Screen printing is more and more widely used in electronic industry, ceramic decal industry and textile printing and dyeing industry. With the continuous progress of society, people have higher and higher requirements for the quality of screen printing products. How to improve the quality of screen printing is a problem that people in the industry are very concerned about. In the entire process of screen printing, from pre-press plate making to printing to post-press processing, every link will affect the quality of the printed matter. We must give sufficient attention to the factors that may cause the quality of the printed matter in each link. Pay attention, only in this way, can the quality of screen printing be guaranteed and improved. This article mainly discusses the influencing factors of screen printing quality from the following aspects: the selection of manuscripts, the selection of screen frame, the selection of screen, the selection of photosensitive glue and Choice of plate making method.
The selection of manuscripts
The manuscripts used in screen printing can be mainly divided into line manuscripts, continuous manuscripts and halftone manuscripts. Screen printing requirements for manuscripts are as follows:
① When selecting line manuscripts for screen printing, it is required that the lines should not be too thin, otherwise it will easily cause the printing mesh to not be exposed. Even if it is forced to be exposed, it is difficult to print because it is easy to block the screen.
② The thickness of the ink film of screen printing can be as high as several hundred microns, and the color is bright, so it is difficult to print subtle color changes. When selecting the original, pay full attention to this point.
③ The fine-level ability of halftone printing is far less than that of offset printing, gravure printing and flexographic printing. The matching relationship between the number of screen lines and the number of screen meshes is more complicated, and it should be fully considered when selecting manuscripts. Generally, the number of meshes is 3 to 4 times the number of added mesh lines.
④ The image on the film must be firm, and it is not allowed to fall off during the process of copying and saving.
⑤ The original text and lines require a certain contrast to ensure high definition.
⑥ The layout of the manuscript should be quite clean, free of dirt and dust, otherwise the printing quality will be affected during printing.
⑦ The color of the color continuous tone original is basically the same as the color requirements of the printed copy, and the color range of the printing should be fully considered.
Second, the selection of the screen frame
The selection of the correct screen frame is very important for plate making and printing. Generally, when selecting a screen frame, the following factors need to be considered:
①Strength of the mesh frame, the mesh frame used for stretching the mesh must have a certain mechanical strength.
②Light weight, on the basis of ensuring a certain mechanical strength, the screen frame should be selected as light weight as possible, which is easy to operate and use. Therefore, the general aluminum frame is also hollow.
③ The sturdiness of the screen frame. The screen frame requires good resistance to resistance. When it comes into contact with water, solvents and other substances during use, and is affected by environmental changes, it is required that the screen frame does not have obvious deformation, otherwise, it will affect the normal function. .
④ The size is reasonable. The size of the screen frame selected should be suitable for the format of the graphic and text to be printed, and the material configuration should be appropriate, that is, it is sufficient and does not waste materials.
⑤ The roughness of the screen frame surface, the side where the screen frame and the wire mesh are combined should have a certain degree of roughness, which is conducive to the good adhesion of the wire mesh and the screen frame under the action of the glue, which is generally used in this area. The joint surface is polished to strengthen the joint fastness of the wire mesh and the screen frame.
3. Selection of wire mesh
In the whole process of screen making, the selection of screen is very important. Generally, the following aspects should be carefully considered:
1. Selection of wire mesh material
Screens of different materials are suitable for printing objects and the effect of printing may be very different. When selecting screen materials, you must first determine the printing objects, etc., and then make a reasonable selection. For example, when printing thermoplastic ink, heating is required, so the plate making needs to use a wire mesh that is electrically conductive and thermally conductive.
2. Mesh number of wire mesh
The selection of the screen mesh is one of the most important technical indicators of the printing effect. Before plate making, it must be measured and selected according to the quality requirements of the printed products.
When the resolution of the printed image is high, the mesh number is required to be high, and vice versa, it is low. It is required that the ink layer is thick and real-time, and the screen with low mesh number and large ink output should be selected. We will also choose the mesh number according to the characteristics of the ink: if the ink is thicker, use a lower mesh mesh, and vice versa, choose a high mesh mesh. Printing functional inks, such as fluorescent inks, foaming inks, etc., should use a low-mesh screen. When printing ink with high viscosity, a low-mesh screen can be used.
3. The warp and weft of the wire mesh
The warp and weft of the screen and its related parameters will also affect the printing effect. It must be selected reasonably and correctly in combination with the accuracy of the printed matter, the color requirements of the printed matter and the thickness of the ink layer. For example, when the printing ink layer is required to be thicker, it is best to use the screen of hinged textile.
Generally speaking, the screen used for screen printing plate production should also have the following properties:
High tensile strength, low elongation, good resilience, good stability to temperature and humidity changes, good ink passability, good abrasion resistance, good chemical resistance, etc.
Fourth, the choice of photosensitive glue
There are two types of photosensitive adhesives for screen printing and printing: single-liquid photosensitive adhesive and two-liquid photosensitive adhesive. Among them, the single-component photosensitive adhesive has already added the photosensitizer into the latex during production, and it can be coated without preparation when using it; the two-component photosensitive adhesive needs to dissolve the photosensitizer in water according to the formula before use, and then mix it in the latex. Stir well and place for 1 to 2 hours before using it until the bubbles disappear (it needs to be prepared before use).
Photosensitive adhesive is the most widely used photosensitive material in the screen printing industry at home and abroad. At present, a new type of diazonium salt sensitizer is widely used. The advantages of the diazonium salt sensory glue are that the resolution is very high, the image of the plate is very clear, the photosensitizer can be used for a long time when added to the latex, the printing resistance is high, and it is non-toxic and non-polluting.
