Post-printed paper transparent hologram printing technology
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Process principle
Analysis of imaging mechanism of paper substrate laser hologram
The imaging mechanism of the paper substrate laser hologram is basically similar to other traditional substrates (plastic film, aluminized film), which are holograms obtained by holography, and then the hologram is turned into a holographic metal plate, and then passed through the heat. The molding process copies the hologram to the paper print.
However, the surface properties of paper prints are significantly different from those of plastic or aluminized films. The film which can be used for making holographic products has the property of thermoplastic deformation, and the surface is flat, and can directly perform holographic embossing, especially after aluminizing, the hologram can be greatly enhanced due to the reflection effect of the metal aluminum layer. Depending on the effect. However, paper prints are not the same, neither the hologram and the embossing of the ink can be directly applied, and the printed matter can not be aluminized.
Therefore, in order to print a holographic effect on a paper print, a special printing medium is required. This medium should have excellent thermal deformation properties, and can obtain stable fine holographic grating stripes after deformation. At the same time, the medium must have sufficient transparency to ensure the visibility of the original printed graphics and colors. This special holographic substrate is called a holographic coating.
In order to obtain a good hologram visual effect on the holographic coating, the 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, wear resistance, post-processing suitability, etc. are also very important.
From the optical principle, the hologram recorded on the holographic coating uses the diffraction effect of light to reproduce the holographic effect, while the original ink printed image uses the reflection effect of light to express the printing effect, due to the use of light. In different ways, the hologram does not interfere with the original ink printing effect. On the contrary, the transparent holographic coating enables the two printing effects to be simultaneously expressed and forms a special decorative effect.
According to the imaging mechanism of paper substrate laser hologram, the basic principle of "printing" hologram on paper substrate is: coating a layer of thermoplastic holographic coating on paper or paperboard with existing printing ink, drying After calendering, a bright and smooth coating is obtained, which is used as a memory layer of the hologram, and then holographic molding is performed on a special holographic paper molding machine, and 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 method for directly printing holographic images on paper substrate prints is studied.
(2) Development of holographic coatings: The formulation of solvent-based and water-soluble environmentally-friendly paper substrate holographic printing coatings suitable for different application conditions and preparation methods thereof.
(3) Holographic replication technology: For the general offset printing paper or paperboard, a special holographic compression molding equipment-platform paper holographic molding machine is developed.
Transparent holographic printing process
Under the premise of not affecting the original printing effect, a process for directly printing holographic images on paper substrate prints was developed. This process does not affect the normal printing process, and can make full use of existing post-printing processing equipment to meet the requirements of industrial production. The process flow is as follows:
The coating process is to uniformly apply the holographic paint to the surface of the paper substrate printing ink through a coating device, and the coating quality directly affects the holographic effect. The coating method adopts a three-roll coating method, and has the advantages that the glazing coater of the existing post-press processing can be fully utilized, and the coating and drying machine can be used at both ends with low cost. The drying method of the coating is infrared radiation heating and air drying.
After the holographic coating is leveled and dried, and then calendered by a continuous rolling calender, the smoothness and gloss of the holographic coating can be greatly improved and improved, so that the desired hologram effect can be obtained in the holographic molding process. .
Holographic coating
The main role of holographic coatings is two: flat and smooth paper surface and printed hologram.
The quality of the coating directly affects the holographic effect. The main technical requirements for coatings are: sufficient adhesion to paper, excellent n-sex deformation characteristics, and excellent physical and chemical properties and post-processing suitability.
For different application conditions, we have developed two types of paper holographic special coatings: QXC alcohol soluble holographic coating and QXS water soluble holographic coating.
(1) Technical characteristics of alcohol-soluble holographic coatings
Select AB, C, and D with higher refractive index and transparency as comonomer, and reasonably match hard monomer, soft monomer and reactive monomer, and select E and F as initiator and cosolvent. The reaction conditions are strictly controlled, and a constant temperature, an initiator concentration, a monomer concentration, a solvent concentration, and an inert gas protection are maintained as much as possible in the polymerization process, and the obtained copolymer has an average molecular weight of about 90,000. The colorless transparent resin is added to the compounding resin I (high speed and flexibility), auxiliary agent and solvent to form a paper substrate anti-counterfeiting holographic coating with high comprehensive performance such as high refractive index, high gloss, abrasion resistance and flexibility.
(2) Technical characteristics of water-soluble holographic coatings
The water-based resin II has a high refractive index and can print a hologram well, but the coating film is brittle and has poor water resistance. To this end, we have synthesized an aqueous emulsion with high solid content, low scale, good flexibility and excellent water resistance. The organic combination of the two, supplemented with additives, to produce a high refractive index, high gloss, wear resistance, flexibility, water resistance and other comprehensive performance, suitable for paper anti-counterfeiting, environmentally friendly holographic coatings.
Platform holographic paper molding machine
Whether holograms can play a role in economics, culture, art, anti-counterfeiting and daily production, one of the most critical issues is the ability to replicate in large quantities to achieve industrial production. Since the printing of the general lithographic printing method is a sheet-fed paper except for the gravure printing, the conventional film-winding molding machine cannot be used for the holographic molding of the flat sheet or paperboard. To this end, we have integrated the advantages of the existing wide-format soft presses and hard presses, and designed and manufactured the platform-type holographic paper molding machine. Its outstanding features are:
(1) holographic copy replication of a sheet of paper that can be used for printing;
(2) having an automatic overprinting device;
(3) a paper stripping mechanism with an anti-adhesive plate;
(4) It has automatic paper feeding, delivery and point function.
