Composite 3D printing: all you need to know about composite materials

What are composite materials and what are the main types of composite?

Composites are materials created by the combination of two materials with different physical and chemical properties, designed to improve the qualities of the neat material. They are composed by matrix and reinforcement.

Based on the nature of the matrix, there can be three main types of composite:

• PMC (Polymer-Matrix Composite): composites with a polymer matrix, usually thermoplastics like for example PA, PEEK and PP or thermosets like epoxy resin;
• MMC (Metallic-Matrix Composite): composites with a metal matrix, generally Aluminum, Titanium, or other alloys;
• CMC (Ceramic-Matrix Composite): composites with a ceramic matrix, commonly silicon carbide or alumina.

The most common composites are polymer-based because they guarantee both low density and increased performances. Thermosetting polymers have been preferred in the past because they preserve their rigidity up to pyrolysis temperature. Their use has been substituted in the recent years over thermoplastics because they are generally recyclable, more tough and can be stored for longer time.

In most cases, the binder phase, namely the matrix, is more ductile than the discontinuous filler phase, that results stronger or stiffer. To make the reinforcement strengthen the matrix, it is necessary to have a substantial volume fraction of the reinforcing phase (usually at least about 10%).

Composite 3D printing: what is the reinforcement and what types of reinforcement can be used?

The composite reinforcements can be dispended in different ways inside the matrix, and it has the scope to ensure rigidity and mechanical resistance, taking on itself most of the external load.

Based on the type of reinforcement, composites are divided in:

• particle composites;
• fiber-reinforced composites;
• structured composites (e.g., sandwich panels, laminated composites, and aluminum composite panels).

The latter are very common in the aerospace sector but are considered a special case of composites.

What is the difference between particle and fiber-reinforced composites?

Fiber reinforcement is a more effective mean of strengthening composites than particle reinforcing. Both reinforcements are used to achieve gains in stiffness, strength, and toughness, but the improvements achieved with fiber reinforcements are higher.

The main advantage of particle reinforced composites is the good surface quality and ease of production and forming. Furthermore, they have a low cost and can be used where a high wear resistance is required. An example of particle composite is sphere reinforced composites. Since spheres have a higher contact surface, hence a higher matrix/reinforcement interface, they absorb higher amounts of energy, resulting in an improved behavior when subjected to vibrations.

What is a fiber-reinforced composite?

Fiber-reinforced composites can be classified in three families, based on the size of the fibers and their arrangement:

• continuous (or long) fiber composites;
• discontinuous (or short) fiber composites, with fibers aligned with each other;
• discontinuous (or short) fiber composites, with fibers arranged randomly.

Discontinuous fiber composites, with fibers arranged randomly are highly anisotropic. On the other hand, discontinuous fiber composites, with aligned fibers, have a weaker anisotropy, being the fibers more equiaxed. But what is the influence of the fibers’ alignment on the mechanical properties?

Theoretical mathematical modeling is used to calculate the properties of composite materials. Since composites are anisotropic, their properties depend on the orientation of the applied loads in respect to the fiber/matrix interface direction. There are two main loading conditions that can hit a composite and affect its strength: isostress and isostrain.

• Isostress: theoretical model used to calculate the composite mechanical properties in a direction orthogonal to loads. When loads are applied in a direction that is perpendicular to the matrix and fibers direction, there is stress continuity across all layers. Fiber and matrix operate as they are working in series: they bear the same load but undergo different deformations. This results in the weakest condition.
• Isostrain: theoretical model used to calculate the mechanical properties of a composite in the direction that is parallel to the load direction. When moments and forces are applied in the same direction of the fibers, displacement continuity is guaranteed across all layers. Fiber and matrix operate as they are working in parallel, so even if the deformation is the same, the withstood loads are different. Being the fiber stronger than the matrix, the composite can bear higher loads than the ones that the matrix could bear itself. This results in the strongest condition.

Composite stiffness is maximized when fibers are aligned with the loading direction, but this is also the configuration that increases the possibility of fiber tensile fracture when the tensile strength exceeds that of the matrix.

When controlled, anisotropy can be an advantage: the material performances can be strengthen based on the loaded directions. With molded parts, anisotropy can hardly be controlled, while 3D printing technology can be beneficial because the material properties depend on the deposition direction, thus can be controlled and customed based on the applied load directions.

What is the interface in composites?

The interface is the primary dimension of any composite. The composite interface is the contact area between the matrix and the reinforcement.

The interface must be strong and with high compatibility between the two materials to form a successful composite. 

The mechanical properties of composite materials are strongly influenced by the properties of the fiber-matrix interface: having a good interface is essential to get good performance of the composite to stress loading.

Once subjected to loads, the fibers tend to realign themselves, inducing states of compression and traction in the matrix that can cause detachment between the fiber and the matrix, with a consequent reduction in the strength of the composite. The parameters that measure these phenomena are the debonding strength and sliding resistance of the fiber/matrix interface, that are considered the most important parameters to determine the properties of any composite.

What type of fibers can be used to reinforce a polymer?

There are diverse fibers that can be used to reinforce a polymer, here a short list:

• Carbon fibers (graphitic or amorphous carbon): thermally and electrically conductive, provide high mechanical resistance at high temperature. Lightweight and low density, expensive.
• Glass fibers: thermally and electrically insulative, provide high stability, high tensile stiffness and strength, small bending stiffness. Medium density, medium weight, low cost.
• Ceramic fibers (e.g., silicon carbide or alumina): highly thermally conductive, grant thermal stability, mechanical strength, high-temperature creep resistivity, low density, and stability against oxidation.
• Aramid fibers (such as Kevlar): high impact resistance, high strength, high modulus, toughness, and thermal stability. Kevlar fibers are highly crystalline, their surface is chemically inert. Kevlar is used in a wide range of applications due to its high strength-to-weight ratio, and it is five times stronger than steel on an equal weight basis.
• Basalt fibers: brittle, high modulus of elasticity, excellent heat resistance, significant capability of heat and acoustic resistance and outstanding vibration isolators.
• Wood fibers: environmentally friendly material, strong in tension and flexible.