The requirements for the photosensitive adhesive used in screen printing are: good plate making and easy coating; there must be a suitable photosensitive spectral range, the general wavelength range is 340 ~ 440nm, if the photosensitive wavelength is too long, the printing operation and screen printing The storage of the plates needs to be carried out in a strict darkroom; if the photosensitive wavelength is too short, the selection of printing conditions and the protection of the staff will become very difficult; the sensitivity of the photosensitive glue is required to be high, which can save energy and improve plate making efficiency. After exposure, the photosensitive adhesive is required to have good developing performance and high resolution; photosensitive adhesive requires good stability, easy storage, and reduced waste; photosensitive adhesive is also required to be economical, non-toxic, hygienic and pollution-free.
The requirements of screen printing for photosensitive adhesives are: the film formed by the photosensitive adhesive should meet the performance requirements of different types of inks; it can be scraped multiple times with the squeegee and has a certain printing resistance; the photosensitive adhesive film cannot be used during the printing process. If the film fails, it requires a strong bond between the photosensitive adhesive and the mesh; in order to facilitate the recycling of the screen printing plate, the photosensitive adhesive is required to be easy to peel off.
Five, the choice of plate making method
At present, the most widely used screen printing platemaking method is the photosensitive platemaking method, and a few screen printing factories use the digital platemaking method.
1. Photosensitive plate making method
The photosensitive plate making method uses the photochemical reaction of photosensitive glue, that is, the photosensitive glue has a photo-crosslinking and hardening reaction when exposed to light and is firmly combined with the screen to form a plate film. Through the mesh, and form the screen printing plate. The screen printing plate produced by the photosensitive plate making method has high image quality, good effect, and is economical and practical. Therefore, this screen printing method is the most widely used and the most important one in modern screen printing. Methods.
The photosensitive plate-making method can be divided into three types: direct method, indirect method and direct method. These three screen plate-making methods have the same technical requirements in essence, but the basic process method of coating photosensitive adhesive or film is still the same. Differently, these three methods are the most commonly used screen printing photosensitive platemaking methods.
The direct method is currently the most widely used method, which is to directly coat the photosensitive adhesive on the stretched screen to form a photosensitive adhesive film. The coating operation can be done manually or by an automatic coating machine, but in order to obtain a smooth and uniform layer of photosensitive adhesive, skilled operators are definitely inseparable. The disadvantage of this plate-making method is that coating and drying need to be repeated. In order to obtain the required plate film thickness, a certain amount of time for coating and drying operations is required, and the efficiency is not high.
(2) Indirect method.
The indirect plate-making method means that the positive film and the photosensitive film are closely combined, and the desired image is formed by successive exposure and development, and then the image is transferred to the stretched screen, and finally the film base is peeled off after drying. The plate film was prepared. Compared with the direct plate-making method, the indirect plate-making method is easier to obtain fine images and does not require a very special screen frame. The advantage is that it is easy to operate and saves time. The disadvantage is that the service life of the printing plate is relatively short, the cost is high, and the plate film is particularly easy to stretch. Compared with the direct method, the printing plate obtained by the indirect method has higher relative precision and does not require special printing equipment. The indirect method is different from the direct method. It is to first put the positive image film and the photosensitive film with photosensitive properties on the photosensitive film, after printing and developing the required image, and then repost it on the screen. of.
(3) Straight-line method.
Mixing the direct method and the indirect method is the direct plate making method. It is to stick the photosensitive film on the screen, after drying, tear off the film base on the photosensitive film, and close it with the positive film film, and then make the required screen printing plate after exposure, development and drying.
The direct method is different from the direct method or the indirect method. It obtains the final thickness of the screen printing plate through the thickness of the diaphragm, but the direct method depends on repeatedly coating the photosensitive film to determine the thickness of the screen printing plate; The direct method is to paste the film first and then dry it, but the indirect method is to make the image first and then paste the film.
A photosensitive film that is pre-coated with a certain thickness is used in the direct-to-plate method, so it can save a lot of time. Moreover, since the photosensitive adhesive is coated on the film base in advance, the flatness of the screen printing plate can be guaranteed.
No matter which photosensitive platemaking method is used to make the screen printing plate, the quality inspection of the screen printing plate should be carried out. The general requirements are as follows:
Film quality: whether the lines are complete, whether the edges are clear, and whether the zigzag is serious.
Through-hole quality: The transparent place should be transparent.
The adaptability of the plate film: the organic solvent-based ink selects the solvent-resistant photosensitive material; the water-based ink selects the water-resistant photosensitive material.
In order to obtain a screen printing plate with relatively good quality, it is best to use the straight-line method plate-making, which has relatively good dot reducibility.
2. Digital plate making
The digital plate-making method of screen printing, namely CTS, is a computer-to-plate-making method of screen printing. It does not require film, and the computer directly outputs the image to the screen, and then develops the printing plate. It is directly controlled by the computer and output on the stencil or silk screen, which realizes the digitization of the image in screen printing.
The traditional screen printing plate making method requires more than a dozen processes, but after using the CTS process, the digital workflow from designing the graphic and text layout to making the screen printing plate only requires four simple steps, which greatly improves the efficiency of the process. The efficiency of screen plate making is improved, and the quality of the printing plate is easier to control, and there is no need to prepare film in advance, which greatly saves the cost of plate making.
In order to obtain good quality in screen printing, the quality of plate-making must be strictly controlled. The best plate-making effect is digital plate-making, but the cost of plate-making may be slightly higher. Which plate-making method to choose in the end should be selected reasonably according to the specific situation.
There are many factors that affect the quality of screen printing. In addition to controlling the quality of printed products from the above aspects, the performance requirements of printing inks and the technical level in the printing operation process are also places that cannot be ignored that affect the quality of screen printing.