Carbon Printers RPU 61 Rigid Polyurethane is a two-component, dual-cure photopolymer resin designed for Carbon Digital Light Synthesis (DLS) additive manufacturing systems. The resin is supplied as a two-part liquid that is blended by the printer’s automated metering and recirculation hardware before being exposed to the CLIP process, in which UV light polymerizes a methacrylate scaffold in the build zone while oxygen inhibition creates a liquid interface. After printing, the green part is washed in solvent and thermally cured in a forced-air oven to complete the isocyanate-polyol reaction network. The final product is a black, rigid polyurethane with a density of 1.10–1.12 g/cm³ under ISO 1183-1:2019 and a Shore D hardness of 74–78 under ASTM D2240-15. Published datasheet values place tensile strength between 35 and 39 MPa and tensile modulus between 1.1 and 1.4 GPa under ASTM D638-14. These properties position RPU 61 as a stiff but not brittle material for functional prototypes, manufacturing aids, and low-volume production components.

The distinguishing feature of RPU 61 is its elongation at break of 8–10%, which is higher than typical highly crosslinked rigid polyurethane and epoxy DLS resins. The impact resistance, measured as notched Izod under ASTM D256-10, falls in the 40–50 J/m range. This performance is achieved without fiber reinforcement, so the material maintains more isotropic behavior across build planes. In practice, Z-direction tensile values are within 5–10% of XY-direction values when the part is printed at 100 µm layer thickness and fully cured. The absence of glass or carbon filler also permits smoother down-facing surfaces in concave geometry than filled resins produce, though it limits upper-use temperature and creep resistance under sustained load.

Compared with RPU 70, the more commonly specified rigid polyurethane in the Carbon portfolio, RPU 61 trades heat deflection temperature for ductility. RPU 70 exhibits a heat deflection temperature at 0.455 MPa of approximately 65–70 °C, while RPU 61 is typically documented at 50–55 °C under ASTM D648-18. This does not make RPU 61 an inferior material for all cases; in applications where the highest part temperature does not exceed 40 °C and the dominant failure mode is crack initiation at a snap-fit or clip, the RPU 61 network often shows fewer brittle fractures than RPU 70. The product also differs from flexible polyurethane resins such as FPU 50, which are specified for elastomeric applications with elongation above 50% but far lower modulus. At the molecular level, the dual-cure system separates the build-speed function from the final mechanical performance function. The UV-cured acrylate structure fixes the part form during printing and provides green strength. The subsequent isocyanate-polyol reaction builds a high-molecular-weight polyurethane network that contributes impact resistance and moderate chemical resistance.

What Is the Comparative Property Profile of RPU 61, RPU 70, and Epoxy-Based DLS Materials?

Direct substitution between rigid resins requires comparison of the published property window. The table below lists typical representative values for three Carbon DLS materials after full cure and conditioning at 23 ± 2 °C and 50 ± 10% RH. The values should not be read as guaranteed lot-specific properties; they are supplied as engineering reference points only.

The comparison shows why RPU 61 is often selected for housings and fixtures. EPX 82 offers high strength and high thermal stability, but its lower elongation at break and lower notched Izod values make it more prone to brittle crack propagation in thin-walled features. RPU 70 sits close to RPU 61 in stiffness but has lower elongation and higher heat deflection temperature. The selection question is therefore not one of overall superiority but of whether assembly strain or elevated temperature is the limiting design factor.

On Carbon DLS production platforms, RPU 61 is dispensed from twin cartridges through a recirculating system that maintains the resin at a controlled temperature. The uncured liquid has a viscosity that supports high-resolution printing; typical print layer heights for RPU 61 are 25 µm, 50 µm, or 100 µm depending on the part’s surface-finish and build-time requirements. The CLIP process creates an oxygen-inhibited liquid layer that prevents part adhesion to the optical window and enables continuous or semi-continuous build. During this process, RPU 61 green parts have enough strength to retain fine features after removal from the platform, but they remain chemically unfinished until the secondary cure is completed.

Build preparation should avoid fully enclosed cavities unless drain holes of at least 2–3 mm diameter are provided. Trapped uncured resin inside a closed cavity becomes a containment risk during oven cure because the exothermic reaction can generate internal pressure and cause wall rupture or surface cracking. In field audits of failed RPU 61 parts, a recurring root cause was insufficient washing of deep blind holes, leaving residual monomer that cured into a brittle skin and altered the part’s local mechanical response. The washing protocol should therefore use a two-stage solvent bath with agitation and, for small internal channels, positive-pressure flushing.

