Thixotropy refers to the fluidity of the ink under the action of external force stirring, and the performance of the liquid gradually reduces to the original state after the agitation is stopped.
Thixotropy and printing:
Ink suitable for offset printing should have good thixotropy, In printing, the extrusion pressure, shear stress between the roller, ink viscosity decreased, mobility will increase, the ductility increases. This is conducive to the smooth and uniform transfer of ink, and improve the transfer rate of ink. when the ink transfer to the surface of the substrate, the external force disappeared, the ink flow became smaller, gradually from dilute thickening, viscosity rise, so as to ensure the clarity of ink imprinting. especially dot screen printing can avoid dot gain and so on.
However, if the thixotropy of the ink is too large, the ink supply will be impeded, and the ink supply will be interrupted seriously, which will affect the ink supply and ink uniformity of the continuous printing. therefore in the printing operation, often mixing ink in the ink fountain, increased mobility of ink in the printing process, to ensure normal supply.
4. fluid degree & fluidity
Fluid degree refers to the ability of expanding the diameter of ink flowing under the action of pressure. It shows that the ink is thin or thick and the structure of ink is loose or tight. Fluidity is the comprehensive property of viscosity, yield value, thixotropy and fluidity of ink. Under general circumstances, when the viscosity of ink is high, yield value is large and thixotropy is large.
The size of the fluid degree must be adapted to the printing, the flow of ink is too large, the performance of ink is good.
the impact on printing:
(1) if fluid degree is strong, the printing pressure was generated spreading phenomenon, transferred to the paper surface imprinted size expansion, especially in the screen printing, character printing, affect the clarity, levels and color printing performance.
(2) after printing, the ink layer becomes thinner, the brightness of printing color decreases, and the hue becomes lighter.
(3) after printing, a large degree of dilution, increased the permeability of the material, serious ink penetration of the various disadvantages.
(4) the fluid degree of ink is too large, often caused by excessive oil content in the connecting material, such ink in the transmission process will produce a lubrication phenomenon, so that the edge of the printing plate text traces of sliding. the sliding phenomenon between the rollers and plates will make the plate image edge of radial spiny spreading, reflected in the printed matter is called the phenomenon of shelter.
(5) excessive fluid degree of ink in the offset printing prone to emulsification, resulting in printing float. different kinds of ink have different degrees of flow, which is based on the requirements of printing.
The flow value of commonly used flat offset printing ink is 28-36mm, and that of web offset ink is 30-40mm.
The transferability of printing ink refers to the ability to transfer ink from one surface to another when the ink layer is separated between two contact surfaces in the process of ink transfer. the transferability of ink can be expressed by transfer rate.
The transfer rate of ink refers to the ratio between the ink transferred to the surface of the paper and the amount of ink on the plate, expressed as a percentage:
Z= x 100%
Type: transfer rate of Z- ink%
X- the amount of ink on the plate before printing, g/m2;
The amount of ink on the paper after Y- printing, g/m2
The ink transfer rate shows the amount of ink transfer.
The relationship between ink transfer and printing:
Printing process essentially is the transfer process of ink, after the output from the ink transfer roller, then transfer to the plate, and to blanket again, finally transfer to the substrate, the printing process complete .Therefore, the transferability of ink is shown throughout the printing process, it is related to the distribution of ink components in the ink separation in the process of ink transfer, the amount and uniformity of ink transferred to the substrate surface after several times of transfer are determined, it affects the degree of thickness, ink film printing ink, the full extent of the bright color and gloss.
the good transferability ink in the separation of printing machine ink roller will be distributed evenly, in multiple passes can increase to the printing surface of the ink layer uniformity, ink concentration and degree of satiation.
The poor ink transferability, transferred to the printing plate or blanket ink is insufficient, easy to cause the printed text based exposure, the formation of flower version, or cause the ink roller between the ink unevenness, ink is too thick, resulting in shades of ink, ink flying, the roller surface drying fault.
Contact: Charles Zhang
Phone: +86 15942392116
Tel: +86 159 42392116
Add: no162, Shenbei district, Shenyang ,Liaoning, China. PC:110164