Successful Fabrication of Screens for Screen Printing Ceramics and Glass
Quality screens can be made by focusing efforts on the environment, components and processes.
A quality print can only be produced by using a quality fabricated screen. By focusing efforts in three areas—environment, components and processes—quality screens can be made consistently and repeatedly.
The environment has the single largest impact on a screen’s finished quality. The biggest issue is contamination (e.g., dirt, debris, etc.), which compromises the integrity of the glue and the emulsion, and might even distort or alter the artwork being imaged on the screen. The screen needs to be produced in a temperature- and humidity-controlled environment. Temperatures that are too cold, too hot or constantly varying affect factors ranging from from mesh tension to glue and emulsion integrity. This creates the possibility of decreased performance, durability and longevity of a screen.
Taking basic steps, such as keeping doors closed, walling off dirty processes, and installing sticky mats and a well-running HVAC system, all go a long way in controlling the process. Having the proper environment for screen fabrication aids in successful print runs and prolonged screen life.
Screen components need to meet high standards, as they directly affect the overall quality of the screen—and thus the final ceramic or glass products. Components should be checked against specifications, inspected for defects, and stored in the proper environment. Four key components require attention: frames (the screen foundation); mesh (the controller of ink transfer and detail); emulsion (the enabler of the idea); and the artwork (gives the screen its image).
The first component of screen fabrication is the selection of a frame. The frame is the foundation of the screen, and is typically made from aluminum, steel or wood. Aluminum is the preferred material for its all-around strength, durability, weight and cost. Frame profile selection is determined by the overall size of frame required. Frame stock material comes in different widths, with larger frames requiring a wider width for stability, and for a larger adherence area for the mesh. This ensures consistent mesh tension and adhesion, as well as performance on press. Wood or metal tubing is cut to length, then joined together by glue, staple, or welding. Frames should be approximately 40% larger than the artwork. This will allow enough flex in the mesh during the printing process to prevent distortion.
Core component number two is the mesh, which is the single largest component to a screen. Mesh comes in a variety of counts (threads per inch), and colors (white, orange, yellow). The selection of mesh is determined by the desired deposit of ink, the intricacy of the image, and the substrate to be printed on (glass, metal, etc.). The mesh type also needs to be decided, as it is available in nylon, polyester and stainless steel. The higher the mesh count, the more detailed print with a lower ink deposit can be attained. The opposite is true for lower mesh counts (see Figure 1).
Emulsion is the third component. It gives the screen the ability to do what it was intended to do: print a specifically designed pattern. Emulsion has the largest number of variables to consider when fabricating a screen. The selection of emulsion is based on the print application (substrate, image and ink to be used), mesh, imaging method, emulsion thickness and cleaning method. Different emulsions have different exposure rates, and are also optimized for different exposure methods. For example, computer-to-screen (CTS) technology requires a significantly faster exposing emulsion than that of the standard metal halide lamps.
Emulsion thickness is also a deciding factor in exposure time for imaging, and it aids in the total ink lay-down required during printing. Exposing emulsion is like microwaving food. The ultraviolet (UV) rays start by exposing the outer layers, while slowly moving inward. The thicker the emulsion on the screen, the longer it takes to expose. Emulsion thickness also determines the maximum amount of ink being printed in fine detail areas, as it acts as the gasket between the substrate and the mesh. The type of ink (solvent, water or UV) and the cleaners used to clean the screen influence the type of emulsion needed. Make sure the emulsion is compatible with the materials being used.
The final component to fabricating a screen is the artwork that will give the screen its image. The vast majority of today’s screen printing industry uses some form of drafting or illustration software to create the image. It is then printed out on a Mylar film, which is used for imaging the screen. The film needs to be clean and well maintained, and stored so as to prevent damage and dirt from building up. This prevents unwanted “alterations” from being imaged onto the screen during the imaging process, which creates poor quality and an inaccurate image.
Over the past several years, CTS technology has allowed electronic files to be directly imaged onto the screen without the use of film or other similar medium. The benefit of this technology is having first-generation artwork—usually cleaner and more detailed—imaged onto a screen. However, this technology is very expensive, and is used by high-volume and/or specialty manufacturers.
Once the environment has been set up and all the components selected, it is time to start putting everything together in the process. Over the years, many technical advancements have been made, allowing unparalleled precision. Even with these advancements, much of the process still comes down to the eyes, hands, and especially the knowledge of the people on the floor.
Screen fabrication begins with the preparation of the frame. When a frame is selected, it should undergo an inspection for size, structural integrity, flatness and squareness. If any of these do not meet specification, they can compromise the overall quality of the finished screen. This can also cause failure of the frame later in the process, or even on press. Warped or out-of-square frames can lead to registration issues when trying to line up on the press.
