illegal Screen

[johnyd]

I am confused about illegal screens. Is it possible to have a Violation and not a foul when calling an illegal screen?
Example; Setting a pick to the rear of a defensive player (Blind Screen) and not allowing room for the defender to take on normal step back. Or setting a screen and then sliding sideways to further blaock the defender. Are these violations?
vs.
Setting a screen and as the defender runs by, throwing out an elbow to stop or slow him down, which would be a foul, not an illegal screen. Correct?