The empirical judgment method of fluidity is to provoke the ink with a dipstick knife, and then let it flow down and pull it into ink. If the ink is long, the fluidity is good, and vice versa.
Surface Tension The surface tension of the ink is related to the transfer properties of the ink and the stability of the print on the printing surface. This relationship is even more pronounced when printed on glossy materials, especially plastic surfaces. For example, when the surface tension of the ink is greater than the surface tension of the printing surface, the imprinting will shrink, and even fisheye-like holes will appear. If the surface tension of the printing ink and the ink is printed, the printing will be blocked. These conditions can be adjusted by adding surfactants, microcrystalline wax, silicide, etc., so that the surface tension of the ink is equal to or less than the surface tension of the printing surface to obtain a good printing effect.
Fineness Fineness of ink refers to the degree of dispersion of solid powders such as pigments and fillers in the binder. The fineness of the ink is not the same as the fineness of the solid powder, which refers to the basic particle size of the powder, which is small, see Table 4-5. In general, the basic particle size of the pigment is less than 1 μm. In fact, the pigment is aggregated with several elementary particles (up to 50 μm in size) and added to the binder of the ink. After grinding, the aggregates disperse and the dispersion becomes better. The smaller the ink particles (fineness). The fineness of general ink is about 5μm, ultrafine up to 1-2μm. Fineness affects the sheen of the ink film and the rheology of the ink.
Several types of pigment particle size Table 4-5
Pigment Name Particle Size (μm)
General pigment <1.0
Titanium dioxide 0.25
Carbon black 0.01-0.1
Fluorescent pigment 0.01-0.1
Gold and silver powder 1.2-50.0
Thickness: 0.1-2.0
Diameter: 1-200
Color foil pigment>200
In terms of the printability of silk screen printing, the fineness should be compatible with the aperture of the printing plate mesh, ie, the ink particle size should be less than or equal to 1/3 of the screen size.
Dryness refers to the drying speed of the ink. It has two requirements: the slower the ink dries on the plate, the faster it will dry on the printing surface. Fast drying on the plate will affect the ink viscosity, rheology and the uniformity of the imprint, and even cause blocking of the network, also known as netting, the time for the ink to start netting should not be less than 5 minutes, the slow drying on the print, then affect Production speed or increase drying costs.
It should be said that online slow-drying and imprinting fast-drying inks are the ideal drying performance, resulting in the use of inks such as photo-setting, thermosetting and hot-inking.
Viscoelastic Viscoelasticity refers to a kind of mechanical property that the material has both solid elasticity and liquid viscosity (plasticity) under external force. Polymers are almost all viscoelastic, so most also have this property. Viscoelasticity is closely related to the splitting of the ink during printing, the drawing conditions, and the smoothness of the surface of the print.
The difference in viscoelasticity of the material can be described by the relaxation time. Simply put, when the external force is less than a certain time, the material shows elasticity. When the external force is longer than this time, the material will change. This time is called relaxation time Ï„. The relationship between Ï„ and material properties is:
When τ → ∞, the object is completely solid and only elastic;
When τ → 0, the object is a liquid and it is only sticky;
<τ <∞, the object is viscoelastic.
The value of Ï„ reflects the size of the intermolecular binding force of the object. Therefore, the Ï„ value of the linear structure polymer is small, and the Ï„ value of the polymer is higher in the network structure or the cross-linking point.
Relaxation time is very useful to adjust the printing speed, especially the separation speed between printing plate and printing surface, such as the separation speed should not be less than Ï„, otherwise the ink transfer is elastic and the transfer rate is low. There is no sticky ink with elasticity, which is difficult to transfer when printing; there is no elasticity only with sticky ink, or drawing is serious, or the flow is out of control, and no good print can be obtained.
The elasticity provided by the viscoelasticity allows the split ink to snap back quickly, ensuring a good impression.