Carbon Printers RPU 70 Rigid Polyurethane is a dual-cure photopolymer resin formulated for Carbon Digital Light Synthesis systems in which a UV-initiated acrylate network builds green strength and a thermal urethane reaction completes the crosslinked network. The material carries a nominal hardness of 70D under ASTM D2240-15 and is specified for rigid functional prototypes, assembly fixtures, robotic end-of-arm tooling, and short-run end-use parts where moderate stiffness, machinability, and impact resistance are required. Unlike elastomeric polyurethane grades in the same portfolio, RPU 70 resists indentation and compressive creep under metal fastener preload when service temperatures remain below 45 °C. On Carbon M-series DLS hardware, the resin prints at a pixel resolution of 75 µm; vertical accuracy is determined by build orientation, support density, green-state resin drainage, and thermal post-cure. The uncured resin requires controlled handling, and published data for this specific configuration is limited where long-term UV aging or subambient storage are concerned.

What Mechanical Property Envelope Does RPU 70 Occupy After Full Dual-Cure Conversion?

Mechanical characterization is performed on Type I tensile bars per ASTM D638-14 after manufacturer-recommended thermal post-cure. The tensile modulus is reported in the range 1.5–1.8 GPa, and ultimate tensile strength is reported in the range 34–38 MPa; elongation at break is 15–25%. Flexural modulus measured by ASTM D790-17 three-point bend is 1.3–1.6 GPa. Notched Izod impact under ASTM D256-10 falls between 30 J/m and 50 J/m, with sensitivity to post-cure completeness and notch preparation. Heat deflection temperature is 52–60 °C at 0.45 MPa and 45–50 °C at 1.8 MPa per ASTM D648-16. The gap between the two HDT fiber stresses indicates a broad glass transition for the urethane phase, and continuous load-bearing service above 40 °C requires creep testing to ASTM D2990 rather than reliance on HDT values.

The values in the table are typical manufacturer-published values and should not be used as design allowables. Impact performance is not fully isotropic across build orientations; XY-plane specimens typically differ from Z-axis specimens because the dual-cure network retains anisotropy from the print direction and from thermal cure. Under sustained compressive preload near 0.5 MPa average stress at 23 °C, polyurethane networks exhibit time-dependent relaxation; RPU 70 is therefore unsuitable for spring-like clamping force retention without mechanical verification. Published data for this specific configuration is limited where long-term creep of printed polyurethane is concerned.

Secondary operations on fully cured RPU 70 include CNC machining, thread-forming, and adhesive bonding. Carbide end mills at speeds below 10,000 rpm with air coolant prevent local surface smearing; petroleum-based mist coolants are avoided because they can plasticize the urethane surface. Thread-forming screws are preferred over thread-cutting screws because chip formation at the thread root can initiate brittle fractures in high-strain sections. For adhesive bonding to aluminum or steel, plasma or flame surface treatment improves wetting and removes residual process films; lap-shear testing under ASTM D5868-01(2014) is required for structural load paths. Painted and coated surfaces require scuffing and a primer selected for low-surface-energy thermosets.

Residual stress accumulates in monolithic printed sections because the thermal cure exotherm raises internal temperatures before final crosslinking. Sections thicker than 6 mm should be ribbed, hollowed, or subdivided to reduce thermal mass; otherwise post-cure warpage can exceed 0.5 mm across a 100 mm span. The effect is more severe in asymmetric geometries where one side remains thin and the opposite side is thick. Support removal in the green state can also initiate microcracks at layerless interfaces if force is applied rapidly. Published data for this specific configuration is limited, but the failure mode is repeatable on production builds of large blocks and should be included in process risk assessments.

In automotive and industrial automation production cells, RPU 70 appears most frequently in assembly nests, inspection gauge bodies, robotic end-of-arm tooling, and protective covers. At 75 µm pixel resolution on Carbon M-series hardware, a printed fixture with 3 mm nominal wall thickness and ribbed geometry holds positional tolerances of ±0.15 mm after post-cure when the build is conditioned at 23 ± 2 °C and 50 ± 5% RH per ASTM D618-21. Green-state support removal with side cutters is feasible, but sharp impact can initiate subsurface cracks that become visible only after thermal cure. In high-humidity fabrication rooms above 60% RH, open resin trays display viscosity drift over an 8 h shift; the resulting polymerization depth can deviate by more than 50 µm on fine negative features. Dry-air blanketing or sealed cartridge handling is therefore imposed on production lines running features below 0.5 mm.

