Carbon Printers FPU 50 Flexible Polyurethane is a dual-cure polyurethane-based photopolymer supplied for Carbon Digital Light Synthesis systems, including Carbon M2, M3, and L1 printer configurations validated for flexible polyurethane service. The liquid resin is shaped by UV-initiated crosslinking through the printer’s oxygen-permeable optical window, then thermally post-cured to complete urethane network development. The suffix 50 is a portfolio identifier, not a direct Shore A durometer value. Supplier-reported typical mechanical values for fully cured FPU 50 include a Shore A hardness of 72 per ASTM D2240, ultimate tensile strength of 32 MPa per ASTM D638, elongation at break of 250%, tear strength of 74 kN/m per ASTM D624, and compression set of 20% after 22 h at 70 °C per ASTM D395. The material is used for flexible dust boots, robotic gripper pads, gasketed enclosure seals, cable strain reliefs, vibration isolators, bellows, and living hinges in low- to mid-volume manufacturing where cyclic flexure and cut-growth resistance are required.

What Distinguishes FPU 50 from EPU 40 and RPU 70 in the Carbon Polyurethane Portfolio?

The difference is not a simple hardness shift. FPU 50 increases tensile strength relative to EPU 40 by a factor of approximately 4.2 and tear strength by a factor of approximately 3.4, while elongation falls from 330% to 250%. EPU 40 remains the appropriate grade for high-rebound cushioning and soft-touch applications because it can accommodate greater strain without fracture. FPU 50 is specified where a clamped seal or flexing web experiences high local stress at edge details; the higher tear strength delays crack initiation. Against RPU 70, FPU 50 trades tensile strength and dimensional stiffness for low hardness and high elongation. RPU 70 is used for structural housings, brackets, and fixtures requiring tight geometric retention under load. FPU 50 is not a rigid engineering polymer and should not be used for load-bearing structural elements unless wall thickness and deflection limits are explicitly validated.

The following methods are typical for FPU 50 characterization on production parts and test coupons.

In production on an M2-class Digital Light Synthesis system, FPU 50 is delivered as a reactive liquid resin with controlled viscosity. The build basin is maintained at printer-controlled temperature because resin reactivity and oxygen inhibition are temperature-dependent. The continuous liquid interface process suppresses layerwise topography by maintaining a polymerization-inhibited zone at the optical window. Green parts are removed with residual uncured resin film and transferred to a supplier-approved solvent wash station. Reagent-grade isopropyl alcohol is commonly used, but alternative wash solvents may be specified for higher-throughput operations. Solvent immersion time must be limited: thin sections below 2 mm can swell during extended washing, and the resulting dimensional offset may exceed downstream assembly tolerances. After washing, parts are dried with filtered compressed air until residual solvent is no longer visible on surfaces.

Thermal post-cure is not optional. FPU 50 is a dual-cure resin in which the UV step produces a green-state network and the thermal step drives the remaining urethane crosslinking. Under-cured parts exhibit lower tensile strength, higher compression set, and reduced solvent resistance. The post-cure oven should be a forced-air or inert-gas convection oven with calibrated temperature mapping; the specific ramp rate, hold temperature, and part spacing are given in the current FPU 50 technical data sheet. Production lines using multiple oven zones have observed batch-to-batch variance in compression set when thermocouple placement does not match the validation load. Operators should log resin bath age, ambient humidity, and wash-solvent condition because aged resin can increase viscosity and reduce green-part dimensional accuracy. Viscosity drift can be monitored per ASTM D2196; a rising trend above the supplier specification is a process warning for surface defects and incomplete cure at constant exposure settings.

Support removal for FPU 50 differs from rigid photopolymers because the material is elastomeric at room temperature. Cutting with metallic blades can propagate nicks at support interfaces; abrasion or cryogenic deflashing may be required for parts with large support contact areas. Parts with internal channels or trapped volumes should be washed and dried with the channels oriented to avoid solvent pooling. Printing orientation is selected to place support marks away from sealing surfaces and flexural hinges. Build failures on production equipment are most often traced to insufficient oxygen permeation, degraded optical-window membranes, or contaminated resin rather than to the material itself.

Orientation dependence arises from the continuous build direction. Green parts may exhibit different tensile elongation in the Z-axis because of polymer network alignment during the build pull. Manufacturer datasheets commonly report XY-oriented values. If a gasket or diaphragm is built vertically, the sealing faces should be tested for compression set and tear at the actual build angle. Downward-facing surfaces can develop microgrooves related to oxygen-dead-zone thickness rather than layer lines, and sealing-surface roughness should be measured per ISO 4287 when leak rate is critical. Minimum wall thickness for self-supporting features should be determined by build trials, not by the printer’s nominal voxel size, because elastomeric thin walls can curl during washing and drying.

