Pad Print Process: Technical Papers

 

 

Controlling Static Electricity

Winter weather provides problems for pad printers, especially in those areas of the country where the low humidity associated with winter produces static electricity. Static problems are most prevalent when humidity is low, because the moisture on the surface of many materials we print makes a good electrical conductor. When humidity rises, so does electrical conductivity, thus reducing static.

There are many things that you can do to help eliminate static problems. The plant wide approach to controlling static means installing humidity and temperature control systems. This will not only control the static problem, but will serve to minimize the effects of seasonal changes on the entire process by also controlling temperature. Press setup can be reduced, because the evaporation of the solvents from the inks will be much more uniform and predictable.

Short of an expensive climate control system, a standard household type cool air humidifier can help. If your printer is in the middle of a big, dry room, you may need to make a simple enclosure with plastic to make the humidifier more effective. I recommend getting a little hygrometer/thermometer and keeping it near the printer to keep track of conditions. You can find these at a hardware store. Ideally, you want to be between 68 and 72 degrees Fahrenheit with 45-55% humidity. Static starts to be a real problem below 20% humidity.

Remember, moisture on the surface of the substrate makes static worse. Make sure that you allow the substrate time to acclimate to the temperature and humidity of the press room before beginning production.

Slowing down the printer can also help reduce static problems. Printing at high speeds can generate a static charge on the cliché, which can transfer to the pad, which can then transfer to the substrate. By running more slowly, the electrical charges around all the machine's moving parts are held more stable.

There are two static reducing additives for inks distributed by Service Tectonics:

1. 100-VR-1241 Special Anti-Static Solvent can be added at 1-5% by weight. It is available in liters for $38.00.

2. 180-PE-0013 Anti-Static Gel/Thixotropic Raster Paste can be added at 10-20% by weight. It is available in kilos for $47.00.

Contact Service Tectonics at 517-263-0758 to order.

Other methods of controlling static include grounding, using anti-static coatings or cleaners, and ionization.

Grounding the charged material (substrate) is difficult because bringing all areas of the charged material close enough to contact the grounding object ( a metallic nesting fixture) is not always an option. Grounding the machine is only effective if the material being printed is conductive.

Anti-static coating and cleaners can be applied to the substrate prior to printing. These are usually chlorinated hydrocarbon solutions that cling to the substrate, reducing its surface resistance, thus reducing static. These can be expensive, and can require repeated applications to the substrate for each printing operation.

Ionization involves removing electrons from oxygen molecules called ions. These ions attach themselves to the substrate to neutralize static. Ionized air can be created by using air knives, which create a shower of ionized air under which the substrate passes prior to printing. Air knives require a power source. They can be obtained from a static control equipment manufacturer.

Ionizing air nozzles can be attached to a compressed air line for the purpose of blowing the substrate off prior to printing. Some of these also require a power source. Nuclear neutralizers use radio isotope emissions to ionize the air without a power source. These are not harmful to the operator, and are effective provided you can get close enough to the substrate for them to work (anywhere from 1 to 1.5 inches). These can also be obtained from a static control equipment manufacture.

 

 

Monthly Newsletter

 

COLD WEATHER ADVISORY!

Are you pad printing with a two-component ink system, then immediately shipping the printed parts to your customer? If you are, and you or your customer are in an area of the country where the temperature is near or below freezing this winter, be advised.

Two-component inks may be dry to the touch within seconds of printing, however, they may not be fully cured for up to 72 hours. When certain types of two-component inks are subjected to extremely cold temperatures prior to the completion of their manufacturer's recommended cure schedule, the chemical reaction that cures the ink may stop. In most cases, once it stops, nothing can restart it. This means that if you are printing and shipping immediately, and your parts are subjected to the cold while they are sitting in a truck or warehouse, the ink film may not ever achieve the desired adhesion and/or mechanical and chemical resistance.

To be safe, we recommend that you submit some sample parts to cold temperatures similar to those involved with storing or shipping, then test them for adhesion and resistance. You can either do this by shipping the samples to your customer to have them tested, or by simply putting a few in the refrigerator or freezer overnight and testing them yourself.

