Working Properties

Introduction:

The study of the flow of liquids is known as rheology. The printer is interested mostly in the flow of liquids and the control of viscosity, body, and tack. Viscosity refers to the resistance of flow under sheer stress: the greater the resistance of flow, with a given applied force, the greater the viscosity. Viscosity and the flow of ink, varnish, ink vehicles, or other liquids are measured with a viscometer. There are several kinds of meters including efflux cups like the shell cup, zahn cup, and ford cup. There are also the falling-rod viscometer, rotational viscometer, vibrating reed viscometer, and the cone and plate viscometer. Viscosity effects printing by affecting the yield value. Presses need to be warmed up so the ink can flow easily, as well as keeping the press room at the same temperature. Having a low yield value and low viscosity like flexography and gravure inks will enable inks to feed properly. High yield value with low viscosity will also be able to feed. High yield value and high viscosity would result in the ink backing away and improper feeding. Length is the property of an ink that enables it to be stretched into a thread. Length is important to printing because it affects image quality. If the length of screen printing ink was long it would cause strands to form on the screen when printing resulting in poor image quality. Offset inks need to be long to distribute ink on to the rollers, but if the length is to long it will cause misting. Tack is the stickiness of the ink. Tack is the resistance of the ink film to splitting. Tack is the force required to split a thin film. Inkometer and finger tapping are ways to measure tack. Having ink that is to tacky will result in pulling of the substrate to the blanket causing linting on uncoated and picking on coated paper. If the ink is not tacky enough it will cause poor transfer, poor image, and dot size. If CMYK inks are not tacky the inks will not to adhere to each other resulting in poor trapping. Cyan ink is normally printed first because it is the tackiest.

Experiment:

Two ink samples, A and B, were already prepared by adding different amounts of solvent into water-based blue flexography ink. First the viscosities of both samples were taken using a shell cup #2 and a stop watch. The clean and dry shell cup was submerged in the ink and lifted vertically out. The stopwatch was started as the cup broke the surface and stopped when the stream broke. Three measurements for each sample were taken and recorded in seconds. The averages were then calculated and recorded.

Upon measuring the efflux, a test print was made using a Harper Phantom Proofer of each ink sample on uncoated paper. The reflection densities of both test prints were taken using an X-Rite SpectroDensitometer 528. Three measurements were taken and the averages calculated. Using a Triple Beam Balance two 3-gram