What’s the Difference Between Kevlar® and Carbon Fiber?

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Kevlar‍

What do you think about when you hear the name Kevlar? Most people think of Bullet Proof Vests. You might not think DuPont™, but this material was developed by DuPont™ back in 1964 by a Polish-American chemist Stephanie Kwolek.

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Did you know that Kevlar has a number of different uses across industries? You can find it in everyday applications like:

- Tires for cars and bicycles

- Plates for weights

- Sails and rope

- Gloves, socks, and luggage

- Work boots

- Brake pads

- Protective equipment

- Sealants

Kevlar is a synthetic fiber that is in the aramid fiber’s group which is heat resistant. Kevlar and Nomex are a part of this group. Synthetic fibers are fibers synthesized through chemical synthesis, as opposed to natural fibers derived from living organisms. Synthetic fibers are created by extruding fiber-forming material through spinnerets, forming fiber.

Kevlar has a high-modulus type used primarily in fiber optic cable, textile processing, ropes, cables, plastic reinforcement and composite applications for aerospace, automotive, defense, energy, consumer, electronics, medical, and heavy industry to name a few. The Naval Facilities Engineering Command explored Kevlar rope capabilities for use with ocean engineering and construction, resulting in innovative designs and applications provided by Kevlar ‘s incredible tensile strength and buoyancy. Kevlar fiber has a tensile strength comparable with that of carbon fiber, a modulus between those of glass and carbon fibers and lower density than both.

Kevlar aramid is used for high-performance composite applications where lightweight, high strength and stiffness, damage resistance, and resistance to fatigue and stress rupture are important. Markforged finds that reinforcing Onyx, Onyx FR, and even Nylon White with Kevlar allows engineers and part designers to create extremely versatile parts. Kevlar can undergo significant changes in low-temperature environments, as low as 320°F (-196°C) and show no embrittlement or degradation, and environments with electron radiation, as electron radiation is not harmful to Kevlar. However, Kevlar is sensitive to UV (ultraviolet) light.

Designers can develop parts that are safe, strong, stiff, lightweight, and tolerant of the environment, application, and loading conditions by printing with continuous fibers. By designing parts with the CFF (Continuous Filament Fabrication) process designers can leverage Kevlar's tensile strength (stretching or pulling) that is over eight times greater than that of steel wire.

Reinforcing with CFF enables any designer to build composite parts with metal strength, increase the part’s durability (lifetime) and optimize the part’s strength where it is needed most. Kevlar also has a very long plastic deformation range and when it fails it does so one strand at a time and will even bend or fall over instead of snapping. It has a much more predictable and forgivable failure mode compared to other fibers like carbon fiber.

Unique properties of Kevlar fibers:

- Very low stretch

- High tensile strength

- Very high strength-to-weight ratio

- Excellent fatigue resistance

- Good performance over large temperature range

- Does not melt; will decompose at 800°F - 900°F (427°C to 482°C)

- Low creep

- No shrinkage

- Good chemical stability

- Highly abrasion resistant

- Weak strength in the transverse direction (weak in compressive strength)

- The least catastrophic failure mode of all Markforged filaments

Impact Resistance

Kevlar is 8x more impact resistant than ABS while remaining 15-20% lighter than our other reinforcement fibers.

Flexural strength

In three-point bending, 3D printed Kevlar is 3x stronger than ABS and 6x stronger than nylon.

Flexural stiffness

3D printed Kevlar is 12x more rigid than ABS and 30x more rigid than nylon.

Kevlar possesses excellent durability, making it optimal for parts that experience repeated and sudden loading. As stiff as fiberglass and much more ductile, it can be used for a wide variety of applications tailored for additive manufacturing, such as:

- Athletic footwear

- Robotics and cradles

- End effectors/grippers

- Smartphone cases, personal electronics

- Parts designed to be driven by hydraulics or pneumatics

- Protective gear, helmets; combat, motorcycle

- Brake levers, clamps, mounts

- Fixtures, tooling, workholding, soft jaws

- Gears, wrenches, drones

- Sporting goods & accessories, carabiners

- End-use parts, consumer products, etc...

Dixon Valve grippers are printed with Onyx and reinforced with Kevlar. The material must be strong enough to transmit the clamping force, durable through repeated loading cycles, and non-marring to the valves.

Composites in 3D printing take advantage of the compressive strength of the plastic matrix — the support structure which comprises most of the part volume — and the tensile strength of embedded fibers. These two materials are mutually dependent: without fiber, the plastic part is only as strong as the adhesion within and between extruded plastic strands. Without the matrix, the fiber has no structure and therefore won’t maintain its shape. The matrix creates space so that the fiber has a lever arm to stabilize against the load. When combined, they synergize to form a composite with greater strength in both compression and tension than either can offer individually. This is true for all of our fibers; Kevlar, carbon fiber, fiberglass, and HSHT fiberglass.

