Packaging printing is often the first to print the base colour in order to accentuate the high quality of graphic decorations in pursuit of high value-added goods. Found in practice, such a printing order is very easy to produce ink crystallisation phenomenon. In the end, what is the cause of this? And how to solve this problem?

 

Causes of ink crystallisation (crystallisation) of the reasons for analysis

First, in order to achieve the purpose of bright and vivid colours, generally in the printing of the ink layer printed very thick or reprinted once or increase the printing pressure, dry oil is also added more. Although the ink layer completely covered the carrier, but because of the drying too fast and lead to the printing ink film after the surface becomes very smooth ink film layer, as if the glass is as difficult to overprinting fine, so that after printing on the ink printed on the uneven or completely printed on the ink, cover (stacked) printed on the ink in the base colour on the presentation of bead-like or a piece of the colour of the printing of the very weak pattern, the ink link is very poor, and some can even be wiped off. Printing industry called ink film crystallisation, glass or mirror.

 

In order to improve the clarity of the edge of the graphic, in recent years, most manufacturers in the ink system to add silicone oil, but too much silicone oil tends to cause vertical shrinkage of the ink film.

 

Currently there are several different insights into the causes of ink film crystallisation as follows. According to the theory of crystallisation, crystallisation is the process of forming crystals from the liquid state (liquid or molten body) or the gaseous state. Solubility with the reduction of temperature and a significant reduction in the substance, through the cooling of the solution can be saturated and then crystallised; solubility with the reduction of temperature and the reduction of the substance is not large, when part of the solvent evaporates and then cooled and crystallised. Some people believe that the packaging printing graphics (ink film layer) of crystallisation that is recrystallised …… printing ink film system solvent evaporation (volatile) and then cooled to form, also known as recrystallisation.

 

Second, there is also a part of the people that packaging printing ink crystallisation (crystallisation) is mainly caused by the crystallisation of pigments in the ink system. We know that when the pigment crystallisation is anisotropic, the crystalline state is needle, rod. When the ink film is formed, the length direction is easily arranged along the flow direction of the resin (linker) in the system, thus generating a larger contraction; while there is no directional arrangement in the case of globular crystals, so the contraction is small. Inorganic pigments in the packaging printing ink system usually have spherical crystals, such as cadmium pigment packaging printing ink, its shrinkage (crystallisation) is also small. The particle size also affects the moulding shrinkage and moulding shrinkage ratio, when the pigment particles are large to a certain degree or small to a certain degree, the moulding shrinkage and shrinkage ratio are minimum. On the other hand, the crystalline large spherical crystal resin moulding shrinkage is small, on the contrary, the crystalline large, non-spherical crystal moulding shrinkage is large. In conclusion, whether it is a mixture of colour pigments or a mixture of colour and light, the correct use of pigments is not only related to the chemical structure, but also depends on most of its physical properties, such as the distribution of crystalline phase size, cohesion phenomenon, solid solution and other influencing factors; it should also be inorganic pigments and organic pigments have their own strengths and weaknesses to make a fair evaluation, so that the two coexist, and the latter occupies a primary position.

 

In the process of choosing packaging printing ink (pigment), we must also consider its colouring power (the finer the degree of dispersion, the higher the colouring power, but there is a limit value, over which the colouring power will be reduced), covering power (pigment’s own light-absorbing properties, the difference in the refractive index of the pigment and the resin linker required for colouring, the size of the pigment particles, the pigment’s crystalline form, the symmetrical molecular structure of a high degree of symmetry than symmetry of the low-crystalline form), low covering power; the crystalline form is the most important factor. Covering power is low; crystal type is flaky than rod covering power, high crystallinity pigment than low crystallinity covering power, so the greater the covering power of the packaging printing ink ink film, the greater the chance of failure of its vitrification), heat, migration, weathering, solubility, and polymers (ink system of the resin) or add additives, etc. are not to be underestimated.

 

Third, some operators believe that if the choice is not appropriate, will also cause crystallisation failure. Is because the base colour ink dry too hard (dry), the surface free energy is reduced. The previous colour printed after too long a storage time, the workshop temperature is too high or printing ink desiccant is too much, especially cobalt desiccant, it if the use of rapid and intense drying methods, such as drying and so on will produce crystallisation phenomenon.

 

Prevention of ink crystallisation fault method

1, with more solvent-containing fast-drying ink overprint (not wait for it to dry on the print of the second colour, although the effect is better, but this method is laborious, time-consuming and costly), the use of solvents can be immersed in and soften the next layer of the ink film layer (but sometimes it will not help).

 

2, master overprinting time, in the previous colour printing should be as fast as possible after printing the second colour.

 

3, in the ink to add some slow drying co-polymer (such as hydroquinone) or slow drying materials (petroleum jelly, lanolin, wax additives).

 

4, in the packaging printing decorative pattern design can be used in different easy to overprint the base colour.

 

5, in the ink formula should be less cobalt desiccant.

 

6, can use fast-drying synthetic resin-type linker to reduce the amount of ink desiccant.

 

7, enhance the adhesion of the second colour ink, the purpose is to destroy the ink film has been crystallised. Such as the use of zero ink blending oil or alkyd resins, epoxy resins and polyamide resins are more adhesion materials, add ink can enhance its adhesion, but these resins are not good with the miscibility of offset printing ink, can not be added, because too much accession tends to dilute the colour of the printing ink, and too little of its effect will not be good.

 

8, in the printing ink to add organic solvents to dissolve the ink film layer has been crystallised. This is one of the methods commonly used by printing operators, but the crystallised ink film layer has become the main body of the smooth ink film layer of the mesh structure is not easy to be dissolved, too much when the effect is not good.

 

9, in the printing ink with alkali or soap to enhance the polarity, so that it is easier to print on, but at that time, although it seems to be printed on, but after drying is not firmly attached and very easy to wipe off.

 

To sum up the above methods and preventive measures to ensure the quality of packaging printing graphics is to grasp the printing ink dryness, in order not to let the ink film dry over the start of printing the second colour as the best.

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