I. Introduction Screen printing (hereinafter referred to as screen printing) is a classification other than offset printing, letterpress printing, and gravure printing, and this type of application is particularly extensive. It is used in paper, plastic, organic glass, leather, wood products, ceramics, textiles and other industries. At present, a large number of screen printing methods are used to serve the electronic and telecommunications industry, such as the manufacture of printed circuits and nameplates. Despite its wide range of uses, silk screen printing is just an old and young industry that has redeveloped within 30 years. Therefore, people only have a preliminary understanding of silk screen printing, but they still don't know much about the quality of silk screen printing. How to ensure the quality of silk screen printing has become the scope of our current discussion. This article focuses on discussing some quality problems in screen printing from the aspect of process requirements, and how to control it so that the level of screen printing can be further improved and the products can be better decorated. 2. Screen printing template and its quality Process flow of two methods of screen printing template manufacturing 1. Direct method Preparation of wire mesh frame stretched net-> cleaning treatment of silk screen-> scraping the photoresist multiple times-> covering the plate after drying with the photographic master plate-> developing-> drying-> repairing-> sealing. Matters needing attention in operation: (1) The screen must be thoroughly degreased before applying photosensitive adhesive: (2) The number of times of applying photoresist should be taken into consideration of the accuracy of the printed pattern and the level of screen mesh; (3) Be sure to cover the photographic master plate and the photosensitive adhesive layer closely when printing; (4) After cleaning the wire mesh, it should be used immediately. 2. Direct photosensitive film method (apply capillary film method) Wet the film or apply photosensitive adhesive-> dry-> peel off the base-> plate-printing-> development-> plate revision, etc. Matters needing attention: (1) The storage film should be protected from light or yellow light; (2) The film should be attached after using the corresponding photosensitive adhesive. Net material. From the quality requirements, compare the performance of commonly used mesh materials. A. Nylon wire mesh: (1) It has certain wear resistance and tensile strength; (2) Good resilience and permeability of printing materials; (3) Not resistant to acid solution; (4) Great hygroscopicity, so poor dimensional stability; (5) When using, pay attention to increase the pulling force on the screen to print fine products. B. Polyester wire mesh: (1) All advantages of nylon mesh are: (2) Strong acid resistance and chemical resistance; (3) Since the hygroscopicity is lower than nylon yarn, the dimensional stability is better. (4) The decrease in strength of polyester yarn after being exposed to sunlight is smaller than that of nylon yarn; (5) Disadvantages: The binding force with the currently used photosensitive adhesive (film) is slightly worse, and the diameter of the monofilament is thicker than that of nylon filament. Therefore, under the same porosity ratio as nylon yarn, the performance of the polyester screen ink through the screen is worse than that of nylon screen. Mesh-refers to the number of meshes per centimeter of length. Inch system refers to the number of meshes per inch; Perforation rate-refers to the percentage of the mesh area of ​​the entire screen area. Mesh width-refers to the side length of the square mesh; Thickness-refers to the thickness of the wire mesh; Wire diameter-refers to the diameter of the monofilament of the braided wire mesh. [next] 3. Stretching net requirements In the screen printing process, the screen must also be fixed to the frame, so that after the screen is stressed during the scraping process, the blade edge of the scraper comes into contact with the screen and the substrate. Once the pressure disappears, the screen immediately Leave the substrate. Otherwise, the clear pattern required will not be printed. Therefore, the wire mesh itself must have a certain elasticity, and it is not suitable to be too large or too small. In order to make the wire mesh have a certain elasticity, the wire mesh must be stretched on the frame, so people think that to get an ideal template, the stretched mesh must be: (1) Tension must be uniform throughout the wire mesh. (2) The area of ​​the wire mesh is basically close to a square (3) The stretched wire mesh ensures that the tension is stable during use. Before stretching the net, the wire net and the net frame must be subjected to stress treatment to stabilize their cold deformation. (4) In the process of stretching the net, the force applied to the screen must be uniform and uniform. If possible, use stages to increase the tension of the screen at intervals to promote a stable "aging" process. (The company sells all kinds of wire mesh, sizes and sizes, from 80 mesh to 420 mesh) In order to achieve the above ideal conditions: (1) It is best to use special stretch net equipment with a tensile test table. It must be noted that the stretch net cannot be stretched once, and the tension should be increased 2-3 times within a certain period of time and placed naturally for 2-4 hours "Aging", after the wire mesh is deformed and stabilized, the tension is measured again, and the wire mesh can be fixed on the frame only after reaching the standard. (2) There is a silk screen version required for overprinting. The texture, cutting direction and tension of the silk screen must be consistent. (3) When the net is stretched, the warp and weft of the wire mesh should be perpendicular to each other as much as possible under stress. (Iv) Give the frame a certain amount of stress before stretching the net. By doing the above points, a satisfactory wire mesh can be stretched out. 4. Requirements for black and white manuscripts Drawing black self-made manuscripts is the basic preparation process