Screen making is the area that I focus on heavily since the quality of the print will be very influenced by, if not dictated by the quality of the screen. Many shops make the same mistakes, and the most obvious one was the way they dried their screens, so following is a step by step of what I would do in making a screen from the beginning:
1-Stretch a screen to a specific tension (all shops used retensionable screens). To make life easier, I prefer working to one tension range for each of the mesh counts and staying there. Following are the tensions I would like to see maintained:
a-up to 110 mesh 50 N/cm or higher
b-110 mesh - 158 mesh 45-50 N/cm
c-175 mesh - 230 mesh 35-40 N/cm
d-above 230 mesh 23-25 N/cm
The mesh counts above are based on standard “S” and “T” thread diameters and not special “thin” diameters or Super tension meshes. The lower your mesh tension, the lower the mesh count needs to be in order to get a decent print. There really isn’t a substitute for high tension.
2-Record the tension on the bottom of the mesh using a Sharpe Marker and draw a fat line after it. It is important to document the number of times a screen has been retensioned and this is an easy way of following it. This information will be used in selecting screens for a job since it is very important to use screens that have been retensioned the same number of times in order to not loose registration during printing. New mesh looses tension much faster than an old screen that has been retensioned many times. If using rigid frames, new screens will allow for a crisper image than old ones since their tension will be higher than the old ones. In either case, having screens of a similar age, number of times used, reclaimed and retensioned will make for a better looking print.
|3-Wet the screen. It would be best if a shop has two sinks, one for degreasing and washout and the other for reclaiming because of contamination problems that could arise from the “splash back” of chemicals. This is usually where the “fish eyes” come from. If you only have one sink, make sure it is washed down completely before going on.|
|4-Abrade the screen if it is new, if using capillary film or if the screen has been used for five or more reclaimings and has not been abraded. The abrading scratches the surface of the mesh and allows the emulsion to bit on to it for better adhesion. Abraders come as a powder or paste and there are some degreasers that have an abrader in them and if using one, then the abrader by itself is not necessary. The brush used for abrading a screen should only be used for that purpose, as should the brushes for each of the other steps in this process. A maximum of five brushes may be needed.|
|5-Rinse the screen and apply degreaser working it in with its brush. Degrease both sides of the screen.|
|6-Dry the screen. One of the best ways is to vacuum off the water and then put the screen in a drying cabinet. The quicker a screen is dried, the less chance there is on dust settling on it and creating problems. Never use compressed air to dry a degreased screen. That air is dirtier than the screen was before it was degreased because of the oils and other containments in the compressor.|
7-Coat the screen on the outside first (the side that faces down and touches the garment), then coat the inside of the screen. Only put on the amount of emulsion necessary. Scraping off excess emulsion wastes time and creates a lot of little bubbles in the scoopcoater which can cause little streaks on the next screen and weak spots. Coat screens until you notice a fair amount of bubbles. Empty the emulsion into a clean empty container. This emulsion will be used later once the bubbles have burst. The should be a second empty container to put the emulsion into from the first container so there will not be bubbles present when starting the coating process.
|8-Dry the screen in a drying cabinet with the outside facing down. The reason we do our last coat on the inside of the screen and face the outside down when drying the screen is to have a smooth surface on the inside for the squeegee to glide over and not get caught on the edge of the emulsion or drag on it. The drying cabinet should maintain a temperature of 95 degrees F if using pure polymer emulsion or 105 degrees F if using a diazo or dual exposing emulsion. With those temperatures, a screen should be dry in 10-15 minutes. Put a thermometer into your drying cabinet to check the temperature. Setting the thermometer to a particular temperature doesn’t mean the inside of the drying cabinet reaches that temperature. Do not use fans to dry your screens. They throw dust at the wet screen and create problems.|
|9-Once the screen is dry; it is time to expose it. If the screen was dried and put into storage, then a check of the relative humidity is necessary before exposing the screen. The ideal relative humidity is about 40%. Having a hygrometer in the screen storage area is necessary for this reason. If the relative humidity is above 50%, put the screen back into the drying cabinet for a few minutes (3-5 minutes) prior to exposing the screen. Emulsion pulls moisture from the air and can re-wet a screen. A “wet” screen takes longer to expose properly than a dry one and this will ensure your screen is dry. Many of the screen breakdown problems are a result of screens being under exposed because of the moisture in them. When conducting exposure tests using a calculator, make sure you are using dry screens. Touching a screen to see if it is dry will not give you that information. Screens coming out of a drying cabinet or screens stored in an area where the relative humidity is less than 50% should be used. (Next month I will write about how to conduct exposure tests and the telltale signs of under exposure).|
|10-Wet the screen and let it stand for a few seconds before washing it. The emulsion will start to soften in the areas that are supposed to wash out. Using a 1000-PSI washer from about three feet away (one meter) wash the screen. If the exposure was correct, the emulsion will not breakdown. If the emulsion does breakdown, this is a better place to find out the screen wasn’t made correctly than while on press.|
11-Dry the screen. A vacuum with a special nozzle is my favorite way of doing it, but compressed air works and so does putting the screen in a drying cabinet after blotting off the excess water, just make sure the image area is free of water so the is no scumming (the shiny clear film that forms in the image area from unexposed emulsion settling there) that will dry in and not let the ink through it. If this happens, use a damp cloth to open the area.
12-Blockout and tape up the screen.
|13-After the screen has been used, remove and clean all ink. Put the screen in the reclaiming sink and wet it. Apply ink degradent and work it in with a brush, then blast off the excess ink using a 1000-PSI washer.|
14-Apply and work in the reclaimer. Blast with a 100-PSI washer.
15-Haze Remover could be used at this point. Use haze remover about every five reclaimings if at all (I don’t like haze removers because they are very caustic and most employees don’t want to wear the correct protective gear when using haze removers). If haze remover was used, spray the screen down with a mixture of 50% white vinegar and 50% water to neutralize the alkaline in the haze remover. Haze Remover is responsible for breaking mesh by slowly eating away at it. That is why sometimes a screen breaks while it is not being used.
16-Retension the screen and document the tension on the screen.
17-Go back to number 3 and start to prep the screen for use again.
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