Curing Common Mistakes in Pad Printing
Troubleshooting the pad-printing process requires the identification and control of associated variables. The tips presented here will help you avoid some of the persistent problems you encounter on press.
The most common mistake made in pad printing is to change several variables in the process at once. To know what to avoid, we should first understand what variables commonly cause problems. For example, if you have a problem and think that it may be directly related to a particular setting or variable, make only one change to that single variable and review the result. If the change is made to multiple variables at once, the results get confusing and you run the risk of never solving the essential problem. If the problem goes away after several changes, you still may never know what the real problem was or its related solution.
Case in point
After printing an image onto a smooth plastic substrate, the image only partially transfers with small, irregular missing parts. I look at the pad and see what missing pieces are not there. Logically, those parts were not picked up by the pad; therefore, they could not be transferred. I decide to replace the pad and to re-etch the cliche. I choose a harder pad, thinking that it would do a better job. Next, I re-etch the cliche a little deeper because I think that the etched area is not deep enough, and that is why the image has not been picked up. I remix the ink. I do not record the viscosity, but I remember to document the viscosity and the amount of thinner used. After all this, the image comes out perfect.
I have no idea what fixed the problem, but everyone is happy. The next day, while working on the same job, my operator mixes the ink and uses a different formula—mixing the ink with retarder as well. She uses the same pad and cliche that I had changed to the day before. This time the image looks terrible. The ink that is missing on the image is left behind on the pad. The operator then grabs the old pad and cliche and replaces her ink cup with her ink mix-ture in it. The image comes out perfect. What happened?
If I had changed the ink formula to something with more thinner or added some retarder the day before, I would have realized that the ink was too dry and that the deeper cliche I made helped to mask the problem. The true problem was simply that the ink was drying out on the cliche. When I deepened the etch on my cliche, I compensated for a drying problem. Changing the pad made no difference. I spent three hours messing around with possible solutions, and we ended up running the print job at a loss.
Thinking about what is happening and the cause-and-effect of what you intend to do, or have done, is the right approach. Additionally, there are usually several solutions to a problem. The question is really: How do I find the simplest, least costly solution? In this case, making a new cliche costs a lot more than simply adding some retarder to the ink.
The following list identifies some of the most common variables in pad printing. Controlling these variables prevents common mistakes from hindering your productivity.
• Pad size and shape (curvature), durometer, and material choice
• Ink composition and viscosity
• Composition and evaporation speed of thinners
• Substrate type (plastic, metal, glass), texture (roughness), and shape (curved, flat, compound)
• Cliche type (steel, polymer, aluminum), etch depth, and etching technique
• The artwork/positive, including orientation, screening, detail, image type (surface coverage), and method of printing
• Machine settings, including speeds, pauses, strokes, and positioning (pad tooling, orientation)
Pad printers often overlook the importance of pad selection and its overall effect on the success of the printing process. Too often I consult with an operator who is encountering problems with ink transfer. Common difficulties include partial image transfers, pinholes throughout or in portions of the print, and/or distortion of the text or logo. Often these problems are caused by incorrect pad selection for the particular print project.
Three major considerations when choosing a pad for a project include, but are not limited to, the geometry of the pad, the size of the pad, and the durometer or hardness of the pad. When choosing a pad, the operator must first consider the shape of the object he is printing on and the print position on that object. If the shape of the pad tends to trap air between the pad and the object being printed, then the ink will fail to transfer. Many operators opt for a flat pad, which causes this problem to occur.
One should not shy away from pointy pads (Figure 1). Ideally, an operator should choose a pad that will lead with a point or edge that will begin to compress onto the etched area of the cliche, slowly rolling out so as to prevent air bubbles from being trapped between the pad-and-ink interface, thus preventing the pad from picking up the molecules of ink and evenly pushing air out from underneath it in all directions, similarly onto the object being printed, therefore allowing the ink to release again without air separating the ink from the printed surface.
The shape of the object and the print on it will determine whether it is best to use a round or rectangular pad, an extremely pointy pad, a rooftop-shaped pad, or a gradually compounded curved pad. When setting up your print, always check for the location of the print on the pad. Try to avoid the print on the lead point of the pad where air can be trapped on the initial contact of the pad on the cliche. When there is no way of avoiding print at the point, opt for a pad with a more gradual curve in the center.
When choosing the size of the pad, always choose one that is at least 20% to 25% larger than the image being printed. If a pad is too small, your image pick up will be too close to the edge of the pad where the silicone is not as firm and where it tends to stretch more. This can cause distortion such as the smiles or frowns of the image one often encounters (Figure 2). A pad that is too large can pick up excessive ink often left on the cliche around the inkcup travel. Therefore, it is important to make sure that the image you are printing is not so large that it comes too close to the edge of the ink cup outline.
A pad that is short requires more pressure to compress. If you are trying to pick up an image that covers a large area of the pad, and especially if the pad is fairly steep (pointed), pick one that is taller. In this case, your pad will not exert too much force onto the cliche etching. Ink tends to move in the direction of pad roll out.
Higher durometer (harder) pads tend to work better than softer ones because softer pads vibrate slightly as they move back and forth on the machine. This can cause an effect known as pinholing (Figure 3).
Harder pads tend to trap less air due the pressure they exert onto the etched surface of the cliche. They also allow the image to be transferred off the pad surface more readily, thus allowing for crisper images with sharper definition. A simple rule of thumb: As the pad gets softer, it should be pointier. When printing onto rougher surfaces, harder pads tend to drive the ink into the nooks and crannies more readily and thus prevent the image from looking pinholed. Always review the image on the pad before and after the print.