If you can't avoid having to ship parts prior to the completion of the inks recommended curing schedule, the use of a hot air drying system may sufficiently accelerate the chemical reaction. Here again, we recommend that you cold-test the hot air dried parts to ensure ink performance.

In conclusion, if your customer is seeing performance problems, it may not be you or the ink. It may be the weather.

 

 

Pad Printing Custom Colors

 

There are many variables involved with correctly reproducing custom colors with pad printing inks. For a color match to be correct, it is necessary for the person color matching to have access to information regarding the following variables:

1. The exact color you wish to match, whether it is a P.M.S. (Pantone Matching System) color, or a proprietary color master.

2. The color(s) and surface characteristics of the substrate you will be printing on.

3. The number of times you intend to print the color on the substrate.

4. The type or series of ink you intend to use, whether or not you intend to use a hardener, and if so, at what ratio.

5. Whether you want the dried ink film to have a gloss or matte finish.

Let's review why all of these variables are important for proper color matching.

THE COLOR MASTER

The color(s) you wish to match will either be a color chosen from a color matching system or a proprietary color that is already printed on a part or color chip. The most popular color matching system in the United States is the P.M.S. or Pantone Color Matching System. While this system offers a lot of colors to choose from it does have a number of important limitations.

One problem with the P.M.S. color matching system is that all of the colors are printed on white paper. This is an important consideration, since chances are you won't be pad printing on white paper. P.M.S. colors are also classified as being either a "C" for being on coated, or "U" for being on uncoated paper, resulting in it being either a glossy (coated) or matte (uncoated) finish. Both the color and gloss (or texture) of the substrate have an effect on the appearance of the dried ink film once it is on your specific substrate. Therefore, the exact reproduction of some P.M.S. colors on your substrate is not guaranteed. It may be necessary to first print white on some dark substrates prior to printing bright or florescent colors.

The same holds true for proprietary colors. The color master for proprietary colors may be represented by a spray painted color chip, or some other type of color master that was produced using pigments, applications processes or substrates other than the one you will use in production. All of these variables can have an effect on the appearance of the dried ink film.

As a rule, the more closely the materials and process used to generate the color master parallel those used in the pad printing process, the better the chances of obtaining an acceptable match.

SUBSTRATE COLOR AND SURFACE CHARACTERISTICS

The color of the substrate is important because, as I've already mentioned, it will have an effect on the appearance of the color once it is printed and dried. Pad printing deposits a very thin layer of ink (about 20% of the etch depth). When we look at a color we are "seeing" the light that reflects back off of the ink film and substrate. Since the ink layer is thin some light makes it all the way to the substrate and back through the ink to our eye, resulting in our eyes picking up some of the substrate's color. Furthermore, the texture of the substrate has an effect in that different amounts of light are refracted or directed in directions other than back to our eyes. For this reason, it is necessary to take the color and texture of the substrate as compared to the color master into account when color matching.

NUMBER OF PRINTS

The number of times you intend to print is important because it determines how much ink you'll be putting on the substrate, and thus how much the substrate's color will effect the perceived color of the dried ink film.

INK TYPE AND GLOSS REQUIREMENTS

Finally, the type or series of ink you intend to use and whether you want the dried ink film to have a gloss or matte (flat) finish is also important. Ink formulations use different resins and pigments that can effect the finished appearance, as can the addition of catalysts (hardeners) and/or matte agents. For example, the addition of a catalyst that is clear can "dilute" the color, and it can also increase the gloss level. Since some inks require catalysts and some don't, it is helpful to know which type you intend to use so that the color technician can add the required amount of catalyst to the formulation during color matching.

The only other major variable is the amount of thinner that you will eventually add in production. The more thinner you use, the more the ink is "diluted", and the more transparent it can appear on the finished part. In most cases, however, the amount of thinner will not have a major effect on the finished color.

Keeping these variables in mind can eliminate a lot of surprises in correctly reproducing colors with the pad printing process.

