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Article

Comparing Degradation Mechanisms, Quality, and Energy Usage for Pellet- and Filament-Based Material Extrusion for Short Carbon Fiber-Reinforced Composites with Recycled Polymer Matrices

by
Marah Baddour
1,
Chiara Fiorillo
1,2,
Lynn Trossaert
1,2,
Annabelle Verberckmoes
1,
Arthur Ghekiere
1,
Dagmar R. D’hooge
2,3,
Ludwig Cardon
1 and
Mariya Edeleva
1,*
1
Centre for Polymer and Material Technologies, Department of Materials, Textiles and Chemical Engineering, Ghent University, Technologiepark 130, 9052 Ghent, Belgium
2
Laboratory for Chemical Technology, Department of Materials, Textiles, and Chemical Engineering, Ghent University, Technologiepark 125, 9052 Ghent, Belgium
3
Centre for Textile Science and Engineering, Department of Materials, Textiles, and Chemical Engineering, Ghent University, Technologiepark 70a, 9052 Ghent, Belgium
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2024, 8(6), 222; https://doi.org/10.3390/jcs8060222
Submission received: 8 May 2024 / Revised: 20 May 2024 / Accepted: 5 June 2024 / Published: 12 June 2024
(This article belongs to the Special Issue Recycled Polymer Composites: Futuristic Sustainable Material)
Browse Figures
Versions Notes

Abstract

:
Short carbon fiber (sCF)-based polymer composite parts enable one to increase in the material property range for additive manufacturing (AM) applications. However, room for technical and material improvement is still possible, bearing in mind that the commonly used fused filament fabrication (FFF) technique is prone to an extra filament-making step. Here, we compare FFF with direct pellet additive manufacturing (DPAM) for sCF-based composites, taking into account degradation reactions, print quality, and energy usage. On top of that, the matrix is based on industrial waste polymers (recycled polycarbonate blended with acrylonitrile butadiene styrene polymer and recycled propylene), additives are explored, and the printing settings are optimized, benefiting from molecular, rheological, thermal, morphological, and material property analyses. Despite this, DPAM resulted in a rougher surface finish compared to FFF and can be seen as a faster printing technique that reduces energy consumption and molecular degradation. The findings help formulate guidelines for the successful DPAM and FFF of sCF-based composite materials in view of better market appreciation.

