B ack Pressure is also important. It's defined as the pressure the screw must overcome to retract in the barrel. The greater the back pressure, the longer it takes for the screw to retract in the barrel, thus causing a longer cycle time. If the back pressure is too great, degradation of the plastic can occur. Most often, little or no back pressure is used unless a colorant is used.
Back pressure helps to mix colorant in the plastic.
I njection speed is another key parameter in molding. The faster the injection speed, the greater the amount of sheer occurs in the mold. The greatest amount of sheer is seen at the gate since it has the smallest flow area and highest pressure. If the injection speed is too great, burning may occur at the gate. Injection speed also affects stresses in a plastic part.