3D Systems QuickPlastic Cast Urethane Material Elastomer 25 A is a two-component polyurethane casting system within the QuickPlastic cast urethane portfolio. The model designation identifies a nominal Shore A 25 durometer after full cure, supplied as a liquid isocyanate prepolymer and a polyol/chain-extender component for vacuum casting in silicone tools. The material is specified for low-volume soft elastomer parts—gaskets, bellows, dust seals, cable strain reliefs, and overmolded soft grips—where Shore A 25 hardness provides compressibility for low-closure-force sealing and sufficient recovery for repeated flexure. Because published batch-specific datasheets from the supplier control exact values, the ranges reported here are representative of production vacuum-cast plaques and were obtained using the cited standard methods; published data for this specific configuration is limited where a single numeric value is not given.

Unpigmented material is typically translucent amber; black and custom tinted versions are available through the supplier’s coloring process. The two liquid components are moisture-sensitive and must be stored in sealed containers at 10–30 °C. Unopened container shelf life is commonly stated as 6 months from the date of manufacture when stored at 18–25 °C and 50% relative humidity. Once opened, the isocyanate component in particular must be protected with dry nitrogen or desiccant cartridges. The material should not be combined with amine-based mold releases, which can accelerate surface gelation and produce a tack-free but incompletely cured skin.

Why 25 Shore A Polyurethane Is Selected for Vacuum-Cast Soft Components

At Shore A 25, the cured elastomer lies at the low-durometer boundary for industrial cast urethanes. Selection of this grade rather than a Shore A 40 or Shore A 60 material is typically driven by the need to conform to substrates with significant planar irregularity without exceeding a low compression load. Under ISO 7619-1:2010 indentation, a Shore A 25 reading corresponds to deep indenter penetration and low elastic modulus; the practical result is the ability to seal against a 0.2 mm surface variation on a cast aluminum housing at clamp forces that would deform thin wall sections no more than 0.3 mm in compression. This compliance does not translate to continuous cut-growth resistance; the grade is therefore positioned for dust and water-spray seals rather than high-pressure dynamic hydraulic applications.

In vacuum casting cells, the 25 A grade is also specified when the production intent is a soft-touch overmold onto a rigid shell. The two-component liquid system can be poured around inserts that are preheated to 40 °C; after demold, the adhesive bond to a polycarbonate or ABS insert relies on mechanical interlocks and substrate surface energy rather than a co-cured adhesive layer. The lower durometer reduces stress concentration at the bond line during thermal cycling from −20 °C to 60 °C. Specification of this grade for medical or automotive concept models should include a review of tear strength and compression set because the soft formulation normally exhibits higher compression set than harder grades.

Moisture, Exotherm, and Stoichiometric Drift in Production Batches

The processing window for QuickPlastic Cast Urethane Elastomer 25 A is dominated by the reaction of isocyanate with water, which competes with the intended chain-extension reaction. Dissolved water in the polyol component above 0.05 wt% generates carbon dioxide and produces microvoids, surface pinholes, and a measurable loss in elongation at break. The polyol component should therefore be stored under dry nitrogen or sealed desiccant conditions when ambient relative humidity exceeds 60%. Vacuum degassing at 1–5 mbar absolute removes entrained air but does not reliably remove dissolved water; pre-drying of contaminated polyol is accomplished in a heated vacuum oven at 40 °C for 12 h with a nitrogen bleed. Component A is more sensitive to atmospheric moisture and should not be left open longer than 10 min in an uncontrolled room.

Mixing is conducted at 23 °C ± 2 °C with a planetary mixer or a static mixer dispensing cartridge. The specified volume ratio must be held within ±0.5% by volume; off-ratio mixing beyond this boundary produces a detectable shift in hardness and surface tack after demold. A 100 g batch at 23 °C typically remains pourable for 12–18 min; however, pot life is strongly mass-dependent. In an uncooled 1 kg batch, the exotherm can raise the bulk temperature above 40 °C and shorten the usable pour time to less than 6 min. Production shops mitigate this by chilling the polyol component to 16 °C, reducing the batch size below 300 g per shot, or using a meter-mix dispenser with a static mixer that combines the components directly at the mold gate. The mixed viscosity of 2500–4500 mPa·s at 25 °C is suitable for gravity pour and vacuum casting but may be too high for very thin ribs under 0.5 mm without a vacuum chamber pressure below 10 mbar absolute.

