Chapter 11
Getting The Screen Ready For Press
The first task at hand is to check for pinholes in the stencil. Place the finished screen on a light table or hold it up against daylight and look for any unwanted holes or breaks in the emulsion due to dust or foreign debris. It is best to use a light table so that you may see what you are about to do. We will want to fill any pinholes or other mistakes with block out. Block out is an emulsion like chemical you can use to fill pinholes. It is not light sensitive and only needs to dry. Be sure it is compatible with your solvents and inks.
You can use some of the emulsion used to make the screen or old emulsion as block out. You can keep a small container of it so that you are able to take it out and work in daylight using it as needed. If you use photo emulsion to block out pinholes or any portion of your screen, you will need to do a post cure. You can use anything from the tip of a pencil to small fine art brushes and small plastic smoothers/scrapers to apply emulsion as block out.
It is most often applied to the substrate side but it is a good idea to make sure it is filled on both sides but remaining relatively flat. Do not create bumps or dimples by leaving large drops of emulsion to fill pinholes. Keep it smooth so the squeegee will not get caught up on anything.
VIDEO: Preventing & Fixing Pinholes
VIDEO: Using Blockout To Fix Pinholes
Post Curing The Stencil
I mentioned this earlier but it is important enough to mention again when working with photopolymer emulsions. Many screen printers employ a technique known as the post cure. This is often done whether a photo emulsion was used as block out. If you have used a photo emulsion as block out, then this is a perfect time to cure that. In general, this is done to ensure that the ink well side of the screen is fully cured. It prevents premature stencil breakdown and stops pinholes from forming during printing. The screen is simply put back on the exposure unit, this time with the ink well side facing down toward the light source, and exposed again. The top is pulled down but the vacuum is not necessary for this. We are just making sure all the emulsion is fully cured and hard. Dual cure emulsions do not respond to post cures. You can post cure both sides of the screen for the same exposure time you used for making the stencil for good measure.
VIDEO: How To Post Expose An Emulsion Stencil
Post curing prevents screen lock from making the emulsion very difficult or impossible to reclaim. Screen lock occurs when chemicals and inks are absorbed by under exposed emulsions. If you are unsure if your photopolymer emulsion is fully cured, a post cure will be sure it is. If you have not done the post cure of your photopolymer emulsion previously as we discussed then do it now. The screen is now ready to tape off and go to press.
Taping Off The Screen For Printing
Tape off any areas on the sides of the emulsion toward the frame edge where there may be no emulsion present. Typically, a scoop coater for a 20 X 24 screen is about 16 inches wide. This will leave about 1 inch or less of open mesh on the length sides near the frame edge. Make sure to tape off all 4 sides to ensure that stray ink will not pass through unwanted areas of open mesh. Some screen printers prefer to do this with block out. I find using masking tape or screen tape is best and costs less. You can tape off the edges of a screen on either side. It is your preference. I tape off my screens on the substrate side because I work in such a way with my ink that tape on the ink well side gets in my way. This is because I prefer to use the more controlled, efficient, and ergonomic pull flood and push stroke.
Standard 2-inch masking tape will suffice for most of your taping needs on press. Screen tape is often good to use to tape off the screen so that it tolerates more solvents. Masking tape will become more difficult to remove with more exposure to inks and solvents. 2-inch screen tape will remain very easy to remove without leaving a ton of adhesive on the screen that will need to be cleaned off during the reclamation process, if that is where the screen will end up. Point being, that sometimes spending a little more money on screen tape and using it wisely, (just to tape off the edges), can save you time and labor later. However, I do use regular masking tape most of the time.
If you have multiple designs on one screen, you will want to obviously cover the designs you are not printing while you print another. If the stencil is not inked, I will tape it off on the ink well side so that it stays clean and I can easily line it up on the pallet for use next. If it something that has already been printed, tape it off on the substrate side to cover the ink already in the stencil. I use pieces of glossy magazine paper with masking tape. Just tear a piece of the glossy magazine paper to fit the design and tape down with masking tape to prevent unwanted ink passage. The longer you leave this on the screen the more difficult it may be to remove and clean out the mesh in the design you have covered during a print run. This is especially true if the area gets ink soaked and ink passes to the magazine paper. It will get oily looking but it will not leave ink on the shirts. It will clog the screen if left for long periods of time but it is possible to clear it out with screen opener.
VIDEO: Screen Taping Tips
VIDEO: Masking Tape Tips
VIDEO: How To Re-Use Screen Tape More Than Once
VIDEO: Screen & Ink Changes Production Run