Effect of ink printability on ink transfer

In the printing process, the printing machine ink system transfers the ink from the ink fountain through the ink roller and printing plate to the surface of the printing material, forms an ink film, and then adheres to the surface of the printing material through fixing and drying. In order for the ink to successfully complete the transfer from the printing press to the surface of the printing material and adhere securely, the ink must have the appropriate printability.

1. The rheological behavior of ink

The operational suitability of the ink in the printing process is closely related to its rheological behavior.

There are a wide variety of printing inks and many different properties. Due to the different printing methods, the rheological behavior of the ink in the printing process is also different. High-viscosity inks are used in lithographic printing plates. Rheological properties are similar to those of plastic fluids. Low-viscosity inks are used in gravure printing and flexographic printing. The rheology is close to that of pseudoplastic fluids, and the fluidity of the intaglio printing inks is nearly inflated. Sexual fluids.

1 plastic fluid. Plastic fluid refers to a liquid that must start flowing after the applied external force is large enough. Its flow form is called plastic flow.

2 pseudoplastic flow. This fluid can flow immediately when an external force acts, but the shear stress is not proportional to the velocity gradient. When it is agitated, it can flow well. However, once the fluid structure is destroyed, it will not be restored. That is, this process is irreversible.

3 Inflation fluid. Inflation fluid refers to a fluid that increases the flow of force (increases the viscosity) when stirred or sheared, but returns to the original flow state after standing for a certain period of time.

2 Thixotropy of ink.

When the ink is stirred by an external force, the ink becomes thick and thin as a result of stirring, but when the stirring is stopped, the ink thickens again from thinning. This phenomenon is called thixotropy.

Due to the thixotropy of the ink, the fluidity of the ink becomes greater under the action of the shear force generated when the ink roller rotates, and the transfer performance is improved; but when the ink is transferred to the surface of the paper, the ink loses the effect of the external force. The thickening from the thin, but not overflow to the imprint, to ensure the accuracy of the imprinting.

The process of transferring the ink from the ink fountain to the ink distribution roller via the transfer roller is called the ink process. In the ink feeding process, if the thixotropy of the ink is too large, the stirrer is placed in the ink fountain to prevent the “ink blocking” phenomenon of the ink, reduce the apparent viscosity of the ink, and improve the flow condition of the ink.

3, the drying of ink

The speed of ink drying has a great influence on the print quality. If the drying is too fast, the ink will be pasted on the printing plate or ink roller to form a crust, causing the ink on the printed matter to accumulate, destroying the luster, and causing ink spots and the like. Drying is too slow and prints may appear dirty on the back.

The drying speed of the ink is related to the drying mode of the ink. There are mainly three types of ink drying methods:

1Infiltration Drying of Ink refers to the drying method in which a part of the binder penetrates into the paper when the ink is dry, and a part of the binder is fixed on the surface of the paper together with the pigment. The letterpress ink mainly relies on osmotic drying.

In actual production, the ink penetration goes through two stages: At the moment of imprinting, the ink penetrates into the surface layer of the paper under the pressure of the printing pressure, which is called pressure infiltration; after the paper leaves the embossing area, the ink remaining on the paper surface is Paper penetration into the deep layer of paper is called free penetration.

2 Volatile Drying When the ink layer is dry, the solvent in the binder is partially volatilized. The rest of the resin and the pigment form a solid film layer and is fixed on the surface of the substrate. Gravure inks and flexographic inks are typical volatile drying inks. In the volatile drying ink, the volatilization of the solvent is affected by the binder and the pigment and is more complicated than the pure solvent. For example, the greater the solubility of the resin, the more the solvent evaporation rate decreases. Therefore, when the resin is selected, the resin solvent should be required to have a high release property. Otherwise, the dryness of the printed product may be caused due to the poor evaporation of the solvent. In addition, the greater the percentage of pigment in the ink, the lower the solvent volatilization rate; the smaller the pigment particles and radius, and the larger the surface area, the lower the solvent volatilization rate.

3 Oxidative conjunctiva drying. An ink that uses a dry vegetable oil as a linking material undergoes an oxidative polymerization reaction after absorbing oxygen molecules in the air. The ink is converted from a liquid state to a solid state, forming a glossy, abrasion-resistant, firm ink film. Lithographic printing inks, iron printing inks, and screen printing inks are oxidized conjunctiva drying inks.

The drying of the oxidized conjunctiva of the ink is accomplished by an oxidation bridge polymerization reaction and a conjugated double bond addition polymerization reaction, which occurs on the double bond of the unsaturated acid. The more double bonds the dry vegetable oil has, the faster the oxidative conjunctiva is dried. In order to increase the drying rate, a certain amount of desiccant is usually added to the dry vegetable oil to accelerate the production of peroxide.