I was recently asked about back pressure…
My employees were asking me to explain the following statement ‘Back pressure causes additional material to flow over the flights of the screw’. I wanted to hear your explanation before discussing it with them.
During screw recovery, the screw is pumping material forward down the flights of the screw. If unhindered, the material will flow to the front of the screw, thus pushing the screw backward to build the next shot. Back pressure provides resistance to the backward movement of the screw, causing some of the material pumped forward to either 1) flow back within the screw channels or 2) flow back over the flights of the screw between the flights and the barrel.
There will always be some degree of backflow within the channels and over the flights, but increased back pressure will significant increase this effect.