A conventional injection moulding machine is a hydraulic machine first. Its pump, valves and cylinders clamp the mould, inject the melt and eject the part, cycling every few seconds, shift after shift. Pump losses and a hot shop heat the oil, and proportional or servo valves react badly to dirt. When the oil is wrong, too hot or dirty, the first sign is often inconsistent shots and rejected parts, long before anything breaks.
Most injection moulding machines run on an anti-wear hydraulic oil (HLP or HM) of ISO VG 46; some makers specify VG 68 for hot shops or heavily loaded machines. Keep the oil between about 40 and 55 °C with a working cooler, filter it to the cleanliness the valves need, and change it on condition, using oil tests and filter behaviour, rather than only by the calendar.
| Situation | Grade (follow the machine maker) |
|---|---|
| Most machines in a normal shop | ISO VG 46 anti-wear (HLP 46 / HM 46) |
| Hot shops, heavily loaded or older machines | ISO VG 68 where the maker allows it |
| Servo-pump (hybrid) machines | The grade the maker specifies, usually VG 46 |
HLP (DIN 51524-2) and HM (ISO 6743-4) describe the same family of anti-wear hydraulic oils. Our guides to hydraulic oil 32 vs 46 vs 68 and HL, HLP, HM and HVLP oils explain the choice. Whatever the grade, keep to one oil per machine and top up only with the same oil.
Hydraulic oil works best at roughly 40–55 °C. Above about 60 °C it oxidises much faster; a widely used rule of thumb is that oxidation speed roughly doubles for every 10 °C. Hot oil also thins, which increases internal leakage and makes the machine less repeatable.
Keep the oil cool:
Our article on hydraulic oil overheating walks through the causes one by one.
Proportional and servo valves have very fine clearances. Dirt causes sticking, erratic movement and wear.
Hot, oxidising oil forms sticky deposits called varnish on valve spools and in the tank. Symptoms are valves that stick when the machine is cold, drifting cycle times and filters that block quickly. Prevent it by controlling temperature and keeping the oil clean. If it is already present, a lab test such as MPC shows how bad it is, and a full oil change with cleaning may be needed.
Change the oil when tests show it, not just because a date has arrived. A simple oil analysis checks:
If you do not test the oil, follow the machine maker's change interval in hours. Either way, drain the oil warm, clean the tank, change filters and fill through a filter.
Oil temperature during production, oil level, cooler inlet and outlet temperatures, filter indicators, leaks at hoses and cylinders, and any change in cycle time or shot consistency. Write it down; trends warn you early.
For the full range of lubricants for moulding, extrusion and blow moulding plants, see our page on plastics industry lubricants.
Which hydraulic oil is used in injection moulding machines?Most use an anti-wear hydraulic oil (HLP or HM) of ISO VG 46; some makers specify VG 68 for hot shops or heavily loaded machines. Use the grade in the machine manual.
What temperature should injection moulding hydraulic oil run at?Roughly 40–55 °C. Above about 60 °C the oil oxidises much faster and the machine becomes less repeatable.
How often should the hydraulic oil be changed?On condition, using oil tests for viscosity, acid number, water and particles, or at the maker's interval in hours if you do not test.
Why does an old oil cause inconsistent shots?Dirt and varnish make proportional and servo valves stick, and hot, thin oil increases internal leakage. Both make cycles less repeatable.
Running injection moulding or extrusion machines? Tell us the machine makes, oil volumes and shop temperature, and our team will recommend grades and an oil-care routine.