In architectural massing model production on full-color powder-bed platforms, the dominant failure mode is not green-part breakage during printing but nonuniform salt recrystallization after the first aqueous cure bath. Production-scale equipment in this class—ProJet CJP 660Pro and 860Pro configurations with layer thickness of 0.1 mm and build envelope up to 508 × 381 × 229 mm—runs hollow shell geometries with wall thickness between 3 mm and 5 mm to reduce powder consumption and shorten depowdering time. The hollow core is interrupted by internal drainage channels because trapped air pockets cause incomplete solution ingress during salt cure immersion and subsequent efflorescence on exterior surfaces. The salt bath is prepared at 1 kg of VisiJet PXL Salt Water Cure powder per 2 L of process water; immersion is limited to 20–30 s for this shell thickness. After removal, parts are dried at 20–25 °C and 40–50% RH for 24–48 h to avoid rapid skin drying that creates a salt crust and inhibits internal recrystallization. Airborne gypsum-dust exposure during depowdering is controlled below the OSHA 29 CFR 1910.1000 Table Z-1 nuisance dust limit of 15 mg/m³ total dust and 5 mg/m³ respirable fraction; the powder feedstock is managed under REACH (EC) No 1907/2006 with site-specific SDS exposure scenarios. Terminal output types include 1:100 and 1:200 scale city block massing models, façade detail models, and sectional study pieces. The operational boundary is wall-thickness dependent: walls thinner than 2 mm show variable salt uptake along vertical and horizontal build axes, producing visible mottling; reducing immersion time to 10 s controls mottling but lowers edge hardness, so such thin-wall regions are frequently printed solid rather than hollow.

Foundry Pattern Burnout and Ash Residue

For room-temperature vulcanization silicone tooling, a salt-cured VisiJet PXL positive master is serviceable only when the molding process remains below the dehydration threshold of gypsum dihydrate. Moldmakers often attempt to use these masters in heat-cured silicone vulcanization at 120–180 °C; that is outside the material’s stable window because calcium sulfate dihydrate begins losing water of crystallization above approximately 60–80 °C, increasing surface microcracking and dimensional drift. The recommended downstream route is condensation-cure or addition-cure RTV silicone poured at 23–25 °C. The printed master is produced with binder saturation of 35–45% in the outer 3 mm shell and 15–20% in the internal core, followed by the standard 1 kg/2 L salt bath and a two-part polyurethane or acrylic infiltrant applied in 0.2–0.3 mm wet coats. The infiltrant is required because untreated salt-cured gypsum remains porous and will adhere to poured silicone; a release agent alone does not close surface porosity. Compressive strength before and after salt cure is compared under ASTM C472-20 after oven drying to constant mass at 40 °C, because residual moisture depresses measured values. Production tooling shops report that incomplete drying before silicone pouring causes bubble formation at the master–silicone interface, creating mold defects that appear only after the first 10–20 casting cycles. Terminal products include low-volume polyurethane resin prototypes, wax investment patterns, and polyurethane foam packaging inserts produced from the resulting RTV cavity. Published data for direct lost-wax burnout of salt-cured PXL masters is limited; the material is not recommended as a sacrificial pattern where low ash residue is a metallic alloy cleanliness requirement.

When a hand-held consumer appliance housing is printed with full CMYK graphics, the critical process conflict lies between binder saturation and salt-cure color shift. High binder saturation in shell zones increases green strength but intensifies lateral color bleed at thin wall features; the machine profile is therefore set to 30–35% shell saturation and 10–15% core saturation for walls from 1.5 mm to 3.0 mm. After bulk depowdering with compressed air regulated to 2–3 bar, the salt-cured VisiJet PXL part is immersed in the salt-cure bath for only 10–15 s because longer exposure creates visible salt bloom on dark blue and red printed areas. Drying proceeds at 20–25 °C for 24 h before a clear polyurethane topcoat is applied at a dry film thickness of 0.05–0.10 mm; this coating is measured on metal reference coupons with an eddy current gauge, though the gypsum substrate limits absolute transferability of the reading. The print service is operated under ISO 9001:2015 process control, and the finished mock-up is not placed into service as an electronic enclosure; therefore IEC 62368-1 and RoHS 2011/65/EU assembly declarations do not apply unless functional electronic components are introduced. Terminal products include full-color appearance models for home appliances, personal care packaging, footwear color studies, and retail display mock-ups. Operational limitation: salt-cured parts exposed to relative humidity above 70% can exhibit surface tack and reduced topcoat adhesion; such parts require a sealed display environment or an alternative non-aqueous infiltrant.

What Limits Color Fidelity After Immersion in Brine-Based Curing Solutions?

