The ISG consists of individual mixing elements that are stacked in a tubular housing. The ends of the elements are shaped so that adjacent elements form a tetrahedral chamber. Four holes bored through each element provide the flow paths. The holes are at oblique angles so that material near the periphery (i.e. near the pipe wall) on the inlet side of the element emerges near the center on the outlet side. The materials from each of the four holes recombine in the tetrahedral chamber only to be split again in the preceding element. Product in the center will exit near the wall. This radial mixing eliminates the wall smearing effect that can be otherwise encountered when mixing viscous materials.