ETEC (EnvisionTEC) E-RigidForm PU 77 is a rigid polyurethane-like photopolymer formulated for 405 nm continuous digital light projection systems in the ETEC Envision One and Xtreme 8K equipment families. The designation PU 77 denotes a nominal post-cured hardness of 77 Shore D under ASTM D2240-15. The material is supplied as a black, medium-viscosity liquid based on urethane acrylate oligomers and monofunctional reactive diluents. It cures by free-radical photopolymerization under mask-projected UV irradiation and is used for rigid functional prototypes, short-run jigs, fixtures, housings, and connectors. The urethane-bearing backbone differentiates the product from epoxide-functional cDLP resins that rely on cationic ring-opening polymerization; the polyurethane character provides a balance of stiffness and machinability but does not replicate the high elongation of thermoplastic polyurethane grades.

Mechanical property envelope under ASTM D638-14 conditioning

Representative mechanical data are generated from specimens printed at 50 µm z-resolution and post-cured in a 385–405 nm UV/VIS chamber until the manufacturer-defined dosage is reached. Values are conditioned at 23 °C and 50 % RH for at least 24 h before testing. The table below consolidates manufacturer-reported typical values; these values are not design minimums and do not replace lot-specific acceptance tests or end-use part validation.

Post-cure conversion of residual acrylate unsaturation raises crosslink density and shifts the material toward brittle fracture. Thin walls below 2 mm exhibit higher apparent tensile strength but lower elongation because of size effects and surface curing gradients. Detailed fracture toughness data for the exact formulation are not widely published; designs with snap-fit engagement or living hinges should be validated under ASTM D5045-14 or ISO 13586:2018 using printed or machined notches rather than relying on un-notched tensile data alone.

What limits the continuous DLP processing window for E-RigidForm PU 77?

The black pigmentation reduces working-curve depth of penetration compared with clear urethane resins. The Jacobs working-curve parameters, critical exposure energy and depth of penetration, are part-specific constants calibrated on each ultraviolet light engine because irradiance varies from 2 mW/cm² to 15 mW/cm² across cDLP platforms. The filled formulation also raises viscosity and can retard leveling after blade or recirculated-meniscus recoating. Vat temperature control is therefore a process variable rather than a peripheral setting; a drop below the recommended plateau increases viscosity and produces incomplete layer leveling, while an elevated vat temperature accelerates dark polymerization and progressively increases viscosity over the build.

On bottom-up membrane-based cDLP systems, the green part is attached to the build head and separated from the oxygen-permeable membrane after each exposure cycle. Excessive exposure leads to strong adhesion to the membrane and high separation force; insufficient exposure produces soft green layers that distort during separation. Because the oxygen inhibition layer is inversely related to irradiance, continuous printing modes require the oxygen-permeable film to maintain a stable dissolved-oxygen concentration. Accumulated carbon black pigment, suspended by recirculation, can abrade the membrane; filter clogging is a reported failure mode on long unattended builds. Published membrane lifetime data for this specific resin configuration are limited.

Supports printed with E-RigidForm PU 77 are stiff and tend to fracture at contact points rather than yield during removal. Closed-tip punctual contacts with neck diameters of 0.4 mm to 0.6 mm are therefore used on rigid polyurethane-like cDLP systems to balance release force and surface damage. This practice is not unique to the formulation but follows the brittle green-state behavior reflected in ASTM D638-14 tensile tests.

When E-RigidForm PU 77 replaces injection-molded PU connectors

The substitution is most applicable to low-run connectors, harness clips, and enclosure hardware where injection tooling delivery time exceeds the production window. The thermoset network of urethane acrylate crosslinks does not exhibit the melt-reprocessing behavior of thermoplastic polyurethane. It also does not offer the high recovery elongation or tear propagation resistance of injection-molded TPU grades; elongation at break in manufacturer data is below 10 %. Dimensional accuracy in the build plane is limited by pixel discretization and resin shrinkage during post-cure, while z-axis accuracy depends on layer thickness and post-cure exposure. Critical pin spacing and snap-fit deflection should therefore be measured on printed parts under ISO 527-2:2012 and ASTM D256-10 rather than inferred from CAD dimensions.

Chemical exposure data for the specific PU 77 formulation are not fully published. General polyurethane-like photopolymers are often compatible with aliphatic hydrocarbons and dilute acid solutions in short-term contact but are not automatically suitable for ketone, chlorinated solvent, or ester-based immersion. Compatibility with service fluids should be tested under ASTM D543-21 or ISO 175:2010. Amine-based additives, strong oxidizers, and chlorinated solvents should not be introduced into the vat without compatibility testing because they can accelerate decomposition or inhibit free-radical polymerization.

For design engineers, the principal difference between PU 77 and clear rigid epoxy acrylate cDLP resins is ultraviolet penetration depth. The black filler in PU 77 reduces light penetration, limiting maximum reliable layer thickness and requiring higher exposure per layer. Clear rigid acrylate formulations can be printed thicker but often exhibit more brittle green-state handling and lower notched impact resistance. Published comparative process-response data across material families are limited; equivalency must be established on the target equipment under identical orientation and post-cure conditions.

Unopened containers should be stored between 5 °C and 30 °C. Before use, the resin should recirculate or be stirred with a low-shear mixer to redisperse pigment. High-shear mixing introduces air entrainment that creates microvoids during exposure. Open-vat systems require monitoring of relative humidity; water uptake above 60 % RH can alter the oxygen inhibition balance and compromise the recoating film. Direct contact with nitrile gloves is preferable because latex can introduce thiol-based contaminants that suppress free-radical polymerization.

Post-processing includes an initial solvent wash using isopropanol or a dedicated tripropylene glycol monomethyl ether-based wash, followed by UV/VIS post-cure. Undercuring leaves residual acrylate deformability; overcuring can cause embrittlement and surface oxidation. The required post-cure dose is not universally fixed and must be calibrated against part mass, section thickness, and the specific UV chamber irradiance. Published data for this exact resin in production-scale post-cure tunnels are limited.

In continuous digital light processing applications, the resin is typically validated on a printer-specific basis. Calibration builds are run first to determine exposure settings within the variable irradiance envelope, then tensile bars are printed in multiple orientations to characterize anisotropic response. The material is not recommended for applications requiring long-term outdoor weathering, hydrolytic stability, or significant elastic recovery without additional validation under ASTM D638-14, ASTM D570-98, and ISO 4892-3:2016. Published field data for the exact product are limited across many end-use environments; qualification remains equipment-specific rather than generic.