimensional surface finish so that the ink trace can be printed consistently. Like with all printing, matching the surface energies and surface tensions of the substrates and inks is equally important.

Registration

Registration in most Flexo and Gravure presses are .003 to .004” (75 to 100 microns). Some PE related products can be printed with this registration but designers are stating that there are more products to be printed in an ideal printing press at registration at 5 to 20 microns. There are press manufacturers that are claiming a 20-micron tolerance. With this said, it appears that, for the majority of potential products, a narrow web press around 13” (330 mm) print width will be ideal. With that platform, a hybrid press with Flexo, Gravure and Digital decks would be included in the press. The registration system is one of the major press challenges. Again, as earlier stated, speed in itself can be an advantage as long as the quality is maintained. However, speed can also waste precious materials faster.

Surface Engravings

Metering and engravings have been challenging in both flexo and gravure. In gravure, the cylinder manufactures have been able to make 5-micron trace lines. However, trying to meter to keep hazing to a minimum without pulling the ink out of the trace lines has been challenging. It is much harder to meter across machine lines that are parallel to the cylinder axis without pulling the ink out with the doctor blade. Designing the circuit so that lines are diagonal to the cylinder axis is critical. In many cases, curved trace lines do better than straight lines. Gravure printers have found doctor blade materials and tips that do not wipe graphic ink clean will work on conductive inks. As stated earlier, applying only the minimal amount of ink is critical in both flexo and gravure. In some cases involving flexo printing, the reverse metering typically used may tend to shear the ink. So, a forward doctoring chamber or open system would be best.

In gravure, closed cell structures that perform spectacularly in graphic printing do not necessarily allow the deposited ink to be continuous. Etched or laser engraved channels and open cell structures are required over mechanical or electro-mechanical engraving in many applications. We have developed a laser engraved XOS (Extreme Open Structure) structure to overcome many of the challenges. Also, it appears a softer gravure impression rubber roll (60 Shore A) helps with the printing line details.

In the last year, Flexo has come on as the easier choice with the introduction of 8,000 DPI-RIP (raster imaging processor) for plate imaging. An illustrator created vector image, when converted digitally, has been a problem in creating straight edges in the trace lines. With the 8000 DPI-RIP, the imaged plate line is straighter and cleaner, allowing the relief lines to be narrower and placed closer together. However, photopolymer plates may not hold up to specialty inks that are toluene based. There are flexo plate alternatives available in laser engraved EPDM rubber. The other challenge with Flexo from the image carrier is controlling line width growth from impression. What we see in graphics as dot gain can become a problem when printing precise conductive trace lines. This can be helped with keeping the relief between .015 to .021” (380 to 530 microns).

With the availability of anilox rolls in almost any combinations of engraving specifications, flexo has the advantage over gravure in its ability to dial in the ink film thickness for a particular pattern since the pattern is on the plate and the anilox is just the ink carrier. Anilox volumes for printing nano or flake inks do differ, where nano inks are printing nano or flake inks do differ, where nano inks are printed 1 to 3 BCM ( 1.55 to 4.56 cm3 /m2 ), oxide inks in the 3 to 6 BCM ( 4.56 to 9.3 cm3 /m2 ), and flake inks in silver and carbon or graphene 9 to 26 BCM ( 14 to 40 cm3 /m2 ). These are ranges that we have seen but that may not be suited for all inks and substrates. Volume testing is again easier with flexo over gravure. Flexo has the challenge from inks that have low percentage of solids since it has a second ink split over gravure.

Print Environment

The traditional pressroom environment can be an issue. The more detailed the pattern or the more chemically unstable the printed fluid, the cleaner the room and air needs to be. In many cases, the ambient air may need to be a given temperature and humidity in order to not contaminate the printing. Or, air-born dust, if codeposited, may change the functionality of the printed pattern. In depositing advanced materials and, in some cases, liquid electrolytes, corrosion to the equipment would require the printing equipment to be made of different materials than with traditional graphic inks.

The three-dimensional printed trace line is like a wire and the size of the wire is critical toward its physical conductive or resistive properties. As a simplified example, if the printed lines are to carry the current from a 3v battery to a 2.8v LED the voltage drop is calculated in the circuit design. But, if the conductive material is thinner or narrower than calculated, the resistance will increase. The delivered current decreases, the brightness of the LED changes, and the life of the battery is affected.

Conclusion

I covered a wide range to topics to help with your thinking about the challenges associated with starting to contemplate printing functional sensors, devices or circuits. Some of the best advice I can give is to align yourself with a project team that can support the science and physics side of the project so you can just concentrate on the printing side. What I have seen in many incidents is that the advanced materials and electrical engineering project managers do not understand the basics of Flexo or Gravure printing and, in turn, the printers do not understand the basics of electricity. Remember that electrons do not jump very well and variations in ink film thickness and printed line width changes the electrical properties where it can cause the device to not function. Consult the suppliers and universities who have spent the time and money to learn what is needed to make the printed electronics work. As earlier stated, the designs and the required function will be the determining factor in what printing process is best for a given product design.