In geospatial terrain reproduction, the primary colorimeter-measured defect is not hue drift in planar areas but edge darkening at sharp ridgelines. This defect arises because the cure solution accumulates at concave slope intersections by capillary action, leaving higher mass loading of recrystallized salt per unit area. A production run using a digital elevation model with 10 m grid spacing and 1:5,000 horizontal scale produces ridge walls often below 1 mm thickness; the printer’s binder saturation is lowered to 18–22% in the core to reduce soluble binder residue, while the shell saturation is held at 28–32% to retain feature geometry. Post-print, bulk powder is removed using a vacuum system with 20–25 kPa negative pressure; brush-only cleaning is insufficient on steep terrain because unremoved powder becomes cemented by the salt bath. The salt bath is the supplier-certified 1 kg/2 L ratio, but immersion is kept to 5–10 s for models with thin ridge features. Terminal products include hydrographic basin models, urban heat-island study surfaces, and transportation corridor relief displays. A documented limitation is that the material itself carries no validated ISO 19115-1:2014 metadata claim because that standard governs geospatial data, not physical model output; any deliverable metadata therefore belongs to the source DEM, not the printed part.

When Surgical Planning Models Require Repeated Handling Without Enclosure

Surgical planning models produced from DICOM segmentation inherit the anatomical noise of thin trabecular structures, and the salt-cure step creates a surface hardening gradient that is useful only if the model is intended for repeated hand-held review. In this workflow, the segmentation threshold is set to preserve bone boundaries at 0.5–1.0 mm minimum wall thickness; thinner walls are shelled manually or supported by neighboring anatomy to prevent delamination during depowdering. Binder saturation is set to 35% in the outer 2 mm shell and 20% in the core, with salt bath exposure limited to 10 s for models with fine sinuses, nasal conchae, or thin orbital floors. Dimensional verification is performed against the source DICOM dataset using a calibrated noncontact scanner; tolerances are documented per the receiving facility’s quality procedure rather than implied by a generic ISO 2768-1 tolerance class. Quality management under ISO 13485:2016 and risk analysis under ISO 14971:2019 apply when the printing service operates within a hospital or contract manufacturing pathway; in U.S. supplier pathways, 21 CFR Part 820.30 design controls may apply if the model is supplied as a patient-specific device used for diagnostic or treatment planning. The model itself is not an implantable or invasive device, so ISO 10993-1 biological evaluation is not triggered by transient contact with intact skin; mucosal or invasive use requires a case-specific assessment. Terminal products include craniofacial surgical rehearsal models, cardiac anatomy study models, and patient-specific teaching aids. Repeated steam autoclaving is prohibited; salt-cured gypsum loses dimensional stability above 60 °C and is not compatible with hydrogen peroxide plasma sterilization cycles.

Within museum reproduction workflows, full-color powder-bed processing maps captured surface texture at 0.1 mm layer increments, but the salt-cure bath can generate efflorescence on dark ochre and carbon-black regions within 48–72 h if drying is not uniform. The standard 1 kg/2 L salt bath is used at a reduced immersion time of 5–10 s; published data for alternative lower-concentration archival baths is limited, so deviation from the supplier-certified ratio is not recommended. Binder saturation is set to 25–30% in the shell and 10–15% in the core to balance color vibrancy against disintegration during depowdering. The printed replica is sealed with a water-based acrylic dispersion at a wet film thickness of 0.1 mm, then conditioned at 20 °C and 45% RH for 72 h before transfer to display. Handling compliance follows institutional risk assessment; the object is not suitable for children’s full-mouth contact because EN 71-3 migration limits are not evaluated on this product. Terminal products include tactile teaching replicas, artifact reproduction for exhibition, and archaeological site dioramas. Ultraviolet light accelerates binder-dye fading; display illumination is filtered to exclude wavelengths below 400 nm and kept below 50 lux for water-sensitive colorants.

When engineering teaching laboratories produce tactile, color-coded anatomical or mechanical models in large batches, the binding constraint is dry time rather than surface finish. Class sets are printed with low shell saturation of 20–25% and core saturation of 10% to reduce binder consumption; this produces a weaker green part that requires gentler depowdering at 1–2 bar air pressure before the salt bath. The salt bath uses the standard 1 kg/2 L concentration, but dwell time is capped at 15 s because student-handled models with protruding features can trap solution and form salt crusts in joint recesses. After drying for 48 h at 20–25 °C, parts are marked with heat-transfer identifiers if required; no solvent-based paints are used because residual salt can cause adhesion failure. The instructional facility operates under general workshop dust controls aligned with OSHA 29 CFR 1910.1000 Table Z-1; the material is not certified under ASTM D4236-94(2016) as a children’s art material. Terminal products include color-coded anatomy teaching models, exploded mechanical assembly trainers, and science fair demonstration parts. Batch-to-batch variation in ambient humidity can shift drying time by ±12 h; racks should be spaced to allow continuous airflow rather than stacked in enclosed trays.