  When two plasmas collide, their interaction can be mediated by collisionless
plasma instabilities or binary collisions between particles of each shell. By
comparing the maximum growth rate of the collisionless instabilities with the
collision frequency between particles of the shells, we determine the critical
density separating the collisionless formation from the collisional formation
of the resulting shock waves. This critical density is also the density beyond
which the shock downstream is field free, as plasma instabilities do not have
time to develop electromagnetic patterns. We further determine the conditions
on the shells initial density and velocity for the downstream to be
collisional. If these quantities fulfill the determined conditions, the
collisionality of the downstream also prevents the shock from accelerating
particles or generating strong magnetic fields. We compare the speed of sound
with the relative speed of collision between the two shells, thus determining
the portion of the parameters space where strong shock formation is possible
for both classical and degenerate plasmas. Finally, we discuss the
observational consequences in several astrophysical settings.