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About the Author: Starting off in paper machinery manufacturing in 1974, 2015 makes 41 years of Industrial experience from manufacturing management and line supervision, plant and facility engineering, manufacturing engineering, production and safety training, technical customer support, application sales, and product development. The last 25 years has put him in pressrooms as a student and a teacher. He has countless hours working as a trouble-shooter and problem solver in pressrooms relating to anilox rolls and gravure cylinders, doctor blades and chambers, pressroom operation, and technical customer support.

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To view the original Coating International article click HERE.

Examining Options & How to Make The Right Choice

First published in FLEXO Magazine, June 2015 issue.

Written by Harper GraphicSolution Director, Alexander James

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Anilox engravings have evolved from a rudimentary ink carrier—originally a 45 degree quad engraving, primarily for solid ink coverage—to a highly sophisticated carrier for a wide variety of applications from varnishes and laminates to specialty inks.

With all the various needs and different application requirements in our industry, thankfully today there is also a wide range of anilox engraving and specifications to choose from. They include:

• XLT 60 degree

• 70 degree hex

• 45 degree quad

• 30 degree channel

• Weave

• KatRon (elongated cell)

• 45 degree trihelical

• 60 degree XTR

• 70 degree XTR

• 89 degree XTR

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COATINGS

Laminates, varnishes and adhesives: Each has a different purpose and when determining what anilox specification is needed, it will first be necessary to request the Technical Data Sheet (TDS) from your coating supplier. The TDS will provide useful product specific information, such as the application method, appropriate substrate, recommended coat weight, viscosity or pH, percentage solids, dry rate, storage and handling.

For the anilox supplier, the useful information on the TDS includes the target coat weight, percentage solids and the weights per gallon. Coat weight can be based on lbs./ream, gram/square meter or target thickness in microns. Knowing the application method will also be necessary; is it a gravure coating station or traditional flexo application? Will the application be in a two roll or chambered system? Providing this information will enable your anilox supplier to make the right recommendations.

As an example, if the coating is very thick and is going to be applied to an uncoated substrate, then it makes sense to use large cell engravings, such as the 45 degree quad, 70 degree hex, KatRon or one of the channel engravings. Uncoated substrates will necessitate heavier coatings because of the absorbent characteristics of the material. Weave engravings, 30 degree channel, 45 degree trihelical, 60 degree XTR, 70 degree XTR or 89 degree XTR; channel engravings naturally will lay down lots of product due to the continuous open channel patterns. Some very thick coatings will need the channel engravings in order to achieve the recommended coat weight. Coatings require thicker application and, naturally, the larger open engravings are a good fit. With the information provided on the TDS and a Coating Application Worksheet, your anilox supplier will be able to recommend the correct anilox specification for your needs.

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SPECIALTY INKS

Specialty inks, such as fluorescent, thermochromatic, metallic and pearlescent, will require different anilox specifications. Some may need volumes ranging from 4.0 bcm to 8.0 bcm and others may need more. For this category of inks, formulation and pigment grind size will be factors that must be considered. Again, the TDS will be most helpful.

Communication between the customer, ink supplier and anilox supplier will be the most important factor. For each specialty ink, it is critical to discuss the expectations, goal and production setup. You can have the desire to print a brilliant, rich fluorescent color, or apply sparkling pearlescent ink; but do you have the right setup to dry the inks at production speeds?

From the anilox recommendation side, the critical criteria will be the particle size. A good rule of thumb is to keep the cell opening a minimum of twice the size of the largest particle size. For example, if you are applying pearlescent inks with 15-µ. to 20-µ. size particles, your anilox should have, at the minimum, an opening of 40-µ.

Once the minimum opening has been determined, you need to look at the anilox volume. When thinking of volume for specialty inks with large pigments, don’t get distracted with volumes that may have very deep engravings. After all, the pigments will need to be easily released from the anilox in order to achieve the desired effect. Anilox engravings that are very deep will invariably have a conical shape that can easily trap some of the large specialty pigments. There will need to be a balance between the anilox opening and the volume (depth). Try to keep the cell depth to opening at a ratio in the range of 23 percent to 33 percent.

To calculate the depth to opening ratio, use the following formula: D/O Ratio = Cell Depth/Cell Opening x 100.

You will need to consult your anilox supplier for the depth and opening information, which is to be provided in microns. Logically, if you have deep engravings when applying large pigment, the anilox roll will have a tendency to plug easily. Sometimes, as in the case of pearlescent inks, keeping the ink in suspension for proper application will be part of the challenge.

