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| Wednesday Mar. 10, 2010 |  |
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 Support Articles
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Let me make it clear right at the start: I am not an ink technologist or even
a chemist. I am writing this article from the printer's viewpoint. In order
to get satisfactory results with your pad-printing inks, you must have some
basic information about selecting, mixing, and using them. Having read this
article, you will know more about pad-printing inks than most ink reps. You
will finally understand why they behave and why sometimes they appear to misbehave
(the inks, not the reps!). Pad-printing inks are similar in make-up to screen-printing inks, although there are some important differences. Screen-printing inks are formulated to resist rapid evaporation so they don't dry in the screen. Pad-printing inks are designed to do just the opposite, since rapid solvent evaporation during the printing process is critical. Also, pad-printing inks don't have the advantage of being applied in very thick films like screen-printing inks (figure 1).
Figure 1: Pad Printing Ink-Film Characteristics Pad printing achieves very thin ink deposits in comparison with screen printing. Only about half of the ink in the cliché is picked up by the pad; once deposited, the ink film is further reduced by solvent evaporation. Pad-printing formulations are usually decorative in nature, though they can be used to conduct or resist the flow of electricity, carry pure gold, store magnetic messages, or simply be invisible. Pad-printing inks consist of the following components (figure 2): 
Figure 2: Typical components of solvent-based pad printing inks
Pigments, usually supplied in powder form, impart the color and opacity to the finished print. They are incorporated into the ink by the manufacturer through a mechanical dispersion process. Dyes are sometimes used in place of pigments - in sublimation inks, for example, or in conventional formulas where a special transparent shade is needed. Solvents enable the resin-pigment mixture to be transferred to the substrate during printing. Pad printing requires fast-evaporating solvents to enable the ink to transfer, the advantage being that wet-on-wet multicolor printing is possible. The exact choice and amount of solvents will depend on the resins and pigment used in the ink. In a number of cases, the substrate also plays a role in what solvent should be used. For most inks, no single solvent will have all the desired properties, so a mixture of solvents is used. The inks you buy may not have the amount or type of solvent required for this to enable you to "fine tune" the system to suit the ambient conditions. Even in a controlled environment, different solvents may be needed depending on the application. For example, the drying speed of the ink may be influenced by whether you are doing single-color work or wet-on-wet multicolor printing. It may be necessary to use a faster-drying solvent for the first color printed, an intermediate-speed solvent for the second, and a retarder for the last. Minor additives include plasticizers and surfactants designed to improve the ink's flexibility, flow, pigment stability, and other characteristics. These additives are essential for adequate performance. Without them, the ink would suffer such defects as brittleness, poor film strength, pigment separation, and unsatisfactory flow. Ink types The ink systems currently available to the industry fall into seven different catergories:
In oxidation-drying inks, the resin absorbs oxygen from the atmosphere and undergoes a polymerization process, producing a very tough, flexible, weather-resistant ink film. They have limited uses in pad-printing applications due to their slow drying speed, but they are excellent for printing onto metal and glass. Two-part or reactive (catalyst curing) inks, used extensively in pad printing, also contain resins capable of polymerization. However, the required catalyst is either blended into the ink by the manufacturer or supplied separately and mixed in by the printer when required. Either way, the inks have a restricted shelf life once the catalyst is added. Two-part inks cure very rapidly when heated. They are generally printed on difficult substrates such as metals, some plastics, and glass, and are particularly popular when good chemical and abrasion resistance is required. Particular care must be taken when mixing the base ink with the catalyst. Manufacturers specify an exact weight to be added, so you must always weigh the components when mixing - no exceptions! Inaccurate mixing can give inconsistent adhesion and product-resistance characteristics. Another important factor to be aware of with two-component inks is that after printing and prior to complete curing, the temperature of the printed ink film must not drop below 59°F (15°C). Should this occur, the ink will cease curing and cannot be restated. This may not be a problem if the curing may nearly be complete. However, if curing takes place in storage over a period of time, the ink film is vulnerable. A dry ink film is not necessarily cured. It takes time, temperature, or combination of both to effect a complete cure. With two-component inks, curing typically takes five days at 68°F (20°C) or 10 min at 212°F (100°C). This information is available on the ink's technical data sheet, which unfortunately, most users do no bother to read. Baking inks need a certain minimum temperature and time to cure. The cure time varies inversely with the