 

Pad Printing Textured Surfaces

 

Textures vary according to their individual depth, the degree of draft on the side-walls, and in the frequency of peaks and valleys for a given surface area. Some textures are more difficult to successfully print than others, and there are certain textures that are simply impossible to completely cover.

In an attempt to find a correlation between these variables and "print-ability", I printed a series of sixty-odd black, "visual texture standard" plaques (provided by Mold-Tech). The plaques were molded out of ABS (Acrylonitile-Butadiene-Styrene). I used an automotive approved, two-component, white ink thinned 15% by weight, a steel cliché with an etch depth of .001", and a 60 durometer (Shore scale A) transfer pad. Each plaque was single printed at one end and double printed on the other, then allowed to dry per the ink manufacturer's recommendations.

Each single and double printed image was visually inspected for coverage under a uniform, non-directional (unfocused) light source at a distance of 18" for approximately 10 seconds (per Ford Motor Company visual inspection procedure for automotive interior parts.) Acceptance or non-acceptance was determined by the presence of any visible defect or void resulting from insufficient coverage of the texture.

What I found was that the frequency of the texture played a significantly larger role in achieving an acceptable print than the depth of the texture, or the angle of the side-walls. One texture having a depth of .0055" and 8 degrees draft was successfully single and double printed, whereas another texture that was only .0015" with 2.5 degrees draft couldn't be successfully printed at all. The difference was that the number of peaks and valleys in the texture within the image area. The obvious conclusion: the higher the frequency, the lower the likelihood of acceptance.

The data from my experiment only covers a limited number of known textures in a sea of millions. Unless you're considering printing a part that came from a mold textured by Mold-Tech, it doesn't help much. So how can you determine if your texture is printable? Print it yourself.

In experimenting, it is important to use a machine, cliché, ink formulation, transfer pad and cycle time that accurately represents what you plan to do in production. For example, a machine that generates more compression than the machine you intend to use for production may compress the pad further into the texture, resulting in an acceptable print that your production machine can't recreate. The type of cliché, the size and depth of the image also need to be as real as possible, as does the ink formulation (i.e..: ink:hardener:thinner ratio). Finally, use the same pad shape and durometer as you'll use in production to conduct your test. Setup your test machine to run with production settings for speed and compression. When starting out, set your compression on the cliché and the substrate at the minimum amount necessary to pick up and transfer the image.

If you can't get an acceptable print using the process you prefer, there are several things you can change. The easiest thing to try is a harder pad. Contrary to what logic would dictate, a harder pad penetrates the texture further than a soft pad does before the ink releases.

If a harder pad doesn't do the trick, try slowing the speed with which the pad compresses on the substrate. This can make the displacement of the air in the valleys of the texture more efficient, leaving less potential-pinhole-causing air under the ink film. Allowing the pad to dwell on the surface of the substrate for a few seconds may achieve the same result. Some machines, such as those having a programmable stepper motor, can be programmed to do this. Other machines require that the pad be over-compressed on the part, resulting in the machine actually stalling out. This can result in image distortion or worse yet, undue wear and tear on the machine, the pad, and the part being printed. In the event that the pad you're trying doesn't work regardless of what durometer you use, or how you compress it, perhaps you need to experiment with one having a different angle. In this case, I have found that the steeper the angle, the better.

On finely grained textures I have found that if you fail to cover the texture with a single pass, the chances of covering it with a second pass aren't very good. This is because the thickness of the ink layer that you lay down only makes the voids (or valleys) that much deeper than they were initially. When this occurs, I usually revert to "bridging" the texture, rather than continue trying ot fill it in.

Bridging textures can be achieved by changing how the ink film releases from the pad. If you are lucky, this can be achieved by simply using less thinner, or by slowing down the machine so as to allow more solvents to evaporate from the ink film while it is on the pad. This increases the tackiness of the ink, making it leave the pad in favor of the substrate sooner. When the ink releases sooner, it adheres more to the peaks and less down the side-walls and into the valleys of the texture. In the event that you can't sacrifice the speed, you can try directing some low velocity airflow to the surface of the pad in between image pick up and transfer. The increased airflow accelerates evaporation of the solvents. If you're double printing, you may also wish to direct some air at the surface of the part to dry the first hit a little before the second pass.

