Correct understanding of influencing factors and solutions of viscosity of water based ink

Ink is an indispensable and important material in carton printing. In recent years, with the increase of environmental problems, Water Based Ink has attracted the attention of packaging and printing industry. Compared with Solvent Based Ink, water based ink has no VOCs emission in the printing process, which is more environmentally friendly and safer, and the cost can be saved by about 30%. Therefore, in a few short years, water based ink ranks first among other packaging and printing inks with its dosage of 35%.

Viscosity is one of the important quality indexes of water based ink, and it is also an important factor to determine the printing quality. The viscosity of water based ink is too low, which easily leads to poor transfer, insufficient ink supply, uniform stripes in printed matter, insufficient transfer ink and white exposure. If the viscosity is too high, the printed text will be unclear, and the ink film will be too thick due to too much paste and transfer ink, which will easily lead to some disadvantages such as color fading and difficulty in ink delivery.

1.Correct understanding of viscosity of water based ink
The viscosity of water based ink is generally standardized in water based ink products according to the category suitable for printing, and it is not necessary to prepare before use; For personalized products, they can be customized or deployed on site according to the actual situation of users. In the application of water based ink, some users have some misunderstandings about viscosity, mistakenly understanding that water based ink with high viscosity can be diluted in a high proportion, thus reducing the printing cost, resulting in a series of quality defects in printed products.
Water based ink is mainly made of water-soluble resin, organic pigment, solvent and related additives through compound grinding. Water based ink is especially suitable for packaging and printing products with strict hygiene requirements such as cigarettes, wine, food, beverages, medicines and children's toys.
As the main solvent of ink painting, water exists in a strict proportion. Adding water to ink painting at will increases the proportion of water and dilutes ink painting, which actually changes the reasonable proportion of various components in ink painting, changes the viscosity of ink painting, destroys the original performance of ink painting and the state of pigment in ink painting, affects the transfer performance, drying speed and conjunctival quality of ink painting, and will bring a series of problems to printing operation and printing quality. Therefore, it is wrong to add water to ink at will, which will inevitably affect the quality of printed matter.
Different ink viscosities should be used for different printing requirements, and it is more important to maintain the stability of ink viscosity in printing operations.
2.Relationship between printing viscosity of ink and common quality problems
In plastic printing, it is very important to control the printing viscosity of ink. Especially in high-speed rotary gravure printing, whether the viscosity control of ink printing is appropriate or not has a great relationship with most quality problems such as pigment transfer, dirty plate, gloss of printed matter, fastness of ink layer, static electricity and so on, which affects the yield and production efficiency of printed matter. How to control the printing viscosity of ink can give consideration to many aspects and ensure the quality of printed products, which requires us to understand the relationship between the printing viscosity of ink and these common quality problems.
2.1 Ink printing viscosity and pigment transfer effect
Plastic film Printing Ink is composed of organic solvent, binder, pigment, additive and auxiliary material. In the printing process, the effect of pigment transfer is mainly related to the printing viscosity of ink. Practice has proved that the printing viscosity of ink has a certain range (10 ~ 25s, 3# Zahn Cup). In this range, the greater the printing viscosity of ink, the worse the effect of pigment transfer. When the printing viscosity of ink is too high, the whole ink system is in a state of supersaturation, and the liquidity of pigments and other substances is poor, so they can't be dispersed evenly, but appear in groups and are easy to pile up together, so that pigments can't smoothly enter the printing plate mesh holes, even can't enter the mesh holes at all, let alone transfer. This is the so-called blocking phenomenon. Therefore, we only need to make the colloid system composed of resin, pigment and organic solvent not in supersaturated state, but in saturated or unsaturated state, so that pigments and other substances can be well dispersed in it to form a uniform and delicate colloid system, so that pigments can smoothly enter and exit the mesh holes, and the problem can be solved.
Some ink manufacturers suggest that the printing viscosity of ink should be controlled at 15 ~ 18s (3 # Zahn Cup), but in practice, especially in high-speed gravure press, in order to ensure good transfer effect, print for a long time and improve production efficiency, the ideal state point of printing viscosity of ink is generally found between 11 ~ 15s (3 # Zahn Cup). The actual optimum viscosity needs to be obtained after comprehensive analysis and trial operation in combination with specific conditions such as materials, inks and use conditions.
2.2 ink printing viscosity and dirty plate fault
The dirty plate is poorly wetted by the printing plate, causing the blank part to be inked. The reason is that pressure is one of the important conditions of offset printing and relief printing, and moderate printing pressure is also an effective technical measure to ensure the normal production and improve the printing quality of products. However, if the operation technical measures are not in place and the printing pressure is not suitable, it will cause some printing failures.
In plastic film gravure printing, the phenomenon is that the non-printing pattern parts of the printing plate cylinder will also be stained with a hazy layer of resin (with pigment in the resin) and transferred to the plastic film, which will appear as sheet or linear dirt on the printed matter. The root of the dirty plate phenomenon is not only related to the hardness, pressure, contact angle of the scraper and the quality of the plate cylinder, but also related to the printing viscosity of the ink. When the printing viscosity of ink is too high, the fluidity of pigments and other substances is poor, and it is easy to pile up together. Under the relatively high-speed movement of the scraper and the printing plate cylinder, the scraper and the printing plate cylinder are easy to be damaged, which is easy to cause linear dirt.
At the same time, because the resin is also in supersaturated state, a large number of resins coalesce together, and the affinity of the resin to the printing plate cylinder, under the pressure of the scraper, the resin will tightly stick to the printing plate cylinder (with pigment in the resin), so that flaky dirt will appear. Generally speaking, the smaller the printing viscosity of ink, the less likely it is for dirty plates to occur.
3.What is the ink viscosity detection method?
Stir the ink sample for testing evenly, then immerse the viscosity cup into the sample ink, and then lift it out at a uniform speed. When the viscosity cup just pulls out of the ink surface, press the stopwatch to observe the ink outflow, and press the stopwatch when the ink just breaks.
At this point, the time on the stopwatch is the viscosity value of the ink. If the viscosity is high, of course, the number of seconds is large, otherwise, the number of seconds is small. Of course, the viscosity of ink will change with the temperature, and the detection temperature of general test is fixed at around 25℃. There is no obvious correlation between the temperature and the color of ink, but it has some influence on the performance of ink when it is printed on the machine.
Relationship between printing speed and ink viscosity. In high-speed printing, low-viscosity ink should be used, and fast-drying solvent should be used for dilution; On the contrary, in low-speed printing, high-viscosity ink should be used and diluted with slow-drying solvent. Therefore, the ink should be adjusted to a suitable viscosity when operating on the computer, and this viscosity should always be kept constant.
Dilution method of ink: At the beginning of dilution, special thinner must be added to the new ink to make the ink reach the viscosity suitable for printing.
While stirring the ink, slowly pour in the solvent, because the pigment particles in the ink are wrapped in the resin and evenly distributed in the whole system. If a large amount of solvent is used and diluted rapidly, after the ink is impacted by this solvent, the evenly distributed pigment particles often become bare, lose their due gloss and transparency, and the naming printing effect decreases. In this regard, special attention should be paid.
In the printing process, in order to get printed matter with the same hue, it is necessary to maintain a certain printing viscosity, because the viscosity of ink, solvent drying speed and printing speed are closely related. Usually, low-viscosity ink is used in high-speed printing and high-viscosity ink is used in low-speed printing, but it is necessary to select the most suitable printing viscosity for printing according to the plate depth, whether there is hierarchical change or not, and combined with the transfer, fluidity and concentration of ink.

