Proto3000 supplies the Formlabs Silicone 40A Resin as a 100% silicone stereolithography resin in a light-shielded cartridge for Formlabs low-force stereolithography platforms. The material is recognized by PreForm for the Form 3, Form 3+, Form 3B, Form 3L, and Form 3BL printer variants; cartridge data chips restrict use to validated process profiles. The uncured resin is a single-component formulation, amber in appearance, and is dispensed through the standard resin tank interface. Manufacturer-published typical properties place cured specimens at Shore 40 A hardness by ASTM D2240, tensile strength of 5.5 MPa and elongation at break of 440% by ASTM D412, and tear strength of 11 kN/m by ASTM D624 Die C. The resin is supplied in 1 L cartridges for desktop builds and 5 L cartridges for the Form 3L platform. The cured state is a crosslinked siloxane network rather than a thermoplastic elastomer; this distinction controls both the post-processing route and the service response under compression and heat.

Because the green part after printing contains only partially crosslinked polymer, mechanical data generated before thermal post-cure are not representative. The manufacturer workflow directs operators to wash printed parts in isopropyl alcohol or the solvent system specified by the PreForm profile, allow the alcohol to evaporate, and then expose the part to elevated-temperature cure. The effect of skipping this step is not a simple decrease in stiffness; residual reactive groups remain available, the surface remains tacky, and tear strength may remain below the datasheet threshold because the network has not established the intended siloxane crosslink density.

Because the formulation is solvent-free and more viscous than standard resins, the material’s recoat behaviour is temperature-dependent. Cartridges stored below 20 °C should be warmed to 25–30 °C before an extended build; otherwise the resin tank may show incomplete recoating, air entrainment, and delamination in the first printed layers. Cartridge changeovers are performed without vigorous shaking because entrained air can translate into voids in thin seal lips.

The printed silicone is not a drop-in replacement for compression-moulded or room-temperature-vulcanizing silicone rubber of the same nominal Shore hardness. Layerwise construction produces interlayer boundaries and surface roughness that are absent in a homogeneous cast network. Tensile specimens printed in the Z orientation may show reduced elongation relative to XY-plane specimens from the same cartridge. Consequently, substitution of a transfer-moulded silicone rubber by Silicone 40A should be validated with printed test plaques and service-strain tests rather than by Shore hardness alone.

Why does a 100% silicone stereolithography resin require a different thermal cure envelope?

Acrylate-rich flexible resins reach serviceable properties after short Form Cure exposure at temperatures around 60 °C. Silicone 40A is formulated around a silicone crosslinking mechanism that is thermally activated only after the initial laser-based layer formation. The manufacturer-set thermal cure operates at 120 °C; the PreForm profile varies dwell time with part wall thickness, but typical runs require a longer dwell than standard flexible acrylate resins. The elevated cure is not optional because the target compression set value, measured by ASTM D395 Method B after 22 h at 70 °C, assumes a fully established siloxane network. Parts removed from the Form Cure before the specified soak interval can show a compression set that is two to three times the datasheet value, because the initial elastic response is overwhelmed by residual unreacted material migrating to the contact surface.

Solvent washing before thermal cure has a competing effect. Isopropanol removes partially cured resin from recessed seal channels, but prolonged exposure can swell the green network and distort fine ribs. Operators should use the minimum dwell prescribed by the material profile, maintain solvent bath cleanliness, and allow full solvent evaporation before the 120 °C cycle. Residual alcohol trapped in thick sections can vaporize during cure and create internal voids. This failure mode has been observed in prints with wall thicknesses above 10 mm when the ambient drying interval was shortened; the defect is not visible on the surface but produces a lower tensile break force when tested according to ASTM D412.

Form Cure chamber load also affects the silicone resin more than room-temperature standard resins. Because the final Shore hardness and compression set are sensitive to the last 10–15 °C of cure, an overloaded cure chamber can create part-to-part hardness variation of 3–5 A across one batch. Production validation should include a calibrated thermocouple embedded in a sacrificial part or logged within the chamber near the load centre. The cure cycle is considered complete only when the entire mass of the part has reached the specified temperature, not when the chamber air has reached set point.

In production cells, the most frequent processing bottleneck is not the cure chamber but the resin tray and recoater. The resin’s viscosity is higher than that of clear standard resins; at ambient room temperatures below 20 °C, the recoat floor may be insufficiently wet, producing entrapped air at the build surface. Climate-controlled lines typically hold the cartridge or resin compartment at 25–30 °C for at least 4 h before starting a build. Lot-to-lot variation within manufacturer specification can still shift peel force enough to require re-optimization of part orientation on low-adhesion build platform surfaces. Because the material is an elastomer, part removal from the build platform must avoid tearing thin sections; flexible spatulas or compressed air are preferable to rigid metal tools.

Silicone residue on the resin tank release film can accelerate haze formation and raise peel force. The tank film should be inspected under magnification after each build for micro-tears and adhered particle inclusions. Any partially cured particle left in the vat can obstruct the light path and produce a local uncured zone in the next build. Filtering through the manufacturer-approved mesh is recommended after failed builds, but repeated mechanical filtration can increase shear exposure and alter the high-viscosity formulation; therefore frequent full-vat filtration is not a substitute for maintaining clean build platforms and calibrated LPU optics.

