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The viscosity of hot melt adhesives is greatly affected by temperature and shear rate. Most hot melt adhesive viscosities are determined with a rotational shearing viscometer accompanied with a temperature controlled heating unit. It is also defined as the ratio of shear stress and strain rate. The unit of viscosity is in poise, centipoises, or mPas. The definition of viscosity is: “A measure of the ability of a liquid to resist flow”. Viscosity variances are the easiest way to determine that your adhesive is not the same product. In other words, viscosity is simply useful for quality assurance (QA) and quality control (QC). The truth is that viscosity in hot melt manufacturing is simply an indicator to determine the consistency between produced batches or lots. Most hot melt adhesive users believe that the value of “viscosity” is an important parameter to various application techniques.