Additive manufacturing can be used with a wide range of materials, from high-performance thermoplastics like PEEK to aerospace-ready metals like titanium.
Sometimes, however, engineers want to combine the properties of two different materials, and one of the best ways to do this is by using composites. Used in processes like FDM and SLS (as well as novel technologies), composite materials typically contain a base thermoplastic material and a reinforcing element such as carbon fiber. The ratio between the two elements can vary, as can the method of integrating the reinforcing material.
As 3D printing technologies continue to improve, the use of 3D printable composites is becoming more widespread. And their use is not limited to industry either: while some composites (SLS powders, for instance) are targeted mainly at industrial users, others (like chopped fiber reinforced thermoplastics) can be used in mid-price FDM 3D printers for consumers and professionals.
This is article serves as an introductory guide to 3D printing composite materials. It looks at some popular composite materials and composite printing technologies, explains the difference between chopped fiber and continuous fiber composites, and considers the main applications and advantages of 3D printing composites.
What are composite 3D printing materials?
Simply put, a composite material is a material made up of two or more different materials. Composite 3D printing specialist Markforged defines composites as being “made up of more than one material that, when combined, have properties different from their original materials.”
Examples of composites outside of 3D printing include plywood (layers of different wood veneer) and reinforced concrete (concrete strengthened by steel bars).
Within 3D printing, composites are typically a combination of a thermoplastic base material (a matrix) and a reinforcing element such as carbon fiber, fiberglass, graphene, or kevlar. (Note that a mixture of two thermoplastics, such as PLA + ABS, is typically referred to as a blend, not a composite.) Base materials can be virtually anything, but industrial users typically use premium thermoplastics that have good material characteristics even without reinforcing elements; such plastics include PC, nylon, and PEEK.
Composites can take different materials forms, the most common being mixed powders for selective laser sintering (SLS) and mixed filaments for fused deposition modeling (FDM). Less common but particularly exciting are composites made up of a base material combined with continuous fibers that, using novel processes, can be threaded or woven into thermoplastic matrices while they are being printed. Such processes typically require two nozzles: one to deposit the thermoplastic, and one to distribute the continuous fibers. Finally, there are also a limited number of composite resins for vat photopolymerization 3D printing processes like stereolithography (SLA); with this technology, thermoset resin base materials can be cured around a reinforcing fiber skeleton.
Popular composite 3D printing materials
Whether in powder, filament, or other form, composite 3D printing materials are usually developed for their high strength, high stiffness, good dimensional stability, and good strength-to-weight ratio. Fibers are exceptionally light, and they can drastically improve the strength of a thermoplastic without adding to its mass. 3D printed engineering composites can even be used as a substitute for metal.
The base thermoplastics for FDM 3D printing composites range from commodity polymers like PLA and ABS at the cheaper end of the scale, to high-performance polymers like PEEK at the premium end. Nylon is the main material used for composite SLS powders (as it is the main material used in laser sintering generally), but high-performance materials like PAEK can also be used.
When looking at composites overall (including outside of 3D printing), fiberglass is the most popular reinforcing material — and it is also widely found in 3D printing composites. Within additive manufacturing, however, carbon fibers are much more widely used than glass, since composite 3D printing is used for many critical applications where the superior strength of carbon is worth the added expense. Other reinforcing materials include Kevlar and graphene.
- Polyamide / nylon (PA):
- Acrylonitrile butadiene styrene (ABS)
- Polylactic acid (PLA)
- Polycarbonate (PC)
- Polyetherimide (PE)
- Polyphenylene sulfide (PPS)
- Polyether ether ketone (PEEK)
- Polyaryletherketone (PAEK)
- Carbon fiber
- Glass beads
- Other metals
Examples of branded composite materials on the market include EOS’s PA 640-GSL, a PA 12 nylon SLS powder reinforced with glass beads and carbon fibers; 3DXTech’s CarbonX PETG+CF, a PETG FDM filament reinforced with chopped carbon fiber; and Markforged’s Onyx, a nylon base material filled with carbon fiber that can also be reinforced with continuous fibers using Markforged’s proprietary composite printing technology.
3D printing processes for composite materials
Aside from a few exceptions, the key 3D printing technologies for composite 3D printing are fused deposition modeling (FDM), selective laser sintering (SLS), and novel technologies for continuous fiber printing.
FDM is the most widely used 3D printing process for plastic parts, and many mid-price machines are capable of processing composite materials like carbon fiber-reinforced ABS. Composite materials for FDM consist of a thermoplastic base material and (usually) chopped, discontinuous fibers. These fibers can strengthen and stiffen printed parts but in larger quantities can also make the filament harder to print and can negatively impact surface quality.
SLS is another plastic 3D printing process suitable for the production of composite parts. Due to the cost and complexity of SLS systems, this technology is mostly used by industrial users. Materials are a mix of thermoplastic powders (often nylon) and reinforcing elements like chopped fibers or glass beads. Note that composite SLS powders are not universal; hardware manufacturers like EOS make machines dedicated to composite printing.
Novel continuous fiber processes
One of the most cutting-edge areas of additive manufacturing is the printing of composites with continuous fibers — a concept explained in the following section. Companies like Markforged, Desktop Metal, Orbital Composites, 9T Labs, and Anisoprint have all developed composite additive manufacturing hardware that can integrate continuous fibers into thermoplastic parts during printing, typically by feeding continuous fibers into each individual layer using a separate nozzle.
Chopped fiber vs continuous fiber composites
Composite 3D printing materials with reinforcing fibers can be divided into two distinct categories: chopped fiber composites and continuous fiber composites. So while two different composites might contain exactly the same constituent materials, they might perform in a very different way depending on whether they have chopped or continuous fibers.
- Chopped fibers are tiny strands of reinforcing material like carbon or Kevlar. Typically measuring less than a millimeter in length, these strands can be easily mixed into a thermoplastic matrix (PLA, ABS, etc.), giving the ordinary plastic increased strength and stiffness. Chopped fibers are incredibly useful because they are highly versatile: they can be mixed with a wide variety of thermoplastics, and the resulting composites can be printed on ordinary 3D printers. However, when chopped fibers are mixed into a base material, each individual fiber takes on a random orientation, which makes the material less strong than a material with continuous fibers.
- Continuous fibers, on the other hand, are longer, unidirectional strands of reinforcing material that, when integrated into a thermoplastic matrix, provided vastly superior strength compared to chopped fibers. This is because a strand can absorb and distribute loads across its entire length, so a longer continuous length has greater load-bearing capacity than a tiny chopped strand. Only in the last five years or so has continuous fiber composite 3D printing become mainstream, with several manufacturers developing their own technologies. Needless to say, continuous fiber composite printing is more expensive than chopped fiber composite printing, as dedicated hardware is required.
Applications of composite materials
Composite materials have many uses across many industries, and applications span prototypes, end-use parts, and tooling.
Sectors that use additive manufacturing and 3D printing composites include aerospace, automotive, electronics, consumer goods, medical, and industrial. Most of these industries use composite 3D printing to make high-stiffness parts that are small in scale (although some large-format extrusion hardware is capable of printing composites).
Tooling — on a small or large scale — is a particularly relevant application for composite 3D printing, as reinforced materials are ideal for molds and end-of-arm tooling.
In terms of particular technologies, continuous fiber 3D printing offers the greatest scope of potential applications (compared to FDM or SLS), though it is still in its infancy and not as widely used as chopped fiber 3D printing techniques.
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