Industrial Digital Printing for the Converting Industry Dene Taylor A & Vince Cahill B September, 2012 2012 AIMCAL Fall Conference Introduction: Digital printing has had phenomenal success for one reason above all else: it has enabled people to do things they couldn t do before. In the 70 s there was telephone bills and bank statements, in the 80 s desk top printing, from the 90 s we saw signage & point of purchase, and during the 00 s it has been labels, rigid panels and high quality direct mail. Digital print has a number of attractions (Table 1) but what has been most attractive recently has been improved service, reduced production times and often lowered overall customer costs. In only a few areas has digital totally replaced traditional printing in many it is complementary to offset, flexo and gravure. But in all of these it has been the catalyst for totally different applications and therefore new markets. Printed electronics is digital printing s latest frontier, and it is seeing tremendous investment. Whereas graphics is about visual effects and productivity, printed electronics requires highly functional responses, so its developments use a wide variety of different materials in controlled and precise combinations to form continuous, uniform layers, i.e., coatings. In comparison, speed is not a priority. We believe that combining graphics productivity with the precision and materials of printed electronics provides the basis for applying digital printing technologies to the paper film and foil converting industry. Digital printing technologies All four digital printing technologies, direct thermal, thermal transfer, electrophotography and inkjet, are finding application in non-traditional fields. But when it comes to converting paper foil and film inkjet has the broadest application, in large part because it can easily be turned to coating, it is non-contact and there are few limits to the materials it can process. Fluid Deposition by Inkjet Inkjet printing is computer controlled drop-wise application of a fluid to a surface 1, 2. The drops are ejected from nozzles in printheads (Figure 1). The heads together with the associated electronics, ink delivery, ink drying or curing and media transport, are the basic printer components (Figure 2). However, it is the printhead that defines the performance. Printheads are distinguished primarily by their standard droplet size, spacing, and firing frequency (Table 2). These combine to give a maximum flow rate (ml/min/inch), which is a limit for productivity at any target coat weight. Nozzles frequently are arranged in rows on the printhead (Figure 3). The largest volume heads may have only a single row, most have two, while new MEMS technology is being used to manufacture a 8.25 inch wide head with 5 rows. The rows may be fed from the same supply channel or, like the MEMs head, have their own. For coating purposes, heads where the fluid is being cycled continuously through the supply channel have proven more reliable. A Dene Taylor, Specialty Papers & Films, Inc., 101 Old York Road, New Hope, PA 18938, (215) 862 9434. dene@spf-inc.com B Vince Cahill, VCE Solutions, 11458 Country Circle Drive, Waynesboro, PA 17268, (717) 762-9520 vince@vcesolutions.com
Three mechanisms are used for ejecting drops. Piezo-electric (PIJ), thermal (TIJ) and continuous (CIJ). CIJ has a continuous stream of liquid flowing from each nozzle. This is normally impinging on a barrier so the ink can be returned to the ink supply and recycled. On signal the flow is directed past the barrier as one or more droplets and hits the substrate. CIJ was used for early production printers but a continuous flowing system has limitations for applications where a machine is frequently idle. PIJ and TIJ function by drop-on-demand. Behind each nozzle is a small chamber containing ink. A sharp short pressure pulse is generated by electrical signal, and pushes fluid out the nozzle. PIJ heads have a chamber wall made of piezo-electric material, which deforms very rapidly with applied voltage. The pulse in a thermal head comes from a tiny electrical heater vaporizing the adjacent water forming a bubble, and pushing out fluid. The bubble collapses immediately after the signal. For both processes the return to normal state draws in replacement fluid from the supply. Thermal is limited to water-based inks, but as PIJ ejection is a purely mechanical action it has few limitations. PIJ heads have been made from a wide range of materials, including stainless steel. And they can be designed to be heated to 50 