Classification and introduction of offset printing ink

The first impression of printed matter is the true reproducibility of colors, which is expressed by ink. So ink is the most important printing material. As an offset operator, you must fully understand the composition and printability of offset inks so that you can use the inks properly and selectively.

Composition and classification of offset printing ink

Offset inks are mainly composed of a homogeneous mixture of colored pigments (dyes, etc.), binders, fillers and additives.

Pigment is a solid powdery substance that is neither soluble in water nor soluble in oil or binder, and has a certain color. It is not only the main solid component of the ink, but also the colored body visible on the object. To a large extent determines the color, thinness, light resistance and other properties of the ink.

The linking material is the heart of the ink, a sticky fluid, as the name implies, it plays a role of linking. In the ink, powdery pigments and other substances are mixed and connected to form a glue with a certain degree of fluidity and viscosity after grinding and dispersion. The binder is the main fluid component in the ink, which determines the fluidity, viscosity, dryness and printability of the ink.

Filler is a white, transparent, translucent or opaque powdery substance. It is also a solid component of ink and plays a major role in filling. Appropriate selection of some fillers, in addition to reducing the amount of pigment and cost, it can also adjust the properties of the ink, such as thinness, fluidity and so on.

The additive is an additional part in the ink, and can also be used as an additional material in the finished ink. As a printing aid to change or improve certain properties of ink, such as: drying, abrasion resistance, etc.

Offset inks are divided into the following categories:

The most commonly used offset printing inks are:

One offset printing ink

Offset bright ink is a colloidal ink composed of synthetic resin, dry vegetable oil, high boiling alkane oil, high-quality pigments and additives. It is used for printing high-quality and exquisite prints such as picture books and pictures on coated paper by single-color, two-color or multi-color offset presses. The gloss of the ink is mainly caused by the conjunctive film after the drying of the connecting material. In order to improve the gloss of the ink, the ink film needs to have good leveling property. It should be fixed on the surface of the substrate to form a plane, which reflects the incident light directionally, that is, it has a mirror effect , This requires that the bonding material be left on the surface of the substrate to the maximum extent. Printing on coated paper with low absorption, it is easy to get a good gloss, and has good film forming, at the same time, the pigment and filler components in the ink does not affect the mirror effect of the film forming of the connecting material.

The proportion of resin and dry oil in the offset printing ink is large, the proportion of solvent is small, and there are many types of resin. Low softening point resins have good gloss and good leveling properties. Bright inks also require pigments. In order to make the proportion of ink binders higher, the coloring power of the pigments should be as large as possible. It is better to be non-flocculating. The gloss of the pigment itself also plays a big role in the gloss of the printed matter, but the pigment is required to have no metallic luster, the transparency should be as strong as possible, the pigment particles are smaller, and the wettability in the binder is better. Similarly, the required particle size of the filler for offset bright ink is as small as possible, and ultra-fine colloidal calcium carbonate is often used as the filler.

The colors of domestic offset printing bright ink are divided into six colors: white, yellow, green, blue, red, and black. Among them: P01233 (medium yellow) P01234 (transparent medium yellow ink) P01422 (sky blue ink) P01412 (peacock blue ink) P01655 (pink ink) P01656 (pink ink) P01808 (black ink) can be used for four-color printing. [next]

Two offset resin ink

Offset resin ink is made of synthetic resin, dry vegetable oil, mineral oil, high-quality pigments and fillers, which are prepared by grinding. It is used for printing various pictures and trademarks on offset presses.

The characteristic of offset resin ink is that it has less vegetable oil content and a high proportion of solvents (15-25%), usually high boiling alkane, which plays a role in viscosity reduction in the ink. After the ink is transferred to the paper, this part of the solvent quickly penetrates into the paper fibers under the action of the capillary, speeding up the drying speed of the ink, and reducing the smearing of the printed matter. Because the solvent content in the ink is too high, it has a certain destructive effect on the rubber roller and the blanket, which affects the quality of printing. The drying of this fast-setting ink is ultimately carried out by the dry form of oxidized conjunctiva. Therefore, a small amount of lead or cobalt desiccant needs to be added to the ink. The drying speed is fast, but the residual viscosity is large.

