Process principle

Analysis of Imaging Mechanism of Laser Holographic Image on Paper Substrate

The imaging mechanism of the paper substrate laser hologram is basically similar to other traditional substrates (plastic film, aluminized film). Holograms are obtained by holography, then the hologram is turned into a holographic metal plate and then passed through the heat. The molding process copies holograms onto paper prints.

However, the surface properties of paper prints are significantly different from plastic films or aluminized films. The film that can be used for making holographic products has thermoplastic deformation properties, and the surface is smooth, and can directly perform holographic text imprinting. Especially after aluminum plating, due to the reflection effect of the metallic aluminum layer, the hologram can be greatly enhanced. Depending on the effect. However, paper-based prints are not the same. It is not possible to imprint holograms directly on inks, and it is impossible to apply aluminum-plating on prints.

Therefore, to print holographic effects on paper prints requires a special printing medium. This medium should have excellent thermoplastic deformation properties and be able to obtain stable fine holographic grating stripes after deformation. At the same time, this medium must have sufficient transparency to ensure the visibility of the original printing graphics and colors. This particular hologram media is called a holographic coating.

In order to obtain a good hologram visual effect on the holographic coating, the smooth and smooth surface quality of the coating is a key factor, which requires the holographic coating to have high gloss and good leveling. 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 optical point of view, the hologram recorded on the holographic coating is to use the diffraction effect of light to reproduce the holographic effect, while the original ink printing graphic is to use the reflection effect of light to express the printing effect, due to the use of light Different ways, hologram text does not interfere with the original ink printing effect, on the contrary, a transparent holographic coating allows two kinds of printing effects can be expressed at the same time, and form a special decorative effect.

According to the imaging mechanism of paper substrate laser hologram, the basic principle of “printing” a hologram on a paper substrate is that a layer of thermoplastic holographic coating is first applied to the paper or paperboard that has been printed with ink, and is dried. After the calendering process, a layer of bright and smooth coating is obtained as a memory layer of the hologram, and then holographic molding is performed on a special holographic paper molding machine. Finally, a transparent 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: Under the premise of not affecting the original printing effect, a process method for directly printing holographic text on paper substrate prints is studied.
(2) Development of holographic coatings: Development of a solvent-based, water-soluble, environmentally friendly paper substrate holographic printing coating formulation suitable for different application conditions and its preparation method.
(3) Holographic replication technology: A general-purpose holographic press-molding device, a flatbed paper holographic press, is developed for general offset sheet or paperboard.

Transparent holographic printing process

On the premise of not affecting the original printing effect, a method for directly printing holographic text on paper substrate prints was developed. This process does not affect the normal printing process, can make full use of existing post-printing processing equipment, and meet the requirements of industrial production. The process flow is as follows:

In the coating process, the holographic paint is uniformly applied to the surface of the printing ink of the paper substrate by a coating device, and the coating quality directly affects the holographic effect. The coating method adopts the three-roller coating method, which has the advantages of making full use of the existing post-printing coating coater, and coating and drying one machine for two purposes with low cost. Drying of the paint is dried by infrared radiation heating.

After the holographic coating is leveled and dried, it is then calendered by a continuous rolling calender. The smoothness and gloss of the holographic coating can be greatly improved and improved so that an ideal holographic text effect can be obtained in the holographic molding process. .

Holographic coating

The main role of holographic coating is two: smooth and clean paper surface and printing hologram.
The quality of the coating directly affects the holographic effect. The main technical requirements for the coating are: sufficient adhesive fastness to paper, excellent plastic deformation characteristics, and excellent physical and chemical properties and post processing suitability.
For different application conditions, we developed two types of paper holographic special coatings: QXC alcohol-soluble holographic coatings and QXS water-soluble holographic coatings.

(1) Alcohol-soluble holographic coating technology features

Select the refractive index and transparency of AB, C, D comonomer, and hard monomer, soft monomer, reactive monomer reasonable match, select E, F initiator and co-solvent. Strictly control the reaction conditions, keep the temperature, initiator concentration, monomer concentration, solvent concentration in the polymerization process as much as possible, and fill with inert gas protection. The average molecular weight of the obtained copolymer is about 90,000. Colorless and transparent resin, in addition to the addition of compound resin I (high-speed flexibility), additives, solvents made of a high refractive index, high gloss, wear resistance, flexibility and other comprehensive performance excellent paper substrate anti-forgery holographic coating.

(2) Technical Features of Water-Soluble Holographic Coatings

The water-based resin II has a high refractive index and can well print a hologram, but the film is relatively brittle and has poor water resistance. For this reason, we synthesized a water-based emulsion with high solids content, low scale, good flexibility, and excellent water resistance. The organic combination of the two, supplemented by additives, to produce a high refractive index, high gloss, wear resistance, flexibility, water resistance and other comprehensive performance, suitable for paper security, environmental protection type holographic coating.

Flatbed holographic paper press

Whether the hologram can play a role in economy, culture, art, anti-counterfeit and daily production, one of the most critical issues is to be able to replicate in large quantities in order to achieve industrial production. Due to the fact that except for gravure printing, the general lithographic prints are plain sheets. The traditional film web presses cannot be used for holographic stamping of flat sheets or cardboard. To this end, we integrated the advantages of existing wide-format soft presses and hard presses, and designed and manufactured a platform-type holographic paper press. Its outstanding features are: (1) can be used for hologram copying of post-press sheetfed copying; (2) automatic overprinting device; (3) peeling mechanism with anti-sticking plate; (4) automatic feeding , paper collection and points function.

