When a steel spring fractures in a progressive die, the line stops and the mechanic searches the open tool cavity for fragments of broken coils. A polyurethane spring eliminates that scenario: it does not shatter, but gradually settles and loses stroke, so wear is visible in advance. This article covers where polyurethane outperforms steel and rubber in stamping tooling, which numbers support this, and how to order a spring to your drawing.
What Polyurethane Springs Deliver in a Die and Press
A polyurethane spring returns the die’s clamping element to its home position the same way a steel coil does, but without the risk of fatigue fracture and with a softer force characteristic. It is placed in strippers, blank-holders, ejectors, and mould buffers, where the part works in compression through thousands of cycles in succession.
The primary advantage lies in the operating character. A steel coil has a rigid linear characteristic and concentrates stress in the coils, causing it to crack near the end-coil over time. Polyurethane deforms throughout its entire volume, stores energy, and returns it on unloading. Rebound elasticity of cast polyurethane is 35 to 65 percent, meaning the material returns most of the applied energy rather than dissipating it as heat and plastic deformation.
For the tool designer this means compactness. Where a steel spring requires a long pocket for full coil travel, a polyurethane element delivers the required force on a short working stroke. This is critical in closely packaged progressive dies where every millimetre of stack height counts.
Media resistance in the die environment also matters. The tooling surface is continuously wetted with lubricant and cutting fluid, which attack and weaken ordinary rubber. Cast polyurethane has excellent resistance to technical oils, so it does not swell and does not lose force from contact with them, and the operating range from -60 to +100 °C allows it to work in a cold shop and during tooling warm-up under intensive stamping.
Polyurethane versus Steel Spring and Rubber: the Numbers
The direct answer: polyurethane wins on wear resistance and compression durability; steel wins on maximum force and temperature range; rubber loses to both on service life. To make the decision evidence-based, key properties are compared against reference data.
| Property | Cast polyurethane (PU) | NBR rubber | Carbon steel (St3, St45) |
|---|---|---|---|
| Abrasion, DIN 53516, mm³ | 38-39 | 80-150 | n/a |
| Operating temperature, °C | -60…+100 | -30…+100 | -40…+500 |
| Density, g/cm³ | 1.05-1.25 | 1.0-1.3 | 7.7-7.9 |
| Tensile strength, MPa | 39-87 | 10-25 | 370-700 |
| Rebound elasticity, % | 34-61 | 20-45 | n/a |
Two numbers in the table resolve most disputes in polyurethane’s favour. By DIN 53516 abrasion, polyurethane loses 38-39 mm³ versus 80-150 mm³ for NBR rubber, meaning it lasts significantly longer in real operating conditions. Against the canonical TIMOL benchmark, polyurethane wear resistance is 2-5 times higher than standard rubber, which is why springs made from it do not require monthly replacement.
Steel wins on two counts: density 7.7-7.9 g/cm³ gives greater mass, and tensile strength 370-700 MPa gives a very high limiting force. So where the maximum clamping force is needed in a heavy press, steel remains in place. But steel does not damp impact, corrodes from cutting fluid, and fails by fatigue. Polyurethane is six to seven times lighter at the same envelope, is unaffected by oils, and operates in the range -60 to +100 °C.
Engineer’s tip: select spring hardness for the required force, not the other way around. For a light stripper, use polyurethane around 80 Shore A; for a firm blank-holder, closer to 95 Shore A or the lower end of the Shore D scale. Leave clearance for lateral expansion: under load polyurethane increases in diameter, and if the seat is an exact fit the part will work in side pressure and fatigue faster.
Where Springs Are Fitted and How to Install Them
Polyurethane springs are installed in die tooling pockets on a guide post or into a blind bore, where they work in compression between the moving and fixed plates. The most common positions: strip strippers, blank-holder/draw-bead pads during deep drawing, finished-part ejectors, and slide-travel-limiting buffers.
Installation is straightforward: the spring is slipped over a guide rod or inserted into a counterbore so that it works strictly on the compression axis. The primary requirement is coaxiality and absence of misalignment, since lateral loading is harmful to any elastomer. Part end faces are made flat and parallel so that force is distributed evenly rather than concentrating at the edge.
Maintenance consists of periodic inspection. Polyurethane does not crack suddenly — it gradually settles and loses its original height, so by monitoring working stroke reduction a replacement can be planned before clamping force drops below specification. This eliminates the emergency stoppages characteristic of steel coils that fail without warning. Further wear-resistant solutions for friction nodes are in the catalogue.
How to Select Hardness, Cross-Section, and Stroke
Selection begins with the required force and allowable stroke, and hardness and cross-section are then selected to match. TIMOL polyurethane is manufactured in the hardness range from 50 Shore A to 70 Shore D, so the same geometry can be made as a soft damping element or a stiff load-bearing one.
Element stiffness depends on three parameters: material hardness, cross-sectional area, and working height. The thicker the wall and the higher the hardness, the greater the force at the same compression. The taller the part, the greater the available stroke. So within a limited pocket envelope the designer balances hardness against cross-section to achieve the required characteristic.
It is important to allow for compression reserve. The working compression of a polyurethane spring is normally kept within limits that do not bring the material to full volume enclosure, otherwise force rises sharply and service life drops. An experienced factory technologist will calculate these limits for your cycle if you supply the force, stroke, and pocket dimensions. Hardness selection for specific chemistry and abrasive conditions is covered in detail in the reference section.
Manufacturing Polyurethane Springs to Drawing at TIMOL Factory
Polyurethane springs for a specific die can be ordered directly from TIMOL factory in Dnipro. Parts are cast by the free-casting process, which allows manufacture of both simple cylindrical springs and profiled elements with guide holes, flanges, and seating lands to your pocket geometry.
Free-casting technology requires no expensive metal die tooling for each size, making it economical for a single repair part and for a production series. Polyurethane solutions are used in 12 industrial sectors, from metalworking to mining equipment, and in every case the part is made to the actual node conditions.
Send a drawing, sketch, or sample of the worn steel spring, and we will select the hardness, calculate the force, and confirm the lead time. For consultation and calculation for your tooling, contact us via the contacts page.