For direct-shelling investment casting patterns in gas turbine repair cell operations, the build configuration is defined by ceramic shell expansion stresses rather than photochemical conversion alone. DSM Somos ProtoGen™ 18920 liquid photopolymer is built on a 355 nm vat photopolymerization system with a 0.10 mm layer thickness, a 0.15 mm laser spot diameter, and a hatch spacing of 0.08 mm to 0.12 mm. The pattern is generated as a hollow shell with an internal hexagonal lattice, a wall thickness of 2.5 mm to 3.5 mm, and drain holes of 3 mm to 5 mm diameter. After solvent rinse in tripropylene glycol monomethyl ether and UV postcure at 1.5 J/cm² UVA, the pattern receives a thermal postcure at 70°C for 2 h in forced air. The thermal step raises surface conversion above 92% before ceramic investing. Insufficient postcure leaves residual acrylate unsaturation that softens during autoclave exposure at 150°C and causes shell distortion. Foundry protocols require linear shrinkage measurement per ASTM D2566-86 on a 100 mm bar before primary slurry application. Shell cracking during pattern burnout is managed by targeting ash residue below 0.05% and ramping from 200°C to 900°C at 2°C/min under an oxidizing atmosphere. Published data specific to ProtoGen 18920 in industrial turbine blade shelling is limited; production foundries validate each batch by casting a 15 mm wedge section and sectioning it for porosity.

For small rotating parts, the pattern is oriented 20° to 40° from the vertical so that recoat blade pressure does not induce layer delamination on unsupported leading edges. Support contact marks are removed by wet sanding with 600-grit silicon carbide paper before applying a 0.5 mm primary slurry coat of fused silica and colloidal silicate. The primary coat pH is held between 9.5 and 10.5 to avoid acid hydrolysis of residual photopolymer at the pattern surface. Secondary slurry viscosity is 8 s to 10 s on a Zahn #4 cup. Terminal cast parts include turbine blade replacement components and pump impellers in 17-4PH stainless steel and Inconel 625. The pattern is not suitable for direct resin-burnout in microwave dewax systems where local temperatures exceed 1,200°C before oxygen can penetrate the thick shell.

How Does Platinum-Cured RTV Silicone Tooling Constrain the Postcure Sequence for ProtoGen 18920 Masters?

Master patterns for room-temperature vulcanization silicone tools are prepared from ProtoGen 18920 at a layer thickness of 0.05 mm when the average surface roughness of the tool must be below 0.8 µm Ra per ISO 4287. Platform parameters shift to a 250 mW laser power at 355 nm, and a 0.05 mm hatch spacing. Published data for this exact photopolymer in platinum-cured RTV applications is limited. However, residual amine or sulfide species in photopolymers are known to poison platinum-catalyzed hydrosilylation at the master interface. To reduce inhibition risk, the master pattern undergoes UV postcure at 1.8 J/cm² in a 360 nm to 420 nm broadband chamber, followed by thermal postcure at 75°C for 4 h and an additional 30 min at 90°C under vacuum. Outgassing of residual low-molecular-weight acrylate species is monitored with a vacuum pressure rise test; a master is not released to tooling until chamber pressure remains below 1.0 kPa after 10 min isolation. If tin-condensation-cure RTV is substituted, inhibition is avoided but tool shrinkage increases to 0.6% to 1.2% compared with 0.1% to 0.3% for platinum cure. Terminal outputs are polyurethane cast parts in 45 Shore A to 65 Shore A elastomer. The pattern is frequently molded with a 10 mm to 15 mm silicone wall over a rigid epoxy backup shell. No silane-coupling release agent is used because it alters interfacial wetting of platinum RTV.

Dimensional control for the silicone mold is referenced to ISO 16916:2016 for tool-in-use shrinkage. The master is kept in a desiccated cabinet at 23°C and 35% RH for 48 h before molding to stabilize moisture expansion. Build orientation for flat master plates is 15° from horizontal to reduce cupping. After thermal postcure, the surface is lightly polished with 3 µm aluminum oxide slurry; deep polishing cuts through the highly crosslinked surface skin and can expose less-cured core material, which increases silicone inhibition.

Medical Diagnostic Housing Prototypes, ISO 13485 Design Control, and Outer Surface Cleanability

Design verification groups producing portable diagnostic instrument housings with ProtoGen 18920 operate under ISO 13485:2016 document control, but the photopolymer is not designated as a body-contact material. The housing prototypes are used for benchtop usability testing, board fitment checks, and drop-tower evaluation. Mechanical property acceptance is anchored to ASTM D638-14 tensile values obtained on solid specimens built in the same build orientation as the housing walls. The material is postcured with UV at 1.2 J/cm² followed by thermal cure at 65°C for 2 h. Surface cleanability is tested by wiping with a 70% isopropyl alcohol / 30% deionized water solution for 60 cycles; unpigmented photopolymer surfaces may exhibit slight tack after repeated exposure to strong solvents. The manufacturer has not published a full ISO 10993-5:2009 cytotoxicity dataset for this product, so biocompatibility screening is outsourced only when the prototype contacts intact skin for more than 24 h in a formative study. The housing prototypes are assembled with brass heat-stake inserts. Insert retention strength is measured at 23°C using a screw pull-out speed of 5 mm/min. The material is not recommended for steam autoclave sterilization at 121°C because the heat deflection temperature under 0.46 MPa falls below this temperature for most unfilled stereolithography resins.

