For a long time, the use of offset printing products printed on the back of the dirty accidents are common, resulting in the loss of paper is less than a hundred, more than a dozen orders, resulting in poor product quality or resulting in a lack of supplemented version, direct and indirect economic losses are considerable, In addition, the quality of the dirty backside of the offset paper is much higher than that of coated paper. Why is this anomaly? Here to analyze the causes and treatment methods.
First, the problem of sticking on the back of the offset paper has not attracted enough attention.
Most people generally think that coated paper is smoother than offset paper, and it is prone to sticky backs. Therefore, it is more important to use coated paper for printing, and corresponding preventive measures are more effective. In contrast, some operators believe that the surface of the offset paper is better than the coated paper, and it is not easy to produce a back-adhesion accident. It is easy to think ideologically about the “low enemyâ€. Not only does the operator have this idea, but even production schedulers, technicians, and color-ink staff all have this idea. Therefore, from the technical measures, it is often overlooked to prevent back sticking. Some adjustments to the deployment of spot colors, the ink is adjusted lighter, resulting in over-thick ink when printing, if the operator does not check and deal with in a timely manner, there will be back dirty accident.
Second, do not pay attention to the difference between proofing paper and printing paper.
The ink layer density value of the offset color image product is generally 1.05 for yellow ink, 1.3 for red ink, 1.4 for black ink, and 1.6 for black ink, which achieves such a solid density value. The ink layer thickness on paper is about 1.2 μm. This value is very important. Generally, when the thickness of the ink layer is controlled to be less than 1.2 μm, the offset paper will not cause the back surface to be dirty. When the thickness is 1.2 μm, it is easy for the back side to be dirty.
In the process of trial printing before the official opening of the seal, the offset printing is always identified against the proof. It is always because if the proof is made of imported coated paper, and the printing paper is offset paper, the operator must increase the amount of ink in order to match the product he has printed with the proof color. The proofing on the coated paper to achieve the field density requirements, the ink layer thickness of 1.2μm, the ink on the plate as long as 2.8g/m on the line. However, the use of coated paper for printing, to meet the requirements of proofing the field density value, the version of the ink to reach at least 4g / m, and even more than this value, in this case of ultra-ink printing, ink transferred to the paper often A threshold of more than 1.2 μm is one of the risk factors for creating a dirty page. From this point of view, if the problem of the difference between proofing paper and printing paper cannot be solved, it is feared that the accident on the back of the offset paper will be difficult to eliminate. Because the difference between imported paper and domestic paper is too big, often inexperienced operators will suffer.
Third, the prevention measures taken to produce a dirty period of paper are weak.
After testing, we divided the backside sticking problem into two important stages to distinguish and recognize, namely, the initial sticky dirty stage and the solidification stage sticky dirty stage.
1, the initial stage of slime. The initial stage of sticky dirt refers to the fact that the paper is conveyed to the delivery pile during the printing process, and there is a stack of 3-5 sheets of paper, and the back side is immediately dirty. The reason for this backside sticking is that the water in the printing is large and the ink is too much; the spot color ink is acidic and the ink layer is too thick; the color sequence on the multicolor machine is reversed and the ink is adjusted too thinly. Caused by other reasons.
It is reasonable to say that the back side of the initial stage is relatively easy to find, from the take-up pile on the 3-5 sheets of paper to the stand, due to a put, a variety of force enough to make the print adhere to the above A piece of paper on the back, if you can put the back of the dirty is included in the item should be checked, open the following sheets of paper to see the back, can certainly check out, and immediately take measures to prevent dirt, the loss can be reduced to a minimum. The problem is that at the beginning of printing, due to many tasks such as school coloring and checking, it is easy to overlook the back side of the inspection, resulting in the occurrence of sticky ills from the outset. The operator has not yet estimated that there will be several thousand later. Printing paper is printed. Therefore, when printing offset paper, it is very important that the inspection of the back side is performed in the initial stage of printing.
2. The back of the imprinting stage is dirty. After the printing ink is transferred from the liquid state to the paper, it gradually begins to solidify, finally reaching a final film that is resistant to friction and drying. This process takes approximately 4-12 (h) hours (the specific time it takes to form a dry film is related to the room temperature, humidity, dry oil ratio, paper properties of the press room). After the ink is printed on the paper, the continuous oxidation polymerization reflects that, on the one hand, the heat is released on the one hand and the temperature of the paper stack rises. On the other hand, due to the limited supply of air, the unheated ink film is plastic. It is still in a softened state, and this period is a very dangerous period when the back side is dirty. If measures can be taken to loosen the paper stack once, then there will be no back-scratching accident. However, the problem is that offset paper stacks are generally high, often 5-8 orders. In many offset printing plants, there are no measures and requirements to allow the paper stack to loosen once. This is another reason why the offset paper is prone to back-smearing. After 4 hours of permeation, the back surface of the above-mentioned coagulation stage is extremely rare. Therefore, if you can determine some necessary bleeder measures, it will be of great help to prevent sticky backside of offset paper.
In short, when printing offset paper products with a large image area and a large amount of ink, the operator must take full consideration of such issues as printing coated paper, and in addition to paying attention, take necessary preventive measures, such as adding anti-adhesive agents to the ink, control it well. The thickness of the ink layer, the ventilation of the middle, etc., all can reduce the occurrence of sticky accidents on the back side of the offset paper. (Li Wen) (From "China Packaging Newspaper and Print Weekly")