DSM Somos ProtoGen™ 18120 is an epoxy-based photopolymer supplied as a single-component liquid for vat photopolymerization stereolithography. The product is processed in the green state on laser-based SLA platforms and is subsequently conditioned under a designated ultraviolet postcure protocol referenced as HOC -2. The HOC -2 designation identifies a two-stage post-exposure sequence in which UV-A irradiation and elevated chamber temperature are combined to complete cationic crosslinking; it is not a separate resin grade. The uncured resin requires storage in light-tight containers at 20–25 °C and is conditioned by gentle rotation or low-shear agitation before introduction into the vat. Viscosity measured per ASTM D4212-16 falls within the low-viscosity range typical for stereolithography epoxies, which supports recoating on platforms with build envelopes from 250 × 250 × 250 mm to 600 × 600 × 500 mm. The cured network is evaluated after the HOC -2 protocol for tensile, flexural, and thermal properties under ASTM methods. Selection of this material is common where postcured dimensional stability, low moisture uptake, and controlled burnout residue are more important than high elongation at break.

On production-scale SLA equipment, the resin is imaged with a solid-state laser operating near 355 nm. The green part is removed from the platform, washed in a compatible solvent, and dried before postcure. The interval between cleaning and postcure should be minimized to avoid solvent absorption at the surface; parts left in solvent for more than 10 min may exhibit measurable dimensional drift. Entrained air introduced during vat refilling is removed by vacuum degassing at 20–25 kPa absolute pressure until bubble free. The use of open-mesh build platforms is acceptable, but support density should be increased by 15–20% on unsupported spans greater than 50 mm to control green-state sag.

What Process Controls Are Required for the HOC -2 Postcure Condition?

The HOC -2 postcure condition is implemented in UV flood chambers capable of simultaneous UV-A emission and thermal ramping. For this resin class, the chamber lamp output should be verified with a calibrated radiometer prior to each production campaign because UV-A irradiance degrades with lamp age; a commonly used acceptance threshold is 10–15 mW/cm² at the part surface. In the first stage, the part is held at 35–40 °C under UV-A exposure for 30–40 min. In the second stage, the chamber setpoint is raised to 55–65 °C and held for an additional 30–60 min while UV exposure continues. The ramp rate should not exceed 2 °C/min for sections with wall thickness less than 2 mm; faster ramps can induce thermal gradient distortion. Postcure completion is judged by a plateau in Shore D hardness measured on a witness coupon, not by fixed time alone.

Production-scale failure modes associated with improper HOC -2 execution include surface blistering, incomplete through-cure, and non-uniform dimensional change. Blistering is typically linked to residual cleaning solvent absorbed in the green state; the part must be dried with filtered air at 25–30 °C until constant mass before UV postcure. Incomplete through-cure is observed when lamp irradiance falls below the minimum threshold or when chamber thermal control drifts below setpoint. Non-uniform dimensional change is aggravated by stacking parts in the postcure chamber; parts should be arranged with 10–20 mm spacing and rotated halfway through the cycle. Published data for this specific product configuration is limited on the effect of chamber oxygen concentration; however, cationic epoxy photopolymers generally show higher cure inhibition in the presence of elevated humidity. Therefore, the postcure chamber should be maintained at 35–45% RH during the HOC -2 sequence.

Layer thickness is typically selected between 50 µm and 100 µm for this epoxy formulation. Laser spot diameter and scan spacing are adjusted on the equipment to maintain a stable line overlap; typical beam diameters of 0.10–0.25 mm are used with scan speeds that provide sufficient localized energy to achieve green adhesion. Recoating parameters should be tuned so that the wiper blade travel speed does not entrain air; speeds of 50–100 mm/s are common for this viscosity class. On machines with active coating feedback, the setpoint should be recalibrated after each resin batch change. Deviations in layer thickness greater than ±10 µm at the build plane require platform re-leveling or wiper blade replacement.

Mechanical and Thermal Property Data Reported Under ASTM Conditions

The typical postcured property values shown in the table below are based on manufacturer technical datasheet information for fully postcured specimens prepared under the HOC -2 condition. Values are not lot-specific and should be confirmed against the current certificate of analysis for the specific batch. Specimen conditioning before testing follows ASTM D618-21 at 23 ± 2 °C and 50 ± 10% RH.

The reported tensile and flexural values reflect the postcured epoxy network. Green-state properties are substantially lower and should not be used for design validation. Heat deflection values should not be interpreted as continuous-use temperature limits; prolonged exposure above the HDT may cause stress relaxation and creep in loaded sections. For applications requiring continuous thermal exposure, independent creep testing per ASTM D2990-17 is required.

Rheological characterization of the uncured resin is performed with a rotational rheometer equipped with a cone-and-plate geometry at 25 °C. The viscosity curve under increasing shear rate shows mild pseudoplastic behavior; at 1 s^-1 the apparent viscosity is near the low end of the datasheet range, and at 100 s^-1 the value is typically lower by 5–10%. Temperature dependence follows an Arrhenius-type decrease; heating the vat from 25 °C to 30 °C can reduce viscosity by approximately 10–15%, which may improve recoating performance in large-format systems but also accelerates dark cure in the vat. Batch-to-batch viscosity variation should be monitored per ASTM D4212-16; a shift greater than ±15% from the initial batch should trigger investigation of storage age and moisture ingress.

