Application Of Printing Viscosity Index In Printing Process

　　 Printing viscosity The index is referred to as PVI (Printing Viscosity Index), which is the ratio of viscosity of the same paste under two different shear rates (usually shear rate ratio to 10:1), which is used to measure the rheology of the paste. Viscosity can be determined by NDJ. 1 rotary viscometer. The rotary viscometer is composed of two cylinders inside and outside, and the two cylinders are placed between the sample. The outer cylinder remains stationary, the inner cylinder (rotor) rotates at a certain angular speed, and the torque is calculated from the torsion angle of the suspension spring, and then the fluid viscosity is obtained. The inner cylinder is divided into five kinds, the speed is 60r/min and 6r/min or 100r/min and 10r/min, that is, the PVI value is 60 or 7700 respectively. The structural viscosity of the paste can be expressed by increasing the shear rate and decreasing the viscosity. The PVI value of the general printing paste is 0.1 - 1 and the Pw value equals 1, which is a Newtonian fluid. The PVI value is related to the structural viscosity index, and the smaller the PVI value, the structural viscosity of the original paste. index The bigger.

　 PVI value The larger the paste, the smaller the viscosity changes when the shear rate changes. Therefore, the printing paste containing this type of paste has little change in viscosity when printing speed changes, and can maintain a more consistent printing effect. On the contrary, the paste with smaller PVI value, with the change of the speed of printing, the viscosity change is significant, the printing effect is not easy to control. At this time, the paste must be properly improved. Solid content 。

Printing and dyeing workers in Europe and America have studied rheological properties through the fluidity of printing paste, while Japanese and India countries have solved some problems in printing process by calculating the viscosity index of printing.

1. mix and use all kinds of raw paste.

Single paste is difficult to meet various requirements in printing production due to its single characteristics. For this purpose, two or more than two kinds of mixed pastes are often used to compensate for their deficiencies.

The most prominent problem when mixing paste is the change of viscosity and the change of rheological properties. Usually, the paste with low solid content and high viscosity is mixed with the paste with high solid content and low viscosity, for example, the emulsion paste is mixed with low alginate alginate. In order to meet different printing requirements, all kinds of processed paste sold in the market are mixed mixtures which have been mixed in advance, for example, mixing all kinds of etherification locust bean gum.

After mixing, the viscosity of the paste will generally decrease, but the rheological properties will be improved. The viscosity index of paste will also change accordingly. Usually, the viscosity index of printing paste can be calculated by the following formula.

In the formula, Wa and WB are the blending weights of raw paste A and raw paste B respectively. The more the raw paste is mixed, the better the linear relationship of PVI value. But after mixing, the viscosity coefficient of the original paste can not be reflected from the PVI value, but it must be seen from the original paste diagram. {page_break}

2. sketch map of raw paste

Using the PVI value of the original paste as the abscissa and drawing the logarithm of the apparent viscosity (LGN) as the ordinate, it is the original paste diagram. The PVI value of the original paste on the right side of the indicator diagram is large, and the apparent viscosity of the original paste is measured by the rotary viscosimeter. When the speed difference is 10 times, the change amplitude of the paste is smaller (vertical line is shorter), indicating that the speed change of the printing locomotive has little influence on the original paste, and the original paste on the left side of the indicator diagram is opposite. The indication of commonly used paste is shown in Figure 1.

3. relationship between printing method and PVI value

The shear stress of flat screen printing and roller printing is different, so the rheological properties of printing paste are different. Color paste used for flat screen printing does not necessarily apply to roller printing. The test of commonly used printing pastes showed that the viscosity index PVI of different figures was suitable for different printing methods.

Screen printing uses a smaller PVI value (i.e. a smaller structural viscosity index) and a higher viscosity. The original paste suitable for flat screen printing should be pseudoplastic fluid, that is, flat screen printing is through the extrusion of the scraper (or magnetic rod), the color paste is pressed over the mesh at a lower speed (the smaller shear stress), and the dye paste printed on the fabric is permeated and diffused freely. Therefore, in order to maintain the smoothness of the outline, the color paste must have higher viscosity and uniformity, and a certain degree of rheology. When printing fine lines, it is necessary to have a high thixotropy, which is different from that of printing large blocks.

In roller printing, the color paste stored on the copper roller web or diagonal line is transferred to the fabric in the presence of pressure after scraping and scraping. Usually, after the transfer of the color paste, it is extruded by the copper roller in the back, and then quickly dried. Therefore, the fluidity of the color paste and the permeability of the semi finished products are particularly important.

Rotary screen printing is to dye the paste into the inner wall of the nickel round net, then squeeze the color paste through the scraper (or magnetic rod), and print it from the mesh wall of the circular net to the fabric. Because the rotary screen printing is continuous printing, the color paste is completed by one time scraping and printing. Therefore, its printing paste property is between the flat screen printing and roller printing, that is, the solid content is high and the viscosity is low.

4. relationship between fiber material and PVI value

When the hydrophobic fabric is printed, the paste Pw value and the field must reach the minimum saturation value of the color paste, so that the printing effect can be satisfied in this area. When the hydrophilic fabric is printed, the paste density and the mouth should not exceed the maximum saturation value of the paste, otherwise it will not be able to achieve satisfactory printing effect.

Of course, the above is not absolute. In actual use, the printing effect is related not only to the size of mesh mesh, the mesh opening rate of the mesh, the method of engraving the copper roller, the permeability and levelness of the printing paste, but also on the transmission of the flower pattern, the color paste and the color paste on the fabric. Although several indicators can not be used to predict the actual printing effect at present, according to the performance test data of all kinds of paste, combined with the actual production conditions, the printing process can be adjusted so that satisfactory printing results can be obtained.