Increasing Pad Printing Productivity and Efficiency
Written by John Kaverman, President, Pad Print Pros
Specializing in the pad printing process for nearly twenty years, I’ve had the opportunity to visit hundreds of companies, review thousands of different applications, and train thousands of people at various levels within each organization.
What I’ve learned is that many of the problems that companies experience with the pad printing process are the result of the same, common mistakes. Some of these mistakes are due to ignorance, whereas others are the direct result of ill-conceived efforts to “save money”. This paper will review some of these common mistakes, their impact on the process, and immediate actions for correcting them.
The production environment plays an important role in pad printing, just as it does in any wet ink film transfer process. Ideally, pad printing operations should be conducted in a controlled environment, with temperatures between 68-72OF, and relative humidity of 55%, +- 10%. The inks, plates, pads and materials to be printed should also be acclimated to these conditions.
If quality throughput is your goal, a controlled environment is a necessity. Otherwise, pick one: Quality, or throughput. While printing with quality results in an uncontrolled environment is entirely possible, the quality almost always comes at the expense of throughput. In an uncontrolled environment time is invariably lost “dialing in” and maintaining ink transfer efficiency.
If controlling the entire production environment isn’t cost effective, consider at least purchasing a printing platform that features a controlled enclosure. In Europe, where utility costs make controlling the environment almost impossible to justify, companies use printing platforms that incorporate temperature and humidity control as well as HEPA filtration, as shown in Figure 1.
Figure 1: Pad printing platform with climate control and HEPA filtration.
Regardless of whether you’re using a laser to engrave or a film to photo engrave or chemically etch your clichés, nothing you can do downstream in the process can compensate for bad artwork. Therefore, it is imperative that you start with correctly formatted (vector) graphics.
Typically, these are generated in Adobe Illustrator, CorelDraw or similar programs that convert all text and lines to curves which can be filled 100% black. Exporting or “saving as” graphics from programs without first converting to curves will not produce the desired results. For example, design and 3-D modeling programs, word processing and photo manipulating programs are not capable of producing or exporting as correctly formatted vector graphics, so the art department of your pad printing supplier will have to redraw your files. That means time and money.
If you’re creating film for use in exposing clichés for either photo engraving (photopolymer) or chemically etching (steel) clichés, it is important that your film and / or film output device be capable of producing an image with sufficient density (opacity).
Photopolymer coatings and photoresist harden (cross-link) from the bottom up. When your image is not dense enough and some percentage of the U.V. light gets through, the result is a cliché that is too shallow or completely unusable.
Often companies that screen print wish to use their screen film output device to create films for pad printing. While this is sometimes successful, those occurrences are the exception, rather than the rule.
To be safe, always output a film and send it to your pad printing cliché supplier, asking them to do a test exposure to verify that the density is correct.
The orientation of the film used in cliché exposure should be “emulsion down”, as illustrated in Figure 2
Figure 2: “Emulsion Down” Film Orientation.
If the emulsion of the film is on the top of the film the light from the exposure will undercut the image, resulting in poor edge resolution and / or the complete loss of fine lines and details such as trademarks.
For the best finish over the surface of the cliché, and therefore the cleanest possible surface to doctor, cut both the image and line screen films so that they cover the entire cliché surface, using the outline of the cliché’s dimensions for alignment. This practice will ensure that cut marks don’t etch into the cliché, and the image is at least as consistently aligned as possible on subsequent clichés using the same film positive.
The storage, exposure, and development of photopolymer clichés needs to happen in an environment that is free of U.V. light. U.V. light can be filtered out from overhead florescent lighting using sleeves, or U.V. free light sources can be used.
Cliché exposure(s) should be consistent for each cliché material type. You should never try to adjust the “depth” of your clichés by changing your exposure times. In photopolymer clichés you control the depth, you control the amount of ink that a given surface area of the cliché can hold by varying the line screen used in the second exposure.
Typically 150, 120 and 100 line / cm2 line screens are used for the second exposure. 120 line is most frequently used. On rare occasions, 80 line / cm2 film is used, but only in cases where you need to transfer an unusually thick ink film.
The exposure for the image film and the line screen should be the same length, resulting in a finished cliché where the peaks in the line screen are level with the top surface of the cliché. Too short a second exposure will result in a cliché where the line screen is below the surface, which can result in doctoring issues (scooping) and print quality issues.
Developer is one thing that people always try to save money on.
The developer used for alcohol wash photopolymer clichés is 98% pure, with a specific denaturing agent. What it is most assuredly not is the denatured alcohol solvent that you can buy at Lowe’s or Builder’s Square. Box stores buy their chemicals in bulk, and they are always diluted with up to 40% water.
