Process principle
Analysis of the mechanism of laser hologram imaging on paper substrate
The imaging mechanism of laser holograms on paper substrates is basically similar to other traditional substrates (plastic films, aluminum-plated films). Holograms are obtained by holography, and then the holograms are turned into holographic metal plates, and then through heat The embossing process copies the hologram to the paper print.
However, the surface properties of paper prints are significantly different from those of plastic films or aluminized films. The film that can be used to make holographic products has thermoplastic deformation properties, and the surface is flat, which can be directly embossed for holograms, especially after aluminum plating, due to the reflection effect of the aluminum metal layer, the holograms can be greatly enhanced Visual effects. This is not the case with paper-based prints, which can neither directly emboss the ink with holograms, nor apply aluminum plating to the prints.
Therefore, to print the holographic effect on paper prints, a special printing medium is needed. This medium should have excellent thermal deformation properties, and stable and fine holographic grating stripes can be obtained after deformation. At the same time, this medium must have sufficient transparency to ensure the visibility of the original printed graphics and colors. This special hologram text printing medium is called holographic coating.
In order to obtain a good hologram visual effect on the holographic coating, the smooth and smooth coating surface quality is a key factor, which requires high gloss and good leveling of the holographic coating. In addition, other physical and chemical properties of the coating, such as folding resistance, abrasion resistance, and post-processing suitability, are also very important.
From the perspective of optical principles, the holograms and texts recorded on the holographic coating use the diffraction effect of light to reproduce the holographic effect, while the original ink printed texts use the reflection effect of light to express the printing effect. Due to the use of light In different ways, the hologram text does not interfere with the original ink printing effect. On the contrary, the transparent holographic coating enables the two printing effects to be expressed at the same time and forms a special decorative effect.
According to the imaging mechanism of laser holograms and texts on paper substrates, the basic process principle of "printing" holograms and texts on paper substrates is: apply a layer of thermoplastic holographic coating on paper or cardboard with printing ink, and then dry After the calendering process, a layer of bright and smooth coating is obtained, which is used as the memory layer of holograms, and then holographic molding is carried out on a special holographic paper molding machine, and finally a diffractive hologram is formed on the surface of the printed matter.
Therefore, the main content of this study has the following three aspects:
(1) Transparent holographic printing process: On the premise of not affecting the original printing effect, a method of directly printing holograms on paper substrate printed materials is studied.
(2) Development of holographic coatings: Development of solvent-based and water-soluble environmentally friendly paper substrate holographic printing coating formulations and preparation methods adapted to different application conditions.
(3) Holographic copying technology: develop a special type of holographic molding equipment—platform paper holographic molding machine for general offset printed flat paper or cardboard.
Transparent holographic printing process
On the premise of not affecting the original printing effect, a process method for directly printing holograms and texts on paper substrates was formulated. This process does not affect the normal printing process, can make full use of existing post-printing processing equipment, and meets the requirements of industrial production. The technological process is as follows:
The coating process is to apply the holographic coating uniformly to the printing ink surface of the paper substrate through the coating equipment, and the coating quality directly affects the holographic effect. The coating method adopts the three-roller coating method, which has the advantage that it can make full use of the existing glaze coating machine for post-press processing, and the coating and drying can be used for one machine and two purposes, and the cost is low. The coating drying method is infrared radiation heating and air drying.
After the holographic coating is leveled and dried, it is calendered by a continuous rolling calender, which can greatly improve and improve the smoothness and gloss of the holographic coating, so as to obtain the ideal holographic effect in the holographic molding process .
Holographic paint
There are two main functions of holographic coating: smoothing the surface of smooth paper and printing holograms.
The quality of the coating directly affects the holographic effect. The main technical requirements for the coating are: sufficient adhesion fastness to paper, excellent n-type deformation characteristics, and excellent physical and chemical properties and post-processing adaptability.
For different application conditions, we have developed two types of special coatings for paper holography: QXC alcohol-soluble holographic coating and QXS water-soluble holographic coating.