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WHITE INK

White ink for most printers is the highest percentage of ink consumed. Part of today’s trend is to have high opaque whites, using UV, water and solvent based inks. When making recommendations for white ink, it is always best to have a specific opacity target. Your opacity target will have an impact on the anilox specification.

When needing volumes in the 8.0 bcm to 12.0 bcm range, the logical choices are the 60 degree hex and the 30 degree channel engravings. If you are applying white to an uncoated substrate, then you should consider larger cell engravings such as the 70 degree hex and the KatRon (elongated cells) engravings. As you move on to the high opaque whites, you’ll need more volume; in that situation the various channel engravings are great options: Weave, 45 degree trihelical, 60 degree XTR, 70 degree XTR or 89 degree XTR.

In today’s market, combining the latest solid plate pattern options with your anilox engraving choices can have an impact on your opacity. The solid plate pattern can result in better laydown—mottle free—and also lower the anilox volume needed in order to achieve your desired opacity target. This is exciting, because that can result in significant white ink savings.

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SOLID & COMBINATION PRINTING

With solid and combination printing, the XLT 60 degree hex is a great choice to start with. However, when printing on uncoated surfaces, it is an opportune time to take advantage of the variety of engraving options. Rest assured that, depending on your ink system and substrate, there is an engraving that will meet your needs. In every situation, it is key to have a defined print target with tolerances. This will enable your anilox supplier to be able to consistently provide the anilox specification that meets your needs.

Combination printing, solids and vignettes require a balance between hitting your target solid color and also printing a clean vignette. In that situation, there will be a need for your anilox to have enough volume to achieve the solid, and also a high cell per inch specification, so you can also print a clean vignette. XVR 60 degree anilox engravings with higher cell per inch specification and deeper volumes are the best fit. These engravings, along with proper ink formulation, work well for combination printing. If your minimum dot is large—4 percent or higher—then you can consider other options like the 70 degree hex and other channel engravings. This is especially applicable for the paperboard and corrugated market segments.

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PROCESS PRINTING

Process printing requires defined print targets (with tolerances). Having specific print target information will always make it easier on your anilox supplier. Along with your print targets (and tolerances), your anilox suppliers will also need to know the type of ink, substrate, plate dot lpi and target minimum dot percentage. With this information, your anilox supplier will be able to make a sound recommendation.

Consistency and repeatability are part of the print requirement for process printing and for anilox suppliers, the option that delivers those needs, as well as best engraving, is the XLT 60 degree hex. The structured qualities of this type of engraving will assure that you will be able to hit your print targets with the most consistent product. It is critical to know what your minimum dot lpi will be and it is important to measure its diameter. Once you know the specification of minimum dot diameter (in microns) you should keep the anilox cell opening equal to or less than that diameter.

Keeping the anilox cell opening equal to or less than the minimum dot diameter will ensure you can achieve the desired quality of clean printing, especially on highlights and vignettes. With those criteria, your anilox will deliver the right amount of ink to your plate and not over ink your minimum dots, avoiding what is sometimes referred as “dirty printing.”

By far the most popular anilox engraving use in our industry is the XLT 60 degree hex. This is due in large part to the fact that the 60 degree hex has the most support structure compared to the other engravings. Regardless of what your print needs are, rest assured there is an anilox engraving that will work for you. Sometimes this may require banded roll testing in order to determine which engraving and specification will work for your situation. This is especially true when testing a new application and product, or a change in your print process.

In most cases, the key things that your anilox supplier will need to know in order to make the correct anilox recommendations are the following:

• Print target (with tolerances)

• Plate lpi

• Minimum dot diameter (in microns)

• Type of ink

• Press type

• Production speed

• Type of substrate

When you need an anilox recommendation for coatings, specialty inks or white inks, it will be important to clearly communicate the TDS information of the products you plan on using, as well as the goal or print target and your measuring method. Measuring methods, unfortunately, are often assumed, and not knowing exactly what method is used can lead everyone involved astray. Anilox specifications of cpi and bcm have numerical values and tolerances that can correlate to your print target and tolerances. Work with your anilox supplier to determine the right engraving choice and specifications that will meet your needs.

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About the Author: Alexander James is the director of Harper GraphicSolutions, the technical division of Harper Corporation of America, and has a Masters of Science degree in graphic communications from Clemson University, as well as a Bachelor of Fine Arts degree from the University of North Carolina at Asheville, NC. With more than 20 years experience in the graphic arts industry, Alex has participated in many industry related events as guest speaker/presenter. He is also FIRST Level III Implementation Specialists Certified and has authored numerous technical flexographic related articles.

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To view the original FLEXO Magazince article click HERE.

A Guide to Preparing, Proofing & Using Specialty Inks

First published in FLEXO Magazine, June 2014 issue.

