DSM Somos ProtoTherm™ 12120 Water-resistant resin for stereolithography, UV Postcure

DSM Somos ProtoTherm™ 12120 is a water-resistant liquid photopolymer resin for laser-based vat stereolithography, supplied as a single-component photoreactive formulation for processing at ultraviolet laser wavelengths. The resin is used where functional stereolithography parts must tolerate humid air, intermittent water contact, or short-duration aqueous immersion while retaining dimensional and mechanical stability. The commercial designation places the material in the high-heat-deflection segment of the manufacturer’s stereolithography resin platform rather than in the high-elongation or general-purpose segment. It is not a sealant grade, and continuous exposure to pressurised hot water above the wet thermal service range requires application-specific qualification.

The product name carries two explicit process conditions: stereolithography and UV postcure. Stereolithography refers to layerwise photopolymerisation in a vat, using a frequency-tripled solid-state laser or equivalent source at 355 nm. UV postcure is required after part cleaning because as-built conversion of the reactive groups is not complete. Green parts retain residual unsaturation and epoxide functionality; without a controlled postcure step, the published tensile modulus, heat deflection temperature, and water absorption behaviour are not achieved. The resin therefore occupies a processing window in which green-state handling, solvent removal, and final radiant exposure collectively control the delivered property envelope.

The commercial resin is supplied in light-blocking containers and should be conditioned to the machine temperature before use. Viscosity at 30 °C is sufficiently low for recoat operations on standard stereolithography platforms. If the room relative humidity exceeds 60 %, the vat surface can pick up atmospheric moisture over extended idle periods. The recommended mitigation is active dehumidification of the build chamber or nitrogen blanketing, because water in the vat can reduce interlayer adhesion and shift the green-state modulus.

What Limits the Use of Standard Stereolithography Resins in Moisture-Exposed Functional Testing?

Standard stereolithography photopolymers typically exhibit measurable moisture uptake when exposed to humid air or water, producing dimensional swelling, edge softening, and loss of heat deflection under load. In vehicle fluid-handling prototypes and pump test fixtures, this behaviour can obscure the performance of the design being evaluated because the plastic component, rather than the intended metal or composite production material, becomes the source of leakage or dimensional shift. ProtoTherm 12120 is formulated to reduce that uptake. Manufacturer-published water absorption values for the postcured material fall below 0.6 mass% after 24 h immersion at 23 °C when tested under ASTM D570-98(2018) or ISO 62:2008 protocols, depending on part thickness and postcure uniformity. The corresponding dimensional change is lower than that of general-purpose stereolithography grades; however, the resin is not hydrophobic and should not be treated as an immersion membrane.

In water-pump housing prototypes, the build orientation is chosen so that the sealing face is not supported by overhanging supports that generate rough surfaces. The material’s water resistance prevents the wicking of water along layer lines during the test, but layer lines are not eliminated. For sealing applications, the printed face is typically post-machined or coated with a thin sealant because stereolithography layer striations can provide capillary paths regardless of resin hydrophobicity. This limitation is not unique to ProtoTherm 12120 and should be included in design reviews.

Postcure Requirements and Green-State Property Drift

Green-state parts removed from the vat contain solvent-wash residues and unreacted photoinitiator fragments. The postcure step is conventionally performed in a UV flood chamber equipped with UVA sources such as mercury arc lamps or LED arrays operating at 365 nm or 405 nm. Uniform irradiance across vertical walls and internal channels is a practical bottleneck; sections thicker than 10 mm may require staged exposure because light attenuation limits through-cure. Production schedules often specify a solvent pre-clean in isopropyl alcohol or a dedicated resin washer, followed by air drying to remove residual alcohol before UV exposure. If residual alcohol remains, surface crazing can appear during postcure, producing microcrack networks that invalidate water-resistance measurements. The manufacturer recommends verifying postcure effectiveness by measuring the 0.46 MPa heat deflection temperature or the tensile modulus; parts that fall below the lower end of the published range are returned to the postcure chamber.

The relationship between postcure and water resistance is not linear. Under-cured sections show higher equilibrium moisture uptake because residual polar functional groups and low crosslink density increase free volume. Overexposure in a thermal postcure can embrittle the network and raise notch sensitivity without additional improvement in water uptake. The practical processing window therefore treats postcure as a fixed cycle rather than a variable to be maximised. Lot-to-lot variation in photoinitiator content is small but sufficient to shift the optimum by several minutes on older lamp-based units.

The table below summarises representative postcured property ranges reported for DSM Somos ProtoTherm™ 12120 in manufacturer technical literature. Exact values are lot-, build-orientation-, and postcure-dependent.

