A hydraulic cylinder starts to leak fluid when the seal loses its elasticity or wears against the rod: pressure drops, the line stops, and the repair comes down to finding a seal of the right profile. A standard rubber seal is often not on hand in the required size, and its service life in a dynamic assembly is short. Polyurethane seal manufacturing solves both problems: cast polyurethane holds a seal several times longer than rubber and is cast to any profile from a drawing. Below I explain why a polyurethane seal is better than a rubber or PTFE one, how to choose the hardness, by what technology it is made, and what you need for an order at the factory.
Why a polyurethane seal outlasts a rubber one
Polyurethane combines wear resistance with high tensile strength, so a seal made from it holds tightness in a moving assembly longer than a rubber one and resists being extruded into the gap under pressure better. This is the main reason for switching to polyurethane seals in hydraulics.
The working surface of the seal is constantly rubbing against the rod or the cylinder wall, and it is wear that determines the service life. The abrasion of cast polyurethane per DIN 53516 is 38-39 mm³, whereas for NBR rubber, the most common seal material, it reaches 80-150 mm³. All else being equal, a polyurethane lip wears down several times more slowly. The second factor is tensile strength: for polyurethane it is 39-87 MPa versus 10-25 MPa for NBR rubber. Higher strength means the seal does not tear at the lip and is not extruded into the gap between the rod and the guide under pulsing pressure.
Polyurethane also holds oils and hydraulic fluids on a par with oil-resistant rubber, yet significantly outperforms it in wear. The working range of -60 to +100 °C covers most industrial hydraulic systems, from machinery in the cold to heated assemblies.
Another advantage is tear resistance at the lip. The sealing lip of the seal constantly works at its limit, wrapping around the rod on every stroke. Rubber with low tensile strength quickly nicks this zone, and the seal starts to leak in a thin trickle even while the main body is intact. Polyurethane, with its higher strength, keeps the lip intact much longer, so the seal fails gradually and predictably rather than in a sudden leak. For hydraulic systems where a sudden seal failure means an emergency stop, this predictability of service life is often more important than the wear-resistance figure itself.
A numbers comparison of materials for seals
For seals, three parameters matter at once: wear resistance, tensile strength and resistance to the environment. Below are the reference characteristics of the materials most often used to make seals and oil seals.
| Material | Abrasion (DIN 53516), mm³ | Working temperature, °C | Hardness | Tensile strength, MPa | Oil resistance |
|---|---|---|---|---|---|
| Polyurethane (TIMOL) | 38-39 | -60…100 | 85A-95A | 39-87 | excellent |
| NBR rubber | 80-150 | -30…100 | 40A - 90A | 10-25 | good |
| Silicone rubber | 150-350 | -60…230 | 20A - 80A | 5-12 | moderate |
| PTFE | 200-500 | -200…260 | 50D - 65D | 15-35 | excellent |
The numbers explain why polyurethane dominates in dynamic seals. Silicone rubber withstands a wide temperature range, but its abrasion of 150-350 mm³ and strength of only 5-12 MPa make it unsuitable for loaded moving seals; it is a material for static heat-resistant gaskets. PTFE is chemically inert and works up to +260 °C, yet its abrasion of 200-500 mm³ and creep under load limit its service life in assemblies with friction. NBR rubber is cheap and oil-resistant, but it is 3 to 4 times worse than polyurethane in wear and half as strong. Polyurethane gives the best balance precisely for moving seals under pressure.
For rod and piston seals, do not lower the hardness for the sake of softness. A lip that is too soft is extruded into the gap between the rod and the guide under pressure and fails quickly. For moving seals, take the higher hardness values; this directly extends the service life. Leave soft grades for static contact seals at low pressure.
The free casting technology for seal manufacturing
Polyurethane seals are cast using free casting technology, which does not require an expensive pressure mould and is therefore economically justified even for a single repair item. This is a key advantage when a seal of a non-standard profile is needed for a specific assembly.
The process begins with preparing a mould for the required seal profile: the mounting diameter, the height, the shape of the sealing lip. Polyurethane of a given hardness is poured into the mould and polymerised, after which the item is brought to exact size if needed. Since free casting allows complex geometry to be reproduced, the factory makes not only seals but also oil seals for sealing shafts and damper rings for hydraulics and pneumatics. The material leaves no marks on mating surfaces and does not attract debris, so the assembly stays clean longer. Read more about the manufacturing service in the Services section.
The choice of hardness at the casting stage is critical. The same polyurethane in different hardness behaves like different materials: a soft lip seats better but holds pressure worse, while a hard one does the opposite. That is why the process engineer sets the hardness to suit the specific sealing conditions.
Free casting is also advantageous in that it allows repair sizes to be made. When the rod or cylinder has already been reground for repair and a standard seal does not fit by diameter, the mould is prepared to the actual size of the mating parts. This saves a worn assembly that would otherwise have to be written off due to the absence of a seal of the right size in catalogues. For old or rare equipment taken out of production, manufacturing a seal to the actual dimensions is often the only way to return the machine to work.
Where polyurethane seals are used and how to order
Polyurethane seals, oil seals and sealing rings work in hydraulic and pneumatic installations, presses, hydraulic pumps, cylinder rods and assemblies with rotating shafts. Their task is to seal a joint, to keep the working fluid from escaping and to protect the assembly from dust and moisture from outside, while ensuring the free movement of the moving parts.
The most convenient way to obtain an accurate seal is to take dimensions from the old seal and the mating parts: the diameter of the rod or piston, the mounting diameter of the groove, its width and depth, and also to record the profile of the lip. If the old seal has survived, it is enough to hand it over as a sample, and the factory will reproduce the geometry without a separate drawing. When a seal fails often, it is worth reviewing the gap in the assembly at the same time: sometimes the cause lies not in the seal material but in a worn guide that increases the gap and provokes extrusion of the lip.
To order the manufacture of a seal for your assembly, provide the factory with a drawing of the seal or a sample of the old seal together with the operating conditions: the type of fluid, the temperature, the nature of the movement and the required hardness. Based on this data, the process engineer will select the polyurethane grade, and production will reproduce the profile using free casting technology. See ready standard sizes of seals in the catalogue, and for a calculation on a non-standard profile, contact the TIMOL factory via the Contacts page.