Yield value is the shear stress in viscosity measurement, but obtained at very low shear. Yield value is the minimum shear stress applied to a liquid to produce flow. When force is gradually applied, the liquid deforms and does not flow. Essentially, liquids behave like elastic solids. Below the yield value, the viscosity approaches infinity. When a critical force (yield value) is entered, the fluid begins to flow.
Yield value is important for understanding the behavior of a decorating fluid once deposited on a substrate. The shear stress on the deposited coating or ink is low. While gravity plays a role in liquids, surface tension is far more important.
If the yield value is greater than the shear stress, no flow will occur. Liquids will behave like solids. In this case, what you save is what you get. Even with a low surface viscosity, the yield value can be relatively high if the coating is uneven. As we will see in the next section, surface tension, while it can be changed, cannot be changed enough to overcome high yield values. Unfortunately, high yield values may be an inherent property of trim materials. In such cases, changing the method of use of the material may be a remedy.
Although a high yield value would render the coating unusable, this characteristic may be a desirable ink for printing . Once the ink is deposited, it should stay where it is placed. A yield value that is too low may allow the ink to bleed, producing poor, irregular edge definition. A value that is too high may cause poor ink flow. Colored inks are not an issue since pigments tend to increase the yield value. Clear protective inks can be a problem, especially when depositing thick films, such as screen printing. When it is not practical to increase the yield value, wettability can sometimes be beneficially altered by surface tension modification. Increasing surface tension inhibits flow and therefore inhibits ink or coating bleeding.
