Some problems easily overlooked in screen printing

In the large-volume screen printing production process, if a specific link does not pay much attention, it will bring a series of troubles to the production, and this situation will not be resolved a day, it will always plague production.

Here we mainly discuss several issues that are often encountered in production but are easily overlooked.

First, the color of the screen

The colors of the screen are white, yellow, amber, red, and the like, of which the yellow screen is preferred. Our production of commonly used Japanese or German imports of 270 ~ 280T yellow screen, for particularly delicate prints, we often import 350T yellow screen.

Why use a yellow silk screen? We know that the brilliant colors of the natural world are all due to the effect of light. Without light, there is no color. The color of light is determined by the frequency of light waves. Red, orange, yellow, green, green, basket, and purple are the light that people's optic nerves can feel, called visible light. The surface of the object absorbs light selectively and reflects it to get different colors. In short, any color we can see is the color of light. That is, when a beam of light hits the surface of a yellow object, only the yellow light is reflected, irritating our visual nerves and coloring, while Other colored light is absorbed, so we see that the color of the object is yellow.

Since the screen printing uses ultraviolet light during the exposure of the printing plate, the purple light is reflected by the mixture of two colors of cyan and magenta in the color, that is, it lacks a yellow hue in the black. At the time of exposure, the yellow of the purple screen mesh is superimposed on black, so the surface of the yellow screen (ie, the bottom of the emulsion layer) is not sensitive or less exposed, so that the phenomenon of “back exposure” does not occur and the resulting image has high definition. Does not produce vignetting.
What is back sun? That is, when light reaches the emulsion layer through the transparent portion of the film, reflections, refractions, and slow shots are generated on the surface of the white screen at the center or bottom of the coating layer, which is just exposed from the bottom and the inside of the coated surface. This phenomenon is called "back sun." It can cause halo when exposed, affecting the reproduction of tones.

Second, the coating of emulsion

The prepared photosensitive liquid was placed in a stainless steel tank, and the photosensitive liquid accounted for about half of the tank. The coating tank is in contact with the lower end of the inclined screen, and while the tank is tilted out of the photosensitive liquid, the tank is slowly and evenly lifted up and applied along the screen surface. The coating speed should not be too fast. If it is too fast, it will cause foaming of the emulsion and pinholes on the film surface. If it is too slow, it will cause uneven emulsion coating. In order to obtain the best coating effect, it is best to apply the coating from the side that is in contact with the substrate during printing. The area to be coated should be slightly smaller than the screen frame, and then turn the screen upside down and apply it again. Finally, The backside of the screen was coated and dried to allow the emulsion to fill the mesh. It is best to dry with 30-35°C hot air so that the drying time can be shortened. If the temperature is higher than 40°C, the screen will produce hot fog.

Sensitive emulsions have low sensitivity in the liquid phase, the sensitivity rises with the drying of the coating film, and can only reach the specified sensitivity after being completely dried. Therefore, sufficient drying is performed before the printing, and the printing is completed in a short time after drying. .

Of course, due to the different production conditions of various manufacturers, there are also differences in their coating methods and times.

Since the screen is covered with dust, the screen will have pinholes, so keep the room clean. At the same time, poor handling of screens can also result in excessive pinning of the screen surface. When the emulsion is coated, the drying place is too bright, which will cause the fog of the screen full-length image, and affect the clarity and saturation of the printed matter.

Third, printing, trimming

Before printing, you should check the photosensitive surface of the film and the glass surface of the printing table for dirty work or dust. It is best to clean it with cotton and alcohol before cleaning. The surface of the film with silver halide attachment is frontal, and the glossy side of the film is negative. When printing, the film surface of the film and the emulsion surface of the screen should be closely attached, leaving the bite line for exposure. Exposure time should be based on the type of photosensitive thorn, the coating thickness of the photosensitive film, the degree of blackness of the positive picture, and the printing requirements. The thicker the coating layer of the photosensitive film, the longer the exposure time required for the film to be completely cured. If the image on the Yang picture is not solid enough, ie gray but not black, the exposure time should be shortened and the development time should be extended accordingly so that the unilluminated image part is completely transparent.

