Printing ink manufacturers take into account multiple factors before they concoct an ink formula. Rising press speeds and a broad variety of substrates must be accounted for in the chemical formula. The main considerations when it comes to physical properties include scruff resistance, tack, viscosity, gloss, and pigmentation level. Only if these are correct then the ink will perform as it should be. It is important to have standard equipment. The only way to ensure a better quality of ink is to have a world class ink laboratory. Other than print ink manufacturers, printers and even ink suppliers now have invested in ink laboratories.
Elasticity and viscosity
Measuring elasticity an viscosity requires Laray Viscometer and Duke Falling-Rod Viscometer. These are the most common equipment used. The viscosity could be measured at a number of shear rates. The yield value can also be measured. This is the minimum shear amount needed to get the ink to flow. Problems with ink viscosity will manifest itself in emulsification and transfer problems. Elasticity is another important ink property. This is best done by controlled stress rheometer. This can measure elasticity and viscosity at progressively rising shear rates. The behavior of the ink can be understood by its elastic and viscous components.
Tack is another important factor. It represents the force needed to split the ink at the roller nip. With the increase in press speeds, the ink tack must be lower. Too much tack will result in printing problems. Picking happens if the ink gets too tacky. Fibers get pulled off the paper. Linting happens when the fibers travel through roller train. They ultimately end up on blanket or plate. Piling occurs when too much linting happens. Numerous fibers stick to the plate image area. Trapping problems occur due to tack.
For uncoated substrates, excessive tack is a major issue. This is as these substrates like newsprint are not particularly designed to possess superior holdout properties. It follows these do not need high rub resistant characteristics which are critical in the coated stock ink solutions. These inks must possess low picking and linting properties. Stability of tack is a time function. An offset press has multiple rollers. The ink thus is subjected to immense splitting and spreading. It must be made sure that the ink does not get tackier through the oil being driven out. This will result in the ink drying up.