Voids in extrusion can be caused by air entrapment, moisture, poor venting, and non-uniform cooling. Voids caused by non-uniform cooling are typically referred to as shrink voids; they are common in thick-walled products (2+ mm). They result from non-uniform shrinkage that occurs in the cooling of the extrudate. The shrinkage occurs as a result of the increase in density as the polymer cools. This paper presents an experimental and theoretical study of void formation. The cooling process is analyzed using the simulation software chillWARE. The paper will outline measures that can be taken to eliminate shrink voids.