temperature: the higher the temperature, the shorter the drying time. The flexibility of the ink film is another factor to consider with these inks. Ink films that must retain their flexibility require lower temperatures, for a higher temperature can cause brittleness. Sublimation inks involve a special process where a solid turns into a gas when heated. When these inks are applied to an appropriate surface and then heated to the specified temperature (approximately 392°F or 200°C), dyes in the ink sublime, the surface of the material becomes porous, and the dyes pass into the material. This actually changes the color of the base material. Once the material has cooled, the ink is sealed into the surface. Pad printing is a suitable process for sublimation since the ink deposit must be kept to a minimum to prevent color bleeding. Applications include keyboards and other areas where abrasion resistance is critical. Care must be taken when selecting colors, as certain inks are sensitive to UV light and fade very quickly. The range of colors is very limited, and matching Pantone colors is almost impossible. Another limitation is that the substrate color must be lighter than that of the ink. Ceramic and gas thermoplastic inks are used quite successfully in the pad-printing industry. These inks are similar to the ones used in screen printing in that at ambient temperatures, the ink is solid (like candle wax). It becomes fluid when raised to 176°F (80°C), which is accomplished in the ink reservoir and cliché (Figure 3). 
Figure 3: Pad printing with thermoplastic inks Thermoplastic inks used for glass and ceramic printing are solid in ambient conditions and must be heated to become liquid. Presses used for these applications have heated clichés and ink reservoirs. Unlike conventional pad-printing inks, pad wetting doesn't occur due to solvent evaporation, but by the cooling effect of the pad when it comes into contact with the ink in the etching of the cliché. Similarly, the ink transfers from the pad to the substrate because the outer ink surface becomes tacky when exposed to the air, making it tackier than the ink on the surface of the pad. The cooling effect of the glass or ceramic substrate enable a complete transfer. This process allows fairly heavy deposits of ink to be printed because the etch depths of the cliché are greater - 30 to 50 microns. Once printed, the ink must be fired onto the surface at approximately 1075°F (580°C) for glass and 2130°F (1200°C) for ceramic. These inks are dishwasher safe, but if that isn't a requirement, baking inks and reactive systems are very effective for printing onto glass and ceramics. UV-curable inks widely used in the screen-printing industry, offering faster curing speeds, ease of printing, and fewer environmental problems than solvent-based systems. In certain applications, a pad printing ink will consist of a UV formulation with solvent added to it to enable the ink to transfer conventionally. The hard coats used to protect products such as keyboards from abrasion are an example. Pigmented UV pad-printing inks for decorative applications have been available on a limited basis, but questions about the technology remain. Most of the major ink suppliers are working to develop a UV ink suitable for the pad-printing process. I am aware of at least four that are currently testing products in the field. Machinery manufacturers are working with ink manufacturers, and I've been told of equipment that reportedly has print speeds up to 4000 iph using these inks. The initial target market is the compact-disc industry. Pad printing's use on CDs has been declining in recent years, but this development could bring new life. Precise details regarding ink technology and application are still somewhat sketchy. Since UV inks contain no solvents, they can't rely on solvent evaporation in the printing process as ordinary inks do. The manufacturers achieve the tackiness necessary for pad ink rheology. Although the cliché etching is about 20 microns, only a percentage of the ink - approximately 6 microns - sticks to the pad. However, because the inks contain no evaporative liquids, the cured ink film is much closer to the wet ink film than with conventional inks. The thickness of the ink film must be carefully controlled or else curing will be very difficult. This is still in the experimental stage and the manufacturers involved specify different pad materials, hardnesses, and cliché etch depths. The range of printable substrates will likely be less extensive than solvent-based inks, and ink opacity will be compromised. In some applications, a white background may be necessary to give the required color intensity. Also, printing wet on wet would not be possible, so intermediate drying would be required. For a pad printer, the big advantage of UV technology is that small changes in ambient conditions won't affect it, giving much greater stability to the process. It will likely be most popular in four-color work where thin films of transparent ink are necessary for satisfactory results. More details will be available in the near future. Ink selection - Substrate concerns All manufacturers provide technical data sheets that state what substrates can be printed with that ink. The sheets also specify any special requirements such as pre- and post-treatments. But to select the correct ink, you must be aware of a wide range of conditions and requirements. A customer's request to "print a blue ink on this plastic component" poses a whole range of questions that must be answered to fulfill the application satisfactorily. 
