Analysis of ink properties in inkjet printing
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Ink-jet printing is a topic that printers are currently paying attention to. It is a kind of digital printing. With digital data storage images, the output of the image can be arbitrarily changed before being output. It has high speed, can be copied in a small amount, is easy to combine, realizes variable information printing, and has low pollution. The characteristics of inkjet printing in the printing format, ink and printing medium diversification have expanded the connotation of traditional printing. At present, digital inkjet printing, digital proofing, digital inkjet market and so on have been formed.
Inkjet printing is a non-contact, pressure-free, non-printing printing technology. The earliest development was in 1878, and in 1951, Siemens applied for a patent for converting liquid ink into ink droplets. There was a problem and the effect was not good until it was officially entered into the practical stage in 1960. With the continuous development and advancement of technology, the market continues to expand. According to the forecast of I.T. Strategies of the United States, by 2009, the total revenue of the global inkjet market (including hardware, inkjet printing media and ink) will exceed $57 billion.
The color performance and high image quality of inkjet printing have been satisfactory to most consumers, but the printing time is still slow, and the requirements for paper are also high. Although there are many inkjet special papers on the market, However, the price is too high, it is difficult for the general family and the student population to be widely used. Therefore, the inkjet technology can be widely applied to various papers. It is the most urgent technology to be developed now. If it is to make the special paper of special materials cheaper. It's not easy, so developing new ink ingredients is a direct and effective way.
The ink used for inkjet printing is a special ink with moderate viscosity. It has non-toxic, stable, non-clogging nozzles, moisture retention, good jetting properties, and non-corrosive properties to metal parts of the nozzle. At present, inkjet inks are mostly dye-based inks, and inks of inkjet printers must have sufficient surface tension to prevent ink from being sputtered out when ejected. In addition, the ink must be fast-drying to prevent smearing on the paper, so the ink itself is one of the factors that affect the printing quality. In the printing, select the appropriate ink, adjust the printing parameters, in order to get a satisfactory print.
First, the composition, type of inkjet ink
Ink is a very important component in inkjet printing. The development of inkjet ink also explains the development of inkjet printing technology to a certain extent, because ink not only determines the quality of inkjet image, but also determines the ink droplets. Shape and reliability of the printing system. Inkjet inks generally consist of colorants, binders, and additives. The ink binder can be water, oily substance, solvent, resin, etc., which determines the dynamic properties of the ink distribution; in the inkjet ink, the binder accounts for 40%-90%. The colorant mainly plays a role in color development in the ink, generally accounting for 1%-10%. The rest are referred to as additives and are mainly used to improve the viscosity, adhesion, thermal stability, light resistance and surface tension of the ink.
In general, there are many types of inks used in the inkjet printing process, and there are many classification methods. At present, it is generally possible to classify different aspects of the properties exhibited by the ink, and generally divide the ink into water-based inks, oil-based inks, solvent-based inks, and thermosetting inks according to different binders.
1. Water based ink
Commonly used on perforated and uncoated substrates, such as paperboard and paper, are also often used in the printing of direct mail products and other commercial printing markets. It can also be divided into pigment type and dye type. Because the pigment is insoluble in water, it will precipitate. Therefore, the pigment particles should be ground very fine. If it is as small as 30μm, it will have better printing effect. Otherwise, when using high resolution. It is easy to cause blockage. The weather resistance, light fastness, washing fastness and abrasion fastness of the pigment itself are very good. The advantage of the dye is that there is no problem in solubility, and there are many kinds of colors, and the vividness is good, but the fastness is poor.
2. Oil based ink
It is most suitable for printing on perforated materials because it is dried by absorption. They are formulated with vegetable oil or mineral oil. It is especially useful in the field of cardboard packaging; it is not suitable for use in confined office spaces because Air pollution is generated. The colorants used can be divided into two types: pigment type and dye type. Oil-soluble type inks are characterized by fast drying, so they are commonly used in industry.
3. Solvent based ink
It is used in a wide range of applications, from sign printing to bar code printing to outdoor advertising. It is economical and fast-drying and can be printed on a variety of materials, especially for non-coated materials such as vinyl and wire mesh. , glass and paper.
4.UV/EB thermosetting ink
The ink is suitable for printing on a variety of soft and rigid substrates, such as corrugated cardboard and metal foil. UV/EB inks cure faster and have higher durability under the action of UV light. Ideal for DOD applications such as retail point display ads, calling cards, credit cards and frequently used shopping cards.
In addition to sorting by means of connecting materials, in practice, there are some other classification methods: according to the color material, divided into organic dye type ink, polymer dye type ink and pigment type ink; if used according to ink The different drying mechanisms can be divided into volatile inks and polymeric inks. Water-based and non-aqueous inks are water or other solvents used as binders. Drying is accomplished by the penetration and absorption of ink by the substrate. If ordinary copy paper or flat paper is used, the ink and its binder are absorbed together by the substrate. This absorption reduces the color density and sharpness of the print. The thermosetting ink means that the ink is solid at normal temperature, and is a hot melt liquid when ejected from the nozzle, and rapidly solidifies upon reaching the surface of the substrate. This rapid solidification prevents the ink from diffusing or penetrating into the inside of the substrate. To ensure the quality of printing.
Second, choose the right ink
1. Reasons for using inks supplied by inkjet digital press manufacturers
In the actual production process, producers generally choose the ink provided by the inkjet equipment provider. In fact, this has advantages and disadvantages. Of course, different digital printing opportunities use different technical methods, such as continuous inkjet (CIJ) and drop-on-demand (DOD). These two inks are not interchangeable because the two technologies are completely different. CIJ technology means that ink droplets are continuously produced, forming a droplet flow, which is imaged by a deflector; while DOD technology is a digital signal generated as needed. Some ink droplets, rather than an ink stream, change the ink at will, which inevitably causes confusion in the ink. Therefore, in production, the premise of maximizing the effect of the printing press is to have a full understanding of the ink used, which requires consideration of various factors.