(1) Technical characteristics of alcohol-soluble holographic coating
Choose AB, C, and D with high refractive index and transparency as comonomers, and match hard monomers, soft monomers, and reactive monomers reasonably, and choose E and F as initiators and co-solvents. Strictly control the reaction conditions, as far as possible to maintain a constant temperature, initiator concentration, monomer concentration, solvent concentration during the polymerization process, and filled with inert gas protection, the average molecular weight of the prepared copolymer is about 90,000. The colorless transparent resin is mixed with mixed resin I (high speed and its flexibility), additives and solvents to make a paper substrate anti-counterfeiting holographic coating with excellent comprehensive properties such as high refractive index, high gloss, wear resistance and flexibility.
(2) Technical characteristics of water-soluble holographic coating
Water-based resin II has a high refractive index and can print holograms well, but the coating is brittle and has poor water resistance. To this end, we synthesized a high solid content, lower scale, good flexibility, and excellent water resistance emulsion. Combining the two organically, supplemented with additives, a high refractive index, high gloss, wear resistance, flexibility, water resistance and other comprehensive properties are produced, which is suitable for paper anti-counterfeiting and environmentally friendly holographic coatings.
Platform type holographic paper molding machine
Whether the hologram can play a role in economy, culture, art, anti-counterfeiting and daily production, one of the most critical issues is to be able to replicate in large quantities to achieve industrial production. Except for gravure printing, the general flat printing method is plain paper, and the traditional film roll molding machine cannot be used for holographic molding of flat paper or cardboard. For this reason, we have integrated the advantages of existing wide-width soft presses and hard presses, and designed and manufactured a platform-type holographic paper press. Its outstanding features are:
(1) It can be used for embossing and copying of hologram text on a single sheet of paper after printing;
(2) With automatic overprinting device;
(3) Peeling mechanism with anti-sticking plate;
(4) It has functions of automatic paper feeding, paper receiving and counting.
The platform-type holographic paper molding machine is composed of four parts: the host, the oil heating system, the hydraulic system, and the electrical control system. Its basic working principle is: the high-temperature thermal oil is heated in the oil tank, and the oil is pumped to the hollow molded holographic roller, The heat is transferred to the holographic roller to keep the roller at a certain temperature. The holographic roller and the rubber roller directly below form a pair of rotating pressure pairs. When the molding work starts, the hydraulic system pushes up the rubber roller and presses it on the holographic roller to apply a certain pressure to the holographic roller. The sheet of paper is fed one by one between the two rollers, and the rotation of the two rollers drives the cardboard forward. During the movement, under the action of a certain temperature and pressure, the holographic coating softens and deforms, and the hologram on the holographic roller is copied, and after cooling, a stable holographic effect is formed.
Technical Difficulties
Compared with the traditional laser hologram imaging of plastic substrates, the laser hologram imaging of paper substrates is more difficult, requires high technology, and is more technical.
Mainly manifested in three aspects:
(1) Due to the fiber structure, the paper surface itself cannot directly print the hologram n, but must use a special coating.
(2) When the holographic molding is copied on the surface of the cardboard, the thickness of the cardboard is much larger than that of the plastic film, and its surface quality is also worse than that of the film or aluminum coating. In addition, the movement of the flat cardboard during the molding process lacks effective traction And tension control, it is easy to cause paper "sticking" phenomenon, these adverse factors will affect the quality of the copied hologram.
(3) Except for gravure printing, the paper is cut into flat sheets of a certain size during printing. The existing holographic molding equipment for thin films cannot be directly used for the production of holographic molded paper after printing. Design a dedicated paper molding machine.
Experiment and discussion
In the research process, a lot of experimental work was carried out, including the surface properties of the substrate, the physical and chemical properties of the coating, the quality of the hologram, and the production process parameters, etc., due to space limitations, only the analysis of the molding and hologram performance.
