Screen printing 150 questions (43-48)

â–¼43. How does a vignette come about?

Vignette is a common phenomenon in screen printing plates. The main form of expression is that after the printing, the screen part on the screen printing plate is not fully formed into a through hole after development, and part of the mesh hole is still frozen by the photosensitive adhesive, and the graphic edge is not uneven. Qi, some meshes are translucent, and the edges of the images are like the wind rings around the moon. With this kind of screen printing printing, the edges of the imprints are not neat, and some of the text lines are missing. The size also changes. This reduces the print quality. There are two main reasons for producing vignetting. When using direct platemaking, a white screen is used. The white screen has reflection and scattering effects on the printing plate light source during the printing, so that the photosensitive material at the edge of the picture is sensitive to light. The degree of non-uniformity, resulting in vignetting; plate printing plate and the screen plate does not fit tightly, making the picture edge of the photosensitive material is not uniform, resulting in the emergence of vignetting.

â–¼44. How to prevent vignette?

In order to prevent the occurrence of vignetting phenomenon and improve the quality of plate making and printing, the following measures can be used to control it.

1 The use of colored silk screens in the plate-making process as much as possible without white screens, and use red, yellow, orange and other colored screens for plate making, use the color of the screen itself to absorb the light source of the printing plate, reduce the screen when the screen light Reflection, scattering, to prevent the appearance of vignetting.

2 Pre-painted varnish-preventing solution Pre-painted vignette-preventing solution is a solution of ethanol-cresol and ethanol-soluble red dyes that are blended at a given ratio. Before applying this solution, the screen must be cleaned and dried

Dry, and then apply varnish to prevent the liquid and then dry, dry and then coated with photosensitive adhesive and printing, so that it can play a role in preventing the appearance of vignetting.

3 During the printing, pay attention to the plate and the screen plate to be affixed and tightly attached to ensure that the light-sensitive material is evenly distributed, so as to prevent the occurrence of vignetting.

â–¼45. Why are there pre-treatments for screens? What are the main treatments?

In the process of manufacture, transportation, storage, and use of the screen, dust, oil stains, etc. adhere to the surface of the screen. After the screen surface adheres with dust and oil, the combination of the screen and the photosensitive adhesive will be seriously affected, resulting in the quality of the screen printing plate. To reduce, in order to ensure that the screen and the photosensitive adhesive closely integrated, the screen should be pre-treated before coating the photosensitive adhesive. Screen processing is an important process for making screen printing plates. There are usually two methods for the pretreatment of the screen. Physical treatment and chemical treatment.

Physical treatment method: The purpose of this method is to polarize the surface of the screen to facilitate the combination of the screen and the photosensitive adhesive.

This method is pulverized using a pumice stone and a silicon carbide powder and treated by mechanically treating the surface of the mesh. Or the wire mesh is treated with silicone carbide powder, with the powder particles below 20 microns. After treatment, the screen is washed and dried with water and then used for plate making.

Chemical treatment: The phenol, cresol, and phosphoric acid corrosives are usually used to process the surface of the synthetic fiber mesh before the plate making process to roughen the surface of the mesh and achieve the purpose of tight bonding with the photosensitive resin. However, the use of phenol, cresol Phosphoric acid-based corrosives tend to damage the screen, so pay full attention to chemical treatment.

In addition to the polarization treatment on the screen surface, the screen can be degreased and cleaned with caustic alkali, cleaning agents, etc. The de-esterification cleaning method can effectively remove the oil, dust and other substances on the surface of the screen, making the screen and photosensitive adhesive Bonding is strong. After washing with the above solvent, rinse with water, dry it, and use it for plate making.

â–¼46. What is the principle of screen printing plate imaging?

From the foregoing, it can be seen that the screen is formed by weaving a certain material, and the latitude and longitude lines intersect to form a mesh. During the process of plate-making, a certain thickness of photosensitive resist is coated on the screen and dried, and a photosensitive film is formed on the screen, and then the positive-texture plate-making negative film and the coated screen are attached and exposed by a human exposure machine. In this way, the graphic portion of the screen is not cured due to light reception, and the non-graphic portion is cured by light. After developing and rinsing, part of the uncured photosensitive material is washed away, and the screen hole is a through hole. The non-graphic part of the photosensitive material solidifies the mesh. When printing, the silkscreen hole in the graphic part leaks ink, and the non-graphic part does not leak ink, thus obtaining the printing ink. This is the imaging principle of screen printing plates.

â–¼47. How is the screen printing plate developed and fixed?

Screen printing plate production process is relatively simple, with the continuous emergence of new photosensitive materials this feature is more prominent. Use modern photographic materials to make screen printing plates, use photographic plates and stencils coated with photosensitive materials to fit the printing plates, and put the frame in water, warm water or pharmaceutical water for about 10 minutes after printing, and the graphic shows Come out (different photosensitive material development time is also different). Then, the screen printing plate was gently scrubbed with water in a sponge, so that the mesh part of the picture and text basically became a through hole. Then the pressurized water is sprayed to remove the unexposed material left in the graphic part, so that all the screen holes in the graphic part are changed. Finally, the water mark is wiped and dried to make a screen printing plate.

â–¼48. How to determine the thickness of screen printing plate?

The thickness of the screen printing plate refers to the thickness after coating the photosensitive resin on the screen. That is, the thickness of the screen printing plate is equal to the sum of the thickness of the screen and the thickness of the photosensitive film.

The thickness of the screen printing plate is determined according to the printing requirements. Usually the height of the ink layer after printing is closely related to the thickness of the screen printing plate. Under normal circumstances, when the screen printing plate used is thicker, the ink layer after printing is used. On the other hand, if the screen plate used is thinner, the ink layer will be lower after printing. In special cases, it is required to use screen printing plates of thick-film photosensitive materials with a thickness of up to 1 mm. For example: Making Braille The thickness of the screen printing plate is generally between 0.1-0.250 mm.

With the direct platemaking method, the thickness of the screen printing plate is obtained by coating the photosensitive resist several times. The coating is applied once and dried once. The required thickness of the screen printing plate can be obtained by performing the cycle operations in sequence.

In the case of indirect plate-making or direct-to-plate printing, the thickness of the screen printing plate is obtained by selecting different thicknesses of the photosensitive film according to the printing requirements.