Written by Harper GraphicSolution Director, Alexander James

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Printers continue to strive for ways to get both the print buyer’s and the consumer’s attention. As a printer, have you been looking for something that makes your printed product stand out among the rest? Are you vying for an edge in obtaining market share in the label business? You should consider the expanding field of flexographic specialty applications. Specialty applications that provide the “pop” in your print cover a wide range of visual stimulations that differentiate a product from the rest.

Some of these include:

• Glitter

• Metallic

• Fluorescent

• Pearlescent

• Holographic

The hard part is getting these inks from plate to substrate. A lot of that has to do with testing, proofing, proper anilox selection and an understanding of the inks themselves.

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PICKING “POP”

Before starting on any print project, it is imperative to define the goal. What is the effect you are trying to create? Are you trying to match a print sample or simply wanting to create a dynamic effect? Let’s look at a couple of applications as examples.

Specialty inks are special for a reason. They are unique not only in effects but also in setup, handling and testing. Make sure you understand the requirements to print and plan to run testing before committing product to a customer. We often find that rushing to completion on a specialty ink project has about the same low success rate as a poorly planned fingerprint.

You might start with a popular recent admission to flexography: The realm of tactile coatings. Tactile effects create an interesting interaction between the consumer and the product. Raised bubble areas or dew drops and raised grid patterns on your graphics can create that surprise “wow” factor that helps differentiate your product from the competition by capturing the consumer’s attention as he/she explores the feel of the packaging or label.

You might also consider a specialty effect like a color shift of the graphic image using thermochromatic inks, whose color shift is triggered by temperature change from physical touch or a change in the temperature of the substrate or content of the package. While these are exciting developments for flexo, consider a word to the wise: Before you proceed, be sure to match the ink’s ability with your end print expectations. Accomplish this by obtaining printed examples from your ink supplier that demonstrate printing methods that are at least adaptable to your existing capability.

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PREPARING “POP”

So what do we need to do first? Set up a meeting with your ink supplier, anilox supplier and production staff to discuss goals and review how to pre qualify the variables prior to going to production. Request the technical data sheet and MSDS sheets for your specialty ink. These are very informative documents that you and your anilox supplier will need to make decisions. You can expect to find information regarding particle sizes, which can be particularly helpful when dealing with glitter, pearlescent and metallic inks. Often the particle size determines volume requirements because the particles have to be able to fit and sometimes flow within the cells.

These sheets will also indicate an ink’s shelf life, which is often much shorter than standard ink systems. You may be surprised to find a batch life with an expiration date only a month from receiving. Many of these inks have longer lead times because they are not kept on the shelf, but are made to order when needed, due in large part to the short shelf life. Plan your tests accordingly. Don’t order the ink and then push off the testing for months. Make the commitment to test.

Specialty inks may also be somewhat less stable thermally, so the inks must be kept within their temperature window. This means no freezing, no high humidity and storage in a stable window of operating temperature. Make sure you know what you can or cannot add to these inks for reduction and what to use for cleanup. Shocking a batch of ink can be very frustrating in the middle of a trial or during an anilox cleanup. You will also need to verify the type of substrate you are using to make sure the ink you want to use is compatible with the substrate.

Learn what drying requirements are needed. If you are working with an older press running a water based application where you have only successfully dried ink films from 10 bcm volume aniloxes, and you are now going to put in a 30 bcm roll for glitter, you will find out that your drying capacity becomes a great hindrance to run speed and may not dry enough between stations. You are going to want to fully represent your printing process in the trial run. Testing should include the substrate you plan on running in production, the same inks and, where possible, the same setup. This will also give you some indication of potential drying or curing issues you may need to consider when going to production.

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ANILOX SELECTION

Once you have decided what your goal is and have the technical information on your ink or coating, it is time to select your anilox. Keep in mind the particle size of your ink grind when dealing with glitter, pearlescent and metallic inks. A good rule of thumb when selecting an anilox is to maintain a minimum anilox opening to particle size ratio of 2:1. This method helps to select an anilox specification with a large opening and shallow depth. With this minimum ratio you will have a greater chance of enabling your ink particles to flow into and out of your anilox, transfer to your photopolymer and result in your desired specialty effect. Guidance from your anilox supplier will help determine which type of engraving will be the best fit.

Thankfully, today there is a wide range of different anilox engravings to choose from:

• 30 degree channel

• 45 degree trihelical channel

• 60 degree hexagonal

• Elongated hex

• 70 degree hexagonal

• 90 degree XTR channel

Regardless of the type of engraving chosen, keep in mind that with this kind of application, you will need to lay down a lot of ink to achieve the desired effect. For example, working with your anilox supplier, if the pigment grind of your ink is 35-40 microns, you will need to select engravings with minimum cell openings of 70-µm. to 80-µm. and 280 cells per inch (cpi) or lower. At 280 cpi, the cell opening is 85- µm. and engravings with lower specifications will have even larger cell openings. There are exceptions to every rule, so you must also inquire about the particle shape.