The ink and substrate must be compatible and adhesion should be suitable for the product under print. Often an ink with hardener is used when adhesion requirements are not that high. Inks that have hardeners added may be tossed out and replaced once or more per shift, therefore unnecessary wasting hardener (catalyst). This can waste money. The reverse is true as well. Do not get a job rejected because you used a single-component ink on a glass substrate that commonly requires an ink with hardener added.
Ink/thinner ratios should be ad-justed to match ambient environmental conditions. Many pad printing inks come with standard recipes on the label: add 25% thinner, etc. Proper ink mixing is key to successful pad printing. Following simple documented recipes can really save time.
Thinners flash (evaporate) more quickly in warm humid climates and slower in cool dry conditions. It is better to mix ink to a specific viscosity. This requires seasonal adjustments in thinner/retarder content, but results in a consistent ink mixture (and consistent printing results) throughout the year. If you were to follow the standard recipe on the ink container, ink could dry up prematurely on a hot day and remain too wet to transfer properly on a cold day. Documenting mixes that achieve the best results under specific conditions saves time spent searching for the ideal mixture–especially if you have found it once before. Moving pad printers into the climate-controlled area will be a money saver in the long run.
There are many tools available to mix and maintain a specific viscosity, from an inexpensive hand tool used as a gauge during mixing to an in-inkcup sensor that can measure and control viscosity automatically.
Choose between fast, medium, and slow-drying thinners (also referred to as retarders). Combine them as needed, especially in multicolor prints on a shut-tle conveyor where final colors tend to dry out before the transfer is complete. We often use a fast-drying thinner for the first print stations and a slower-drying thinner for the latter. This is not a concern with elliptical racetrack or over/under conveyor styles, where fresh ink is brought to the pad with each cycle advance.
Chemistry plays an important role in pad printing. If it is a plastic, is it an inert material, such as a polyolefin, nylon, or polyester? Does it need to be pre-treated with a corona or flamer or post-treated in a heat tunnel? Familiarize yourself with the idiosyncrasies of the materials involved. Not all plastic is identical.
Treat metal, glass, ceramic, and other substrates with respect and find inks and thinner combinations that are specific to those substrates. Contamination control is just part of the process. There may be additives present that will leach to the surface, or the pad may have been sprayed with a mold-release agent.
If you are printing on a plastic part that is painted, you are not really printing directly onto plastic. The surface that contacts the ink is the substrate. Wood that is painted with polyurethane is a polyurethane substrate and ink adhes-ion is very different on raw wood versus urethane.
Always test print by producing a number of samples prior to production. Run tests as required by your customer to guarantee proper adhesion (Figure 4).
Cliche and support-table inspection and cleaning
Clean the underside of the plate as well as the plate support. Even tiny amounts of debris can affect print quality, eventually damaging your cliche. Consider the concealed areas—the underside of the cliche and the support on which the cliche sits. Even something as small as a grain of dust lodged under the cliche is enough to cause a bump on the surface. Bumps can lead to pooling marks, which transfer to your part.
One of the more common occurrences of pooling (Figure 5) stems from ink build-up on the cliche leaking over the edge and seeping into the gap bet-ween the cliche and support table. A typical symptom of this problem is phantom marks transferring to the printed parts. These marks return almost immediately, no matter how frequently the press oper-ator cleans the cliche’s surfaces. The operator should also check the underside of the cliche, as well as the corresponding support table, as built-up ink dries under the cliche surface and creates a speed bump, which results in unwanted ink on the surface. The offending debris is almost always discovered and, after removal, normal and smooth print quality returns.
Printing larger images
One of the challenges pad printing en-counters is transferring a quality image when printing a large, open area. As the cup passes over the open area, the ring tends to scoop out some of the ink from the etched area, resulting in less ink being picked up and transferred (Figure 6). The print will then appear faint in those areas. Several steps can prevent scooping and improve print quality.
1. Select the correct line screen and halftone pattern. The halftones in the etched area help prevent the ink from being removed as the inkcup passes over the plate. Increasing the dot size and arranging dots in cross patterns prevents much of the ink from being removed. Most pad printers choose either a 300-line/in. 90% or 200-line/in. 80% screen when creating the cliche plates. This allows for a deeper etch with the peaks still supporting the cup.
2. Align the registration of the artwork. Ensuring that the shorter side of the image faces the operator is a technique that works with some graphics. For example, when printing an image that is rectangular, register the image so that the ceramic ring travels in the direction of the longer side.
3. When possible, add lines or text to break up the open area to help prevent scooping. An open circle often presents problems.
Using excessive pressure during ink pick-up from the cliche and downward force of the pad on the object is a common mistake. Decrease ink pick-up and print pressure to the minimum force needed to pick up the complete image and transfer it completely. This prevents overstretching the pad and smudging/squishing out ink during pick-up or printing.
Check the pad after its pick-up stroke to see if it picked up the complete image off the cliche. Many times the cliche stroke is too short and the extremities of the image are incomplete. Use a mirror to see if the image is complete. In the case of rough substrates, you may need to reconsider the rule of minimum force, as the down stroke on the print side may have to be excessive to improve print quality. The down stroke on the cliche should be as conservative as necessary to pick up the complete image. Center the image on the pad—that also helps to eliminate the need for excessive pressure in the down stroke.
If your ink is drying too fast, could you speed up the machine? Know what your equipment is capable of in terms of programming and use those features. Simply adding a pause before the print phase often helps to cure image-transfer problems caused by a deeply etched cliche or ink that is a bit thin. Whatever you do, be sure to document press settings.
The single most expensive mistake pad printers make is not thinking about problems and the possible solutions. The second biggest mistake is not asking for help when needed. Both of these can quickly become quite costly.
Julian Joffe is CEO of East Dorset, VT-based Pad Print Machinery of Vermont.