Kolon Inkrayon Flex 3D Printing Polymer is categorized as a flexible photopolymer resin within the Inkrayon vat photopolymerization line for 385 nm and 405 nm LED DLP/mSLA platforms. The product identifier is the Inkrayon Flex grade; no separate numeric model designation was identified in accessible product-family literature. The resin is supplied as a UV-curable liquid; an independently accessible technical datasheet for this exact grade was not located at the time of writing, so vendor-reported Shore hardness, tensile strength, and elongation are not reproduced here. Lot-specific certificates of analysis should be used to confirm viscosity, density, and reactivity before production. Based on the flexible grade designation and common vat photopolymerization classifications, the intended application range includes elastomeric prototypes, gaskets, seals, and low-volume functional parts requiring repeated flexing and compression recovery after UV post-cure. Mechanical qualification should follow ASTM D638-14 Type IV tensile testing, ASTM D2240-15 or ISO 7619-1:2010 durometer measurement, and ISO 37:2017 where elastomer-specific tensile behavior is relevant. Conditioning at 23 ± 2 °C and 50 ± 10 % relative humidity is mandatory before reporting comparative data.

For process validation, the first critical measurement is viscosity at 25 °C with a cone-and-plate rheometer at 10 s⁻¹ and 100 s⁻¹. In open-vat mSLA machines using a recoating blade, low-shear viscosity above 2500 mPa·s is associated with microvoid entrapment and build-platform damage, while high-shear values above 1500 mPa·s can slow recoating and increase print time. The resin should be conditioned at 25 °C for 24 h before printing because a ±5 °C temperature shift can change viscosity by 15–30 % in unfilled acrylate and urethane-acrylate systems. High-shear mixing above 2000 rpm may introduce air and generate localized thermal excursions; if mixing is required, a planetary mixer at 100–300 rpm followed by vacuum degassing at −0.08 MPa gauge is preferable. Filtration through a 100 µm stainless steel mesh reduces cured-particle contamination before vat loading.

When flexible photopolymer vat printing is transferred to production scale, failure modes include delamination at the build plate from insufficient bottom exposure, raft peeling during the transition from bottom to normal layers, and microvoid formation when recoating speed exceeds 120 mm/s at viscosity above 1500 mPa·s. On DLP systems with fixed vat heating, temperature overshoot above 35 °C may accelerate thermal initiator decomposition and shorten working life. A fluoropolymer release liner degrades with accumulated UV dose and can cause localized adhesion defects when its haze exceeds 5 % or surface roughness increases beyond 2 µm Ra. These operational symptoms are magnified for elastomeric resins because soft green-state parts are more susceptible to peel-force distortion than rigid green parts.

How Does the Flex Grade Differ from Rigid Inkrayon Photopolymers?

Rigid photopolymer grades in the same product family are typically formulated for flexural modulus above 1000 MPa, tensile elongation below 20 %, and brittle fracture. The Flex grade is instead intended for lower Shore hardness and higher strain recovery, with class-level elastomeric photopolymer formulations showing tensile elongation in the 30–120 % range and Shore A values between 50 and 90. These are industrial photopolymer class ranges, not product-specific values. The mechanical difference arises from lower crosslink density, a flexible oligomer backbone, and a reduced multifunctional monomer fraction; this also lowers green strength and can reduce overcure tolerance. Consequently, exposure latitude for elastomeric resins is typically narrower than for rigid resins, particularly at layer heights below 50 µm. Printed flex parts may require a higher bottom-layer exposure increase and more careful support placement because tear-prone thin walls exhibit lower green tear resistance during build.

On DLP systems with 405 nm LED engines and measured irradiance of 2–4 mW/cm² at the build plane, initial normal-layer exposures for flexible resins are commonly screened from 0.8 s to 3.5 s at 50 µm layer thickness. For mSLA platforms with monochrome LCD masks and array irradiance of 3–10 mW/cm², the practical range shifts upward to 2–6 s because LCD light attenuation reduces the dose reaching the resin. A working-curve evaluation should be performed because published data for this specific configuration is limited. The cure depth must be at least 2× the layer height to overcome oxygen inhibition at the free surface and to achieve interlayer adhesion. Bottom exposure for the first 6–10 layers is generally increased by 5–10× the normal exposure to anchor the part without excessive overcure, which can raise peel force and distort fine features near the build plate.

For component design, unsupported overhangs below 45° may survive in rigid grades but tear in elastomeric grades because green tear strength is low. Supports should use thicker contact points of 0.8–1.2 mm diameter and rounded tips to minimize notch-induced tear during part removal. Hollow sections should include vent holes of 2–3 mm diameter to prevent suction cup effects against the release film. Feature sizes below 0.5 mm may collapse during cleaning or post-cure if the green modulus is insufficient; thin walls should be validated with the actual cleaning solvent and post-cure support fixture. Surface tack after printing is a known limitation because oxygen inhibition at the part surface leaves an uncured layer. Isopropanol or ethanol rinses shorter than 5 min may leave tack, while solvent exposure above 10 min can extract reactive components and reduce tensile elongation. A two-stage rinse in fresh solvent followed by compressed air drying at 0.2–0.4 MPa is typical before UV post-cure. Post-curing while parts remain wet with solvent can create white surface haze and may interfere with degree of conversion. If tack persists after a validated post-cure, the exposure window should be adjusted rather than increasing thermal exposure above 60 °C, which can accelerate oxidative degradation and dimensional warpage in low-crosslink-density resins.

