Carbon Printers RPU 60 Rigid Polyurethane is a dual-cure resin formulated for Carbon Digital Light Synthesis platforms. In the CLIP process, ultraviolet exposure in the 385 nm band converts the liquid resin into a green part, and subsequent forced-air thermal cure completes the polyurethane network. The material is classified as a rigid polyurethane because post-cured specimens exhibit Shore D hardness above 70 under ASTM D2240-15e1 and tensile elongation at break generally above 100% under ASTM D638-14. It is positioned for impact-loaded housings, brackets, low-pressure ducts, and snap-fit closures where cyclic deflection exceeds the capability of brittle acrylate photopolymers.

RPU 60 is supplied as a single-component liquid prepolymer resin for CLIP-compatible trays and requires closed-loop control of tray temperature and oxygen-permeable window conditions. The oxygen-permeable window maintains an uncured dead zone, typically 20–100 µm, that prevents part adhesion during continuous build motion. If oxygen permeability or irradiance drifts outside the calibrated window, the dead zone thickness changes and z-axis dimensional error appears. On production-scale Carbon M2 and L1 cells, z-axis compensation offsets are applied to control feature deviation below ±0.2 mm for dimensions under 50 mm. Resin bath age and photoinitiator depletion shift polymerization kinetics during long campaigns, so resin-level sensing and wipe cycles are required to limit batch-to-batch variance.

Because RPU 60 contains no fiber fillers, pigment or colorant additions require validation. Light-scattering particulates reduce cure depth and can alter green-part mechanical integrity before the thermal cure step. The uncured resin viscosity is temperature-dependent; elevated tray temperature improves recoating dynamics, but excessively high temperature accelerates photoinitiator consumption and shortens resin bath life. Production cells that operate below the supplier’s recommended tray temperature frequently observe irregular resin layer formation, visible as horizontal banding on large-area surfaces. Such banding is not cosmetic only; it indicates locally reduced monomer conversion and correlates with lower z-axis tensile strength retention in printed specimens.

What Changes During Thermal Post-Cure of RPU 60?

UV exposure in the DLS cell establishes part geometry but leaves the polyurethane network only partially converted. The forced-air oven step, typically conducted at 120°C for 2 h under manufacturer-directed protocols, drives chain extension and crosslinking through reaction of residual isocyanate-functional groups. Oven temperature uniformity should be held within ±5°C of setpoint because under-cure at the low end depresses heat deflection temperature and increases solvent swell, while over-cure at the high end can accelerate oxidative discoloration. For sections thicker than 6 mm, staged ramps or extended dwell are used to limit thermal overshoot; published cycle-time data for very thick configurations remains limited, so production validation is required.

Green-part support removal is performed before the thermal cure step because the uncured network is softer and less prone to brittle fracture. Aggressive support removal after cure can create subsurface microcracks that reduce notched Izod impact by more than 10% relative to properly separated specimens. Cross-section microscopy of under-cured parts shows a gradient in network density from the outer surface to the core; the outer skin can pass hardness checks while the core remains under-converted. This condition is detected by differential scanning calorimetry residual exotherm or by solvent uptake under ASTM D543-21, which exceeds the datasheet range when cure is incomplete.

Relative to RPU 70, RPU 60 occupies a lower-stiffness, higher-elongation region of the Carbon rigid polyurethane series. Representative tensile modulus for RPU 60 is below 1.5 GPa under ASTM D638-14, while RPU 70 is reported above that threshold; the elongation at break of RPU 60 is approximately two times the value typically reported for RPU 70. This distinction directs RPU 60 toward snap-fit covers and impact-loaded brackets that tolerate flexural compliance, whereas RPU 70 is selected where thinner walls must maintain tighter dimensional stability under load. Compared with epoxy-based Carbon grades such as EPX 82, RPU 60 shows lower tensile modulus and lower heat deflection temperature but higher notched Izod impact and reduced brittle failure tendency; EPX 82 is specified for stiff structural components with elevated thermal exposure, where RPU 60 would approach its upper thermal limit.

Impact Resistance, Snap-Fit Insertion Force, and Abrasion in Automotive Housings

Automotive housing programs select RPU 60 for parts that must endure repeated service access and moderate impact. Notched Izod impact under ASTM D256-10e1 is reported above 60 J/m for post-cured specimens, contrasting with unfilled epoxy photopolymers that frequently fall below 30 J/m in equivalent specimen geometries. Snap-fit insertion force is governed by flexural modulus, beam deflection, coefficient of friction, and undercut depth. The lower flexural modulus of RPU 60 reduces peak insertion force for two-arm cantilever latches while retaining latching force through larger allowable undercut deflection. Abrasion resistance measured by Taber or rotating drum mass loss places RPU 60 between elastomeric polyurethane and rigid epoxy grades, making it suitable for shrouds, cable guides, and positioning brackets subject to repeated contact.

