A heated kettle for melting and mixing a thermoplastic resin into a homogeneous mixture has a helically-formed agitator which rotates about a central shaft in close proximity to the internal wall of the kettle. Fixed to the shaft at approximately the center of the kettle is a cluster of radially extending paddles. The paddles and agitator have opposite pitch when the agitator and paddles are rotated, the resin circulates along the wall of the kettle and thence through a central region of the kettle. The circulation action serves to thoroughly melt and mix the resin into a homogeneous liquid mass.