The 3D Systems QuickPlastic Cast Urethane Material Elastomer 32 A is a two-part liquid castable polyurethane system within the QuickPlastic materials portfolio. The model designation identifies the material as a cast urethane elastomer with a nominal Shore A hardness of 32 A when tested under ASTM D2240. The grade is intended for low-volume production of soft rubber-like components using silicone RTV tools that are typically generated from stereolithography or CNC-machined master patterns. In contrast to harder QuickPlastic grades such as 40 A, 60 A, or 80 A, the 32 A variant is specified where deformation, elongation, and conformability dominate over load-bearing or abrasion resistance.

The product class is a thermosetting polyurethane formed by the reaction of a polyol side with an isocyanate prepolymer. The hardness suffix is not a standalone specification; mechanical performance is characterized by a set of standardized tests including ASTM D412 for tensile behavior, ASTM D624 for tear resistance, ASTM D395 for compression set, and ASTM D792 for specific gravity. Product-specific values are controlled by the manufacturer’s lot documentation, and the following representative ranges are drawn from published data for commercial Shore 32 A cast urethane systems rather than from a single lot certificate.

What Limits the Processing Window of the 32 A Cast Urethane?

The usable working time is limited by the exothermic reaction between the isocyanate and polyol, and by the competing side reaction with atmospheric moisture. At a mix mass of 250 g held at 25°C, commercial Shore 32 A systems of this chemistry often exhibit a pot life of 5–10 min; at 1 kg or larger, the temperature rise can reduce the flowable window below the nominal value. Published data for the QuickPlastic grade under non-standard masses is limited, and the manufacturer’s processing bulletin should be read as the controlling document.

Metering accuracy is critical because an offset of more than ±0.5 wt% from the specified ratio can shift cured hardness, increase residual unreacted isocyanate, and reduce tear strength. For laboratory and pilot-scale mixing, a planetary mixer operating at 500–1500 rpm for 30–60 s is appropriate for 100–500 g batches, followed by vacuum degassing at −95 kPa or better for 1–3 min. Degassing must be completed before gelation; otherwise the vacuum cannot remove bubbles trapped by the rising viscosity.

Residual moisture is a more significant process conflict for this soft grade than for higher-durometer cast urethanes. Polyol-side moisture above 0.05 wt% generates carbon dioxide through the isocyanate-water reaction, creating microvoids that can survive demolding and reduce tensile elongation. When ambient relative humidity exceeds 60%, pre-drying of any added fillers or colorants is required, and the use of dry nitrogen blanketing during dispensing is observed on production lines to limit surface haze and bubble formation. Avoid amine-based additives unless they are part of the curative package because they can accelerate gelation and destabilize the working window.

The mixed viscosity of this hardness class is low enough for gravity casting of thin walls but not so low that the material leaks through parting-line gaps. Rheological behavior is Newtonian until gelation; shear thinning is not relied upon for mold filling. Fill rates should be slow and consistent. For parts with wall sections below 2 mm, a pressure-assisted vacuum casting machine can improve fill, but the pressure must not exceed the mold clamping force or the silicone tool deformation limit.

On automated vacuum casting machines with twin-component metering and dynamic mixing, batch-to-batch variability is controlled primarily by flow meter calibration and material viscosity at the dispensing temperature. Manual mixing of 100 g batches shows greater property scatter in soft grades because small weighing errors become proportionally more significant. Production lots should be monitored by casting a standardized 2 mm tensile sheet per ASTM D412 and a 6 mm tear specimen per ASTM D624 from each newly opened kit; if the obtained hardness deviates by more than 2 A from the nominal value, the metering and degassing step should be checked before further production parts are poured.

Soft 32 A castings generate lower tearing forces on silicone tools than harder urethane grades, but demolding is complicated by high surface friction and undercut retention. Silicone RTV tools with durometer 20–40 A are typically selected because they permit rolling removal of soft parts without tearing the cured urethane. Draft angles above 1.5° are recommended on vertical walls, and undercuts should be limited to those that can be opened by mold flexure rather than by part stretching. Because linear shrinkage of soft cast urethanes can range from 0.5% to 1.5%, cavity scaling is typically developed from a test plaque rather than assumed from generic datasheets. A solvent-free silicone release agent is applied in a thin film; excessive release agent transfers to the cast surface and interferes with subsequent bonding or painting.

Shrinkage in thick sections is non-linear and is influenced by the local exotherm. Sections above 20 mm may exhibit sink marks or internal voids if the material gels before full packing. This is managed by splitting thick areas into multiple pours or by using a vented overflow reservoir to supply liquid resin during the initial cure. Mold temperature is commonly held at 60–70°C for the first stage of cure, but the optimal schedule must be derived from the manufacturer’s recommended cure envelope. Green strength at demold is often lower for 32 A grades than for harder grades. If the part is removed before the full cure cycle, the cured network may have sufficient hardness but insufficient tear strength to survive demolding of deep undercuts. A two-stage cure, with an initial room-temperature gel followed by oven post-cure, is common when the silicone tool cannot tolerate 70°C for extended cycles.