After the development is completed, the surface of the screen should be quickly removed with a sponge or a highly absorbent paper to perform hot air or dry naturally. If the heating temperature is too high during drying, the image on the printing plate will produce errors to a certain extent or cause the screen plate to loosen, affecting the overlay accuracy during printing.

In order to strengthen the stocks, improve the water resistance and solvent resistance, and prevent the joints of the screen and the frame from leaking ink during printing, the edges of the graphics may be wrapped and reinforced with a wide tape, and plugged with the sealant or emulsion. Bonding sites, pinholes on the screen, and unwanted openings. For the small stain on the pattern part and a little of the sensitized adhesive, you can use a wide tape to stick it, you can also use light cyclohexanone or alcohol to gently wipe it, you can also use a small needle to gently point, make the screen to keep Transparent. However, the action is soft and gentle, and the action is too large and will damage the screen.

Fourth, printing

In most manufacturers, screen printing is not done by one person alone, but by many people working together to complete the division. Only the upstream process is responsible for the next process, and the self-inspection and self-examination of each process are done well, and the quality is strictly controlled, so as to produce a screen printing product satisfactory to the user. The perfect match between screen, ink, and printing is a prerequisite for good print. The quality of the printing plate directly determines the quality of the printed product. Printing is only a test of the quality of the plate. However, under the premise of stable quality of the printing plate, due to the lightness of the printing staff, the ideal print can not be obtained, but will result in More waste, defective or waste products.

Before printing in batches, the printed screen should be fixed first, and the ink should be poured into the test to check the level, ink, ink thickness, specifications, and registration accuracy of the copy. If the registration accuracy is poor, you can adjust it according to the fine adjustment knob on the front of the frame; if the hue of the ink is not correct, check whether the wrong ink is used and replace it immediately; if the ink is bad, use a solvent to wipe the graphic part of the printing plate to make the ink transparent. Over-performance and wettability of the substrate have been improved; if there is a large difference in the tone and level of the graphic, the printed screen should be replaced immediately. The following points must be noted during the printing process.

(1) Look at the rules. As the printing plate moves back and forth during printing, the printing plate and the frame are subjected to collision and vibration in the back-and-forth operation. A small displacement occurs between the printing plate and the frame, resulting in inaccurate overprinting and affecting the appearance quality of the product. Therefore, we must always look at the rules, discover them in time, and adjust them in time.

(1) Mostly due to the sticky plastic film on the plate or the ink crust drying to block the screen. If the ink is found to be uneven or faded, the viscosity of the ink may be too high. The viscosity of the ink should be shut down immediately by diluting the viscosity of the ink. At the same time, the graphic portion of the plate should be wiped with gasoline to prevent bleeding or blocking of the net so as to facilitate continuous operation.

(3) The stacking of semi-finished products should be neat. In the process of stacking semi-finished products, it is necessary to let the two sides of the paper fall down. Otherwise, the paper is bowed due to its own gravity, the two sides of the paper are upturned and not flat, and the paper is difficult to convey during printing, and the phenomenon of breakage often occurs. When the sheet is uneven, it can be leveled with cardboard. At the same time, the height of the sheet must be adjusted with the adjustment knob of the sheet separation mechanism so that the paper can be smoothly conveyed and the printing process can proceed smoothly.

(4) The blade of the drying device has been damaged due to long-term use, and some places are not synchronized with the operation of the mainframe, often resulting in spurious prints on the paper frame. At this time, we should try to get rid of the disease quickly and immediately remove it from the bottom of the chaotic sheet. It is better not to make the ink film face to avoid stickiness or defective products.

Therefore, only by attaching importance to all aspects affecting the quality of printing, can we produce beautiful printed matter, increase the yield, reduce the consumption and increase the benefit, and make the enterprise stand invincible in the market competition.

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