Some particles require depth, as well as opening because the shape may be more round where most particles are a rod or flat flake and fit in the cells. Round particles need room to fit inside the cell.

Using the engraving specification of 280 cpi with an opening of 80-µm., we can then review how much volume will be required. Consulting your ink supplier, if it happens that you’ll need 7.0 bcm for the specialty ink with flat or rod type particles to work, then review where 280 cpi, 7.0 bcm falls into the chart. You will see 7.0 bcm falls close to the end of the standard (green) engraving range.

Keep in mind the particle “ size of your ink grind when dealing with glitter, pearlescent and metallic inks—A good rule of thumb when selecting an anilox is to maintain a minimum anilox opening to particle size ratio of 2:1.

As the anilox supplier, I would suggest moving to a lower engraving specification, such as a 220 cpi with 7.0 bcm. The reason for making this adjustment is so the engraving is now to the middle of the volume range and the cell opening is a little more than two and one half times the pigment grind. This choice should result in easy ink laydown and less issues with plugging. Note that you can have same volume with different cell counts and cell openings, so consult your ink supplier and anilox supplier. Making the right choice will dramatically impact your chances of success. Make sure you know the condition of the anilox you are going to use. You can easily have the anilox specifications but not get the results due to wear or cell plugging

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PROOFING “POP”

It was mentioned earlier that it is always a good idea to conduct as much testing as possible prior to going to press when embarking on new projects. If you are not 100 percent confident in the ink and anilox combination, then consider another step in preparation. This step involves proofing. Proofing is a more cost effective way to determine whether the desired effect is achievable. There is a variety of tabletop testing equipment available that will enable you to conduct proof print trials. You want your proofing device to mimic your press, so proofing that incorporates your ink, anilox and substrate is preferred for repeatability in press. In the case of UV based inks, you can also test curing requirements with small tabletop curing units. Testing will provide a snapshot of potential challenges you may face before going to production if you are considering different run options.

For example, on a tabletop proofing device it is rather easy to conduct a double hit of your ink using different anilox volumes and drying or curing settings. At this stage you can also conduct very cost effective experimentation with a variety of different substrates. In contrast, webbing and running an entire roll of material on press is wasteful, when a few passes on a proofer can achieve the same results. Proofers need to be simple to use. Some tabletop devices have quick anilox switch out options that make it rather easy to change out and test different anilox engravings and specifications. This can be important, especially with considering the need for a double bump of the same material or multiple passes like a primer or opaque white. These various passes will likely use different types of anilox specifications in order to achieve your desired goal. Having it all there for proofing makes good economic sense.

Once you have a good handle on what you want the effect to be, you can then be comfortable making any final tests of design at the press. The “pop” of any specialty ink can be enhanced further by how you include the effect in the design of the product.

If you plan to utilize graphic options and not just print a solid, you may have to balance these requirements with the desired effects. You can use a banded roll that brackets the original volume and line requirements to enhance the printability. Pair a graphic design or designs you want to test with the banded roll and run them at the desired production speeds to test process requirements.

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FINAL CHECKS

Other things to keep in mind as you transition from testing to a production setup are ways to maintain the inks, drying and curing options. Make sure you start with an ink that has just been thoroughly blended to get all particles in suspension. All glitter and pearlescent inks must be kept in suspension to create the desired effect and it is reasonable to think some settling will occur, so blend. You may also have to look at ways to keep the inks moving in the pan. Make sure the base system the particles are in is one that actually suspends them. Often we find using a pumping system instead of letting inks sit statically in the tray is better for print consistency.

Depending on your production equipment you may need to do some adjustments with your doctor blade. For example, in single blade setup when applying a thick layer of tactile ink, your goal is to allow the doctor blade to float or butter the coating—not shear it. This will increase the layer of ink that makes it to the polymer and then to the surface. You may need to adjust the web path as well to allow sufficient time for drying or curing. Confirm you have sufficient drying capacity or curing capacity (a minimum 400-w.) or that your curing lamps are well maintained and in good working order. It would be worth having your drying system or UV cure lamps inspected, cleaned and revamped, if necessary.

Think about cleanup, too: What type of chemicals will you need to clean up at the conclusion of your printruns? This will depend on the type of ink or coating used. Each will require a different cleaner and any that are fast drying will require special attention at cleanup time. Keep in mind that regardless which type of ink you use, it is always easier to clean when the ink is still wet. While the ink is being saved for future use, focus on cleaning the anilox before any other component.

