The rod seal and the wiper seal sit in the front head (gland) of a hydraulic cylinder, where the piston rod comes out. The rod seal keeps the oil in, the wiper seal (scraper) keeps dirt out, and the guide ring keeps the rod centered. On an injection molding machine these are the cylinder seals you can see failing: an oily rod or a puddle under the clamping, injection or ejector cylinder usually means the rod seal set needs attention, once a leaking hose or fitting has been ruled out.
The rod seal set
In the drawing, the rod seal set is in the front head around the piston rod. From the oil side outward, it usually contains:
- Guide rings (wear rings or a bushing): the bearing for the rod. They take side load and keep the rod centered, so the seals are squeezed evenly all round.
- Rod seal: usually a polyurethane Y-ring, or a PTFE seal with an O-ring energizer, with its lips facing the oil. Some cylinders add a second seal, a buffer seal, in front of it to absorb pressure peaks.
- Wiper seal: the outermost seal, with its lip facing out. It scrapes dust, plastic particles and moisture off the rod as it retracts. Double-lip wipers also catch the thin oil film the rod carries out.
- Static O-rings on the outside of the front head, which seal it against the barrel.
The piston rod seal set we supply includes the wiper seal, rod seals and guide rings, in polyurethane, NBR and PTFE, rated up to 35 MPa.
How a rod seal works
Like a piston Y-ring, the rod seal is fitted with a slight squeeze and pressure pushes its lips harder onto the rod. As the rod extends it carries a very thin film of oil out with it, which lubricates the seal and the wiper. A good rod seal lets that film back in as the rod retracts, so a healthy rod looks slightly damp but does not drip. That only works on a smooth, undamaged surface, which is why the rod is ground and hard chrome-plated.
Signs of worn rod seals or wiper
- Oil drips from the front head, or runs along the rod and collects under the cylinder.
- An oil ring builds up on the rod at the end of each stroke.
- Dirt, plastic dust or rust marks on the rod, and grit at the wiper lip: the wiper is no longer scraping.
- Fine lines along the rod in the direction of travel.
- Uneven wear on one side of the old seals and guide ring when they are removed, a sign of side load.
Why rod seals fail
- Rod damage is one of the most common causes: scratches, dents from impacts during mold changes, weld spatter, pitting from corrosion, and flaking chrome. Each damaged spot cuts the seal on every stroke.
- A worn wiper lets abrasive dust in, which wears the rod seal and the rod.
- Side load and worn guide rings: the rod runs off-center and the seals wear on one side. On a toggle machine, worn crosshead guides can load the clamping cylinder rod sideways.
- Heat and dirty oil: see oil temperature and hydraulic oil.
- Fitting damage: a seal pulled over the rod thread or a sharp groove edge without protection.
Inspecting the piston rod
- Clean the rod and look along it under good light, turning it if you can.
- Run a fingernail across any mark. A mark you can feel will cut new seals.
- Look for pitting, flaking chrome and rust, especially on the part of the rod that stays outside the cylinder.
- If the rod has been hit or bent, check its straightness on V-blocks with a dial gauge and compare with the limit in the machine manual or from the cylinder maker.
A damaged rod is re-chromed and ground, or replaced. For the injection cylinder, see the injection piston rod we supply.
Replacing rod seals
On some cylinders the front head can be removed and the rod seals changed with the cylinder in place; on others the cylinder has to come out. The full job, including safety and testing, is in replacing cylinder seals. The rod seal steps are:
- Make the machine safe: stop the pump, lock out the power and release all pressure, including the accumulator if fitted. Support anything the rod holds up.
- Remove the front head as the machine manual describes.
- Take out the old seals with plastic or brass tools so the grooves are not scratched. Keep them in order to see how they were fitted.
- Clean the grooves and check the bore of the front head and the guide ring seats.
- Fit the new seals: rod seal lips toward the oil, wiper lip facing out. Y-rings for internal grooves can be bent gently into a kidney shape to insert them, then pushed into place.
- Oil the seals with clean hydraulic oil and slide the front head back on over a fitting sleeve, or tape, that covers the rod thread.
- Tighten the bolts evenly to the torque in the manual, then cycle the cylinder slowly and check for leaks.
What to send for a replacement quote
Send the machine brand and model, which cylinder it is, the rod diameter, the groove dimensions in the front head for the rod seal and the wiper, and any codes printed on the old seals. Photos of the front head and the old seals in order help, and an OEM seal kit number is the quickest. Request a quote. For the seals on the piston, see piston seals, and for the rest of the cylinder the clamping cylinder guide.
Updated by the IMSPARES technical team.