DSM Somos 8110 Epoxy Photopolymer is a cationically curable epoxy resin supplied as a single-component liquid for vat photopolymerization platforms operating at 355 nm. The material is specified through a combination of ASTM and ISO methods: viscosity under ASTM D4212 or ISO 2884, density under ASTM D4052, tensile properties under ASTM D638 or ISO 527-2, flexural properties under ASTM D790 or ISO 178, heat deflection temperature under ASTM D648 or ISO 75, Shore hardness under ASTM D2240, and water absorption under ASTM D570. It is used for functional prototypes, investment casting patterns, wind-tunnel test articles, and prototype injection mold inserts where part accuracy and controlled post-cure distortion are evaluated against spatial tolerance classes defined in ISO 2768-1. Because polymerization proceeds through cationic ring-opening rather than free-radical chain growth, volumetric shrinkage and oxygen inhibition are lower in the vat than for many acrylate photopolymers, but the resin requires stricter moisture and contamination control.

Because the material is an unfilled epoxy photopolymer rather than a filled hybrid, the vat resin does not require continuous agitation to maintain particulate suspension. However, viscosity is temperature dependent, and the build chamber must be maintained within the manufacturer’s recommended process window. In machine configurations using compliant recoat blades, resin temperature below 18 °C may produce incomplete layer wet-out and visible drag marks; above 30 °C, dark-cure propagation accelerates and may raise the vat resin viscosity over successive builds. Operators should record vat temperature and recoat speed before process adjustments because surface defects from low-temperature resin are often mistaken for laser exposure errors.

How Does the Cationic Cure Profile of Somos 8110 Affect Part Accuracy?

Unlike free-radical acrylate photopolymers, Somos 8110 polymerizes through a cationic mechanism initiated by photogenerated Brønsted acids. The process is not subject to the same order of oxygen inhibition at the liquid surface, so edge acuity and surface film formation can be controlled when scan parameters are matched to the resin’s critical exposure thresholds. The laser exposure necessary to maintain a stable green layer is machine-dependent and is encoded in the equipment manufacturer’s build style; published data for this specific configuration is limited when users alter scan spacing, hatch pattern, or layer thickness outside the recommended range.

Cationic systems continue to propagate after the laser has moved, a behaviour referred to as dark cure. This can reduce green-part distortion but also makes processing delays chemically active. A green part left at ambient temperature before post-cure may continue to crosslink non-uniformly if it is exposed to uneven room lighting or if residual cleaning solvent remains in thin sections. For critical tolerance work, post-cure should begin within 24 h of vat removal unless the manufacturer’s protocol explicitly permits longer staging. Cleaned parts should be dried with filtered compressed air at a dew point below -20 °C before UV flood post-cure; residual alcohol in blind holes or thin slots can create local under-cure and dimensional growth during oven exposure.

Moisture is a significant boundary condition. At relative humidity above 60%, absorbed surface moisture can interfere with residual oxonium propagation and produce a softer, under-cured skin. Pre-drying or immediate post-cure is required in these conditions. Amine-based release agents, epoxy-amine adhesive residues, and some solvent-borne mold releases inhibit cationic cure and should not contact the liquid resin or green part. Cleaning is typically limited to isopropanol or proprietary solvents approved by the manufacturer; solvent immersion beyond 10 min can induce microcracking in thin unsupported sections. The use of acetone, methylene chloride, or aggressive alkaline strippers is incompatible with this resin family.

Thermal, Mechanical, and Dimensional Stability Parameters

Mechanical property comparisons should be based on specimens conditioned to ASTM D618 at 23 ± 2 °C and 50 ± 5 % relative humidity. Tensile strength, tensile modulus, and elongation at break are usually reported according to ASTM D638 Type I or ISO 527-2 1A specimens; flexural properties are reported according to ASTM D790 or ISO 178. Because stereolithography builds are anisotropic, values measured on specimens oriented parallel to the build plane and along the z-axis should be compared separately. Published datasheet averages for a single orientation do not transfer directly to thin walls, small holes, or load-bearing inserts.

Heat deflection temperature measured under ASTM D648 is a single-point thermal test, not an upper-use temperature. The value is sensitive to post-cure completeness, sample thickness, and applied stress. A lower HDT compared with filled grades reflects the unfilled epoxy network; the material should not be specified for continuous hot-air or hot-liquid service above the manufacturer’s published HDT unless creep and chemical resistance data under the actual load are available. Published data for this specific configuration is limited in long-term thermo-oxidative aging.

