Replacing the seals of an injection molding machine cylinder

Injection molding machine cylinder seal kit replacement means taking the hydraulic cylinder apart, fitting new piston seals, rod seals, wiper, wear rings and O-rings, and putting it back together without damaging them. The job is the same in principle for the clamping, injection and ejector cylinders: make the machine safe, open the cylinder, inspect the bore and the rod, fit the new kit the right way round and test it slowly. The torques, lifting points and the order of dismantling vary from machine to machine, so keep the machine manual at hand.

When to replace the seal kit

• The rod leaks oil from the front head (see rod seals and wipers).
• A bypass test shows oil passing the piston (see piston seals).
• The cylinder is opened for another reason: a rod repair, a loose piston rod nut or a scored bore.
• At a planned overhaul.

Replace the whole kit, not only the seal that leaked.

What is in a cylinder seal kit

• Piston seals: Y-rings or a compact seal, and the piston wear rings.
• Rod seal set: rod seal, wiper and rod guide rings.
• Static seals: O-rings and back-up rings for the end caps, the front head and the joint between the piston and the rod.

We supply complete hydraulic cylinder seal kits and Y-rings and the piston rod seal set. Before you start, lay the new kit beside the old seals once they are out and compare sizes and profiles.

Tools and preparation

• The correct seal kit.
• Lifting equipment rated for the cylinder or the rod and piston assembly.
• Plastic or brass seal picks; never screwdrivers.
• Fitting sleeves or tape to cover threads, ports and sharp edges, and a sizing sleeve if the kit contains PTFE seals.
• A torque wrench, clean lint-free cloths, clean hydraulic oil, and caps or plugs for every hose and port.
• A clean bench away from grinding and welding.

Step by step

1. Make the machine safe. Stop the pump motor, isolate and lock out the power, and release the hydraulic pressure, including the accumulator if the machine has one. Support anything that could move under its own weight, such as the moving platen, the crosshead or the injection unit.
2. Clean the outside of the cylinder and the hose ends, so dirt does not fall in when they are opened.
3. Disconnect the hoses. Mark them, collect the oil and cap every hose and port.
4. Open the cylinder. Remove it from the machine if the manual says so, or work in place if it allows. Remove the front head bolts or retaining ring and draw the rod and piston out straight, supported along their length, so the piston does not drop and score the bore.
5. Remove the piston. Release the locking of the piston rod nut first, then undo the nut and slide the piston off the rod.
6. Remove the old seals with plastic or brass tools. Keep them in order: they show which way round the new ones go.
7. Inspect everything. Check the bore for scoring, corrosion and wear, the rod for chrome damage and straightness, the piston grooves, the front head bore and the threads of the rod and nut. Measure the bore and the rod and compare with the original size in the manual or drawing.
8. Read the old seals. Hard, cracked seals point to heat; cuts point to damage at fitting or a scored surface; seal material squeezed into the gap points to pressure peaks or wear; wear on one side points to side load.
9. Clean all parts in clean solvent or oil, dry them with lint-free cloths and blow out the ports.
10. Fit the new seals. Oil them with clean hydraulic oil, fit the lips of each seal facing the pressure it seals (the wiper lip faces out), and use sleeves over threads and edges. Do not twist O-rings. Resize PTFE seals as the seal maker instructs.
11. Reassemble the piston. Fit the piston on the rod with its new O-ring, tighten the rod nut to the torque in the manual and lock it.
12. Refit the rod and piston into the barrel straight, using the bore's lead-in chamfer so the seals are not pinched. Refit the front head and tighten its bolts evenly, in a cross pattern, to the specified torque.
13. Reconnect and bleed. Reconnect the hoses, top up the oil and move the cylinder slowly at low pressure through several full strokes. Check for leaks, then return to normal speeds and pressures. Check again for leaks after the first production shift.

Common mistakes

• Fitting a Y-ring the wrong way round.
• Pulling seals over threads, ports or keyways without a sleeve.
• Reusing old O-rings and back-up rings.
• Prying seals out with a screwdriver and scratching the groove.
• Fitting new seals onto a scored rod or bore, which makes them leak again soon.
• Letting workshop dirt into the open cylinder or hoses.
• Leaving out the rod nut locking, or not tightening the nut to the specified torque.

If the cylinder still moves slowly after new seals, the cause is probably elsewhere in the circuit: see troubleshooting hydraulic pressure loss.

What to send for a seal kit quote

1. The machine brand, model and serial number, and which cylinder the kit is for.
2. The bore and rod diameters.
3. The groove dimensions, or the codes printed on the old seals.
4. Photos of the old seals laid out in order.
5. The OEM seal kit number, if you have it.

Request a quote. For how the cylinder fits into the clamping unit, see the clamping cylinder guide.