1. Introduction

Additive manufacturing (AM), which is also known as 3D printing, allows the fabrication of polymeric parts with complex geometry. Additionally, it enables quick and cost-effective prototyping, so that manufacturers can rapidly test and cultivate new product designs. The essential step to advancing AM technologies is the selection of the material, as 3D printing is sometimes limited to only certain polymers. Consequently, improvements are needed for both techniques and feedstock materials to align with the required mechanical standards [1,2,3,4,5]. Thomas et al. [6] highlighted that thermoplastic olefin (TPO) and polycarbonate (PC) are two of the most significant exterior automotive plastic substrates, driven by the rise in electric vehicles, environmental regulations, and functional needs. Their review explores the evolving automotive coating market, focusing on the possibilities and challenges of current and future cure systems for plastic substrates, noting that PC is set to replace glass in many automotive parts. Additionally, combining carbon fiber (CF) with thermoplastic materials is common due to its low density, high strength, and high elastic modulus, making it ideal for aerospace, wind turbines, automotive, and sports equipment applications [7,8]. In this context, polymer-based composite materials already find applications in AM, although the 3D printing of composites remains complicated [9].
The need to control material properties for 3D printing is clear, e.g., by focusing on semi-crystalline materials prone to deformation through shrinkage and warping due to crystallization after the melting process and subsequent cooling [10]. Notably, carbon fiber-reinforced polymers (CFRPs) can enhance the mechanical properties of composite parts produced through 3D printing, particularly improving both their strength and stiffness [11]. In more detail, the flexural properties of the printed composites can experience a two- to threefold improvement compared to the parts produced without CF reinforcement. However, the combination of carbon fibers (CFs) with a matrix material characterized by a different thermal expansion coefficient raises the melt viscosity, consequently weakening the interlayer adhesion and leading to an increased occurrence of voids. The way fibers are oriented is also pivotal in determining the mechanical behavior of the composite [12,13,14]. For example, as shown by Jiang et al., the incorporation of short carbon fibers (sCFs) into polyethylene terephthalate glycol (PETG) as a polymer matrix can result in a considerable enhancement in the modulus of elasticity reaching 313%, which was only clear at the 0° print orientation linked to the alignment of CFs in the loading direction [15].
Parallel to these AM material developments, attention should be paid to the identification of the most suitable AM technique. Currently, fused filament fabrication (FFF) is the leading AM technique, due to its simplicity and affordability. FFF works by melting a thermoplastic filament through the extrusion nozzle and then placing it onto a moving bed, layer by layer, resulting in the final products, as shown in Figure 1A. Recent work has shown that, upon consecutive deposition, heat control is relevant between layers [16].
Related to FFF is the more recent direct pellet extrusion-based AM (DPAM), which is also known as fused granulate modeling (FGM). DPAM involves the direct 3D printing of polymer pellets using a single-screw extruder (SSE) following the same layer-by-layer approach as FFF. The process involves heating the polymer pellets in a hopper using a heating element and then propelling the melted and homogenized material through a die, using, e.g., an SSE with three heating zones, as shown in Figure 1B. This technique employs only one extrusion cycle, eliminating the need for producing the filament, which usually reduces the material degradation and improves the properties of the printed parts, particularly upon using thermosensitive polymers [17,18,19,20,21]. Ghabezi et al. [22] demonstrated, in their research on using upcycled basalt fiber-reinforced PP thermoplastic from waste materials, that polymer degradation occurred after melting during extrusion. Therefore, DPAM can potentially alleviate polymer degradation by bypassing the filament manufacturing step.
It should be mentioned that screw design is important in DPAM systems, as it is responsible for the efficient melting of the polymer pellets and steady deposition of the melt on the printing bed [23]. Despite this, SSE in DPAM is at first sight similar to conventional extrusion; DPAM-based SSE stands out as a specialized system designed specifically for extrusion-based additive manufacturing (EAM) applications, for which very precise control over the extrusion process is crucial, due to its reduced dimensions [24].
DPAM distinguishes itself from FFF by accommodating a wider range of polymeric materials. One of the key advantages is its capability to produce parts with complex geometries and internal structures, while also allowing for the fabricating of components with varying material properties through feedstock material composition adjustments. It can be more easily scaled up for mass production, making it a great option for large-scale manufacturing operations. With its unique combination of versatility, precision, and cost-effectiveness, DPAM holds the potential to become the new leading AM technique. Ongoing research and development in this field are expected to further enhance its capabilities, making it an essential tool for a wide range of manufacturing applications [3,17,21,25,26,27].
Another AM challenge is controlling the printing process as such. Many extrusion-based 3D printing systems are, e.g., based on an open-source design and have been developed by the operators and researchers themselves to flex the dimensions, the raw material, and the platform. The most emphasis is nevertheless on single-component materials. Silveira et al. [28], for instance, designed a mini-head extruder that allows the use of various raw materials, including powders, pellets, and solid forms. Tseng et al. [29] successfully engineered a screw extrusion-based 3D printing system. They achieved the high-quality printing of polyether ether ketone (PEEK) with consistent flow properties and high printing speeds, demonstrating the system’s reliability and capability. Furthermore, La Gala et al. [17] successfully integrated a Programmable Logic Controller (PLC) control system into a patented micro-extruder-based 3D printing setup. They compared pellet- and filament-based AM with injection molding (IM) for two types of thermoplastic polymers, namely acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA). For ABS specimens, both AM techniques deviate from IM, with FFF showing a smaller deviation, highlighting the need for further DPAM design. For PLA, all AM results closely match those of conventional IM. The FFF process has a clear effect on degradation compared to DPAM, while PLA’s degradation is less affected by the manufacturing technique. Similarly, Ceretti et al. [30] compared FFF and pellet-based AM (PBAM) for the 3D printing of polystyrene (PS) and ABS. The selected PBAM conditions have successfully produced printed specimens of PS with good mechanical properties and reduced degradation. During the processing, including velocity and pressure variations, materials undergo significant stress variations. These stresses may induce molecular degradation, thereby impacting the flow behavior and negatively influencing the properties of the final product [31]. However, for ABS, an additional optimization of the PBAM process is still desired [30].
In addition, Alexandre et al. [20] evaluated the performance of FFF and fused granular fabrication (FGF) for recycled and virgin PLA in terms of economic and technical dimensions using a desktop 3D printing setup. They used five types of recycling feedstocks: commercial filament, pellets, distributed filament, distributed pellets, and shredded waste. They found that FGF has an economic advantage, reducing the printing cost by 65% for the commercially recycled filament. Moreover, there were no statistical differences between the small FGF-printed samples compared to the FFF samples. The results are very promising for further advancing the application of recycled materials for 3D printing. Nevertheless, it is important to note that FGF is still a growing technique that requires further development, especially regarding its sensitivity to granule properties such as form and size during the printing process.
It should be stressed that studies on FFF and DPAM with composite materials are rare. An example is the Dexter robot arm development, which is almost entirely made of Markforged printed parts, including components from composite materials like carbon fiber for enhanced strength [32]. Another example is the 3D printing of a Boeing 777x wing tool with carbon fiber and ABS composites starting from pellets [33]. Despite advancements, industries are still rather slow to adopt FFF and DPAM, missing out on their full benefits and various material potentials. Specifically, the extensive studying and optimization of DPAM can give a push to the industry to further employ this technique for composite part production [34].
In this contribution, the aim is to utilize mechanically recycled polycarbonate/ABS (rPC/ABS) and recycled polypropylene (rPP) to create sCF composites both via DPAM and FFF. As mechanically recycled materials likely already experienced a significant degree of (molecular) degradation, it is of high importance to not further degrade them during additional processing. Hence, polymer degradation was studied in detail for DPAM and FFF in the first step via GPC, FTIR, and SOAS to then, in the next step, produce composite materials, thereby upcycling the recycled polymers and expanding the application of 3D printing techniques. Two levels of sCF were used as filler, specifically 2.5% for rPC/ABS and 20% for rPP, to investigate the limits of additive levels in composite materials that can still be effectively processed via DPAM and FFF. Additionally, it is demonstrated that DPAM is a more energy-efficient method, and with further dedicated parameter optimization, it has the potential to enable the production of parts with acceptable mechanical properties.
In summary, as environmental challenges increase and the demand for sustainable manufacturing grows, combining 3D printing with recycled materials offers a promising solution. This research enhances sustainable materials science in AM by effectively integrating recycled polymer matrices into advanced composites, providing molecular, rheological, thermal, morphological, and material property analyses.

2. Materials and Processing Parameters

2.1. Material Selection and Preparation Composites

In this contribution, two post-industrial waste polymeric materials supplied by the Maier S.Coop group (Bizkaia, Spain) were utilized, namely Bayblend® T65 XF, a ‘Polycarbonate/acrylonitrile-butadiene-styrene (rPC/ABS) blend and SABIC® Polypropylene (rPP 108MF10). To enhance thermal stability, 1% of Hostanox® P-EPQ® powder (Muttenz, Switzerland), known for its highly efficient phosphorus-based secondary antioxidant properties, was first introduced into the rPC/ABS blend. Subsequently, a 2.5% volume fraction of short carbon fibers (sCFs) was incorporated into the rPC/ABS blend using twin-screw extrusion. For the rPP material, reinforcement was achieved by adding a 20% volume fraction of sCF through a twin-screw extrusion process. To address the brittleness associated with higher sCF volume fractions, a 4% Polymer Process Improvement (PPI) agent, an additive from Plastics Science by Design (PSBD) (Flanders, Belgium), was added. Table S1 in the Supporting Information shows the standard properties of the used matrices.

