Crucial for executing a high-quality print is the proper preparation of the printing form, commonly known as the screen. Below you will find several important issues that you should familiarize yourself with before starting your adventure with screen printing.
Choosing the right mesh
The choice of mesh depends on the type of printed surface (flat, irregular, non-flat), the type of print (simple elements, thin lines, rasters, full coverage – solid), and the type of ink (conventional, pigment pastes, UV, glues, flocks). In connection with the above, the mesh density expressed in the number of threads per centimeter as well as the mesh tension on the frame should be taken into account. For example, dense meshes 130-180T (screen ruling 20 lpi and more) are used for raster printing, while meshes 49T-79T are used for printing with plastisol inks on t-shirts.
The mesh color is also important – light falling on the threads of a white mesh is scattered, causing undercutting of the copy layer (emulsion) and thus the formation of unsharp edges. In the case of using a yellow mesh, the light is completely absorbed by the dyed threads and the phenomenon of light scattering does not occur.
Preparing the mesh
Before coating the mesh with photosensitive material, it must be degreased using an agent intended for this purpose and spread with a nylon bristle brush on both sides. Then the preparation should be rinsed off and the mesh left to dry. Degreasing is very important and prevents the formation of so-called fish eyes as well as the detachment of the emulsion during stencil development.
On such a prepared mesh, we apply the photosensitive emulsion using a special scoop coater. During the application of the emulsion, the frame with the mesh should be slightly tilted from the vertical (it is best to lean it against a wall and secure it against sliding). We start “coating” the mesh with emulsion from the print side and then from the squeegee side, remembering that the last coating must always be done from the squeegee side. Typically, meshes are coated in a 1/1, 1/2, or 2/2 system, but depending on the printed substrate and the ink used, a thicker copy layer (emulsion layer) may be needed.
After applying the emulsion, dry the mesh in a horizontal position, print side down. Two-component emulsions (with a sensitizer) must be mixed beforehand and degassed (free of air bubbles). All operations related to the application of the emulsion should be performed under yellow light.
Preparing the positive (film)
In the case of printing simple, non-raster graphics, a regular laser printer and tracing paper are sufficient to prepare the positive. Such prints should be additionally blackened with a special preparation before starting the exposure. If we intend to make raster prints, a good printer (e.g., Epson SC T3200 or Epson L1300) along with RIP software and appropriate ink for separation as well as a special matte film is necessary.
Exposure
We attach the prepared film using clear adhesive tape to the mesh on the print side, place it on the glass of the exposure unit, and start the exposure. During this process, pay attention to ensure that the positive adheres exactly to the mesh. Establishing the correct exposure time should be done experimentally, using different times. The length of the exposure time depends on the type of light, the distance between the mesh and the light source, the photosensitivity of the emulsion, the thickness of the applied emulsion, the mesh density, and the humidity of the emulsion. To precisely select the exposure time, use an exposure calculator.
Developing
After the exposure process is complete, the unexposed (not hardened by light) elements of the copy layer (covered by the positive) are washed out with water. Wet the mesh on both sides with a gentle stream of water and wait about a minute. Then, wash out the unhardened parts of the emulsion with a stronger stream until the pattern is clearly visible and the water flowing from the other side of the mesh has no traces of foam. You can also use – with caution – a pressure washer from the print side so as not to damage the copy layer. Collect excess water from the mesh surface, because after drying it can form thin films blocking the passage of ink during printing. This can be done using a sponge or lint-free paper.
Such a prepared printing form should be dried in a horizontal position, print side down, at a temperature of 30-40°C. Note that drying both the already exposed mesh and the mesh coated with emulsion at a temperature above 40°C can cause its thermal hardening and consequently problems with emulsion decoating.
Retouching
All damage to the exposed mesh (e.g., pinholes) resulting from improper preparation can be painted over with a special retouching agent or sensitized emulsion. In the latter case, the mesh must be exposed again.
Decoating
After finishing printing, collect the ink and thoroughly clean the mesh and frame using organic solvents or special ink removers. After washing off the ink, the screen must be thoroughly degreased, because solvent residues remaining in the pores can block access to decoating solutions. Use a decoater intended for the given type of emulsion, in accordance with the manufacturer’s recommendations.
source: own study