Avoid potential issues by purchasing developer from a reputable supplier. If you want to dilute it, you usually can with up to 15% distilled water.
Distilled water is what should also be used to develop water wash cliché materials. Tap water, or well water, may contain contaminants and chemicals (fluoride, chlorine) that can adversely affect development.
When developing either alcohol or water wash photopolymer clichés, do not scrub with image area. Use a soft, clean paint pad and apply only the pressure required to gently float the pad over the surface of the cliché. Of course, this implies that you need enough developer in the tray to completely submerge the clichés.
Post-Exposure and Drying
In polymer cliché making the post exposure step is extremely important for maximizing the operational life of the cliché.
The development step, unexposed polymer is gently loosened with brushing, and floats out of the image area. As a result, the sides of the pattern produced by the line screen are yet to be exposed to U.V. light, so they are still soft. The post exposure step serves to harden the image area.
Drying is also recommended to remove residual developer from the polymer coating. Skipping the drying step and going straight to press can result in the doctor ring damaging the cliché beyond repair.
Photopolymer clichés are hydroscopic, so it is also a good practice to dry clichés that have been stored for long period before using them again. Of course, the more humid the season of the year, or region of the country that you live in, the more important the drying step becomes.
Pad printing is utilized on a wide variety of substrate materials. Many of these materials, (especially polyolifins) have surface energies that are too low for pad printing inks to successfully adhere to.
The industry standard “minimum” surface energy is 38 Dyne / cm2, with 42 Dyne / cm2 or higher being the preference of most ink manufacturers.
Do yourself a favor: buy some Dyne test pens and test your substrate to determine whether pre-treatment is required. If the pens say it needs pre-treatment, do it. Make sure parts are clean and free of contamination before testing. Never use mold release agents in producing parts that will eventually require pad printing.
If you are printing on top of a protective hard coat, or two-component paint system, be advised that such coatings usually have a “window of opportunity” during which it is possible to get an ink to adhere. It is entirely possible that if you print coated parts too soon and / or too long after the paint is applied the ink may not adhere, not matter what.
Pad Printing Inks
There are many inks that are specifically formulated for pad printing. Basically there are two three types of conventional solvent based inks, and U.V. inks. Conventional solvent based inks can be single-component (no hardener required), two-component (hardener required) or a combination of both (hardener optional).
U.V. pad printing inks are really modified U.V. inks in that they contain some percentage of thinner. (True U.V. inks don’t contain thinner.) The thinner is necessary to make the ink printable via the pad printing process.
Choosing the Right Ink
Waiting until it’s time to go into production to choose an ink is one reason a lot of companies experience failure. The best way to determine whether an ink is suitable for your application is to send your ink supplier physical samples of the parts you wish to print. If you can’t send actual parts, at least try to send plaques or something representative of the material and texture you want to print.
Relying on the “ink compatibility matrix” in the manufacturer’s catalog, or assuming an ink will work now because it worked on a similar material or coating in the past isn’t always a good idea. These days there are simply too many variations of materials and coatings. Testing is always the best and safest method.
Of course, the ink guys need to know what kind of performance specifications the ink needs to meet, so be sure to supply them with that information up front.
Mixing Pad Print Inks
One mistake many companies make when mixing their pad printing inks is a failure to weigh the components. Pad printing inks differ in weight by volume from one shade to another. To mix them consistently it is necessary to weigh all of the components: ink, hardener (if required), and thinner.
Mixing “by eye” or by volume can cause significant problems with transfer efficiency, adhesion and performance.
When mixing two component inks it is necessary to strictly follow the manufacturer’s recommended ratio of ink to hardener. Using too much, or too little hardener can result in problems too numerous to mention. Add the hardener to the ink, then blend those two components together thoroughly before adding thinner. Adding hardener and thinner simultaneously can significantly reduce the ink’s operational pot life.
Always mix your inks in disposable containers, using disposable stir sticks. Containers should be made of no-wax paper or solvent resistant plastic such as PET. Never used wax-coated paper cups, as the wax will dissolve into the ink, resulting in lots of issues.
Disposable cups and sticks are a lot less expensive than the expense of cleaning and potential issues of reusing mixing supplies.
Always Prepare Enough Ink
The most frequent mistake that companies make is failing to prepare enough ink to adequately fill their ink cups. Every machine design requires a given volume of ink within the ink cup(s) to efficiently flood the image and doctor the cliché. It is important to realize that in addition to providing the color, ink acts as a lubricant in doctoring systems.
