Tough vs flexible resin: choosing by documented properties

Two documented examples to define your requirements

According to its Formlabs page, Flexible 50A is a soft, translucent elastomer meant for prototyping parts normally made from softer rubbers and silicones: wearable straps, compressible buttons, compliant features for robotics, stretchable enclosures and casings, soft tissue anatomy models, and special effects props. Formlabs files it under the Elastomeric category.

The Tough 1000 page describes a ductile, impact-resistant material with strength, stiffness and toughness comparable to high-density polyethylene (HDPE), listed under Impact Resistant. The suggested uses are impact-resistant jigs and fixtures, squeezable prototypes, and low-friction assemblies such as gears and ball joints.

To define your requirements, describe the part’s shape, intended deformation, and loads to check, then look for the corresponding properties of the exact product. A prototype strap and a fixture cannot be specified by the same marketing keyword alone. The applications Formlabs lists are starting points for that reading, without qualifying the geometry you will print. Our 3D printing resin hub extends the search to other documented formulations.

Shore hardness: the product name versus the measured value

Shore hardness provides a concrete data point, provided you separate the product name from the measured value. The product name Flexible 50A includes 50A, and the overview states a 50 Shore A durometer, and the marketing copy repeats that figure. The material properties table on the very same page, however, lists a measured Shore hardness of 55 A. Both statements sit side by side on the manufacturer's page.

Do not reconcile that gap yourself. Keep the product name and record the table value separately, with the stated specimen conditions. The footnote says the data came from parts printed on a Form 3 at 100 microns, using Flexible 50A settings and the Flexible 50A post-processing steps. A thin wall, a different orientation or a different cure can land somewhere else.

The Tough 1000 product page reviewed does not list a Shore value among the properties shown; that does not establish that Formlabs never measures it. The page instead lists other indicators, including tensile and flexural data, notched Izod and Gardner impact. Do not substitute one property for another: if hardness matters, look for a hardness value for the exact formulation rather than assuming modulus replaces it.

Elongation and tensile data: read each figure with its protocol

Elongation at break can be misread when separated from the other results: Flexible 50A lists 160% and Tough 1000 lists 180%. These figures describe specimen elongation before break under the stated conditions; they are not enough to rank part or resin softness. Read them alongside the corresponding stress and modulus measurements, without turning a specimen value into the force required by your geometry.

The pages report several separate results. Flexible 50A lists 3.4 MPa ultimate tensile strength, 0.9 MPa stress at 50% elongation and 1.7 MPa at 100%. Tough 1000 lists 26.3 MPa ultimate tensile strength, a 932 MPa tensile modulus, 29 MPa flexural strength and a 761 MPa flexural modulus. Formlabs describes Tough 1000 as ductile, but these figures alone do not describe the deformation or load of a particular part; keep tensile strength, stress at a stated elongation and flexural properties distinct.

These two datasets were not produced on the same bench. The Flexible 50A footnote cites parts printed on a Form 3 at 100 microns. The Tough 1000 footnote cites parts printed on a Form 4 at 100 microns and post-cured for 12 minutes at 70 °C in a Form Cure V2. You can list both sets with their scope, but they are not a controlled head-to-head test and they do not support a ranking.

Repeated cycles, fatigue and tear: what each page claims

The Flexible 50A page lists tear strength of 12.3 kN/m, compression set of 3.1% after 22 hours at 70 °C, and says its parts can bend, stretch and compress through repeated cycles without tearing and spring back quickly. These are Formlabs statements for this formulation and its specimens. They do not guarantee recovery for another geometry, sealing performance or skin contact.

For Tough 1000, the page lists a maximum stress intensity factor of 1.94 MPa·m½, notched Izod of 72 J/m and Ross flex fatigue above 100,000 cycles at 30° deflection and 1 Hz, at 23 °C. Work of fracture appears with conflicting units on the same page: 3,200 J/m² in descriptive copy and the Mechanical Properties block, but 3,200 J/m in the Tech Specs row. Do not reconcile that unit; omit the value or report both labels. The Ross conditions are Formlabs’ test conditions, not a service-life guarantee for your hinge.

The page also contains two Gardner impact discrepancies. Its overview quotes 128 in-lb; Tech Specs lists 13.1 J at 1/32 in (0.97 mm), while the comparison table gives 13.1 J, or 116 in-lb, at 1/32 in (0.8 mm). The page does not reconcile either the in-lb figures or the metric thickness labels; preserve the exact source block instead of silently converting them. It also lists 14.5 J at 1/16 in (1.9 mm). Notched Izod remains a different test, with no conversion to Gardner.

Post-processing: wash, dry and cure as each page describes

Both pages describe the same skeleton. First, wash the part in isopropyl alcohol or an ether-based solution to remove uncured resin. Then post-cure to finish the chemical reaction that printing started. Formlabs stresses that post-curing is essential for engineering and specialty resins to reach full strength and function. Wash settings are published for Form Wash and Form Wash L, and cure settings separately for Form Cure V1, V2, L V1 and L V2.

For Tough 1000, Formlabs recommends air drying for at least 30 minutes after washing and before handling for the best surface finish. This instruction is specific to Tough 1000; for Flexible 50A or another formulation, consult the applicable instructions instead of extending this duration.

Likewise, the 12 minutes at 70 °C in a Form Cure V2 protocol is the condition under which the Tough 1000 values were measured. It is not a universal profile and not the setting for a different Form Cure generation. For your own parts, follow the setting published for your specific unit, and see our guide to resin post-processing for the order of operations and how to handle parts that are still tacky.

Printer compatibility and selection method

The compatibility lists overlap but are not identical. Flexible 50A is listed for Form 4, Form 4B, Form 4L, Form 4BL, Form 3, Form 3L, Form 3B, Form 3BL and Form Studio 1, with the tanks and platforms shown on its page and a 100 µm minimum layer thickness. Tough 1000 is listed for Form 4, Form 4B, Form 4L, Form 4BL and Form Studio 1, with the Form 4 Resin Tank, Form 4L Resin Tank or Form Studio Resin Tank Pro. Form 4 and Form Studio 1 therefore appear on both lists; verify accessories and conditions for the exact resin. These pages do not document other SLA brands.

For these two examples, start with the use stated on the product page and then check relevant data for the exact formulation. Flexible 50A is presented for elastomeric prototypes; its page lists measured hardness, tear and stress at stated elongations. Tough 1000 is presented as ductile and impact resistant; its page lists tensile data, moduli, Izod and Gardner with the caveats above. Printing, post-cure and the properties required by your part must also match. This method does not rank every resin labeled “flexible” or “tough.”

If you do not own a printer yet, the resin 3D printer buying guide and the Formlabs Form 4 page help you confirm compatibility before you pick a material. To budget the full chain of tank, resin, wash and cure hardware, the startup budget tool keeps the list honest.

Sources and limits

• Flexible 50A Resin | Formlabs
• Tough 1000 Resin | Formlabs

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.