At present, the plate material used in water offset printing in China is basically the positive version of PS. The base of the PS version is aluminum. The key of the PS version is to maximize the tone level of the dots. As the printing operator, it is necessary to provide qualified printing plates for the post-process. Operators of offset presses need to use good printing plates so that they can print good quality prints. However, in reality, there are often contradictions between the printing process and the printing process for printing plate quality problems. It is sometimes difficult to clarify who is responsible for competition, but there are always rules to follow. In general, the failures in the printing process can be summarized as: virtual, deep, shallow, poor, and low. To be specific, the graphics are fictitious, deep, shallow, and the blank part is dirty, the inking of the graphics and text is poor, and the printing plate has low durability. Most of these faults are found during the printing process. As an offset press operator, it is necessary to understand the causes of some printing faults.
The main operations of the printing process are aspiration, exposure and development. It is necessary to master the exposure time, and to develop the concentration and time of the developer.
1. False text
(1) The vacuum pump of the printing machine has poor suction performance and insufficient vacuum, and the original printing plate and the plate material are not in good contact.
(2) The graphic density of the original printing plate is not enough, and the graphic exposure makes the graphic false.
2. The picture is too deep
(1) Underexposure (2) The developer concentration and temperature are too low, the development time is too short, and the developer is fatigued.
3. Graphic is too shallow
(1) Overexposure or uneven exposure of the PS version is too long. Due to the diffraction of light, the photosensitive layer that should not be seen also sees the light, causing light decomposition, reducing the dots, thinning the text lines, and even breaking.
(2) The developer concentration is too large, the temperature is too high or the development time is too long.
4. The blank part is dirty
(1) The printing glass is dirty (2) There is dirt or high haze on the original printing plate (3) The printing time is not enough (4) The development is insufficient or the development alkalinity is too strong (5) Improper removal
5. Poor inking of printed graphics
(1) Overexposure (2) In most cases, the development time is too long, the development concentration or the temperature of the developer is too high will cause a small amount of photosensitive layer in the graphic part to be dissolved, causing 2% dot loss. Therefore, the development time must be appropriate. Generally, the newly developed developer has a shorter development time when the development temperature is high, otherwise the development time must be appropriately increased.
6. Roasted version causes low printing resistance
Nowadays, in order to pursue durability, many printing companies bake the printed plates. The key to baking is temperature and time. After baking PS version, the normal color is reddish brown. If the layout is tea green or dark green, it means that the baking temperature is too low or the baking time is too short, and the resin in the photosensitive layer is not fully cured. At this time, the baking or extending baking time should be increased until the color of the layout reaches red brown . If the plate surface is dark brown, it indicates that the baking temperature is too high or the baking time is too long, so that the resin in the photosensitive layer is partially carbonized. The darker the color, the more carbonized resin, the lower the printing durability of the printing plate.
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