Extrusion of rubber compounds were made on a Monsanto Processability Tester (MPT) to examine the relationship between the instable flow and the parameter variables in extrusion process. An empirical equation distinquishing the critical extrusion rate for instable flow was presentee! as follows:where L/D is the ratio of length-to-diameter of capillary die; a

is apparent shear viscosity of the melts

yw

the shear rate at wall; r is the melt characteristic relaxation time and tR

the resident time of the melts in die;△P is pressure drop. The re-sultsof regression analysis and experiment verification showed that the calculation values using equation (5) were better correspondent with the data measured.