for photo-engraving. It is an indispensable process equipment before silk-screen printing. Therefore, it is required that the size of the black and white manuscript is accurate, the underlined edges are clean, and the manuscript contrast Large, clear and beautiful characters and symbols, well-proportioned stroke arrangement, etc. In order to improve the accuracy of black-and-white manuscripts, it is necessary to consider the scalability of the paper used, manuscript storage, storage, etc. when drawing the manuscript. For this purpose, the following requirements are proposed; Requirements for drawing black and white and its tools, materials, equipment paper: heavy-weight Daolin paper or coated paper with no reflection above 250g, no impurities, stains, etc. on the surface, absolutely smooth. Drawing method and requirements of black and white manuscript: The ratio depends on the device and is generally drawn at 2: 1 and 4: 1. Workpieces that require screen printing on the front and back sides, who have requirements on the location, are seated on the front and back sides of the same paper, such as double-sided printing plates. For black-and-white manuscripts used for multiple overprinting, it should be considered that the lines of several papers are consistent when drawing, the selected positioning standards are consistent, and the environmental and climatic conditions are basically consistent as much as possible. Take pictures etc. Computer sketches and films are generally used today. [next] 5. The quality of silk screen (also called screen) In recent years, as screen printing has been widely used, the materials for providing screen printing are also increasing. On the contrary, there are new requirements for the quality of screen printing. For this reason, the requirements for screen printing stencils should be stable in size. Therefore, the manufacturing of silk screen dies occupies a very important position in the entire silk screen process. Here is a brief discussion on the main issues of silk screen printing. (1) The jagged edge of the line: Due to insufficient exposure, the photosensitive material cross-linked by light absorption is not completely cross-linked, and development causes swelling and partial dissolution of the photoresist. During the development process, the pattern is subjected to external forces, such as rubbing, etc., to pull off the line edge. High-precision patterns but low mesh screens will also produce burrs. If the above situation occurs, the solution is to overcome it one by one, but the key point is to choose the correct exposure time. (2) When the exposure time is appropriate. In the screen version, the part that should not be removed is peeled off or there should be residual glue left in the part that should be displayed. Check the following problems. Whether the contrast of the photographic negative is accurate, and whether there is any contamination, etc. Whether the drying temperature of the photosensitive film attached to the silk screen before exposure is too high, causing thermal cross-linking of the adhesive film and not showing it, or due to insufficient drying of the adhesive film, development and so on. The photographic master and the photoresist film are not in close contact with each other, which has a refractive effect. The relationship between the distance between the light source and the workpiece and the exposure time is not adjusted well, causing overexposure and underexposure. The position between the light source and the screen is not correct, resulting in unequal exposure within the same time. The photosensitive glue (film) is uneven, and the quality is different at different positions on the same screen. (3) Network collapse and network collapse mold a After the screen is made, the drying temperature is too high, especially the deformation of the wooden frame is large (twisting, bending, irregular expansion and contraction) causing the collapse of the mold. b Due to changes in the weather, the tension of the wire mesh causes the collapse of the mesh and the collapse of the membrane, especially after repeated application and cleaning of the mesh for many times. c. Improper treatment of the silk screen, especially the cleaning process before the paste (coating) of the photosensitive adhesive film is not completely used, and it is easy to collapse. (4) The photosensitive film (glue) is not firmly combined with the screen. ①Decontamination and degreasing of the wire mesh are not complete. ② Insufficient exposure, check the lighting system. ③The selected wire mesh is incorrect, the mesh is too thick or the development and washing are not suitable. ④ After sticking the film or applying photosensitive adhesive, it dries too fast, so that the film or adhesive will disappear quickly. ⑤ The performance of the photosensitive material is not good. (5) Pin holes, pits, etc. appear in the template. ①Check the photo base. ② Dust causes contamination of the silk screen and photosensitive film (glue) and uncleanness on the printing glass. ③ Underexposure, uncontrolled, light source, distance and time. ④Development and excessive washing. ⑤ The film is too thin and the uniformity of the melt of the photosensitive colloid and the sensitizer is poor. In short, the quality problems of the net mold are sometimes caused by various factors. Therefore, each process should be analyzed and checked and resolved accordingly. 