Rigid plastic parts such as PS form a dense corrugation on the surface near the gate, centered on the gate, sometimes called a shock pattern. The reason is that when the melt viscosity is too large and the mold is filled in a stagnant form, the front end material quickly condenses and shrinks as soon as it contacts the surface of the cavity, and the subsequent melt expands the contracted cold material to continue the process. Constant alternating causes the stream to form a surface grain in the forward direction.
Solution:

(1) Increase the barrel temperature, especially the nozzle temperature, and increase the mold temperature.

(2) Increase the injection pressure and speed to make it quickly fill the model cavity.

(3) Improve the size of the runners and gates to prevent excessive resistance.

(4) The mold exhaust should be good, and a large enough cold well should be set.

(5) Do not design the parts too thin.