Holographic molding
The experimental results show that under certain conditions of molding pressure and machine speed, within a certain range, the holographic coating exhibits four states: glass state, highly elastic state, plastic state and viscous flow state with increasing temperature. For example, the alcohol-soluble holographic coating has a brittle temperature Tg≈50 ° C, a softening temperature Tm≈65 ° C, and a degradation temperature Td≈120 ° C. When molding and copying, the holographic coating should be in a plastic state, and the moldability of the coating is the best in this temperature range. If the hot pressing temperature is higher than the lowering temperature, due to the increased viscosity of the paint, it will cause a sticking phenomenon, that is, the cardboard cannot be peeled off in time, and is rotated on the holographic pressing roller. This situation should be prevented during production.
Hologram performance
In order to test the holographic effect of paper substrates, we entrusted China Packaging Research and Test Center to test the produced holographic paper products with reference to GB / T17000-1997 "General Technical Conditions for Anti-counterfeiting Holographic Products". The inspection items are appearance, diffraction efficiency and signal-to-noise ratio. In the testing process, we found that the ink color of the sample has a great influence on the diffraction efficiency. The general trend is that the diffraction efficiency is the highest on a white background (68%), the diffraction efficiency is the lowest on a black background (0.52%), and the diffraction efficiency for other background colors is somewhere in between. After a preliminary analysis, the main reasons for this phenomenon are: â‘ The absorption of light by the color reduces the signal intensity, so the measured diffraction efficiency of the hologram of the paper substrate is lower than that of the aluminum coating; â‘¡ Different background colors The different light absorption rate makes different diffraction efficiency for different areas of the same sample.
It can be seen that the existing holographic effect detection standards are formulated for aluminized holographic film or aluminized holographic paper, and its index system and detection method are not fully applicable to transparent holographic paper with ink printing colors. The influence of ink color on performance indexes such as hologram diffraction efficiency and its regularity has yet to be further studied.
in conclusion
This technology solves the problem of copying holograms on post-printed paper. Its main features are:
(1) A laser holographic image can be generated on the printed paper surface. After a reasonable decoration design, the transparent holographic rainbow image is matched with the ink printed image to obtain a special printing effect;
(2) What is obtained is a transparent hologram, which does not destroy the original printing effect and does not require the evaporation of aluminum (or any other inorganic) layer; it can make full use of post-press processing equipment and be integrated into the existing printing process;
(3) As a post-printing processing method, it also has the function of protecting the printed surface, and has the characteristics of improving brightness, water resistance, acid and alkali resistance, and wear resistance;
(4) The holographic coating involved does not contain toxic substances, and the resulting holographic product does not contain plastic films or metal media, and is an environmentally friendly paper product that can be fully recycled.
Its research significance is mainly manifested in the following three aspects:
(1) Improve the anti-counterfeiting of packaging. The anti-counterfeiting function of this technology is as follows: â‘ Holographic anti-counterfeiting: by making difficult holographic images, using appropriate encryption and other technologies, it can increase the difficulty of imitation; â‘¡ process anti-counterfeiting: counterfeiters must master a full set of process technology and have all production equipment ; â‘¢ Material anti-counterfeiting: holographic paint, holographic paper, and cardboard all have anti-counterfeiting functions; 5 Overall anti-counterfeiting: the final product itself is an anti-counterfeiting unit, and can be combined with other anti-counterfeiting technologies (such as structural anti-counterfeiting, ink anti-counterfeiting, etc.) to form a comprehensive anti-counterfeiting System to realize the overall anti-counterfeiting in the true sense.
(2) Expand the application of laser holography. This technology successfully uses natural light as the medium, reproduces on paper and can copy a large number of color holograms with depth of field; color transparent holography and ink printing graphics and colors can form a wonderful overall decorative effect, which is for printing , Packaging and decoration, advertising and other industries, its significance is extremely far-reaching.
(3) Huge social and economic benefits. The technology has strong anti-counterfeiting strength, exquisite decorative effect, and simple manufacturing process. The lower production costs will bring considerable social and economic benefits.
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