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Grinding is the step that makes it possible to refine the dispersion of pigment initiated at the premix stage, when the size of the aggregates is reduced. In fact, the pigment elements coated with binder obtained at the premix step can be dispersed even more finely using various types of mills. This step is essential to develop the colour strength of the pigment, because the more finely dispersed the pigment, the more intense the colour and the higher the gloss.
Different types of Grinding processes and equipments can be used. The most widely used are three roll mills and bead mills.
Three roll mill:
This type of milling is used to manufacture what are known as “paste” inks (conventional offset and UV inks). As its name suggests, the mill comprises three cylinders through which the ink circulates. Normally, the ink is passed through the mill three different times. Certain more difficult to mill pigments may need to be passed through the mill an additional time. The three passes follow on from each other with increasing pressure between the cylinders that perform the milling: the first milling stage, which fines down the premixed ink, the second milling stage, which breaks up any remaining agglomerates and the final milling stage, which completes the dispersion.
Three roll mill - Source: Buhler
Bead mill:
All types of inks (conventional offset, UV and liquid) can be milled by this procedure, with only the configurations of the mills differing. In this case, the milling is assured by the balls (of variable diameter and made out of various materials) within the mill being brought into movement within a cylinder (stator) via the rotation of an axle (rotor). The stator and rotor are fitted with “fingers”, which are cooled to avoid too much overheating. The ink circulates inside the milling chamber and the agglomerates are broken up under the impact of the balls. The quality of the milling is optimised by adjusting the temperature and flow rate of the ink.
Bead mill - Source: Buhler
In the case of “paste” inks, one passage is generally sufficient. It may, if necessary, be followed by a passage through a three roll mill (this cools down the heated ink and removes any air bubbles). Liquid inks sometimes need to be recirculated several times.
The finesse of the milling is a key factor that has to be controlled on each batch. This control is carried out using a North gauge, which determines the particle size of the milled ink.
North gauge
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