UV printing field and printing process adaptability

UV ink refers to the ink that cures instantaneously upon exposure to UV light. The main components of UV inks include pigments, oligomers, monomers (reactive diluents), photoinitiators and various additives. Among them, resins and reactive diluents play a role in fixing pigments and providing film-forming properties; pigments give inks a moderate color and hiding power to substrates; photoinitiators require that they can still absorb photons to initiate polymerization under the interference of pigments.

From the printing process, UV ink has a fast fixing speed, good conjunctiva performance, good adhesion to various printing materials, and scratch resistance, wear resistance, acid resistance, alkali resistance and other properties are better than ordinary inks. Therefore, Although its price is much more expensive than ordinary ink, it is still widely used.

The fields currently printed with UV inks are as follows:

1. Non-absorbent material printing

Such as polyethylene (PE) film, polyvinyl chloride (PVC) film, polypropylene (PP) film, metal foil and non-absorbent special materials. If these printing materials are printed with ordinary solvent-based inks, a certain drying time is required. Usually, powder spraying, drying rack devices or other additives are added to the ink, and UV inks have no such trouble.

2. Metal material printing

When printing with UV ink on the surface of metal materials, the curing process can be shortened and the original curing device can be simplified. At the same time, the good curability and conjunctive properties of the UV ink surface layer can not only improve the post-press processing characteristics, but also improve the appearance quality of the product.

3. Commercial paper printing

The printing materials and inks used in commercial paper printing are special, such as carbonless copy paper, thermal paper, magnetic ink, anti-counterfeiting ink, etc., and require ink to have certain resistance to discoloration, heat resistance and stability, so it is best to use UV Ink or UV varnish. At the same time, due to the speed of the bill printing equipment and the small amount of ink, it also has the favorable conditions for printing with UV ink.

UV printing process adaptability

1.UV printing (offset)

Offset printing can be divided into sheet-fed offset printing and web-fed offset printing according to the state of the substrate. Sheet-fed offset printing is suitable for paper, cardboard, plastic sheet and metal foil printing; rotary offset printing is faster and suitable for large numbers of books and newspapers , Business forms and stickers printing, etc. The UV inks used in offset printing mainly consider the following issues: ink curing of non-absorptive printing materials; ink emulsification may hinder the adhesion of ink and substrates; the speed of rotary printing presses requires the curing speed of ink to be in line with the printing speed Matching, especially self-adhesive printing and security printing using special inks, UV ink printing effect is better than resin-based offset printing ink.

Offset printing is mostly used for fine product printing. UV curing can reduce the loss of small dots and improve the ink transfer rate of overprinting. When selecting the ink raw materials, the influence of each component on the UV ink should be comprehensively considered to ensure the best printing effect. Since the offset ink film is generally 2 to 3 μm, although it is conducive to light curing, the ink film is thin, and the hiding power of the pigment on the printing surface will be affected to a certain extent, so it is generally necessary to increase the pigment content in the formula; while increasing the pigment content, the ink film The light transmission performance will be reduced, which is not conducive to UV curing; and the ink film itself is thin. If the wettability of the pigment is not good, the pigment and filler are easily exposed to the surface of the ink film, which affects the gloss of the ink film. Good paint. The following points should also be noted when using UV offset printing.

(1) Different types of substrates should choose different inks, and comprehensive consideration should be given to the transparency of the ink, the curing rate, the coverage of the ink, and the gloss of the surface of the printed product.

(2) In color printing, the ability of various pigments to absorb UV photons is different, and the transmittance is M, Y, C, BK from high to low, so the curing degree of each color ink is also different. Transmittance directly affects the excitation energy of photons to photoinitiators, so it is advisable to arrange the printing color sequence as BK, C, Y, and M, so that the ink with poor transparency can absorb as many photons as possible and enhance its curing effect.

(3) The use of alcohol dampening system can reduce the surface tension of the ink and promote the curing effect. In the printing process, the dampening liquid can be used to strengthen the oleophobic area (blank part) of the printing plate to ensure that the lipophilic area is sufficient Ink-friendly, hydrophilic areas are not inked.