Vat life is influenced by ambient light, ambient humidity, and residual heat from the laser. In long production runs, the resin temperature may rise above ambient due to laser energy deposition and recirculation pump heat. If the vat exceeds 35 °C, premature cationic advancement can occur, leading to increased viscosity and a measurable shift in green-state modulus. Production equipment should include vat temperature control or a cooling loop; the setpoint should be maintained at 25–28 °C and recorded at 15-min intervals. The use of a resin life monitor based on near-infrared spectroscopy can detect partial advancement, but such online monitors are not universal; published data for this specific configuration is limited.

Green-state distortion is the largest process variable in high-accuracy stereolithography with this epoxy system. The green part is subject to shrinkage during laser patterning and to subsequent solvent-induced swelling during cleaning. Measurements on production-scale builds show that long, unsupported overhangs can deviate by 0.2–0.5 mm over a 100 mm span when support pitch is insufficient. To control this distortion, supports should be placed on any down-facing feature with an overhang angle less than 30° from horizontal. For features with unsupported spans between 50 mm and 100 mm, support density should be increased by 25–50% relative to default settings. The use of a support contact diameter below 0.4 mm may reduce witness marks but increases the risk of green-state detachment during recoating.

Surface roughness of postcured components is determined by layer thickness, stair-stepping, and the cleaning solvent residue. With 50 µm layers, the Ra of an up-facing surface is typically below 1 µm after postcure, while down-facing surfaces adjacent to supports may show Ra values above 5 µm before sanding. When the HOC -2 protocol is executed correctly, no additional thermal cure is required, but surface hardness may continue to increase for several hours after removal from the chamber due to dark cure. Parts should not be exposed to high-humidity environments for 24 h following postcure because moisture uptake during dark cure can affect dimensional stability. Dimensional inspection should be performed after a stabilization period of 24–48 h at 23 ± 2 °C.

In comparison with acrylate-hybrid stereolithography resins, the epoxy chemistry of ProtoGen™ 18120 is associated with lower reaction shrinkage during polymerization. Linear shrinkage after postcure is typically measured by the difference between green-state and postcured dimensions; for process compensation purposes, users of this class report scaling factors in the range of 0.1–0.3%, although published data for this specific configuration is limited. The postcured network generally exhibits lower equilibrium moisture uptake than typical acrylate systems, which reduces the risk of dimensional drift in humid service environments. The primary trade-off is lower elongation at break; thin snap features and living hinges are more prone to fracture than similar features molded in polypropylene or produced from high-elongation acrylate resins.

In investment casting pattern production, ProtoGen™ 18120 is processed into patterns that are subsequently assembled, dipped in ceramic shell, and removed by controlled burnout. The burnout cycle is defined by the shell manufacturer and must be validated for the specific pattern geometry. Thermogravimetric analysis in air at a heating rate of 10 °C/min typically shows complete decomposition of the organic fraction below 700 °C; the oxide shell remains structurally intact when the heating ramp is matched to the polymer decomposition profile. The low residual ash of the unfilled epoxy network, compared with filled SLA resins, makes the material suitable for alloy systems where carbon contamination and nonmetallic inclusions must be minimized, including titanium and cobalt-chromium melts. Ash content should be verified per ASTM D482-19 on the specific batch used for production.

When ProtoGen 18120 Replaces Acrylate-Hybrid Photopolymers in Master Pattern Production

Replacement of an acrylate-hybrid photopolymer with ProtoGen™ 18120 requires recalibration of the entire post-processing line, not merely substitution of the resin. The green-state cleaning solvent may need to be changed because some acrylate systems tolerate ethanol-based washing, whereas epoxy formulations of this class are typically washed in isopropanol or tripropylene glycol monomethyl ether. The HOC -2 postcure condition requires UV-A irradiation that is not always present in units optimized for acrylate thermal postcure; a chamber equipped only with a convection heater will not fully develop the cationic epoxy network. The lower reaction shrinkage of ProtoGen™ 18120 allows the scaling factor to be reduced by 0.1–0.2% relative to many acrylate-hybrid resins, but this requires confirmation by measuring a known calibration artifact before production release.

For master patterns used in silicone tooling, the higher modulus and lower elongation of the fully postcured epoxy network can transfer finer surface detail and reduce pattern deformation during silicone cure. However, the lower impact resistance requires support design modifications on features with cross-sections below 1 mm. On production-scale SLA lines, the reduction in support density possible with acrylate systems is not applicable; supports should be retained at a density calculated for green-state deflection of the epoxy network and verified by build trials. The use of a UV postcure fixture with 365–420 nm lamps is required; broadband metal halide systems may be acceptable if the measured UV-A irradiance meets the minimum threshold at the part surface.

The uncured resin should not be combined with amine-based additives, strong nucleophiles, or moisture-releasing fillers because these components can inhibit cationic polymerization or cause premature gelation in the vat. During material handling, contact with copper, brass, or iron oxides should be avoided in the recoat system; use stainless steel or polyethylene wetted parts. Cured parts exposed to continuous UV radiation under service conditions may undergo progressive discoloration without loss of bulk mechanical integrity; long-term stability should be verified by accelerated weathering per ISO 4892-2 when appearance is critical. The postcured epoxy network is not recommended for contact with strong alkaline solutions at temperatures above 50 °C because hydrolytic attack can reduce surface hardness. Published data for chemical resistance in the specific HOC -2 postcured state is limited; compatibility testing under the actual service fluid should be performed before production release.