2.2. Fused Filament Fabrication and Direct Pellet-Based AM of sCF Composites

For FFF, a BCN3D Epsilon W27 3D printer (Barcelona, Spain) with a 0.4 mm nozzle diameter and a 2.85 mm filament (average) diameter was used to print the samples. This machine has a closed-build chamber, resulting in a passive heating system and humidity-controlled environment. The filament spools are placed inside a separate chamber where the humidity is controlled to prevent moisture absorption. Additionally, the printer is equipped with two extruder heads that can function independently or together, enabling the printing of both primary and support materials simultaneously [35].
To print the polymer pellets, an in-house developed machine consisting of 4 motors was utilized. The first two motors are used to move the bed in the Z-direction, and the other two are used to move it in the X and Y direction (Figure S1A,B of the Supporting Information). These motors are connected to movement screws so that the bed can move in all three directions. To maintain the printing bed in a horizontal position, the system uses the support of two linear rods that work together to keep the bed level. To prevent moisture and contamination during printing, the printer has a closed printing chamber. Additionally, lubrication for the X and Y axes is crucial to maintain low noise levels and prevent engine lock-up.
The machine is based on a single conical-shaped rotating screw (Figure S1C,D of the Supporting Information) with a dedicated engine. Machine geometry and dimensions for extrusion are reported in Table S2 of the Supporting Information [25]. The printer head had a 0.6 mm nozzle diameter and a TwinCAT 3 software (Version 3.1, Build 4024.56) program-based PLC was used, as developed in previous work [26], to regulate both the micro-extruder and bed temperature in addition to the frequency and the clearance (Figure S2 of the Supporting Information). To control the movement of the Cartesian system, Pronterface software (Version 2.0.1) was used to execute the G-code instructions and move the bed accordingly.

2.3. Printing Parameter Optimization

To determine the optimal printing parameters for the FFF technique, four tests were conducted. Firstly, temperature tests were performed by extruding the filament at different temperatures without dedicated deposition to establish a temperature range for successful extrusion and to identify any potential nozzle-clogging issues. Thin-walled bars measuring 20 × 20 mm, with a height of 10 mm (Figure 2A), were printed at various temperatures within the determined range and, in some cases, the cooling fan was enabled to ensure proper printing.
Secondly, the optimal printing speed was assessed by printing a velocity tower (see Figure 2B) which is a part for which the printing velocity increases incrementally each 12.5 mm as follows: 20, 40, 60, 80, and 100 mm/s.
Thirdly, the filament retraction settings were examined to address the occurrence of stringing (Figure 2C), which refers to the formation of thin polymer strings during unsupported rapid nozzle movements [36]. Different retraction distances in multiples of 1 mm and retraction velocities in multiples of 5 mm/s were tested. The occurrence of stringing was also influenced by nozzle temperature and fan speed design.
Finally, a benchmark part was printed using predetermined settings for printing temperature, fan speed, printing velocity, and filament retraction to evaluate the overall printing quality (Figure 2D). The parameters used for defining the FFF process settings are presented in Table 1, and Figure 2 shows the main printing test shapes.
For the DPAM technique, a methodology based on trial and error was used. The temperature optimization involved extruding the generated filament without deposition at various temperatures to achieve homogeneous extrusion. The screw speed, which affects the surface finishing, was set between 1 and 5 rpm, considering the nozzle diameter. For the bed temperature design, either the same temperature as for FFF or a slightly higher temperature, specifically 130 °C, was employed to ensure proper adhesion between the printed part and the bed [37]. Regarding the bed speed, a range of 20 to 40 mm/s was tested. During the trials, the most important aspects to be considered were the examination of excessive material or overly fluid material, the presence of gaps between printed lines, and adhesion issues between the part and the bed. Successful results were achieved in cases where these factors were well-managed. The optimal printing parameters for FFF and DPAM are summarized in Table 2.

2.4. Rheological, Morphological, and Thermal Property Characterizations

2.4.1. GPC Analysis

Gel Permeation Chromatography (GPC) was performed to study the chain length distribution (CLD) of the rPC/ABS polymer matrices before and after processing. The GPC unit is equipped with 3 Resipore columns in series, and a refractive index, viscosimetry, and light-scattering detector. THF was used as an eluent at a flow rate of 1 mL min−1. The column compartment was thermostated at 30 °C. Column calibration was performed via polystyrene standards (162–1.5 × 106 g mol−1 range). For the analysis, ca. 2 mg of the sample was dissolved in 1 mL of THF and kept in a shaker for 1 h. The samples were filtered through a (PTFE) filter prior to analysis.

2.4.2. Rheology Analysis

Rheological measurements for the rPC/ABS and rPP materials with and without sCF before and after printing were performed for compression-molded disk-shaped specimens with 25 mm diameter and 1 mm thickness manufactured in a hot press (Fontijne Holland, Vlaardingen, The Netherlands) at 200 and 250 °C. The frequency sweep tests were performed in an MCR 702 rheometer (Anton-Paar, Graz, Austria), using the parallel plate configuration with a 25 mm diameter and 1 mm gap. The storage modulus (G′), loss modulus (G″), and complex viscosity (η*) were monitored as a function of the angular frequency (from 600 to 0.1 rad/s), with a strain amplitude of 0.1% under a nitrogen atmosphere. The strain amplitude was defined employing amplitude sweep tests and all the materials were in the linear viscoelastic regime. All materials were dried at 60 °C in a vacuum dryer overnight before molding and rheological testing.

2.4.3. Optical and Scanning Electron Microscopy Analysis

Optical microscopy for the printed parts was performed on a VHX-7000 Keyence OM (Keyence International NV/SA, Mechelen, Belgium), employing its software to assess the printing quality. Scanning electron microscopy (SEM) images were obtained on a Phenom Pro electron microscope (Benelux Scientific, Ede, The Netherlands). The images were taken at 5 kV.

2.4.4. Carbon Fiber Length

To evaluate the carbon fiber length, the dissolution of the matrix at 150 °C was performed. In the case of rPC/ABS composites, chloroform was utilized for a two-hour heating period. For rPP composites, xylene was chosen to dissolve the matrix. Subsequently, the mixture was filtered using filter paper and left to dry at room temperature for a day. The resulting samples were then examined under an optical microscope VHX 7000 by Keyence (Keyence International NV/SA, Mechelen, Belgium) with a 200× magnification lens.