6. The following aspects should be evaluated to measure the quality of a screen template: First, during the screen printing process, the resilience of the screen is relatively stable. During the screen printing process, the screen mold is close to the original tension before the screen printing. Second, the printing material has good performance. Third, the edges of the pattern lines are smooth, without jagged or pitting; 3. Selection of printing materials Due to the development of the silk screen process with the national economy. It needs to develop, and the printing material in the screen is an indispensable part of the entire screen printing. The screen printing must be based on the screen (template) and the printing material. There are indeed many printing materials currently used in screen printing, but they are divided into three categories; 1. Water-based printing materials 2. Solvent-based printing materials (1) Physical drying type (2) Oxidation drying type (3) Chemical dry solid type [next] 3. UV-curable printing materials Due to the applicability, no printing material is omnipotent. The printing material with the highest bonding strength and the best screen printing performance should be selected according to the requirements of the workpiece material to ensure the quality of the screen printing. In order to achieve the goal, according to the specific requirements of the workpiece, the printing materials must meet the following conditions: (1) In the screen printing process, the printing material can smoothly pass through the screen, and it has fidelity and good leveling performance (refers to fineness, printing ability and coverage performance) without drawing. (2) The printing material cannot dissolve or swell the omentum. The premise is that under the specific conditions of the workpiece, the printing material is selected first, and then the screen die. (3) The color of the printing material should have obvious color difference with the workpiece, and should have good adhesion with the workpiece. (4) The printing material should have suitable viscosity and dryness at the time of printing. ①It is not easy to dry during silk screen printing, and it will dry quickly after printing. ②After the printing material is dried, no bubbles (bubbles) or cracking are allowed. ③The thixotropy performance is better, (even if the imprint is very thick, but it will not dissolve, out of shape, no pinholes, etc.). (5) Printing materials should be selected with low toxicity, low cost and wide resources. (6) The printing material should have high chemical stability, and it will not dissolve, peel or blister in special etching materials. (7) Special printing materials should have high insulation resistance, high temperature resistance, etc. (8) The screen template after screen printing is easy to clean, and it is used as temporary printing material, which is convenient and simple to remove. 4. Pretreatment of substrates (workpieces) In the screen printing, the substrates (workpieces) include paper, acrylic, ceramic glass, metal, textile, and paint surface, as well as the development of the electronics industry in which plastic products are replaced by plastic. In order to increase and improve the bonding strength between the printing material and the substrate, the screen-printed surface of the substrate should be consciously subjected to auxiliary treatment. It is not possible to treat the surface of the substrate before screen printing (including removing dust). The purpose of the pre-treatment is to improve the adhesion between the printing material and the substrate, as well as post-screen printing post-processing, such as anodizing penetration type ink, ink and micro-bead foam ink, etc., must be post-processed to meet the requirements. According to the different materials of the substrate, the corresponding processing method is adopted. Fifth, the parameters of the scraping movement in screen printing In screen printing, the screen (die) and the substrate should have two relative positions, one is static (that is, there is no scraping action), and the two are in a non-contact state; the other is the screen and the substrate when scraping It is in a non-contact state; and this line always moves at a constant speed relative to the substrate. Without the transition and change of these two states, the desired pattern cannot be printed. If you can accurately grasp the template and the substrate in the silk screen The change in the relative position between objects is the theoretical basis for ensuring the quality of the silkscreen during the silkscreen process. In other words, the screen printing operation must be carried out under two prerequisites. First, at rest, a certain distance is maintained between the screen of the screen and the substrate, and the size of the distance has an impact on the quality of the screen. Second, the screen of the screen during the screen printing is kept in close contact with the substrate—a line contact. If there is a small contact, what result will be brought to the quality of the screen printing and so on. What is the principle of screen printing Screen printing is composed of five major elements, namely screen printing plate, squeegee, ink, printing station and substrate. The basic principle of screen printing is to use the basic principle of screen printing to make the screen part of the text permeable and non-graphic part of the screen impermeable to ink. During printing, pour ink on one end of the screen printing plate, apply a certain pressure on the ink portion of the screen printing plate with a scraper, and move toward the other end of the screen printing plate. The ink is squeezed by the scraper from the mesh in the graphic part to the substrate while moving. Due to the viscosity of the ink, the imprint is fixed within a certain range. During the printing process, the squeegee is always in line contact with the screen printing plate and the plexiglass screen printing substrate. The contact line moves with the squeegee movement. Maintain a certain gap with the substrate, so that the screen printing plate during printing generates a reaction force against the scraper through its own tension. This reaction force is called the rebound force. Due to the resilience, the screen printing plate and the substrate are only in mobile line contact, while the other parts of the screen printing plate are in a detached state from the substrate. The ink and the screen are broken, which guarantees the dimensional accuracy of the printing and avoids smearing the substrate. When the squeegee is scraped across the entire plate, it is lifted, and the screen printing plate is also lifted, and the ink is gently scraped back to the original position. This is a printing trip.
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