Three UV offset printing inks

UV ink is used for printing in offset printing, which is usually called UV offset printing. UV offset printing ink is a type of ink that uses ultraviolet (UV) radiation energy to make liquid chemicals through rapid crosslinking and curing into an ink film. It is one of the energy curing inks.

As a kind of offset printing ink, UV offset printing ink must first be printed in the form of offset printing, so UV offset printing ink should also have some characteristics of ordinary offset printing ink to meet the requirements of printability. These performance requirements include fluidity, dispersibility, anti-emulsification, viscosity, low flyability, good transferability, and drying and post-printing processability.

1. Characteristics of UV light curing ink

Ultraviolet light is an electromagnetic wave with a shorter wavelength than visible light, and different wavelengths have different energies. Generally, the UV light wave length used for curing UV light curing ink is 250-400nm. The ultraviolet light sources commonly used in the industry are high-pressure mercury lamps and metal halogen lamps, which have the highest efficiency in the range of 200 to 300 nm and 300 to 400 nm, respectively. Therefore, colorless UV light curing inks are commonly used in high-pressure mercury lamps, while colored UV light curing inks are commonly used in metal halogen lamps.

The fundamental characteristic of UV light-curable inks is different from other types of inks: the film formation of the ink layer is irradiated by strong ultraviolet light, and the film-forming material is initiated by the photoinitiator and undergoes rapid polymerization, within a few seconds to tens of seconds Finished. If it is not exposed to strong ultraviolet light, even if heated, the ink will remain in a viscous state for a long time and cannot be cured. The main advantages of UV light curing ink are:

(1) The thicker ink layer can be cured at a time, and the maximum thickness can reach 100 ～ 150um ..
(2) Short curing time, less energy consumption, saving printing space and space, and high production efficiency.
(3) Free of organic solvents, almost 100% cured to form a film, no organic solvent volatilization, reducing environmental pollution.
(4) Suitable for printing on substrates with large thermal capacity.
(5) The printed matter is exquisite, good quality, high grade, the ink film after curing has good heat resistance, solvent resistance and scratch resistance.

2. Composition of UV offset printing ink

(1) Photosensitive resin, also known as oligomer, is an oligomer with unsaturated groups at the molecular terminal, which is the main film-forming substance of ink, and its content is about 30% -50%. The most commonly used photosensitive resins are epoxy acrylate, urethane acrylate, polyester acrylate, polyether acrylate, various types of modified acrylate, etc. Their performance largely determines the physical properties, chemical properties, printability and printability of the ink. The type with the largest dosage is epoxy acrylates, which is characterized by good pigment wetting, strong adhesion, good compatibility with other resins, fast light curing speed, high hardness, high gloss and scratch resistance after curing. Injuries, heat resistance and chemical resistance are excellent. The application range of urethane acrylates is also relatively wide, and its characteristic is that the toughness of the ink film after curing is better than that of epoxy acrylates, but some varieties have poor weather resistance. The photosensitive resins used in inks for different purposes are not the same, and are selected according to the performance required by the ink, used alone or in combination with several resins.

(2) Reactive diluent This is a type of reactive monomer compound that is initiated by a photoinitiator to form an ink film. The main feature is an acrylic ester with unsaturated monofunctional groups or polymonofunctional groups at the molecular end. Its main function is to adjust the viscosity of the ink for printing. In the ink accounted for 40%-~ 60%. The reactive diluent also has a great influence on the performance of the ink. When selecting, it should take into account the dilution effect, curing speed, ink film adhesion, ink film hardness and other properties. Usually several diluents are used together for better results.