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About the Author: Alexander James has a Masters of Science in graphic communications from Clemson University and a Bachelors of Fine Arts degree from the University of North Carolina at Asheville, NC. With more than 20 years experience in the graphic arts industry, Alex has participated in many industry related events as guest speaker/presenter. He is the director of Harper GraphicSolutions—the technical Division of Harper Corporation of America—and is also FIRST Level III Implementation Specialist Certified. He has authored numerous technical flexographic related articles.

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To view the original FLEXO Magazince article click HERE.

First published in Converting Quarterly, Quarter 2: 2014 issue.

Written by Harper product development engineer, Tony Donato

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Introduction

From the beginning of flexography – originally called aniline printing – its strength was the process’s ability to print on a very wide range of substrates in a web-fed, roll-to-roll (R2R) method. This strength has lead flexography to be the dominate process of choice for cost, quality and versatility in the packaging segment in North A merica, and it is on track to dominate package printing worldwide. What makes flexo different from other processes is the use of a relief-imaged flexible plate that – after being attached to a cylinder – receives ink from contact with a uniformly engraved roller called an anilox roll. The anilox roller originated as a full coverage gravure cylinder that was mechanically engraved into a copper layer and, after engraving, given a top coat of hard chrome plating. This served the process well, as long as it was doing simple text printing. As the users of flexography saw the potential of the process and the packaging industry experienced the impact of the Nutrition Labeling and Education Act of 1990 [1], the elements of the process improved exponentially through the 1990s, led by the use of an anilox roll that was laser engraved into a ceramic surface.

Because an anilox roller can be used with different imaged plates, pressure was put on the engravers to produce a longer lasting engraving surface. Initially, the mechanical engraving moved from copper and hard chrome to other metals – such as mild and stainless steels – and eventually the top layer of chrome was replaced with a thermal-sprayed ceramic top coating (see Figures 1-3). This increased surface life but still was limited in screen count and volume due to what mechanical engraving tools could be produced, and because the top ceramic coating also partially filled the mechanically engraved cells.

Introduced in the late 1980s, a new anilox-roll surface was developed that used a computer-controlled, pulsing laser that blind-hole-drilled a controlled pattern into a thermal-sprayed, Cr²O³ (chromium oxide)-coated surface. The laser-engraved anilox roller was born, and its evolution has continued moving it beyond the 550 screen-count limit of the mechanically engraved pattern to screen counts of 2,000. Today, the laser-engraved, ceramic (LEC) anilox rolls have proven themselves as the most consistent means of regulating the amount of ink film thickness being deposited onto a given substrate – guaranteeing the desired color densities.

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LEC rollers apply to direct-gravure coating

With the evolution of the LEC roller, processing equipment and methods improvements have resulted in increasing the density of the ceramic, improving surface finish, providing variations in engraved geometries and – with the latest 500-watt fiber lasers – have given engravers the ability to engrave very low line screens (20 lpi) with consistent wall thicknesses and volumes to the 100-bcm (billion cubic microns) range. These improvements also have allowed the LEC roller to become used more frequently as a direct-gravure cylinder for coating, laminating, long-running patterns in product gravure, such as décor images and texts on tail-end printers that use a fixed repeat.

The advantages of chromium oxide begins with the coating thickness, the Cr²O³ being applied in a controlled-plasma, thermal-spray process that converts a granulated powder into a coating that can be applied as thick as needed, so the entire engraving is in the ceramic. This gives the engraving the advantage of having the same hardness through its entire depth instead of having only 0.0005 to 0.002 in. of hard chrome to protect the mechanically engraved cells. Note: hard chrome has a hardness of 900-1100 Hv but needs a minimum of 0.001-in. thickness to reach the hardness, and the wear resistance drops below 0.001-in. thickness.

Traditional gravure cylinders can be reprocessed into an LEC surface. First, after the old coatings are removed, the steel cylinder is coated with chromium oxide using the plasma, thermalspray method. The chromiumoxide material is a minimum of 0.005 in. thick and is ground to a T.I.R. (Total Indicator Run-out) of 0.0005 to 0.001 in., then is diamondpolished to 4 Ra to make it ready for laser engraving. The advantage of laser-engraved, ceramic cylinders is their longer life – between 10 to 20 times that of a traditional chromed cylinder.

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Advantages of LECs for gravure

One of the biggest advantages of LEC is its wear resistance to the metering of steel or ceramic doctor blades and abrasive pigments, such as titanium oxide and for direct-gravure abrasive substrates. The ability to resist abrasion also allows LECcylinders to be used in traditional gravure-forward metering or where the coating station has a fixed repeat metered in the reverse-blade direction. By reverse-metering, the consistency of the coating thickness is controlled only by the cylinder volume and is not affected by the forward blade angle. Reverse metering helps to keep the process under control longer through the use of an enclosed inking chamber with fixed-blade contact angles and removes the need for an open ink bath as in traditional gravure metering. Removing the open ink bath also helps to keep the ink/coating viscosity more controllable.

