DSM Somos 7120 Epoxy Photopolymer, UV & Thermal Postcure is a rigid epoxy-based vat photopolymerization resin formulated for 355 nm stereolithography systems. The product designation defines a two-stage property-development route: UV irradiation initiates cationic epoxy ring-opening through a photoacid generator, and thermal postcure drives additional propagation, crosslinking, and residual oxirane consumption. Manufacturer-reported viscosity at 30 °C is approximately 160 mPa·s, with a density near 1.13 g/cm³. These values support recoating at layer thicknesses between 0.050 mm and 0.100 mm on conventional blade or vacuum recoater platforms. The cured material is used for investment casting patterns, vacuum casting masters, dimensional prototypes, and moderate-temperature functional fixtures. All mechanical values cited in this document are based on ASTM or ISO methods and refer to parts tested after completion of the manufacturer-recommended UV and thermal postcure sequence.

What postcure sequence drives oxirane conversion?

Green parts removed from the vat are not final parts. Residual photoacid remains active, and unreacted cycloaliphatic epoxy groups retain significant ring strain. The transition from green state to cured state is governed by two process variables that cannot be interchanged: UV dose and thermal cure temperature. In production-scale postcure, UV exposure is performed in multi-lamp chambers with UVA output in the 315–400 nm band, commonly delivering 30–60 J/cm² per side depending on wall thickness and lamp intensity. Thermal postcure follows in a forced-air convection oven at a set-point between 50 °C and 80 °C for 1–3 h. The heated stage is diffusion-limited: as the network vitrifies, the glass-transition temperature rises and the propagation rate constant decreases. Ramping too quickly through the glass transition introduces bulk stress from transient temperature gradients. Insufficient thermal dwell leaves a sub-90% conversion layer that depresses heat deflection temperature and solvent resistance. Residual reaction enthalpy can be measured by differential scanning calorimetry using ISO 11357-2:2020.

Build parameters on 355 nm solid-state laser platforms are typically derived from working curves generated on the target machine. Users should derive critical exposure and penetration depth by vat-specific windowpane testing because published working curves for this specific configuration are limited. Comparable cycloaliphatic epoxy systems commonly exhibit critical exposure values between 8 mJ/cm² and 15 mJ/cm² and penetration depths between 0.10 mm and 0.15 mm. Vat temperature is usually maintained at 28–32 °C to reduce viscosity and improve recoat uniformity. Galvanometer scan speed, hatch spacing, and laser power are adjusted to maintain overlap without excessive overcure. Overcure in downward-facing layers increases edge growth and degrades fine-feature accuracy. On production equipment such as a 3D Systems SLA Viper Pro or equivalent 355 nm platform, the resin’s low viscosity permits faster recoating than filled or tough-modified epoxy photopolymers.

Laser patterning parameters and recoat dynamics

Orientation-dependent anisotropy must be considered when interpreting tensile and flexural data. Tensile bars tested per ASTM D638-14 after full postcure generally report tensile strength in the 50–60 MPa range, tensile modulus near 2.4–2.7 GPa, and elongation at break between 5% and 8%. Flexural strength measured per ASTM D790-17 is typically higher because the laser-scalloped outer skin has a higher degree of crosslinking than the interior. Hardness measured by ASTM D2240-15 is usually Shore D 82–86. Heat deflection temperature under 0.46 MPa stress is typically 50–65 °C depending on thermal postcure completion; under 1.82 MPa, values can fall to 45–55 °C. These values are representative of the manufacturer’s published range and are not lot-release specifications. Users should request a certificate of analysis for acceptance testing of production lots.

Moisture sensitivity is a defined operational boundary. Epoxy photopolymers absorb water and plasticize; extended immersion at 23 °C can reduce modulus and heat deflection temperature. The material should be stored in sealed containers away from UV and heat sources, and resin in open vats should be protected from ambient humidity above 60% RH. Avoid contact with strong bases and amines because such additives can quench the photoacid and leave surface tack. Postcure chambers should be vented to control ozone and low-molar-mass photolysis byproducts. Parts intended for tolerances tighter than 0.1% linear dimension should be preconditioned in the service environment before metrology because moisture uptake can cause dimensional drift. The resin’s storage life in unopened containers is typically 12 months from date of manufacture under dark, dry conditions.

When investment casting patterns require shell-safe burnout

Investment casting patterns built from Somos 7120 require a staged burnout schedule to prevent ceramic shell cracking from differential thermal expansion. Unfilled epoxy photopolymers generally leave lower residual ash than filled high-temperature grades, but burnout performance depends on pattern hollowing, drain channels, and oven ramp rates. Production foundries typically program slow heat-up through 300–600 °C to allow decomposition and volatilization without excessive gas pressure. Published data for this specific configuration is limited, so foundry-specific burnout trials are required before committing to production quantities. Pattern walls should be uniform and free of thick section changes that produce trapped expansion stress. Vent holes of at least 5 mm diameter are often used to release gases from hollow sections.

Compared with clear epoxy photopolymers such as Watershed XC 11122, Somos 7120 is not specified for optical clarity or water-white appearance; it is selected where dimensional stability and moderate thermal performance are more important than transparency. High-heat grades such as ProtoGen 18420 shift heat deflection temperature upward through a more densely crosslinked or filled network, but they often carry higher viscosity and slower recoating. Toughness-modified grades such as Somos NeXt raise elongation and Izod impact above the glassy epoxy range, whereas 7120 remains a rigid system with elongation below 10%. The lower viscosity of 7120 relative to filled or tough grades permits finer feature reproduction and faster recoat on platforms with blade or vacuum recoating systems.

Solvent resistance is controlled by postcure conversion, not formulation alone

After full postcure, the epoxy network resists short-term exposure to many solvents, but ketones, chlorinated solvents, and hot oils can cause swelling and stress cracking. Chemical compatibility studies should follow ASTM D543-21. Aqueous resistance is moderate; water absorption after 24 h is 0.4–0.6% per ASTM D570-98. Aggressive alkaline cleaners and high-pH baths above 60 °C should be avoided because ester or ether linkages in the polymer network may hydrolyze. Long-term aqueous exposure at elevated temperature accelerates modulus loss and should be quantified for each application.

Vacuum casting masters produced from 7120 require a stepped postcure before silicone mold exposure. Residual uncured monomer can inhibit platinum-catalyzed addition-cure silicone rubber. A thermal postcure protocol at 60–80 °C for 2 h after UV exposure reduces the risk of cure inhibition. The master surface should be sealed or coated if used in repeated high-temperature vulcanization. The material’s glass transition limits continuous service under load to below 50–60 °C; above this range, creep and stress relaxation become significant. RoHS and REACH status must be verified against the supplier’s current safety data sheet and lot-level declarations before export or medical prototyping.