Effect of Peak Cavity Pressure on Unreinforced Polyamide Linear Shrinkage Rates
Elevating peak cavity pressure compresses unreinforced polyamide melt, suppressing specific volume and reducing post-moulding linear shrinkage rates.
Hydraulic Pressure Drop
Meaning
Fluid resistance within the main cylinder and hydraulic lines of an injection molding machine determines the force required to move the ram at the specified velocity. This hydraulic pressure drop represents the difference between the pressure generated at the pump and the pressure acting on the injection piston. If this loss is excessive, the machine cannot maintain the programmed injection speed, causing erratic fill rates.
System designers minimize this resistance by using short hose runs and large diameter valves.
System Resistance
Internal friction in the hydraulic oil creates a measurable loss of energy that is converted into heat. As oil temperature rises during a production run, viscosity decreases and the hydraulic pressure drop through the circuit changes. This change affects the repeatability of the injection stroke and alters the transition point from velocity to pressure control.
Keeping the hydraulic fluid within a tight thermal band helps stabilize these internal pressure losses.
Process Stability
Insufficient hydraulic pressure limits the maximum injection speed during the mold filling stage. When the hydraulic pressure drop is too high, the machine suffers from hesitation in the screw movement. This hesitation can cause premature freezing of the polymer at the gate.
Equipment Selection
Machine specifications must provide enough pressure reserve to overcome both the internal hydraulic loss and the melt resistance of the polymer. When hydraulic pressure drop consumes too much of the total system capacity, the moulder cannot process high viscosity resins like polycarbonate. Sizing a press with a larger hydraulic valve block allows the oil to flow with less resistance, which restores the necessary pressure at the injection cylinder.
Proper maintenance of hydraulic filters prevents additional pressure losses and extends valve life during continuous manufacturing.