Thermal post-cure is carried out in a forced-circulation oven with a temperature uniformity of at least ±2 °C across the load. RPU 61 follows a time-temperature protocol within the 100–120 °C band for 2–4 h, with thicker sections held longer to bring the internal material to within 5 °C of setpoint. Under-cure is easily detected by a Shore D reading below 70 or a tensile elongation below 6% on sacrificial coupons. Overheating above 130 °C can lead to oxidative yellowing and embrittlement, with elongation at break dropping below 5% under ASTM D638-14. Production quality systems should include a cure-tracking log, oven temperature mapping study, and weekly tensile or hardness coupon testing because dual-cure polyurethane properties are sensitive to resin lot age, wash solvent quality, and ambient humidity.

Production bottlenecks often occur at the post-cure oven because RPU 61 parts cannot be stacked densely without losing cross-flow uniformity. If parts are packed too closely, the center of the batch may not reach the target cure temperature, while edge parts fully cure, producing batch-to-batch hardness variation. A maximum loading density of 40–50% of oven volume is recommended for forced-circulation ovens, with spacing of at least 25 mm between parts. Temperature mapping of the loaded oven with thermocouples embedded in sacrificial RPU 61 blocks is the most reliable method for qualifying a cure cycle. Lot-to-lot variation in dual-cure resins can also occur due to isocyanate monomer purity and polyol water content. Users should require a certificate of analysis for resin shipments and monitor Shore D hardness of standardized post-cure coupons. A hardness drop of 2 points on the Shore D scale may indicate under-cure or resin aging.

Environmental storage for unprinted resin should be maintained between 15 and 30 °C and protected from moisture ingress. The isocyanate component reacts with atmospheric water; containers left open in ambient conditions above 60% RH may form surface skin and increase viscosity. Printed parts should be post-cured soon after washing. If cured parts are stored in high-humidity environments, moisture absorption remains low, but continuous contact with liquid water at temperatures above 40 °C can reduce tensile modulus by 5–10% over extended exposure.

When Impact-Loaded Snap-Fits, Clips, and Protective Housings Are the Primary Application Targets

RPU 61 is specified for components in which assembly forces and repeated impact events determine service life. Design rules require the snap-fit insertion strain to remain below approximately 5%; higher strains may exceed the material’s yield point and create whitening at the root even if the beam does not immediately break. Wall thickness between 2.0 and 3.5 mm provides a useful balance of stiffness and impact resistance. Thinner walls below 1.5 mm are printable but show a greater scatter in notched Izod results due to layer-boundary effects. Sharp internal corners should be replaced with a root radius of at least 0.5 mm; field studies on DLS polyurethane components identified root radius omission as the most frequent cause of early snap-fit failure.

End-of-arm tooling, robotic gripper jaws, and palletizing fixtures have been produced from RPU 61 in low-volume manufacturing cells. In these applications, components operating at room temperature with localized compressive loads under 20 MPa and maximum service temperatures below 40 °C have shown service lives between 10,000 and 30,000 cycles when inserts are used at bolted connections and the parts are not dropped onto hard edges. Published data for this specific configuration is limited, and users should validate cycle life with instrumented testing rather than extrapolating from general material toughness.

RPU 61 also appears in protective covers for handheld instruments and interior automotive clips. For automotive interior use, the lower heat deflection temperature is acceptable for cabin surfaces that remain below 50 °C, but parts should not be placed near airbag deployment paths, engine-compartment heat sources, or painted panels that undergo bake cycles above 80 °C. The black pigmentation provides some UV protection, but outdoor use exceeding 500 h accelerated weathering under ISO 4892-2:2021 may require a clear coating to prevent yellowing and surface microcracking.

RPU 61 should not be combined with amine-based mold releases, amine-cure epoxies, or strong basic cleaning agents, because residual isocyanate groups on the part surface can react prematurely and produce haze, increased crosslink density, or reduced hardness. For adhesive bonding, solvent-based primers should be screened for compatibility; methyl ethyl ketone and similar ketones can soften or craze the surface. Mechanical fastening with threaded inserts is preferred for applications requiring repeated disassembly because tapped threads in RPU 61 may lose preload after multiple thermal cycles between 20 and 45 °C. For regulated applications, no food-contact, medical, or long-term skin-contact claim is implied by the standard datasheet. Users must determine whether cured RPU 61 meets REACH, RoHS, or FDA 21 CFR 177.1680 requirements under their specific printing, washing, and post-cure conditions, because residual monomer content is process-dependent.