In subsonic wind tunnel development for underbody aero components, ProtoGen 18920 models are built as scale 1:4 and 1:5 shells with a 0.10 mm layer thickness. The surface must be sealed because the as-built stereolithography stair-step roughness, typically Ra 2.0 µm to 3.0 µm at 0.10 mm layer thickness, exceeds the Ra 1.0 µm requirement for attached flow visualization. Post-processing includes solvent rinse in 2-propanol, UV postcure at 1.5 J/cm², thermal cure at 60°C for 1.5 h, then hand sanding with 320-grit abrasive and a polyester spray filler. The filler is wet-sanded to 600-grit. The wind tunnel model is tested at airspeeds up to 40 m/s because the heat deflection temperature of this unfilled photopolymer limits use at higher stagnation temperatures. The part is mounted on a six-axis force balance through a 10 mm aluminum backbone cast into the model. Pressure taps of 0.8 mm inner diameter are produced as sacrificial holes and cleared with a 0.8 mm reamer. The terminal output is an aero load map for front splitter and rear diffuser development. The photopolymer shell is not used for transonic or hot-cycle testing where surface temperatures exceed 50°C. Build orientation is set at 5° to 15° from the airflow direction to minimize separated flow from layer steps. Published data for this exact use is limited; teams typically run a baseline test with an aluminum reference plate to correct for model flexure.

Because the model is a hollow shell, internal ribs are spaced at 25 mm to 30 mm and wall thickness is 2.0 mm to 2.5 mm. Drain holes are placed in low-pressure regions. The resin is not loaded with abrasive fillers, so machining of pressure tappings must use new high-speed steel drills at 4,000 rpm with soluble coolant to prevent heat buildup. Model weight is kept below 3.5 kg for the balance load cell. The postcure is necessary to reduce creep under aerodynamic loads; uncured or UV-only postcured models show time-dependent deflection of 0.2 mm over a 60 s load period at 30°C.

PCB Depaneling Fixtures Demand Predicted Creep Modulus at 23°C and Controlled Electrostatic Discharge

Assembly lines that depanel thin FR-4 boards use router fixtures with vacuum channels and clamp nests produced from ProtoGen 18920. The fixtures are designed with a 0.10 mm or 0.15 mm layer thickness, depending on whether locating features are smaller than 2 mm. Thermal postcure is set to 80°C for 2 h in a recirculating oven to maximize the glass transition onset and reduce creep at room temperature. The material is an electrical insulator with surface resistivity typically above 10¹³ Ω when dry, so it is not inherently static-dissipative. An engineered conductive carbon-loaded acrylic coating is applied to the upper surface to achieve a surface resistivity between 10⁶ Ω and 10⁹ Ω per IEC 61340-5-1. The control plan requires the fixture to be marked with a lot number and the coating thickness to be verified at 25 µm to 40 µm by an eddy-current probe. Terminal use is a router fixture for depaneling 150 mm by 220 mm boards with 1.6 mm thickness.

Board locators are built to +0.05 mm / -0.00 mm over a 165 mm span. Because unfilled stereolithography resins absorb moisture from air, fixture dimensions shift by 0.1% to 0.2% when stored at 80% RH for 48 h; a conditioning protocol of 23°C and 50% RH for 24 h before coordinate measuring machine verification is mandatory. The router spindle creates a lateral cutting force of 25 N to 40 N at 40,000 rpm; the fixture walls are stiffened with 8 mm ribs on a 25 mm grid.

When PETG Sheet Contact Temperatures Exceed the Heat Deflection Temperature of an Unfilled Photopolymer

Prototype thermoforming tools for PETG blister packaging are occasionally produced from ProtoGen 18920 for short runs. PETG is processed at sheet surface temperatures of 110°C to 130°C. The unfilled photopolymer has a heat deflection temperature that is field measured according to ISO 75-2:2013 at 0.46 MPa. In most unfilled epoxy/acrylate stereolithography resins, the HDT at 0.46 MPa ranges from 50°C to 70°C. A tool made from ProtoGen 18920 therefore operates in a transient heat transfer mode; the aluminium tool base acts as a heat sink, and the photopolymer face must not remain in continuous contact with the heated sheet for more than 15 s per cycle. Vacuum holes of 0.6 mm to 0.8 mm are drilled through the face after thermal postcure at 70°C for 3 h. The tool face is coated with a PTFE dry-film lubricant to reduce PETG sticking. The mould is used for 20 to 50 cycles, not for production volumes exceeding 1,000 parts. Published data for this specific configuration is limited; a trial tool is validated with a thermocouple embedded 1.0 mm below the face, and face surface temperature is kept below 55°C with forced-air cooling between cycles.