Jetting is a result of the inertial effects of material exiting at a very high velocity from a restrictive gate.
Jetting is caused by restricted gates and poor placement of gates in the mold. The material must have something to stick to, or impinge on, as it enters the cavity. If a restricted gate is in-line with a relatively thick cavity (as shown below) the inertial effects of the fast flowing plastic will cause it to “jet” into the cavity. This might also happen because of the injection rate being too high for the viscosity of the material. Jetting could be remedied by adjusting the design of the gate or moving the gate to a better location.