Bridging textures can result in the dried ink film having less mechanical resistance, especially when the texture is a deep one. Since the ink is really only adhering to the peaks of the texture, there are tiny voids under the ink film in between Ink that becomes brittle can fracture at these points more easily. Therefore, it is important to keep the end use of the part you are printing in mind when you are deciding to bridge or not to bridge.

Finally, some textures are just plain impossible to completely cover. In these cases it is necessary for the texture to be modified to make it printable. This typically isn't up to the people that have to do the printing; it is up to the people manufacturing the parts. If you've conducted some of the simple experiments outlined in this article you'll be better able to communicate the reasons why the texture in question isn't printable, and your findings will undoubtedly be an asset in determining what the texture should be.

 

Pre-Treating Plastics for Pad Printing

 

Certain types of plastics require pretreatment in order to obtain optimal ink adhesion. Plastics from the polyolifin family are among those that require the most extensive treatment; with the most popular members of the family being polyethylene and polypropylene.

The molecules of polyolifins are linear, and thus have no concentration of positive or negative charges at their ends. The coatings, inks, dyes and adhesives that need to be applied to the plastic are polar, adhering through the attraction of unlike charges. Since polyolifins are non-polar, the inks molecules cannot adhere.

Translation: You know the last time you waxed your car? When you sprayed water on the hood for washing it "wet out", or sheeted over the surface. This would indicate that the surface had a high surface energy (or was polarized). Once the hood was waxed the water beads up. This would indicate that the waxed surface had a low surface energy. Polyolifins are to pad printing ink what a waxed car hood is to water. To change that condition, we pre-treat using one of three methods: liquid primers, flame treating, and corona treating.

Liquid primers are blends of aggressive solvents such as toluene and xylene that slightly "bite" the surface of the plastic while cleaning it. The most popular method for applying a liquid primer is to wipe it on with a clean rag. (Preferably one that does not leave a bunch of fibers behind.) If the parts are dirty or have contaminants such as mold release it is necessary to use a different part of the rag for successive parts and to discard the rags regularly to avoid simply transferring contaminants from one part to another. The effects of liquid primers do not last long... two to maybe four hours max. After treating you should avoid touching the print area. Make sure you have sufficient ventilation and wear proper protective clothing (chemical resistant gloves, goggles, and in the absence of good ventilation, an organic vapor respirator may be necessary).

Flame treating is probably the most popular form of pretreatment. A carefully controlled flame burns contaminants off the substrates surface while significantly increasing the materials ink receptiveness. Proper burner design and treatment times are essential in ensuring that the surface is consistently treated. Too concentrated a flame pattern, or too much time, and the substrate can be irreversibly damaged. The effects of flame treatment can last from several hours to months, depending on the specific type of plastic. Once again it is important not to touch the print area after it has been pretreated.

Corona treatment (also referred to as plasma, gas plasma or suppressed spark) could be compared to hitting the substrates surface with a controlled lightning bolt. The treatment of the surface results from bombardment and penetration of ionized particles into the molecules on that surface, polarizing the molecules and increasing ink receptiveness.

The actual bombardment of the substrates surface can be achieved several ways. Thin flat sheets, films or parts can be place between an electrode and a ground, so that when the charge travels across the gap it treats one or both sides. This is commonly what people mean when they use the generic term "corona treatment". Parts having irregular surfaces such as balls, cylinders and blow molded containers are treated by means of plasma (a.k.a. gas plasma), where the charge is actually blow onto the part with forced air. Finally, running them through a suppressed spark environment, which is essentially a corona filled tunnel, can treat parts that require "all over treatment". The method used to apply corona discharge is thus dictated by the nuances of each particular application. The effects of corona treatment can last months, however the majority of treatment degradation takes place within 48 to 72 hours, so printing should be accomplished within that time frame whenever possible. In most cases the treatment equipment is attached to the printer or part feeding system, so this is rarely an issue. Remember, do not touch the print area between the time it is treated and printed.

