Kolon Inkrayon Transparent 3D Printing Polymer is supplied as a clear liquid photosensitive resin for vat photopolymerization platforms operating at 385 nm to 405 nm. The material is used in downstream applications where visible-light transmittance, low haze, dimensional stability, and leachable content are process-critical. The following application scenarios are separated by the governing specification set, not by a unified processing template. Exact exposure windows, post-cure dose, and surface finishing parameters must be derived from the resin supplier’s technical datasheet and validated on the target printer because batch-to-batch photoinitiator content can shift cure depth by more than 0.02 mm per pass.

Automotive Forward-Lighting Prototype Lenses and Optical Bench Evaluation

Exterior lighting development uses transparent 3D printed polymer for pre-production inner lenses, light guide plates, and collimator mock-ups mounted on a goniophotometer. The Kolon Inkrayon resin is printed on a DLP platform with a 405 nm LED source and a build envelope of 192 mm × 108 mm × 200 mm. Layer thickness is set to 50 µm, and the exposure energy per layer is controlled between 3.0 mJ/cm² and 6.0 mJ/cm² as measured by a calibrated UV radiometer. The recoating sequence is delayed for 2 s to 8 s depending on ambient temperature. Below 22 °C, the uncured resin viscosity increases to a point where the recoat blade can entrain air bubbles at the edges of large flat optical surfaces, producing visible voids after post-cure. The printed lens blank is washed in two successive 99.9% isopropanol baths for 3 min and 2 min, dried with filtered compressed air at 1.5 bar, and post-cured in a 405 nm LED flood chamber at 40 °C for 20 min. Total post-cure dose is held between 6 J/cm² and 10 J/cm². Dose above 14 J/cm² produces a measurable increase in yellow index because residual photoinitiator photolysis products absorb in the short-wavelength visible region. Optical acceptance is based on total luminous transmittance and haze measured according to ASTM D1003-21 using a spectrophotometer with an integrating sphere. The prototype lens is not a homologated production part under FMVSS 108 or ECE R149, but it is used to validate photometric distribution before metal tooling is released. No solvent or reactive diluent is added to the resin for this application. If lower viscosity is needed for high-speed recoating, the build chamber temperature is raised to 28 °C instead of adding diluent, because even 2 wt% of a non-reactive plasticizer reduces crosslink density and shifts the refractive index. The terminal product is a clear inner lens or light guide prototype with polished surfaces, typically finished with 1 µm diamond paste to bring surface roughness below Ra 0.2 µm before goniophotometric measurement.

Microfluidic flow visualization chips impose different constraints because the printed part contacts aqueous buffers, cell culture media, or fluorescent tracers. For live-cell microscopy and particle image velocimetry, the resin is used neat to avoid introducing non-photopolymerizable contaminants. If tracer particles are required, fluorescent microspheres are added externally to the working fluid rather than blended into the resin. A transparent micromixer is printed on a 405 nm LCD system with 35 µm XY pixel pitch and 50 µm layer thickness. Channel widths below 200 µm are not recommended without validating minimum recoating clearance, because under-cured resin in narrow dead-end channels can reduce the optical path by diffuse scattering. The printed chip is washed in 99.9% isopropanol for 5 min under ultrasonic agitation at 25 °C, followed by a deionized water rinse for 2 min. Post-curing is performed in a 405 nm chamber at 30 °C for 30 min to minimize leachable acrylate monomers. Leachable content is assessed according to ISO 10993-5:2009 with an L929 mouse fibroblast cytotoxicity assay when the chip is intended for live-cell microscopy. The exposed microchannel surface is treated with oxygen plasma at 100 W for 60 s before bonding to a glass coverslip or PDMS slab. Bonding is completed at 70 °C for 30 min under a 0.2 kPa vacuum. The terminal part is a microfluidic imaging chip with a transparent top window that permits epifluorescence observation at 20× magnification. Operating pressure is limited to 1.5 bar for solvent-free ports because the layer interfaces can delaminate at higher internal pressure. The resin is not evaluated for continuous-flow organic solvent compatibility, and published data for this specific configuration is limited.

Why Does Over-Cure Shift the Yellow Index in Transparent Acrylate Resin?

Display light pipe prototypes present a process window in which the critical defect is not surface deformation but the optical b* value that appears when the total radiant dose exceeds the optimum conversion window. Transparent photopolymers formulated with acylphosphine oxide or alpha-hydroxy ketone photoinitiators can show a yellow index increase when the post-cure dose is pushed beyond the point where methacrylate double bonds have reached near-complete conversion. In practice, a printed light pipe is produced on a 405 nm DLP machine at 25 µm layer thickness, washed in isopropanol, and post-cured in two steps. The first post-cure is 4 J/cm² at 30 °C; the second is 2 J/cm² at 60 °C with a 10 min dwell. Total dose above 10 J/cm² can produce a measurable shift of more than 1.5 b* units when measured according to ASTM E313-20 under illuminant D65. This shift is caused by residual photoinitiator photolysis products and oxidation of tertiary amine synergists. The terminal component is a light pipe with a 0.8 mm to 3.0 mm wall section, designed with draft angles above 3° to reduce layer-line shadowing. Surface roughness from the stair-step effect is removed by polishing with 0.5 µm diamond compound, and a UV-curable acrylic hardcoat is applied at 5 µm to 10 µm thickness to restore surface gloss. The hardcoat is diluted with propylene glycol methyl ether acetate in a 1:1 ratio before spray application at 2 bar. Compliance for consumer electronics prototypes requires that the cured material meet RoHS 2011/65/EU and REACH substance restrictions. Halogen-free flame retardancy is not an inherent property, and the part is not suitable for direct contact with high-power LED dies exceeding 85 °C unless the junction temperature is controlled. Optical transmittance is measured by ASTM D1003-21; refractive index is measured by ASTM D542-14 using an Abbe refractometer. The process boundary is explicit: under-cure leaves tacky surfaces and low glass transition temperature; over-cure increases yellow index and haze. Long-term UV aging is not established for this specific configuration, so ISO 4892-2:2013 or ASTM G154-23 testing is required before outdoor-facing use.