Now, let’s explore our understanding of carbon fiber a little bit more.

Carbon Fiber

Carbon fiber filament is made up of carbon atoms organized into a crystalline structure. Because of its very high stiffness and strength, it is widely used in the aerospace and automotive industries. It has one of the highest strength-to-weight ratios in existence — higher than both steel and titanium.

Read our free Composites Design Guide

Strength-to-weight ratio

Compared to 6061 aluminum, 3D printed carbon fiber has a 50% higher strength-to-weight ratio in flexure and 300% higher in tension.

Flexural Strength

In a three-point bending, our 3D printed carbon fiber is 8x stronger than ABS and 20% stronger than the yield of aluminum.

Flexural Stiffness

3D printed carbon fiber filament is 25x more rigid than ABS and 2x more rigid than the rest of Markforged’s reinforcement fibers.

Carbon fiber material characteristics:

• Markforged 3D printed carbon fiber is equal to the yield strength of 6061 Aluminum
• It fails at the same stress Aluminum starts to plastically deform at.
• Carbon fiber will return to its original shape after a load is removed while Aluminum plastically deforms
• High stiffness and high strength-to-weight
• Conductive to electricity
• Corrosion and heat resistant
• Stiff until fracture (failure is abrupt and not predictable)
• Ideal loading is constant - supporting a known force all of the time.
  

Carbon fiber’s incredible properties allow it to be used as a metal replacement in applications where weight saving is important. Every industry now has the ability to leverage CFF with carbon fiber and print incredibly strong parts. Generative Design also offers advantages when combined with Markforged CFF that allows designers to explore multiple optimized solutions and have the ability to select the best design tailored for its use from both design and strength perspectives.

Carbon fiber can be used for a wide variety of applications; aerospace, automotive, architecture and construction, consumer goods, medical, energy, defense, electronics, industrial machinery, etc. tailored for additive manufacturing and there’s no end to this list, so here a just a few:

• Robotics and robotic arms
• End effectors, grippers, and soft jaws
• Inspection fixtures, welding fixtures, and CMM fixtures
• Forming tools
• Bicycles and their components
• High-end motorsport applications

Take a look at the Haddington Dynamics' use case, a 3D printed robotic arm reinforced with continuous carbon fiber filament, stiff and lightweight enough for the robotic arm to have precision of 50 microns. Using a carbon fiber 3D printer, the company were able to reduce part count from 800 to less than 70.

Please reach out to us for further help or advice on which is the most appropriate reinforcement fiber for your application. Request a Kevlar or carbon fiber sample today.

Reference List:

1. Ferer, M. Kenneth and Swenson, C. Richard, “Design Guide for Selection and Specification of Kevlar Rope for Ocean Engineering and Construction,” Pages v, 9, 39, https://apps.dtic.mil/dtic/tr/fulltext/u2/a163255.pdf, Jul. 1976, Naval Research Laboratory, Naval Facilities, Engineering Command, Washington, DC.
2. Smith, F. William, 1996, “Principles of Materials Science and Engineering, Third Edition,” McGraw-Hill, Inc., Page 774, ISBN- 0-07-059241-1.
3. DuPont™. DuPont™ Kevlar Applications - "The Kevlar® Journey, Top To Bottom". YouTube, Aug. 2014. https://youtu.be/hIqKoZLL4QU?t=90.
4. DuPont™. 2017, Kevlar Aramid Fiber Technical Guide, Pages 12, 14, 16, https://www.DuPont™.com/content/dam/DuPont™/products-and-services/fabrics-fibers-and-nonwovens/fibers/documents/Kevlar_Technical_Guide_0319.pdf
5. Science Channel. “Watch In Slow-Motion As Kevlar Fibers Are Put To The Test.” YouTube, Jun. 2017, https://youtu.be/ybgMEjl9j-g.
6. Yeung, K. K., and Rao, K. P., “Mechanical Properties of Kevlar Fibre Reinforced Thermoplastic Composites,” Page 411, https://pdfs.semanticscholar.org/fa3f/845bb8b7230c6d82b29392c8c5baf7da10d5.pdf, Jan. 29, 2010, Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong SAR, China
7. Wikipedia contributors. "Kevlar." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 11 Aug. 2019. Web. 16 Sep. 2019, https://en.wikipedia.org/wiki/Kevlar#History.
8. Wikipedia contributors. "Aramid." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 8 Sep. 2019. Web. 16 Sep. 2019, https://en.wikipedia.org/wiki/Aramid.
9. Wikipedia contributors. "Synthetic fiber." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 7 Sep. 2019. Web. 16 Sep. 2019, https://en.wikipedia.org/wiki/Synthetic_fiber.
10. Williams Bryan, Attwood Louise, Treuherz Pauline, 2017, “Design and Technology: All Materials Categories and Systems, Fire Resistant Materials,” 2017.‍