How can you tell if a part requires pretreatment, or if the treatment was successful? A water - wet test can be easily be conducted in the field. Untreated polyolifins have no affinity for water (just like the waxed car hood). Once the material to be tested is free of dust, dirt or other surface contaminants it should be immersed or flushed with water (preferably distilled) for at least three seconds. The part should then be removed from the water and placed in a vertical position. The length of time the water remains on the test area is indicative of the degree of surface tension. If it all runs off quickly, it requires treatment. If the water forms a consistent film over the entire test area and remains there for 5 to 10 seconds it is probably going to accept ink. A very high level of treatment can result in the film of water remaining intact until it completely evaporates.

Another test that can easily be conducted is to use Dyne level test pens. These pens contain a mixture of liquids and dyes that have specific surface tensions. You simply swipe the pen across the surface and observe whether it wets out or beads up. For pad printing the minimum surface tension should not be below 38 Dynes / cm sq. (42 is preferred.) These pens are available from corona treatment equipment manufacturers and laboratory supply companies.


 

Quality Artwork for Cliché Making

 

What is quality artwork? Quality artwork is the cornerstone of any printing process. Usually, it means a solid black image on a clean, white or clear background. The graphic arts industries refer to such artwork as being "camera-ready". The term "camera-ready" comes from the pre-computerized days when all artwork had to be photographed with a process camera in order to generate a film positive.

If you are sending artwork to Service Tectonics, Inc. for use in making clichés, it must be camera-ready. If possible, print your artwork out on a laser printer two to four times size, and with the highest possible resolution. Having the image two to four times size and at the highest possible resolution ensures that the image quality will be at its best once it is shot down to actual size on camera, since any defects in edge quality will be minimized. A bubble jet printer may work if you use the best possible resolution, but a laser printer is better.

Examples of artwork that are not camera-ready include: photocopies, facsimiles, business cards, or CAD plots. Photocopies and facsimiles do not provide an image that is solid enough to be photographed without voids. Business cards are generally printed on paper that is textured, and logos are usually too small. CAD plots usually don't have the images filled in solid, and the line weights are generally too small. In applications where this is all we get it is necessary to scan the image into the computer and redraw it, which takes time, and thus costs you money at a rate of $50.00 per hour.

Electronically transferring art files via the Internet can be a hit or miss business. It is not always possible to download files without discrepancies due to the possible incompatibilities of our system and the one you are using. We use Corel Draw 8.0. Using this program we can open or import many types of files. To ensure that your file can be opened correctly it is a good practice to contact Bob Thompson in the art department to ask him which formats he can use. It is also a good idea to then send a copy of your artwork via facsimile to Bob for comparison, as it is not unusual for little details to get lost or misinterpreted by different programs, regardless of what format they are transferred in on.

In addition to quality artwork, proper cliché making requires a layout. Whether you are mailing or electronically transferring your artwork to us, it is useful to have a layout showing the location and orientation you desire. In the event that we do not receive a layout with the artwork, we will generate one and fax it to you for approval prior to etching your cliché, just to make sure we are putting the image exactly where you want it.

Following these artwork guidelines serves to aid everyone involved in meeting the requirements of you and your customers.

 

 

Warm Weather Printing Problems

 

With summer's higher temperatures and humidity come ink related printing problems. Higher ambient temperatures cause inks to dry more quickly, and high humidity can cause surface tension problems. Since the entire pad printing process revolves around being able to control these ink characteristics, knowing how to compensate for changes in temperature and humidity becomes necessary.

There are a number of ways to compensate for higher temperatures. For those who prefer to stick with one thinner year round, it is necessary to increase the amount of thinner, or decrease the amount of time between image pickup and transfer by speeding up the machine.

There is a fine line to negotiate here, since too much thinner can cause the print quality to deteriorate significantly. The image generally becomes fuzzy or blurred, and colors look washed out, or less opaque. If you cannot obtain an acceptable print with a combination of more thinner and a faster machine speed, it becomes necessary to go to option two, which is "slowing" the ink down. I'll explain how this can be accomplished after I explain the difference between thinners.