The physical forces of doctoring a cliché result in the ink flowing back and forth within the confines of the ink cup. If you’re double printing or otherwise printing at an elevated rate (on continuous cycle, for example) and you don’t have enough ink in your cup, the ink never has enough time to flow completely over the cliché surface before the next doctoring cycle. A sure sign of that is when you get a crescent-shaped haze on the front and rear of the cliché as illustrated in Figure 3.
Figure 3: Crescent-shape hazing / not enough ink.
This is what happens when the ink cannot re-wet the entire doctoring area due to insufficient volume. As the haze dries, the ink cup starts to ride up onto the haze, causing excessive leaking and other problems. Think of ink in a pad printing machine as being like the oil in your car. If your car requires four quarts of oil, would you take it for even a short drive if you knew it was 2 or 3 quarts low, and expect it to function properly?
Drying vs. Curing
Failure to sufficiently cure printed ink films is a big problem, largely due to ignorance as to the difference between drying and curing. Drying is simply evaporating thinner from the printed ink film. Curing is the chemical reaction that takes place until the ink has reached its maximum level of performance with regard to adhesion as well as chemical and mechanical resistance.
Ink manufacturers specify separate drying and curing schedules on technical data sheets, yet few people follow the recommendations. Everyone wants an ink that cures immediately, but it doesn’t exist. Even U.V. curable inks are somewhat post-curing, meaning that they too should not be tested for adhesion and / or chemical and mechanical resistance immediately upon exiting the curing unit.
Ink manufacturers will normally tell you to wait 24 hours before testing the performance of single component inks. Once you add a diisocyanate hardener to a two-component ink, the ink can require 72 hours or more to completely cure. In that case, water vapor in the air reacts with the hardener and the ink’s resin system, so curing is independent of heat and essentially impossible to “speed up”.
U.V. inks cure by polymerization when subjected to U.V. radiation. First, you need to evaporate the thinner from the ink, then you need to polymerize the ink film. For this reason, many U.V. pad printing applications feature I.R. drying prior to U.V., specifically to remove the thinners that are added to U.V. pad printing inks
Why U.V. ink isn’t always the answer to your curing problems:
I am frequently asked whether U.V. ink can be used to circumvent the lengthy curing requirements of conventional, solvent based pad printing inks.
The answer is, “It depends.” U.V. is great for small format, single color applications. Because U.V. inks remain wet until they are exposed to U.V. radiation, it is not possible to print them “wet-on-wet” or even one print in close proximity to another without intermittent curing. For this reason, multiple color applications and / or large format (larger than about 150 mm square) cannot be easily processed in pad printing systems without significantly increasing the cost and footprint of the machine.
For example, you can easily use U.V. ink on a small, single or multiple color parts when you can integrate small (150mm square max.) U.V. curing units between print / color stations, however it isn’t feasible for something large, like a multiple colored appliance panel. The system would have to be enormous, and slow, to accommodate U.V. curing.
Pads are often the one component required to make a pad printing application successful, yet they are the most overlooked.
Transfer pads come in numerous shapes, sizes and colors. When I need to choose a pad for a new application I always start with the largest (most mass), steepest angled, hardest pad I can efficiently compress in the machine I am going to use.
- Pad Size At a minimum, I want the pad to be 20% larger in diameter or height and width than the image that I need to print. If my image requires more than 80% of the pad’s surface, I know that I am flirting with distortion near the edges of my image / end of my compression stroke.
- Pad Shape When choosing a shape, I look at two things: the shape of the image (is it round, square, short and narrow, etc.) and the contour of the part in the print location. Ideally, I want a conical shaped pad for round images, a rectangular shaped pad for rectangular shaped images, a roof-top shaped pad for short, wide images and straight lines of text, and a square pad (or conical pad with the edges molded square) for square shapes. The contour, or angle of the pad, should be as steep as possible. For compound angles, this pretty much means I want a pad shaped as close to the opposite of the angles of the print area as possible. As a pad compresses to pick up and transfer the image it has to roll out evenly from its point or ridge, displacing the air from between the surface of the pad and the surface of the cliché (during pick-up) and part (during transfer). If the angle isn’t steep enough air gets trapped between the pad and the cliché or part, and you end up with voids in your print.
- Pad Shore Pad hardness is expressed as “shore”. There are various shore scales, but in every case the higher the number, the harder the pad. Shore is determined by the amount of silicone oil that is used in manufacturing the pad. The higher the percentage of silicone oil, the softer and more pliable the resulting pad. Conversely, the lower the percentage of silicone oil, the harder and more rigid the pad. Softer pads last longer and deposit more ink than harder pads, but harder pads produce sharper image resolution, less distortion, and superior coverage on textured surfaces than softer pads.
