There are many bumps on the surface of things printed by 3D printer. What's the reason?

Unsolved

Reward points:0 - From the end of the problem as well

Let's show the picture which will more intuitive. The printing material is PLA. It's estimated that the temperature will not be too high, because I set the temperature at 190 degrees and the printing speed at 30. The bumps are the most serious at the corner.