3D Systems Accura Amethyst™ Plastic for Viper™ SLA Systems is a low-viscosity stereolithography resin qualified for the 355 nm solid-state laser source used in the Viper SLA platform. The liquid is processed at a vat temperature of 30 °C, at which the supplier lists viscosity as approximately 250 cps. That viscosity class is lower than filled SLA resins and enables faster recoating, improved drainage from internal channels, and reduced solvent carryover after part removal. The product is not qualified for 385 nm or 405 nm systems; the photoinitiator package is matched to the Viper laser wavelength. Layer thicknesses of 0.05 mm and 0.10 mm are used depending on the build style. The resin is supplied for general-purpose prototyping, master patterns, and investment casting patterns where low-viscosity handling and fine sidewall detail take priority over high elongation or high heat deflection.

Liquid density at 25 °C is approximately 1.13 g/cm³. The density difference between liquid and cured states contributes to volumetric shrinkage during polymerization, so build styles for Viper systems include scale factors and beam compensation values. The low viscosity also means that the recoating blade can form a continuous film at higher speed than high-viscosity filled grades, but the same low viscosity causes drainage from blind pockets if the build platform is not sealed or if the blade gap is set improperly. Production use requires the vat heater controller to hold temperature within ±1 °C of the setpoint. Temperature drift below 28 °C raises viscosity and can produce wave defects on vertical sidewalls; temperature drift above 32 °C lowers viscosity and can reduce edge definition on fine ribs.

Laser working curve and recoating boundary conditions

The relation between cured depth and exposure is described by Cd = Dp ln(E/Ec), where Cd is cured depth, Dp is penetration depth, E is laser energy dose, and Ec is critical exposure at 355 nm. On Viper systems, Dp and Ec are stored in the resin calibration file and are lot-specific. Manual adjustment of these parameters outside a formal working-curve calibration is not recommended. When a 0.05 mm layer is used, the applied energy must exceed Ec by a sufficient factor to ensure cure through the layer and adhesion to the previous layer. Undercure caused by a dirty laser window or an aged resin surface manifests as interlayer separation at the corners of bosses and along leading edges. Overcure increases lateral cure and closes small channels; the diameter reduction is most apparent in channels below 0.8 mm. A blade gap value of 0.05 mm is typical for 0.05 mm layers; gap settings below 0.03 mm may introduce oxygen bubbles at the leading edge of the film. Published data for this specific configuration is limited, and process validation on the specific Viper vat is required.

Representative post-cured mechanical data are shown in Table 1. The values are obtained from supplier-published datasheets and are not production specifications. Build orientation, layer thickness, post-cure chamber uniformity, and wall thickness shift the measured values; lot-specific certificate data should be consulted before release of a production part.

The tensile modulus of approximately 2,400 MPa places Accura Amethyst in the rigid unfilled SLA class. The elongation at break near 4% is lower than many snap-fit resins, and thin sections below 0.8 mm with sharp internal radii below 0.25 mm can fracture during assembly. The heat deflection temperature of 56 °C at 0.45 MPa means that continuous load-bearing service above that temperature is outside the resin’s operational boundary. Transient exposure to 60 °C can be acceptable for unstressed parts, but dimensional creep should be evaluated by a heat sag test. The notched Izod impact value of 16 J/m limits the material to general-purpose housings and patterns, not high-impact service. These boundary conditions are not defects; they define the substitution window against tougher SLA grades.

Dimensional stability after post-cure is not immediate. Parts measured immediately after UV post-cure can show additional linear shrinkage of 0.1–0.3% over 48 h at 25 °C. Dimensional inspection should be delayed or conditioned at 23 ± 2 °C and 50 ± 10% RH for 24 h before measuring. Moisture uptake is lower than polyamide SLS but not zero. Published data for this specific configuration is limited; therefore, critical dimensions should be verified after conditioning rather than immediately after removal from the post-cure chamber.

What breaks when post-cure is skipped in silicone tooling?

Incomplete UV post-cure creates a process conflict in RTV silicone tooling. Uncured monomer and low-molecular-weight photoinitiator residues on the pattern surface migrate into the addition-cure silicone and complex with platinum catalyst sites. The visible defect is a sticky inhibition layer that remains uncured within the normal 24 h room-temperature cure. The risk is highest when the pattern has been cleaned only with alcohol and immediately transferred to the mold box. The corrective control is a uniform UV post-cure to a tack-free surface, followed by a Shore D hardness check: if two consecutive post-cure intervals of 10 min change the surface hardness by less than 2 Shore D units, the part is considered stable. If the part is still increasing in hardness, it is not ready for silicone contact. Because published data for this specific configuration is limited, the hardness criterion should be validated with the specific RTV grade and Viper build style.

