Carbon Printers SIL 30 Silicone Urethane Elastomer is a liquid photopolymerizable resin formulated from silicone and urethane precursors for Carbon Digital Light Synthesis additive manufacturing. The material is supplied as a 100% solids photopolymer and is processed in layerless DLS builds, where patterned light exposure creates a green-state part and a subsequent forced-air thermal post-cure completes network formation. The 30 designation corresponds to a nominal Shore A hardness of 30 after full cure when measured according to ASTM D2240. The resin is specified for compressible elastomer components requiring low hardness, high elongation, tear resistance, and skin-contact documentation. Typical part categories include wearable device cushions, earbud tips, medical device housing interfaces, gaskets, respiratory mask seals, and soft-touch industrial covers. Mechanical property values are lot- and process-dependent and should be obtained from the current material datasheet, production-representative test coupons, and first-article qualification reports.

Silicone Urethane Backbone Chemistry and Dual-Cure Network Formation

The uncured material is a homogeneous liquid blend of silicone-rich and urethane-rich domains. During DLS operation, the oxygen-permeable optical window creates a continuous dead zone at the resin–window interface. Dissolved oxygen inhibits polymerization within this zone, allowing resin reflow beneath the forming part while patterned light exposure triggers radical photopolymerization in the adjacent layer. The cured green-state part remains chemically underdeveloped and must not be placed into service. Its mechanical integrity is sufficient for removal from the build platform and solvent washing, but terminal hardness, tensile resistance, tear strength, and compression-set behaviour do not appear until the thermal post-cure has been completed.

Dual-cure architecture introduces a process boundary. Residual unreacted monomers and low-molecular-weight oligomers are removed or reduced by the solvent wash, but final property development depends on oven temperature uniformity, air turnover, part loading, and soak time. An under-cured SIL 30 part exhibits reduced tensile stress, elevated compression set, increased extractable mass, and higher surface tack after ageing. Processors should record oven set point, measured air temperature at product-level thermocouples, load mass, and part location for each cure cycle. Batch logs that omit these variables are not sufficient for medical or high-reliability production.

The hybrid network combines a low-modulus silicone-like tactile response with the processability of a urethane. In contrast to condensation-cure or platinum-catalyzed room-temperature-vulcanizing silicones, SIL 30 does not require a mold dwell and can be built directly on DLS platforms with undercuts, internal channels, and lattice structures that would require multi-piece tooling in conventional silicone molding. The silicone-rich domains contribute softness and a low-tack surface after complete cure, while the urethane domains provide tear initiation resistance and dimensional stability during solvent washing and downstream handling.

What Distinguishes SIL 30 from EPU 40 and FPU 50 in Production?

The principal comparative boundary is polymer class. SIL 30 is a silicone urethane elastomer, whereas EPU 40 and FPU 50 are urethane-only systems. This distinction controls solvent uptake, thermal-mechanical creep, tactile character, moisture absorption, and the documentation package available for skin-contact devices. Table 1 summarises product-positioning data reported in Carbon material literature.

The silicone-rich domain in SIL 30 reduces hardness below the EPU 40 envelope and provides surface character closer to a silicone elastomer. The same domain reduces plateau stress at high elongation, making SIL 30 appropriate for strain-dominant parts that must conform to irregular body contours rather than support high mechanical load. EPU 40 and FPU 50 generally provide higher load-bearing capacity but are not positioned with the same skin-contact documentation set. Material substitution should not be made on Shore A hardness alone because wash protocol, thermal post-cure schedule, secondary machining behaviour, and compatibility with downstream adhesives differ among the three systems.

Following the build, parts are removed from the platform and washed in a Carbon Smart Part Washer or equivalent solvent-based equipment to remove uncured resin from enclosed channels, lattices, and reentrant features. Green-state tear during the wash cycle is a primary failure mode observed in production. The risk increases when wall thickness is below 1.0 mm and when high-aspect-ratio channels are oriented parallel to solvent drainage. Small unsupported flaps and sealing lips may fold during agitated washing; these features should be oriented to avoid direct impingement from solvent jets or supported with temporary ribs that are removed after cure if the geometry permits.

