In automotive HVAC duct sealing prototypes, VisiJet SL Flex is evaluated as a substitute for injection-moulded sealing geometry in flange gaskets, connector boots, and low-pressure air damper collars. The resin is processed on stereolithography platforms with 355 nm solid-state lasers or 405 nm LED light engines; print parameters should remain within the current 3D Systems VisiJet SL Flex build profile. Open parameter edits outside the qualified envelope produce green parts with inconsistent crosslink gradients and tear behaviour. After build completion, the parts are washed in a two-stage tripropylene glycol monomethyl ether or isopropanol protocol. Prolonged immersion in isopropanol beyond the supplier-specified window causes edge swelling and dimensional drift because the low-crosslink-density urethane acrylate network absorbs polar solvent. Forced-air drying at 23 °C to 30 °C for 30 min precedes UV post-curing in a 405 nm LED flood chamber with rotating turntable. Incomplete post-curing leaves surface tack and a Shore A reading that drifts downward over 7 days at 23 °C. Hardness is measured to ASTM D2240-15 on specimens at least 6.4 mm thick using a 1-second durometer reading. Public datasheets report Shore A values typical of soft elastomeric photopolymers; current lot-specific certificates should be checked because photoinitiator batch variation shifts final crosslink density.

For flange sealing evaluation, initial compressive strain is generally limited to 15–25% for short-term low-pressure air or water circuits. Higher compression on sharp sealing beads promotes crescent-shaped tearing near the bead root after thermal cycling. Compression set is not consistently published for VisiJet SL Flex under ASTM D395-16e2 Method B; production-seal design therefore requires internal testing on printed coupons. Build orientation influences the failure mode: sealing flanges printed with the flange plane parallel to the build platform may develop interlayer shear cracks at the sealing edge. Orienting the flange plane perpendicular to the build platform moves the weak plane away from the compressed region. Validation on production-scale stereolithography equipment has shown that support tip penetration into sealing beads creates local stress risers; support tips should be placed away from the bead crown. The resin is not rated for continuous contact with automotive fuels, brake fluid, or high-pressure hydraulic oil. Compliance documentation should be verified against EC 1907/2006 and 2011/65/EU Annex II for EU-bound parts; SDS section 15 lists current regulatory status.

What Limits Shore Hardness Retention in Wearable Electronic Housings?

Wearable device housing components are subjected to cyclic flexure, sebum exposure, and repetitive snap engagement. VisiJet SL Flex is used for external shell components, decorative band segments, and gasket-like compression ribs that are not in direct mucosal or open-wound contact. Thin walls below 2 mm cure faster at the surface but may retain lower crosslink density at the print-layer interface. Flexural modulus is screened to ISO 178:2019 at 2 mm/min, although flexible photopolymer parts often fall outside the linear strain range used for rigid plastics. Cyclic flex testing is more relevant for living-hinge sections and snap arms. Test conditions should record force drop after 5,000 or 10,000 cycles at 23 °C and 45 °C. Published data for this specific configuration is limited; lot-specific screening is mandatory before build approval. Environmental aging can be run under IEC 60068-2-78:2012 damp heat at 85 °C/85% RH for 168 h, followed by Shore A measurement to ASTM D2240-15. Under-cured parts typically show hardness drop and surface tack after damp heat exposure because residual unreacted acrylate groups plasticize the network. On stereolithography platforms with 50 µm layers, field failure mode in flexural screening is edge delamination near support tips. Reducing support tip penetration or changing orientation moves the failure plane away from tensile stress concentration. VisiJet SL Flex is not classified as biocompatible under ISO 10993-1:2018 unless validated for the exact printed and post-cured part configuration. If skin contact is intended, a secondary skin-safe elastomer barrier or biocompatible coating is required.

For consumer electronics packaging where rigid ABS and flexible TPU regions meet, snap-fit undercuts and living hinges are prototyped with VisiJet SL Flex before rigid-to-soft overmould tooling is cut. The resin provides tensile elongation required to simulate low-cycle soft TPU behaviour. Tensile properties are measured to ASTM D638-14 using a Type IV specimen at 50 mm/min crosshead speed; current supplier datasheets should be consulted for conditioning protocol. Living hinges thinner than 0.8 mm are at higher risk of surface cracking after repeated open-close flexure; published cycle life is limited, so in-house fatigue screening should be performed before design sign-off. The material is not a replacement for injection-moulded polypropylene or TPU in production hinges. For early ergonomic studies, printed parts are trimmed with end mills designed for soft polymers; burr formation is lower when parts are cooled to 5–10 °C before trimming. Support removal on snap-fit features is performed with flush cutters, then surfaces are sanded at 320–400 grit. Sanding dust from UV-cured acrylate is controlled through local exhaust ventilation as described in the SDS section 7. The printed snap-fit geometry is compared against injection-moulded ABS substrates in assembly force tests on a motorised test stand with a load cell calibrated to ISO 7500-1:2018. For undercut depths above 1.5 mm, insertion force values are strongly influenced by print orientation; only comparative A-B screening is meaningful, not absolute force prediction.