2.4.5. DSC Analysis

The thermal properties of the polymer matrices and composites were studied via Differential Scanning Calorimetry (DSC) with a DSC 214 Polyma device (NETZSCH-Gerätebau GmbH, Selb, Germany) under a nitrogen atmosphere. All the materials had a weight ranging from 9 to 11 milligrams and were heated from 25 to 250 °C with a 10 °C·min−1 ramp considering two heating–cooling cycles.

2.4.6. TGA Analysis

Thermogravimetric analysis (TGA) was performed with NETSCH STA 449 F3 Jupiter (NETZSCH-Gerätebau GmbH, Selb, Germany). An aluminum crucible for rPP samples and a platina rhodium crucible for rPC/ABS samples were used, with maximum temperatures of 600 °C and 800 °C, respectively. The test ran under a nitrogen atmosphere. The temperature profile started below 40 °C and increased with 10 °C·min−1 to maximums of 500 °C and 800 °C, respectively.

2.4.7. FTIR Analysis

FTIR in attenuated total reflectance mode (ATR) was recorded using a Bruker Tensor 27, Billerica, MA, USA. The test was conducted at a resolution of 4 cm−1, with the wavenumber ranging from 4000 to 600 cm−1.

2.5. Measurement of Mechanical Properties

2.5.1. Tensile Property Measurements

All the tensile parts were printed following the ISO_572-2_Type_1A 170 mm standard [38] with 100% infill density. The tests were performed with an “Instron 5565” machine (Instron, Norwood, MA, USA) equipped with an Instron 2630-107 extensometer. The tests were performed in a controlled environment with a 2 kN load cell. The pulling speed for rPC/ABS, rPC/ABS composites, and rPP composites was 2 mm/min, and for rPP was 50 mm/min, as using a speed of 2 mm/min took too long a time to reach failure after removing the extensometer.

2.5.2. Flexural Property Measurements

The ISO-178 standard [39] was employed for the determination of the flexural properties of the printed parts. The distance between the two supports was set to 64 mm, which led to a span/width ratio of 16. The displacement control was set to 1.5 mm/min. The machine used was an Instron 4464 (Instron, Norwood, MA, USA) with its associated Bluehill 3 software.

2.5.3. Impact Property Measurements

Impact tests were executed according to the Charpy ISO 179 standard [40] with a Tinius Olson IT 503 impact testing machine (Philadelphia, PA, USA). This test is suitable for thermoplastic and thermoset materials that can be reinforced by fibers. The type of test is specified in ISO-179-1/1 e A [40].

2.6. Energy Consumption

An Energy Logger 4000 device was used to calculate the energy consumption of the printed components. The average recorded power per hour was calculated and, subsequently, the following equation was applied:
E = P × ( t 1000 )
in which E is the energy measured in Joules or kilowatt-hours (kWh), P is the power in Watts, and t is the time over which the power or energy was consumed (the printing time).

3. Results and Discussions

In the following sections, the molecular properties of the polymers and composites are analyzed before and after printing via DPAM and FFF, to evaluate the relevance of thermal and oxidative degradation. Material properties are then measured to establish the influence of degradation on the tensile and flexural properties. An evaluation of DPAM and FFF in terms of energy consumption is also conducted to provide guidance for efficient techniques for the 3D printing of recycled materials.

3.1. GPC: Degradation by Scission

Figure 3 shows the number average molar mass (Mn) and mass average molar mass (Mm) of the rPC/ABS composites before and after processing via DPAM and FFF, as determined via GPC. Pellets with and without sCF, as processed via DPAM, were compared in the first stage, and an acceptable decrease was observed in Mn and Mm, with the Mn value decreasing by ca. 30% and the value of Mm decreasing by ca. 15% for the material without sCF, whereas the decrease is less pronounced (20% for Mn and 6% for Mm) in case of the material with sCF. In the second stage of FFF, the decrease in molar mass characteristics was even more acceptable for the material with sCF, as it experienced a ca. 20% drop for Mn and almost none for Mm. In both techniques, Mn was more affected during processing, indicating the formation of short oligomers.
As the decrease in average molar masses in the case of DPAM is more pronounced compared to FFF, this indicates a chain–scission process with a higher degradation degree at first sight. However, GPC only allows an analysis of the soluble fraction of the polymer material, meaning that the effect of chain crosslinking, which is the main mechanism of the thermo-mechanical degradation of ABS [30], cannot be fully accessed via GPC. In this context, the thermo-mechanical and oxidative degradation of the materials are also addressed with rheometry, which allows us to study the sample as such and account for the insoluble fraction of polymer chains as well.

3.2. FTIR: Identifying Degradation Products

Figure 4A,B show the FTIR spectra of rPC/ABS and rPC/ABS/sCF composite pellets before processing and after 3D printing via DPAM and FFF. To evaluate the thermo-oxidative degradation, the signal of the C=C bond at 1637 cm−1 and the signal of the oxidation product’s carbonyl group at 1770 cm−1 were followed, as it is known that butadiene units in the structure of ABS are most prone to thermo-oxidative degradation [41,42]. The oxidation product’s C=O signal for ABS, however, overlaps with the signal of the carbonyl group of PC, complicating the analysis at first sight. The intensity of the latter can, fortunately, be considered constant, thus not affecting the analysis.
Based on the experimental data and the signal intensities of 1637/1770 cm−1, it can be noticed that the polymer matrix (no sCF yet) experienced degradation after being processed via DPAM and FFF, as the ratio between the specific intensities changed from that of the pellets in both cases around 20%. This change indicates a notable alteration in the polymer matrix characteristics, which is similar for both processing techniques. Remembering that GPC faced limitations and FTIR gives the same conclusions as GPC, it should be again put forward that one should be careful when solely analyzing GPC and/or FTIR data.
Furthermore, according to FTIR, the addition of sCF does not affect the frequency of the polymer matrix signals, indicating no specific interactions between the matrix material and carbon fibers. Processing in the presence of sCF does also not affect the oxidative degradation of the matrix material, as the ratio of C=C and C=O signals does not change due to the presence of sCF.
Figure 4C–E, in turn, show the FTIR spectrum of rPP and rPP/sCF composites. Both materials show peaks at the same wavelength, so adding sCF did not significantly affect the chemical compositions or structural features. No extra peaks were found in the range of 1700 cm−1, which indicates nearly no oxidative degradation during processing, at least on an FTIR basis.