The exotherm of one kilogram of mixed material cast into a single block can exceed 80°C in the center of the mass. This accelerates the reaction but also creates thermal gradients that can cause internal stresses and sink marks. For large soft castings, the material should be poured in layers or the mold should be cooled to maintain a centerline temperature below the manufacturer’s maximum exotherm limit. Shore A measurements on cast urethane are sensitive to specimen thickness and substrate. The ASTM D2240 method requires a minimum thickness of 6.4 mm and a flat specimen; hardness values measured on thinner curved surfaces are not comparable. Durometer indenter dwell time also influences readings; hardness may drop by 1–3 A between instantaneous and 15 s readings for soft urethanes.

Compression Set, Hydrolytic Stability, and Dynamic Loading Boundaries

Compression set measured under ASTM D395 Method B at 70°C for 22 h is the conventional screening method for gasket and seal applications. For Shore 32 A cast urethanes, permanent set is generally higher than for 60 A or 80 A grades of the same chemistry because lower crosslink density permits greater viscoelastic flow under load. Components that must maintain sealing force at temperature should be tested with the actual flange geometry and thermal cycle, not specified by hardness alone.

Cured cast urethanes are hygroscopic and susceptible to hydrolytic degradation in hot water or saturated steam. Continuous immersion above 60°C is outside the typical service boundary for standard ether-based cast urethane elastomers; ester-based variants are less resistant to hydrolysis but may offer better oil resistance. The QuickPlastic Elastomer 32 A grade should be evaluated under ASTM D471 or ISO 1817 for the relevant process fluid before use in aqueous sealing, coolant lines, or outdoor exposure. Aromatic urethanes may also yellow under UV exposure, which is primarily an appearance and surface embrittlement issue rather than a bulk tensile loss.

Dynamic loading conditions require more than a Shore A value. DMA per ASTM D4065 provides storage modulus and tan delta over the expected temperature and frequency range; for a soft cast urethane, the glass-transition region and the onset of viscoelastic heating should be identified before the part is used in high-frequency flexure. Heat rise in thick rubber-like components under cyclic loading can exceed 20°C above ambient at high strains, which then lowers stiffness and alters the part geometry.

Post-curing is required for applications where dimensional stability, low outgassing, or stable Shore hardness are specified. A typical post-cure for two-part cast urethane elastomers is 60°C for 4–16 h, but softer grades may continue to drift by 2–3 A during the first 48 h at room temperature as residual isocyanate groups react with ambient moisture. The time–temperature profile is affected by section thickness, mold thermal mass, and stoichiometry; therefore, the manufacturer’s cure schedule should not be shortened without verification by ASTM D2240 hardness after 24 h and 7 days.

Common application fields include low-volume bellows, grommets, sensor boots, impact pads, soft-touch grips, and sealing profiles where injection molding of an equivalent 32 A thermoplastic elastomer would require high clamp force or high shear processing. The cast urethane route allows complex cross-sections without weld lines, but cycle time is longer and mold life is limited to the silicone tool life. Prototype testing should include ASTM D412 tensile extension at the maximum service strain, ASTM D624 tear resistance at the smallest notch radius, and ASTM D395 compression set at the upper operating temperature.

When Silicone Alternatives Are Rejected Due to Contamination or Abrasion Requirements

In clean assembly environments where siloxane volatiles or silicone oil migration are prohibited, a cast urethane Shore 32 A part can replace RTV silicone for seals, flexures, and soft housings, provided that the chemical environment does not include hot water above 60°C, strong polar solvents, or continuous UV exposure. Silicone elastomers generally offer lower compression set and better high-temperature stability, but they are often excluded in optical, electronic, and painting environments due to contamination concerns. Urethanes offer higher abrasion resistance than most RTV silicones of comparable hardness and can be painted, bonded, or coated with less aggressive surface preparation.

Compared with 32 A thermoplastic polyurethanes processed by injection molding, the cast urethane grade avoids high-shear melt processing and permits thicker walls without sink and flow-induced anisotropy. The trade-off is that cast urethane is a thermoset and cannot be re-melted, reground, or re-processed. It is also more sensitive to moisture during processing than a thermoplastic compound. Compared with harder QuickPlastic cast urethane grades, the 32 A material has a lower modulus and higher elongation, but lower tear and lower cut resistance. The choice between grades should be based on a strain-rate-matched tensile test rather than hardness alone.

Compliance statements for standard cast urethane grades are lot- and supplier-specific. A Shore 32 A cast urethane is not automatically food-contact, medical, or flame-retardant. If the cured part is intended for skin contact, ISO 10993-5 and ISO 10993-10 testing must be performed on the actual cured material, including any post-cure and cleaning process. REACH and RoHS status should be confirmed from the material safety data sheet and the supplier’s regulatory declaration for the specific QuickPlastic product code, not inferred from generic polyurethane chemistry. The absence of phthalate plasticizers or halogenated flame retardants is not assumed; the supplier certificate controls.