Most ink manufacturers offer more than one thinner for an ink series. You usually have a choice between a "fast", "medium" and "slow" speed thinners. When people refer to speed they mean evaporation rate. For example, if you took equal amounts of fast, medium and slow speed thinners and poured them into puddles having equal surface areas, the fast thinner would evaporate first, followed by the medium thinner, and finally the slow thinner. You also hear the word "retarder" frequently. A retarder is simply a "very slow" thinner.

Slowing your ink down can be accomplished by either using a slower evaporating thinner in place of your current thinner, or by mixing the two together. For example, let's say you are using a medium speed thinner at 15% by weight and you're still having problems with the ink drying on the pad before image transfer. If your ink has a slower thinner you might try 10 or 15% of that thinner by itself, or if that is too slow, perhaps 10% medium with 5% slow thinner mixed together. This way you can slow down the ink's drying rate without sacrificing image quality. If you're not sure, check with your ink manufacturer or distributor to see if there is more than one compatible thinner for your ink, and if they can be mixed.

Humidity can also cause problems. When the relative humidity is high, everything gets water on it. Even though you can't see it, there is water on your pad, cliché, ink film and substrate. Water and pad printing inks don't mix. The ink loses its ability to flow out into a smooth film, and its surface tension (tackiness) becomes inconsistent over the surface area exposed to the air. As a result, you begin to see areas of the image "drop out", which means they either fail to be picked up by the pad, fail to transfer to the substrate, or both. To the untrained eye, this condition looks a lot like ink that is insufficiently thinned. While in some cases the addition of thinner can compensate, it is often safer to either eliminate the water vapor by introducing a low volume of heated airflow (a blow dryer) to the pad, or by increasing the ink's ability to flow by using a flow additive, such as a thixotropic paste.

Thixotropic paste is commonly used in four color process printing to prevent changes in dot sizes brought on by a lack of flow. (In addition to increasing the flow characteristics of the ink it can also alleviate static problems, which makes it equally useful during the winter, when relative humidity is too low.)

High humidity can also make the ink "spiderweb". Spiderwebs are those tiny, stingy little wisps of ink that protrude from the edge of the print. These form when the ink is beginning to get too dry, when static is present, and when humidity affects flow. Spiderwebs can sometimes be eliminated by slowing down the speed with which your pad lifts off the cliché in image pick up, and off the substrate in image transfer. What happens when the pad lifts too quickly is this: the outside edge of the image is the last area to adhere to the pad during image pick up, and the last area to transfer during printing. As the pad compresses, air is pushed out the the way and the ink adheres to the pad or substrate. When the humidity is higher, it results in water condensing on the ink film and pad. This condensation acts as a "barrier" to the displacement of the air by the pad, and it makes the ink resistant to flow. If the pad lifts away too quickly (before the ink film can overcome the affects of the increased humidity), the ink still wants to stick to the pad, stretching as the pad lifts away and ultimately breaking to form spiderwebs.

If you're printing the same job all year round, I recommend collecting some data to make ink speed adjustments a breeze. If you don't have a scale for weighing ink and thinner, a thermometer, and a hydrometer (for measuring relative humidity), go buy them. Then, each day, record the temperature, relative humidity, and the amount of thinner (or blend of thinners) you have to add to achieve an acceptable print. After a while, you will gather enough data to make what I call an "Ink Mixing Matrix". By making a chart with the relative humidity being one axis and temperature being the other, and then filling in the points where the lines intersect with the amount of thinner you had to use, you create a visual aid for thinning your ink. Once the Ink Mixing Matrix is created, all the operator needs to do is check the temperature and humidity, refer to the chart for the correct amount of thinner (or blend of thinners), and add it to the ink. Presto!

Dealing with fluctuations in temperature and humidity can be frustrating. By experimenting with different thinners, additives and machine speeds, and gathering some information, you can learn to control your process accordingly.

 

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