3.3. Rheological Behavior: Relevance of Crosslinking

Rheology is a valuable tool for investigating the viscoelastic characteristics and microstructure of materials, which significantly influence their mechanical properties. Various components such as fillers, stabilizers, and antioxidants affect the rheological behavior of materials [43]. They can provide valuable insights into the degree of degradation during processing for recycled materials in cases where GPC data are less available, e.g., for grafted or cross-linked polymers. For example, the frequency of the crossover point, i.e., the frequency at which G′ and G″ have the same value (G′ = G″), indicates the molecularly driven relaxation time of the material, which is calculated as the inverse of the crossover frequency [44]. In more detail, this point marks the characteristic relaxation time of the material when shifting occurs from a more rubbery state (G′ > G″) to a viscous state (G″ > G′) with all the polymer chains being fully relaxed [45].
Figure 5A,C show the G′ and G″ moduli as a function of the angular frequency of rPC/ABS and rPC/ABS/sCF composites. Both the storage and loss moduli of those materials were measured before and after processing. G′ shows a decrease in its absolute value as a consequence of degradation (Figure 5A,C, black vs. blue and pink symbols). In both cases, the processed materials present lower values of the moduli compared to the materials before processing. The absolute values of moduli for the materials processed via DPAM and FFF are nearly the same but not at the crossover point. This point is shifted towards higher values after processing (Figure 5B,D); thus, it is possible to verify an earlier switch to the rubbery phase of the material, indicating the faster relaxation of the disentangled chains and the increased melt complexity. This can either be due to the increased number of shorter chains and/or an increased amount of branching, both arising from the degradation process. In the case of FFF-processed materials (both with and without sCF), the shift of the crossover point is more pronounced, indicating a higher degree of degradation [45].
Hence, the results of SAOS seem to contradict those of GPC, as the decrease in molar mass is more severe in the case of DPAM (Figure 3) but the change in relaxation behavior is more pronounced for the FFF-processed material (Figure 5). To understand these results correctly, one needs to keep in mind that, during processing, two molecular degradation pathways can take place, i.e., chain scission leading to shorter chains and chain crosslinking resulting in longer, more branched, or even crosslinked chains. The latter are often filtered off before GPC analysis and such degradation products can thus be masked. In other words, the sample preparation before GPC analysis can lead to the underestimation of Mn and Mm values. On the other hand, for the SAOS analysis, the samples do not need to be treated so that the contribution of high-molar-mass polymer chains is not affected. Consequently, for the evaluation of polymer degradation, the results of GPC should be analyzed together with other techniques, e.g., SAOS, to obtain an unambiguous conclusion. ABS degradation is affected by crosslinking and, hence, it can be concluded that, for the DPAM process, one has a more scission-driven degradation, also bearing in mind the mechanical forces of the screw, and, for the FFF process, two steps with the latter likely deliver crosslinking to a significant degree, consistent with the work of Ceretti et al. [46].
Furthermore, the difference between the values of the frequencies of crossover points is higher among the samples with sCF than in the cases without sCF, meaning the composites’ melt exhibits increasing elastic behavior with sCF [47].
Figure S4A,B of the Supporting Information show the complex viscosity as a function of the angular frequency of rPP and rPP composites. It can be noticed that rPP presents a lower complex viscosity upon processing, indicating polymer degradation via chain scission [48]. Additionally, distinct patterns are visible for rPP composites, and the complex viscosity is slightly higher compared to the complex viscosity of rPP attributed to the presence of CF, which contributes to increased resistance to shear deformation (Figure S4B of the Supporting Information). Filled polymers generally exhibit higher viscosity at low shear rates and an increase in filler concentration may lead to yielding [49].
The G′ and G″ variations in rPP before and after processing with DPAM and FFF are depicted in Figure 6A,C. G′ shows a decrease in its absolute value similar to the rPC/ABS case. The degradation is more prominent for the material with sCF, which highlights again the similarities with the rPC/ABS case (Figure 6B,D). Again, more pronounced degradation in the case of FFF-processed materials is observed upon checking the crossover point variation with chain shortening, which is thus linked to material degradation [50,51].
Additionally, the effect of the flow enhancer (PPI) on the rheological properties was investigated for the PP case. Figure 6C (last two blue vs. pink symbols) shows the storage and loss moduli of the materials in the presence of a flow enhancer. Upon processing, an increase in the crossover point frequency is again registered, as shown in Figure 6D, noting that, to add PPI, a processing step is needed. However, upon considering the material without PPI overall, a higher increase in the crossover frequency is observed upon going from pellets to the material processed via DPAM, whereas, with PPI, an immediate increase is obtained with the actual printing being characterized by negligible extra degradation. Despite a larger increase in the cross-over frequency in the case of DPAM processing without PPI compared to FFF processing with PPI, it is important to stress that the printing of the rPP/20%sCF filaments via FFF was impossible. This indicates the additional advantages of the DPAM process, as it requires less manipulation for processing composites with a high content of fillers.