C, 180 C or even 1000 C. Developing a coating fluid for inkjet is now mostly about meeting the rheological requirements. Coat Thickness/Coat Weight: The primary factor in coating is of course coat weight. Wet thickness by inkjet is the product of surface droplet density and droplet volume. For example a 360 dot per inch (dpi) head operating at its native resolution (360 x 360 dpi MD & CD) shooting 30 picoliter (pl) droplets, gives wet coating thickness of 6.0 µm. Thicker coatings need larger droplets, or overlapping dots, e.g., by using multiple heads or firing at greater frequencies. Thin coatings can be made with smaller droplets, depositing at lower surface concentrations, or by doing both (Figure 4). Dry coat weight is of course the product of the thickness, fluid density and solids content. Dilute mixtures can give sub-micron layers, while 100% solids fluids (hot melt or UV cure) in multiple layers may produce structures a millimeter high. Thickness uniformity Most printing is dot-based so it is not necessary to have the fluid form a continuous film, the desirable state for a coating, and droplet spreading (usually referred to as dot gain) is controlled to actually prevent that. Getting thickness uniformity on the droplet scale for a functional coating requires different thinking. So in addition to droplet surface density, drop volume and solids, a great deal of attention must be paid to the fluid flow or dot spread. Dot spread depends upon the rate of wetting and the time to drying or curing. The rate of wetting in turn depends upon fluid contact angle, viscosity and thickness. Given the low viscosity of jettable fluids, it is obvious that producing thin coatings on plain or simple-pretreated films with aqueous or solvent systems will be much easier than making thick. Drying, of course, can enhance artifacts from the application, or even introduce its own. For example, the solid matter may accumulate at the dot perimeter, so the dot looks line a ring. Fluids which have been heated build viscosity rapidly on striking a cool substrate, which reduces their lateral flow. UV cure and hot melt inks typically have a matte or microscopic lumpy appearance. Hotmelt inkjet prints are commonly hot rolled to reduce this effect. Types of Fluid
As noted above, there are heads for most fluid types, not just solvent/uv cure or aqueous, but also even moderately strong acids and strong bases. UV cure inks drove the demand for heads operating at 50 C, while hotmelt technology established the 150-200 C family. Containment of the fluid from the supply to the nozzle further increases the types of fluids that can be handled. Thus chemical reactivity per se is not necessarily a limitation to jetting, and a large number of materials of interest to the convertor are routinely printed or coated (Table 3). There are three areas where rheology is important. The ink supply is low shear flow through the nozzle is high shear, and the break up of the ejected stream has an extensional component. Most heads require Brookfield viscosity lower than 15 cps at the nozzle, while there are a few functional with 20 to 50 cps fluids. Low viscosity can come from the fluid itself (e.g., a monomer mixture), from a carrier, diluent or solvent, or by heating. Inks which are solutions or dispersions in a carrier fluid are invariably dilute, with solids content seldom more than 10%. It is extremely difficult to obtain dry layers of more than about 0.5 µm unless the fluid is immobilized rapidly. This is why inkjet substrates for desktop printers are absorbent. Non-absorbent substrates can receive thin solvent or aqueous coatings. They may also be coated with multi-component fluids that mix on the surface. The most reliable fluids for nonabsorbent materials though are 100 % solids, i.e., UV cure or hot melt. UV-cure inks are invariably heated to jet; they cool and gain viscosity rapidly on the substrate, which as mentioned above, leads to a texture. Coatings have more freedom with monomer selection so not all formulations need to be heated, and surface uniformity is easier to obtain. Nozzle diameters are obviously very small, so normal coating and printing pigments usually cause blocked nozzles, or jet outs. It is standard practice to filter through 1 µm or even 0.5 µm absolute filters when manufacturing inks. Grinding to sub micron average particle size is necessary. Additionally, dispersions need very good stability so that aggregation or settling in the supply line or the head does not occur as they too cause jet outs. Commonly inks have not more