---
title: "Hydraulic cylinder piston seals and Y-rings"
url: "https://www.imspares.com/injection-molding-machine-parts/clamp-cylinder/piston-seals/"
---

# Hydraulic cylinder piston seals and Y-rings

What the piston seals of an injection molding machine cylinder do, Y-ring and compact seal types, wear rings, signs of bypass leakage, testing and replacement.

A hydraulic cylinder piston seal is the seal in the grooves of the piston that stops oil passing from one side of the piston to the other. On injection molding machines the clamping, injection and ejector cylinders commonly use **Y-rings** (also called U-cups or lip seals), with **wear rings** beside them that keep the piston centered in the bore. When the piston seals leak, the cylinder loses force and speed and can creep under load, but no oil appears outside.

## Where the piston seals sit

In the drawing, the piston seals are in the grooves around the piston, between the two oil chambers of the cylinder. The piston is held on the end of the rod by the [piston rod nut](https://www.imspares.com/injection-molding-machine-parts/clamp-cylinder/piston-rod-nut/), and a static O-ring usually seals the joint between the piston and the rod. The [rod seals](https://www.imspares.com/injection-molding-machine-parts/clamp-cylinder/rod-seals/) in the front head keep oil inside the cylinder; the piston seals keep the two chambers apart.

## How a Y-ring piston seal works

- The Y-ring has two lips. It is fitted with a slight squeeze, so it seals at low pressure.
- As pressure rises, oil pushes into the space between the lips and presses them harder against the bore and the bottom of the groove.
- A Y-ring seals in one direction only. A double-acting piston therefore carries two of them back to back, each with its lips facing its own chamber.
- **Wear rings** (guide rings or guide bands) take the side load and keep the piston from touching the bore. They do not seal; if they wear out, the piston rubs the barrel and the seals are pushed out of shape.

## Types of hydraulic cylinder piston seal

| Type | How it seals | Where it is used |
|---|---|---|
| Y-ring (lip seal), polyurethane or NBR | One direction per seal; two back to back on a double-acting piston | Common on injection molding machine cylinders |
| PTFE seal with an O-ring energizer | One ring seals both directions; the O-ring under it presses it onto the bore | Where low friction and smooth slow movement matter |
| Compact seal (rubber seal with anti-extrusion and guide rings) | One assembly seals both directions | Heavy-duty pistons that need a single groove |

Which type your cylinder uses depends on the machine and the cylinder maker, so match the new seals to the old ones. The [hydraulic cylinder seals and Y-rings](https://www.imspares.com/parts/mc-sb-08/) we supply are made of polyurethane, NBR and PTFE, are rated up to 35 MPa and from -30 °C to +100 °C, and come as complete seal kits or single Y-rings.

## Signs of worn piston seals

- The cylinder is slower than it used to be at the same settings, especially under load.
- It **creeps or drifts** when its valve is in the neutral position.
- It loses force: the mold does not break open, the clamp is slow to lock, or injection pressure drops.
- The oil runs hotter than usual, because oil forced through a leaking seal turns pressure into heat.
- Pressure falls on the gauge while the cylinder holds a load.

A leaking directional valve, check valve or proportional valve gives the same symptoms. Test before you open the cylinder.

## Why piston seals fail

- **Dirty oil**: particles score the bore and cut the lips. See the [oil filter](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-filter/).
- **A scored or corroded bore**, often from dirt or from water in the oil.
- **Heat**: oil running too hot hardens rubber and weakens polyurethane, and hot oil with water in it attacks polyurethane. See [oil temperature](https://www.imspares.com/injection-molding-machine-parts/hydraulics/oil-temperature/).
- **Pressure peaks** that push the seal into the gap between the piston and the bore.
- **Worn wear rings** that let the piston rub the bore.
- **A loose rod nut** that lets the piston knock on the rod.
- **Fitting damage**: lips cut on sharp edges or threads, or a seal fitted the wrong way round.

## Testing for piston seal bypass

The usual test checks whether oil passes the piston when the cylinder is held at the end of its stroke. Follow the machine manual and your site's safety rules, keep clear of moving parts and use a pressure no higher than the manual allows for testing.

1. Run the cylinder to the end of its stroke in manual mode.
2. Stop the pump, isolate the machine and release all pressure, including the accumulator if there is one.
3. Disconnect the hose from the port on the side that will not be pressurized. Plug the hose and lead the open port into a clean container.
4. Start the pump and pressurize the other side, so the piston is held against the end of its stroke.
5. Watch the open port. A steady flow of oil means oil is passing the piston seals.
6. Release the pressure, reconnect the hose and bleed the air out by moving the cylinder slowly.

If the cylinder passes the test but the movement still drifts, the leak is in the valves: see the [directional valve](https://www.imspares.com/injection-molding-machine-parts/hydraulics/directional-valve/) and the [hydraulic check valve](https://www.imspares.com/injection-molding-machine-parts/hydraulics/hydraulic-check-valve/). Our article on [troubleshooting hydraulic pressure loss](https://www.imspares.com/blog/troubleshooting-hydraulic-pressure-loss/) covers the rest of the circuit.

## Replacing piston seals

Piston seals are changed with the piston out of the barrel. The full procedure is in [replacing cylinder seals](https://www.imspares.com/injection-molding-machine-parts/clamp-cylinder/cylinder-seal-replacement/). Points that matter for the piston:

- Replace both Y-rings, the wear rings and the O-ring between the piston and the rod together.
- Fit each Y-ring with its lips facing the chamber it seals.
- Do not overstretch the seals. Follow the seal maker's instructions for PTFE seals, which are usually resized after fitting.
- Inspect the bore before fitting new seals. A scored bore needs honing or a new barrel, or the new seals will soon leak again.
- Check the rod nut and its locking when reassembling.

## What to send for a replacement quote

Send the machine brand and model, which cylinder the seals are for, the bore diameter, the groove dimensions (inside diameter and width), and any code printed on the old seals. The OEM seal kit number, if you have it, is the quickest. Photos of the old seals laid out in order show the type and which way they were fitted. [Request a quote](https://www.imspares.com/request-quote/), and see the [clamping cylinder guide](https://www.imspares.com/injection-molding-machine-parts/clamp-cylinder/) for how the piston seals fit with the rest of the cylinder.

## Questions

### What is a Y-ring piston seal?

A Y-ring, also called a U-cup or lip seal, is a seal with two lips in a Y-shaped profile. Oil pressure pushes into the space between the lips and presses them against the bore and the bottom of the groove, so the seal grips harder as the pressure rises.

### What are the signs of a leaking piston seal?

The cylinder moves more slowly or with less force than before at the same settings, it creeps when its valve is closed, and the oil may run hotter. No oil appears outside, because the leak is from one side of the piston to the other.

### Which way round does a Y-ring go on a piston?

The open side with the lips faces the pressure it has to seal. A double-acting piston usually has two Y-rings fitted back to back, each with its lips facing the chamber on its own side.

### Should piston seals and rod seals be replaced together?

Yes, in most cases. Once the cylinder is open, the extra cost of the rod seals, wiper and O-rings is small compared with the work of dismantling it again, and all the seals have the same age.

## Replacement parts

- [MC-SB-08 Hydraulic Cylinder Seals & Y-Ring](https://www.imspares.com/parts/mc-sb-08/): Hydraulic cylinder seal kits and Y-ring piston rod seals for injection molding machine cylinders.