After the criteria have been checked, the fabrication process can begin. The first step is to check the frame for any rough edges, nicks or burrs that need to be removed or ground smooth. These could cause a tear in the mesh, with the potential of causing failure of a process or the finished screen. For new frames, the gluing surface needs to be roughened, sandblasted or ground to aid in the adhesion of the fabric to the frame. For reprocessed frames, all labels, ink, adhesive and old mesh material needs to be removed. Finally, wipe down and clean off the frame to remove debris. This will help to prevent contamination in later processes.
Core process number two is stretching. The mesh is brought up to a specific tension and adhered to the frame. This is done by placing the frame in the middle of the stretching system, with the desired mesh rolled out over the top. If the mesh is to be stretched on an angle (bias), then the mesh will need to be positioned at the appropriate angle or the frame rotated depending on the type of stretching system being used. The mesh is then cut off the roll, along with any excess material. It is then locked in the clamps on all four sides. Operating the controls of the stretching system, the mesh is slowly stretched to the desired tension. It should not be stretched too quickly, as it can create excess stress and cause the mesh to tear or rip. The tension is then checked in several places to ensure the required tension is met and consistent across the frame.
Adhesive is then applied with a brush or paddle by working it back and forth through the mesh. Once adhesive is applied around the entire frame, it is left to dry for several minutes. This allows bonding and gives the adhesive time to build strength. The tension is released in the stretching system, and the excess mesh is cut off, finishing the stretching process.
Coating is the third process, and involves the application of the selected emulsion to the stretched screen. Before the emulsion can be applied, the screen must go through a degreasing process. The screen is degreased by rinsing it with water and applying a degreasing chemical on both sides. The chemical is then worked into the screen and thoroughly rinsed. This process removes chemicals from the mesh manufacturing and cleans off any other contamination before coating. Some degreasing chemicals also help to promote the adhesion of the emulsion to the mesh.
Coating works by using troughs filled with emulsion. They are pressed up against the mesh, tilted at an angle and then slowly moved up the screen. To build up the emulsion to a desired thickness, multiple passes may be needed. Coating should be performed on both sides to ensure complete encapsulation of the mesh. The print side (the side touching the substrate) should have the largest build-up of emulsion, with the squeegee side being thinner. This ensures that the screen provides a good gasket during the printing process. It also aids in creating a smooth surface for the squeegee to run across.
Application of emulsion can be done manually or with an automatic coating machine. Emulsion type, speed, pressure and the number of passes determine the end emulsion thickness of the screen. Although automatic coating machines are expensive, they yield significantly more consistent and repeatable results. They are also able to coat both sides of the screen simultaneously. Once the coating process is finished, the screen will need to be completely dried before the imaging process.
The standard way of screen imaging, the fourth process, is to tape the artwork on the screen in the desired location and place it into a vacuum table. A vacuum table is used to ensure 100% contact from the film to the screen, which guarantees that light scatter will be minimal. Lamp power, lamp distance, mesh color, emulsion type and emulsion thickness all determine the length of time that a screen needs to be exposed. For normal metal-halide bulbs, integrators are used to ensure the screen receives the same degree of light as the lamp ages and begins to lose power. When it has been determined that a secure vacuum is achieved, the lamps are turned on and the screen is exposed with the artwork in place. Once the screen has been imaged for the pre-determined length of time, it can be removed from the vacuum table and the artwork returned to storage or used on another screen.
Once exposure (imaging) is finished, the screen requires developing (still considered part of the imaging process). Using a garden hose with a sprayer attached, the screen must be continuously sprayed, alternating between the front and back of the screen. The uncured emulsion will wash away and leave the finished image on the screen. This step is followed with a pressure-washer rinse on the print side, staying about 16 in. away from the screen. Both sides of the screen are flooded with low pressure to wash away any remaining small particles. Once the screen has been allowed sufficient time to dry, block-out may be used to cover images (such as title blocks, revision dates, or press alignment marks) that are not meant to print, but still visually useable.
The final process in screen fabrication is inspection. This takes place in two parts: physical inspection and visual inspection. Physical inspection involves checking the frame size, mesh tension, emulsion thickness, image placement and other physical attributes of the screen to make sure they are within acceptable tolerances. Documentation of these attributes should be recorded for troubleshooting purposes should there be a problem downstream. Visual inspection is the inspection of the image itself: checking that it’s correct and fixing where needed, within acceptable limits. This may include removing unwanted emulsion, to filling in areas where it washed away, such as pinholes. These spaces should be no bigger than a pin point. After a screen has passed the inspection process, and all the above steps have been taken, performing a high-quality printing job should be a simple matter.
For more information, visit www.schillinggraphics.com.
Editor’s Note: This article is based on a presentation given at the Deco ’14 meeting of the Society of Glass and Ceramic Decorated Products (SGCDpro). For more information, visit www.sgcd.org.