Abstract: This shear mixer for polymeric material has two mateable bodies that, when pressed in mating condition together, form a reverse flow path for the movement of the material being mixed. The mixer has two mixing gaps, each defined by a primary wall surface on one body and an opposing wall surface on the other. Contractive movement of the two bodies together effectively causes the linear length of each mixing gap to increase and also causes one wall of each mixing gap to move in a direction opposite to the flow of material through the mixing gap. The mixing method involves not only increasing the length of a mixing gap as material is pushed through it for mixing, but also causing flow of material through one mixing gap to be in a direction opposite but parallel to the flow of material through the other mixing gap.