Covestro Addigy LPU Rigid 341-02 IM is a liquid photopolymer resin based on acrylate-functional polyurethane oligomers, reactive diluents, and a photoinitiator system activated in the 385–405 nm waveband. The material is supplied for vat photopolymerization platforms, including digital light processing and liquid crystal display systems. The IM suffix is retained as part of the supplier’s grade identifier; it is not used here as a processing descriptor. The uncured liquid is controlled by the ratio of reactive diluent to oligomer, and its rotational viscosity is reported by the manufacturer under ISO 3219 or equivalent methodology. The cured network is a crosslinked thermoset, not a thermoplastic polyurethane; after polymerization it cannot be melt-reprocessed or solvent-welded in the manner of injection-moulded TPU grades. The product therefore occupies a distinct class: a rigid, Shore D-scale polyurethane photopolymer positioned between brittle epoxy acrylates and highly elastomeric urethane photopolymers.

In vat photopolymerization, layer thickness and recoating behaviour are the first process variables to control. For rigid polyurethane resins of this class, layer thickness is commonly set between 25 µm and 100 µm, but the optimum must be confirmed on the intended machine. Thinner layers improve vertical resolution but increase build time and do not compensate for insufficient exposure. Build chamber temperature should be held within the range specified on the technical datasheet, typically 22–30 °C, because viscosity changes with temperature and affects recoat speed. If ambient relative humidity exceeds 60 %, resin containers should be promptly resealed because water uptake can interfere with free-radical chain growth and alter green-part mechanical strength. The material should be stored in sealed opaque containers at 15–30 °C and protected from wavelengths below 420 nm.

What processing variables control the vat photopolymerization window?

The photopolymerization response is governed by the semi-logarithmic Jacobs working curve, C_d = D_p ln(E/E_c), where C_d is cure depth, D_p is depth of penetration, E is applied energy dose, and E_c is critical energy dose. For this grade, D_p and E_c are not universal constants; they must be determined on the specific machine because projector irradiance, vat film transparency, and resin temperature affect actual dose. A calibrated 405 nm radiometer should be used to map the build area and establish stable exposure time. Burn-in layers are generally exposed at a higher energy dose than standard layers to anchor the first 100–200 µm of material to the platform. Large solid cross-sections require reduced build speed or adjusted tilting because separation peel force scales with projected area and can cause edge delamination or build collapse. Oxygen inhibition is a further variable: free-radical polymerization is retarded by dissolved oxygen, so enclosed vats, inert gas blanketing, or increased post-cure dose improve surface conversion. Published data for this specific configuration is limited; process qualification should therefore use response-surface testing of exposure time, layer thickness, and post-cure dose rather than nominal printed settings alone.

Green parts exiting the vat are saturated with uncured resin and require a two-stage solvent wash before post-cure. A first wash removes bulk liquid resin, and a second clean solvent wash reduces residual surface monomer. Isopropanol is widely used, but prolonged immersion can swell polyurethane photopolymer networks; the wash step should be limited to the shortest duration that produces a solvent-free surface. Ultrasonic agitation should be validated because acoustic cavitation can heat the solvent and initiate microcracking at sharp corners. After washing, parts should be dried with filtered compressed air or allowed to evaporate under ventilation before post-curing. Residual solvent trapped in the polymer network can become a plasticizer in the cured part and reduce hardness and heat deflection temperature. If a solvent-free washing system using alkaline detergent is used, compatibility must be confirmed, because polyurethane networks can undergo hydrolysis at elevated pH and temperature. Centralized wash stations should monitor solvent temperature and immersion time to keep dimensional variation within the part tolerance band.

Standardised mechanical property reporting for rigid polyurethane photopolymers

Mechanical properties are reported only after post-curing and conditioning. The supplier’s current technical datasheet should be consulted for lot-specific values; the following methods are the recognized framework for interpreting the data. Tensile modulus, tensile strength, and elongation at break are determined per ISO 527-1:2019 with type 1BA specimens or per ASTM D638-14 with Type V specimens. Flexural modulus and flexural strength are determined per ISO 178:2019. Durometer hardness is measured per ISO 868:2003 using a Shore D indenter. Impact response is reported either as notched Charpy per ISO 179-1 or notched Izod per ASTM D256. Heat deflection temperature is evaluated under the two common loads, 0.455 MPa and 1.82 MPa, per ISO 75-2 Method A or B. Water absorption is determined by immersion per ISO 62. Density of the liquid and cured specimens follows ISO 1183-1. Conditioning before testing should follow ISO 291, typically 23 °C and 50 % relative humidity for 24 h.

Test specimens printed flat may produce different values from specimens printed vertically. For design purposes, the z-direction tensile strength should be determined because interlayer adhesion can be lower than in-plane properties. Supplier datasheet values are often generated from fully post-cured specimens printed horizontally; users should not use those numbers directly for vertically loaded features without internal certification.