2. Printing medium and ink selection
For inkjet printing, how to control the printing effect on different substrates, it is necessary to analyze the performance of the substrate. Taking paper as an example, due to the thermodynamic interaction between the paper and the ink, the capillary action of the paper, and the diffusion of the chemical composition of the ink, the absorption of the ink component by the paper, that is, the ink penetration. Among them, the capillary action of the paper is the main cause of ink penetration. Since the paper itself is a porous water-permeable structure, the ink easily spreads inside the paper. As paper surface coating processes continue to evolve, the different distribution of ink within the paper results in different ink penetration effects. Therefore, the inkjet printing paper requires a paper with a certain degree of sizing, the ink sprayed on the surface is evaporated, the ink dots are round, the printed characters are clear, and the graphics are beautiful. For color inkjet printing paper, it is preferable to have a very thin transparent coating on the surface to absorb ink quickly and avoid scattering of light.
Inkjet inks can be printed on a variety of substrates, which is a major advantage, such as fiber, wallpaper, glass, plastic, metal foil and plastic film, and of course on various types of paper. Knowing which type of material is suitable for inkjet technology is only one aspect of the problem. In addition, is the substrate coated or uncoated? Is the substrate porous or non-porous? Does the printing material absorb water or not? What is the effect of the ink on the surface of the substrate? These will all be related to the development of print job plans, increase production efficiency, and ultimately improve return on investment.
3. Selection of dye inks and pigment inks
From a chemical point of view, the dye is present in a single molecular state in the ink, and the pigment is composed of a floc in which a large number of pigment molecules are aggregated. In general, the color performance or color gamut of dye inks is superior to pigment inks. When printing on substrates of better quality, dye inks can achieve the same quality as silver salt photos. However, dye inks have poor stability, especially light resistance, dark storage stability, moisture resistance and water resistance, because the chemical properties of individual dye molecules are very unstable under the conditions of light, moisture and oxidation. It is easy to chemically react, causing the print to fade. Unlike dyes, pigments in pigment inks are present in a state of flocculation in which numerous pigment molecules are aggregated, so their environmental impact is better.
Dye inks are also very picky about print media and often require specialized media support. When printing on ordinary copy paper, since the copy paper is composed of paper fibers which are criss-crossed, it is filled with a gelatinous substance, and the hydrophilic factor in the general dye ink accounts for a large proportion, due to these hydrophilic factors. As a function, the dye ink easily penetrates into the paper fibers, causing blooming on ordinary copy paper, which seriously affects the final printing effect. Pigment inks, on the other hand, replace the hydrophilic factor with a large number of water-repellent factors, so that the ink does not penetrate deep into the fiber, but adheres to the surface of the paper.
According to the above advantages and disadvantages, it can be known that the dye ink has strong performance, bright and uniform color, high brightness level and easy to express color tone, so it is especially suitable for indoor use; pigment ink has good stability and durability, suitable for outdoor use. .
Third, ink control in inkjet printing
In inkjet printing, in order to ensure the results, it is necessary to control the parameters of the printing ink more appropriately. Taking water-based ink as an example, the conditions for printing control are as follows:
1. In order not to block the inkjet head, it is necessary to pass a 0.2 μm filter.
2. The sodium chloride content must be less than 100 ppm.
Sodium chloride will cause the dye to settle down, and sodium chloride is corrosive, especially in bubble jet systems. Although the nozzles are made of titanium, they are still corroded by sodium chloride at high temperatures. .
3. With the effect of a buffer solution.
The pH of the ink should be stable and some dyes will precipitate due to changes in pH.
4. Viscosity control (1 ~ 5 cp).
Micropiezoelectric inkjet systems require higher viscosity and bubble jet systems require lower viscosity.
5. Surface tension (30 to 60 dyne/cm).
Micropiezoelectric inkjet systems have low surface tension requirements, and bubble jet systems require high surface tension.
6. Drying speed should be just right.
It is too fast to plug the inkjet head or break the ink, it is too slow to spread easily, and the dot overlap is serious.
7. Preservation period.
It takes up to three months, usually one year, for the ink to be used from the factory to the consumer. Normally, the ink must pass through a 0.2μm filter for one year.
8. Thermal stability.
The dye for the bubble jet system is more thermally stable because the ink of the bubble jet system is heated to a high temperature of 400 ° C, which will decompose or discolor if the dye is not resistant to high temperatures.
While controlling the properties of the ink, it is necessary to obtain the desired print when using the ink. The method must be used correctly. The performance of the ink is monitored before, during and after the printing. And storage conditions such as temperature, humidity, cleanliness and airflow all have an effect on the performance of the inkjet ink.
In addition, after printing, inkjet inks may also be exposed to undesirable environments such as UV rays, ozone, adhesives, and corrosive environments. These factors can all degrade the quality of the printed product, and select the type of ink as needed to ensure that the printing environment meets the needs of the printing environment. Inkjet inks are chemical products that should be properly handled to ensure safety. The MSDS Material Safety Data Sheet is a guide to ensuring that ink is stored, utilized, transported, and disposed of under normal conditions.
The best equipment does not necessarily give you the best prints. In the case of inkjet printing, choose the right ink, understand the applicable range of various types of ink, adjust the ink parameters, monitor the ink changes during the inkjet printing process, and control the environment of the workshop. Only in this way can the stability of production be maintained. And the quality of the product.