Coudray identifies environmental factors and processing concerns that you need to control in order to produce reliable, high-quality printing screens.
Screenmaking: The Gateway to Image Quality
It is not uncommon for me to receive phone calls from screen printers who are at the end of their rope. They appear to have all the right tools and all the right supplies. They use good chemicals and emulsions. But for some reason, they just can’t seem to get the results they are looking for--image quality doesn’t measure up to their expectations. They say their exposed screens lack sharpness and edge definition and are full of moiré. These common problems most often originate during screenmaking, so this month I’ll describe some ways you can control key aspects of the process, manage your shop’s environment, and explain how you can--in a matter of hours--dramatically improve the quality of your image output.
Back to basics
Following some standard procedures and adding some internal check points will make a majority of the ills vanish. Let’s start with emulsion. Printers who want to produce decent work select quality dual-cure or photopolymer emulsion. The emulsion’s solids content should be more than 35%, which is adequate for good to excellent edge definition. For the photo emulsion to have a fighting chance at maintaining fine halftone dots, or very fine lines, it must be capable of resolving them. This is a problem if the emulsion is too old. If you use a dual-cure emulsion, count on a maximum usable time of four weeks from the time you sensitize it. For the sake of perspective, photopolymer emulsions remain usable almost a year from the time they’re sensitized.
The same concept applies to screens. Once you coat a screen, you have a maximum of one week before it will no longer resolve those 3-4% dots. As time passes, the coated screen experiences a phenomenon known as dark reaction. This is common to almost all photo-grade materials. The term refers to the tendency of photosensitive materials to essentially expose themselves over extended periods of time, even if no addtional light sources are present. So whatever emulsion type you use, get in the habit of recording the date when you sensitized the emulsion. Also mark screens with a coating time and date. This can be very helpful in tracking down problems related to resolution.
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