DSM Somos 9420 is an unfilled epoxy-based photopolymer formulated for vat photopolymerization on stereolithography systems operating at 355 nm laser wavelength. The liquid resin has a nominal viscosity of 270 mPa·s at 30 °C and a density of approximately 1.13 g/cm³ at 25 °C, which permits layer recoating at thicknesses between 50 µm and 100 µm on standard platforms. Cationic ring-opening polymerization during laser exposure yields lower volumetric shrinkage than free-radical acrylate photopolymers, reducing curl in thin unsupported ribs and fillets. Green parts retain a slightly tacky surface until post-cure and require solvent rinsing to remove unpolymerized resin from recesses, blind holes, and internal channels. The cured material is amber and is used primarily for sacrificial investment casting patterns, master patterns for silicone replication, and rigid prototype tooling.

The cationic epoxy mechanism proceeds through photoacid generation and ring-opening of oxirane groups. Unlike free-radical acrylate systems, the reaction is not oxygen-inhibited and continues after the laser writes each layer; this dark cure stabilizes green dimensions but also extends the period in which the part surface remains sensitive to humidity and solvent sorption. The material is formulated without high-load ceramic fillers, so it avoids high-shear recoating defects associated with heavily filled resins and permits smaller laser spot overlap without filler-induced scattering.

Nominal mechanical and thermal values from the manufacturer’s technical data sheet are summarized below. These values are not specification minima; they shift with build orientation, post-cure dose, and ambient humidity.

The 4 % elongation at break positions 9420 as a rigid, low-ductility resin rather than a tough end-use plastic. Thin clips, snap fits, and living hinges below 1 mm section thickness may fracture during ejection from silicone tools. The measured heat deflection temperature places 9420 below high-temperature stereolithography grades such as Somos PerFORM and Accura Bluestone. This limits direct use as an injection mold insert above 40 °C, but the resin remains adequate for low-temperature vacuum casting and sacrificial pattern production. The flexural modulus near 2,750 MPa is high enough to resist handling distortion but low enough to avoid brittle shell loading during burnout. Build orientation influences tensile properties; vertical orientation typically reduces tensile strength relative to horizontal orientation by 5–10 % in stereolithography due to interlayer adhesion, although published orientation-resolved data for 9420 are limited.

The unfilled nature of 9420 separates it from ceramic-filled or nanocomposite SL grades. Without filler particles, laser scattering through the liquid is lower, making the resin suited for fine text, sharp fillets, and thin ribs down to 0.3 mm. The absence of inorganic reinforcement also means that burnout leaves minimal residue, but the room-temperature modulus remains lower than filled high-temperature resins.

What limits the burnout window in ceramic shell investment casting?

In ceramic shell processing, the disposable pattern must expand, soften, and oxidize without exceeding the strain tolerance of the primary coat or backup layers. Epoxy photopolymer patterns exhibit thermal expansion from ambient to the onset of decomposition; if a shell is heated rapidly, differential expansion across thick and thin pattern sections can initiate shell cracking before pattern pyrolysis. Foundry practice with epoxy stereolithography patterns commonly applies a controlled ramp through 120 °C to 300 °C, followed by oxidation at 650 °C to 750 °C in an air-fed burnout furnace. The low manufacturer-reported ash content of ≤0.01 % reduces non-metallic inclusions in ferrous, nickel-based, and cobalt-based castings, but shell permeability and venting must be sufficient for gaseous decomposition products.

Thick pattern sections above 6 mm require staged holds during burnout to avoid local pressure accumulation inside the ceramic cavity. Pattern geometry with enclosed volumes or internal ribs should incorporate vent holes of at least 2 mm diameter unless the shell can outgas through an open cup or riser. Published decomposition kinetics for 9420 are limited; foundries generally validate shell integrity by differential scanning calorimetry to 700 °C and thermogravimetric analysis in air. The low ash value alone does not guarantee clean metal because residual carbon and photoacid components may persist as microporosity or surface scale in reactive alloys such as titanium.

Compared with wax-filled stereolithography patterns, 9420 offers finer edge definition and lower melt-related dimensional movement during shell build. Wax-filled materials soften before burnout and can distort during autoclave dewax, while unfilled epoxy maintains green shape until decomposition. However, epoxy patterns are more rigid and may require relief cuts or hollow sections in large masses to prevent shell loading.

When environmental humidity is not controlled during vat recoating

Cationic photopolymerization is quenched by water, which terminates propagating oxonium species and inhibits surface cure. Production cells above 60 % RH can increase green-part tack, lower interlayer adhesion, and reduce post-cure modulus in epoxy SL resins; published data for 9420 under controlled humidity gradients are limited, but the processing recommendation is to maintain the build chamber at 30–50 % RH and to keep the vat sealed during idle periods. The liquid resin absorbs moisture from air; viscosity drift from water uptake can alter recoated layer thickness and produce layer-to-layer variation above ±0.02 mm across a 600 mm platform build.

Post-rinse with isopropyl alcohol or a manufacturer-approved solvent is required within 30 min of build removal to prevent partially cured resin from swelling green features. Blowing dry with compressed air at pressures below 0.2 MPa removes solvent from blind holes without eroding delicate walls. Exposure to high humidity before post-cure can also induce surface haze, which increases light scattering and reduces dimensional clarity of thin translucent sections.