(3) Photoinitiator A photoinitiator is a compound that is easily excited by ultraviolet light to generate free radicals or cations. Although the content in the ink is small (usually 1% to 5%), the effect on the photocuring performance of the ink is direct and critical. Its role is to absorb the ultraviolet light energy, the photolysis reaction generates free radicals or cations, which in turn triggers a photopolymerization reaction between the photosensitive resin and the active monomer, which causes the ink to solidify into a film.

(4) In addition to coloring pigments in UV offset printing inks, pigments also have a major impact on various properties of the ink system, such as fluidity, storage stability, drying properties, and anti-emulsification properties. Because the bonding material of UV offset printing ink is generally composed of strong polar photosensitive resin and polar monomer. The presence of these polar substances requires that the pigments used in UV inks have a certain polarity, otherwise it will directly affect the dispersion of the pigments.

(5) Auxiliaries are very important for UV offset printing inks. If there are no auxiliary agents, the printing suitability of UV offset printing inks is difficult to guarantee. The commonly used auxiliary agents are:

a The main function of the dispersant can shorten the wetting and dispersing time of the pigment in the binder and reduce the humidity during grinding.
b. Stabilizer UV offset printing ink is sensitive to both light and heat. Excessive light and heat will emulsify the ink and become waste. Therefore, heat stabilizer and light stabilizer are generally added to the finished ink.
c Anti-emulsification aids Because the binder used in UV offset printing inks has strong polarity, and the molecular weight of the resin is low, the anti-emulsification performance of the ink system is generally worse than that of ordinary offset printing inks. If the appropriate amount of anti-emulsification aid is added to the UV offset printing ink, the anti-emulsification performance of the system can be improved.

There are other additives, which are used in relatively small amounts.

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With the development of printing technology, UV offset printing technology will be popularized, UV offset printing inks are more widely used, so what issues should be paid attention to when choosing UV offset printing inks?

(1) Note that the transmittance of UV ink is magenta, yellow, cyan, and black in that order.
(2) Choose UV offset printing ink suitable for the substrate.
(3) In principle, UV inks can be used without adjustment, but 2% to 5% of UV-specific ink adjustment oil or debonding agent can be added appropriately according to the specific situation.
(4) Auxiliary agent or cleaning agent special for UV ink should be used.
(5) Do not mix with oil-based inks. If you must mix with other types of inks, you must check the fluidity, adhesion, printing suitability and gel level of the mixed inks beforehand.
(6) When the UV gold and silver ink is mixed with other UV inks, the fluidity will deteriorate. The gel speed increases, and the gloss is poor, so the mixed ink should be used immediately, not for long-term storage.
(7) The UV ink should be stored in a dark place below 20 °, especially the UV gold and silver ink, which is easy to harden and cannot be stored for a long time. [next]

Four pearl ink

Offset pearlescent ink is a part of lithographic specialty ink. Its printed matter has delicate pearlescent luster and strong light refractive index, which can improve the grade of printed matter. It is mainly used for printing high-end packaging, trademarks and picture albums.

Offset pearlescent ink is composed of pearlescent pigments, binders, additives and other substances, and has a certain degree of fluidity of paste adhesive, evenly mixed by stirring, and can show a soft pearlescent effect after drying on the substrate.

1. Pearlescent Pigment

Pearlescent pigment is a new type of pigment that is neither soluble in water nor binder. This pigment can reproduce the pearl luster possessed by pearls, shells, etc. At present, the most common pearlescent pigment is composed of anatase or rutile titanium dioxide coated mica flakes, which can show a variety of delicate and soft silver-white luster. This elegant and generous color is more and more loved and accepted by people. Pearlescent pigments account for approximately 20% to 50% of pearlescent inks.

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It should be particularly pointed out that due to the characteristics of pearlescent pigments, the pigments are not allowed to be ground like ordinary inks during the dispersion process, otherwise the structure of the pearlescent pigments will be destroyed, affecting the gloss and pearlescent effect of the printed matter. That is to say, do not use the inking machine when adjusting the ink during printing.