On production lines, RPU 60 parts that are ultrasonically welded or adhesively bonded require post-wash surface activation. Residual isopropyl alcohol or uncured monomer can reduce bond strength by more than 20% if not evaporated before bonding. Corona or plasma treatment is generally applied to raise surface energy above 45 mN/m measured by dyne test inks, but the specific treatment window depends on time since post-cure and storage humidity. Assemblers should not use amine-heavy accelerators on RPU 60 joints unless validated, because amine species consume isocyanate groups and shift the network stoichiometry at the bond line.

Immersion testing under ASTM D543-21 at 23°C shows moderate mass uptake in aliphatic hydrocarbon fluids and measurable swell in polar aprotic solvents such as acetone and methyl ethyl ketone; percent mass increase can exceed 10% after 24 h immersion. Continuous water exposure at 60°C initiates hydrolytic degradation of the polyurethane ester segments, and tensile strength retention after 1,000 h is typically below 80%. RPU 60 therefore should not be specified for continuous contact with strong acids or alkalis at pH below 3 or above 11 unless part-level validation is performed. In hydrocarbon splash environments common to automotive powertrains, short-term contact is tolerated, but elastomeric gasket and seal compatibility must be reviewed because plasticizer migration from adjacent rubber can alter RPU 60 surface hardness over time.

When RPU 60 Replaces Glass-Filled Nylon in Low-Pressure Air Ducts

Direct substitution of 30% glass-filled PA66 with DLS-printed RPU 60 has been evaluated for low-pressure air ducts where continuous service temperature remains below the 0.455 MPa heat deflection temperature of RPU 60, typically near 60°C. Glass-filled PA66 exhibits tensile modulus above 6 GPa, so RPU 60 requires increased wall section or rib density to match flange stiffness. The polyurethane network provides isotropic mechanical behavior and eliminates the fiber orientation anisotropy that complicates molded nylon air duct deflection. The DLS route also removes mold tooling cost for low-volume bridge production, but per-part printer time and thermal post-cure capacity limit economic use; when injection molding tooling already exists and annual volumes are high, RPU 60 is not a direct cost substitute.

Design for DLS in duct applications should orient the part so that load-dominated flanges do not rely solely on interlaminar z-axis strength. While published data comparing build orientations for RPU 60 shows limited anisotropy, z-oriented tensile specimens may retain 80–90% of xy strength. Finite-element material cards should therefore use reduced allowables for load paths that cross print layers. Service validation also requires thermal soak testing at the upper intake temperature, because creep deflection in a constrained polyurethane flange under constant load can exceed short-term HDT-based predictions.

Managing Solvent Residue Before Oven Cure

Green RPU 60 parts removed from the DLS build platform are washed in isopropyl alcohol to remove uncured resin, and support structures are separated before thermal cure. If solvent or atmospheric moisture is not removed, the thermal cure step can generate internal porosity because water reacts with isocyanate intermediates and releases carbon dioxide. Production cells apply a pre-drying hold at 40–60°C for 30–60 min before the 120°C cure ramp. Trays should be sealed when idle, and resin exposure to relative humidity above 60% should be limited because the uncured resin is hygroscopic.

Cured RPU 60 water absorption under ISO 62:2008 is generally below 3% at 23°C and 50% RH, a level that affects dimensional tolerance in tight-tolerance snap fits more than it affects bulk mechanical integrity. In humid environments, equilibrium moisture uptake can shift dimensions by 0.1–0.3% linear expansion, depending on section thickness and cure conversion. Assemblies that press-fit metal inserts into RPU 60 should account for this hygroscopic expansion to avoid residual hoop stress that can exceed the material’s tensile strength in very thin bosses. Published data for press-fit insert retention in RPU 60 is limited; validation under expected thermal-humidity cycling is required before release.

Published multiaxial fatigue data for RPU 60 is limited. Component validation under expected duty cycles is required before production release for high-cycle brackets and snap-fit arms. Conservative design practice applies a strength derating of at least 40% from the static tensile value when no component-specific S-N curve is available. The derating is applied to the build direction with the lowest tensile strength, typically the z-axis, and covers the combined effects of interlayer boundary regions and surface roughness.

Regulatory compliance for the uncured resin and cured article must be confirmed against current supplier declarations. The uncured resin contains photoactive and isocyanate-functional components; industrial hygiene controls should follow the safety data sheet, including nitrile glove protection and local exhaust ventilation for oven off-gassing. EU REACH registration under Regulation (EC) No 1907/2006 and RoHS Directive 2011/65/EU conformity should be verified by the supplier for the intended component, particularly for applications sold into electrical and electronic equipment. No food-contact claim is established for RPU 60 unless a separate FDA 21 CFR migration study is completed for the exact curing, washing, and post-cure sequence.

RPU 60 should not be blended with amine-containing additives unless explicitly validated, because amine species consume isocyanate groups and shift network stoichiometry toward incomplete cure. Storage of cured parts in direct sunlight or continuous UV exposure can lead to progressive discoloration and surface embrittlement; UV-stabilized coatings are required for exterior-use validation. The upper service temperature in air is limited by the heat deflection temperature and oxidative stability of the polyurethane backbone; sustained exposure above 80°C is application-specific and requires mechanical property retention testing after heat ageing under ASTM D3045-18 or equivalent oxidation protocols.