This month’s newsletter gets back to the basics of screen making. Spring is the start of the busy season in screen printing with new companies opening their doors and new employees that need to know how to make a screen. Not just any screen mind you, but a screen that will have few pinholes, produce sharp crisp art details, and avoid breakdown on press. So this month’s article is an A to Z look at the products and process to create a durable screen.
Part One – Screen Preparation 1. Cleaning the screen prior to De-hazing – The first step in screen preparation is to analyze the mesh for ghosts of ink that show up from a previous print run. If the mesh is new you can skip to Step 2. But if your screen has a ghost image read on. Ghosts or fabric stains are caused by ink from the last print run. The ink can get caught in the ‘creases’ of the ‘mesh knuckle’ and is usually seen when dark inks are used. Ghosts can cause pinholes or show up in a solid print area on the next job since they can interfere with the ink transfer and leave a ghost in the print. Preventing Screen Haze: The easiest way to minimize this issue is to clean the screens immediately after the print run. This includes plastisol as well as water base, discharge, or HSA inks. Cleaning screens while the inks are still wet will prevent ink from drying in the mesh knuckle creases. This is especially true of discharge, water base and HSA inks that can air dry. Once the ink has dried you will need a haze remover to clean mesh. Murakami’s SC-501, SC-505 and SC-507 are designed to clean the mesh well without chemically flashing the emulsion and making it hard to reclaim. Screen Openers, Acetone, MEK, and other hot solvent chemistries can lock in the emulsion and make reclaiming difficult. Murakami screen cleaners avoid locking in the emulsion which allows the reclaiming process to go easier.
For water base and discharge inks a 5 gallon bucket of warm water works well, for plastisol or other inks the cleaners above work well, with SC-507 designed for graphic and industrial inks as well as textile inks. When companies tell me they have no time to wash the screens after a job has finished on press I point out that screen reclaiming personnel will spend 2 to 3 times as much time cleaning the dried ink compared to a worker cleaning wet screens at the press. Typically all that needs to be done is to card out the inks and wash the image area. This practice makes ghost removal in the reclaiming area an occasional job rather than needing to do it on all screens which wastes chemistry and labor.
2. Reclaiming the Screen: Dip tanks help save emulsion remover and soften the emulsion for reclaiming. Murakami ER-605 or ER-660 can be used in the dip tank. For hardened screens ER-605 can be mixed 15:1 Always reclaim the screens before the emulsion remover can dry on the screen. If you apply emulsion remover and allow it to dry, it may be impossible to reclaim since emulsion remover can lock in the emulsion once it has dried on the stencil.
3. De-hazing: The residual image left by the previous print job can be difficult to remove once the ink has dried in the screen. If left in the mesh it can affect the appearance of the next print run. Discharge prints may not show anything when the ink is wet, yet when cured this previous print image will appear within a solid area of the print when cured. Process and Simulated Process jobs can also be affected. Add to this an increased risk of pinholes and break down in the ghost area and it is clear the screen is better off with the ghost image removed.
Murakami emulsion is engineered to accept complete exposure and still develop extremely fine details. If your emulsion cannot hold details at full exposure then that is a competitor’s product issue. Under exposing to get details will not yield a durable screen. This is a main difference found in Murakami emulsions. Expose them completely, no under exposure needed for details. Click here to continue reading the article: Basic Screen Room Training