F. Last_Name, F. Last_Name Advanced Manufacturing Processes Literature Review Fall 2021

Investigation of Nozzle Diameter and Process Temperature in Material Extrusion Based 3D Printing of Polymer and Carbon Fiber

Abstract

Material extrusion-based 3D printing, otherwise known as fused filament fabrication (FFF), is one of the most widely used additive manufacturing technologies. The FFF method has been adapted to produce thermoplastic composites which are reinforced with continuous carbon fiber. There are advantages to incorporating continuous carbon fiber into additive manufacturing which include increased polymer part strength and increased unsupported span, as well as various limitations to overcome in order to realize the full potential of the carbon fiber in the composites which can be produced, such as ensuring even distribution of the fiber within the polymer matrix, achieving a high fiber content, and ensuring a strong interface (or high infusion) between the polymer and fiber. This paper reviewed some relevant research articles in this field including: (i) interface and performance of 3D printed continuous carbon fiber reinforced PLA composites (Tian et al. 2016), (ii) rapid prototyping of continuous carbon fiber reinforced polylactic acid (PLA) composites by 3D printing (Li & Li 2016), and (iii) manufacturing and 3D printing of continuous carbon fiber prepreg filament (Hu et al. 2017).

Introduction

Fused filament fabrication is a fundamentally simple process, melting a filament of thermoplastic polymer (most commonly nylon, acrylonitrile butadiene styrene (ABS) or polylactic acid (PLA)) in a heated liquefier, also known as the heat block, and using the cool, solid filament outside the liquefier to push (and extrude) the molten material through a nozzle onto a build plate or previously deposited material, where it solidifies, as illustrated in Figure 1. The heat sink may be cooled by a fan to keep the polymer filament below its glass transition temperature before it enters the liquefier. As researchers seek to extend the capability of the FFF method and broaden the type of products that can be manufactured with this technique, they have begun developing ways to incorporate carbon fiber into the process. Fiber-reinforced polymers incorporate high-strength, lightweight fibers into a polymer base, either thermoset or thermoplastic, to produce composite materials with high strength-to-weight properties that can replace steel and aluminum. The most common fibers used are fiberglass and carbon fibers, which are both very strong. Carbon fiber is more rigid and generally used in applications where flexibility is not required; fiberglass is more flexible.

Heat sink

Heated liquefier with nozzle

Heat sink

Heated liquefier with nozzle

Figure 1. Fused filament fabrication process (Source: Pranzo 2018)

Carbon fiber is formed from a chain of carbon atoms generally aligned parallel to the long direction of the fiber. It is the alignment of the atoms that gives the fiber its high tensile strength. Carbon fiber can be added into a polymer matrix as short fibers or as continuous fiber. Because of the small diameter of a single strand of carbon (5 to 10 µm), the continuous fiber is generally sold in bundles of thousands (1,000 to 48,000) of strands of axially aligned carbon fibers called carbon fiber tows. Carbon fiber is very strong longitudinally, so it is frequently woven into a fabric to provide reinforcement in more than one direction. Traditionally, layers of carbon fabric can be infused with a polymer matrix to produce carbon fiber reinforced polymers (CFRP). In some ways, FFF is a natural home for producing CFRP parts, because carbon fiber requires the support of a polymer or resin matrix in order to realize its full strength. In this project, we varied the FFF process parameters of nozzle diameter and liquefier temperature during extrusion of carbon fiber with PLA polymer to study the effect on the distribution of carbon fiber within the polymer and the infusion of polymer into the carbon fiber bundle.

Literature Review

Two different paths have been pursued to incorporate carbon fiber into FFF, as described by Hu, et al. (2017): 1) short carbon fiber and 2) continuous carbon fiber (CCF). Ning et al. (2015) mixed short carbon fiber with thermoplastic pellets in a blender until the fibers were evenly distributed. They then extruded the mixture through a screw extruder to produce short carbon fiber reinforced polymer (SCFRP) filament to be used in FFF. This process achieved modest increases in tensile strength and Young’s modulus for manufactured parts with low carbon content (≤ 7.5 wt %), but higher carbon content parts (≥ 10 wt %) suffered from decreased toughness and yield strength because the mixing process increases the porosity of the SCFRP filament for high levels of carbon content.

