Part line is a typical defects for injection molded parts with many hole features, such as computer key board typically, the hot resin flow around the little inserts and melt flow meet each other on other side where the welding line occurs, this is how welding line occurs, melt flow temperature always drop while melt resin flow over the mold cavity, that is why the welding line can only be optimized but can not be completed removed if you simply use higher mold temperature.

If you have strict requirement on the surface, we suggest to make paiting on surface.