Figure 4: Surface wetting If the surface tension of an ink is greater than the surface energy of the substrate, the ink will bead up rather than flow properly. To make such a substrate wetable, and enable ink adhesion, substrate pretreatment is necessary. In order for an ink to adhere to a substrate, it must be capable of wetting the material. If this is to occur, the surface tension of the ink must be lower than the surface energy of the substrate (Figure 4). Contamination in the form of oil, grease silicones, condensation, etc., will inhibit or stop ink from wetting the surface. Cleanliness is essential. I can't overstate the need to accurately identify the substrate, as small changes in the chemical composition can radically affect the surface of the material. The most difficult situation is where surface conditions vary, either at different points on the substrate or throughout a production run. Plastics that are filled (e.g., glass-filled nylon, talc-filled polypropylene) of the filler rather than those of the plastic will change due to slip-additive migration. This is particularly so in applications where heat is applied to a molded component. Ink that appeared to have adequate adhesion can simply fall off when slip additives migrate to the surface. Substrate pretreatment The term polyolefins refers to a wide range of plastics. Some, such as polystyrene and vinyl, are very suitable for printing. Others, such as polyethylene and polypropylene, can't be printed in their natural state. Polyethylene and polypropylene have a surface energy of 30 dynes/cm². For good printability. The surface must be treated in one of four ways. Liquid printers are probably the least favorite option in the industry. They are limited in the range of plastics that can be successfully treated, and they are inconvenient to apply. Spraying or dipping, not wiping, are the preferred methods. Care must be taken not to inhale the vapors or allow the fluid to come in contact with the skin. Experimentation is necessary, but even then, changes in substrate batches can alter the primer's effectiveness. Corona treatment uses a high-voltage discharge to change the surface energy of the substrate. Materials are treated in line using two electrodes - one over the substrate and one under. The electrodes generate a plasma that ionizes the substrate, altering its surface tension. The distance between the electrodes is critical and should remain constant. Corona discharge treats films most successfully because the distance between the electrodes is small. Larger decorators may have such equipment in house or they may buy corona-treated films from their vendors. Sophisticated systems are available for corona treating three-dimensional objects very effectively. These are used where high volumes can justify the capital costs. The process will not work if there is any break in the surface being treated because the discharge will find the path of least resistance and short directly through the hole. As an alternative, three-dimensional components can be bulk treated in a chamber that is charged with an electrical plasma. This is a very effective method and will treat every surface of a molding no matter what the shape. Decorators can only justify these methods in house for a very high number of parts, however, because the capital expense is high. Flame treating is the most widely used method of pretreatment. Like corona treatment, it may be done in house or by the substrate manufacturer. Flexible and reliable if carefully controlled, flame treating enables uneven and curved surfaces to be treated. This process uses a mixture of air and gas such as butane, propane, methane, etc. For the flame to be effective, it must be oxidizing (that is, blue). Correct flame control is very important. A basic flamer will do simple work, but for regular use and long production runs, specially designed flame-control systems are recommended. These are fitted with gas- and air-control valves to compensate for pressure Fluctuations, ensuring that the mixture is always optimum. Flame-nozzle design is important usually featuring single or double rows of ribbon burners that give a more stable flame shape and characteristic. "Flame throwers" are inefficient and unreliable. Flame control and position of the item in the flame are critical, so setting up the flamer is very important (Figure 5). 
Figure 5: Flame treatment In flame treating, flame shape is critical for the pretreatment to be effective. Over-flaming will damage the surface of the substrate, while under-flaming will cause the ink not to stick. Cold-gas plasma treatment is emerging as an efficient way to treat polymers, dramatically improving their surface properties for high-performance printing. In some cases, plasma treatment provides the only acceptable solution to these common surface treatment problems. The only disadvantage is the expense of the equipment, although some companies offer contract plasma treating. Testing for correct pretreatment to ensure that a substrate has been properly pretreated, you must establish that the surface energy has changed. Kits are available that allow you to test the surface energy of the substrate by applying liquids with known dyne levels and watching the reaction (Figure 6). 