- Pad Life As I mentioned previously, softer pads last somewhat longer than harder pads of the same size and shape due to their pliability. As any pad, regardless of shore, picks up and transfers the image, the silicone oil within the pad is depleted. Eventually, enough oil is depleted that the pad no longer efficiently or “evenly” picks up and / or transfers the image. They dry out. Once the oil is depleted from within the pad material, you cannot effectively replenish it by applying more silicone oil to the surface of the pad.
If you have an underpowered (pneumatic) machine, you’re many times limited as to the size, shape and shore of the pads you can use. Just because a machine can accommodate an ink cup with a large diameter doesn’t mean it can efficiently compress a pad that is suitable for transferring an image that size on a given part profile.
Pad and Part Pre-Drying
Adding low volume, low pressure air blowers to your equipment can significantly increase both speed and quality.
Figure 4: modular hose for pre-drying
Using modular hose as shown in Figure 4 to direct low volume, low pressure air at the image once it is on the pad and prior to transfer can reduce the time required for the ink to become cohesive, increasing throughput.
If you’re double printing, directing low volume, low pressure air at the part helps to dry the first layer of ink during double printing, increasing the transfer efficiency of the second hit.
If you’re printing multiple colors wet-on-wet or in close proximity to one another, directing low volume, low pressure air at the printed images can help transfer efficiency for over-prints, and will reduce the chances of the pad lifting wet ink during compression on subsequent transfers. Heated air isn’t necessary, but clean, dry air is a must. The last thing you want to do is blow wet, oily air across your pad and parts. Be sure to dry and filter the air accordingly. Modular hose kits are available from numerous industrial suppliers.
TWO Sets of Accessories
Having at least two full sets of accessories (ink cups, doctor rings, pad holders, cliché supports, etc.) available for change-over saves time and makes money. Get the old job off press and the new job on faster by having two sets of accessories, and having someone other than the press operator performing make-ready (preparing ink, clichés and pads) and clean-up.
Keeping your production environment organized helps increase efficiency and productivity. Too often time is lost looking for those tools and supplies that are necessary for pad printing.
Figure 5: work cart
Having a simple, plastic cart such as the one shown in Figure 5 available for each press, or operator, helps keep tools and supplies organized, and provides mobility for transporting ink cups and clichés to and from make-ready and clean-up areas. When spills occur, having those areas that come in contact with ink covered with newsprint or contact paper makes cleaning quick and easy.
Spending a few extra minutes cleaning up every shift can save you hours of lost productivity over the long run.
When the moving parts of machines and accessories (especially ink cups) become caked with dried ink and debris like flash and packaging materials, problems are inevitable.
For pad printing machines to operate correctly, clichés need to be clean and flat, and everything needs to be parallel. If the supports or rails that the clichés rest in during printing have dried ink and debris on them, clichés get out of parallel and clean doctoring becomes difficult or impossible. This is especially true when using thin clichés, because even an eyelash between the cliché and the plate that it rests on can cause damage once the ink cup doctors.
Most ink cups are designed in such a way so as to allow the doctor ring to “float” or flex during doctoring. Doctor rings are usually removable for cleaning, and should be carefully removed after each job (when using two-component ink) so that the rings and cup can be thoroughly cleaned. When thorough cleaning isn’t performed, dried ink causes the ink cup and doctor ring assembly to lose its ability to compensate for the physical forces they undergo during doctoring, especially when the cup or cliché changes direction.
Expecting dirty clichés, rails, cliché supports, doctor cups and rings to perform well is like expecting to obtain a smooth ride while driving a car with solid tires and no shocks down a railroad track.
Contrary to what a lot of people believe, pad printing is a science, not an art or craft. To be successful your organization needs to have someone on every shift that is willing and able to take ownership of the process. By ownership, I mean that they need to be properly trained in all aspects of the pad printing process to the point where they can identify the potential cause(s) of problems and systematically work their way through investigation and experimentation to arrive at a solution. Once they’ve found a solution, they need to be able to document it, and teach others how to recognize the problem when it comes around again, so they can fix it by themselves.
Successfully increasing pad printing productivity and efficiency isn’t difficult, or expensive, but it does require organizational direction, oversight and ownership.
Most companies that pad print, and even a lot of companies that manufacture and distribute pad printing equipment and supplies, don’t have people on their staff with the expertise to perform thorough evaluations of their pad printing process, as well as processes upstream and downstream in their facility that can adversely affect their pad printing results.
While this paper just scratches the surface, it can be used as a guide in performing an objective evaluation of your process.