Water absorption under ASTM D570 measures immersion uptake, but dimensional change in humid air is a separate equilibrium moisture condition. Unfilled epoxy resins typically absorb more moisture than acrylate resins and more than filled epoxy grades, which can shift as-built dimensions by several tenths of a percent after prolonged exposure. For pattern work in foundry or investment casting, storage in sealed polyethylene bags with desiccant is recommended until use. Dimensional checks should be made after conditioning to 23 ± 2 °C and 50 ± 5 % relative humidity, not immediately after removal from a dry box or humid shop floor.

Functional selection between DSM Somos 8110 and other photopolymers should begin with cure mechanism and post-processing throughput. Acrylate photopolymers polymerize by free-radical chain growth; they are more prone to oxygen inhibition and often require higher laser doses to overcome surface tack, but they can provide faster green-part handling and lower viscosity. Somos 8110’s cationic epoxy network typically produces lower volumetric shrinkage and better retention of fine features because ring-opening polymerization reduces internal stress accumulation during vat build. The trade-off is sensitivity to moisture, slower dark-cure kinetics, and a narrower cleaning solvent window.

Compared with dual-cure epoxy-acrylate hybrid resins, Somos 8110 does not combine free-radical and cationic mechanisms at the same molecular network density. Hybrid formulations can be tuned for higher toughness and faster cure but may report different oxygen sensitivity and water uptake. For a given target geometry, the correct comparison is not only the datasheet tensile strength but the ratio of green-part tolerance retention to post-cure growth. When evaluating replacements, measure z-direction tensile strength per ASTM D638 on specimens cut from the same build orientation as the production part; thin sections and small radii often fail below bulk values.

Compared with filled composite photopolymers, Somos 8110 has no abrasive particulate filler, so recoat blade wear is lower and the liquid resin does not settle under idle conditions. However, unfilled epoxy photopolymers generally exhibit lower flexural modulus and lower HDT than glass-filled or ceramic-filled systems; they also show higher coefficient of linear thermal expansion. For applications requiring long-term oil or fuel contact, chemical compatibility should be screened by ASTM D543; strong bases, ketones, and aromatic solvents are more likely to degrade the network than aliphatic hydrocarbons.

Investment casting patterns made from Somos 8110 can be shelled directly if the foundry procedure uses low-ash burnout schedules. Epoxy photopolymers can leave higher ash than dedicated investment casting resins if burnout is incomplete; the burnout cycle must hold above 600 °C in an oxidizing kiln until no carbon residue remains. Operators should avoid rapid heating between 250 °C and 450 °C because thermal decomposition gases can crack thin ceramic shells if the ramp rate is too aggressive.

When Somos 8110 Replaces ABS-Like or Polypropylene-Like Resins in Functional Prototyping

Replacement of an ABS-like acrylate or polypropylene-like photopolymer with Somos 8110 is justified when the part requires dimensional stability in thin walls, lower moisture-driven curl, or improved edge definition in small features. The epoxy system is generally more rigid and more brittle than impact-modified acrylate resins, so snap-fit arms, living hinges, and high-deflection clips require conservative redesign. Notched Izod impact testing under ASTM D256 is the appropriate screening method; data should be compared on specimens prepared in the same orientation and post-cure condition as production parts.

In prototype injection mold inserts, the cavity surface temperature is the limiting process variable. Somos 8110 can be used for short runs with low melt temperature resins such as unfilled polyethylene or polypropylene if the tool is treated as a low-conductivity polymer surface. Continuous cavity temperatures above the resin’s HDT produce softening and loss of parting line definition. Published data for this specific configuration is limited, so tool trials should begin with 10–20 shots and use mold release only if chemically compatible. Generous draft angles and venting are required because the polymer tool does not conduct heat away from the melt at the same rate as steel or aluminum.

Electrical and electronic housing prototypes benefit from the resin’s fine detail, but post-cure moisture uptake affects dimensional stability in controlled assembly. Fasteners inserted into printed bosses should follow manufacturer-recommended pilot hole sizes; thread-forming screws in unreinforced epoxy can generate radial stress beyond the material’s tensile elongation limit, causing microcracks. If threaded applications require repeated insertion, use bonded threaded inserts after post-cure rather than cutting threads directly into the green part.

For direct shelling in investment casting, the burnout protocol must be validated because epoxy photopolymers can leave carbon residue when heating rates are too high. The burnout cycle should include a slow ramp between 250 °C and 450 °C to allow thermal decomposition without shell cracking, then an oxidizing hold above 650 °C to remove residual carbon. Foundries that already process epoxy patterns can use existing schedules; foundries moving from wax or acrylate patterns should run a trial tree before committing production volume.