Figure 6: Testing surface Dyne kits allow you to determine the surface energy of a substrate by applying fluids with known dyne levels onto it and watching the reaction. If the surface energy of the substrate is higher than the surface tension of the testing fluid, it will flow evenly rather than break up into globules. If the mixture spread evenly across the surface, then the surface energy of the substrate is equal or higher than the surface tension of the testing fluid. If the liquid beads up and forms into globules, however, then the substrate has a lower surface energy. These tests are essential for establishing that a material can be printed. They work no matter what form of pretreatment has been used. Dyne-testing kits normally consist of six to eight fluids with surface tensions ranging from 28-56 dynes/cm². The lids must be firmly replaced after use, and gloves and goggles should be worn to prevent contact with the skin and eyes. Test kits are also available in felt-tip pen form, which are adequate for ensuring that the substrate has a certain minimum dyne level. Having a dyne-testing kit is imperative for any pad-printing shop. In lieu of one, a few simple rule-of- thumb tests can be tried. First, you can simply hold the object to be printed under running water and then remove it. If the substrate has been properly treated, the water will seem to wet evenly, then de-wet slowly. On an untreated surface, the water will bead up. Another, less reliable test is to mark the substrate with a ball-point pen. Stick a strip of Scotch tape on the pen mark and pull it off. If the substrate is correctly pretreated, most of the ink will adhere to the plastic instead of the tape. This test is far from ideal and should be used only as a last resort. Other factors affecting adhesion Surface pretreatment is a decisive factor when printing onto polyethylene and polypropylene. But other factors affecting adhesion, such as slip-additive migration, can't be detected from a dyne test. You could get favorable results in a dyne test and still suffer ink-adhesion failure. Also, two surfaces with the same treatment levels may show different ink-adhesion characteristics. You need to be aware of these possibilities when troubleshooting a printing difficulty. If you are printing a polyolefin and pretreating it isn't an option, special inks are available that don't require pretreatment. These single-part inks require efficient post-treatment in the form of either IR, forced-air, flame, or UV drying that enhances the final characteristic of the relevant ink system. Without this post-treatment, the highly stable structure of the material makes satisfactory adhesion impossible. Properties of cured ink The substrate alone isn't enough information to determine what ink to use. You must know what is required of the cured ink film by the customer. Color matching is a topic for another article, but a few basic issues should be mentioned. The starting point is either a sample of the color applied to the surface to be printed and/or a reference to a standard color specification, such as PMS , DIN, etc. Remember that the ink film in pad printing is very thin, and you often must print on a colored background that will affect the final color of the ink. Always check the printed color under different lighting conditions. Stability of color under UV light is very important. You'll find information on your technical data sheets regarding the properties of the pigments. Adhesion and abrasion resistance: Cross-hatch tests, rub tests, tabor-wheel abrasion tests, etc., should all be specified before you select an ink. The classical comment "it mustn't scratch off" is not acceptable. The most recognized adhesion test is to cut a cross hatch through the cured ink in 4-mm squares (Figure 7). 