2. Features of offset printing pearlescent ink

(1) Fragility of pearlescent pigments Pearlescent pigments are composed of anatase or rutile titanium dioxide coated mica flakes. The flake structure is very brittle and easily damaged, which affects the pearlescent effect.

(2) Fineness of pearlescent pigments Offset printing pearlescent inks have strict requirements on pigment fineness. The particle size of commonly used mica titanium pearlescent pigments is below 25um. If the pigment particles are too coarse, the transferability of the pearlescent ink is poor, but if the pigment particles are too fine, the pearlescent effect of the printed product will be less obvious.

(3) Transparency of offset pearlescent ink The pearlescent effect of offset pearlescent ink depends on the refraction and interference of incident light. Therefore, the transparency of offset pearlescent ink has a great influence on the pearlescent effect. If the transparency of the ink layer is poor, sufficient light will be absorbed and affect the pearlescent effect. Therefore, when selecting the ink binder and filler, you must choose a raw material with good transparency.

(4) Viscosity and fluidity of offset pearlescent inks The speciality of offset pearlescent ink pigments requires that the inks have lower viscosity and greater fluidity than other common inks, and can meet the floatability requirements of pearlescent pigments during printing to achieve the ideal Pearlescent effect.

3. Special attention should be paid to the printing of offset pearlescent ink

(1) Not suitable for mesh tone printing. The pigment used in pearlescent ink is flake-shaped anatase or rutile titanium dioxide coated mica flakes. Therefore, the pearlescent pigments need to be arranged in order to achieve satisfactory pearlescent effect. The mesh tone printing is not conducive to the orderly arrangement of pigments, therefore, pearlescent ink is more suitable for the printing of solid color patches.

(2) Printing sticky pearlescent ink printing is particularly prone to sticky printing. This is because offset pearlescent ink has low viscosity, high fluidity, and large particles of pearlescent pigments. It is mostly used for printing on the ground and the amount of ink used is large. Generally 2 ~ 3 times of ordinary offset printing ink. The printing ink layer is relatively thick and prone to sticking. Therefore, it is necessary to spray powder properly and control the appropriate stacking height.

(3) Drying speed The printing ink layer of the pearlescent ink is thicker, and the drying speed of the ink is slower than that of the ordinary offset printing ink. At this time, the method of adding a desiccant cannot be adjusted, but the temperature and humidity of the paper and printing environment need to be controlled. The ambient temperature should be controlled at 25 ± 2 ℃, and the relative humidity should be controlled at about 55%.

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Because the offset pearlescent ink is relatively soft and easy to emulsify during the printing process, it is more appropriate to control the amount of water supply to the minimum and adjust the pH value of the dampening fluid to about 5 without dirty plates.

Ink for five web offset press

The ink used in web offset presses is what is usually called offset printing ink. This ink is basically divided into the following two types:

1. Penetration drying type Due to the high printing speed of the web offset press and the fast drying speed of the ink, the most commonly used inks are the penetration drying type. The composition of the ink is basically the same as that of the sheet-fed offset printing press, but the amount of resin added is relatively reduced, and the content of the aliphatic hydrocarbon solvent is high. Its rheological properties should meet the requirements of high-speed printing, so the fluidity is better. In order to prevent paper fluffing, the viscosity should be relatively low to improve the performance of ink transfer. In addition, this ink should not be placed with a desiccant.

2. Thermosetting ink The drying device is added in the second half of the printing device of the modern web offset press. In order to adapt to this production condition, there are thermosetting inks. This ink is heated in the drying device and the solvent in the ink volatilizes. The ink film can be dried quickly. The content of dry oil (such as linseed oil) in thermosetting ink is less, and the content of petroleum agent is more. The other ingredients are slightly the same as the ink of sheet-fed offset printing press. The paper used in the ink type is of better quality than the paper used in the penetrating dry ink.

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