The Fused Filament Fabrication (FFF) printing process is incredibly adaptable—however, it doesn’t work for every plastic. As a result of the tight constraints required to precisely extrude plastic out of a tiny nozzle, traditional plastics originally optimized for injection molding do not print. The plastics that are printable, however, cover a massive range of compositions, print constraints, and material properties. To find the right material, you need to match the requirements of your applications to the properties of the materials you can print with. In this article, we discuss the strengths and weaknesses of a variety of thermoplastics.
In addition to printing thermoplastics, Markforged also adapts the FFF process to print non-plastic materials. In Continuous Filament Fabrication (CFF), an FFF printer with a specialized second nozzle lays down continuous carbon fiber, fiberglass, or Kevlar® into a part. Atomic Diffusion Additive Manufacturing (ADAM) builds on the existing metal fabrication technology of Metal Injection Molding (MIM), by using an FFF based process to print metal powder encased in a plastic binder. These printed parts are placed in a solvent bath to remove binding material and sintered into fully metallic parts.
As 3D printing has expanded rapidly, so has the variety of printing filaments. Despite this boom, most FFF 3D-printable thermoplastics fit into three categories: basic thermoplastics, niche thermoplastics, and superplastics.
Basic thermoplastics: These plastics don’t have any excellent qualities, but are the most popular printing thermoplastics available. PLA, the most common printing plastic, prints well and possesses decent mechanical properties—however, its complete lack of heat resistance and its low durability makes it impossible to use in industrial environments. ABS has superior heat resistance, but isn’t particularly strong and reacts poorly with most manufacturing chemicals. PETG, a printing subset of polyethylene, is a cross between the two: a bit stronger than ABS and a bit more heat resistant than PLA, but still not robust enough for most manufacturing environments.
Niche thermoplastics: Niche thermoplastics have one or two excellent facets, making them very useful in specific applications. Nylon is a perfect example. It’s not stiff or particularly strong and it carries virtually no heat resistance, but it’s extremely durable and has remarkable chemical resistance. As a result, it’s used in applications where flexibility and durability are most important. TPU (or TPE) is an extremely ductile material that has similar properties to Nylon, with a bit more flexibility. Polycarbonate is an excellent plastic in many respects—quite strong and heat resistant—but is only moderately durable and chemically resistant.
Superplastics: These materials possess all the aspects necessary to thrive in manufacturing environments. PEEK and Ultem are both strong, stiff plastics that have extremely high heat resistance and chemical resistance. Engineers used them heavily in manufacturing before they were 3D printable, and now use printers to create custom, robust fixtures out of these materials.
A filled thermoplastic is a material in which a standard plastic is impregnated with tiny particles of a second material. The concentration of the second material may vary, but it’s still primarily a thermoplastic by composition and material behavior. Adding particles of a stronger material to thermoplastic can alter many material properties (though chemical resistance is still wholly dependent on the plastic).
Filled thermoplastics fall into two camps: exotic material filled plastics and chopped fiber reinforced plastics. Exotic material filled plastics are most similar to niche plastics, as their secondary materials (coffee, wood, and other materials) alter texture and appearance more than mechanical properties. Chopped fiber reinforced plastics, on the other hand, utilize industrial grade fibers to improve the material properties of printing thermoplastics. The most common of these is chopped carbon fiber reinforced nylon, a nylon base plastic impregnated with microscopic pieces of carbon fiber.
Adding the right amount of carbon fiber to nylon transforms it from a niche plastic to a superplastic. The fibers boost strength, stiffness, and heat resistance significantly, and the dimensional stability that the carbon fiber adds makes the plastic print as well as any FFF plastic in existence. Parts 3D printed in this material are excellent in a wide variety of applications, and often are mistaken for parts that aren’t printed. Markforged’s Onyx material is a carbon fiber filled plastic of this variety.
Overfilling nylon with carbon fiber yields a completely different material. The additional carbon fiber boosts strength and stiffness further, but at the cost of decreased print quality. As the concentration of carbon fiber particles rises, the binding thermoplastic cannot flow through the printing system properly, leaving parts that have visible defects and rough surface texture. These materials, while remarkable in their own right, are closer to niche plastics than superplastics.
Continuous fibers (CFF)
While chopped carbon fibers are remarkable in their ability to augment thermoplastics, continuous fibers can add far more strength to parts. Markforged uses a combination of FFF printing and Continuous Fiber Fabrication (CFF) to lay down long strand fibers in conventionally printed thermoplastic parts. This technology is also extrusion based and prints via a secondary nozzle, but instead of melting the whole filament, it uses the heat of its nozzle to “iron” down fibers into a thermoplastic layer. Fibers do not melt—instead, they’re captured by the thermoplastic matrix in a similar way that thermoset adhesives like epoxy capture fibers in traditional fiber fabrication methods.
The resultant parts are an order of magnitude stronger, stiffer, and more durable than plastic (filled or not) and maintain the heat resistance, chemical resistance, and print quality of their thermoplastic matrix material. With Markforged, you can print chopped carbon fiber reinforced nylon (Onyx) with continuous fiber reinforcement.
To print with metal, the Markforged Metal X prints with a specialized form of FFF called Atomic Diffusion Additive Manufacturing (ADAM). Similar to filled thermoplastics, the machine extrudes a filament comprised of a plastic binding agent filled with small metal powders in a process nearly identical to FFF. The resultant part is washed to remove binding material and sintered to yield a metal part. These parts are fully metal, but carry similar geometric properties to conventional FFF plastic parts. To learn more about the process, check out the Metal X.
Written by Markforged
Markforged is transforming manufacturing by addressing 3D printing as a holistic problem. Their process innovations are only possible by a combined effort in advanced cloud computing, cutting-edge materials science, and industrial design.