DSM Somos ProtoGen™ 18120 is an epoxy-based liquid photopolymer supplied for vat photopolymerization stereolithography using 355 nm solid-state laser scanning. The material designation includes UV postcure at HOC +3, identifying a postcure protocol offset of +3°C relative to the baseline HOC reference. The resin does not follow free-radical acrylate polymerization. Photoacid generation under laser exposure initiates cationic ring-opening of epoxide groups, and the network continues to propagate during dark cure and thermal postcure. This mechanism gives the material a strong dependency on postcure temperature control and produces rigid, opaque parts after full cure.

Manufacturer-published representative values for fully postcured specimens are listed in the following table. The values are typical results obtained with the test methods shown, not specification limits. Batch-to-batch variation on production platforms is controlled primarily by resin age, vat humidity, laser energy calibration, and postcure temperature uniformity.

The two heat deflection temperature values define the practical thermal design envelope. The 0.46 MPa HDT of approximately 110°C indicates tolerance for short-duration contact with heated fixtures, silicone cure cycles, and low-pressure autoclave processes. The 1.82 MPa HDT of approximately 68°C is the more restrictive limit for load-bearing service because it approximates the temperature at which the network softens under structural stress. The tensile modulus near 2900 MPa places ProtoGen 18120 in the rigid structural prototyping class, distinct from elastomeric SLA materials and from filled epoxies that sacrifice elongation for higher heat resistance.

Specific gravity near 1.14 is typical for unfilled epoxy formulations; filled grades may be denser but introduce settling and viscosity penalties. The notched Izod value of 25 J/m is lower than many impact-modified acrylate resins, so design rules must account for notch sensitivity at sharp corners and gate vestiges.

How Does HOC +3 Influence Final Conversion and Thermal Response?

Postcure temperature is not a secondary step for ProtoGen 18120. The HOC +3 offset moves the chamber thermal condition +3°C above the standard HOC reference to compensate for heat loss from part fixturing and to raise segmental mobility in partially converted epoxy domains. In a chamber equipped with 365 nm fluorescent or LED lamps and convective heating, the thermal setpoint should be verified with a contact thermocouple placed on a sacrificial control part. Air-temperature readings alone do not capture the exothermic rise from continued cationic propagation inside thick sections.

Published data for HOC +3-specific postcure performance is limited to equipment manufacturer process documentation rather than a formal ASTM protocol. The narrow processing window is nevertheless clear from epoxy SLA process experience. Local part surface temperature should remain within ±3°C of the HOC +3 target during the first 15 min of postcure. Deviation beyond ±5°C can create differential conversion between thick and thin walls, producing after-cure bow and heat deflection temperature scatter. Production-scale SLA service operations have reported that controlled thermal soak is more effective than UV dose alone in reducing residual epoxide in this resin class, because dark-cure propagation is diffusion-limited at ambient temperature.

High-output UV chambers vary in spectral output and fluence rate. A radiometer calibrated at 365 nm should be placed at the part location because reflective walls, part shadowing, and lamp age create local fluence differences. The HOC +3 offset does not compensate for inadequate UV dose; photoacid generation depends on absorbed UV energy, while the thermal offset drives subsequent dark-cure propagation. Both parameters are coupled: insufficient UV dose produces a low acid concentration, and no practical thermal soak can fully polymerize an under-exposed green part. Differential scanning calorimetry of partially postcured parts typically shows a residual exotherm that declines below 5% of total reaction enthalpy after complete HOC +3 curing. Prolonged HOC +3 exposure beyond 120 min can over-advance the network and reduce notched Izod impact, especially in thin sections. The postcure recipe should therefore be qualified with both HDT and impact specimens on the actual chamber, not transferred from another machine without radiometric and thermal mapping.

Vat handling and recoating parameters control scrap rate before postcure. At 30°C, the liquid viscosity is approximately 260 mPa·s, which permits stable recoating at layer thicknesses between 0.05 mm and 0.15 mm on standard 355 nm SLA platforms. At ambient relative humidity above 60%, the uncured resin surface absorbs moisture and green-part dimensions can shift before postcure. Build chambers should be maintained with dry air purge or the resin surface blanketed with inert gas when the vat remains open for extended runs. Storage temperature should remain between 15°C and 30°C, and the resin should be protected from stray 405 nm light to preserve photoacid generator stability.