Application programmers typically select ProtoTherm 12120 for underhood fluid-system models, water-pump housings, manifold segments, and static sealing surfaces where test fluids include water/glycol mixtures at temperatures below 60 °C. In these applications, the material’s value is not high elongation but the retention of bolt-load and seal alignment under intermittent moisture, combined with a heat deflection temperature above the local temperature of an engine test cell. Parts built on 355 nm stereolithography systems with 100 µm or finer layer thickness are hand-finished in support regions and then postcured in UVA cabinets. Dimensional tolerance after postcure is not identical to the green-state build; vertical and horizontal surfaces may differ by several hundred micrometres because volumetric shrinkage during final conversion is anisotropic. This behaviour is consistent with layerwise polymer orientation and is accommodated by scaling factors in the build file rather than by mechanical compensation.

Evaluating the Influence of Postcure Irradiance on Moisture Uptake

Postcure irradiance is a more sensitive control variable than total exposure time because the resin undergoes both free-radical and cationic reactions. Low irradiance may generate radicals that terminate before the slower cationic network builds sufficient molecular weight, leaving a rubbery core. High irradiance at 405 nm can produce high surface conversion but insufficient penetration through thicker sections, creating a gradient in water absorption and glass transition. The result is a part that meets surface hardness but exhibits lower heat deflection in the core. For this reason, postcure fixtures intended for 20 mm wall thickness and above use multi-angle lamp arrays or rotating stages. Radiant exposure should be recorded at the part surface with a calibrated UVA radiometer; the use of an uncorrected timer alone is inadequate for critical water-resistant prototypes. Relevant measurement standards include ASTM D648-18 for thermal distortion and ASTM D570-98(2018) for water uptake, but the postcure condition itself is machine-specific.

A recurring field failure mode in production-scale stereolithography of this class of resin is under-cure near the build platform side when postcure light is incident only from one direction. Floor-side surfaces may show 0.2–0.4 % higher moisture uptake than the exposed top surface for thick blocks. The practical correction is to flip parts after the first postcure interval or to use a wire rack that permits light transmission through the bottom. Batch-to-batch variance in photoinitiator concentration is low, but calibration drift in UVA lamp arrays is a more frequent cause of rejected parts. Typical industrial maintenance intervals specify a radiometric output check at monthly intervals, with lamp replacement below 80 % of initial irradiance.

When the Design Load Involves Sustained Water Contact Under Flexural Stress

Water absorption values alone do not define performance when the part is subjected to sustained flexural load. Absorbed water acts as a plasticiser, lowering the glass transition onset and accelerating creep below the static heat deflection temperature. For ProtoTherm 12120, the 0.46 MPa heat deflection temperature is a short-term heat resistance parameter under dry conditions; it does not represent a creep-rupture limit in humid environments. Prototypes that will run for extended periods in circulating water should be evaluated under load at the intended service temperature. Published data for continuous immersion creep of this specific configuration is limited, so test fixtures frequently use reduced-section tensile bars or cantilever specimens in the actual water bath to validate the design before committing to full printed housings. Where cyclic water spray and thermal cycling are combined, an industrial practice is to add drain slots or pressure-relief openings in the printed part to prevent trapped-water damage during oven drying cycles.

The material imposes handling constraints that differ from general-purpose stereolithography resins. Supports are brittle; removal must occur before full UV postcure, when the green part is still sufficiently compliant. After full postcure, aggressive support removal can cause edge chipping because the network has reached its final crosslink density. Solvent cleaning should be limited to 10–20 min in an ultrasonic bath to avoid solvent-induced microcracking. Drying at 40–50 °C for at least 2 h is typical before postcure; parts with closed internal channels require longer because residual solvent can persist in thin cavities. These are operational boundaries, not defects; failure to observe them produces measurable reductions in water resistance and flexural modulus.

Moisture resistance changes the selection logic among stereolithography grades

Within the manufacturer’s stereolithography resin portfolio, ProtoTherm 12120 is differentiated from general-purpose resins by lower equilibrium moisture uptake and higher dry heat deflection temperature. It is differentiated from impact-modified stereolithography resins by lower notched Izod energy absorption and lower elongation at break. The dry tensile modulus range of 2,000–2,600 MPa is intermediate between rigid unfilled grades and higher-modulus ceramic-filled grades, but the water-resistance characteristic changes the selection logic for humid functional testing. A part built in a general-purpose stereolithography resin may pass a dry dimensional check but fail a water-flow test through edge swelling; the same geometry in ProtoTherm 12120 is more likely to retain its sealing face and bolt preload over the test interval. The trade-off is a narrower postcure window and reduced resistance to impact loading. The resin should not be selected for snap-fit prototypes or parts requiring high strain to failure, because elongation at break in the 5–10 % range is insufficient for many mechanical interlock designs.

Switching from standard grades frequently requires adjustment of support removal timing because the green-state hardness and final hardness after UV postcure are different. The material also has a lower wet modulus retention loss than non-water-resistant grades; in comparative tests under humid ageing, the retained flexural modulus after water exposure is higher, but published data for this specific configuration is limited. The resin is therefore specified for moisture-exposed functional testing where moderate elongation, thermal resistance, and dimensional stability are more important than impact toughness or rapid build turnover.