The wash solvent is not only a cleaning agent but also an extraction medium. Low-molecular-weight silicone-rich species are extracted at different rates depending on solvent polarity, solvent temperature, and exposure time. An overly aggressive or extended wash can micro-texture the surface and shift post-cure Shore A hardness upward. An insufficient wash leaves residual monomer on the surface, which can form a brittle skin during thermal cure and reduce elongation at break. Production-scale processors often establish wash time by tracking mass loss from test coupons and comparing the loss against the supplier’s recommended mass-loss band. Solvent saturation in the wash bath must be monitored because resin-laden solvent leaves an invisible film that becomes tacky after oven cure and compromises subsequent coating, printing, or adhesive bonding.

After washing, parts are dried to remove residual solvent and then thermally post-cured in a forced-air oven. The post-cure profile is material-specific and must be matched to the DLS equipment generation and part cross-section. Convection uniformity is critical. Ovens with poor airflow produce cold zones that leave under-cured regions in thick sections or tightly packed builds. Under-cured regions show local Shore hardness depression and elevated compression set. Batch-to-batch moisture ingress in the resin container is another known handling issue. The material should be stored in sealed containers and brought to ambient temperature before pouring to avoid condensation on cold resin surfaces. Entrained water during mixing produces microvoids and shifts the thermal cure profile, particularly in thick parts. If bubbles persist after a standard stir and rest cycle, the resin lot should be tested for viscosity against the supplier’s acceptance range before use.

When Shore 30A and Tear Resistance Dictate the Geometry of Wearable Seals

Elastomeric parts specified for skin-contact wearables require more than a nominal Shore 30A hardness. Compression set, tear initiation at stress risers, and cyclical strain recovery determine service life. SIL 30 parts should be evaluated according to ASTM D412 for tensile stress and elongation at break, ASTM D624 for tear strength, ASTM D395 for compression set, and ASTM D2240 for hardness. Test specimens should be built in the same orientation, wash protocol, and oven load configuration as production parts. Specimen-level values from unsupported XY flats often overstate mechanical properties relative to parts with vertical walls, internal cavities, or pronounced z-axis build sections.

DLS elastomers exhibit orientation-dependent tear behaviour. A sealing lip built in the XY plane may show different tear initiation than the same lip built at a 45 degree angle to the build plane. This anisotropy arises from residual stress gradients created during the continuous build and subsequent thermal cure. Tear coupons should therefore be built in the same orientation as the production sealing lip, not in a flat XY configuration. When sealing flanges are used, compression set results must state test time and temperature. A value reported without ASTM D395 conditions is not a valid comparator. Shortened solvent exposure can leave plasticizing residue that temporarily improves flexibility but degrades long-term recovery and increases compression set after repeated loading.

For wearable seals, internal channel diameters below 0.5 mm are printable, but the wash and post-cure steps must force solvent and air sufficiently through the part. Accumulation of uncured resin or wash solvent in blind channels creates outgassing, local hardness depression, and residual volatiles that may irritate skin or interfere with adhesive bonding. If the seal contains a blind retention groove, the design should include a drainage or vent path positioned at the lowest point of the build orientation. Sharp inside radii below 0.25 mm should be reviewed because solvent diffusion and residual stress can initiate edge cracking during post-cure.

Biocompatibility Verification and Post-Cure Accountability

Carbon has published material-level biocompatibility data for SIL 30 covering cytotoxicity under ISO 10993-5 and skin irritation and sensitization under ISO 10993-10. These results apply to the fully post-cured resin. Green or partially post-cured parts must not be represented as equivalent. Finished-device biocompatibility remains the responsibility of the device manufacturer under ISO 10993-1. Material-level data do not account for assembly adhesives, surface treatments, sterilization residues, or later manufacturing contaminants introduced downstream.

When a medical or skin-contact application is planned, the post-cure oven log should be retained as part of the device history record. Missing oven logs are a common audit non-conformance for DLS elastomer lots and can invalidate subsequent mechanical or biocompatibility batch certification. The log should include timestamp, oven set point, air temperature measured at product-level thermocouples, load mass, part location, and the operator identifier.

In assembly environments, cured SIL 30 parts should not be exposed to amine-containing primers or tin-catalyzed condensation-cure silicones without compatibility testing. These agents can alter surface energy, plasticize the near-surface network, or initiate unintended crosslinking at the bond line. When bonding to rigid substrates, surface activation by plasma or primer should be followed by lap-shear testing under ASTM D3163 or a comparable adhesive-standard method after conditioning at the maximum service temperature. If the application requires repeated steam sterilization, published data for this specific configuration is limited; the processor should verify Shore hardness, tensile strength, elongation, and compression set after each anticipated sterilisation cycle rather than relying on ambient bench properties.