Low-compression-set elastomer data under ASTM D395 Method B

The values in the following table are manufacturer-published typical data for fully post-cured material; they are not lot-release specifications and are not to be used as design allowables without printed-coupon validation. The test specimens include flat XY-oriented bars and Die C tear specimens. The table places Silicone 40A between Elastic 50A and Flexible 80A on hardness but gives a significantly different elongation profile.

Shore hardness is a useful quality-control indicator but it does not predict sealing retention. The compression set of Silicone 40 A is tested under ASTM D395 Method B with specimen deflection maintained at 25% for 22 h at 70 °C; the published typical value is approximately 22%. This means the part retains more of its original thickness after sustained deformation than typical acrylic-rubber blends with comparable Shore hardness. The value should not be extrapolated to long-term dashpot or flange applications without additional stress-relaxation testing using ISO 3384 or equivalent.

Printed part anisotropy must also be considered. The datasheet values are derived from specimens printed in the XY plane; Z-oriented specimens can exhibit lower elongation because interlayer boundaries act as crack-initiation regions under tensile loading. For critical sealing parts, printed tensile bars should be included on the build plate at the same orientation as the production part, and the measured values should be compared with the XY datasheet values before selecting the material.

For gas-tight or liquid-tight sealing surfaces, layer stack lines can create microscale leak paths. A Shore 40 A material is soft enough to conform to some roughness, but the leak rate depends on the surface profile of the printed layer lines and closing force. Optical profilometry of the sealing rib should be performed for applications requiring a pressure differential below 10 kPa. If roughness exceeds the specified value, orientation changes, polishing, or post-process coating may be required; the elastomer should not be assumed to fill every leak path.

This material is specified for seals, gaskets, vibration isolators, custom keypads, and soft-touch housings where the service temperature or chemical resistance of urethane-acrylate flexible materials is insufficient. The cured siloxane network is hydrophobic and retains flexibility after short-term exposure at 125 °C; however, continuous immersion in nonpolar hydrocarbon fluids can cause volume swell. Parts destined for oil exposure should be tested by ASTM D471 using the actual fluid and soak duration, because datasheet hardness and tear values do not capture swollen-state behaviour. Direct chemical adhesion to metals and most polymers is limited; sealing parts should be retained by mechanical compression, dovetail grooves, or validated primers. If bonding is required, adhesion should be evaluated by ASTM D429 or ISO 813 on the cleaned and cured surface.

No biocompatibility, food-contact, or implantable status is asserted for this resin. Regulated applications require device-level testing under ISO 10993 or applicable food-contact standards, because the final part includes print residues, wash solvent residues, and post-cure by-products that vary by facility. Published data for repeated steam autoclave cycles for this specific resin is limited; therefore steam-sterilization compatibility must be established by the end user.

When a true siloxane backbone is the requirement, not a polyurethane-soft-segment mimic

Flexible 80A and Elastic 50A belong to the acrylate-rich flexible resin family. They are supplied with Shore hardness values of 80 A and 50 A, respectively, and their mechanical spectra differ from the silicone system. Silicone 40A is not a diluted version of Flexible 80A; it contains siloxane functionality that changes the failure mode under repeated compression. The datasheet comparison shows tensile strength is lower than Flexible 80A but higher than Elastic 50A, while elongation at break is approximately 190 percentage points above Elastic 50A and 320 percentage points above Flexible 80A. Tear strength is higher than Elastic 50A but less than Flexible 80A; therefore the silicone material should not be selected for high abrasion or high-tensile flexural applications where Flexible 80A is more appropriate.

In a flange-sealing context, the material choice is driven by compression set and contact pressure retention. Acrylate-rich flexible resins may exhibit increased stress relaxation after thermal aging; the siloxane network is designed for lower compression set at 70 °C. Where the design requires a harder wear surface or snap-fit behaviour, Flexible 80A is the harder candidate. Where the design must accommodate large strain with a soft touch, Elastic 50A is the lower-strength but softer candidate. Silicone 40A occupies the middle hardness region but provides the siloxane-specific chemical resistance and low compression set that neither acrylate-rich resin replicates.

Thermal aging must be evaluated according to the actual environment. The datasheet may permit short-term exposure at 125 °C, but permanent set and elongation loss after hot-air aging depend on both the antioxidant and catalyst residues in the cured network. Dry-heat aging tests can be performed using ASTM D573 or ISO 188 at the upper service temperature; tensile properties and hardness should be remeasured after the specified soak interval. Swelling in silicone-compatible solvents is not necessarily a failure criterion unless dimensional stability is required, but extracted low-molecular-weight siloxanes can contaminate optical or electrical surfaces in sealed assemblies.

Uncured resin handling follows the resin manufacturer’s safety data sheet. Nitrile gloves, local exhaust ventilation, and chemical splash goggles are required when handling cartridges, build platforms, and wash baskets. Wash solvent contaminated with silicone resin is a hazardous liquid fraction and is not to be poured into municipal drains. Storage of the cartridge outside the manufacturer-specified temperature range may shift viscosity and build reliability, even if the cartridge remains within its unopened shelf life. If ambient relative humidity exceeds 60%, condensation on the build platform should be prevented by keeping the printer covered and minimizing door-open time. Regulatory status under REACH, RoHS, or other chemical frameworks must be confirmed for the specific cartridge lot through Proto3000, because the cured part, uncured resin, and waste solvent can fall under different classifications.