Resin Handling Boundaries and Batch-to-Batch Verification

Closed-container storage in low-actinic amber containers at 15–30 °C is recommended for UV-curable elastomeric resins. Open-vat operation at ambient relative humidity above 60 % may increase water uptake and alter polymerization rate, final elongation, and haze. After printing, the vat should be covered or the printer enclosure closed to limit ambient UV and oxygen exposure; residual overhead lighting can initiate surface gelation. Combination with amine-containing colorants or mercapto additives should be avoided unless explicitly validated; acrylate-functional photopolymers can undergo premature gelation or reduced shelf life. Batch-to-batch variance in flexible photopolymers is commonly observed in pigment dispersion stability, viscosity, and post-cure Shore hardness. For production control, retain a 100 mL reference sample from each lot and record viscosity at 25 °C, post-cure Shore hardness, and exposure needed to reach the target working depth. If lots are mixed, the blend must be revalidated by ASTM D638-14 tensile testing because nonlinear hardness shifts greater than 5 Shore A points may occur from reactive diluent ratio changes.

When a Flexible Photopolymer Replaces Machined Elastomers or FDM TPU

Substitution of a machined silicone or cast polyurethane component with Inkrayon Flex should be evaluated through tear strength, compression set, and cyclic deformation rather than Shore hardness alone. FDM thermoplastic polyurethane parts typically exhibit z-axis tensile strength that is 50–80 % of in-plane strength due to interlayer coalescence limits. Vat photopolymerized elastomers generally have lower anisotropy but may carry higher crosslink density, reducing elongation and increasing tensile set under repeated strain. The relevant cyclic benchmarks are ISO 815-1:2019 or ASTM D395-18 compression set measured at 70 °C for 24 h, and tear strength measured by ASTM D624-00 Die C. In the absence of vendor data for Inkrayon Flex, fabricated parts should be exercised for 1000 cycles at the expected service strain and inspected for surface microcracking, delamination, or Shore hardness drift greater than ±3 Shore A points.

Tear Strength and Compression Set Are Not Interchangeable with Shore A Hardness

Flexible photopolymer parts that pass a Shore A durometer specification may still exhibit unacceptable tear propagation when notched or when printed with thin walls and sharp corners. ASTM D624-00 Die C tear strength for elastomeric photopolymers is frequently lower than that of cast polyurethane elastomers of equivalent Shore hardness because the photopolymer network contains higher methacrylate crosslink density and less energy-dissipating entanglement. Compression set under 70 °C conditions is often the limiting property for sealing applications because residual stress relaxation and thermal aging can alter contact pressure. A qualified part should be tested with the actual post-cure schedule, not an abbreviated cure, because surface conversion and core conversion differ in sections thicker than 6 mm. Hardness measurements taken within 1 h of post-cure can drift by 2–5 Shore A points during dark equilibration; mechanical data should be collected after 24 h at 23 ± 2 °C to avoid overstating stiffness.

Seal, gasket, and flexure prototypes produced with flexible photopolymer resins are typically post-cured under 385–405 nm LED flood lamps at 5–20 mW/cm² for 10–30 min, followed by 24 h dark equilibration. For parts with variable wall thickness, tensile and tear coupons should be printed in the same orientation and section thickness as the service component because anisotropic cure light scattering and internal stress may differ. Chemical compatibility testing under ISO 175:2010 is required for applications involving polar solvents, fuels, or alkaline solutions; mass and dimension change should be recorded after 7 days at 23 °C. Skin-contact or wearable applications may require ISO 10993-5:2009 cytotoxicity and ISO 10993-10:2021 sensitization data, which are not supplied automatically with the resin.

Regulatory documentation requests should include REACH registration status, RoHS Directive 2011/65/EU Annex II restricted substances, and absence of Substances of Very High Concern in the as-supplied resin. A supplier safety data sheet under CLP Regulation 1272/2008 provides hazard classification, but it does not supply mechanical or biocompatibility data. For medical device prototyping, a resin may be tested to ISO 10993-5:2009 and ISO 10993-10:2021; certified biocompatibility is a device-level determination and cannot be assumed from resin test reports alone.

Before release of parts to production, complete a three-batch validation that includes tensile stress-strain curves, Shore A hardness, tear strength, compression set, and a printability run on the target mSLA or DLP equipment. The printability run should use the actual production orientation, layer height, and post-cure schedule. Surface tack, support tear-out, and build-plate delamination should be recorded as binary process indicators, and any lot change should trigger a reduced validation battery consisting of viscosity, Shore hardness, and ASTM D638-14 tensile testing. For applications that require lot traceability, mark each part with the resin lot number and post-cure date; do not rely on visual surface appearance as an indicator of cure uniformity.