The injection cylinder of an injection molding machine is the hydraulic cylinder that drives the screw forward to inject the melt into the mold. It is double-acting: oil on one side of its piston pushes the screw forward for injection and hold pressure, oil on the other side pulls it back for decompression, and restricting the oil that leaves it during screw recovery creates back pressure. The drawing highlights it as item 8, at the back of the injection unit behind the barrel.
Where the injection cylinder sits
Two designs are common on hydraulic machines:
- Single in-line cylinder. One cylinder sits directly behind the screw, on the same axis. Its piston is connected to the screw through the drive shaft, and the plasticizing motor is mounted at the back.
- Twin cylinders. Two cylinders sit side by side, one on each side of the screw axis, fixed to one plate of the injection unit. Their piston rods are attached to a moving plate that carries the screw drive, and the plate slides on guide columns. The forward stroke of the plate pushes the screw.
Both do the same job. Which one your machine has is in its manual and parts list. The injection unit guide explains how the cylinder works with the other parts in the cycle.
How it is built
- Cylinder tube with a honed bore that the piston seals run against.
- Piston with piston seals and guide (wear) rings that keep it centered in the bore.
- Piston rod, hardened and chrome-plated, fixed to the piston with a nut; see the injection piston rod.
- Rod-end gland or head with the rod seals and a wiper seal.
- End caps and ports for the oil, and on many machines a mounting for the position transducer.
The injection cylinder we supply is made of 45# or 40Cr steel with a honed bore, with the bore size specific to the machine.
How it works
Oil flow sets the screw's speed, and oil pressure sets the force on it. Because the piston area is much larger than the screw cross-section, the pressure on the melt is higher than the oil pressure by the ratio of the two areas, the intensification ratio. The flow and pressure come from the pump and are controlled by proportional valves or a servo pump; see the hydraulics guide. A linear transducer measures the stroke.
During recovery, the screw is pushed back by the melt, and the oil leaving the cylinder passes through a valve that holds the set back pressure. See back pressure.
Signs of wear or failure
- Injection is slower or less even at the same settings.
- Injection or hold pressure does not reach its set point, or drops during hold.
- The screw moves when the valves are closed and it should hold position.
- Oil leaks where the piston rod leaves the cylinder.
- The hydraulic oil runs hotter than usual, because oil leaking across the piston turns pressure into heat.
- Knocking or scraping noises, or scoring marks on the piston rod.
An unstable cushion or a screw creeping forward during hold is most often a worn check ring, not the cylinder. Rule that out first.
Why injection cylinders wear
- Dirty oil. Particles cut the seals and score the bore and the rod. See hydraulic oil problems.
- Hot oil. Seals harden, shrink and crack when the oil runs too hot for long periods.
- Side loads. Worn guide bushings or misalignment push the piston and rod sideways and wear the seals and guide rings on one side.
- Damaged piston rod. A scored, pitted or rusty rod destroys rod seals quickly.
- Age. Seals wear with every stroke.
Inspecting and repairing the cylinder
- Stop the machine, switch off the pump, and release all hydraulic pressure. On machines with an accumulator, discharge it as the machine manual describes.
- Lock off the power and support any parts that could move.
- A hydraulic technician can check for internal leakage by pressurizing one side of the piston at the end of its stroke and watching for oil from the other port, following the machine manual.
- To repair, disconnect the hoses, the transducer and the rod end, then remove the gland and withdraw the piston and rod.
- Inspect the bore and the rod for scoring, pitting and wear, and measure the bore at several points along its length.
- Replace all seals and guide rings as a set; the procedure is in cylinder seal replacement. Our hydraulic cylinder seal kits include piston seals, rod seals, guide rings and wipers.
- Reassemble with clean oil on the seals, tighten to the torques in the machine manual, and bleed air from the cylinder before running at full pressure.
Light marks in the bore can sometimes be honed out. A deep score, an oversized bore or a cracked tube means a new cylinder.
What to send for a replacement quote
- The machine brand and model, and the injection unit size or screw diameter.
- Single or twin cylinders.
- The bore diameter, rod diameter and stroke.
- The mounting, the port positions and thread sizes, and the transducer mounting.
- The OEM part number or a drawing, and photos of the cylinder and its nameplate.
Request a quote with these details.
Updated by the IMSPARES technical team.