Improving the ink formulation and adding corresponding additives can increase the hydrophobicity of the ink, but too strong hydrophobicity will cause ink shrinkage at the edge of the image, loss of fine layers, unclear dot boundaries, etc., affecting the quality of printing.

(4) UV printing has high requirements on the surface strength of the paper. If the surface strength is not enough, it is prone to hair failure. When printing with gold or silver cardboard, the surface of this type of paper is smooth, and the affinity for the ink is small. Pull off the first color ink. Therefore, in UV printing, paper with a large surface tension should be selected, the color sequence should be reasonably arranged, and the relevant process parameters in the printing process should be adjusted to avoid and reduce the occurrence of the above phenomenon.

2. UV printing (flexible version)

Flexographic printing is a printing process that has developed rapidly in recent years. The graphic part of the printing plate is higher than that of the non-graphic part, and it has flexibility, which can better express the subtle levels and contours of the graphic part. In addition, its applicable printing materials are very wide. In addition to traditional paper, some non-absorbent printing materials, such as plastic film, aluminum foil composite paper, cellophane, etc., can also be used for flexographic printing.

In addition to the problems of coloring, curing, and adhesion, the use of UV inks for flexographic printing must also pay attention to the effects of ink components on ink viscosity and rheology, so as to obtain better printability and stability.

The flexographic printing process determines that the UV ink used should not have a high viscosity. Traditional flexographic printing inks are solvent-based and water-based. A large amount of solvent or water is required to adjust the viscosity. Due to the large amount of solvents used, most flexographic printing inks are similar to Newtonian fluids in terms of rheology, which will cause the ink to settle and become unstable. Therefore, the appropriate thixotropy can be given to the flexographic printing ink to prevent pigment flocculation and sedimentation.

UV flexographic printing inks use reactive diluents to reduce the viscosity of the system. Therefore, the overall viscosity may be higher than solvent-based flexographic printing inks. Although the viscosity of UV flexographic printing inks is lower than screen printing and offset printing inks, the addition of pigments will increase the viscosity, especially when the wetting, dispersion, and stability of the pigments are not well handled. In addition, the effect of pigments on ink rheology is more important, and proper thixotropy is beneficial, because when the molecular crosslinking inside the ink reaches a certain degree and pseudo plasticity occurs, the ink system without pigment dispersion stabilizers, in On the high-speed running ink roller, the viscosity will drop sharply, but when it leaves the printing roller and enters the printing part with relatively low speed, the viscosity will rise quickly, and the wetting effect on the substrate surface is too poor to be suitable for printing. The ink containing a suitable dispersion stabilizer has a certain stability, and its viscosity is not high. After high shear, the viscosity rises slowly, and the printing adaptability is good.

3. UV printing (screen)

The solid content of UV printing (screen) ink is 100%, there is no component volatilization, the thickness of the ink film is basically unchanged after curing, which is beneficial to obtain a higher film thickness, the printing and curing process is easy to control, and blockage can be avoided. Faults such as spots reduce downtime.

Although UV screen printing has many advantages, the determination of the photo-initiated conditions affects the curing quality of the ink layer. If it is not properly selected, it only cures faster on the surface of the ink film. On the other hand, UV screen printing has its advantages, that although the ink film is thick, the low pigment content can achieve high hiding power, which is a favorable condition for the curing of the bottom layer. Without hindering pigment saturation and hiding power, properly adding a light-transmitting filler with good refractive index, increasing the concentration of the light-transmitting medium in the ink layer, and also helping to increase the light absorption of the bottom initiator. Due to the thick film printing ink, the printing accuracy is not very high, and the particle size of the pigment can be larger. Therefore, without sacrificing the dispersion of the pigment, the particle size of the pigment can be appropriately increased to increase the actual light transmittance of the ink, which is beneficial to The bottom layer is cured.

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