Polyurethane (PU) elastomer selection is governed by a short list of coupled variables: Shore hardness band, isocyanate index, polyol family, and the dynamic duty cycle the part will see in service. Hardness is the cheapest screening number, but it is not the design number — tensile strength, elongation at break, hysteresis under cyclic compression, and glass-transition temperature are the four properties that drive whether a part survives 18 months or 18 years.
For reference-grade background on the polyurethane elastomer chemistry, the linear guide wear-stack context, and the pressure transmitter sealed-diaphragm analogy, the same engineering rules of elastomer compatibility apply across those adjacent component decisions.
Hardness Bands and the Sizing Trade-off
Industrial cast PU elastomer stock is normally quoted in five Shore bands: 80A, 90A, 95A, 60D, and 75D, with 70A sometimes used for soft rolls and bumpers. The widely cited rule of thumb is that tensile strength roughly doubles between 80A and 95A, then plateaus or declines modestly into the 60D-75D range as the hard-segment fraction crowds out soft-segment mobility. The inverse trend holds for elongation at break, which commonly drops from roughly 600% at 80A to 250-300% by 75D in MDI-polyester systems. That is the first sizing lever: pick the softest hardness that still meets the load-deflection and wear-life targets, because softer grades absorb impact and outlast harder grades in dirty or misaligned contact. [S1]
Dynamic compression tests on MDI/PTMEG/BDO PU blocks (63.5 × 63.5 × 15.9 mm), run at 18.5 Hz for 200,000 cycles under a peak load of 24.5 kg with a 10:1 minimum-to-maximum load ratio, produced internal hysteretic temperatures between 50 °C and ~110 °C depending on hard-segment content [S5]. A practical rule: every 5-point Shore A increase above 90A can raise the steady-state hysteresis temperature by 10-15 °C under sustained cyclic load, which shortens life in roll, wheel, and screen-deck service.
Isocyanate and Polyol Family Selection
The three isocyanates that dominate the cast-urethane market — MDI (methylene diphenyl diisocyanate), HDI (hexamethylene diisocyanate), and IPDI (isophorone diisocyanate) — give noticeably different parts. MDI/PTMEG (polytetramethylene ether glycol) is the default for wheels, rollers, and screens where abrasion and resilience dominate; MDI/polyester systems give higher tensile and tear strength but poorer hydrolysis resistance; HDI and IPDI systems, both aliphatic, are specified where UV stability and colour hold matter, and HDI/poly(ε-caprolactone) (PCL) systems are documented in biomedical-grade formulations where extractables and biocompatibility drive the spec [S2].
For procurement teams comparing silicone rubber suppliers and manufacturers against PU, the chemistry distinction is the discriminator: silicone caps out near 80A in practical hardness bands and loses tensile above ~150 °C, while PU holds strength to roughly 90 °C continuous and reaches well past 60D. Where the duty is sub-zero flexibility down to -30 °C or below, polyester-PUs become brittle and polyether/PCL grades are the documented choice. Aromatic MDI systems will yellow in UV; aliphatic HDI/IPDI grades hold colour but cost 1.5-3× the kilo price of commodity MDI parts.
Cast vs Millable vs TPU: When Each Family Fits
Cast elastomers (CPU) are poured into open moulds and cured in situ, dominate the large-part market — rolls, screens, conveyor scrapers, mining liners — because the chemistry allows wide hardness and modulus tuning without a moulding press. Millable polyurethane (MPU) is a rubber-processable PU that can be milled, calendered, and extruded on conventional rubber equipment; it is specified where the part is small, geometric, and high-volume, and where compression set at moderate temperatures matters [S3]. Thermoplastic polyurethane (TPU) covers the injection-moulded and extruded-film cases.
