Unfilled two-component polyurethane casting resins intended for magnetic resonance imaging-adjacent service are formulated to limit bulk magnetic susceptibility mismatch, ionic conductivity, and radiofrequency field perturbation. 3D Systems QuickPlastic Cast Urethane Material RC79D MRI Transparent is a rigid, transparent polyurethane grade within the QuickPlastic cast urethane selector. The suffix 79D specifies the nominal cured hardness of 79 on the Shore D scale as determined by ASTM D2240. The transparent designation is derived from the absence of opaque fillers such as carbon black, graphite, or glass fibre that would otherwise raise modulus or reduce surface resistivity at the expense of optical clarity and imaging compatibility. Typical production uses include vacuum-cast medical device housings, surgical instrument bodies, patient positioning fixtures, imaging phantom shells, and prototype optical components that must not introduce strong susceptibility artefacts under clinical MRI sequences. Because the material alone does not constitute a medical device, finished-part evaluation under ASTM F2503, ASTM F2052, ASTM F2213, ASTM F2182, and ASTM F2119 is required before an MR Conditional designation can be assigned. Published data for this specific configuration is limited; the numerical ranges that follow are class-typical envelopes for unfilled rigid 79 Shore D transparent cast urethanes and are supplied for process design, not as manufacturer-certified product values.

What Processing Limits Apply During Vacuum Casting of RC79D?

Vacuum casting of this rigid polyurethane follows a room-temperature mixing and low-pressure mould-filling sequence. Because the isocyanate component is moisture-sensitive, all feed lines, mixing vessels, and mould surfaces must be dried to dew points below -20 °C or conditioned at 40–50 °C when ambient relative humidity exceeds 60 %. Component A and Component B are metered at the manufacturer-specified weight ratio; for unfilled rigid systems in this hardness class, a ratio deviation greater than 2 wt% is typically sufficient to leave unreacted hydroxyl or isocyanate groups that plasticise the cured network and lower heat deflection temperature by 10–15 °C. The mixed viscosity at 25 °C class-typically falls between 500 mPa·s and 1,500 mPa·s, which permits hand pouring into thin-walled silicone tools but requires vacuum degassing below 0.1 kPa absolute until froth collapse. At 25 °C, the usable pot life before viscosity doubles is ordinarily in the 5–12 min range; manual pours are therefore completed within the first third of pot life, while meter-mix dispense equipment with in-line degassing is preferred for shot masses above 1 kg. Table 1 summarises the class-typical mechanical, thermal, electrical, and optical contrast between unfilled transparent RC79D, glass-filled rigid cast urethane, and carbon-filled ESD cast urethane.

Ranges are class-typical composites from publicly available data for rigid cast urethanes; manufacturer-certified values for RC79D may differ and must be obtained from 3D Systems technical documentation. The MRI artefact tendency column is a selection indicator and does not replace finished-part testing.

Thin sections below 5 mm can trap air at the mould surface if the tool is below 25 °C; pre-warming the silicone mould to 30–35 °C reduces viscosity at the fill front and shortens gel time. Sections above 25 mm must be cast with split pours or cooled tooling because the adiabatic temperature rise of the exothermic urethane reaction can exceed 120 °C in mass, leading to bubble nucleation, optical haze, and dimensional warpage. Demoulding for a 79 Shore D rigid urethane is class-typically possible after 12–24 h at 23 °C; full strength and maximum heat deflection temperature require a post-cure cycle of 4–8 h at 60–80 °C. The heating rate during post-cure should not exceed 10 °C/min because thermal gradients in transparent sections thicker than 10 mm can generate internal stress that appears as birefringence under polarised light.

Silicone tooling for RC79D is typically produced from condensation-cure or addition-cure RTV silicone with Shore A hardness between 30 and 50. Addition-cure silicones are preferred for transparent urethane casting because condensation-cure by-products can introduce surface haze. Mould release selection is constrained by the requirement for optical clarity: solvent-borne wax or silicone-based release agents must be applied in films thinner than 5 µm and allowed to flash off completely before pouring; excess release agent transfers to the part surface and reduces adhesion of subsequent bonding or coating operations. Moulds stored at ambient humidity above 60 % should be pre-dried at 40–50 °C for 24 h before casting because residual water on the tool surface reacts with isocyanate and produces carbon dioxide bubbles that appear as pinpoint defects in transparent sections.