The platform-type holographic paper mold press consists of four parts: the main unit, the oil heating system, the hydraulic system, and the electrical control system. The basic working principle is that the high-temperature heat-conducting oil is heated in the oil tank and pumped by the oil to the hollow molded holographic roll. Pass the heat to the hologram roller so that the roller maintains a certain temperature. The hologram roller forms a pair of rotary pressure pairs with the rubber roller directly below it. At the beginning of the molding operation, the hydraulic system lifts the rubber roller up and presses against the holographic roller to apply a certain pressure to the hologram roller. Single sheets of paper are sent one by one to the two rollers, and the two rollers rotate to move the cardboard forward. During the exercise process, under a certain temperature and pressure, the holographic coating is softened and deformed, and the hologram on the hologram roller is copied and cooled to form a stable holographic effect.

Technical Difficulties

Compared with the traditional plastic substrate laser hologram image imaging, the paper substrate laser hologram image imaging is difficult, the process requirements are high, and the technology is stronger.

Mainly in three aspects:

(1) Due to the fibrous structure, the surface of the paper itself cannot be directly hologram-printed, but must rely on a special coating.

(2) When the holographic molded paperboard surface is replicated, because the thickness of the paperboard is much larger than that of the plastic film, the surface quality is also worse than that of the film or the aluminized film. In addition, the movement of the flat cardboard during the molding process lacks effective traction. And tension control, easily lead to paper "sticky" phenomenon, these adverse factors will affect the quality of the hologram copied.

(3) In addition to the gravure, the paper is divided into flat sheets of a certain size at the time of printing. The existing hologram-molding equipment used for the film cannot be directly used for the holographic-molded production of the sheet-fed paper after printing. Design a dedicated paper press. $Page break $

Experiments and discussions

During the research, a lot of experimental work was carried out, including the surface properties of the substrate, the physicochemical properties of the coating, the quality of the hologram, and the production process parameters. Due to space limitations, only the properties of the molding and hologram were analyzed here.

Holographic Molding

The experimental results show that, under certain pressure and machine speed, in a certain range, the holographic coating presents four states of glassiness, high elasticity, plasticity and viscous flow state with increasing temperature. For example, the alcohol-soluble holographic coating has a brittle temperature of Tg≈50°C, a softening temperature of Tm≈65°C, and a degradation temperature of Td≈120°C. The holographic coating should be in a plastic state during the compression molding, and the coating has the best moldability in this temperature range. If the hot pressing temperature is higher than the lowering temperature, due to the increased viscosity of the coating, a sticking phenomenon will occur, that is, the cardboard cannot be peeled off in time, and it is wound on the holographic pressing roller. Production should prevent this from happening.

Hologram performance

In order to test the holographic effect of the paper substrate, we commissioned the China Packaging Scientific Research and Testing Center to inspect the holographic paper products produced in accordance with GB/T 17000-1997 "General Technical Conditions for Anti-counterfeit Hologram Products". The inspection items were appearance, diffraction efficiency, and signal-to-noise ratio. During 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 highest (68%) in the white background, and the diffraction efficiency is the lowest (0.52%) in the black background. The diffraction efficiency of the other background colors is in between. After preliminary analysis, the main causes of this phenomenon are: 1 The absorption of light by light reduces the signal intensity, so the measured diffraction efficiency of the paper substrate hologram is lower than that of the aluminized film; 2 different background color pairs The absorption rate of light is different, so that different regions have the same diffraction efficiency for the same sample.

It can be seen that the existing holographic effect detection standards are developed for aluminum-plated holographic films or aluminum-plated holographic papers, and the index system and detection method thereof are not completely applicable to transparent holographic papers with ink printing colors. The influence and regularity of the ink color on the hologram diffraction efficiency and other performance indicators have yet to be further studied.

in conclusion

This technology solves the problem of hologram copying on post-print paper. Its main features are:
(1) The laser holographic image can be generated on the surface of the printed paper. After a reasonable decoration design, the transparent holographic rainbow image matches with the ink printing image, and a special printing effect can be obtained;
(2) The result is a transparent hologram, which does not destroy the original printing effect, and does not require vapor deposition of aluminum (or any other inorganic) layer; and can fully utilize the post-press processing equipment, and is integrated into the existing printing process flow;
(3) As a printing post-processing method, it also has the function of protecting the printing surface, and at the same time it has the characteristics of improving brightness, water resistance, acid and alkali resistance, and abrasion resistance;
(4) The holographic coating involved does not contain toxic substances, and the obtained holographic product does not contain plastic film or metal media, and is an environmentally-friendly paper product that can be completely recycled.

Its research significance is mainly reflected in the following three aspects:

(1) Improved packaging security. The anti-counterfeiting function of this technology is shown in: 1 holographic anti-counterfeiting: through the production of difficult holographic images, the use of appropriate encryption and other technologies, can increase the difficulty of imitation; 2 anti-counterfeiting: counterfeiters must master a full set of process technology, and have all production equipment 3 material security: holographic coatings and holographic paper, cardboard have anti-counterfeiting capabilities; 5 overall security: the final product itself is a whole anti-counterfeiting, and can be combined with other anti-counterfeiting technologies (such as structural security, ink security, etc.) to form a comprehensive anti-counterfeiting System, to achieve a true sense of the overall security.

(2) Broaden the application of laser holography. The present technology successfully uses natural light as a medium to reproduce on a paper surface and can reproduce a large number of color holograms having a depth of field; a color transparent hologram can be formed with a graphic and color matching of ink printing.