First, the new situation
The in-depth development of the socialist market economy and the inevitable trend of economic globalization have prompted the printing industry to closely follow the development of high-tech industries in the world, adopt advanced digital technologies and continue to develop new and superior materials. The printing process subjects are printed by newspapers and periodicals with a certain number of the same contents and printed in the same manner at one time, and they are turned to many types, fewer quantities, high quality, content and layouts, and the need for surface finishing and complex forming packaging after printing. print. The prepress production of digital transmission and high-resolution full-force scanning and direct plate making technology, together with a multi-color high-efficiency printing press with an hourly speed of more than 15,000, greatly shortens the printing cycle.
The position of paper selections for book covers, merchandise packaging, and personalized prints tends to be increasingly advanced, super-polished, fine paper or cardboard.
Commercial prints have higher requirements for the beautification of printed substrates and surface finishes. These are the new processes that have impacted the current process and technology of the post-print finishing solid-phase coating. Studying these new situations, proposing corresponding new technologies and solutions, and promoting continuous innovation in the technology of post-press coating, can improve the overall level of the printing industry and open up a vast market for the printing industry.
Second, the impact of the new situation
1. In order to harmonize with international publication standards, the National Press and Publication Administration informed that since May 1, 2000, books and magazines and their sizes and sizes must comply with the GB/T788-1999 standard. With the increase in the size of books and magazines, textbook publishing departments in many provinces and autonomous regions have changed their paper to offset paper, and illustrations and cover sheets have been upgraded to coated paper.
Commercial printing tends to be high-quantitative coated paper and super-polished paperboard.
Coated paper and non-coated paper, high-quantity coated paper and low-weight coated paper, the basic difference is the smoothness of the paper. To improve printing quality, high-smooth coated paper or coated paper is used, and the printing effect is reflected in rich color expression and realistic image reproduction.
The coated and calendered coating layer and full ink printed layer on the base paper substrate will cause the surface tension of the paper to drop, which will affect the wettability and adhesion of the adhesive when the film is coated and compounded. The dense and dense coating layer and the external force of calendering on the coating increase the density of the paper while increasing its density. Under the same process conditions, the operation of the coating technology will not reach the diffusion, penetration and dislocation anchorage of the past adhesive molecules to the paper substrate. Not only does the adhesive strength of the composite under this surface state decrease, but even metallic inks such as gold, silver, etc., in which ink pigment particles are coarse, are easily peeled off.
2. The color design of a large number of publications and packaging products adopts a large area of ​​color blocks and two primary colors and black (four colors) as the main tone. The solid color of the high-density dots or the overlapping color of the dots increases the thickness of the ink print. Due to the basic conditions such as lack of time and area in contact with the air, the oxidative drying of the ink can only be performed on the surface of the printing layer at the instant when it is separated from the printing press.
The layer of ink that is dry on the surface and cannot be dried on the inside will only dry gradually after extensive contact with the air, after it has been coated, bound, or formed into a box. In this process, ink thinners (solvents) that evaporate to dry out will not only cause blistering and stripping of the coating film that is processed by the wet glue compounding process, but also cause the odor migration of the solvent to volatilize, and make the goods appear inside the package. Odor or deterioration.
3. The significant reduction in the printing cycle is reflected in the fact that the moisture content of the paper after finishing in the finishing process is still high. Under the hot-pressing working conditions, the water vapor generated by the volatilization of the moisture contained in the paper is not only the release agent that reduces the composite adhesive force, but also the surface crust of the hot melt adhesive, and the leveling property and permeability are destroyed.
4. The elimination of the reform of the binding process of the iron wire binding has made the wireless glue binding process an important technology for the four printed pages and more. The glue-adhesive hot melt with softening point ≥170°C puts forward new requirements for the heat resistance of film-covered products. Solvent-sprayed quasi-precoating films with a compound temperature of about 70°C can easily cause localized defoaming at the back and side of the book.
5. The diversification of post-printing die-cutting and indentation has caused the film products to be impacted by different physical shapes and strengths at certain locations. These local impact forces are far greater than the composite pressure. If the composite pressure is small and the film tension is large, it will cause the film at the local force to bounce off or break.
Third, the corresponding new measures
Over the past two years from 1999 to the present, we have been studying new technologies for printing and binding, box-making (cassette) technology, and changing and upgrading printed-substrate substrates.
1. Apply new process
The improvement of the surface smoothness of the printing substrate, the shortening of the printing process cycle, the increase of the water content of the paper, and the residual solvent of the ink not completely dried, overlaps with the organic solvent, which remains a certain amount of the product after the production of the wet glue compounding process. The composite strength of laminated products is further damaged. Ensuring zero pollution and high-strength compounding has become a necessity for the reform of the lamination process.
Selecting a solvent-free melted and coated hot melt adhesive pre-coated film (smell without odor, hand-touch non-stick) as a composite material, and using a heat-pressed dry composite process are basic measures.
