Inside the Hydraulic System of a Vertical Injection Molding Machine

Proportional control

The hydraulic system needs proportional pressure and flow valves, or a closed-loop variable pump system, to provide different working pressures and flows. Whether the pump is fixed- or variable-displacement, proportional valves control pump output pressure and flow with linear adjustment.

Separate clamp and injection circuits

On direct-pressure clamping machines, injection begins the instant high clamp pressure is built, while oil is still filling the clamp cylinder. If the two circuits were shared, the moving platen could be pushed back by fill pressure and mold-opening force, the pump could overload, and injection speed and stability would suffer. The clamp and injection circuits must therefore be separated.

Fast response for high-speed molding

High-speed vertical machines often mold at high injection pressure and speed — for connectors or keypads, for example — with fill times of only a few tenths of a second. The system must respond quickly; otherwise output lags the command signal, process setting becomes difficult and part dimensions and weight become unstable.

Response depends on controller scan time, pump response and valve response. Systems with servo valves have a clear advantage in response and repeatability. On variable-pump machines, pressure and flow are adjusted by the pump swashplate angle, giving fast response and pump repeatability better than 0.2%.

Mechanical, electrical, hydraulic and instrumentation integration

Modern machines are fully computer-controlled, with operator dialog, fault self-diagnosis, process monitoring and SPC quality monitoring. The hydraulic system powers every movement, the controller manages it, and instruments monitor machine status — and a robot can be added for automated operation.