The key to digital printing is ink. Inkjet inks must meet certain physical and chemical standards and have specific properties to form droplets. They are suitable for specific inkjet printing systems and provide excellent image and color vividness. The performance of the ink not only determines the effect of the printed product, but also determines the shape characteristics of the droplets ejected from the nozzle and the stability of the printing system.
The basic performance requirements for reactive dye inkjet printing inks are as follows:
The effect of surface tension on ink drop formation and print quality is extremely significant.
From the observation of the presence or absence of spillage around the nozzle during the inkjet test, the length of the droplet breakage, the stability, the droplet velocity, and whether it is running in a straight line, the quality of the droplet formation can be evaluated, all of which are subject to surface tension and viscosity. influences.
Too high surface tension makes the print head surface not easy to be wetted, the ink is not easy to form small droplets, and may have a long break length, or break into a “tailing” shaped droplet, and the ink build-up around the nozzle will affect the fineness. The linear operation of the droplets and the reproducibility of the printing effect.
If the surface tension is too low, the droplets will be unstable and even form a “satellite”, creating a sputtering spot. It is generally required that the surface tension of the inkjet ink is lower than that of the printed substrate, and the contact angle of the ink droplet with the substrate is greater than 140 degrees to obtain a clear pattern.
In summary, a suitable surface tension requirement is that the ink can be easily infiltrated into the interior of the fabric without causing percolation of the ink droplets on the surface of the fabric, and also needs to be tailored to the equipment used.
As previously mentioned, the viscosity of the ink, together with the surface tension, affects the formation of ink droplets.
The high viscosity causes the broken tail to be stretched and drawn in a draw shape; if the viscosity is too low, the droplets are easily broken.
Another important parameter that is affected by viscosity is the jet velocity of the droplets. If the viscosity is too high, the droplet velocity will decrease and even the ink droplets will not hit the same point on the substrate being printed.
Basic tests and actual printing results have shown that a viscosity of less than 10 mPa·s (preferably 2 to 4 mPs·s) is a suitable range for obtaining a satisfactory droplet shape and printing effect.
In continuous jet printing equipment, the ink droplets are deflected by the charge, so the ink must have the ability to be charged and electrically conductive.
The ORIC printing machine requires an ink conductivity of 750 siemens or more.
In fact, as an aqueous ink system, it has good electrical conductivity. It is only necessary to add a special conductivity modifier in a solvent-based ink system. Studies have shown that the effect of conductivity on droplet formation has not found a significant dependence.
The pH of the heat transfer ink should be stable. Some dyes will precipitate due to changes in pH. Generally, the pH should be adjusted at 7~8. The ink is acidic and will corrode the nozzle and metal parts. A suitable pH value facilitates the combination of dye and fiber reaction and promotes print clarity.
There are other technical requirements for inkjet printing inks, such as good dyeability, so that the color of the fabric is high; the purity of the dye is high, so as to ensure the vividness of the color; the thermal stability and the frost resistance are good, so that the storage Stability can be improved.