2. Specification operation
When applying a film to a calendered coated paper substrate with a small surface tension, ensure that the adhesive has a good leveling property. In the case of synthetically modified thermoplastic adhesives, a good solid-to-liquid phase transition begins only under thermal conditions not lower than 80°C, and tackifying resins and primary tackifying resins can function. At this time, the thermal power provided by the composite machine is also continuously absorbed by the composite substrate paper. If the substrate has a large amount of sizing, a small tightness, a high humidity, a thick ink layer, and a heavy color, the heat absorption is greater. In addition, the heat is unstable, uneven, and the supply is not enough, and the heat energy acting on the adhesive is also attenuated. Therefore, the compound production process is often in the heating process. The temperature set during the production operation is appropriately increased in consideration of the above-mentioned factors and the actual influence of various conditions such as temperature, humidity, ventilation energy, and the like in the production environment that cause the composite thermal drop. The lowest value of the general compound set temperature should not be lower than 85°C, and the highest value should not exceed 115°C. When the composite working pressure and the running speed of the workpiece are determined and the continuous production operation is performed, the heating system needs to be kept at a constant temperature state, and it is normal for the composite product to have good brightness. The temperature raising process that reaches the lowest value at the end of the set temperature should be avoided.
Film is produced by hot pressing. The guarantee of its heat needs pressure cooperation. The working pressure is preferably 8-15Mpa. Depending on the basic conditions of the workpiece, the pressure supply can be appropriately increased, but it should not be reduced.
The speed of the workpiece determines the actual value of the compound working conditions (heat, pressure). When the temperature and pressure are constant values, the main engine speed is 300 r/min as the benchmark, in the adjustment area of ​​300-600r/min, the host speed is increased by 100 turns per minute to adjust the vehicle to adjust. . The composite effect was tested, and each speed was increased by one grade. Compared with that before the increase, the temperature was decreased by 3.4% and the pressure was decreased by 9.4%. The rate of change was still within the adjustment range of 600 to 800r/min. The temperature of the grade falls by 5.2% and the pressure is 11.1%. When this grade is increased to 900r/min, the rate of temperature and pressure decrease to 800r/min. 8% or 13%.
Based on the actual conditions of the current composite substrate (composite of printed matter and BOPP film), in order to ensure a good initial viscosity and better after-effect, the running speed of the workpiece should not exceed 12m/min (600r/min for the main engine rotation speed). Composite products with high brightness and good fastness prevail.
3. Surface pretreatment
The above two measures are to adapt to changes in process and technology operations after the smoothness of the substrate surface is improved.
For a large area of ​​deep color, printing thicker print film, technical adjustment alone can not achieve perfect results. It is important to be able to pretreat the surface of the substrate, increase the surface tension without changing the appearance and color performance of the printed product, and provide reliable operation and post-curing durability for the composite initial strength. Practice has proved that the electrostatic pretreatment of the surface of the substrate is another technological innovation in the coating process in recent years.
The principle is:
The conventional power supply of 220V or 380V, 50HZ generates high-voltage high-frequency currents of 20 to 30 kV and 15 to 20 kV through a thyristor-type circuit, and is applied to a pair of electrodes with a dielectric body (usually quartz) as an interval to form an electrode. Silent diffuse blue-violet glow discharge.
This kind of blue-violet EDM with uniform and strong dispersion between the electrodes has an impact on the media of the paper that passes through it. The electrostatic field can adsorb and remove the dust, oil, water and gas on the surface of the paper, and can also stimulate the activity of the resin molecules in the surface coating and ink combination. In addition to attaching debris on the surface of the cleaning paper, the surface properties are improved, the surface tension is increased, and the affinity of the paint or ink layer with the adhesive is improved.
Oxygen gas ionization through the electrode gap generates ozone during discharge. Ozone is an excellent oxidant. The chemical reaction process of the oxygen ionization in the air during the full opening of the printed work piece can promote the drying and solidification of the offset resin ink, and change the dry and concretion of the ink print surface after printing.
4. Heat drying moisture
The shortening of the printing cycle makes it difficult to volatilize the moisture absorbed by the paper. The water that has not yet evaporated is generated as water vapor in the hot press working interface. Through the very short moment of the interface (0.11S at the instant when the workpiece running speed is 12m/min) and a very small included angle (almost coincident with the covered film), it escapes from the paper substrate into steam. Moisture, at the same time, is sandwiched between the film and the paper in a gaseous state, and is integrated with the nature of the separation layer. Affects the transparency and bonding strength of the composite.
Before the film is coated, drying the prints to reduce the moisture content of the printed matter has become another basic new technology of the post-press processing. Not only can it improve transparency and composite strength, it can also help to reduce the occurrence of compound wrinkling problems and wasted waste.
For small batch processing, the prints can be spread evenly in a laminating machine or other heat source environment. It can also be used to blow hot air or dehumidify by air conditioner or clip adjustment.
Large quantities of printed matter can be changed from a baking paper room. In a room with closed doors and windows, a paper drying device consisting of several infrared electric heat pipes and fans is installed. The heat generated is blown by the fan from different angles to the stack. Ventilation fans and automatic temperature control devices for external ventilation can maintain indoor humidity and temperature of 50% and 30°C, respectively. Printed products that have just been removed from the machine can meet the laminating requirements in 1 to 6 hours in this environment. Pay attention to the uniform dispersion of hot air. Heat-drying and dehumidifying printed products are first used in batches and then used in batches. Pay attention to the control of indoor temperature and humidity, timely check the dry condition of the paper, to avoid the fast dry fast edge or excessive brittle folding.
The new situation and corresponding technology faced by today's lamination
December 23 08:07:45, 2023
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