Liquid crystal printing (liquid crystal printing) is a printing method that uses ink mixed with liquid crystal to make the printed product have a reversible reaction or an irreversible reaction.
LCD printing principle
Liquid crystal is an intermediate state that certain organic substances exhibit in a certain temperature range. In this state, due to the special orientation of the molecules and the specific movement of the molecules, the liquid crystal has both the fluidity and surface tension of the liquid and the optical properties of some crystals, such as the temperature is higher than the upper limit of the temperature of the liquid crystal phase. The liquid crystal becomes an ordinary transparent liquid and loses its optical properties. If the temperature is lower than the lower limit of the temperature of the liquid crystal phase, the liquid crystal becomes an ordinary crystal and loses its fluidity. The nematic liquid crystal in the liquid crystal is used as a display material in the electronics industry, and the cholesteric liquid crystal can be used for temperature indication.
The liquid crystal is irradiated by natural light and artificial white light, and the color light of a certain wavelength strengthens the reflection due to the refraction phenomenon. As the temperature rises, the color of the long wavelength changes from the color of the long wavelength to the color of the short wavelength (that is, the change from red → green → cyan). The color-forming mechanism is formed by the selective reflection of liquid crystal to light of a specific wavelength. The liquid crystal must be printed on a black or dark background. Now the liquid crystal can reflect between -10 ℃ and + 70 ℃, with an accuracy of +/- 0.5 ℃.
When printing, the liquid crystal is enclosed in microcapsules, and printing is added with ink, which can prevent the pollution of the liquid crystal itself, can play a protective role, extend the protection life, facilitate long-term storage, and can improve the printability. The amount of liquid crystal added to the ink is 40% of the total amount of ink, and the diameter of the capsule is between 10 and 30 microns.
Composition of liquid crystal ink
Liquid crystal inks have added compounds with crystalline properties. Under the influence of weak current and temperature, the liquid crystal displays light and dark patterns and colors due to changes in the crystal lattice. Packaging and decoration products printed with this ink can respond to changes in temperature. If used to print calendars, they will show different colors with changes in temperature during the four seasons.
Liquid crystal ink is mainly composed of water-soluble resin, liquid crystal capsules, additives, linking materials, and defoaming agent.
printing art
Screen printing or gravure printing can be used. When using liquid crystal ink, the color change temperature range should be determined according to the purpose of use and the durability of the liquid crystal should be considered according to the application.
The key of the liquid crystal printing technology is: liquid crystals that can display bright colors at different temperatures, and the color gamut value should be a series; the production technology of liquid crystal microcapsules is also the key. The microcapsules need to be small and uniform, the caps are transparent and thin, and the ink mixed requires solvent resistance, stability, reliability, and long life; during the printing process, the liquid crystal microcapsules must not be crushed; the surface is covered with a protective film. Most of the liquid crystal ink printing uses screen printing. The screen printing process is as follows:
A. Screen printing
The general wire mesh can be nylon wire mesh or nylon wire mesh. If a large amount of ink is required on the printing plate, a stainless steel screen can be used for plate making. The screen mesh is generally 100-175 mesh. The diameter of the microcapsules is generally between 10-30 μm. A photosensitive emulsion with strong water resistance is used in plate making.
B. Screen printing liquid crystal ink requirements
In terms of its function, LCD printing is mainly used for temperature and humidity, and color change.
1. The printing plate should be filled with sufficient ink at one time, and it is best not to refill the ink halfway to prevent blistering.
2. As the microcapsule ink is easy to block the plate, if the network is blocked or foamed during printing, it should be stopped immediately and washed thoroughly with alcoholic water.
3. Pay attention to the uniform and sufficient ink supply during printing. Insufficient amount of ink on the printed substrate will affect the color development effect.
4. The printing pressure should not be too large to prevent the liquid crystal capsules from being broken, which will also reduce the color development effect.
5. The background color of printing should be ink color or dark color. The base color can be used with water-soluble ink or organic solvent ink. It must be fully dried after printing, and no organic solvents should remain in the ink film.
6. The printing ink layer should be smooth and smooth, the thickness of the ink layer should be controlled to 15-35μm, uneven or too thin ink layer will reduce the color effect.
7. The printing sequence can use black (or dark color) ink to print the background color first, and then use liquid crystal ink to print the desired pattern; can also use liquid crystal ink to print the thin substrate first, and then use water-based ink (or dark color) ink to print Overlay the negative picture pattern to improve the hair color effect.
8. The surface of the printed matter is best coated with varnish or adhesive protective film to protect the gloss of the ink layer and improve the wear resistance and temperature resistance.
C. Drying after printing
1. The best drying method is natural drying. It can also be dried with warm air around 40 ℃.
2. Do not press heavily when stacking printed materials, and do not apply pressure when cutting.
Features and main uses of LCD printing
Liquid crystal inks do not use pigments and have a viscosity between 4000 and 6000 centipoise. They have the characteristics of low thixotropy and good leveling. It is mainly used for the printing of substrates with temperature display requirements such as thermometers, automatic temperature recorders and detectors. It is also widely used in printing materials such as advertisements, trademark labels, arts and crafts and other decorations that need to change color. It is printed as a calendar picture, and the picture will change to different colors at different temperatures throughout the year; posted on some fear For hot products, you can learn about the quality of the contents according to the change of the color of the pattern. If the bright picture on the candy packaging paper becomes gray, you should eat the candy as soon as possible; LCD printing can also be used as a reminder card for temperature preservation. Foreign countries also use liquid crystal printing paper tape as a disposable temperature tester. Over time, LCD printing will have a wider application field.
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