LCD printing principle and characteristics

Liquid crystal printing is a printing method in which a liquid crystal-incorporated ink is used to give a reversible reaction or an irreversible reaction to a printed product.

(a) LCD printing principle

Liquid crystal is an intermediate state that certain organic substances are present in a certain temperature range. In this state, due to the special orientation of the molecular arrangement, there are specific laws of molecular motion, so that the liquid crystal not only has the fluidity and surface tension of the liquid, but also exhibits the optical properties of some crystals, such as the temperature is higher than the upper limit of the liquid crystal phase temperature, The liquid crystal becomes an ordinary transparent liquid and loses its optical properties. If the temperature is lower than the lower limit of the liquid crystal phase temperature, the liquid crystal becomes an ordinary crystal and loses its fluidity. Nematic liquid crystals in liquid crystals are used as display materials in the electronics industry, and cholesteric liquid crystals 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 enhances reflection due to refraction phenomenon. As the temperature rises, the long-wavelength color changes to a short-wavelength color (ie, changes from red to green to cyan). The mechanism of color formation is due to the selective reflection of light at a particular wavelength of light by the liquid crystal. The liquid crystal must be printed on a black or dark background. Now the liquid crystal can respond between -10°C to +70°C with an accuracy of +/-0.5°C.

During printing, the liquid crystal is encapsulated in microcapsules and printed with ink, which can prevent the contamination of the liquid crystal itself, can play a protective role, extend the protection life, facilitate long-term storage, and can improve printability. The amount of liquid crystal added to the ink is 40% of the total ink volume, and the diameter of the capsule is between 10-30 microns.

(b) Composition of liquid crystal ink

Compounds with crystalline properties were added to the liquid crystal ink. Under the influence of weak current and temperature, the liquid crystal displays bright and dark patterns and colors due to changes in the crystal lattice. The packaging used for printing with this type of ink can react with changes in temperature. If it is used to print a calendar, it will show different colors as the temperature of the four seasons changes.

The liquid crystal ink is mainly composed of a water-soluble resin, a liquid crystal capsule, an additive, a binder, and a defoamer.

(III) Printing Process

The use of screen printing or gravure printing can be used, the use of liquid crystal ink should be based on the purpose of use to determine the color temperature range and depending on the use of liquid crystal durability.

The key to liquid crystal printing technology is that it can display brightly colored liquid crystals at different temperatures, and it needs to be a series in color gamut. The production technology of liquid crystal microcapsules is also key. The microcapsules are small and uniform, and the caps are transparent and thin. Incorporation of the ink requires solvent resistance, stability, reliability, and long life; during the printing process, it is also ensured that the liquid crystal microcapsules are not crushed; the surface is covered with a protective film. Most of the LCD ink printing uses screen printing, and the screen printing process is as follows:

A. Screen printing plate

In general, screens can be made of nylon or nylon mesh. If the plate needs to carry a large amount of ink, stainless steel mesh plate can be used. The mesh size is generally 100-175 mesh. The diameter of the microcapsules is generally between 10-30 μm. A water-resistant emulsion is used for plate making.

B. Screen Printing LCD Ink Requirements

In terms of its function, liquid crystal printing is mainly used for temperature, humidity and color change.

1. The plate should be filled with enough ink at a time, and it is best not to fill the ink in the middle to prevent foaming.

2. Since the microcapsule ink is easy to clog, when the web is blocked or foamed during printing, it should be immediately stopped and completely washed with alcohol.

3. The printing should pay attention to giving the ink evenly and adequately. The lack of ink on the printing substrate will affect the color effect.

4. The printing pressure should not be too large to prevent the liquid crystal capsules from being broken and also reduce the color effect.

5. The ink color or dark color should be used for the printing background. The background color can be water-soluble ink, organic solvent ink can also be used. After printing, it must be fully dried and the ink film should not leave any organic solvent.

6. The printing ink layer should be smooth and smooth. The thickness of the ink layer should be controlled to 15-35μm. The ink layer is not even or too thin and will reduce the color effect.

7. The printing sequence can be printed with black (or dark color) ink and the desired pattern can be printed with liquid crystal ink. The thin substrate can also be printed with liquid crystal ink first, then printed with water-based ink (or dark color). Overlaying the negative pattern can improve the color effect.

8. The surface of the printed matter is preferably coated with a varnish or a protective film to protect the gloss of the ink layer and improve the wear resistance and temperature resistance.

C. Postpress drying

1. The best drying method is natural drying. It can also be dried with warm air at about 40°C. Do not heat rapidly at high temperatures.

2. Don't pressurize the printed matter when it is stacked.

(4) Characteristics and main uses of liquid crystal printing

Liquid crystal inks do not use pigments and have a viscosity of 4000 to 6000 centipoises, and they are characterized by low thixotropy and good leveling properties. It is mainly used for the printing of substrates such as thermometers, automatic temperature recorders, and detectors that require temperature display. The application of advertisements, trademark labels, arts and crafts, etc., which require changes in the color of the substrate, is also widely used. Printed in calendars, the screen will change in different colors at different temperatures throughout the year; On the hot products, according to the changes in the pattern color, to understand the quality of the contents of the contents, such as candy on the bright screen of the paper becomes gray, you should eat candy as soon as possible; LCD printing can also be used as a reminder card to save the temperature. Foreign countries also use liquid crystal printing tape as a one-time body temperature tester. Over time, liquid crystal printing will have a broader application area. (Text / He Jing)