Composites in 3D printing

Initially, plastic 3D printing started with thermoplastics — ABS, PLA, PA, PC, etc. — materials that allow for a few cycles of melting and curing while keeping their chemical properties intact. They are very convenient for extrusion and injection, melt easily when heated, are flexible in creating different shapes, and most of them are recyclable.

However, they give rise to a number of drawbacks. All of the properties that make thermoplastics so good for extrusion are actually a poor fit for engineering applications — they are not strong and durable enough to become reliable functional parts of a machine or a mechanism. The melting points for ABS and PLA are as low as ~200°C, they are not chemically resistant, and they cannot be a substitute for metal constructions.

Since plastic parts lack stiffness and strength, it is necessary to reinforce the plastics with some filler to make these parameters suitable. Filler materials need to meet several requirements and solve specific issues.

Enhancing mechanical properties

First of all, they need to add to the mechanical properties of the resulting part. These materials are preferably used to:

• make the produced parts lighter,
• cut manufacturing costs,
• have low absorption,
• be non-toxic and chemically/thermally resistant,
• be non-flammable.

Enhancing usability

Together with that, their usability is also very important:

• dispersibility of the material,
• a certain degree of flexibility that will allow smooth extrusion,
• they must mix with the polymer without pores, bubbles, or other defects,
• and they should not deteriorate or change their properties during transportation, storage, or use.

This list does not leave us with too many options for reinforcement materials.

Nowadays, there are several types of fibrous reinforcement available: carbon fiber, fiberglass, basalt, and kevlar. We are specifically interested in carbon reinforcement as the most promising type. Carbon fibers offer the highest performance in terms of strength and stiffness. And the main point of CFRPs (carbon fiber reinforced polymers) is in (relative) strength and stiffness. CFRPs – continuous fibers – are the best-known materials in terms of relative strength and stiffness properties.

The fiber and plastic mixed together are referred to as composite materials. Parts made of such materials feature excellent stiffness and strength, and are lightweight compared to metal.

As stated above, there are two ways to use carbon fibers: chopped (and filled in the plastic) and continuous (placed layer by layer).

3D printing chopped carbon fibers

Chopped carbon filament is a good starting point for improving a part’s strength-to-weight ratio. It is a very popular printing material as the extrusion technology is the same as for the usual plastics, so one does not have to buy another machine or extra equipment to start printing with such composites (aside from abrasion-resistant nozzles).

However, it all depends on the particular material; in practice, parts printed with chopped fiber-filled filaments often show even lower mechanical properties than unfilled ones.

The principal difference between chopped and continuous fibers is that composites filled with chopped fibers do not give a tangible increase in strength. We can leverage or significantly improve only stiffness. At Anisoprint, we use both types of material. Though anisoprinting is based on continuous fibers for manufacturing reinforced anisotropic composites, we use chopped ones for improving surface quality, stiffness, wear resistance or improving the look of a part.

Artem Naumov, Head of materials at Anisoprint

Depositing a continuous strand of carbon fiber

In contrast to chopped fibers, there exists a method to reinforce plastics with continuous carbon fiber. The very idea of reinforcing with continuous carbon fiber rests on its unique properties. Continuous carbon fiber tensile strength is very high, even compared to metals. It allows for the creation of anisotropic parts, as users can manipulate the fiber in the required directions in the structure of a printed part. 

That reinforcement is implemented with different technologies and gives rise to a range of adjustable parameters of the part. The common feature of continuous carbon fiber 3D printed structures is that they are more strong and more reliable than metal ones, cheaper in production, and several times lighter.

Continuous fiber 3D printing is a relatively fresh idea that has not spread in the market widely as it requires special technologies and consequently specific equipment that allows 3D printing with continuous fibers.

First of all, there are several technologies for producing filament with continuous fiber. Fiber strands should be impregnated with plastics properly — without any pores or defects. Otherwise, it will result in poor mechanical properties and easy breaking (brittleness). Adding more continuous fiber makes printed parts very strong and stiff, featuring greater strength than aluminum, for instance. However, packing continuous fibers too densely makes it hard to produce filament without cavities or pores.

At Anisoprint, we developed a technology to impregnate fiber strands with a thermoset: due to capillarity, or wicking, the thermoset quickly fills in all the space between fibrillas and forms a stiff string (after curing) that is ready for coating with a thermoplastic of any type.

This enables a fiber share as high as 60% of the total volume (fiber-volume fraction). Structures with continuous fibers can be used for lattices.

That is not only a new tech design opportunity, but also a good way to cut costs due to the economical use of material, it drastically reduces the weight of such parts too. Lattices improve the stiffness of structures, make them more cost-efficient, and provide ready design solutions.

Yet another prominent feature of continuous carbon fiber is boosting mechanical properties. For polymers that possess high specific wear rates in the unreinforced condition, almost any type of reinforcing fiber results in significant reductions in wear and improvement in mechanical properties. However, above all, continuous carbon reinforcement grants the best tribological properties, such as a low wear rate in severe operating conditions and a friction coefficient of 0.1-0.2.

Orienting fibers allows the fiber properties to focus in one direction, while a random mixture dissolves the properties in every direction. That’s why continuous fiber reinforced composites have extremely high properties in the direction of the fibers and short fiber reinforced polymers have fairly modest properties in all directions. This difference is an order of magnitude at least.

Fedor Antonov, Anisoprint CEO

Carbon fiber 3D printing: multiple technologies

As we mentioned above, chopped and continuous fiber printing require different technologies, and a conventional FDM (fused deposition modeling) print head cannot be used for continuous fiber reinforcement.

The extrusion process for plastic and chopped fibers is the same, while continuous fiber placement requires either (a) a separate nozzle to supply the reinforced filament or (b) a single nozzle head with a specific construction for mixing the fiber with the matrix. Some technologies make use of a compaction roller to press the reinforcement in. Using a heating laser that melts the layers together is usually referred to as direct energy deposition (DED).

Last but not least, here the software plays a major part. Correctly set parameters prevent clogging and provide optimal slicing based on the matrix material and the model itself. Different parameter sets are capable of producing different parts in terms of fiber-volume fraction and the way it is layered in the matrix material. 

The anisotropy, the property of a material that allows it to change or assume different properties in different directions, and fiber share can be manipulated and produce a part with very diverse internal structures that exhibit different behavior. For instance, the volume ratio of fibers and matrix can be 60% to 40%, 70% to 30%, 80% to 20%, or other. It is important when we know what kind of load the part should bear and in what direction it will be stressed or bent.