Specialised care for injection moulding machine hydraulics: holding pressure, temperature control and leak management

Unstable hold pressure shows straight up in the parts: sink marks, flash, dimensional variation. Diagnose in this order: read the pressure curve (modern machines all have one), then check the relief and proportional valves, and last the cylinder internal leakage.

Proportional valves are extremely sensitive to oil cleanliness. On a machine with drifting hold pressure, eight times in ten the proportional valve spool is slightly sticky or the oil is dirty. Checking the oil and cleaning the valve is far cheaper than replacing it — and usually works.

The accumulator is critical in the hold-pressure and fast-motion circuits. Wrong pre-charge shows as slow motion and large pressure swings. Measure the nitrogen pre-charge every six months.

The mould and process parameters also affect hold-pressure behaviour. If the machine checks out but the pressure is still unstable, work through it with the process people — do not keep circling inside the hydraulics.

Injection moulding machine leaks concentrate in a few places: the injection cylinder rod seal, the platen tie-bar area, the power-unit pipe joints, and the mounting faces of the proportional and directional valves. Going through them one by one beats blindly changing seals.

For a leaking rod seal, first check the rod for scoring and the guide bush for wear. Changing the seal on a damaged rod just leaks again in days. Light rod scoring can be dressed with an oilstone; severe damage needs re-chroming or a new rod.

Pipe-joint leaks usually come from installation strain or vibration loosening. A hard pipe fitted under strain (forced into alignment) puts continuous stress on the joint and leaks sooner or later. Refit without the strain, switching to hose or adding an adapter where needed.

For a leaking valve mounting face, check the O-ring for ageing, flattening or wrong size, and check the face flatness. Out-of-flat mounting faces leak even with a new seal.

The ideal oil temperature for an injection moulding machine is 40–55 °C. High oil temperature drops viscosity, increases internal leakage, ages the seals, and destabilises hold pressure. Machines exceeding 60 °C in summer need focused attention.

The main heat sources: relief losses (the pump still relieving at high pressure while idle), throttling losses, and internal leakage. A servo-pump or VFD retrofit greatly cuts idle relief losses, typically saving over 30% on power — the mainstream direction for injection moulding energy retrofits.

Falling cooler efficiency is a common cause of rising oil temperature. Water-cooled: check water flow and scaling. Air-cooled: check the fan and fins. Cleaning the cooler yearly is a necessary maintenance item.

Analyse the hydraulic oil on interval. Injection moulding machines run heavy loads and hot oil, so the oil ages faster than in general equipment — sample viscosity, acid number, water and cleanliness every six months.

The clamping unit is the most dangerous part of the machine. The safety-door interlock, the safety valve (mechanical clamp safety block) and the emergency-stop circuit must be proven effective at every service. A failed safety device here is unacceptable.

Check the tie bars for scoring and for even stretch. Uneven loading across the four tie bars tilts the platen, hurting part quality and mould life. Measuring tie-bar stress with strain gauges is the professional approach; day to day, look first for abnormal wear on the bar surfaces.

Confirm the central lubrication system delivers oil at every point, especially the moving-platen slide shoes, the mould-height adjuster and the ejection unit. Poor lubrication scores the guideways and is expensive to repair.

On dry-lubricated machines (wear-material slide shoes), check the clearance to the maker's requirement; adjust or replace the wear plates when it is out of tolerance.

Injection molding machine hydraulics differ from general industrial hydraulics in three special features, and the maintenance plan must be designed around them. The first is the holding phase: after injection completes, the system must hold relatively stable pressure to guarantee part density and dimensions. Unstable holding directly causes part defects (sink marks, dimensional variation, internal stress), and its hydraulic root is usually internal leakage - leakage anywhere in the holding circuit's check valve, holding valve or cylinder seals means pressure will not hold. That is why testing holding performance (pressure decay rate during hold) is a dedicated item in injection molding hydraulic inspection.

The second is the strict requirement on oil temperature control: temperature swings on a molding machine directly change oil viscosity, which in turn affects motion speed and holding stability - so temperature control demands are stricter than for ordinary hydraulic equipment. Another trait is heavy heat load: the barrel heating zones are right there, ambient temperature is high, and with cyclic high-pressure work the oil easily runs hot. Cooling system checks (water or air), cooler cleaning and temperature control valve verification are mandatory before summer.

The third is cyclic impact loading: a molding machine's cycle is high-frequency (tens of seconds per cycle), and each cycle contains rapid clamping, injection, holding, mold opening and ejection stages, with pressure and flow changing violently. This duty affects component fatigue life far more than steady loading, which makes condition checks on accumulators, cushioning devices and shock suppression all the more important. Long-term hydraulic shock (water hammer) loosens pipe joints, damages seals and fatigues components; the countermeasures are adjusting shift speeds, adding shock-suppression circuits, and checking accumulator precharge pressure.

Injection moulding machines are among the biggest electricity consumers in a plant, and energy retrofits on their hydraulic systems usually pay back clearly. A conventional fixed-displacement machine works like this: within one cycle only part of the phases need high pressure and high flow, and in the rest the surplus energy is dumped through the relief valve as heat - that relief loss is the main source of waste. The mainstream retrofit is variable frequency drive: convert the fixed pump motor to an inverter-duty motor and adjust speed to the actual demand of each phase, eliminating relief loss; the saving depends on the process but is normally considerable. Going further, a servo pump package (a servo motor driving a fixed-displacement pump) responds faster, holds better accuracy and saves more energy, and is now the mainstream configuration on new machines.

The basis for the retrofit decision is the load curve: first record the pressure and flow demand of each phase, then calculate what share of energy goes over the relief valve - the larger the loss, the better the return. Machines with long cycles and a large share of holding time usually gain the most; short-cycle, fast-motion machines gain relatively less. This calculation needs measured data and cannot be estimated from the machine model - the first step of any energy project we take on is always measuring the data. Work out the numbers first, then decide whether to retrofit; never retrofit for the sake of retrofitting.

Beyond drive retrofits there are several low-cost energy and maintenance actions: restoring accumulator function (with correct pre-charge pressure it absorbs shock and assists supply, reducing pump load), improving cooling efficiency (oil temperature held in a reasonable band keeps viscosity stable, and stable viscosity means less internal leakage), leak control (internal and external leaks are both energy losses, so sealing leaks is saving energy), and optimising motion parameters (setting speeds and pressures to what the process actually needs rather than blindly high). These items need little investment, show results quickly, and suit a first step before any retrofit.