3.4. Thermal Properties

The DSC results of all materials before processing via DPAM and FFF are provided in Table 3. It can be noticed that the glass transition temperature (Tg) of rPC/ABS and the melting temperature Tm for rPP are close to the ones reported in the literature for virgin materials (Tg, PC/ABS = 105 for ABS; 140 °C for PC [44] and Tm, PP = 160–166 °C [52]). These results suggest comparable thermal characteristics between the recycled and non-recycled materials, implying similar processing parameters [53]. Furthermore, it is noteworthy that the introduction of sCF had no significant influence on Tm or Tg for any of the materials examined. Additionally, the addition of PPI did not show any significant impact on Tm (see Figures S5–S8 of the Supporting Information).
Crystallinity calculations were not performed for rPC/ABS due to its amorphous nature [54]. The crystallinity degrees X c % of the rPP and rPP composites were, in turn, calculated using the following equation [55], with the results presented in Table 3.
X c % = ∆ H m − ∆ H c c Δ H m ∞ . w × 100 %  
In Equation (2), ∆ H m is the melting enthalpy, ∆ H c c is the cold crystallization enthalpy, Δ H m ∞ is the melting enthalpy, and w is the weight fraction of the sample in grams.
The crystallinity degree increases by 8% from rPP to rPP/ sCF and by 22% from rPP to rPP with sCF and PPI. CF thus acts as a heterogeneous nucleation agent for rPP. In more detail, the presence of CF enhances the nucleation density within the matrix, leading to an improvement in the crystallization ability of molecular segments [56]. The presence of a flow enhancer facilitates the re-orientation of the polymer chains, further enhancing the crystallinity.
The TGA results for rPC/ABS, rPC/ABS composite, rPP, and rPP composite samples processed via DPAM and FFF are shown in Figures S9–S13 of the Supporting Information and are summarized in Table 4. A comparison of the temperature of 1% mass loss (T99%) was specifically conducted to assess the thermal stability of the materials [30]. The DPAM-processed samples exhibited better thermal stability (entries 1–2 and 5–6 in Table 4), with 4.6% and 1% higher values than FFF for rPC/ABS and rPP, respectively. This is due to the lower degree of degradation during processing, as explained above by the inclusion of rheological analysis.
However, for the samples with sCF (entries 3–4 and 7–8), the FFF-processed samples demonstrated better thermal stability compared to DPAM by 3.4% and 30.6%. The significant influence of CF on the thermal stability of the material has been indicated in the literature and is dependent on the dispersion quality [57]. Hence, one could expect that a morphological analysis (next subsection) should reveal a more random CF incorporation for DPAM. Moreover, in the case of rPP parts, the effect on T99% is even more pronounced due to the higher percentage of CF added, which was 20%.

3.5. Morphological Properties

The surface quality of the printed parts was examined via optical microscopy in the horizontal and vertical directions. The entire surface was measured and the sample’s surface profile was traced using data obtained from the VK-X Series. Several methods can be used to quantify microscopic asperity; for example, one can find the level difference between the highest and lowest points within a 1 mm square area [58]. This height profile was used to compare the different printing techniques (DPAM and FFF) and the outcomes are included in Figure 7, Figure 8 and Figure 9. These results are displayed in a box plot, wherein the colored box represents the 25–75% height range and a T-shaped line indicates the maximum value. The minimum value is always recalculated to 0 µm to facilitate comparisons. The explicit value in the graph is the disparity between the highest and lowest peaks of the curve element along the sampling length.
In general, parts produced via the DPAM technique exhibited a rougher surface, which is attributed to the variable forces applied on the screw and the increased layer thickness due to the larger nozzle diameter.
However, rPC/ABS + 1%PEQ parts without sCF produced via FFF are characterized by a slight shrinkage compared with DPAM parts, as shown in Figure 7A (black parts). This could be explained by the higher nozzle temperature in the FFF process, causing additional residual stress. An increased printing temperature may cause over-melting and material degradation, leading to dimensional irregularities that subsequently affect the overall shrinkage of the printed part [59], while the DPAM printer with a three-zone heating system improved heat dispersion along the extruder. On the contrary, as shown in Figure 7B (white parts), the rPP parts produced via DPAM showed greater shrinkage, due to the incomplete optimization of the printing settings, as it is likely that too high a temperature was employed in the three zones of the DPAM machine extruder.
A potential solution to this issue involves using a heated chamber rather than just relying on insulation. This chamber would cool down slowly and more uniformly after the printing process is completed; hence, the temperature distribution becomes more uniform. This, in turn, can alleviate internal stresses, leading to a reduction in the likelihood of warping and shrinkage in the printed objects [60]. Such phenomena can have a significant effect on the structural stability of rPP parts, as they are produced from highly crystalline materials. In these two examples, we see that the crystallinity of the matrix material significantly impacts the optimal printing conditions of a part: for amorphous rPC/ABS, the degradation of the material induced by FFF is crucial for the structural stability of the part, whereas, for highly crystalline rPP, the uniformity of cooling is important in reducing the warpage of the parts.
Introducing sCF can boost dimensional stability and reduce shrinkage by enhancing material properties, lowering thermal expansion, and positively impacting finishing surface quality [61]. This positive impact on surface quality is evident for FFF-printed parts with sCF, as seen in Figure 8 and Figure 9 (black parts), where reduced shrinkage is observed compared to parts without sCF in Figure 7 (black and white parts) for both the horizontal and vertical directions.
In general, printed parts with sCF through DPAM exhibited rougher surfaces in comparison to those produced via FFF. This is visible via the lessened variation in surface roughness of FFF parts seen in Figure 8 and Figure 9. The enhanced surface quality in FFF parts can be explained by the reduced clogging of sCF in the nozzle, ensuring consistent material flow during extrusion. Additionally, printing with smaller layer thicknesses resulted in superior properties, facilitated by well-aligned carbon fiber reinforcements [62]. The DPAM parts exhibited rougher surfaces due to the need for continuously adjusted print speeds to prevent overflow or under-extrusion, necessitating constant monitoring of force changes on the screw.
rPP parts produced via DAPM with sCF showed less shrinkage and better quality in both horizontal and vertical directions compared to rPP parts without sCF, as the difference between the highest and lowest peaks was less with sCF (red columns in Figure 7B and Figure 9A,B). Consequently, in agreement with the observations above, the presence of sCF enhances the thermal conductivity of the material, which ensures uniform cooling and an absence of shrinkage. Upon comparing rPC/ABS with sCF parts produced through DPAM, a significant increase in the explicit values (the difference between the highest and lowest peak) is clear. In rPC/ABS parts, this value measures 215.32 µm horizontally and 382.63 µm vertically. However, with the addition of 2.5% sCF, these measurements expand significantly to 715.04 µm horizontally and 625.85 µm vertically, more than doubling the distance (red columns in Figure 7A and Figure 8). This may be attributed to the formation of small bubbles or thickening above the fibers, resulting in surface roughness as the polymer shrinks during the cooling process.