When Flexible Photopolymer Replaces Cast TPU in Short-Run Interior Trim Fixtures

When VisiJet SL Flex replaces cast TPU for initial haptics and fit validation, the printed master must be post-cured to full hardness before silicone moulding. Residual surface tack after primary post-cure causes RTV silicone cure inhibition at the mould interface; the failure presents as a gummy film in the cavity after demoulding. To avoid this, post-cure in a 405 nm LED chamber until surface tack is eliminated, typically after supplier-recommended durations. Haptic panels are tested with a Shore A durometer per ASTM D2240-15. Scratch and mar resistance is not equivalent to production TPU/TPO; the low-crosslink-density surface shows visible whitening after 5–10 scratch strokes on textured areas. A Taber scratch test or ISO 1518-1:2019 can be used for comparative screening, but data spread is high across print orientations. For short-run vacuum-cast polyurethane parts, the VisiJet SL Flex master can be moulded once or twice before dimensional degradation; silicone rubber tools typically record 0.2–0.5% shrinkage after first cure. Continuous service temperature is a boundary condition: supplier heat deflection data for flexible SLA resins should be reviewed. The part may soften and mark interior trim surfaces if left in a closed vehicle cabin above 60 °C for extended periods. Chemical resistance to diluted isopropanol and mild surfactant solutions is acceptable for limited contact, but prolonged contact with undiluted solvent-based trim cleaners will swell the surface. Published data for fogging per DIN 75201:2011 are limited; this resin is not intended for final in-cabin parts without additional material qualification.

Relevant test methods and compliance boundaries are consolidated in the matrix below.

Overmolding Insert Stability Is Governed by Tear Propagation

Elastomeric overmolding prototypes for industrial hand tool grips and instrument bezels use VisiJet SL Flex as the low-volume surrogate for cast TPU or silicone. The insert stability in a subsequent vacuum-cast polyurethane overmold is governed by tear propagation at the interface and at surface notches. Tear strength is screened to ASTM D624-00(2020) Die C. Printing orientation has a first-order effect: parts built with crack propagation parallel to print-layer interfaces typically exhibit lower tear resistance. The magnitude should be quantified on the actual build platform; published data for this specific configuration is limited. In vacuum casting, the 3D-printed core is placed in an RTV silicone mould. A two-component polyurethane with Shore A 60–90 hardness is then poured around the core. Exotherm during polyurethane cure raises insert surface temperature. If the mould cavity exceeds 65 °C, the VisiJet SL Flex insert may soften and deform under clamping pressure. Mould trials should include thermocouples at the cavity wall to log peak exotherm. Release agents containing amine-functional moisture scavengers should be avoided because amine residues can cause surface hazing and inhibit future UV post-cure repair. After overmoulding, peel adhesion between the printed insert and cast polyurethane is tested with a 90° peel fixture under constant crosshead speed; data are reported with both force and failure mode. The resin is not suitable as a direct insert in thermoplastic injection moulding where melt temperatures exceed 180 °C; heat distortion and surface decomposition occur at the interface.

Lattice Midsole Compression and Recovery Screening

In footwear development, midsole lattice prototypes are printed to evaluate cell geometry, wall thickness, and compression response before EVA or TPU foam tooling is approved. Compression testing uses a flat platen press and force-displacement recording at 10 mm/min; repeated loading-unloading cycles to 50% peak compressive strain are used for comparative screening. Permanent set after 5 cycles is recorded after a 30 min rest period and compared to control foam coupons. ASTM F1614-99(2018) provides a framework for athletic footwear cushioning, but applicability to lattice photopolymer parts depends on thickness and cell fill. Lattice cell sizes of 1–2 mm are common for prototyping; support removal from small internal channels is difficult, and residual fragments act as crack initiation sites during compression. Closed-cell variants can entrap moisture during washing; parts should be blow-dried with oil-free compressed air before post-cure. VisiJet SL Flex is not a direct substitute for production midsole foam because tensile fatigue, density, and recovery behaviour differ. Published data for footwear-specific cyclic testing are limited; current use is confined to geometry validation, assembly checks, and design review models.