Viscosity characterization of uncured rubber compounds via uniaxial compression testing: Implications for compression molding applications

This article examines the viscosity measurement of uncured rubber compounds using a simple compression test, with application to compression molding (CM) simulations. Since the nature of the local flow in the compression molding is analogous to the squeeze flow, viscosity measurements from such a test can provide reliable viscosity data that are extremely useful for CM simulations. In this article, we first examine the mechanics of flow between two plates using a linear flow regime, considering both perfect-slip and nonslip boundary conditions. The so-called Stefan formula is employed to determine the shear rate-dependent viscosity of rubber, assuming both a general non-Newtonian model as well as a yield flow model. Simple formulas are also given to determine the average equivalent shear rate in the test samples. A simple strategy is also suggested to check the accuracy of the assumed boundary condition at the interface, i.e., perfect-slip or nonslip. Subsequently, the results of several squeeze flow tests of cylindrical uncured rubber samples are reported, whereby the shear rate-dependent viscosity is determined for three uncured compounds. A general power-law relationship between viscosity and the shear rate is demonstrated for a wide range of shear rates. The results are also compared to the values from the Scott formula, which is based on flow equations with a priori power-law viscosity assumption. It is concluded that the compression test can provide a simple, straightforward, and reliable approach to model and measure the viscosity of uncured rubber samples for a wide range of shear rates.