3.6. Tensile, Flexural, and Impact Properties

Apart from printing quality evaluation by morphological analysis, the interfacial interaction of matrix materials with the fibers plays a significant role in the material properties of the printed part. The interfacial interactions were evaluated theoretically in the first step via calculating the surface-free energies of the matrix and filler. Generally, for effective wetting to occur, the surface energy of the filler should be greater than that of the matrix. For the CF and the selected matrix polymers, data from the literature suggest that this is the case when PC/ABSγPC/ABS is equal to 35–46 mN/m and rPPγPP is equal to 30.1 mN/m [63], both lower than the γCF at 53 mN/m [64]. Hence, from a theoretical standpoint, both polymers are expected to exhibit strong adhesion to CF, resulting in an effective distribution of the polymeric melt within the fibers, yielding high strength and stiffness [65].
In the second step, the interfacial interaction was assessed via SEM images (see Figures S15 and S18 of the Supporting Information). As no distinct interface/air gap between the sCF and polymer matrices was observed, good wetting can be assumed. Consequently, we expect no influence of wetting on the mechanical properties of the printed parts.
Figure 10A–C show the mechanical test results of rPC/ABS + 1%PEQ and rPC/ABS composites. For rPC/ABS + 1%PEQ, the FFF parts exhibit a higher tensile strength compared to DPAM parts, as shown in Figure 10A. This could be explained by the lower layer thickness and better adhesion of the produced parts via FFF. Figure 10B,C show that the FFF parts also demonstrate better flexural and impact results than DPAM parts, due to the superior interlayer adhesion and the insignificant voids inside the parts (Figure S14 of the Supporting Information) [66]. Furthermore, the addition of a small amount (2.5%) of sCF did not lead to a significant increase in the tensile and impact results for DPAM parts. This could be explained by the micro extruder reducing the length of the fibers. SEM pictures (Figure S15 of the Supporting Information) show the low dispersion quality of the fibers inside the DPAM parts and shorter fibers after dissolving the matrix, as shown in Figure S16 of the Supporting Information. In addition, regarding the FFF parts, adding sCF did not increase the tensile results for DPAM. However, the flexural and impact strengths were enhanced. This can be explained by the parallel orientation of sCF during filament deposition, which contributed to the absorbed load, as shown in Figure S15 of the Supporting Information [67].
Figure 10D–F display the mechanical properties of rPP and rPP composites. As mentioned above, rPP + 20%sCF exhibited brittleness and was not processable with FFF. Nevertheless, parts could be successfully produced by the DPAM technique, again highlighting a notable advantage over the FFF technique in this context and reminding us that only by adding 4% of PPI to rPP + 20%sCF does it become processable with the FFF technique. Upon comparing rPP samples produced through DPAM and FFF techniques in Figure 10D,E, it can be seen that the tensile and flexural strengths are similar within experimental error margins for FFF parts; however, all the printed parts with both techniques exhibited low void content, which can be seen in the optical microscopy images (Figure S17 of the Supporting Information).
Figure 10F shows that there is a significant increase in the impact strength for FFF parts, which could be due to the smaller layer thickness leading to improved interlayer adhesion, resulting in enhanced overall part strength and impact resistance. However, when the samples were investigated under the optical microscope, no voids were seen, but some interesting results regarding notable color variations were observed at specific locations on the fracture surface of these parts. This distinctive color alteration is referred to as stress whitening [68]. This phenomenon is a result of the applied stress on the part that creates microvoids. These defects change the refraction index (RI), which, in turn, lets the polymer change from translucent white to a bright white color. Stress whitening was visible in the bottom of the FFF samples, which did not break completely after the impact test, while the DPAM samples tended to break completely and showed a slight color deviation at the bottom (Figure S17 of the Supporting Information).
rPP composite parts printed by the FFF technique with 4% PPI gave a tensile strength of 12.3 MPa, which is about 4% higher than the samples manufactured by the DPAM technique (Figure 10D). This is within the margin of experimental error, so it is considered that both parts have similar tensile properties. The results of flexural properties are rather similar, and the difference is also within the error margin. For the impact strength results, the samples manufactured by DPAM had a 4% lower, but acceptable, reduction in impact strength. Furthermore, printed parts without 4% PPI via DPAM exhibited similar outcomes compared with parts printed with 4% PPI via FFF, providing a notable advantage of the DPAM technique, since this material could not be printed using FFF. The SEM pictures showed the good dispersion of sCF in the produced parts, as shown in Figure S18 of the Supporting Information.
Finally, certain deviations between rPP parts with sCF and those without sCF are seen in Figure 10D,F. Composite parts demonstrated nearly similar tensile and flexural properties compared to parts with no sCF. However, the impact test outcomes show a decline, which can be attributed to the addition of a high amount of carbon fibers, which results in pronounced brittleness and decreases the impact resistance.

3.7. Energy Consumption

For energy consumption measurements, the total value included energy needed for heating and, in some cases, cleaning, printing, and steady-state operation. The results depicted in Figure 11A were calculated depending on the time required to print one part and using Equation (1).
Figure 11A shows the energy consumption during the heating, printing, and steady-state steps for FFF and DPAM applied to both matrix materials. The energy consumption pattern for the composite materials is shown in Figure S20 of the Supporting Information. The DPAM machine consumes a higher amount of energy during the heating stage because of its large steel barrel and screw, which require significant time and energy to reach the desired temperature. This machine is equipped with three heating bands that can consume a maximum of 300 W, meaning that, during the heating stage, the machine uses approximately 1000 W of power to speed up the process. After the machine reaches a steady state, the heaters are deactivated. At this point, additional apparatuses, such as the computer, PLC terminal box, PLC screen, and bed control system, consume approximately 115 W. In DPAM, an extra cleaning step is necessary. During this cleaning stage, the temperature increases by 20 °C, and the screw engine rotates at a speed of 10 to 15 RPM. This speed is higher than the maximum rotation speed of 5 RPM used during the printing process, and the material used for cleaning is always low-density polyethylene (LDPE). These additional steps increase the power usage.
The energy consumption for the Epsilon W27 printer used for FFF differs between the printing and heating stages, but the difference is smaller due to the reduced energy requirement of its smaller heat block. It should be mentioned that, for FFF, only the energy consumption for printing was measured. However, one needs to keep in mind that the production of filament requires energy as well, which will further increase the consumed energy. Here, the energy required for filament production was not measured. Thus, the obtained values can be seen as the lowest amount of required energy.
The energy consumption for both techniques was measured based on the printing time required to produce ISO specimens (both tensile and impact/flexural in the same production circle). Consequently, the printing time is influenced by the specific printing parameters used. For FFF, the printing time is longer, as shown in Figure 11B, resulting in higher energy consumption. FFF requires a higher energy consumption compared to DPAM, considering the longer production time in addition to the smaller layer thickness, which allows for greater details in the printed parts. Achieving similar levels of detail with DPAM implies further machine design. Nonetheless, DPAM has been demonstrated to be a fast printing technique that involves a single processing cycle that minimizes energy consumption. Furthermore, the impact properties of the samples produced via both techniques remain similar within the measurement error, as shown in Figure 11A.