Figure 7: Testing Abrasion Resistance The familiar cross-hatch test, shown here on a cylindrical substrate, is the most common adhesion test in pad printing. You then apply Scotch tape and remove it with a sharp pull at an acute angle to the surface. The amount of the ink remaining determines the level of adhesion. Resistance to chemical attack: A printed part may need to withstand the effects of oils, solvents, acids, alkalis, or plain water. Probably one of the most arduous environments is a dishwasher, where there is a mixture of detergent, alkali, and hot water. I am not aware of any non-ceramic ink systems that will withstand dishwashing for extended periods. Another surprisingly aggressive material is melted snow. This contains a whole cocktail of chemicals that are collected as the clouds from and the snow falls through the atmosphere. Always check what your client needs the printed ink film to withstand before you start printing. Read the technical data sheet provided by your ink supplier. If in doubt, run tests with the ink and provide printed samples for your client to approve. If necessary, ask your ink supplier to run trials. Weathering again: The technical data sheet will give you information on this subject. If weathering characteristics are critical, it is possible to carry out accelerated tests on the printed substrate. Either an ink supplier or an independent lab can conduct tests for you if you don't have access to the equipment. Special requirements: Food and toy applications are a good example, both involving ink that may be ingested. The toxicity level of pigments and resins used in such inks is very carefully controlled. Heavy metals such as cadmium are totally unacceptable, and the regulations are constantly becoming stricter, so seek advice from your ink supplier if you aren't absolutely certain. Post-production processes Any post-printing steps the part must go through could affect the ink performance. In screen printing, die-cutting and thermo-forming can both be problematic with certain inks. In pad printing, clear coating is a process to watch. The underlying ink must be fully cured and impervious to any solvents in the clear coat. The clear coat won't stick to under-cured ink, and it may cause the ink to bleed if the solvents aren't compatible. If adhesion difficulties arise, it's important to check all of the following points:
If you've been reading this series of articles, you know how important it is to control the balance of solvent in the ink so they evaporate properly. Ink manufacturers spend vast amounts of time and money developing and formulating inks to satisfy this criterion. Then what do some printers do? Pour the ink from a can into the ink reservoir without checking the label. They squirt in some solvent, and then try to print. When the ink doesn't transfer or stick, who is blamed? The ink manufacturer, of course. Really, there is a better way. Imagine you're receiving a new order. You need some information about the job before you even begin thinking about which ink to use. This information should be on the work order and it might look something like this: Substrate
Before the job goes to production, tests are conducted to check the ink/substrate combination. The trials not only establish that the printing and drying recommendations work, but also the mix of solvents needed in the ink to give optimum printing performance. Now, on to production. The operator is given the production-control card, which details all the setup and ink instructions. He gathers up the necessary materials and put on any protective clothing required (goggles, gloves, and an apron). He reads the labels on the ink, hardener, and solvents to double-check the production control card. He also checks the date to ensure that nothing has passed the shelf life. He opens the ink can and stirs the contents vigorously. He also opens and stirs the hardener. He places a suitable container on the scales and puts the stated amount of ink in. He then adds the hardener to the correct ratio - by weight - and stirs it in. Then adds the mix of solvents, carefully stirring to ensure complete dispersion in the mix. (At this stage, some shops might use a viscometer to check the viscosity. If your mixtures are correct by weight, the viscosity will be acceptable in any case.) The operator then pours the specified amount of ink into the ink reservoir. He keeps the remainder in a closed container for use throughout the run. He makes the other press settings as specified by the production control card. Surprise, surprise: The first print is acceptable. Assuming the press operator maintains the solvent balance during the day by adding measured amounts of solvent at specified intervals, the press will run right on through. Depending on the ink system and the ambient temperatures, the ink will have to be completely replaced with a new batch in 8-10 hr. This is because a two-component ink will begin curing in the ink reservoir and its printing characteristics will change. In very high ambient temperatures, it may be necessary to change the ink more often. It's very important to estimate a two-component ink carefully, since anything not used that day should be disposed of correctly and is expensive to waste. If the job had called for a single-component ink (no catalyst), the ink wouldn't cure in the reservoir and could be used on press for a much longer period. I would still recommend changing it after 48 hr, as contaminants can build up that will alter the ink's properties. Oxidation-curing inks may also undergo chemical changes. By the way, two-component inks are not recommended for use in sealed ink cups, as they gel if they are not agitated and will cure completely if left in the cup for extended periods. Care must also be taken when using reactive ink on screened clichés. Any trace of ink left in the etched portion of the cliché overnight will cure and be impossible to remove the next day. This applies to a greater or lesser extent with any part of the machine or jigging. Conclusion In pad printing, the ink may be the most important element of the process, yet it is constantly abused. Always carry out tests on the substrate before going into production. Materials do change and problems can occur. Take a systematic approach to choosing, testing, preparing, and using your inks, and you'll see a world of improvement in your pad-printing operation. Rules of Using Ink
Peter Kiddell has been in the pad printing industry for more than 20 years and currently works as a pad and screen printing consultant. His company, Product Decoration Services, develops training packages to help companies fine tune their pad printing process. He or his partner, Carol Burnside, may be contacted at: Product Decoration Services Innovation Way Barnsley s75 1JL South Yorkshire England Tel: +44(0)1226 249590 Fax: +44(0)1226 294797 Email: PDS_Consulting@compuserve.com
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