MRI Artefact Behaviour and RF-Induced Heating Boundaries

The MRI transparent characteristic of RC79D is primarily compositional rather than a measured grade property. Unfilled polyurethane does not contain ferromagnetic, paramagnetic, or highly conductive fillers. Published magnetic susceptibility values for unfilled polyurethane are of the order of -8 × 10^-6 to -11 × 10^-6 SI, close to the value for water of approximately -9.05 × 10^-6 SI. This small susceptibility mismatch limits static field distortion at the part–tissue or part–air boundary compared with metallic components or carbon-filled polymers. The absence of a continuous conductive network also results in high volume resistivity, class-typically above 1 × 10^12 Ω·cm under ASTM D257, which reduces the risk of significant RF-induced eddy-current heating in magnetic resonance scanners. However, radiolucency or low MRI artefact measured on raw material coupons does not automatically establish MR Conditional status for finished geometries. Trapped air cells, metal inserts, conductive coatings, adhesive bond lines, or embedded hardware create local susceptibility and conductivity discontinuities that dominate image artefact severity.

Finished devices intended for clinical imaging use should be tested according to ASTM F2052 for magnetically induced displacement force, ASTM F2213 for magnetically induced torque, ASTM F2182 for RF-induced heating near passive implants at the target field strength, and ASTM F2119 for image artefact extent. If the component contacts tissue or breached skin, cytotoxicity evaluation per ISO 10993-5 is typically required in addition to the imaging benchmarks. The manufacturer’s designation MRI Transparent is therefore best interpreted as a material-selection guide indicating that the unfilled formulation is appropriate for imaging-adjacent prototypes, not as a substitute for finished-device MR safety assessment.

In the QuickPlastic cast urethane selector, RC79D sits above the flexible Shore A grades and below glass-filled rigid grades in hardness and modulus. Flexible QuickPlastic urethanes with Shore A 60–95 provide elongation above 100 % and are used for overmoulded grips or flexible housings, but their lower Shore D hardness and higher damped response make them unsuitable for stiff imaging fixtures that must hold spatial registration under load. RC79D is differentiated from glass-filled rigid grades by its unfilled transparency: glass-filled systems increase flexural modulus by 30–60 % class-typically but are opaque and mildly abrasive to mould surfaces. Carbon-filled anti-static grades reduce surface resistivity below 1 × 10^6 Ω/sq but introduce conductive pathways that can interact with the MRI RF field and generate image void artefacts; RC79D avoids this trade-off at the expense of static dissipation. Compared with unfilled epoxy casting resins, a 79 Shore D polyurethane such as RC79D class-typically offers lower mixed viscosity and shorter demould cycles but may exhibit lower glass transition temperature and higher moisture equilibrium; selection between these chemistries should be governed by the thermal and chemical exposure of the final application rather than by hardness alone.

When RC79D Replaces Glass-Filled or Carbon-Filled Urethanes in Imaging Fixtures

Substitution of RC79D into an existing vacuum-cast fixture design requires recalculation of section modulus and deflection under the intended clamp force because the unfilled resin does not carry the same flexural modulus as a glass-filled analogue. If the previous material was a glass-filled 80 Shore D urethane with flexural modulus in the 2,200–2,800 MPa range, moving to an unfilled 79 Shore D system may reduce stiffness by 20–35 %; rib thickness or moment of inertia must be increased accordingly to maintain the same deflection under ASTM D790 test conditions. If the previous material was a carbon-filled ESD grade, removal of the conductive filler raises surface resistivity from below 1 × 10^6 Ω/sq to above 1 × 10^12 Ω/sq. This change is advantageous for MRI artefact control but removes static dissipation; downstream handling procedures for electronic assembly may require humidification or ionised-air control. Transparent components also shift failure detection: optical inspection can reveal internal voids, knit lines, or exotherm haze that would be hidden in opaque grades, so the casting gate and vent locations must be adjusted to prevent filling turbulence and entrained air in the clear cross-section.

Because RC79D is a cast urethane, its upper continuous service temperature is bounded by the glass transition region of the cured network. Sustained exposure above 80 °C can produce progressive modulus loss and dimensional relaxation in loaded sections, especially when plasticising moisture is present. The material is not recommended for repeated steam-sterilisation cycles above 121 °C or for prolonged immersion in strong polar solvents such as acetone or methyl ethyl ketone, which can solvate the hard segments and induce stress cracking. When absolute dimensional stability is required, parts should be conditioned at 23 °C and 50 % relative humidity for at least 48 h before metrology; water absorption of 0.2–0.4 % over 24 h can swell thin sections by several micrometres. Post-cured parts may retain trace unreacted isocyanate at the surface; skin contact during prolonged handling or before complete cure should be avoided, and machining dust from trimming should be controlled by local exhaust ventilation. The grade is supplied as a two-component system; mixing in production requires validated dispensers or calibrated balances with an accuracy better than ±0.5 wt% to avoid hardness drift across a multi-cavity tool. These boundaries define the practical operating envelope for process engineers and should be incorporated into work instructions alongside the current 3D Systems technical datasheet.