Carbon Printers EPU 40 Elastomeric Polyurethane is a photopolymerizable urethane resin produced for the Carbon Digital Light Synthesis platform and qualified for Carbon M1, M2, and M3-series printers. The fully post-cured resin is specified at nominal Shore A 40 durometer, which places it at the low-modulus end of the carbon polyurethane portfolio. Supplier documentation lists a cured density of 1.05 g/cm³ under ASTM D792-20. The material is processed through an oxygen-permeable window during printing, followed by solvent washing, drying, and forced-air thermal post-cure. This workflow distinguishes EPU 40 from thermoplastic polyurethane and cast urethane products, which rely on melt solidification or isocyanate-amine curing rather than vat photopolymerization. The product is used where repeated elastic deformation, low-modulus cushioning, tear resistance, and design-integrated lattice structures are the controlling engineering requirements.

Product designation EPU 40 consists of the elastomeric polyurethane chemistry and the nominal Shore A 40 hardness target. The 40 suffix does not by itself define processing parameters. Each printer platform uses a separate print profile, with resin temperature, light dose, and peel parameters managed by the machine controller. Unvalidated parameter changes can shift the photopolymerization working curve and produce uncured cavities, delamination, or surface tack. In production-scale Digital Light Synthesis equipment, EPU 40 is washed in an approved solvent bath after printing to remove uncured resin from internal channels and lattice surfaces. Inadequate washing leaves residual photopolymer in the green part, which then cross-links into a brittle skin during thermal post-cure and can block open-cell lattice pores. After washing, parts require forced-air drying before entering the oven. Residual solvent that is not evaporated before the 120 °C ramp can induce subsurface porosity at wall surfaces.

Published mechanical data for the post-cured resin, measured on laboratory coupons, report tensile strength at break of 5.5 MPa and elongation at break of 250% under ASTM D412-16 Die C. Tear strength is reported as 22 kN/m under ASTM D624-00(2012) Die C. Compression set is listed at 20% after 22 h at 70 °C according to ASTM D395-18 Method B. Hardness is specified as Shore A 40 under ASTM D2240-15e1. These values refer to solid, fully post-cured specimens and should not be transferred directly to thin-walled lattices without specimen-level validation. Anisotropy in Digital Light Synthesis parts can reduce Z-oriented elongation when compared with XY-oriented coupons, especially in sections printed below 1.5 mm thickness.

The reported values are typical supplier data and are not a substitute for lot-specific testing on production geometry. For gasket and seal applications, compression stress relaxation should additionally be evaluated under ISO 3384-1 using a representative gap and service temperature. Published data for EPU 40 under long-term stress relaxation are limited, particularly for lattice cells and interrupted surfaces.

How Does EPU 40 Differ From Rigid Polyurethane and Epoxy Systems?

At Shore A 40, EPU 40 is separated from rigid polyurethane grades such as Carbon RPU 130 by a hardness gap of nearly 80 shore points. RPU 130 is a structural, higher-durometer resin with glassy tensile response and high flexural modulus, whereas EPU 40 operates in the elastomeric plateau with gross elongation above 200%. Compared with epoxy systems, EPU 40 offers much lower modulus and higher elongation: an unfilled epoxy network may exhibit less than 10% elongation at break under ASTM D638-14, while EPU 40 absorbs local strain through polyurethane segment orientation and domain relaxation. The trade-off is absolute strength and environmental resistance. Rigid polyurethane and epoxy resins retain geometry under compressive loads that would crush an Shore A 40 elastomer unless the part is structured as a lattice or constrained within a rigid housing. The selection of EPU 40 is appropriate when the engineering requirement is not merely load bearing but reversible deformation, impact isolation, or interfacial sealing.

Hardness alone does not capture the difference. A Shore A 40 elastomer can bend around a 1 mm radius without crack propagation, while a Shore D 72 rigid polyurethane cannot sustain that deformation mode. However, the rigid resin will carry a 50 N static load with far less displacement than a solid EPU 40 section of identical thickness. The choice between EPU 40 and a rigid polyurethane therefore depends on whether the part is expected to recover from deformation or resist deformation entirely.

When Thermal Post-Cure Deviates From the Specified 120 °C Window

Thermal post-cure is not a cosmetic step. Manufacturer guidance specifies 120 °C forced-air oven exposure for 12 h for full property development in production parts. Shorter dwell times or lower setpoints produce parts with incomplete photopolymer conversion, elevated compression set, and surface tack. Production ovens with poor airflow, dense rack loading, or inadequate exhaust can delay the center of the batch reaching 120 °C by 1–3 h, depending on part mass and packed volume. Operators running Carbon M2 or M3 lines therefore verify oven uniformity with embedded thermocouples before committing production batches. Overheating above the recommended window can degrade urethane bond structure, causing yellowing, hardness drift, and loss of elongation. Because the window is process-defined, post-cure oven calibration records are part of the quality release for printed EPU 40 parts.