than about 5 to 8% of particulate matter, regardless of whether they are dispersion in water, solvent or monomer. Finally, fluid surface tension has to be within defined limits. It controls the jetting nozzle s liquid meniscus. If too low, fluid floods the nozzle plate and the head leaks. Too high and jetting becomes erratic. It is also a factor in flowing and spreading on the substrate. Substrates Any substrate that can be processed in roll form, or laid on a flatbed or vacuum table can be printed by inkjet. And it probably already is for marking and coding. Productivity Inkjet printer speed maximum is typically determined as the speed at which the placement of dots MD is the same as the spacing of nozzles CD, i.e., at native resolution for the head operating at it highest continuous head firing rate. Therefore it is dependent upon the rate at which the head can jet fluid, i.e., its drop volume and firing rate, and the nozzle density. Head A (Table 2) operating at its native resolution (360 dpi MD and CD) will run at 24 m/min and with 42 pl drops deposit an 8.4 µm thick layer. Head B (600 dpi) operating at 30 khz can run at 75 m/min. With 14 pl drops it gives a wet thickness also of 8.4 µm. The commercial upper limit belongs to head K which is used extensively for text printing at up
to 3000 fpm. But industrial applications differ from printing some are much thinner, and some are much thicker, so the optimum configuration may be radically different. As heads are designed to be assembled in wide arrays, and multiple arrays can be installed on a line, the capability exists to match analog productivity. Drying and curing obviously need to be matched to the deposition. UV curing, especially with LED, has been advanced because of inkjet, but the industry s expertise with drying is quite limited, and generally unsuited for a manufacturing environment. Commercial Printer Configurations Roll-to-roll printers come in two configurations. The less expensive and slower are like desktop IJ devices they have reciprocating heads and the substrate moves forward in a stepwise manner. This structure is available up to 200 inches wide. Fixed head machines can run much faster but they need many more heads and so may be more costly. A 54 inch wide machine was shown at drupa, albeit sheet fed. Table 4 includes relevant data. Few commercial printers operate at more than 500 fpm. But that limit is established by the minimum ink thickness required to get acceptable optical density. Thinner coatings with more droplet spreading can run much faster. Continuous inkjet is capable of the greatest printing speeds. A full color (CMYK) 25 inch wide press prints paper with aqueous ink at 2000 fpm. The same heads have been installed on many web offset presses to add variable print such as postal addresses. The speed limitation for the four color press is drying indeed it is marginal or a limitation for many printing presses as they have been designed with all the print heads together with a single dryer zone after. Of course for a coating application drying or curing at speed will be a basic element of the process design. Although there is a wide variety of capability provided by the inkjet OEM manufacturers, machines suitable for converting applications are not amongst the offering. On the other hand there are independent integrators who have proven capability to produce the types of devices needed, especially for retrofitting into an existing production line. Special Effects/Capability Reciprocating head printers operating with UV cure ink have the lamp attached to the head. The ink is cured with each pass, so with multiple passes structures as thick as (1 mm) can be formed. While the machines were developed to provide decorative effects for packaging, postcards and book covers, it takes little imagination to see how this capability can be turned to advantage for specialized converting applications. Inkjet for Paper, Film and Foil Converting. There are a large number of applications that could be performed by digital technology we are in many instances well beyond determining if something is possible, and without a great deal of effort can show if something is practical. Of course it is far more important to demonstrate that it is worthwhile, so selecting the right applications takes priority. And for that the advantages that digital printing has provided the graphics and printed electronics industries (Table 1) should be reviewed. There are also many examples where it has proven successful which can be used for inspiration (Table 5).