The platform type holographic paper molding machine is composed of four parts: the main machine, the oil heating system, the hydraulic system and the electric control system. The basic working principle is: the high temperature heat transfer oil is heated in the oil tank, and is sent to the hollow molded holographic roller by the oil pump. The heat is transferred to the holographic roller to maintain the roller at a certain temperature. The holographic roller forms a pair of rotating pressure pairs with the rubber roller directly below it. At the beginning of the molding work, the hydraulic system lifts the rubber roller up and presses it on the holographic roller to apply a certain pressure to the holographic roller. The single sheets of paper are fed one by one to the two rolls, and the rotation of the two rolls drives the paperboard to move forward. During the movement, under a certain temperature and pressure, the holographic coating softens and deforms, and the hologram on the holographic roller is copied and cooled to form a stable holographic effect.
Technical Difficulties
Compared with traditional plastic substrate laser holographic image processing, paper substrate laser hologram image processing is difficult, high process requirements, and more technical.
Mainly in three aspects:
(1) Due to the fibrous structure, the surface of the paper itself cannot directly print the hologram n, but must be coated with a special coating.
(2) When holographic molding is carried out on the surface of the paperboard, since the thickness of the paperboard is much larger than that of the plastic film, the surface quality condition is also worse than that of the film or the aluminized film, and the movement of the flat sheetboard during the molding process lacks effective traction. And tension control, easy to cause paper "sticking" phenomenon, these unfavorable factors will affect the quality of the copied hologram.
(3) In addition to gravure printing, the paper is cut into flat sheets of a certain size during printing. The existing holographic molding equipment for film cannot be directly used for holographic molding of post-printed sheets. Design a dedicated paper molding machine.
Experiment and discussion
During the research, 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. Due to space limitations, only the molding and hologram performance were analyzed here.
Holographic molding
The experimental results show that under the condition of constant molding pressure and machine speed, within a certain range, the holographic coating exhibits four states of glass state, high elastic state, plastic state and viscous flow state with the increase of temperature. For example, the alcohol-soluble holographic coating has an embrittlement 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 moldable state during mold replication, and the formability of the coating is best in this temperature range. If the hot pressing temperature is higher than the lowering temperature, the viscosity of the coating is increased, which causes a sticking phenomenon, that is, the cardboard cannot be peeled off in time, and is wound on the holographic pressure roller. This should be prevented in production.
Hologram performance
In order to test the holographic effect of the paper substrate, we commissioned the China Packaging Research and Testing Center to inspect the produced holographic paper products in accordance with GB/T17000-1997 "General Technical Conditions for Anti-Counterfeiting Holographic Products". The inspection items are appearance, diffraction efficiency and signal-to-noise ratio. During the detection process, we found that the ink color of the sample has a great influence on the diffraction efficiency. The overall trend is the highest diffraction efficiency (68%) on a white background, the lowest diffraction efficiency (0.52%) on a black background, and the other background color diffraction efficiency. After preliminary analysis, the main reasons for this phenomenon are: 1 the absorption of light by color reduces the signal intensity, so the measured hologram diffraction efficiency of the paper substrate is lower than that of the aluminized film; 2 different background color pairs The light absorption rates are different, so that different regions of the same sample have different diffraction efficiencies.
It can be seen that the existing holographic effect detection standard is formulated for aluminized holographic film or aluminized holographic paper, and its index system and detection method are not completely suitable for transparent holographic paper with ink printing color. The influence of ink color on the performance index of hologram diffraction efficiency and its regularity have yet to be further studied.
in conclusion
The technology solves the problem of holographic copying on the printed paper, and its main features are:
(1) A laser holographic image can be generated on the surface of the printed paper. After a reasonable decoration design, the transparent holographic rainbow image is matched with the ink printed image to obtain a special printing effect;
(2) The obtained transparent hologram does not destroy the original printing effect, and does not need to evaporate the aluminum (or any other inorganic material) layer; and can fully utilize the post-press processing equipment, and is integrated into the existing printing process;
(3) As a post-printing processing method, it has the function of protecting the printing surface, and has the characteristics of improving brightness, water resistance, acid and alkali resistance, wear resistance and the like;
(4) The holographic coating involved does not contain toxic substances, and the obtained holographic product does not contain a plastic film or a metal medium, and is an environmentally-friendly paper product that can be completely recovered.
Its research significance is mainly manifested in the following three aspects:
(1) Improve the anti-counterfeiting strength of packaging. The anti-counterfeiting function of this technology is manifested in: 1 holographic anti-counterfeiting: by making high-resolution holographic images, using appropriate encryption and other techniques, the difficulty of imitation can be increased; 2 technology anti-counterfeiting: counterfeiters must master the complete set of process technology, and have all production equipment 3 material anti-counterfeiting: holographic coatings and holographic paper and cardboard have anti-counterfeiting functions; 5 overall anti-counterfeiting: the final product itself is an anti-counterfeiting whole, and can be combined with other anti-counterfeiting technologies (such as structural anti-counterfeiting, ink anti-counterfeiting, etc.) to form comprehensive anti-counterfeiting The system realizes the overall anti-counterfeiting in the true sense.
(2) Expanded the application of laser holography. The technology successfully uses natural light as a medium to reproduce on paper and can copy a large number of color holograms with depth of field; color transparent holography and ink printing graphics and color matching can form a wonderful overall decorative effect, which is for printing In the packaging, decoration, advertising and other industries, its significance is extremely far-reaching.
(3) Great social and economic benefits. The technology has strong anti-counterfeiting strength, exquisite decorative effect and simple manufacturing process. The lower production costs and the social and economic benefits will be considerable.