The premix stage involves predispersion of the pigment (organic or mineral) in a mixture of varnishes. In fact, several varnishes with different properties are used in a single ink and it is through their judicious combination that series of inks with completely different properties are obtained. Certain varnishes have very specific properties, while others may be used in the composition of several references.
The pigment, supplied in the form of powder, is in fact composed of a multitude of aggregates of elementary particles. The premix stage aims to divide these aggregates as finely as possible by coating the fine particles with the varnish (the binder of the ink). This “wetting" step allows the air surrounding the particles to be progressively replaced by binder and therefore to disperse the pigment in an optimal manner. Certain properties of the ink such as the colour strength and the gloss depend on the quality of this coating / wetting step.
Premix - Source Buhler
As a function of the type of ink to be manufactured, the dispersion is carried out either with a “disc” type mixer, of variable shape, or with a “butterfly” type mixer. The geometry of the agitator depends on the ink formula, but the principle remains the same: it involves creating sufficient shear at the heart of the dispersion to assure the break up of the aggregates and the coating of the pigment. The speed of agitation (obtaining a vortex), the temperature (rise in temperature due to the shear) and the rate and order of incorporation of the pigments and fillers are parameters that affect the pre-dispersion.