Green-State Handling, Solvent Wash, and Thermal Post-Cure Requirements

Post-print handling of green RPU 70 begins with a two-stage solvent wash to remove uncured resin from recesses and support interfaces. A dirty-solvent bath dissolves bulk resin, and a clean-solvent rinse reduces residual monomer; dense packing of parts with less than 5 mm spacing causes solvent stagnation and swollen surfaces that later appear as dimensional outliers. Forced-air drying at 25–35 °C for 30–60 min removes surface solvent before the thermal cure cycle. The cure cycle is staged to limit internal stress in thick sections, and under-cured parts show reduced HDT, lower Shore hardness, and increased sensitivity to isopropanol and cutting fluids. Production-scale batch variance is commonly traced to insufficient resin temperature control before printing and incomplete solvent saturation during washing; both factors change the final crosslink density distribution and mechanical response.

Resin temperature at the build surface is a narrow processing window. The manufacturer-recommended tray temperature is 25 °C, and a deviation of ±5 °C produces observable changes in thin-wall thickness because oxygen inhibition in the DLS dead zone is temperature-dependent. Published data for temperature-dependent viscosity of RPU 70 is limited, but production lines compensate by using heated trays and sealed cartridges rather than relying on ambient room conditioning. Solvent compatibility testing per ASTM D543-21 shows that prolonged immersion in concentrated amines, hot glycols, or chlorinated solvents is not acceptable because these fluids attack urethane linkages or plasticize the matrix. Aliphatic hydrocarbon oils and moderate pH aqueous solutions are less aggressive, but part-level exposure tests are required for any production fluid because stress-cracking thresholds depend on molded-in residual stress.

Published data for electrical properties of RPU 70 is limited. The material is not specified as an electrical insulator unless verified for the exact wall thickness and operating frequency. For low-voltage enclosure covers, dielectric strength testing per ASTM D149 and surface resistivity per ASTM D257 are recommended before production assignment. Thermal conductivity is typical of unfilled polyurethane, so heat accumulation in thick sections under motor or light-source exposure should be analyzed. Uncoated parts exposed to ultraviolet light may yellow and develop surface chalking; accelerated weathering testing under ASTM G154 is recommended for outdoor or UV-intensive environments.

When RPU 70 Replaces Elastomeric PU or Epoxy Resins in a DLS Production Queue

Selection between RPU 70 and EPU 40 is determined by Shore hardness and tensile elongation. EPU 40 is an elastomeric polyurethane with elongation at break above 300% and Shore hardness in the A scale, making it suitable for gaskets, seals, and cushioning; RPU 70 cracks when repeatedly strained beyond 15–25% under ASTM D638-14. Conversely, EPU 40 lacks the compressive rigidity required for alignment fixtures and robotic tooling. FPU 50 occupies an intermediate flexible regime and is chosen when fatigue resistance and rebound dominate. Compared with epoxy-based EPX 82, RPU 70 provides lower tensile modulus and lower HDT but higher notched Izod impact under ASTM D256-10. Compared with cyanate ester CE 221, RPU 70 is not suitable for continuous service above 100 °C, but it offers greater ductility and easier machining. In humid environments, RPU 70 absorbs more moisture than many epoxy DLS resins; ASTM D570-22 immersion testing should be performed before assigning uncoated parts to outdoor or condensation-prone locations.

Because RPU 70 is a thermoset polyurethane, certain processing and service boundaries are fixed. The material is not intended for direct food-contact use; 21 CFR 177.2600 compliance would require independent migration testing, and the cured polymer may contain residual monomers that are not cleared for food handling. Steam autoclave cycles above 121 °C exceed the HDT and cause deformation, so autoclaving is not recommended. Hydrogen peroxide vapor may induce surface hazing and dimensional drift. For electrical enclosures, products requiring UL 94 V-0 should not use RPU 70 without an additional flame-retardant formulation or redesign. Strong bases, amine-based accelerators, and hot glycol brake fluids are chemically incompatible with cured parts. In dry, room-temperature mechanical applications, the combination of 70D Shore hardness, 1.5–1.8 GPa tensile modulus, and 30–50 J/m notched Izod impact allows RPU 70 to replace machined acetal, filled nylon, and ABS in short-run fixture programs where DLS geometric freedom is the primary advantage.