The hydraulic power unit (HPU) is the part of a hydraulic quick mold change system that quietly determines whether the install runs smoothly or stalls on day two. Most plants think of the HPU as a generic pump-and-tank package. It is not. The HPU has to be sized to the clamp count, working pressure, electrical capacity available at the press, and the cycle duty driven by change-over frequency. Get it wrong, and the symptoms range from slow clamp engagement to electrical breaker trips during change cycles. This article covers what an HPU is, how KINGHOU sizes it for injection, stamping, and die casting QMC retrofits, and the specifications a plant should validate before procurement.
What the HPU Actually Does
The HPU is the hydraulic engine room for the entire clamp system. It contains:
- Electric motor + pump generating the working pressure (typically 250 to 400 bar).
- Oil tank sized for thermal margin and clamp cylinder volume.
- Pressure regulator and relief valve setting and protecting the working pressure.
- Solenoid valve manifold directing oil flow to either the engagement port or release port of each clamp.
- Pressure sensors and feedback loop confirming target pressure has been reached before the press control is allowed to enable.
- High-pressure outlets with quick-disconnect couplers — typically one outlet per clamp pair.
The HPU runs only during clamp engagement and release cycles — not during production. Idle electrical draw is near zero. This is one of the reasons hydraulic QMC has a lower steady-state energy profile than continuous-power systems.
HPU Sizing — The Four Variables
1. Working Pressure
Driven by the clamp force per unit. Larger clamps need higher pressure to develop their rated force from the same cylinder bore:
- 10T to 25T clamps: 250 to 300 bar working pressure
- 40T clamps: 300 to 350 bar
- 60T clamps: 350 to 400 bar
The pump and all downstream components (hose, manifold, valves) are rated to the working pressure with safety margin.
2. Flow Capacity (Litres per Minute)
Driven by clamp count and target engagement time. Standard KINGHOU spec: all clamps engage within 8 to 12 seconds.
- 8 clamps × 0.5 litre cylinder × 60 / 10 seconds = 24 LPM minimum
- For 12 clamps: ~36 LPM
- For 16 clamps: ~48 LPM
- For 20 clamps: ~60 LPM
Undersized pump flow means slow clamp engagement — every change adds 30 seconds of waiting at the clamp engagement step. Fast becomes slow.
3. Tank Volume
Tank volume = 1.5 to 2 times total clamp cylinder volume. The factor provides thermal margin (oil expands with temperature) and air separation time (entrained air rises and escapes the working oil pool).
- 8 clamps × 0.5 litre × 1.8 factor = 7.2 litres minimum tank
- For 12 clamps: 11 litres
- For 16 clamps: 14 litres
- Most KINGHOU standard tanks: 25, 50, or 100 litres — generous margin built in.
4. Electrical Draw
HPU electrical draw is the variable that catches plants out. The motor sizing follows pump flow × working pressure × pump efficiency:
- 250T to 800T presses (8 clamps, 25 LPM, 280 bar): typically 5.5 kW HPU
- 800T to 1,800T (12 clamps, 36 LPM, 300 bar): 7.5 to 11 kW
- 1,800T to 3,200T (12-16 clamps, 48 LPM, 320 bar): 11 to 15 kW
- 3,200T to 4,000T+ (16-20 clamps, 60 LPM, 350+ bar): 15 to 22 kW
Plants must confirm three-phase electrical capacity available at the press location before HPU procurement. Adding a dedicated breaker is sometimes part of the install scope — surveyed and quoted up front, not discovered on installation day.
Control Panel Integration
The HPU control panel handles three core functions:
- Operator interface: push-button for clamp engagement / release, status display showing each clamp pressure individually.
- Press interlock: wired into press cycle control — clamp engaged signal must be green before press cycle is enabled.
- Audit logging: records timestamp, operator ID (badge or keypad), clamp pressures per change cycle. Exports as CSV for plants needing automotive Tier 1 or aerospace certification audit trail.
HPU Placement on the Plant Floor
HPU placement constraints to consider during survey:
- Hose run length: KINGHOU standard limit is 8 metres from HPU to most distant clamp. Longer runs add cost and reduce response speed.
- Floor footprint: standard HPU footprint runs from 0.6m × 0.4m (5.5 kW) to 1.2m × 0.8m (22 kW). Plus 0.5m clearance for maintenance access.
- Three-phase electrical proximity: a nearby panel or sub-distribution board with available breaker capacity.
- Operator visibility: control panel reachable by operator without leaving the press station during change cycle.
- Drip tray and oil leak containment: required in most plant EHS policies.
Maintenance Schedule
- Oil change: annual, or every 2,000 cycle hours, whichever comes first.
- Filter replacement: at every oil change.
- Seal inspection on clamp cylinders: annual visual check. Replace at first sign of weeping.
- Pressure calibration: annual, against a calibrated gauge.
- Electrical inspection: aligned with plant electrical PM schedule (typically annual).
All maintenance can be performed by plant maintenance staff. KINGHOU supplies a maintenance guide and spare parts kit with every install.
Conclusion
The HPU is one of the most under-considered components in a hydraulic QMC retrofit, and one of the most common sources of unexpected install delay. The four sizing variables — working pressure, flow capacity, tank volume, electrical draw — are deterministic from the clamp specification. A plant that captures the clamp spec and electrical capacity in the first email of the quote conversation enables a clean HPU procurement and a smooth installation.
Send us your machine tonnage and platen size. KINGHOU will return both a clamp sizing and an HPU specification within 24 to 48 hours. Reach the engineering team via the contact form, by WhatsApp at +86 18051902698, or by email to kh020@jskinghou.com.
