Yield value is the shear stress in viscosity measurement, but at very low shear. This yield value is the minimum shear stress, applicable to liquids, that produces flow. With force gradually the liquid deforms without flowing. Essentially, a liquid behaves like it is an elastic solid. Below the yield value, the viscosity approaches infinity. Flow begins at critical force input (yield value).
Yield value is important for understanding the behavior of liquids after modification.
deposited on the substrate. Shear stress, acting on the deposited coating or ink, is very low. Although gravity exerts a force on liquids, surface tension is quite important. If the yield value is greater than the shear stress, no flow will occur. A liquid will behave as if it were a solid. In this case, what you save is what you get. Coatings that reject levels, even with low apparent viscosities, may have higher yield values. We will see in the next section that surface tension, although variable, cannot be changed enough to overcome high yield values. Unfortunately, high yield values may be an inherent characteristic of trim materials. In these cases, changing the method of material application may be the best remedy. Although a high yield value can render the coating unusable, it can be used for printing.
ink. Once the ink is deposited, it should stay in place. Yields that are too low can allow ink to bleed, producing poor, irregular edge definition. Ink that is too high may not flow well. Colored inks are not an issue since pigments tend to add value to the finished product. Clear protective inks can be a problem, especially when thick films are deposited, such as screen printing. Raising the yield value when it is not practical , through surface tension modification, can sometimes be a good way to change wettability. Increasing surface tension inhibits flow, so the ink or coating will flow.
