FFF Printing: Composite Filament

Glass Fiber, The oldest and most used in composites
Carbon fiber, most widely used in high performance applications
Kevlar fiber, famous for bulletproof vests

For FFF printing, the mechanical properties of the printing can be improved by using composite filaments using these fibers.

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One way to improve the mechanical properties of printed parts in FFF printers is to use composite filaments filled with aramid fibers, represented by carbon, glass or Kevlar in a base material such as PLA, PETG, nylon, PC, ABS, etc.

In this section, we will look at these three basic fibers used in composite materials.

<Products made of glass, carbon and Kevlar fibers> 

Glass fiber 

The oldest and most used fabric in composites, it is still the most commonly used reinforcement to replace metal parts in most applications.

It has the advantage of being the cheapest among additive fibers, and although heavier and less rigid than carbon fibers, it is strong in impact and has a high elongation at break.

 Carbon fiber 

Carbon fiber is most widely used in high performance applications because of its high tensile strength, low density, high stiffness and thermal conductivity. It is the strongest and lightest fiber compared to Kevlar and glass fiber. For this reason, it is known as the best choice for aerospace and industrial applications and replaces alloys in aerospace components that previously used aluminum or titanium.

On the other hand, the highest stiffness is excellent for maintaining dimensional tolerances under load, but exceeding the maximum stiffness can cause sudden fracture or cracking of the carbon fiber laminate.

It also has a lower impact resistance than glass or Kevlar fibers and can corrode metals it comes in contact with due to its electrical conductivity.

Kevlar fiber 

Kevlar fiber, famous for bulletproof vests, is used as a fabric for safety equipment that requires impact resistance and cut resistance. It has an intermediate tensile strength of glass fiber and carbon fiber, and is light with a weight close to that of carbon fiber.

It has the disadvantage of being weak against moisture and UV rays.

<Glass, Carbon and Kevlar Properties Comparison> 

Composite Filament 

Composite filaments are made by mixing fibers cut short, as short as 30 to 150 μm, in a matrix. Or made from a composite material blended with fibers cut to lengths of up to 7 mm. To achieve the desired properties in composite parts, the adhesion between the fibers and the matrix must be optimized. Adhesion is enhanced using surface coatings (epoxy, glycerin, etc.) known as sizing to create composites reinforced with 'long fibers' up to 7mm.

In the case of a composite material filled with short fibers, the surface area of ​​the fibers is large enough that sizing is not required for adhesion.

When composites are extruded into filaments, the long fibers are oriented within the matrix. In the case of short fibers, they are randomly distributed without any special direction.

When fibers are oriented, fiber properties are concentrated in one direction, whereas random mixtures dissolve properties in all directions. Composites reinforced with long fibers have high properties in the fiber direction and short fiber reinforced polymers have fairly moderate properties in all directions.

<Filaments filled with short fibers and filaments reinforced with composite> 

Composite Filament print 

Fibers added to the composite filament are held in place by a matrix resin to improve the performance properties of the final part, such as strength and stiffness, while minimizing weight, making parts with superior mechanical properties a lightweight, inexpensive alternative to metals. 

Of course, the mechanical properties may be lower than that of metal, but considering where real metals are used, that strength is not always necessary so It can be a good alternative.

And since the printing of composites is possible at a temperature similar to that of the matrix material, this is also very attractive.

There are downsides too. Due to the nature of the filler, the filament may become brittle and the interlayer adhesion may deteriorate. Also, very small diameter nozzles cannot be used because of the risk of nozzle clogging, and hardening nozzles with high wear resistance must be used.

In conclusion, printing with composites can be challenging, but will continue to evolve as a trend towards additive manufacturing because of the advantage of being able to obtain lightweight parts with properties that replace or surpass metals at a fraction of the cost.