On 355 nm laser systems, working curve parameters must be established on the target machine. For this resin class, critical exposure Ec is typically below 10 mJ/cm², and depth of penetration Dp is near 0.15 mm for a 0.10 mm layer, but exact values vary with laser spot size, scan spacing, and resin age. Published data for ProtoGen 18120-specific working curves across all machines is limited, so iterative window-pane exposures are required after vat replenishment. Green parts should be rinsed in isopropyl alcohol or the machine manufacturer's recommended solvent for less than 15 min under air agitation, followed by forced-air drying at 30°C before HOC +3 postcure. Prolonged solvent immersion can plasticize the partially cured network and reduce final flexural modulus.

Unlike free-radical acrylate systems, cationic epoxy cure is not strongly inhibited by ambient oxygen, so surface cure is less sensitive to nitrogen blanketing. The primary environmental interference is moisture, not oxygen. Water reacts with the propagating cationic species and can reduce polymer chain growth. This is why uncontrolled humidity in a production vat appears as lower peak HDT in postcured parts, even when laser energy remains constant.

Replacing Acrylate SLA Photopolymers in Heated Functional Prototypes

ProtoGen 18120 is selected when acrylate SLA materials fail by creep or distortion at moderate heat. Acrylate systems cure by free-radical propagation that is largely complete after laser exposure, while ProtoGen 18120 requires thermal postcure to achieve its published HDT and modulus. The epoxy network exhibits lower volumetric shrinkage and better resistance to moisture-driven dimensional change after cure, but it demands longer postcure and tighter humidity control before the build. The 0.46 MPa HDT of approximately 110°C exceeds that of many unfilled acrylate SLA grades, which commonly deflect below 70°C under the same stress; however, comparisons should be based on datasheets using the identical ASTM D648-18 test condition.

The material is not a drop-in replacement for transparent acrylate grades. ProtoGen 18120 is opaque and cannot be used where optical clarity is required. Within the epoxy SLA family, it is positioned below higher-temperature filled systems in 1.82 MPa HDT but offers lower viscosity and easier recoating. Published direct comparison data is limited, so selection should be governed by end-use thermal stress, impact requirements, and build chamber capacity. The resin should not be combined with amine-based additives or acid-scavenging fillers, because both interfere with cationic propagation and can reduce final conversion.

Impact-modified acrylate grades may show higher notched Izod values and faster room-temperature property development, but they often exhibit lower HDT and higher linear shrinkage. The dimensional stability advantage of ProtoGen 18120 is most visible in long, thin features that would otherwise accumulate shrinkage stress across a build platform. ProtoGen 18120 should not be blended with other SLA resins, especially acrylates, because phase separation and localized inhibition can produce uncured pockets.

When Master Patterns Are Machined After Thermal Postcure

Postcured ProtoGen 18120 stock is rigid enough for secondary machining on three-axis CNC equipment with carbide or diamond tooling. Spindle speeds above 18000 rpm combined with low feed rates can generate frictional heat that locally exceeds the 1.82 MPa HDT, causing edge smearing or rounding. Coolant or compressed air is required for thin walls below 1.0 mm. Because the resin has low thermal conductivity relative to metal, heat concentrates in the chip and workpiece surface; vacuum fixtures are preferred over mechanical clamps for thin sections. The material is used for investment casting patterns and silicone tooling masters because it can be sanded, primed, and bonded without the layered delamination common in some low-strength acrylate models.

Electrical connector housings and short-run functional enclosures introduce notch-sensitivity constraints. The notched Izod impact of 25 J/m per ASTM D256-10 indicates limited resistance to sharp-corner impact; design rules should specify 0.5 mm minimum internal fillet radii where loading is dynamic. Assembly with adhesives requires surface abrasion and solvent wipe. Cyanoacrylate and two-component epoxy adhesives are commonly used, but published lap-shear data for ProtoGen 18120 is limited. Designs requiring threaded inserts should use heat-staking or ultrasonic insertion only after verifying that local frictional heating does not exceed the 1.82 MPa HDT.

For fluid-contact applications, the cured network is generally resistant to mild aqueous solutions and aliphatic hydrocarbons, but polar solvents such as methyl ethyl ketone or dichloromethane can swell the part. Published data for specific fluid resistance is limited; immersion screening per ASTM D543-21 is recommended before use in fuel or brake fluid environments. Long-term outdoor exposure can yellow the unpainted epoxy surface and reduce surface hardness. Parts exposed to sunlight should be coated with an opaque UV-blocking primer. Accelerated weathering data for ProtoGen 18120 under ASTM G154 is limited, so end-use qualification is required.

Regulatory compliance under REACH and RoHS must be confirmed against current supplier declarations, because formulation additives can change without notice. Cured parts intended for indirect food-contact or medical use require separate extraction testing under applicable ISO standards; the standard technical datasheet does not establish biocompatibility. Resin stored beyond 24 months should be re-qualified for viscosity and working curve before production use.