DSM Somos ProtoCast™ AF 19122 Antimony-free Liquid Photopolymer is a vat-photopolymerization resin formulated for the production of sacrificial investment-casting patterns. The cured polymer article is used in an additive manufacturing workflow in which a ceramic shell is built around the pattern, after which the polymer is removed by thermal burnout and the cavity is filled with molten metal. The designation “antimony-free” identifies a compositional boundary: the formulation does not rely on antimony-based additives that are present in certain castable photopolymer systems. This boundary is relevant to foundries that restrict antimony oxide in combustion residues, to casting operations that experience surface inclusions from burnout residue, and to production sites that document metal-oxide emissions from shell removal.

The material is supplied in liquid form and requires stereolithography equipment with a compatible wavelength and recoating system. Typical uses include jewelry investment casting, dental framework patterns, and low-volume industrial castings where complex geometry or small lot sizes do not justify wax injection tooling. Because the pattern is generated additively, the process eliminates the need for pattern-specific tooling and permits gate-and-riser geometry to be produced directly with the part.

What Process Constraints Follow from Antimony-Free Photopolymer Chemistry?

Antimony compounds, when present in castable photopolymers, can act as flame-retardant synergists, thermal stabilizers, or opacifiers. During burnout they may be converted to antimony trioxide, Sb₂O₃, or other non-volatile oxides. Sb₂O₃ has a melting point near 656 °C and can persist as a fused residue in ceramic shell pores. If the residue is entrapped in blind passages or narrow cores, the metal front can displace it and create a surface defect or non-metallic inclusion. The antimony-free formulation removes that specific residue class at the material level.

The absence of antimony compounds is not a direct drop-in replacement for antimony-containing resins in all cases. Antimony additives influence refractive index, optical scattering, and thermal decomposition behavior. The photoinitiator package and cure response in ProtoCast AF 19122 therefore require machine-specific validation. The working-curve constants critical exposure, Ec, and depth penetration, Dp, should be re-established for the actual laser spot size, resin temperature, and chamber atmosphere. Published data for alternative machine configurations is limited, and first-article inspections are required after any change in build orientation, layer thickness, or post-cure protocol.

Specification Fields for Lot Acceptance and Process Control

Lot acceptance for a liquid photopolymer of this class typically includes viscosity, density, cure response, and cured mechanical properties. The values are post-cure dependent, so test specimens should be processed according to the manufacturer’s post-cure protocol and not mixed with specimens from other resin families. Table 1 lists the characterisation fields and the standards commonly applied to antimony-free castable photopolymers. The actual nominal values applicable to the current lot must be taken from the manufacturer’s technical datasheet and certificate of analysis because specification limits vary by formulation revision.

Cured mechanical values are also influenced by post-cure irradiance and total exposure dose. A calibrated radiometer should be used to confirm the UV source delivers the dose specified in the resin datasheet, expressed in J/cm². Under-post-cured patterns can retain unpolymerized monomer that exotherms during shell burnout, while over-post-cured patterns can become brittle and crack during shell handling. Therefore, the post-cure step is considered part of the material specification envelope, not a secondary operation.

Production-scale stereolithography platforms for castable resins commonly use solid-state lasers operating at the wavelength specified by the resin manufacturer. The vat temperature is maintained at the manufacturer’s recommended set point because viscosity and cure kinetics are coupled. Recoat blade speed and wait times are adjusted so that each slice receives a uniform liquid layer before laser scanning. For thin-walled patterns, build orientation should place shell drainage paths in the direction of furnace venting. If the pattern contains closed internal cavities, decomposition gas pressure during burnout may be sufficient to crack the ceramic shell.

When Low Residual Ash Is a Critical-to-Quality Parameter in Shell Burnout

The burnout cycle for ProtoCast AF 19122 is a process-critical operation. Furnace burnout of the photopolymer pattern follows a staged heating profile. In the first stage, the ceramic shell is heated slowly through the glass transition and thermal expansion region of the polymer. An excessive ramp rate produces thermal stress at the polymer-shell interface and can initiate shell cracking. In the second stage, the temperature is held to allow volatile decomposition products to escape through the sprue and vent channels. In the third stage, the furnace is brought to the shell firing temperature and held to oxidize carbonaceous residue. Antimony-free chemistry changes the ash composition but does not eliminate the need for oxygen access.