Solvent Resistance and Environmental Boundary Conditions

FPU 50 displays polyurethane-typical behavior in service fluids. Dilute aqueous acids and non-polar aliphatic oils generally produce limited property change, while strong polar solvents such as methyl ethyl ketone, acetone, and ester-based plasticizers cause swelling, Shore A reduction, and tensile-strength loss. Chemical compatibility should be tested under ASTM D543 or project-specific immersion conditions rather than inferred from hardness alone. Published data for this specific configuration is limited for prolonged exposure to hot hydraulic fluids and biofuels; qualification programs should include mass uptake, hardness retention, and tensile retention after immersion at the upper service temperature.

Thermal service is bounded by compression-set behavior and oxidative stiffening. The supplier-reported compression set value is generated at 70 °C; continuous operation above this temperature can increase permanent set and reduce sealing force in gasketed joints. Creep under load should be measured per ASTM D2990 when the application involves sustained compression. UV exposure can yellow and embrittle polyurethane surfaces; outdoor components should be coated with an opaque UV-stabilized topcoat or tested under ASTM G154 for weathering. The uncured resin and cured part are subject to regulatory documentation under REACH and the RoHS Directive 2011/65/EU; compliance must be confirmed by the supplier declaration for the specific homogeneous material. No food-contact clearance under 21 CFR 177 or medical skin-contact certification should be assumed without written supplier documentation and end-use validation.

Large-format gaskets and bellows are commonly built flat on the build platform to maximize XY dimensional accuracy and reduce support artifacts. Thin gasket walls below 1 mm can be produced, but tolerance control depends on part geometry, wash solvent, and post-cure shrinkage. For snap-fit covers and living hinges, the part design should place the hinge axis parallel to the build platform and maintain a minimum hinge radius sufficient to avoid tearing at the outer fiber; a hinge thickness of 0.8 mm to 1.5 mm is a typical starting range for flexible polyurethane, but dynamic flex fatigue should be confirmed by cycling the actual part. Vibration isolation mounts benefit from Shore A 72 hardness and high tear strength, but loaded resonant response must be measured with the specific durometer and temperature.

When FPU 50 Replaces Cast Urethane or Injection-Molded Thermoplastic Polyurethane

The primary process advantage is tooling elimination. Cast urethane requires a master pattern and mold, while injection-molded TPU requires steel or aluminum tooling and melt-processing pressures often above 50 MPa. Digital Light Synthesis builds directly from a digital model and can produce complex internal channels and undercuts that would require multi-piece molds. The trade-offs are material and economic. FPU 50 is a thermoset after post-cure; it cannot be melt-reprocessed, welded, or thermally re-formed. Threaded inserts and snap features must use mechanical anchoring, adhesive bonding, or printed retention geometry. Compared with melt-processed TPU of similar Shore A hardness, FPU 50 may exhibit lower elongation and similar or higher tear strength, but the exact difference is supplier- and formulation-specific. Published data for direct FPU 50-to-TPU substitution in dynamic flex applications is limited, so the replacement should be validated by part-level fatigue trials.

Throughput is bounded by build area, batch washing, and oven capacity. A Carbon M2-class system may offer a build volume of approximately 190 mm × 118 mm × 326 mm; the usable packing density for FPU 50 components depends on support design and spacing for solvent drainage. In contrast, injection molding after tooling can produce parts quickly but is economically suited to high volumes. FPU 50 is selected for bridge production, service parts, or low-volume replacement parts where cast urethane tooling is cost-prohibitive and injection molding lead time is excessive. Batch records should retain resin lot number, printer serial number, wash-solvent lot, post-cure oven cycle, and operator identification for traceability under ISO 13485 or IATF 16949 where applicable.

For production acceptance, cured plaques or tensile bars are sampled from each build or batch and tested for Shore A hardness, tensile strength, and elongation. Thermoplastic elastomer test methods such as ISO 37 or ASTM D412 may be applied when the part is rubber-like, but FPU 50 datasheets commonly use plastic test method ASTM D638; the two standards are not interchangeable. Dynamic fatigue and cut-growth resistance may be screened with ASTM D813 or customer-specific cycling. Published data for long-term dynamic fatigue of FPU 50 in combined salt-fog and flexural loading is limited; bellows and diaphragm applications should be qualified by periodic tear-strength retention and hardness measurements after accelerated aging.