Accura CastPro Free (SL 7800) is a transparent, antimony-free, unfilled photopolymer resin supplied by 3D Systems for stereolithography-generated sacrificial patterns in investment metal casting. The designation SL 7800 identifies the resin grade within the Accura CastPro Free series rather than a standalone machine platform; it is qualified for vat polymerization on production stereolithography equipment with a UV laser source operating near 355 nm. The phrase “polycarbonate-like” describes the cured material’s optical transmission, rigidity, and handling behavior relative to unfilled bisphenol A polycarbonate; it does not denote the presence of polycarbonate repeat units. The product is used where burnout must leave low inorganic ash, where visual confirmation of internal QuickCast drain paths is required, and where antimony is a controlled impurity in the cast alloy.

Physical and Optical Benchmarks in the Cured Pattern State

Representative supplier-published data for fully cured Accura CastPro Free (SL 7800) are listed below. Specimens are built in the xy orientation at 100 µm layer thickness and conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity before mechanical testing unless the test method requires otherwise. The cured solid density of approximately 1.18 g/cm³ is close to that of standard unfilled polycarbonate, while the liquid density of approximately 1.12 g/cm³ at 25 °C indicates conventional photopolymerization shrinkage between the vat and the solid state. The heat deflection temperature range of 55–60 °C at 0.46 MPa defines the upper boundary for continuous handling under low load; at 1.82 MPa, the threshold drops to 50–55 °C. These values are thermomechanical acceptance indicators, not proof that the pattern can survive uncontrolled autoclave or shell-curing temperatures.

The values are representative datasheet values and do not replace lot-specific certificate of analysis limits. Batch-to-batch variation on production stereolithography equipment is typically narrower in mechanical tensile values than in ash residue, because ash content follows small changes in photoinitiator and stabilizer residuals.

On large-frame stereolithography equipment, Accura CastPro Free (SL 7800) is processed as a low-viscosity liquid that can be recoated at layer thicknesses between 50 µm and 100 µm. The material is normally paired with QuickCast build styles that generate hollow internal lattice supports instead of solid sections. A solid pattern increases burnout time and increases thermal expansion forces against the ceramic shell, so production files incorporate interconnected drain passages and open vent features. The transparency of the cured resin permits visual confirmation of drain continuity before shelling; this is particularly relevant when patterns are nested in arrays and a blind cavity would otherwise retain uncured resin. Drainage after unloading typically combines gravity, low-pressure air, and solvent rinsing with a compatible organic solvent. Residual liquid in a high-aspect-ratio cavity is a process-related risk: during the first elevated-temperature ramp, trapped liquid volatilizes and generates internal pressure that can exceed the green shell strength. Solvent rinsing with isopropanol or tripropylene glycol monomethyl ether is standard; chlorinated solvents should be avoided because they can craze the cured surface.

Accura CastPro Free (SL 7800) is supplied as a single-component photopolymer; no catalyst premixing or filler suspension is required before vat loading. Lower-viscosity resin grades within the Accura family require different recoating parameters, so process engineers should not transfer build parameters from other resins without recalibrating the working curve. On production platforms with variable beam diameters, the cure depth and critical fluence for SL 7800 differ from those of filled or heavily stabilized resins; thin features may require reduced laser power to avoid overcure-induced dimensional offset. This is particularly important when building internal lattice supports, where overcure can partially seal drain channels before the shell is applied. The cure depth follows the standard stereolithography working curve in which cured thickness is proportional to the natural logarithm of incident fluence; the resin-specific penetration depth and critical fluence are established on the target machine class rather than transferred from small-platform systems.

Can Low Residual Ash Be Reached Without an Antimony Synergist?

Antimony-free sacrificial resins reduce inorganic residue because antimony trioxide and related synergists can survive incomplete burnout as discrete particles inside the shell cavity. Accura CastPro Free (SL 7800) is formulated without an antimony synergist; its low ash response after 750 °C burnout is a result of the unfilled matrix and low-residue photoinitiator package. The relevant measurement is thermogravimetric residue under oxidative heating, not visual inspection. A clear pattern that appears fully combusted can still leave sub-visible residues that affect reactive-metal lot acceptance, especially for titanium and nickel-based superalloys where inclusions are controlled by spectrographic methods. When comparing the Free grade to standard Accura CastPro, the distinction is most meaningful in applications where antimony pickup is an audit parameter. Published quantitative differences between the two specific supplier grades are limited outside supplier technical bulletins and end-user qualification reports; the antimony specification should be taken from the lot certificate for the ceramic shell and the casting standard.

Differences from non-photopolymer casting-pattern materials are apparent in softening resistance, dimensional control, and residue behavior. Filled wax blends commonly soften over 35–45 °C; Accura CastPro Free retains a 55–60 °C heat deflection temperature at 0.46 MPa. PMMA-based sacrificial thermoplastics require injection molding or machining; Accura CastPro Free is built directly by stereolithography and can produce hollow QuickCast geometries without tooling. Compared with wax, the photopolymer is rigid rather than ductile, so thin walls below 0.5 mm require careful support placement and shell handling. Compared with opaque or heavily filled sacrificial polymers, the transparent amber state provides a diagnostic advantage for drain-path verification, but transparency is not a proxy for complete burnout.

If the Shell Is Ramped Too Quickly, Where Does the Pattern Fail First?

Thermomechanical failure during pattern burnout is controlled by thermal expansion mismatch, the pattern’s cross-sectional mass distribution, and the permeability of the primary shell coating. The cured polymer expands with a coefficient of 75–90 µm/(m·°C) between 25 °C and 50 °C; above the glass transition near 62 °C, load-bearing rigidity falls rapidly. If the ceramic shell is heated faster than the polymer can flow from the drain path, internal pressure increases. On production-scale shells, the first fracture tends to initiate at the thinnest wall or at the junction between a drain vent and the main pattern body. This failure mode is observed more often in solid patterns than in properly drained QuickCast patterns, because solid cross-sections provide larger thermal mass and lower volumetric escape volume. Burnout schedules for Accura CastPro Free therefore separate the thermal expansion phase below 100 °C from the combustion phase between 300 °C and 600 °C, with an intermediate hold to allow organic volatiles to exit before the shell reaches high-sintering temperature.

For metal casting service, Accura CastPro Free (SL 7800) is assembled onto wax sprue systems with pattern adhesives that must be selected for low residue; adhesive stacks that contain metallic pigments or mineral fillers can reintroduce the very residue the resin is intended to avoid. Ceramic shell construction is typically carried out in humidity-controlled rooms at 20–24 °C; the pattern remains dimensionally stable within that range. The resin is not intended as a functional plastic part, and its heat distortion threshold excludes continuous exposure above 50 °C before ceramic investment. Containers should be stored in opaque packaging at 5–30 °C; stray UV/visible light can initiate dark polymerization and change viscosity. Chlorinated solvents are incompatible with cured surfaces and should not replace the specified rinsing solvent. Batch qualification for a specific casting line should include a full burnout thermogravimetric trace on the actual shell system, because shell composition and vent geometry can shift the apparent residue and crack threshold more than material variation alone.