In the wonderful world of ink, viscosity and viscosity is like a pair of closely linked but each with its own characteristics of the ‘brothers’, jointly dominated by the performance of ink in the printing process.
First, viscosity: fluid molecules of the ‘internal tug of war’
Viscosity, as a key fluid qualities, like a molecular ‘internal tug-of-war’. When the liquid is stirred, its flow rate is proportional to the force applied, just as we push an object, the object moves faster or slower with the size of our force. To put this into a formula: force applied (shear stress) = constant * speed of shear, and this constant is the viscosity of the liquid. In simple terms, viscosity measures the ability of fluid molecules to interact with each other in a way that prevents the molecules from moving relative to each other, or the ‘internal resistance’ of the fluid as it flows. It is measured in Pascals・seconds (Pa・s) and is also often referred to as viscosity.
Inks, for example, have a viscosity that is mainly ‘set’ by its own components. The linking material of the ink is like a ‘skeleton’, and if its viscosity is greater, then the viscosity of the entire ink will increase. In addition, the type and content of solvents, the percentage of pigments, the size of the particles and the dispersion of the pigments in the linker are like ‘puzzle pieces’, which combine with each other to create differences in the viscosity of different inks. For example, inks made with a high pigment content tend to have a higher viscosity; inks with large pigment particles, on the contrary, have a lower viscosity; and when the pigments are uniformly and well dispersed, the viscosity of the ink will also be relatively small. This is like building a structure with different sizes and numbers of blocks, and ending up with varying degrees of stability and ‘sturdiness’ (similar to viscosity).
The ‘silent match’ between viscosity and printing speed
In the printing world, there is a subtle ‘silent match’ between press speed and ink viscosity. Typically, the faster the press speed, the lower the viscosity of the ink needs to be. For example, at press speeds of up to 600EP/h, ink viscosities of between 20 – 50Pa・s are generally optimal. This is because high-speed printing, if the ink viscosity is too high, like a thick glue, it is difficult to quickly and evenly spread on the printing plate and substrate, which can easily lead to uneven printing, ink spots and other problems. On the contrary, low viscosity ink can be more smoothly in the high-speed printing machine ‘running’ to ensure the quality of printing.
Ink viscosity and substrate ‘fit the way’
Different substrates have different surface strength, which requires the viscosity of the ink and its ‘fit’. For example, paper as a common substrate, there are high and low surface strength. For the lower surface strength of the paper, if the use of higher viscosity ink, as a soft person to carry an excessive burden, the paper may not be able to withstand the ‘pull’ of the ink, resulting in the surface of the paper hairy, powder, and even ink can not be firmly adhered to. Therefore, for this type of substrate, only ink with relatively low viscosity can be used to achieve good printing results.
Viscosity and temperature ‘thermal expansion and contraction’ effect
Temperature has a significant ‘thermal expansion and contraction’ effect on ink viscosity. Generally speaking, when the temperature rises, the viscosity of the ink will decrease, just like winter butter becomes smoother when heated; while when the temperature falls, the viscosity of the ink will increase and become thicker. This is because the temperature change will affect the ink molecules active degree of movement, the higher the temperature, the more intense molecular movement, the ‘bond’ between each other the smaller, resulting in lower viscosity.
Second, viscosity: ink film ‘separation game’
Viscosity, can be regarded as a material ‘adhesion’, figuratively speaking, is at a given speed, the two by the liquid stuck together in a certain unit area of the plane separation of the force required, usually measured by the Taek value, which is a relative value, there is no quantitative outline. In a printing press, the ink layer undergoes a ‘peeling’ during each transfer of ink from the ink rollers to the printing plate, and then from the plate to the substrate, and the tackiness of the ink is the ability to impede this peeling of the ink layer.
Viscosity and ink-to-substrate ‘intimacy’ are especially critical in high-speed printing. The viscosity of the ink must be below a ‘critical value’, otherwise it is like glue that is too sticky to apply smoothly, and the ink has difficulty transferring and adhering to the substrate. However, the higher the viscosity of the ink, the better, when conditions permit. For example, when printing small text, ink with high viscosity can better maintain the shape and clarity of the text, so that the print is clear, sharp dots, as with a brush with delicate strokes to outline the delicate lines. Moreover, according to the sequence of printing, the viscosity of the first colour ink is usually the largest, and then gradually reduced, which is conducive to the overprinting of the subsequent colours, to ensure the accuracy of the colour and sense of hierarchy. The size of the tack can be accurately measured by a tackmeter.
Interestingly, there is a correlation between viscosity and tack. For low viscosity inks, the viscosity and tack show a linear relationship, i.e. the greater the viscosity of the ink, the greater the tack. This is like an invisible line that connects the changes of the two closely.
Third, the detection of ink viscosity and mobility ‘dance’
Ink viscosity detection has a set of rigorous methods. Use the cup of Coat No. 4 (QND – 4), drop the inspected ink, and under the precise temperature condition of 25℃ ± 1℃, open the small hole switch at the lower end and let the ink flow down freely. At the moment when the flow of ink is interrupted, the timing is stopped and the number of seconds indicated by the stopwatch is the viscosity data of the ink. Generally speaking, the viscosity index of the ink is between 25 – 70s/25℃, and the ink factory will mostly control the viscosity in the range of 30 – 60s when the product is shipped out of the factory, in order to ensure that the ink can have a more stable performance in various printing scenarios.
There is also a wonderful ‘dance’ between ink fluidity and viscosity. The fluidity of an ink is its property of flowing under external forces and gravity, and it is closely related to viscosity. When the ink increases the viscosity, it is like adding a ‘brake’ to the flowing liquid, and its fluidity will be reduced; on the contrary, lowering the viscosity will allow the ink to improve its fluidity and make it more ‘free-flowing’. However, it should be noted that thicker ink does not necessarily mean higher viscosity, and ink with higher viscosity is not necessarily thicker. Only in the case of the same viscosity, the greater the consistency, the less fluidity of the ink.
If the fluidity of the ink is too small, it will be like a ‘slow-moving dancer’, in the printing process, the distribution is not smooth and uneven, resulting in the same printing surface appears before the light after the deep phenomenon, as a painting with uneven colour gradation; and too much fluidity, the ink will be like a ‘too lively Children’, uneven distribution, making the printing level blurred, ink colour is not full enough, the network is also easy to expand, seriously affecting the quality of printing.
In summary, although the viscosity and viscosity of the ink are two different concepts, but they affect each other, constrain each other, and jointly affect every aspect of the ink in the printing process. In the printing process continues to develop today, in-depth understanding and precise control of the viscosity and viscosity of the ink, for improving the quality of printing, expanding the field of printing applications is of vital significance. We might as well imagine, if the future can develop a smart ink, its viscosity and viscosity can be automatically adjusted according to the printing environment and demand, it will undoubtedly bring a brand new change to the printing industry, to open a new chapter of more efficient, accurate and diverse printing.
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