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Can PP Woven Bags Be Printed?

Can PP Woven Bags Be Printed?

The answer is definitely yes, and woven bags are printed in a variety of ways. Among them the most important are flexographic printing and gravure BOPP lamination printing. According to the different needs of end customers for the printing quality and cost of woven bags, the manufacturers of woven bags should provide customers with corresponding printing options. In this article, we will give a brief introduction to flexographic printing.

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Flexographic printed pp woven bag

Flexographic printing is a printing method which uses flexible printing plate and transmits ink through the screen roller. It is a kind of relief printing technology, which is called as flexo printing also. The picture and text part of the flexible printing plate is raised. During printing, the screen roller evenly applies a certain thickness of ink layer to the graphic part of the printing plate. Then, under the effect of the pressure of the press roll, the ink layer of the picture and text part is transferred to the surface of the substrate to form a clear picture and text.

Name
The name of flexographic printing got its name since it was originally used for corrugated board with very uneven printing surface, so it needs to keep contact with the board, so it should be very flexible. Moreover, the high point on the board that is not printed shall not be printed with the residual ink on the plate, which requires that the non-graphic part of the board has enough depth to meet this requirement.

History
The original flexographic printing is very poor. The improvement of plate materials and the way of plate making have been greatly improved - usually by photographic exposure and chemical corrosion, directly carved by laser. Printing of full color images is also possible.

Flexographic printing was originally called aniline printing, originated in the United States in the early 1920s. Because of its toxic aniline dye ink, it was not developed. Since then, ink manufacturers began to use widely recognized and acceptable pigment agents, and was more known as flexographic process at the 14th Packaging Conference in 1952, It is also translated into Farui letterpress in Taiwan. Since the mid-1970s, due to the progress of material industry, especially the appearance of polymer resin plate and cermet screen roller, the development of flexographic printing has made a qualitative leap. In the world, printing mode has been the fastest growing way, especially in the United States. Both the manufacturing technology of printing press and the application technology of flexograph represent the highest level of flexographic printing. Most of the packaging products seen in the domestic market in the United States are almost flexographic. According to statistics, the market share of flexographic printing in the United States accounts for 70% in the field of soft packaging printing; 85% in label printing; It is 98% in corrugated paper printing; 25% in cartons printing. In Europe, 60, 35, 85 and 2 per cent were also accounted for. Even 20 percent of newspapers in the United States are printed in flexible versions. 75% of the polypropylene woven bags produced in China are still printed by flexographic printing. These numbers are enough to prove the vitality of flexographic printing.

Characteristic
1) The flexo printing applied Ink is a low viscosity volatile dry ink with alcohol and water as the main solvent. It is fast drying and suitable for flexographic printing with high speed and multi-color printing. The application of water-based ink with no pollution and fast drying is very beneficial to environmental protection.

2) Flexible plate is a kind of photosensitive rubber or resin type plate, which has the characteristics of soft bending and elastic. The hardness of shore is generally 25-60, and the transfer performance of ink is good, especially for alcohol solvent inking.

3) The printing is carried out by light printing pressure.

4) The printing materials for flexographic printing are very extensive. Polypropylene woven fabric is one of the typicals.

5) Good printing quality. Due to high quality resin plate, ceramic screen roller and other materials, the printing accuracy has reached 175 lines / inch, and has full ink layer thickness, which makes the product layers rich and bright color, especially suitable for the requirements of packaging printing. Its striking color effect is often not achieved by offset printing. It also has the clarity of letterpress, soft color of offset printing, thick and high gloss of gravure ink layer.

6) High production efficiency. The flexible printing equipment usually adopts drum type material, which can be finished from double-sided multi-color printing to finishing, coating, stamping, die cutting, waste disposal, winding or cutting. In offset printing, more people and equipment are often used in three or four processes. Therefore, flexographic printing can greatly shorten the printing cycle and reduce the cost, For the packaging of agricultural products and related products with low added value, such as flour bags, fertilizer bags, animal feed bags, etc., the use of flexographic printing can enable users to occupy an advantage in the competitive market.

7) Easy operation and maintenance. The printing machine adopts the screen roller ink conveying system, which saves the complicated ink transmission mechanism compared with offset printing machine and convex printing machine, so that the operation and maintenance of the printing machine are greatly simplified, and the ink transmission control and reaction are more rapid. In addition, the printing press is generally equipped with a set of plate rollers which can adapt to different printing repetition length, especially for packaging printing products with changing specifications.

