by John Kaverman
The manufacturing of polymer clichés has become more and more popular for pad printers of all types. Shorter lead times and the need to control costs are two big reasons the market has seen this increase. As with everything else, polymer cliché making has its challenges. However, if the right preparation and equipment are used, it can be a very successful endeavor.
Most challenges people experience are the result of one or more of the following:
- Cliché material
- Film density and orientation
- Line screen
- Exposure equipment
Manufacturing polymer clichés is much like baking a cake. Think of the cliché material, developer, film and line screen as the ingredients, and the exposure equipment as the oven. If you have bad ingredients to start with, not even your dog will eat it. If your oven is too hot, or not hot enough, you’ll end up with either a hockey puck or a cake pan full of lumpy pudding. The same goes for making polymer clichés. If you have a bad developer, expired or “stressed” cliché materials, poor film density, the wrong line screen, or a deficient exposure unit, your cliché won’t turn out any better than your cake.
Fortunately, a series of simple tests exist that you can perform to help you identify potential causes and remedies in your cliché making process.
Depending on the specific type of cliché material you use, you will probably be developing with either water, alcohol, or a mixture of both. More specifically, you will develop with either distilled water, or denatured ethyl alcohol diluted with distilled water.
Tap water is not the same as distilled water. If a recipe called for baking powder, would you add powdered sugar just because it looks the same? You never know what is in tap water. Municipalities add chlorine, fluoride and other chemicals that may not be compatible with the cliché material. Distilled water will be much more consistent for developing the cliché.
Alcohol developing solution is best purchased from your cliché material supplier. You can buy a gallon of “Denatured Alcohol Solvent” from the local hardware or building supply superstore, but how do you know what’s in the can? The chemicals or amount of chemicals have been known to be inconsistent when you purchase it from a retail store. Your cliché supplier will have the right consistency for your application. I can’t tell you how many times I’ve had someone call me after they have been making clichés for two years to complain about what has to be a “bad batch of cliches,” only to find that they ran out of developer and had to use “stuff from the hardware” instead.
One final word regarding developer – it works better warm than it does cold. Use water or alcohol that is at or above room temperature.
To test your developers, simply develop an unexposed piece of cliché material for the recommended time. If the polymer comes off all the way down to the adhesive layer that holds it to the steel backing material, your developer works fine. If it doesn’t develop, either your developer is weak, or your cliché material is expired.
Testing Cliché Material
Cliché material typically has a shelf life. Most manufacturers will recommend storing it in a cool dry place. Prolonged exposure to varying temperatures and periods of extremely high or low humidity can damage clichés. Clichés are light sensitive and can easily be exposed by accident, so be sure to work with them under appropriate lighting conditions, i.e. lighting that is free of ultraviolet wavelengths. Fluorescent office lights give off UV light, so you should always shield them or use an alternative light source.
Cliché materials vary significantly from brand to brand, and between grades from the same manufacturer. Be advised that different materials may have different exposure times. If you can tell your supplier a little information about your exposure unit’s light source (number of bulbs, wattage, distance from cliché) they should be able to give you a well-educated guess of where to start exposure times.
The test for determining if your material is expired or has been accidentally exposed is the same test as you used above for your developer, i.e. if the emulsion comes off in the recommended time your cliché is good.
Testing Film Density
Film density is a killer. Everyone wants to use laser-printed overhead transparencies, vellum or some other alternative that costs less than “real film”. A laser printed film media can work for a high percentage of applications, but a piece of “real film” will normally yield the best result. The first important thing is that the image density is sufficient. To test density, expose the cliché with the film only for the recommended time, and then develop it. Using a magnifying glass (10X), look in the image area. If you can see areas where the emulsion is still in the image area, chances are these areas are not dense enough on your film.
Testing Film Orientation
Film orientation is the other film related culprit. The image has to be on the side of the film that comes in contact with the cliché during exposure. The correct terminology would be “emulsion down film positive,” which means that when you can read or see the image as you wish it to be etched into the plate, the emulsion is “down” or on the back side.