Translucent anatomical models printed from CT or intraoral scan data require dimensional stability through post-processing rather than maximum optical clarity. A maxillofacial reference model is printed with the Kolon Inkrayon resin at 50 µm layers on a 405 nm LCD platform. The STL file is oriented to place the nerve canal axis parallel to the build direction so that stair-step artifacts do not obscure the anatomical region used for implant path planning. The model is washed in isopropanol for 4 min, air-dried, and post-cured at 40 °C for 20 min. A second thermal soak at 60 °C for 30 min is applied only if the model geometry includes thin sinus walls above 0.8 mm; thinner walls can warp during the thermal soak because the material modulus drops near its glass transition temperature. The resin is used neat. Adding reactive diluent changes the leachable profile and invalidates the ISO 10993-5:2009 cytotoxicity assessment. Dimensional measurements are taken with a structured-light scanner and compared to the STL reference with a tolerance of ±0.25 mm on critical canal landmarks. The model is intended as a reference tool, not as an implantable device. Skin contact compliance is verified according to ISO 10993-10:2021 for sensitization and ISO 10993-23:2021 for irritation if the model is handled repeatedly in a clinical environment. Steam autoclave sterilization is not recommended because prolonged exposure to 134 °C saturated steam can soften and deform the part. Low-temperature hydrogen peroxide gas plasma is the preferred disinfection route for reusable models. The terminal product is a translucent surgical planning model with color-stained nerve canals and drill guide sleeves, allowing the clinician to see internal anatomy through the outer structure.

When Transparent Photopolymer Mandrels Replace Machined Acrylic in Silicone Overmolding

When a short-run silicone manifold requires a dimensionally stable core, a transparent photopolymer mandrel allows visual inspection of silicone wall thickness before demolding. The process is constrained by the low heat deflection temperature of unfilled acrylate networks. The mandrel is printed at 100 µm layer thickness on a 385 nm DLP machine, washed in isopropanol, and post-cured with 8 J/cm² at 40 °C. The surface is then sanded from 400 to 1200 grit and polished with 1 µm diamond paste, because any residual layer-line groove will transfer to the silicone inner surface and create a leak path. A water-based mold release is applied by airbrush at 2 bar as a 2 µm to 5 µm film. Thicker release films alter the inner diameter by more than 10 µm. Platinum-catalyzed silicone is mixed at the supplier-recommended 10:1 base-to-catalyst ratio and degassed in a vacuum chamber at -0.9 bar for 5 min. The silicone is poured around the mandrel and cured at 23 °C for 24 h. Elevated curing at 60 °C is avoided because the photopolymer mandrel can soften and distort above its heat deflection temperature, which for this class of material is typically below 70 °C at 0.45 MPa when measured by ASTM D648-18. If the silicone part design requires a curved internal channel, the printed mandrel may be left in place only if the final assembly does not require mandrel removal; otherwise the part is split along a plane to remove the core. The terminal product is a transparent silicone manifold with a smooth bore and visual wall-thickness uniformity.

Chemical process flow cells that require transparent observation windows demand chemical compatibility with aqueous process streams and moderate internal pressure. A monolithic transparent window is printed on a 405 nm LCD platform at 50 µm layer thickness and post-cured under nitrogen to reduce oxygen inhibition on the exposed surface. The post-cure sequence is 6 J/cm² at 30 °C followed by 4 J/cm² at 50 °C. The window is polished to Ra 0.1 µm and sealed into a stainless steel or PTFE flow cell using perfluoroelastomer O-rings. Chemical exposure tests are performed according to ASTM D543-21 for the specific process solvent before production use. The material is not recommended for ketones, chlorinated solvents, or strongly alkaline solutions above pH 10 because ester linkages in acrylate networks undergo hydrolytic degradation. The operating pressure is limited to 2 bar for a 2 mm thick window. Thicker windows can be printed for higher pressure but transmittance may decrease due to internal light scattering. The print is used in a flow cell for mixing studies under a high-speed camera, with the transparent window providing an optical path for shadowgraphy or laser-induced fluorescence. The resin is not post-heat-treated beyond 60 °C, and the final part is not suitable for continuous contact with aromatic hydrocarbons. Published data for this specific configuration is limited, so a compatibility coupon test is required before installation.