Controlling Etch Depth & Large Images
Question: To Screen Or Not To Screen
One important technique that is useful but misunderstood is the proper use of the film screens in making polymer
pad printing cliches or printing plates. Apart from the window screens that keep flies and mosquitoes out of your house, what do screens do and why would you need them in
pad printing?
Graphic arts film screens perform different functions depending on whether the printing plate is steel or plastic. A screened image has tiny raised dots or “bumps” in the etched areas. When added to steel printing plate image, these have two functions.
First, they keep the doctor blade from “dipping” into large, solid images. The tiny screen dots maintain enough support across these large areas to keep the blade from pulling the large volume of solid pad printing ink away.
Second, screens also keep tiny air bubbles from forming at the edge of images. These trapped bubbles are formed when the printing pad “squeezes” a large volume of pad printing ink from an open area to a closed one. The pockets of air appear as voids in the printed images. The screen’s dot texture prevents this.
When we make our steel printing plate, we rarely use screens, except in the above two situations. When necessary, we use a 120 line, 85% screen. This produces the best image quality, while holding up to ink cup doctoring. The slight texture of the screen almost disappears once the image is applied during pad printing.
Most polymer or plastic print plates, on the other hand, require the use of screens to control etch dept. For example, in a standard 86% film screen, 86% of the film is opaque and 14% is clear. The clear areas allow ultraviolet light to penetrate through the film and into the photo-sensitive plate material. Each clear “dot” is a sort of window, so the smaller the window size, the more shallow the penetration of light into the coating (see diagram below). The unexposed material eventually gets “etched” away, leaving the remaining exposed areas as raised “cones” or “pyramids”. These are useful in supporting the doctor blades. Hence, the coarser the line screen, the deeper the etch producing those deep “wells” of ink.
We recommend using a 300 line, 86% screen for most of our polymer print plates, with some bold images; we may use an even coarser screen. With proper exposure using calibrated equipment, a polymer print plate will have an etch depth of 1.2 to 1.5 thousandth of an inch with a 300 line screen. These do-it-yourself clichés include our alcohol-washout Polymer and Express-3 printing plates.
The Express-3 produces an etch depth of 25-30 microns (1.2 thousandth of an inch)and is preferred for fine line artwork or four-color process pad printing. However, addition of a screen is NOT necessary with this material.
In addition, H2-Orange and Hydro-Foil Thin are both water-washout plates that are suitable for screening. These as well as the Polymer and Express-3 can be processed at a customer’s facility using a proper UV exposure unit and recommended materials.
Please note: a critical factor is whether the unit can be calibrated to produce consistent results. We offer a handy kit and instructions for this purpose. Here is a summary of our product line of do-it-yourself cliche materials:
Variable Depth:
· Polymer (most durable; alcohol wash)
· NBAW (does not require baking; less durable; alcohol wash)
· Basic Grey (short exposure time; less durable; alcohol wash)
· H2-Orange (water wash)
· Hydrofoil Thin (water wash)
Fixed Depth:
· Express-3 (no screen, good for fine lines or four-color process; less durable).