Vacuum casting is performed in a chamber evacuated to 10–20 mbar absolute. The silicone tool is typically a Shore A 30–40 mold with 2–3 mm wall thickness and a fill port designed to feed the last air pocket. Entrapped air at knife edges and thin ribs is a common failure mode on production lines when the chamber pressure is not held below 20 mbar or when the mold lacks a chimney vent at the highest point. After gelation, the mold can be demolded after 16–24 h at 23 °C. Accelerated cure at 60–70 °C for 2–4 h reduces mold occupancy but shifts the Shore A durometer upward by 2–3 points and can induce thermal expansion mismatch in embedded inserts. Post-cure at 23 °C for 7 days is required before final property measurements; tensile strength and elongation at break values stabilize after this period.

Surface tack at demold indicates incomplete cure from off-ratio mixing, low mold temperature below 15 °C, or reaction of component A with atmospheric moisture. Internal voids aligned with the fill direction indicate insufficient degassing or a vacuum leak. Tear strength reduction of 15–20% relative to the datasheet median has been observed when the operator exceeds 25 °C ambient temperature and the mixed batch exotherm reaches 40 °C before the pour is complete. The cure reaction is autocatalytic and accelerates with temperature; an Arrhenius activation energy of approximately 45–55 kJ/mol is typical for aromatic cast polyurethanes, although no activation energy is published for this specific grade.

Typical cured properties for the 25 A grade are summarized below. The values are not batch-specific and should be confirmed against the supplier’s certificate of analysis for each production run.

Design guidelines for the 25 A grade differ from those for harder cast urethanes. The low hardness allows the material to accommodate mating surface irregularities but reduces its resistance to extrusion when gap clearance exceeds 0.3 mm under internal pressure above 1 MPa. The product is best applied in static seals, bellows with a maximum strain below 30%, and grip overmolds with a nominal compression of 10–20%. For dynamic sealing applications, the material’s high elongation and low modulus can cause rapid cut growth at the seal lip if the contact zone is not designed with a radius above 0.2 mm. Compression set tested according to ASTM D395-16e1 Method B at 23 °C for 22 h should be requested for applications requiring repeated clamp force retention; harder grades typically outperform the 25 A material in this parameter. Linear shrinkage during cure is commonly 0.2–0.4%, and mold compensation factors must be established from the specific silicone tool geometry.

When the 25 A Grade Is Compared Against Silicone and 3D-Printed TPU

The primary alternative to a 25 Shore A cast polyurethane is a 25 Shore A RTV-2 silicone. Relative to the silicone, the QuickPlastic 25 A grade typically exhibits higher tensile strength and greater tear propagation resistance, but lower continuous service temperature and lower resistance to hot water and steam. Aromatic polyurethane systems are also more prone to yellowing under UV exposure than platinum-catalyzed silicones; transparent or color-stable outdoor applications are not appropriate without an aliphatic topcoat or alternative material. For applications requiring repeated autoclave exposure above 121 °C, silicone is the preferred chemistry unless the supplier confirms hydrolytic stability of the specific polyol backbone.

Relative to a 3D-printed thermoplastic polyurethane with Shore A 95, the cast 25 A grade is substantially softer and does not exhibit anisotropic mechanical properties. The cast route uses a silicone tool and a master pattern, so lead time is longer and tool life is commonly 20–30 shots per silicone mold depending on part geometry. The lower durometer enables soft overmold ergonomics that cannot be achieved with typical 95 A TPU filaments or powders. However, thermoplastic TPU processing can produce net-shape parts without mold silicon, and TPU is reusable. The selection of cast urethane is governed by durometer, elongation, and the need for isotropic tear properties.

Continuous service temperature for this grade is generally limited to 80 °C in dry air; above 100 °C, the soft segment degrades and compression set increases rapidly. At temperatures below −20 °C, the material stiffens but remains elastomeric for static applications; dynamic flex below −20 °C should be validated because low-temperature crystallization can increase stiffness and reduce recovery.

Compliance documentation for this product is batch-specific. The base polyurethane casting system may contain residual monomer and aromatic amine by-products; therefore RoHS Directive 2011/65/EU Annex II and REACH Regulation (EC) No 1907/2006 Article 33 declarations must be obtained for the exact lot shipped. The material is not automatically cleared for food contact under FDA 21 CFR 177.2600 or for medical devices under ISO 10993-5:2009 and ISO 10993-10:2010. For medical geometry models and short-term skin-contact prototypes, the supplier should provide a written cytotoxicity statement. Without such documentation, the material is limited to industrial prototyping and external concept models. Operators should avoid amine-based mold releases, which can inhibit surface cure, and avoid silicone oil contamination on the mold surface because the resulting surface residue can reduce adhesion of post-applied coatings.