Cast polyester-MDI parts hit roughly 40-55 MPa tensile at 90A, climbing toward 60 MPa in 95A grades; millable PU typically sits 10-20% below those numbers but offers tighter compression-set control; TPU trades peak tensile for throughput. For a roller or screen panel above ~300 mm in any dimension, the cast route is almost always the answer because the press tonnage for an equivalent TPU part is uneconomic.
Service Envelope: Temperature, Fluid, and Hysteresis
Polyester-MDI parts tolerate continuous service typically to 80-90 °C with peaks to 110 °C; polyether-MDI parts gain 5-10 °C on the upper end; HDI/IPDI aliphatic systems hold to roughly 100 °C continuous. Sub-zero performance is dictated by the soft-segment Tg: PTMEG (Tg around -80 °C) gives the best low-temperature flexibility, polyadipate (Tg roughly -30 to -40 °C) sits in the middle, and PCL grades (Tg near -60 °C) are common where a balance of low-temperature flex and hydrolysis resistance is required [S2].
Fluid compatibility is the second envelope test. Water and steam attack polyester-PU via hydrolysis of the ester linkage; polyether-PU is the standard pick for humid or aqueous service and is the dominant chemistry in EPDM rubber for water treatment adjacent specifications. Oils, fuels, and many solvents are friendlier to polyester-PU. Acidic or alkaline pH extremes below 4 or above 10 shorten life across all PU families; specifying a post-cure at 100-110 °C for 16-24 h before service is the standard way to drive residual isocyanate below the 0.1% level that drives long-term hydrolysis.
Sizing Procedure for a New Part
Step one: lock the Shore hardness band from the load-deflection and wear-life targets — start at 85A or 90A for general industrial wheels, bump to 95A or 60D only when contact pressure or abrasion rate forces it. Step two: pick the polyol/isocyanate family from the fluid, UV, and temperature envelope. Step three: size the part geometry from the deflection-under-load equation for an elastomeric block, which for small strains scales roughly linearly with hardness when the same polymer family is held constant. Step four: verify the part against a hysteretic-heat test if the duty cycle exceeds 10 Hz or the duty factor exceeds 50% [S5].
A short comparison helps when the spec is genuinely open: MDI/polyester (high tensile, poor hydrolysis), MDI/polyether (balanced, best all-round for wheels and rolls), HDI/IPDI aliphatic (UV-stable, premium), and millable PU (rubber-processing route, compression-set control). On the four decision criteria that most often decide a project — abrasion life, hydrolysis resistance, UV/colour stability, and maximum tensile — MDI/polyether wins abrasion and balances the other three, MDI/polyester wins tensile but loses hydrolysis, HDI/IPDI wins UV but costs the most, and millable PU wins compression-set control but loses peak mechanicals.
Standards, Sourcing, and Maker Map
There is no single ISO or ASTM grade that covers a "polyurethane elastomer" part; standards that govern the spec include ASTM D412 (tensile), ASTM D624 (tear), ASTM D2240 (Shore hardness), ASTM D395 (compression set), and ASTM D5963 (DIN abrasion) — and a competent supplier will quote test data against all five rather than just a hardness number. The millable-PU and monodisperse-segment literature [S3] gives the structure-property grounding that any converter's datasheet leans on, even if the converter does not cite the papers. Chinese cast-PU makers — including Kemaidi in Dongguan and Luoyang Haien in Henan [S1][S4] — typically offer 70A through 75D as standard, with rollers and bearings as catalogue stock, and accept custom hardness/MOQs down to 100 pieces on custom tooling [S4].
For buyers comparing adjacent rubber-and-elastomer categories, the silicone supplier map silicone rubber suppliers and manufacturers is the natural cross-reference, and where the PU part sits inside a hydraulic system, the hydraulic accumulator buying guide covers the bladder and seal-stack choices that bookend a PU roll or wiper. One trackable signal for the rest of 2026: aliphatic HDI/IPDI pricing has been narrowing against MDI on volume orders, which should pull the UV-stable grade out of "premium only" into the standard screening list for outdoor and visible-surface parts by year-end.