Walk onto the floor of any plastics processing plant and you’ll hear it before you see it — the steady hiss and clunk of a hydraulic system doing the heavy lifting behind every shot. Somewhere inside that pump-and-valve setup sits a hydraulic motor, and on an injection moulding machine, that one component quietly decides a lot about how consistent, fast, and profitable your production run turns out to be.
This page isn’t about a specific product. It’s about the job the motor has to do on an injection moulding machine, and what actually matters when you’re choosing one.
What the Motor Actually Does on an IMM
An injection moulding machine asks a hydraulic motor to do several very different jobs within the same cycle:
- Rotate the screw to plasticise and meter the resin
- Drive the clamping unit that holds the mould shut under enormous force
- Power the ejector system that pushes the finished part out
- On some machines, run auxiliary functions like core pulls or rotating tables
Each of these has its own torque and speed signature. Screw rotation, for instance, needs steady torque at a relatively low, controllable speed — too much variation and you get inconsistent melt quality, which shows up later as short shots, flash, or warping. Clamping needs a sudden, strong push, then has to hold steady without drifting.
A motor that’s undersized, or simply the wrong type, doesn’t usually fail outright. It just underperforms — longer cycle times, more scrap, more unplanned maintenance. That’s the part that’s easy to miss when a machine is specified or repaired in a hurry.
What to Check Before You Choose a Motor
Starting torque. The motor has to get the screw moving from a dead stop under load, every single cycle, thousands of times a day. If starting torque is marginal, you’ll see stalling or slow ramp-up, especially once seals and internals wear in.
Speed range and control. Moulding often calls for a wide speed window — very slow for precision packing, faster for plasticising. A motor with a narrow or unstable speed range makes it hard to hold a repeatable process.
Rated and peak pressure. Look at both numbers, not just one. Rated pressure tells you the day-to-day working point; peak pressure tells you how much headroom exists for pressure spikes during clamp-up or fast injection.
Duty cycle and heat. IMMs run continuously, shift after shift. A motor rated only for intermittent duty will run hot, wear its seals faster, and eventually leak or lose efficiency.
Shaft and mounting fit. Even a perfectly specified motor is a problem if the output shaft, spline, or mounting flange doesn’t match your machine’s existing coupling. This is one of the more common — and avoidable — mismatches we see during replacements.
Why Radial Piston Motors Are Usually the Right Fit
Two broad motor families show up in industrial hydraulics: orbital (gerotor-style) motors and radial piston motors. Orbital motors are compact, economical, and perfectly fine for lighter-duty or intermittent jobs. But injection moulding is a heavy-duty, high-pressure, continuous-cycle application — exactly where radial piston motors earn their keep.
Radial piston motors deliver high torque at low speed without needing a heavy reduction gearbox, hold pressure ratings well above what most orbital motors can sustain, and are built for the kind of continuous, cyclic loading an IMM throws at them shift after shift. That combination is why they’ve become the default choice for screw drives and clamping units on mid-size to large moulding machines.
Matching the Motor to Your Machine
Because IMMs vary so much in tonnage, screw diameter, and cycle speed, there’s no single “right” motor size — it’s a matter of matching displacement, torque, and speed range to your specific machine and process. Our Alpha Series radial piston motors were built with exactly this application in mind, across a wide range of frame sizes and displacements.
If you’re specifying a motor for a new machine, or replacing one that’s underperforming, send us your current nameplate data or machine specs. We’ll help you cross-reference the right model rather than guess.