The enclosed inking chamber first used in wide-web flexography has two doctor blades – metering and containment – and end seals; this allows the ink to be metered into the engravings and, at the same time, pumped to and from an ink tank that can be covered to reduce evaporation (see Figure 4).

The open pan typical of gravure requires operator skill to set and adjust the doctor-blade angles and pressure that all affect the life of the chrome plating (see Figure 5).

LEC cylinders can be used in all the gravure process variations, especially where the run length is measured in shifts and where the coating weight or finish is critical over the long run (see Figure 6A-6C).

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LEC engraving variations

Because LEC cylinders get the engraving geometry from the laser-engraver software and not from a mechanical tool or stylus, LEC engraving variations are almost infinite. The software using the rotary encoder of the head stock and linear encoder of the bed can position a focused laser beam to unbelievable precision, so the cylinder surface can be blind-hole-drilled or cut in a manner that allows almost any geometric shape to be created. The controlled pulsing and placement sequence creates the surface geometry, and the power control varies the depth to give the desired volume for a given line screen.

The traditional 45-deg mechanical quadrangle shape is easily replaced with a 60-deg hexagon that will provide 15 percent more cells in a given area than the 45 deg. Packing more cells in a given area reduces the cell walls and helps in a more even distribution of a coating. The engravings (see Figures 7-15) can be closed cells in a 60-, 30-, 45-deg or elongated variation patterns; 70-, 75-deg or open cells looking like straight-lined threads; (trihelicals in almost any angle 45 deg, 89 deg) to weave like engravings that have parallel continuous walls; and variations where opposing walls look like an hour glass. Note: Not all LEC suppliers can produce all the different geometries; some are exclusive. Consult your supplier for their capabilities and the geometry that is best for your application.

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Match LEC geometry to coating application

In choosing the best LEC geometry for your coating application, it is necessary to consider the pigment fl ake size and viscosity. Thinner coatings typically require closed-cell geometry, and the cell opening needs to be suffi cient to allow the pigment to tumble in and out without getting lodged in the cell. For thick coatings, open-cell geometry is most helpful to prevent air from becoming trapped in the cell as the emptied cell returns from the nip to coating/inking system. In most closed-cell applications, due to the difference in the LEC cell shape and the difference in the ceramic surface as compared to a mechanically engraved chrome (MEC) roll, the volume of the LEC will need to be higher than a chrome roll. A good starting point is 15 to 20 percent higher, but the best way to convert a MEC to a LEC is to start with a banded roll. An LEC-experienced technical support professional can review your requirements and put together a 4- to 8-band engraving that will have different geometries, as well as screen counts and volumes.

The 3D images in Figure 16 show how line screen and volume can affect the cell shape and, in the right combination, the cell walls and bottom can be shaped to help coating release and engraving cleaning.

Ceramic is very wear-resistant, so longer-life doctor blades can be used; however, the ceramic has very little impact resistance and can be chipped easily. Unlike traditional cylinders that can be somewhat repaired and re-chromed, the ceramic cylinder cannot be repaired if chipped away from the very ends. It only can be stripped and completely reconditioned. End chips can be repaired with a compatible epoxy, and this repair will help prevent the chip from growing; if the chip is in the end-seal area, this type of repair will help the life of the chamber seal. One other advantage with LEC is the availability of multiple, corrosion-resistant, bond coatings that can be applied under the ceramic to give the cylinder added corrosion resistance.

For coating applications, the LEC supplier will require the lbs/ream or coating dry thickness and process type. Ink and substrate characteristics are important as the ink – percent solids, pigment size, carrier type – helps to choose the type of engraving. For fl exo or offset-gravure application methods, 19 to 23 percent deposition effi ciency is required – a transfer roller is needed. Flexo must have a higher BCM volume. Direct gravure will use a 40 to 45 percent deposition effi ciency and less volume for the same deposition as fl exo. If available, a “smoothing bar” or off-speed reverse method removes lines and levels the coating. Wetting characteristics of the substrate also are important. Wet-out ability is needed to choose engraving type – CPI and BCM. For example, a 30- deg channel reduces pinholing on film.

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Flood coating, laminations, adhesives

Continuous laser-engraved cylinders are perfect for fl ood coatings, lamination and adhesives or primer decks where the cylinder is seldom changed and the repeat stays constant. As mentioned, advantages of the ceramic engraved cylinder is it can be metered by either the traditional forward-doctoring, gravure blade system or a reverseangled, single blade or enclosed chambered inking system. The enclosed chamber application for adhesives, varnishes, primers or laminators also will help control the viscosity of the coating or ink. The LEC cylinder will last so much longer and will allow for the application of tool steel or ceramic doctor blades with a radius tip, so the coating unit’s uptime is maximized with the reduction in cylinder and or blade changes. Also, when the LEC cylinder is fitted with an enclosed chamber, coating efficiency increases due to the ability to pump the coating into and from the chamber, which helps control viscosity and also makes it easier to change coatings due to less inking equipment to be cleaned. In addition, by using a chamber the coating can be pumped from a drum where a mixer can be mounted to help keep the coating components in proper suspension.

