Measurement of the density distributions in quenched polycarbonate specimens by a quantitative schlieren optical technique

A new technique for the determination of spatial density distributions is described. The density is derived from the refractive index gradient as determined with a quantitative Schlieren optical technique. The advantages of this technique are a high spatial resolution and sensitivity. Gapwise profiles can be measured without slicing the sample so that more reliable data are obtained quickly. This is demonstrated for the gapwise density distributions in quenched polycarbonate specimens. It is shown that the density distribution is determined by a competition between the local cooling rate variation and the cooling stress distribution. The residual density is predominantly determined by the coolant temperature. The density in the core is higher than at the surface for high coolant temperatures, whereas for low coolant temperatures the density in the core is lower than at the surface. The observed effects are explained semiquantitatively by a simple model using calculated cooling stresses and cooling rates.