The emulsion side of the film is almost always dull, while the base side is usually glossy. Some film may be slightly matte on the emulsions side. (Matte film is recommended for better evacuation of the air when the vacuum draws down in the exposure unit.)
If you aren’t sure, and there is some emulsion on a non-image area of the film, scratch it with a film knife or scissors. The emulsion will scratch off. Once you determine what orientation the film needs to be in for exposure, it is recommended to cut a little piece of the upper right hand corner of the film off for future reference.
Having your film orientation wrong will usually result in loss of image resolution during exposure. This is especially noticeable in the more finely detailed areas of the image.
Line Screen Film
If you’re using a cliché material that requires a line screen exposure, the manufacturer or supplier should recommend a couple of line screens. For most applications, 120 line/cm, (300 line/inch) 90 percent dot is a good choice. If you want to transfer a little more ink, you may want to consider using a 100 line/cm (250 line/inch) 90 percent dot. If you are wanting to be on the edge of transferring too much ink, such as when printing white on black, 80 line/cm (200 line/inch) 90 percent dot is recommended.
Regardless of the film you use, the important thing is to get the line screen film on the cliché in the right orientation, which is emulsion down. Like the film that holds your image for the first exposure, line screen film is glossy on the base side and dull on the emulsion side.
Keep your line screen clean and free from scratches and kinks. If you damage a line screen, replace it.
Testing Line Screen Film
If you are using line screen film, the best way to know if your exposure time is correct is to perform a step test, using 15 or 30 second intervals. After development and post-exposure/drying, gently draw a fingernail over the image area. If the line screen exposure is correct, the peaks of the dot pattern should be roughly at the same level as the top surface of the cliché. You should be able to hear your fingernail scraping across the rough surface, but you should not be able to catch your fingernail on the edge of the etched image. If you can catch your nail, it means your cliché is too deep, and that your line screen exposure time should be increased.
Using 10X magnification, observe the non-image areas of the cliché. If you can observe a very shallow dot pattern over the entire surface it indicates that your first exposure with the film was not sufficient.
Testing Exposure Equipment
People tend to overlook several equipment related variables, including the power output of the light source, vacuum blanket, and the vacuum pressure. UV light bulbs have a pretty specific lifetime.
Depending on the manufacturer, the power output of the bulbs drops off significantly somewhere between 400 and 700 hours. If your exposure unit has an integrator that senses the output and adjusts exposure times accordingly, you’re fine. If not, then you should know the life expectancy of the bulbs you have and change them on a consistent basis. Generally, the lower the wattage, the better control you have. It is recommended to use 15 or 20 watt bulbs. If you use 40 watt bulbs the power output is not double that of 20 watt bulbs, but rather the square of the output of 20 watt bulbs. This is why people trying to make clichés with a 1000 watt metal halide (screen printing exposure unit) light source have so many problems with some of the higher quality (more sensitive) cliché materials.
Vacuum blanket materials can also have an effect on exposure. For example, if your vacuum blanket is frosted vinyl it will absorb more UV light than clear glass, so your exposure time will need to be adjusted accordingly. If your blanket gets block-out pen, tape, or permanent marking all over it, you should replace it.
Vacuum pressure should be sufficient to eliminate air from being trapped in between the film or line screen film and the cliché surface during exposures. Normally, when people have sufficient density yet lose detail, or have varying etch depths, poor vacuum is the cause of the problem.
You can periodically test your exposure equipment by exposing a cliché with a test film (having sufficient opacity), using the same exposure time each time. That exposure time should be sufficient to result in a developed cliché having a depth all the way to the adhesive layer of the cliché material. If you use the same film and exposure time each time you perform the test, it should always wash out to the adhesive. If it doesn’t, it means that your light source output is deteriorating.
In sum, the tests that we have discussed will help you pinpoint the root cause of polymer cliché making problems. Remember, polymer cliché making is a process, not a craft or an art. Using a systematic approach, you can troubleshoot and successfully correct most situations.
John Kaverman is National Sales Manager for Innovative Marking Systems. Kaverman, who holds a degree in printing technology, has over 14 years of screen and pad printing industry experience. For more information on cliché making, contact Kaverman via e-mail at firstname.lastname@example.org.