Solvent cleaning is an operational boundary. Acetone should not be used on thin walls below 0.5 mm, because solvent attack can create microcrazing that appears only after painting or metallizing. Tripropylene glycol monomethyl ether (TPM) followed by isopropyl alcohol is the accepted cleaning sequence. Ultrasonic cleaning for detailed parts is limited to 5 min at 40 kHz; longer sonication can erode fine ribs and sharp edges. Compressed air drying should be kept below 0.2 MPa to prevent fracture of unsupported thin walls. Ambient humidity above 60% RH during open-vat operation can introduce water droplets at the free surface and alter polymerization in the next layer; the system cover should remain closed between builds and the vat should be kept in a low-humidity enclosure in humid production environments.

Accura Amethyst differs from several other 3D Systems SLA materials by its system qualification and viscosity profile rather than by a single mechanical property. Accura 25 is designed for polypropylene-like ductility and is used for snap-fit prototypes that require repeated bending; Accura Xtreme is selected for higher notched Izod impact in tough enclosures; Accura Bluestone is selected for ceramic-filled high modulus and lower thermal expansion. Accura Amethyst is selected when the dominant requirement is fine sidewall quality at low vat viscosity on Viper systems. The material is not a direct substitute for Accura CastPro in investment casting workflows that require ash-controlled burnout, although it is used for shell patterns where fine internal channels must drain before slurry coating. Users should review the build style and laser calibration file in the Viper build preparation software when replacing one Accura resin with another, because the working curve parameters, recoating settings, and part finish differ even when the mechanical property tables are similar.

In investment casting, a low-viscosity resin reduces retained liquid in a 6 mm internal channel after draining, but the pattern must still be cleaned with TPM and fully UV post-cured before shelling. Exudate from an undercured pattern weakens the primary slurry layer and can create surface porosity in the shell. The shell firing schedule should include a slow ramp through 200–400 °C to avoid shell cracking from thermal expansion of the cured polymer. Published ash content for this specific configuration is limited; a lot-specific ash test is required before the pattern is used in reactive metal casting or aerospace turbine components.

When Accura Amethyst replaces Accura 25 or Accura 60 in a prototype workflow

The substitution decision is governed by the design’s strain at assembly and the thermal load in service. If a snap-fit beam is strained beyond the 4% elongation at break of Accura Amethyst, it should be redesigned or moved to a higher-elongation resin. If the part is a clear optical prototype, Accura Amethyst is not a validated replacement for Accura ClearVue; optical transmittance is lower and the cured part has a visible tint that may interfere with color-validation prototypes. In applications that only require dimensional accuracy and clean sidewalls, Accura Amethyst can replace Accura 25 or Accura 60 provided that the build style is revalidated and the lower impact toughness is accepted. The replacement should not be made without comparing the post-cure heat deflection values of both resins at the relevant stress level.

The cured resin can be sanded, primed, and painted after full post-cure. Cyanoacrylate adhesives are used for small bonds and two-part acrylic adhesives for structural bonds. Surface preparation should include light abrasion with 600-grit paper after full cure; sanding before full cure smears undercured resin into the surface pores and creates adhesion defects. Solvent-based primers containing strong ketones should be avoided on sections thinner than 0.5 mm. Bond strength should be evaluated by lap shear specimens according to ASTM D1002-10, not by visual inspection alone.

Storage of uncured Accura Amethyst follows the supplier SDS. The resin is stored in the original opaque cartridge at 15–30 °C. Exposure to light below 420 nm can cause gelling in the container or vat; windows without UV blocking film should be excluded from the SLA room. Storage above 35 °C accelerates dark polymerization and shortens shelf life. Frozen storage is not used because component crystallization can occur. Cured parts are not food-contact articles by default; compliance must be reviewed under FDA 21 CFR 177 or EU 10/2011 only with the specific lot and post-cure protocol. REACH and RoHS status should be confirmed using the supplier’s compliance statement for the specific lot shipped.

In a final application scenario, Accura Amethyst is used for a low-volume RTV mold master for a connector housing. The master is built at 0.05 mm layers with the Viper si2, cleaned in TPM, post-cured until the Shore D hardness stabilizes within 2 units, and then conditioned at 25 °C and 40% RH for 24 h before silicone casting. The mold surface reproduces the stair-step texture of the layer lines unless the master is sanded and polished; polishing must be done after post-cure to avoid smearing undercured resin. This sequence avoids the platinum inhibition failure described above and produces a usable RTV mold without post-cure exudate. Published data for this specific configuration is limited; therefore, the first production lot should be inspected for inhibition and dimensional drift before release.