A visualization technique was employed for a qualitative study of the mixing capability of six different screws, The technique involved using a "transparent" single-screw extruder, which had a 45-mm diameter screw and eight openings teach covered with a piece of high-quality glass) along the entire metal barrel. Visual observation by the naked eye through the glass windows revealed that, among the screws studied, pineapple and mixing-pin screws appeared to have the best mixing performance. The presence of mixing elements was found to contribute to the increase in pressure, which was an important processing parameter for enhancing mixing quality. The experimental results also indicated that there might exist an optimum location where a mixing element would best be situated for improving mixing characteristics. It was demonstrated that the Maddock screw gave the most balancing overall performance. (C) 2000 John Wiley & Sons, Inc.