The second path, using continuous carbon fiber, has proven rewarding in some ways and difficult in others. Tian et al. (2016) developed a process to use one liquefier and nozzle to lay down carbon fiber (using a 1K tow, or 1000 carbon strands bundled together, which is the smallest carbon fiber tow commercially sold) and PLA polymer simultaneously, as shown in Figure 1. Tian et al. used a nozzle diameter of 2 mm and varied five process parameters (liquefier temperature, layer thickness, filament feed rate, hatch spacing, and printing speed, values listed in Table 1) to identify preferred values (included in Table 1) for this application, using the flexural strength and flexural modulus of the manufactured part as the evaluation metric.

Figure 2. General scheme of the carbon fiber reinforced FFF process (Source: Yang et al. 2016)

Table 1. Parameter matrix of CFR PLA 3D printing process (Source: Tian et al. 2016)

Target parameter

Range

Other process parameters

Recommended value

Temperature of liquifier (T/°C)

180, 190, 200, 210, 220, 230, 240

L 0.65 mm, V 100 mm min−1, E 150 mm/min, H 1.2 mm

230

Layer thickness (L/mm)

0.3, 0.4, 0.5, 0.6, 0.7, 0.8

T 210 °C, V 100 mm min−1, E 100 mm/min, H 1.2 mm

0.4 to 0.6 (constant over this range)

Feed rate of filament (E/mm min−1)

60, 80, 100, 120, 140, 160

T 210 °C, L 0.5 mm, V 100 mm min−1, H 1.2 mm

80 to 120 (fairly constant over range)

Hatch spacing (H/mm)

0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8

T 210 °C, L 0.5 mm, V 100 mm min−1, E 100 mm/min

Use the lowest spacing practical

Transverse movement (print) speed (V/mm min−1)

100, 200, 300, 400, 500, 600

T 230 °C, L 0.65 mm, E 150 mm/min, H 1.2 mm

Not significant

Tian et al. also examined the fractured surfaces of the manufactured parts under scanning electron microscopy (SEM) to identify the microstructures and composition of the parts. These images, of which a sample is shown in Figure 3, highlight some of the main challenges involved in adding continuous carbon fiber to FFF processes: impregnating the fiber with polymer matrix, distributing the fiber evenly and achieving high fiber content. The microstructure images also make a connection between the process parameter values recommended in Table 1 and the carbon fiber content and impregnation for each part. The flexural strength of the part increases with fiber content and strong fiber-polymer infiltration. Fiber pull-out in failure indicates poor bonding between fiber and polymer; load is not transferred well from the polymer matrix to the fiber, so the fibers can slide longitudinally relative to each other and relative to the polymer. The widely separated fiber bundles at high hatch spacing (lower images in Fig 2.) reveal lower fiber content and uneven distribution of fiber through the part. Tian et al. did not include a comparison with unreinforced PLA parts.

Figure 3. Microstructures of fractured cross section of carbon fiber reinforced PLA composites with hatch spacing of 0.6 mm (a, b, c) and 1.6 mm (d, e, f), respectively: (a) and (d) overall cross section, (b) and (e) interface, (c) and (f) fracture pattern, under the experimental condition: T 210 °C, L 0.5 mm, V 100 mm/min, E 100 mm/min (Source: Tian et al. 2016)

