by John Kaverman, Tampoprint
Q: What purpose does the line screen serve in cliché making?
The line screen serves three purposes: to support the ink cup’s doctor ring, to control the volume of ink and to give the ink some resistance.
To support the ink cup’s doctor ring so that it doesn’t “dip” ink out as it passes over the image area.
Most manufacturers use magnets to create the hermetic seal between the ink cup and cliché. When the magnet force is too great, the cliché can be pulled off the base plate, causing defection and ultimately resulting in uneven doctoring and inconsistent ink film thickness. The line screen helps minimize this by supporting the doctor ring as it passes over the image area.
To control the volume of ink within the etched image.
It is not advisable to alter exposure times in an attempt to make the etch deeper or hold more ink.
The proper way to adjust ink volume is to alter the lineage of the screen. Typically, 120 line/cm is used for the vast majority of images. For large text and images, 100 line/cm is used. 80 line/cm screen is used on extremely rare occasions, such as when the operator is printing over an extremely rough or porous texture.
To give the ink some resistance to the physical forces of the pad during pick-up.
The pad should “roll” outward from its point or ridge as it is compressed onto the surface of the cliché during image pick-up. If pad location and compression are not optimal, the force of the pad can “squish” ink in the direction of the roll within the image area. Line screen gives the ink some “traction” so it can better resist the forces of the pad.
Q: Why is it important to post-expose and dry my cliché prior to set-up?
Post exposure hardens all of the little dots resulting from the line screen exposure, as well as the bottom of the etch. Unfortunately, it is too common for operators to skip this step when in a hurry, thus compromising the physical integrity of the dots and resulting in almost immediate image quality issues and a much shorter cliché life.
Drying removes residual water and developer from the polymer film. Even though the light source in post-exposure generates some heat, that heat usually isn’t sufficient to completely dry the polymer. When relative humidity is extremely high (over 75 percent), taking the time to dry clichés before reuse also is a good idea. In either case, allow the cliché to cool down to room temperature before production.
Q: Why can’t I just buy denatured ethyl alcohol from a hardware store instead of buying it from my cliché supplier?
For most alcohol-washed polymer cliché materials, the developer should be 98-percent pure denatured alcohol, with the denaturing agent being 2-percent kerosene. From there, the user is typically advised to dilute 15 percent or so with distilled water.
Hardware and home improvement chains buy their denatured alcohol solvent in bulk. It may or may not be properly denatured, and/or it may contain as much as 40 percent water (from an unknown source).
In short, even though the stuff from a supplier costs more initially, and costs even more to ship, it still is less expensive than remaking clichés due to poor development.
Even if using water-washed cliché material, it still is recommended to use distilled water instead of tap water, since municipal water contains chlorine and other additives such as fluoride, which can cause problems in development.