8) High printing speed. The printing speed is generally 1.5-2 times of offset printing machine, and the high-speed multi-color printing is realized.

9) Less investment and high returns. The modern flexograph has the advantages of short ink transmission route, less parts and light pressure, which makes the structure of flexograph simple and the material used in processing is much saved. Therefore, the investment of machines is far lower than that of offset printing machines in the same color group, and only 30% - 50% of the investment in gravure printing machines in the same color group. For small and medium-sized woven bag processing plants, one or two sets of flexographic printing machines can meet the printing needs of ordinary woven bags manufacturing without high investment.

The characteristics of flexographic plate making: in plate making, the flexible plate making cycle is short, easy to transport, and the cost is much lower than that of gravure printing. Although the printing cost is several times higher than that of the offset PS version, it can be compensated for the printing resistance, because the printing resistance of the flexible version is 500000 to several million (the offset plate has a printing resistance rate of 100000-300000).


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Flexographic printing rubber plates

Printing characteristics
1) Compared with gravure and offset printing, the density curve of flexographic printing is unstable, and the thickness of the printing plate has a great influence on the printing quality, which increases the difficulty of color overprint of flexographic plate.

2) The flexible printing adopts the screen roller to transfer ink. The screen line of the printing plate must match the number of lines of the screen roller, otherwise, the small point in the high light area will be trapped in the ink hole and cannot be printed out. At the same time, the screen angle of the four-color plate cannot be used 45 °, Otherwise, it is easy to produce craze.

3) The dot increase is more serious, because the hardness of flexible plate is low, the text is easy to deform after being squeezed, and the dot increase in printing process is more serious. The general increase range is 16% - 30%, the maximum intermediate adjustment can reach 50%, and the dot of 2% - 3% increases to 15% or higher. Therefore, the level of printing order is difficult to ensure, resulting in the whole plate level to high-key development, the dark part into the field, dot appears hollow, not smooth. The change of dot level will make the image seriously color biased, the whole picture looks unclear, unclear, the fine level is difficult to express, and the stereoscopic sense is not strong. Under the influence of dot increase, the high light level of the image printed by flexible plate is less than 10%, and the dark tone level above 85% is easy to be on the spot.

4) Because the pressure of flexographic printing is very small, the pressure changes slightly will have a significant change to the printing quality, and the control of the press force becomes very important.

5) The color separation parameters need to be reset, and the offset or gravure printing cannot be copied. Most of the flexographic printing equipment introduced in China is unit type, with 6-8 units generally. In addition, the packaging industry adopts spot color more. Therefore, the color separation of flexographic printing should consider multi-color separation to make up for the deficiency of printing Internet point performance.

6) Design reasonable post process. There are many flexographic printing units, and the online multi-channel post-processing, which puts forward higher requirements for overprint. Therefore, how to design the post-processing technology at the same time becomes a big difficulty in flexographic plate making.

7) The increase of flexographic dot is nonlinear, that is, the change is not in positive proportion to the dot area. The offset printing dot is equal distance, while the smaller the flexographic plate Internet point is, the larger the transverse deformation is, the more serious the dot increases due to the small force area. This is the opposite of the requirement that flexographic printing should draw the contrast between color and step. Flexographic dot shows the following trend: in the dot extremely small, or the net (caused by printing); Either increase to 15% or larger outlets (caused by printing). In the dark, dot points are very easy to become on the spot (caused by printing and printing).
8) Flexible plate deformation and compensation. The most obvious feature of flexible plate is its elasticity. When it is installed on a cylindrical cylinder, the plate will bend around the limit of the cylinder. This deformation will involve the graphic and graphic on the plate surface, which makes the printed text and text deviate from the original size. For printing products with low quality requirements, it may not be considered, but for printing products with high quality requirements, measures must be taken to compensate for the change of this size. The compensation methods are as follows: a) In the original design, the corresponding value is subtracted from the variable dimension according to the plate elongation. b) When taking the film, the deformation mirror is used to shorten the dimension of the change direction. c) The computer version is used, only a single zoom instruction is given before the color separation of the ink draft is finished. d) The printing drum is the same size as the plate roller.


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