Servo Valves for Velocity-to-Pressure (P/Q) Control in Injection Molding
Velocity-to-pressure control, also known as P/Q control, is a function in injection molding applications that describes how the machine switches from controlling speed (or velocity) to controlling hold pressure once the cycle is complete. If this transition is unstable or inconsistent, it can lead to flash, short shots, dimensional drift, part-to-part irregularities, unnecessary scrap, and more.
The Impact of Velocity-to-Pressure Control on Injection Molding Presses
During filling, in most presses, the velocity is controlled for predictability. If the machine waits too long to switch from speed to pressure, or if the hydraulic system cannot respond quickly enough when the switchover command is issued, the cavity may be over-packed, causing flash or part distortion. On the other hand, switching too early can leave unfilled areas or sinks.
In plastics manufacturing, specifically injection molding, servo valves control both the injection phase and the clamp behavior by regulating the flow and pressure with high precision. When the valve’s response time is slow, non-linear, or subject to drift, the switchover point won’t match its intended position, which makes narrow process windows more difficult to manage.
How Speed-to-Pressure Transitions Prevent Flash and Short Shots
In a well-controlled injection molding process, the speed-to-pressure transition protects the press from either under filling or overpacking the mold. If the servo valve can respond quickly and consistently, the system can treat this transition as a threshold between these two different functions.
Then, the cavity will be filled completely. When the pack phase begins early enough, the material is not driven into the parting line, preventing flash. At the same time, the pressure is high enough and applied soon enough to avoid short shots or sinks. For more information about material flow and back pressure control, read our article here.
Domin’s direct drive servo valves use onboard electronics and spool-position feedback to monitor and maintain tight control over both the flow and pressure processes, allowing the valve to track this change more accurately than traditional valves.
How Servo Valves Make a Difference in P/Q Control
Domin’s high-performance valves are essential in dynamic environments where that velocity-to-pressure transition window is tight. In these applications, precise control and accurate movement really matter.
Domin highlights three advantages that matter directly to velocity-to-pressure control: fast response rates (as fast as <3 ms), low hysteresis (as low as <0.2%), and integrated electronics for spool feedback. Together, these characteristics make it easier to tune the transitions and phases so that the machine consistency functions exactly as intended.