Carbon Printers EPU 44 Biobased Elastomer for 3D Printing is a single-component photopolymerizable polyurethane resin formulated for Carbon Digital Light Synthesis platforms, including the M3, M3 Max, and L1 printer families. The uncured resin combines urethane acrylate oligomers, plant-derived polyol segments, reactive diluents, and photoinitiators. Exposure to 365 nm ultraviolet light through an oxygen-permeable build window produces a microphase-separated elastomer with Shore hardness typically between 78A and 82A when measured according to ASTM D2240-15. Renewable carbon content is quantified by ASTM D6866-21; the exact biobased fraction varies by production lot and is stated on the supplier certificate of analysis. The material is intended for repeated compressive or flexural loading applications, including gaskets, vibration isolators, cushioning lattices, conformal protective housings, and elastomeric hinges.

Material Qualification and the Basis of Bio-Based Carbon Accounting

Because EPU 44 is supplied as a liquid photopolymer rather than a pre-formed sheet, mechanical qualification must be performed on printed and post-processed specimens. Standard test specimens follow ASTM D638-14 Type IV geometry. Tensile and tear specimens are printed in the XY plane at 2.0 mm thickness, washed in 99% isopropanol, thermally post-cured, and conditioned at 23 ± 2°C and 50 ± 5% relative humidity for 24 h prior to testing. Representative cured properties are shown in Table 1. These values are lot-qualification targets rather than guaranteed minima because the bio-based polyol fraction contributes to batch-to-batch variation in crosslink density, phase separation, and residual monomer content.

After printing on Carbon DLS equipment, green parts retain a surface layer of uncured resin. The standard post-processing sequence uses a two-stage solvent wash: first in 99% isopropanol at 25 ± 2°C for 10 min, followed by a second clean solvent bath for 5 min. Insufficient washing leaves a surface film that becomes tacky after UV post-cure; tactile tack is an indicator of residual acrylate monomer rather than under-cure of the bulk. Following solvent removal, thermal post-cure in a forced-air oven at 80°C for 8 h or 120°C for 2 h completes reaction of residual methacrylate groups. The lower-temperature schedule is preferred for parts with thin elastomeric hinges because it reduces oxidative surface embrittlement. The higher-temperature schedule improves compression set and chemical resistance but typically reduces elongation at break by 10–20% relative to the lower-temperature schedule. After post-cure, parts should cool to below 40°C before removal to minimize warpage in geometries with wall thickness variation greater than 3 mm.

How Does the Unreacted Resin’s Viscosity Influence Recoater Force During Continuous Builds?

The uncured resin exhibits shear-thinning behavior typical of urethane acrylate formulations. Brookfield viscosity measured at 25°C with a small-sample adapter at 10 s⁻¹ is commonly in the 2,000–5,000 cP range; the exact value is controlled by the relative concentration of bio-based polyol and low-viscosity monofunctional diluent. A viscosity below 1,500 cP can accelerate resin drainage from large vertical surfaces and produce under-filled sections. A viscosity above 6,000 cP increases the force required to recoat the build area and may cause release failures on the oxygen-permeable membrane. Carbon printer control software uses a force-sensing recoater system to indirectly monitor viscosity and will pause the build if coating force deviates more than 20% from the baseline. Production lines using M3 Max printers have observed increased force deviations when cartridges are stored below 20°C and installed without a 1 h warm-up to 25 ± 2°C. The same effect occurs when a cartridge is left open in humid shop air because absorbed moisture increases hydrogen bonding within the bio-based polyol phase and raises apparent viscosity.

In comparison with Carbon EPU 40 and EPU 41, the defining shift in EPU 44 is the introduction of bio-based carbon without changing the fundamental acrylate-terminated urethane cure chemistry. Relative to EPU 40, EPU 44 generally exhibits a comparable Shore A hardness range but a lower tear propagation resistance under ASTM D624 Die C; part-level validation is therefore necessary when replacing EPU 40 in elastomeric hinges or snap-fit retention features. Compared with fused filament fabrication TPU, EPU 44 is a thermoset after UV and thermal cure and cannot be reprocessed by grinding or re-extrusion. DLS parts also show reduced Z-axis anisotropy: tensile strength measured on ASTM D638-14 specimens printed in the Z orientation typically retains more than 80% of the XY value, whereas filament-printed elastomers often retain less than 50% when layer-to-layer fusion is incomplete. For silicone rubber replacement, EPU 44 provides a harder, machinable surface but does not match the high-temperature compression set resistance of peroxide-cured silicone elastomers tested at 150°C.

When EPU 44 Is Subjected to Hot-Wet Aging in Gasket and Sealing Geometries

Compression set and hydrolytic stability control service life in static sealing applications. EPU 44, like other polyurethane elastomers, undergoes chain scission in hot water above approximately 60°C. Seals tested under ASTM D395-18 Method B show that compression set at 70°C for 22 h is typically below 20%, but the same property at 100°C for 70 h can exceed 45% depending on part thickness and post-cure state. Published data for this specific configuration is limited, so seals intended for continuous use above 80°C should be subjected to application-specific compression stress relaxation testing. In water-glycol environments, hydrolysis of ester or urethane linkages reduces elongation at break; after 1,000 h in 50/50 water/ethylene glycol at 80°C, retained tensile strength may fall below 60% of the unaged value. Gasket flanges should therefore avoid sharp internal radii below 0.5 mm and excessive compressive strain above 30%.

Printed lattice structures for protective equipment have been produced using cell sizes between 2 mm and 4 mm and strut thicknesses between 0.4 mm and 0.8 mm. The energy-return behavior of such lattice parts depends on relative density and post-cure thermal history. Lower post-cure temperatures near 80°C for 8 h generally preserve elongation at break, while higher temperatures near 120°C for 2 h increase crosslink density and reduce compression set at the expense of impact toughness. In footwear cushioning trials, dynamic mechanical analysis at 1 Hz indicates a glass transition below -30°C, and the material remains viscoelastic across the expected use-temperature range. Designers using lattice optimization software must impose a minimum wall thickness of 0.6 mm for reliable draining of uncured resin from closed cells; insufficient drain holes produce trapped liquid that later exudes and causes surface tack after post-cure.

Compliance Checklist and Operational Boundaries

Raw photopolymer and printed articles are regulated differently. The liquid resin is supplied with a safety data sheet that identifies hazardous components under 29 CFR 1910.1200 and EU REACH. Table 2 summarizes the principal compliance tests and applicable standard methods. Printed parts that contact human skin may require additional validation under ISO 10993-5 and ISO 10993-10; the supplier does not automatically provide biocompatibility certification for every batch.

The material has defined operational boundaries that should not be exceeded without validation. Continuous service above 80°C in wet environments accelerates urethane hydrolysis; contact with strong bases, chlorinated solvents, or aromatic hydrocarbons can cause swelling beyond 10% by volume and loss of dimensional control. Parts should be shielded from long-term ultraviolet exposure unless a UV-stabilizing topcoat is applied because outdoor weathering under ASTM G154 may cause surface chalking and tensile strength loss. The resin cartridge should not be heated above 30°C during storage, and printed parts should not be post-cured in a conventional oven also used for food preparation. These boundaries are specific to EPU 44 as a bio-based polyurethane elastomer; they do not represent limitations of all Carbon resin grades.