UV printing

With the continuous improvement of people’s requirements for packaging of goods, the use of screen printing in the modern packaging industry has become increasingly popular. It not only gives people a three-dimensional sense of touch, but also further simplifies the production process of goods. Producers and consumers love it.

The three-dimensional effect created by screen printing is the direct application of ink to the surface of the substrate. During the entire printing process, in addition to solving screen printing, machine selection, and printing processes, there is an important aspect, that is, ink. Selection and curing process. Because silk screen printing is different from ordinary printing, its printing effect must have a certain thickness, so the process of drying and curing is more complicated, and ordinary drying methods do not work. In this regard, UV printing with the use of UV inks and the application of UV light radiation technology is the best choice for screen printing, and its environmental protection, high-speed printing, and excellent printing effects are gradually being The printing manufacturers agree that it also brings good news for industrial workers in many industries and maximizes the elimination of poisoning caused by solvent evaporation.

UV printing is a collection of chemical knowledge and physical optics knowledge in the printing industry. It is a typical example of the application of modern technology in manufacturing. Its core content is the understanding and adoption of UV inks and its curing system. Now we will give a brief introduction on these two aspects.

- UV ink composition and its photopolymerization mechanism

UV ink is used as a photo-curable ink, which uses a photo-curing resin as a base material to generate free radicals due to ultraviolet light irradiation to cause radical polymerization and photo-crosslinking reaction of the base material, thereby achieving a chemical ink for curing purposes. .

(-), the composition of the ink

1. Photo-setting resin: It is the base material of UV ink. Its difference from the resin in common ink is its photopolymerization. It determines the properties of UV inks, such as: adhesion, viscosity, hardness, abrasion resistance, chemical resistance, solvent resistance, cure speed and so on. The resins used in the photocurable inks generally use saturated or unsaturated dibasic acid esters and acrylic resins, but different inks have different resins due to their different properties, which are determined according to actual requirements.

2. Solvent monomer; it is a compound with a relatively large molecular weight and is chemically active. Can reduce the viscosity of ink to a certain extent, play a dispersing role, determine the ink surface properties, such as gloss, hardness, softness, surface smoothness. It differs from common solvents in that it has a large molecular weight and a low ability to reduce viscosity. It does not evaporate completely like ordinary ink solvents. It not only does not volatilize, but it also participates in the curing cross-linking reaction of the resin, which greatly reduces the harm of the printing industry to the environment and effectively protects the health of the staff.

3. Photoinitiator: It is an organic compound that excites the photo-curable resin to undergo a cross-linking reaction. It can generate very active free radicals after being excited by ultraviolet light, and this high-energy free radical undergoes energy transfer through the collision with a high-molecular polymer during movement to open a pair of macromolecules in the polymer. The bond causes cross-linking reaction of the formed chain compound to form a mesh-like ink film. This is an ingredient that is not found in ordinary inks.

4. Pigment: It determines the fineness, color, and weatherability and transparency of the ink. Due to the different absorption and reflection of different wavelengths of the spectral lines of different colors, the pigment also affects the curing speed of the ink to a certain extent. Therefore, special photoinitiators and their photoinitiator aids should be selected according to the different pigments, so as to achieve a Good curing effect.

5. Additives: It includes thickeners, fillers, accelerators, etc. It affects the curing speed of the ink, the effective period of use, and the ink layer thickness. However, the printability of the ink can be improved.

(b) Light curing mechanism

The first step: the use of ultraviolet radiation, the photoinitiator molecules and some auxiliary molecules are excited, dispersed to form a very active and high-energy free radicals.

Step 2: The first step is to generate active free radicals. During frequent movements, the high energy that they have is transmitted to the resin macromolecules and some diluting reactive monomers through mechanical collisions, causing the double bonds or rings contained in them to occur. Fracture, forming chemically reactive chain free radicals.

The third step: different chain radicals react and cross-link with each other to form a larger, more stable network structure compound to form an ink film around the ink.

Second, the curing system

(-), light curing elements

1, UV lamp power: In the actual operation, to make the UV ink to achieve a good curing effect, it must make the ink absorb enough energy. The radiant energy of the ultraviolet rays (ie, its penetrating power) does not depend on the length of time it irradiates but rather the power of the lamp that emits light. If its power is not enough, the deep layer of ink cannot be fully cured.

2. Curing speed: The curing speed of UV ink should take into account both the full cure of the ink and the best curing speed. The optimal curing speed makes the ink traces 80% of the speed of the ink layer after it has been cured by the UV machine. This is the best curing speed.

3, UV lamp spectrophotometric spectral wavelength range: This requires the UV lamp emits light in the wavelength range of 180 ~ 420nm, the sensitive area is 350 ~ 420nm.

4, the ink layer thickness and color and substrate surface state will affect the light curing.

(II) Curing equipment

1, the type of reflector: focus type, non-focus type, multi-faceted reflective type. The most commonly used is the curing of photocurable inks with focused and unfocused types.

2, UV lamp: its power to meet the requirements (usually 80 ~ 120W/cm); maximum life can not exceed 1200h, generally use 1000h, usually use two exchanges to be used interchangeably, 500h replace once, each use to Change a certain angle, learn to maintain.

The company's TW-UV series UV curing machine is designed and produced in strict accordance with the requirements of this standard. It not only uses imported UV lamp system, light intensity can be adjusted. Its adjustable belt speed allows prints to achieve optimum cure speeds. At the same time, the company's design and production of flat-panel and three-dimensional rotary conveyor belt structure can be very good to meet the different manufacturers of different curing requirements.

Reprinted from: China Screen Printing Forum