Ceramic laser-engraved, imaged cylinders have their challenges with keeping haze to an acceptable level. The quality of the ceramic coating is very important to have less than 0.5 percent of porosity for the LEC cylinder to be used in an imaged application. Ceramic’s porosity can carry ink, which when properly polished, is not a bad thing because it can help lubricate the traditional forward-metering doctor blade as the crosshatching on a copper/chrome cylinder does. Post-engraving finishing is very important to keep haze under control and so is the blade contact angle. For LEC-imaged cylinders, the contact angle of the blade needs to be in the same range as publication gravure – 60 to 64 degs. A thin metering tip is very helpful also; it needs to be matched with the right blade coating or material. When using a thin metering blade tip, the cylinder radius needs to be close to a 1/4-in. to reduce the blade tip from work-hardening on the edges as it oscillates on and off the cylinder.

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Effect of percent solids on required volume

Tables 1-3 show how the percent solids affect the required volume for a direct-gravure coating or laminating application using a LEC cylinder. All data assume a directgravure deposition efficiency of 42 percent, a 3,000 ft2 ream and 9 lbs/ gal. The tables show, in a logical manner, that as the percent solids decrease, the volume inversely increases, and for 100 percent solids energy-cured inks (EB & UV), the achievable coating thicknesses increases. For a 4 lbs/ream coatweight with only 10 percent solids, it takes almost 93 bcm; at 35 percent solids – 26.5 bcm; and at 100 percent solids – only needs 9.3 bcm.

The other issue that goes along with gravure coating is drying capacity. Just because a large volume cylinder is now available does not mean it can be used. Review the amount of drying capacity available and or what is considered the acceptable coating speed required to make the job profitable.

 

 

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LEC cylinder handling, maintenance

When using LEC cylinders in gravure applications, extra care is needed in cylinder handling. Always keep covers on the engraving and protect the cylinders from being bumped into the press frames or other corners. As previously stated, ceramic has tremendous wear resistance, but little impact resistance. When cleaning the LEC cylinders use an anilox stainless-steel brush; do not use a brass brush on ceramic. Off-line cleaning systems such as soda or media blasting, ultrasonic tanks and auto-cycle systems all can be employed, but it is always best to consult the equipment and LEC suppliers to make sure the appropriate chemicals, accessories and parameters are used.

After cleaning with any water-based cleaning system, always fully dry by rotating the cylinder, applying alcohol or use oil-free, compressed air. Drying prevents water spots, which can form on the ceramic surface in the shape of quarters on the bottom side, if the cylinder is not dry when rotation stops. In most cases, water spots will show up in the print and they are easier to prevent than remove.

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Summary

The advantages of using laser-engraved, ceramic cylinders outweigh the disadvantages in applications of flood coatings, laminating and long-running images or patterns of one to three cylinders.

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Advantages:

• Very long working life (10 to 20x)

• Can be metered with steel, tool steel or ceramic-coated blades

• Can be reverse-metered, single blade or chamber-inking system

• Wide range of available engraving geometries

• Closed cells or continuous channels are available

• Can be engraved from TIF (AI) files images or patterns

• Limitless combinations of lpi and volumes (BCM)

Disadvantages: 

• Initial cost (2 to 4x)

• Specification correlation (stylus angle/ percent vs. lpi/BCM)

• Limited number of imaged cylinders – two to three for registered jobs

• Ceramic has very limited impact resistance

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Final considerations

When using LEC cylinders for the first time, a banded cylinder may be required to match the gravure coating requirements to the laser-engraving, ceramic specifications. Finally, as for any new project, assemble an internal and external team consisting of your LEC cylinder, doctor blade and coating suppliers. This will facilitate a faster transition from traditional, mechanically engraved chrome to laser-engraved ceramic. In the proper application, you will find your coating/ laminating costs will be reduced and product quality will not be compromised and, in some applications, actually will be increased.

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About the Author: Tony Donato, product development engineer, Harper Corp. of America (Charlotte, NC), holds degrees from Purdue and Winthrop Universities, has trained in TQM, ISO and environment compliance, and became FIRST-certified as an Implementation Specialist in 2011. He has 40 years of industrial experience in manufacturing and plant management, line supervision, plant and facility engineering, manufacturing engineering, production and safety training, technical customer support, application sales and product development. In 2013, Tony received the “Top Cat” Award from the Graphic Communications Dept. of Clemson University for supporting the program.

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To view the original Converting Quarterly article click HERE.