In this article, we introduce a new type of small scale compounder. The compounder developed is for mixing of polymeric samples of 0.5-1.0 g. It consists of a heated cylindrical metal having two cylindrical cavities connected through a narrow channel and two cylindrical pistons, which squeeze molten polymers from one cavity to the other cavity through the narrow channel. During mixing procedure, the molten polymers flow from one cavity to the other cavity, repeatedly, and this operation generates the extensional flow in the converging and the diverging geometry. Because the compounder has mixing chamber of very simple geometry, the cleaning is very easy and the material lost is very small. We evaluated the mixing efficiency of the compounder by comparing with the commercialized small-scale mixers including a cup and rotor batch mixer, an internal batch mixer, and a recirculating conical twin-screw extruder. It was found that the compounder developed has many advantages over the existing small-scale mixers. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 609-619, 2009