Compared with rigid epoxy-based photopolymer resins, this polyurethane grade is selected where impact toughness and lower brittle failure are the controlling design requirements, while accepting a more moderate thermal-deflection profile. Compared with flexible polyurethane photopolymers, it delivers the Shore D hardness, flexural modulus, and creep resistance needed for dimensional jigs and mating parts. Compared with fused-filament fabrication of rigid TPU or polycarbonate, vat photopolymerized LPU achieves smoother sidewalls and more isotropic mechanical response; however, the crosslinked network has no melt-weld capability. The difference is not merely compositional: epoxy acrylate resins typically exhibit higher crosslink density and high stiffness but may crack under impact, while this PU system uses urethane hydrogen bonding and lower crosslink density to redistribute stress. Peer-reviewed comparative data between this specific grade and filled epoxy photopolymer resin is limited; substitution trials should use identical build orientation, support density, and post-cure protocols, and test specimens per ISO 527-2, ISO 178, and ASTM D256.

When this material replaces epoxy-based rigid resins in production jigs and fixtures

Production-scale use of this resin in jigs, fixture bodies, and assembly guides requires attention to clamping pressure and repeated load cycles. Unlike machined acetal or aluminium fixtures, photopolymer fixtures are more sensitive to thread pull-out in self-tapping screws; brass heat-set inserts are commonly used, and pull-out strength should be tested with the actual insert geometry because no universal insert size applies. Build orientation introduces anisotropic strength along the z-axis because interlayer adhesion can be the weakest plane even with optimized post-cure. For fixtures that encounter continuous load at elevated temperature, heat deflection temperature is not a safe service temperature; creep under load should be evaluated by ISO 899-2 or a reduced-scale fixture test on the production line. Typical failure modes in this class include delamination at under-exposed layers, cracking at sharp internal corners, and softening after solvent contact. On production lines with automated guided vehicles or robotic pick-and-place, the surface hardness of the cured parts supports repetitive part presentation, but edges should be radiused because photopolymers can chip when struck by hardened steel handles.

Thermal resistance is not a single property but a response that depends on post-cure conversion, atmosphere, and load. Under-cured parts may pass Shore D and tensile tests yet fail heat deflection testing because residual acrylate unsaturation lowers the glass transition. Fourier-transform infrared spectroscopy in attenuated total reflectance mode can track the disappearance of the acrylate double-bond peak near 810 cm⁻¹; a plateau in conversion should be reached before thermal testing. Ultraviolet post-cure under inert gas yields a more complete surface cure than post-cure in air, where oxygen inhibition leaves a tacky top layer. Chemical exposure should be evaluated according to ASTM D543 using the actual cleaning and process fluids present in the facility. Polyurethane thermosets are generally sensitive to strong alkaline solutions, ketones, esters, and chlorinated solvents; brief contact with isopropanol or aliphatic hydrocarbon mixtures is often tolerated but should be validated. Prolonged immersion in water above ambient temperature can cause plasticization and dimensional growth; wet service therefore requires water absorption and dimensional stability testing per ISO 62 and ISO 175.

The cured network’s crosslink density determines both thermal stability and chemical resistance

Crosslink density in this thermoset is established by the polyurethane oligomer functionality, the reactive diluent content, and the total UV dose. Dynamic mechanical analysis per ISO 6721-11 provides the glass transition temperature from the tan δ peak and is a more informative thermal criterion than Shore D hardness. The rubbery plateau modulus above the glass transition is proportional to crosslink density; therefore, an increase in post-cure temperature may raise the glass transition up to the point of thermal degradation. Degradation of urethane linkages becomes a concern above 150–180 °C for many polyurethane systems, but for this photopolymer the practical limit is far below that unless a filler is present. In chemical resistance terms, a higher crosslink density restricts solvent swelling but may reduce impact ductility. The grade’s position in the rigid polyurethane photopolymer category means that it balances modulus and toughness through a moderate crosslink density rather than through a highly aromatic epoxy network. When high-temperature open-air exposure is required, parts should be aged according to ISO 188 and tensile properties re-measured per ISO 527-2; thermal ageing data published for this specific configuration is limited.

Typical production applications include short-run injection mould inserts for low-pressure or low-temperature moulding trials, thermoforming tools, and assembly fixtures where dimensional accuracy and surface finish are critical. The liquid nature permits printing of internal drain channels and conformal vacuum holes that are difficult to machine. However, the material is not a direct substitute for P20 tool steel or machined aluminium when mould temperatures approach or exceed the heat deflection temperature of the cured polymer. The safety data sheet identifies uncured resin as a skin irritant and potential sensitizer; operators must use nitrile gloves, sealed rooms with ventilation, and filtered wash-station enclosures. Liquid waste and wash solvents should be disposed of as hazardous waste under applicable local regulations. Suppliers place this grade under REACH 1907/2006 and RoHS Directive 2011/65/EU obligations; compliance for food-contact or medical use must be verified through the appropriate end-product standards, not presumed from raw-material literature.