General reaction-kinetic behavior implies that a 10 °C reduction below the 120 °C setpoint may roughly double the required dwell time to reach equivalent conversion. This does not mean that lower temperatures can be substituted arbitrarily. At temperatures below the recommended window, side reactions become more significant relative to network formation, and the final compression set may remain elevated even after extended dwell. The post-cure process should be validated by measuring hardness, compression set, and tensile elongation on production-representative specimens, not by oven timer alone.

In production-scale Digital Light Synthesis, the green part is washed in an approved solvent bath after printing. The solvent type and immersion duration must be controlled because EPU 40 in the green state has not yet reached its final crosslink density. Solvent exposure can produce swelling that becomes locked in if the part is not fully dried before the thermal ramp. Batch-to-batch variability in green-state hardness is typically below ±2 Shore A for equivalent post-cure conditions; parts printed at the extremes of the build platform may show larger variance because of light dose non-uniformity. These process observations are derived from production-scale M-series behavior and are not substitutes for material lot testing.

Differences From Thermoplastic Polyurethane and Compression-Molded Elastomers

EPU 40 differs from thermoplastic polyurethane in processing route and property control. Thermoplastic polyurethane requires pellet drying, screw plasticizing, and melt temperatures above 180–210 °C, and its achieved crystallinity depends on mold cooling rate. EPU 40 is photopolymerized at printer operating temperature and then thermally post-cured; it contains no melt-processable crystallites. This removes melt-flow constraints from geometry but introduces anisotropic green-state properties and the need for solvent handling. Compared with compression-molded elastomers such as EVA foam, EPU 40 allows solid elastomer density and lattice topology to be decoupled: an EPU 40 midsole can have a local apparent density below the bulk 1.05 g/cm³ through deliberate cell placement. However, compression-molded foam is generally lower in raw material cost and has well-established hydrolytic and fatigue testing for high-volume footwear.

Cast polyurethane can be formulated to a wider range of durometers and often provides higher tear strength, but it requires mold tooling for every geometry iteration. The Digital Light Synthesis route used with EPU 40 removes tooling for low-volume or lattice-intensive parts, at the cost of post-print washing and thermal processing. From a molecular standpoint, EPU 40 is thermoset in nature once post-cured and cannot be reprocessed by reheating, unlike thermoplastic polyurethane regrind. Drilling or cutting fully cured EPU 40 parts at high feed rates may generate local frictional heating above the degradation onset of the urethane network; cooling and slow feed are recommended. Published data for this specific configuration is limited for highly filled or flame-retardant versions.

Footwear Midsole Lattice Collapse and Compression Set

EPU 40 is frequently printed as lattice midsole structures in which cell geometry, not bulk foam expansion, controls local stiffness. The material’s 250% elongation at break and 22 kN/m tear strength support thin cell wall deformation without immediate crack propagation. Compression set, specified at 20% under 22 h at 70 °C per ASTM D395-18 Method B, is the governing property for cushioning durability because repeated loading produces permanent strain in lattice nodes. In contrast to compression-molded EVA foam midsoles, EPU 40 lattice geometries can vary cell density regionally within one printed part, but this capability requires dynamic fatigue testing beyond static mechanical data. Production experience shows that strut diameters below 1.2 mm increase print-to-print dimensional scatter on some M2 systems; lattice designs for volume production are typically evaluated with process capability studies of strut width rather than with coupon tensile data alone.

The low-durometer response is useful for vibration isolators and pads where a low natural frequency is required. However, low hardness also reduces extrusion resistance in gasket and seal applications. For gasket lip designs, compression stress relaxation under ISO 3384-1 should be measured at the upper service temperature because urethane networks can show time-dependent force decay even when static compression set is acceptable. Dynamic mechanical analysis under ISO 6721-1 can provide storage modulus and loss factor data for frequency-dependent cushioning behavior, but published data for EPU 40 across a broad frequency range are limited.

Chemical compatibility data for EPU 40 are limited. Soft segment chemistry in polyurethane materials is generally susceptible to hydrolysis above 60 °C in continuous hot-water immersion, and polar solvents can cause swelling. Before use in automotive or industrial fluid-contact applications, swell resistance should be tested under ASTM D471-16e1 using the actual fluid and service temperature. The supplier material safety data sheet indicates compliance with REACH and RoHS 2011/65/EU, but no food-contact or medical-grade claim appears in the standard product documentation. Components requiring FDA 21 CFR 177.1680 or 177.2600 status must be validated separately with the specific print and post-cure process because the final article, not the raw resin, determines regulatory status.