Somos ProtoGen™ 18120 is a single-component epoxy-based photopolymer formulated for stereolithography systems equipped with 355 nm solid-state lasers. The product is supplied as a low-viscosity liquid resin that forms a green part during laser exposure and achieves final thermomechanical properties only after a combined UV and thermal postcure sequence. Manufacturer-published liquid-state data place the density at approximately 1.16 g/cm³ at 25 °C and the viscosity in the range of 250–300 mPa·s at 30 °C. Because the formulation is epoxy-based, the polymer network develops through cationic polymerization during laser exposure and continues to crosslink during UV flood exposure and thermal annealing. The material is intended for stereolithography builds that require elevated-temperature stiffness, low moisture uptake, and dimensional stability in functionally loaded prototypes, tooling, and short-run components.

The model designation ProtoGen 18120 identifies this specific epoxy chemistry in the Somos resin portfolio. The resin is not a general-purpose acrylate photopolymer; its postcure protocol and handling conditions differ from those of acrylate-based resins, particularly with respect to thermal cure demand and moisture sensitivity prior to cure.

What Processing Parameters Govern the Use of ProtoGen 18120 in Stereolithography?

The liquid resin is maintained at an elevated vat temperature, commonly 28–32 °C. At 30 °C the viscosity permits recoating blades to level layers without excessive carry-over. If the vat temperature falls below 27 °C, the increase in viscosity can produce inconsistent layer thickness; if it exceeds 35 °C, the dark stability of the resin may be affected. Build parameters are matched to a 355 nm solid-state laser. Typical layer thicknesses are 50 μm or 100 μm, although thicker layers up to 150 μm may be used for parts with large flat cross-sections. The required exposure is determined by critical energy and penetration depth; the epoxy formulation exhibits a moderate penetration depth that supports fine detail when layer thickness is held at or below 100 μm. Parts with thin walls and fine features require reduced layer thickness and lower recoating speed.

After the build, the green part is removed from the platform and washed in a two-stage solvent bath. Common wash solvents for epoxy stereolithography resins are tripropylene glycol monomethyl ether and isopropanol. The first stage removes bulk liquid resin, and the second stage removes residual film from fine recesses. Residual solvent must be evaporated before UV postcure because solvent plasticizes the polymer and reduces conversion. On production-scale platforms with dual-edge recoating, 100 μm layer builds of thin-wall housings are reported to be acceptable when support tips are set to lower exposure values.

Green-state strength in epoxy stereolithography resins is lower than final cured strength. Support structures must be placed to resist peel forces during recoating layers. Feature sizes below 0.5 mm may require reduced laser exposure and lower recoating speed to prevent deformation of the green lip. On platforms using a wiper blade, parts with large flat cross-sections oriented parallel to the resin surface generate higher peel forces than angled or vertically oriented parts. In cationic epoxy photopolymerization, initiation is produced by photocationic acid generation from an initiator system; propagation occurs during laser exposure and continues in the dark after illumination ceases. The dark-cure component is significant for green-strength development but also places an upper limit on vat residence time at elevated temperature. The viscosity of the resin is near-Newtonian under typical recoating shear rates; this reduces layer-to-layer thickness variation when blade speed is changed. Critical exposure and penetration depth values are specific to laser spot size and layer thickness; they are supplied in machine-specific build parameter files rather than as universal constants.

After postcure, the cured resin is characterized by a tensile modulus and heat deflection temperature that distinguish it from unfilled acrylate stereolithography resins. The following typical values are derived from manufacturer-published data and apply to fully postcured specimens; they are not specification minima.

The cured network is a stiff epoxy thermoset with limited elongation. The combination of 105 °C heat deflection temperature at 1.82 MPa and low water uptake makes it suitable for short-duration thermal exposure in fasteners, connectors, and underhood prototype brackets. The values are generated on specimens that are built, washed, UV postcured, and thermally postcured according to manufacturer-specified procedures. Variation in postcure dose, oven temperature uniformity, and build orientation shifts these values.

When Thermal Postcure Is Shortened, Residual Conversion and Dimensional Stability Are Affected

Green-state parts contain unreacted epoxide groups and are relatively brittle and solvent-sensitive. If thermal postcure is omitted, the final network conversion remains below the designed value. The glass transition temperature is depressed, and the part may exhibit higher creep under sustained load and greater moisture sorption. The recommended postcure sequence involves a UV flood chamber that exposes all surfaces to UV radiation, typically in the 320–400 nm range, followed by a forced-air or convection oven thermal step.