Here are some suggestions: Any short run printing Tamper evident and security films o Infinitely variability! Short runs of special colors (e.g., Pantone matching with CMYK or CMYRGB) Embossing masters Selective coating of substrates for thermoforming (exposed versus bonding areas) Short run decorative laminates o Print buried o Any substrate Patch coating The opportunity we see with tamper evident films is to incorporate infinite variability into the printing of the differential release part and transform them into security films too. If the pattern is ever changing an additional barrier to copying has been added. And of course it is more difficult again when the weakness is buried under a metalized layer. The uniquity can be enhanced easily by incorporating one or more special features such as whether visible or invisible ink, etc. And more value is added for the convertor for which reward can be obtained. We have been exploring two component reactive coatings to get thin high performance coatings. There are two ways to do this with ink jet. Firstly the substrate may be coated with one of a pair of reactants, and the second is then jetted on to. This is being practiced in the graphics industry to immobilize water on weakly absorbent surfaces like premium offset printing papers. It is also being used to generate masks on photopolymer plates that will be developed after exposure to UV. Alternatively, two separate fluids can be jetted simultaneously or in rapid succession from separate printheads. Coatings of urethane resin and polyisocyanate can be generated this way. Getting Started Our advice for getting started is easily summarized: Understand the application top to bottom and inside out? Define you what you will sell product or service? Determine if your customers will pay for the benefits Find the appropriate technology Ensure it will they meet industrial production standards List what is not known or available that s for your research team Don t reinvent the wheel! What can you buy? Run the numbers again! And for the greatest value find out how it will let you or your customer do something that couldn t be done before? References (see below.)
Table 1: Advantages of digital printing Economical short runs Print on-demand Make the number required Total customization Eliminate set ups and clean ups Computer controlled Ink jet is non-contact Novel products Set ups embedded in job description Integrates with computer controlled finishing Complement analog presses Low first impression cost Same day, next day shipping No overages, no minimums Slash customer waste Every page unique Easy multi-lingual changes Unlimited repeat length No plates, sleeves or cylinders High consistency job-to-job No scuffing, scratching New opportunities Errors reduced No WIP Ship from the end of the press Retrofit Table 2: Fluid delivery related characteristics of individual inkjet heads Parameter Common range Head A Head B Head P Technology Piezo Piezo Continuous Suitable chemistry UV Cure Solvent Aqueous Aqueous Head width Typical: 1 to 4 inches 2.77 4.25 4.16 Nozzle spacing Tightest: 1500 dpi Typical: 60 400 dpi 360 dpi 600 dpi 600 dpi Fluid viscosity 1 to 50 cps 8 50 cps 5 6 cps 6-10 cps Drop Volume 1 to 200 6 to 42 pl 5 to 18pl 12pl Firing frequency 3 to 60 khz 10 40 khz 20-30 khz <1000 khz Table 3: Jettable fluids Aqueous solutions Aqueous dispersions Aqueous latex and emulsion UV cure 100% UV cure solution/dispersion Thermal curable liquids Two component reactive liquids Hot melt Solvent solutions Solvent dispersions Metallic pigments Volatile solvents Mixtures in oil (stable and oxidizing) Molten metals Ceramic frit
Table 4: Variety of commercial printing machines Size (width) Fixed heads ¼ to 72 inches Scanning heads 5 to 200 inches Productivity 1 to 350,000 sf/hr Complexity Layers Fluids Fixing Curing Drying Pricing 1 to many 1 to 10 UV IR, ambient, warm air $100s to $1,000,000s Table 5: Applications for inspiration Printed electronics, membrane switches, batteries & photovoltaic Graphic arts printing and laminating Labels Marking and coding Ceramic tiles Polyolefin bag printing short run Transdermal patch production Security and tamper evident films 3D decoration and 3D prototyping FIGURES Printed dot pattern Print head Droplets Substrate Relative Movement Figure 1: Schematic of inkjet printing CPU Print Engine Ink Supply Print heads Curing or drying + Unwind Printing line Winder +
Figure 2: Primary components of a roll-to-roll inkjet press. Print engine is CPU, ink supply and heads. Figure 3: Print head with two rows of nozzles Close packed 80 pl 16 pl 2 pl Same spacing 80 pl 16 pl 2 pl Figure 4. Relative sizes of drops and dots with limited spread. Coverage is a combination of drop volume, dot spread and dot placement. References 1 Inkjet printing processes for packaging and labeling. Dene H. Taylor, Converting Quarterly, Volume 1(4) 46 49, 2011. 2 Inkjet Innovations: Application to Coating. Dene H. Taylor & Vince Cahill, Converting Quarterly, Volume 2(1) 2012.