Flexible / elastomeric resins — honest review

Published · Researched 2026-09-20

Printed a part that shatters like glass, and now you need one that bends? Flexible resins are the resin counterpart to TPU filament — photopolymers that cure into rubber-like parts: gaskets, seals, grips, vibration dampers, wearable prototypes, soft robotics bits. They span firm-flex (Shore 65D, bendable but tough — Siraya Tenacious) down to true elastomers (Shore 70A and softer — Tenacious Flex Black, Formlabs Flexible 50A) that stretch and snap back. Two ways to use them: print straight for the softest result, or blend 10–30% into rigid resins — the famous Tenacious trick that takes a brittle standard print and makes it survive a drop. This pass dug into what that trick actually costs and where the standalone material falls short.

What's genuinely good: Tenacious is, by acclaim, the community's "fix my brittle prints" additive, and the 10–20% blend trick is the single most-repeated resin tip in hobby forums — it dramatically improves impact resistance while keeping the detail rigid resins are good at. That compatibility with other 405 nm resins is its superpower; the Siraya 13k-member user group is visibly built around this kind of peer troubleshooting. Owners have quantified the working ratios: 3–4 parts tough resin to 1 part Tenacious, or roughly 20–30% Tenacious in standard resin, to toughen brittle prints without turning the whole vat into rubber. One owner found the mix made self-tapping screws hold where plain resin chipped — a genuine functional win that rigid-resin users will recognize immediately. As a standalone material, Tenacious 65D is firm-flex — think tough parts that bend 180° in thin sections without shattering — while Flex Black 70A is genuinely rubbery, fast-rebounding, and built for impact resistance. Pricing is friendly at the hobby end: Tenacious 65D from $44.85 sale / $65.00 list, Flex Black 70A from $32.49 / $49.99 on siraya.tech, with 1 kg of Flex Black at $69.99 via WoodArtSupply (all observed 2026-09-20; Siraya "from" bottle sizes are UNVERIFIED). A 2026 maker materials survey priced the open-printer elastomer landscape at roughly £50–140 per half-liter (Resione F69 Bio ~£50/500 mL, tunable 50A–70A by cure time; 3Dresyns OTO-F1 Flex €140/500 mL; Tenacious £50/500 mL). At the pro end, Formlabs Flexible 50A ($219/kg, Form 3/4-locked) is the pick when a wearable or prototype must survive repeated flex cycles without tearing, with the BioMed Flex 80A variant (£200/L) covering biocompatible applications.

What's actually wrong: the honest version is longer than the marketing version, so take it in layers. First, the flexibility has fine print: Tenacious is flexible only when printed thin — below about 1/4 inch the print stays flexible after curing; thicker sections read as firm-flex, and one retailer reviewer put it bluntly that it's "not like NinjaFlex on an FDM printer." If you're imagining rubber bands, recalibrate. Second, it doesn't take detail as well as rigid resins, and cleanup is famously messy — described as "motor oil mixed with honey," slippery and sticky at once, thicker than most resins you've handled. Third, the blend trick has a tax: once Tenacious is in the mix, parts stay sticky after cleaning unless washed with IPA 99 — Simple Green-style cleaners stop working — and prints come out slightly less sharp. Fourth, do not hollow Tenacious-class resins; the outer shell can collapse. Fifth, print-in-place articulated models are a known failure: one owner found octopus and shark joints break within a few bends — repeated flex at thin hinges is the fatigue case, so design flex as distributed bending in thin sections, not as pin joints.

The beginner trap that dwarfs the rest is temperature. Flexible resin printed cold fails, full stop — Siraya's guidance demands ≥25 °C ambient, and a drafty 18 °C garage will eat these prints. Flexible parts are also harder to support than rigid ones: orient to minimize peel forces and run thicker supports. And the aging caveat from a long-term user (intofpv forum): Tenacious-blend prints get harder and more brittle over time — fine for prototyping, a deal-breaker for parts that must stay flexible for years. That's one user's experience, attributed as such, but it matches the general UV-aging behavior of photopolymers and deserves to be in the ledger.

Two more honesty items. One reviewer reported a bottle that printed extremely brittle "like ABS" and suspected a wrong-bottle factory batch — a single anecdote, but the takeaway is procedural: test-print every new bottle, and if a "flexible" resin prints like ABS, suspect the bottle before your settings. And the pricing oddity: Siraya's category page showed Tenacious 65D "from $25.19 / $35.99" while the sales page showed "from $44.85 / $65.00" — variant/size ambiguity flagged, so verify the bottle size before comparing prices. Tensile and elongation figures for both Siraya resins are UNVERIFIED in this pass (no TDS pulled), so treat bend-180° marketing as marketing until a datasheet backs it.

The social pulse: of all the resin sub-families, flexible has the strongest organic community signal — Tenacious's blend trick is genuine hobby lore, repeated so often it's practically punctuation, and this pass found the ratios, the IPA-99 tax, and the screw-holding win documented across retailer reviews and forums. Facebook itself stayed silent (zero posts in the groups queried), so the signal is forum-and-review shaped rather than group shaped. The debate is practical, not brand-flame-y: exposure settings, support thickness, room temps, the 70A-versus-65D choice for a given part. Formlabs Flexible 50A sits off to the side in pro conversation, respected but rarely argued about because its users bought a validated workflow, not a hack. The overhyped corner is thin — nobody claims these are miracle materials, because everyone's watched one fail on a cold build plate. One useful dissent from the forums: for functional flexible parts where resin detail isn't needed, TPU filament is the cheaper route — flexible resin earns its keep on detail and surface finish, not on price per part.

Value call, September 2026: against each other, Flex Black 70A at $69.99/kg versus Formlabs Flexible 50A at $219/kg is a 3× gap for a material class where the hobby option is genuinely good — Formlabs only wins when repeated-cycle durability under professional standards matters, or when the printer is already a Form 3/4. Against TPU filament, resin-flex buys finer detail and smoother surfaces at the cost of the whole wash/cure/safety discipline and worse UV longevity. For de-brittling rigid resin, a bottle of Tenacious used as a 10–20% additive remains the best value in the entire resin aisle — just budget the IPA 99 and the slight sharpness loss into the math.

Skip this if: your print room is cold and you won't warm it. Skip Formlabs Flexible 50A if you don't own a Form 3/4 — it's locked to those machines. And skip standalone flexible resins if all you want is "less brittle miniatures" — the Tenacious blend in your regular resin is cheaper and easier than re-tuning your whole profile for a flex resin. Skip rubber-band dreams too; that's TPU filament's job.

Closing tradeoff: buy Tenacious as the additive first and as a material second — and heat the room before you blame the resin. The flexibility is real but conditional: thin sections, warm room, fresh bottle, honest expectations about fatigue. Meet those conditions and it's the most useful specialty resin in the hobby aisle.