On stereolithography platforms such as 3D Systems Viper si2 or iPro series machines, the recoater blade gap, laser spot size, and scan speed must be matched to resin viscosity and part cross-section. Build chamber temperature is typically held at 28–32 °C; lower temperatures increase viscosity and cause trailing-edge defects, while higher temperatures accelerate dark cure and reduce useful vat life. The liquid is filtered through a 45 µm mesh after each build to remove partially cured particulates, and the vat bottom is inspected for cured film that may scatter the laser beam. For features below 0.5 mm, a 50 µm layer thickness is preferred because it minimizes stair-step error and improves burnout uniformity in thin casting patterns.

For large flat patterns, support density and contact point size should follow the stereolithography software’s epoxy-family defaults; reduced support contact area may produce delamination at the start of a build because cationic dark-cure green strength develops more slowly than acrylate green-strength. Builds with dense cross-sections above 50 mm should be staggered to allow exothermic heat dissipation and to prevent localized vat hot spots. Beam width compensation and scan pitch should be calibrated using the penetration depth and critical exposure values provided in the material data package. On a 355 nm laser system, deviation in laser power above ±5 % from nominal can shift layer cure depth and produce under- or over-cure artifacts; a power meter with a 10 mm aperture should be used at the build plane, and beam ellipticity should be checked before processing fine wells or ribs below 0.3 mm.

Post-cure thermal scheduling and the shift in heat deflection temperature

A typical post-cure sequence uses UV flood exposure followed by thermal cure at 60 °C for 2 h, but the exact dose and duration are machine-dependent and must be qualified for each SL platform. The manufacturer-reported heat deflection temperature at 0.46 MPa is 52 °C, and at 1.82 MPa it is 46 °C; this limits direct use in boiling-water immersion or autoclave cycles above 90 °C. For tooling in low-pressure molding or vacuum casting, bulk tool temperature should remain below 40 °C to avoid creep and dimensional shift. Over-post-cure increases crosslink density and may raise the glass-transition temperature, but it can also embrittle thin unsupported sections below 0.3 mm and reduce notched Izod impact relative to the nominal value.

For master patterns used in silicone tooling, the cured 9420 surface should be sealed to prevent inhibition of addition-cure silicone. Trace photoacid or uncured monomer can poison platinum-catalyzed systems; a barrier coat or post-cure bake at 40 °C for 12 h reduces this risk. Pattern dimensions are usually offset by 0.1–0.3 % to compensate for silicone shrinkage and pattern wear in short production runs.

Relative to DSM Somos WaterShed XC 11122, an ABS-like stereolithography resin, 9420 exhibits higher flexural modulus and lower elongation, making it stiffer but less tolerant of repeated flexure. WaterShed XC 11122 is selected where end-use toughness and low moisture absorption control performance; 9420 is selected where clean burnout, dimensional rigidity, and fine feature retention dominate. Relative to Somos PerFORM, a ceramic-filled stereolithography grade, 9420 has lower viscosity, lower heat deflection temperature, and less abrasive wear on recoater blades; PerFORM is specified for high-temperature tooling with elevated stiffness. In contrast to Accura Bluestone, an engineered nanocomposite with a reported flexural modulus above 9,000 MPa, 9420 is unfilled and easier to process on standard vats but is unsuitable for sustained exposure above 100 °C.

Solvent exposure, amine contact, and storage temperature impose process boundaries

Uncured 9420 should not be combined with amine-based epoxy hardeners, nor blended with acrylate monomers, because premature crosslinking or phase separation can occur in the vat and damage recoater components. Strong ketone solvents such as acetone can swell green parts and should be avoided unless used as a brief rinse followed by immediate drying; manufacturer recommendations favor isopropyl alcohol or proprietary rinse solutions. The material is stored between 5 °C and 30 °C in sealed opaque containers; freezing or prolonged exposure above 35 °C can initiate viscosity drift and photoacid degradation.

The Safety Data Sheet classifies the liquid resin as a skin and eye irritant under CLP; handling requires nitrile gloves, safety eyewear, and local exhaust ventilation as indicated. Cured 9420 is not food-contact certified under FDA 21 CFR and has no established USP Class VI or ISO 10993 biocompatibility claim in the manufacturer’s standard documentation without additional barrier coating or regulatory assessment. RoHS compliance of the cured photopolymer must be confirmed against the specific homogeneous material limits for the assembled product; the resin itself is not formulated with deliberately added lead, cadmium, mercury, or hexavalent chromium.

The resin is commercially available through Covestro Additive Manufacturing following the acquisition of DSM’s stereolithography resin portfolio; historical datasheets retain the DSM Somos designation. Supply containers should be dated on receipt and acclimatized to build-chamber temperature before opening to avoid condensation on the liquid surface. For reactive metal casting such as titanium or magnesium, residual carbon after burnout can embrittle the alloy. Foundries should validate shell firing and vacuum-assisted melting through pilot castings before serial production; published data for the specific interaction between 9420 decomposition residues and reactive alloy surfaces is limited. For steel and cobalt-chrome investment casting, the low ash specification and dimensional stability of 9420 reduce inclusion-related scrap when burnout profiles are qualified for each part family.