Type of fibers for composites: what are carbon fibers?

Among all the reinforcements used in composites, the most common are carbon fibers. Carbon fibers (CF or carbon fibres) are very small fibers with a diameter between 5 to 10 µm composed mainly of Carbon atoms.

What is the atomic structure of carbon fibers and what are carbon fibers made of?

The atomic structure of carbon fiber is similar to graphite’s, consisting in aggregates of carbon atoms with a planar structure (graphene sheets) disposed according to a regular hexagonal symmetry.

The difference between carbon fibers and graphite is in the way these graphene sheets are disposed.

Graphite has a compact hexagonal structure, formed by the overlap of graphene planes. In a direction that is orthogonal to the plane, there are weak chemical bonds between the sheets that decrease the mechanical properties in between the planes. On the plane, the mechanical properties of graphite are very high due to covalent bonds.

The most accredited model for the definition of carbon fibers is the turbostratic model that states that carbon fiber is made by graphene planes coaxial to the fiber axis.

Carbon fibers: properties and use

Carbon fibers are extremely mechanical resistant, thermal insulators, resistant to temperature variations, with good fireproof properties. Carbon fibers are tough and have high chemical resistance towards various aqueous solutions and different chemical agents. They can degrade when in contact with metals and metal oxides at temperature higher than 730°C.

The density of carbon fibers is about 1.75 g/cm3 and its mechanical properties are very high, with a tensile modulus between 200 and 500 GPa. Thanks to its outstanding tensile properties, carbon fiber offers 2 to 5 times more rigidity than aluminum and steel (of the same weight).

Carbon fibers are made as the union of thousands of cylindric filaments, constituted mainly of Carbon (at least 92%). Due to their nature, carbon fibers are strongly anisotropic because of their low homogeneity, meaning that mechanical properties have a privileged direction.

Carbon fiber reinforced polymers or plastics (CFRP) are composite materials made by a cohesive matrix that protects and holds the reinforcement, in this case the carbon fibers that provide strength and stiffness. The matrix choice has a heavy effect on the properties of the composite.

How to 3D print composite materials

Many different technologies can be used to produce composite materials. One of the most utilized is the lay-up process, but it is very often manual, therefore it requires high labor costs and times.

Another technology that is widely spread for mass production of composites is injection moulding: once the mould has been designed and manufactured according to part requirements, this technology allows for the fast production of parts with fine tolerances and good surface quality. The main disadvantage of this technology in the production of composites is the impossibility to impart a direction to the fibers, resulting in very anisotropic parts with lower mechanical properties.

In the sphere on non-subtracting technologies, better called Additive Manufacturing, there are three main additive manufacturing technologies to produce PMC:

• Fused Filament Fabrication (FFF): easier technology for production, since the composite filament is melted and extruded through a nozzle. FFF technology has an important advantage, when it is about composites: the fibers are aligned following the axis of the extruded material. It means that the printing direction can be adjusted based on the load direction, to optimize the material performances.
• Selective Laser Sintering (SLS): composite powders are melted by a laser to form parts. With this technology is not possible to orient the fibers, resulting in lower mechanical properties of the composite materials.
• Automated Fiber Placement (AFP): strips of tape are pulled unidirectionally along a surface. The tapes can be dry or pre-impregnated with resin and are pulled from the laying tool, heated, and pressed into the shaped mold. The complexity of the shape to manufacture is limited cause it is dependent on the tool’s capabilities, but this technology has the advantage to be able to use tapes with either thermoplastic or thermoset resins.

What is the difference between continuous and chopped carbon fibers composites?

3D printed carbon-fiber-reinforced composites though FFF technology can be divided into two categories:

• Continuous carbon fibers: In the last years continuous carbon-fiber reinforced composites have taken a space in the Additive Manufacturing scenario. Continuous-fiber reinforcement can guarantee good stability and extremely high mechanical properties in the direction of the fiber, that lays on the printer’s x-y plane. Despite this, 3D printing these composites has some disadvantages: limited design freedom and lower properties on the z-axis that give it low adhesion on the z-axis.
• Chopped carbon fibers: easier to print and extremely cheaper when compared to continuous fibers, chopped carbon fibers allow for unlimited designs of the printed parts. Chopped carbon fibers have a preferred load direction, as well as the continuous version, but the discrepancy between the x-y and z direction properties is not as marked as for the continuous fibers.

3D printing composites examples with Roboze technology

Roboze technology allows for the quick and repeatable production of different composites with a very high accuracy. Some examples of composite materials 3D printed by Roboze and are perfect for 3D printing are Carbon PA and Carbon PEEK.

Carbon PA is a Nylon-based composite. Its matrix is made by PA 6.10 reinforced with up to 20% chopped carbon fibers. Adding carbon fibers make it possible to improve the thermal performance of Nylon, giving it outstanding mechanical properties. Since the percentage of reinforcement is not very low, this reinforced PA has a pretty high surface resistivity, when compared to its neat version.

On the other hand, Carbon PEEK is a composite material made with a PEEK matrix (what is peek) and 10% chopped carbon fibers. Its performance is enhanced at high temperatures, up to 280°C, and its Ultimate Tensile Strength (UTS) is up to 120 MPa. Due to the low percentage of reinforcement, its surface resistivity is not too higher than neat PEEK.