4.Factors affecting ink viscosity and their solutions
4.1. Influence of temperature on ink viscosity
The temperature and humidity of the printing environment will directly affect the changes of ink and wash. The rising of temperature and the falling of humidity will accelerate the volatilization of ink and wash, especially when the temperature difference between winter and summer is large, the viscosity is the most sensitive. Therefore, ink and wash suppliers generally make appropriate adjustments to the ink and wash formulas in the current season according to the seasonal changes. For example, if the ink imported in winter is stored for use in summer, the viscosity of the ink must be re-tested and adjusted.
Printers must attach great importance to the influence of ink temperature, and it is very important to stabilize the temperature of ink at the temperature of printing workshop before printing, otherwise the viscosity of ink will change obviously during printing.
4.2. Effect of pH value on ink viscosity
Another index that needs to be controlled in ink painting application is pH value, and its normal range is generally 8.5-9.0. At this time, the printing performance of water based ink is the best and the quality of printed products is the most stable. In the process of printing operation, ink is continuously circulated into the ink bucket in the ink conveying system of the printing machine, and the volatile components and water in ink are constantly volatilized in the air over time. Therefore, the performance of ink used for a long time will inevitably change due to the change of components, resulting in the decline of pH value, the increase of ink viscosity and the slow drying speed.
The influence of pH value on the printability of water based ink is mainly manifested in the viscosity stability and dryness of ink and dot clarity. In the process of use, due to the continuous volatilization of ammonia in the printing process, with the decline of pH value of ink, the viscosity of ink increases and the drying speed slows down, which will inevitably affect the normal printing work. Therefore, the operator needs to add new ink and various additives to the ink from time to time, and the pH value of the ink will change accordingly. The operator must master the principle and technological method of ink blending, and never add or adjust it at will.