Two other research groups explored ways to improve the impregnation of the carbon fiber with PLA polymer during FFF. Li & Li (2016) performed surface modification of the carbon fiber bundle to improve adhesion of the PLA during FFF, while Hu et al. produced a prepreg (or “pre-impregnated”) carbon fiber filament, a PLA-coated 1K carbon fiber tow, for use in FFF. The process by Li and Li involved a chemical process: dissolving a small amount of PLA into methylene dichloride (using magnetic stirring followed by a high speed dispersion and emulsification machine), then slowly adding deionized water (including 1% surface active, emulsifying and antifoaming agents) to create a sizing agent before adding the carbon fiber to the solution to modify the surface condition of the carbon fibers. This surface modification is designed to improve the fiber wetting by the PLA polymer. Alternatively, Hu et al. performed a mechanical process to improve the carbon fiber wetting. They used the coaxial extrusion mold illustrated in Figure 4 to coat the carbon fiber bundle evenly with PLA before using the resulting filament in FFF. The carbon fiber was heated to ensure that it was dry and warm (so that PLA wouldn’t cool on contact) before adding molten PLA, and it was extruded from a 1 mm nozzle, resulting in a 1.2 mm PLA-clad carbon fiber filament. This filament was employed in FFF using a 1.5 mm nozzle to minimize clogging and fracture of the carbon fiber.

b)

Figure 4. a) Schematic for manufacturing 3D-printable CCF prepreg filament b) Cross-section of resulting prepreg filament, showing poor infusion (Source: Hu et al. 2017)

Tian et al., Li and Li, and Hu et al. all found that they did not need to use a motor to feed the carbon fiber – the continuous carbon fiber reinforced PLA bonds with the base plate, which pulls more carbon fiber and polymer out of the nozzle as the nozzle moves. Both Li and Li and Hu et al. found that their methods improved the interface between the carbon fiber and PLA, based on the flexural strength of the parts produced, and SEM micrographs of composite morphology. Li and Li found that their method resulted in uneven distribution of the PLA around the carbon fibers, shown in Figure 5.

Figure 5. Distribution of PLA and carbon fibers during FFF (Source: Li & Li 2016)

Hu et al. also conducted a three-factor, three-level study on process parameters (print temperature, print speed and layer thickness) to develop res

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ponse surfaces based on a quadratic regression fit which predicts part flexural strength as a function of the three study parameters. The values of the study variables are provided in Table 2. The response surfaces are shown in Figure 6, illustrating relative importance of combining low layer thickness and low print speed and relative unimportance of print temperature. SEM micrographs support the response surface results, showing improved infusion of polymer among the carbon fiber at low layer thickness and print speed, indicating that a strong polymer-fiber interface can be achieved with mechanical methods, not just chemical, as demonstrated by Li and Li. Hu et al. recommend that future efforts focus on impregnation quality of prepreg filament.

Table 2. Three-factor (three levels each) composite design used in response surface study (Source: Hu et al. 2017)

Factors

Levels

A: Print temperature (°C)

200

215

230

B: Print speed (mm min-1)

60

90

120

C: Layer thickness (mm)

0.6

0.9

1.2

Figure 6. Response surfaces showing effect of print speed, print temperature, and layer thickness on flexural strength of produced parts (Source: Hu et al. 2017)

References

Tian X, Liu T, Yang C, Wang Q, Li D (2016) Interface and Performance of 3D Printed Continuous Carbon Fiber Reinforced PLA Composites. Composites Part A: Applied Science and Manufacturing  88:198–205.

Li T, Li Y (2016) Rapid prototyping of continuous carbon fiber reinforced polylactic acid composites by 3D printing. Journal of Materials Processing Technology 238:218-225.

Hu Q, Duan Y, Zhang H, Liu D, Yan B, Peng F (2017) Manufacturing and 3D printing of continuous carbon fiber prepreg filament. Journal of Materials Science 53:1887-1898.

Pranzo D (2018) Extrusion-Based 3D Printing of Microfluidic Devices for Chemical and Biomedical Applications: A Topical Review. Micromachines 9(8):374.

Ning F, Cong W, Qiu J, Wei J, Wang S (2015) Additive manufacturing of carbon fiber reinforced thermoplastic composites using fused deposition modeling. Composites Part B 80:369-378.

Yang C, Tian X, Liu T, Cao Y, Li D (2016) 3D printing for continuous fiber reinforced thermoplastic composites: mechanism and performance,  http://dx.doi.org/10.1108/RPJ-08-2015-0098.

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