Shell permeability, pattern cross-section, and the number of shell coats determine the maximum safe ramp rate. Foundries often validate burnout by sectioning fired shells and inspecting for carbon deposits or oxide residues before pouring. The absence of antimony compounds reduces the probability of a specific non-volatile oxide residue, but carbon residue can still form if the furnace atmosphere is stagnant or if the shell is inadequately vented. Burnout profiles must therefore be validated for each shell system and part family; published data for all shell configurations is limited.

Use of a castable photopolymer differs from conventional foundry wax in a key operation. Wax patterns are often removed by steam autoclave dewaxing, but a crosslinked photopolymer cannot be melted out in that manner. Direct thermal burnout is used instead. This changes vent design, shell formulation, and furnace loading. Compared with antimony-containing castable stereolithography resins, the principal distinction is compositional: the formulation does not generate antimony oxide during shell firing. This can reduce the analytical burden for facilities that track antimony in process residues and can simplify waste stream documentation where customer specifications prohibit antimony-bearing additives. It does not automatically satisfy every restricted-substance list because the formulation contains other organic and inorganic components; material compliance documentation should be requested from the supplier for customer-specific requirements.

Compared with general-purpose SLA resins, a castable resin must balance green-part strength with complete decomposition. A resin that leaves silica, glass-fiber, or mineral filler residues is unsuitable for investment casting because the residue cannot be readily removed from the shell cavity. ProtoCast AF 19122 is positioned for the castable category rather than for direct-use engineering parts. Compared with heavily filled or pigmented castable resins, the antimony-free formulation is intended to leave a lower total residue under controlled incineration. Low-residue behavior is nevertheless dependent on shell permeability, furnace airflow, and part mass. Residual carbon from incomplete burnout remains possible if oxygen access is restricted during the heating profile.

Pattern Storage Must Control Moisture Uptake and Dimensional Drift

Moisture uptake in cured patterns is a dimensional risk. If patterns are stored before shelling under high humidity, water absorption can produce small dimensional changes and can contribute to shell cracking if trapped moisture expands during burnout. Pre-shelling operations should verify that the pattern has been conditioned in a low-humidity environment. For production areas above 60 % RH, sealed containers or desiccated cabinets are typically used. Liquid resin is sensitive to ambient light and should be stored in opaque containers. The vat temperature should be allowed to stabilize before the build start; cold resin can increase viscosity and produce layer-thickness variation. The production area should avoid airborne dust that can contaminate the vat and cause surface defects or localised cure inhibition.

Water absorption is evaluated according to ASTM D570-22, but the acceptance criterion is application-specific and should be set from casting trials rather than from generic polymer limits. Patterns that have absorbed moisture beyond the validated threshold should be reconditioned before shelling. Drying temperature and time must remain below the thermal distortion limit of the cured pattern; uncontrolled heating can cause dimensional drift and distortion.

Managing Viscosity During Long Build Campaigns

Long build campaigns require periodic viscosity checks with a calibrated rotational viscometer. The spindle geometry and rotational speed should match the manufacturer’s method because liquid photopolymers can show non-Newtonian behavior at low temperatures or high shear. If viscosity drifts upward, the cause is often partial polymerization in the vat due to stray light, excessive heat, or inadequate filtration after a previous build. The vat should be filtered through the mesh size specified in the equipment manual, and recovered resin should be blended with fresh material only in the proportion permitted by the manufacturer.

Solvent thinning is generally prohibited. Solvent addition alters photopolymerization kinetics, reduces crosslink density, changes the burnout residue profile, and can introduce volatile contaminants into the build chamber. If the vat is left idle, the resin should be recirculated or stirred according to the machine supplier’s recommendation to avoid sedimentation or localised polymerization. Lot-to-lot viscosity variation should be tracked because a shift in viscosity at constant vat temperature changes recoating dynamics and can produce layer-thickness error. If viscosity exceeds the manufacturer’s upper limit, the material should be quarantined and dispositioned through the lot-review process rather than returned to the vat without filtration.