Because UV dose distribution varies with chamber lamp configuration, parts should be rotated and shadowed surfaces re-exposed. The thermal step is commonly performed at 80–120 °C for durations of 1–3 h, depending on part mass and wall thickness. The exact schedule should be taken from the current product technical data sheet because lot-specific adjustments may exist. Forced-air ovens are typical; vacuum or inert-gas thermal postcure is not required but may reduce surface oxidation. The oven load should be arranged to avoid stacked parts that shade UV during the UV step and to allow uniform air movement during the thermal step. Thermal postcure ovens should be validated with a calibrated load thermocouple to confirm that the center of the thickest part reaches the specified temperature for the required duration. Airflow shadowing can produce undercured regions that are not visually distinct. For lot-to-lot control, a hardness or glass transition check on a standard specimen is preferred because residual conversion is difficult to measure directly on production parts.

Thick sections act as thermal sinks; oven ramp-up must be controlled to avoid overshoot that could induce distortion. If thermal postcure is shortened, residual cationic species may remain active, causing slow property evolution over weeks. Differential conversion through the wall can produce internal stress after thermal postcure; tooling builds therefore require pre-scaling of critical dimensions using process capability data from the specific stereolithography platform. Published data for this specific configuration is limited.

Relative to acrylate-based stereolithography resins, ProtoGen 18120 offers a higher heat deflection temperature and lower long-term moisture absorption, but it has lower elongation at break and requires more rigorous postcure. Compared with heavily filled high-temperature stereolithography resins, it has lower viscosity and finer feature resolution but lower abrasive wear resistance and lower maximum service temperature. Compared with an unfilled acrylate photopolymer of similar modulus, the epoxy network in ProtoGen 18120 typically exhibits different failure behavior: higher crosslink density yields higher heat deflection temperature but also lower impact toughness. The low liquid viscosity of 250–300 mPa·s is closer to low-viscosity acrylate resins than to many high-temperature epoxy stereolithography resins that can exceed 1,000 mPa·s at 30 °C; this is the primary processing difference that reduces vat drain time and supports fine-feature recoating. However, the thermal postcure demand is higher than that of many acrylate systems, which may require only UV exposure.

Electrical connector housings and automotive sensor enclosures built from the resin have been evaluated for short-term exposure at temperatures up to 105 °C without softening under the 1.82 MPa heat deflection temperature limit. Creep experiments performed per ASTM D2990 are recommended for continuous load at elevated temperature; published data for this specific configuration is limited. The resin has been used for silicone RTV mold masters, where low surface roughness and dimensional stability after postcure allow multiple casting cycles. The molds should be sealed with a suitable mold release because uncured silicone may bond to epoxy surfaces if left in contact for extended periods.

Moisture Uptake, Chemical Resistance, and Long-Term Dimensional Drift

The cured material exhibits water absorption of 0.35 % after 24 h immersion per ASTM D570-98. This value is lower than many unfilled acrylate-based stereolithography resins, but the material is not hydrophobic. Continuous exposure to high humidity can still produce slight dimensional growth. Components intended for tight-tolerance optical or mechanical assemblies should be conditioned to the service environment before critical dimension verification. Dimensional measurements on conditioned specimens are typically performed after 24 h equilibration at 23 °C and 50 % relative humidity. For parts with critical interfaces, postcure dimensional compensation should be determined by measuring a known artifact on the same equipment, not assumed from nominal resin shrinkage.

The epoxy network is resistant to mild aqueous solutions and many hydrocarbon fluids, but polar organic solvents and chlorinated solvents may attack the thermoset. The epoxy thermoset is also resistant to dilute acidic solutions and aliphatic hydrocarbons, but exposure to aromatic hydrocarbons and ketones can produce swelling. The affected layer is usually the surface; thin-section parts are more susceptible to property loss. Compatibility testing per ASTM D543 or ISO 175 is required before use with proprietary process fluids. The resin is not recommended for prolonged immersion in strong bases or ketones unless validated. A postcure oven with adequate temperature uniformity minimizes residual acid species that can accelerate hydrolytic degradation in humid environments. Fully postcured materials have low creep at temperatures below the glass transition. However, at service temperatures above 80 °C under continuous load, creep compliance increases; the design should use the appropriate creep modulus for the load duration. Because the material is thermoset, it does not exhibit the viscous flow of semi-crystalline thermoplastics; dimensional change under load is dominated by viscoelastic creep rather than creep rupture in the short term.

The operational boundaries of the liquid resin should be observed. The resin should be stored in the original opaque container at 20–25 °C; exposure to light or temperatures above 35 °C can initiate premature polymerization or increase viscosity. Moisture contamination from humid air should be minimized because water can disrupt cationic cure and reduce conversion. The resin is not supplied as a food-contact material, and any medical or food-contact application must be verified against the relevant regulatory requirements, such as USP Class VI or FDA 21 CFR 177, as applicable. The uncured resin is a skin and respiratory irritant; handling should follow the safety data sheet with nitrile gloves and local exhaust ventilation. Avoid mixing with amine-based or metallic additives that may inhibit cationic polymerization or destabilize the liquid resin. The manufacturer’s regulatory data sheets identify compliance status with REACH and RoHS at the date of supply; current status should be confirmed through the supply chain.