4. Conclusions

In this contribution, the application potential of the direct pellet-based additive manufacturing (DPAM) technique, in comparison to standard fused filament fabrication (FFF), for the processing of short carbon fiber (sCF) composites was investigated. As the matrix materials, recycled polymer materials, i.e., PC/ABS and PP, were used, aiming at upcycling polymer wastes. Two levels of sCF loads, namely 2.5% and 20%, were explored.
As the waste feedstock polymer materials have already undergone some degree of degradation, selecting the processing techniques that assure a minimal influence on the molecular properties is essential for the mechanical performance of the parts. Hence, the thermal and oxidative degradation were analyzed in the first phase via different tests before and after processing. Within this scope, different techniques were employed, namely GPC, FTIR, and SOAS, to unambiguously evaluate the degradation degree. The findings of these tests demonstrate the following:
▪
An analysis of GPC data alone can lead to an incorrect evaluation of the degradation of the polymer matrix, as only the soluble fraction of the polymer is analyzed. However, SAOS allowed us to obtain deeper insights into the polymer matrix degradation process and to evaluate how DPAM leads to a lower degree of material degradation during processing.
In the second phase, the printing quality and material properties of the produced parts were estimated via OM, SEM, and ISO standard tests. The main conclusions regarding these tests are as follows:
▪
Despite the lower layer thickness and better surface roughness of the FFF parts, the tensile and impact properties of the parts produced via DPAM and FFF were comparable.
In the third phase, the energy consumption for the printing process was assessed during the printing of standard ISO specimens, and the results show the following:
▪
DPAM offers a significant advantage, as it allows for faster printing. Furthermore, for FFF, only the lower energy consumption limit was calculated as the energy-consuming filament production process was not accounted for.
In summary, we have further highlighted that the DPAM process provides many advantages for the production of composite-based parts, minimizing the degree of degradation during processing and providing fast part printing while preserving the material properties.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jcs8060222/s1, Table S1. Standard properties of the used matrices; Figure S1. (A): A 3D design sketch of the in-house machine, (B): detailed technical drawing of the single screw extruder’s design and components, (C): the actual in-house machine implementation with its physical construction and SSE positioned on top, and (D): the used single screw; Table S2. Machine geometry and dimensions for extrusion; Figure S2. A visual screen of the TwinCAT 3 program; Figure S3. (A): Complex viscosity vs. frequency of rPC/ABS + 1%PEQ: pellets, processed through DPAM and processed through FFF. (B): Complex viscosity vs. frequency of rPC/ABS + 1%PEQ with sCF: pellets, processed through DPAM and processed through FFF; Figure S4. Complex viscosity vs. angular frequency of (A) rPP, and (B): for rPP + 20% sCF and rPP + 20% sCF +4% PPI; Figure S5. DSC analysis results of rPC/ABS + 1%PEQ with and without sCF; Figure S6. DSC analysis results of rPP; Figure S7. DSC analysis results of rPP + 20%sCF; Figure S8. DSC analysis results of rPP + 20%sCF + 4%PPI; Figure S9. TGA analysis results of rPC/ABS + 1%PEQ with: (A): DPAM and (B): FFF; Figure S10: TGA analysis results of rPC/ABS + 1%PEQ + 2.5%sCF with (A): DPAM and (B): FFF; Figure S11. TGA analysis results of rPP with (A): DAPM and (B): FFF; Figure S12. TGA analysis results of rPP + 20%sCF + 4%PPI with (A): DAPM and (B): FFF; Figure S13. TGA analysis results of rPP + 20%sCF with DPAM; Figure S14. Fracture surface images of rPC/ABS + 1%PEQ and rPC/ABS + 1%PEQ + 2.5%sCF using Optical Microscopy (×200) for produced parts with DPAM, and FFF; Figure S15. Fracture surface images of rPC/ABS + 1%PEQ and rPC/ABS + 1%PEQ + 2.5%sCF using Scanning Electron Microscopy for produced parts with DPAM and FFF; Figure S16. SEM pictures of CF length of rPC/ABS + 1%PEQ + 2.5%sCF with DPAM and FFF; Figure S17. Fracture surface images of rPP, rPP + 20%sCF, and rPP + 20%sCF + 4%PPI using Optical Microscopy (×200) for produced parts with DPAM, and FFF; Figure S18. Fracture surface images of rPP, rPP + 20%sCF, and rPP + 20%sCF + 4%PPI using Optical Microscopy (×200) for produced parts with DPAM, and FFF; Figure S19. SEM pictures of CF length of rPP + 20%sCF, and rPP + 20%sCF + 4%PPI with DPAM and FFF; Figure S20. (A): Power usage as a function of time and (B): Printing time to produce tensile and flexural/impact parts.

Author Contributions

Conceptualization, M.B., L.C. and M.E.; software, M.B., D.R.D. and L.C.; investigation, M.B., C.F., A.G., L.C., D.R.D. and M.E.; writing—original draft preparation M.B. and M.E.; writing—review and editing, M.B., C.F., L.T., A.V., A.G., D.R.D., L.C. and M.E.; supervision, D.R.D., L.C. and M.E.; and funding acquisition, D.R.D. and L.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the European Union’s Horizon 2020 research and innovation program entitled ‘Recycling and Repurposing of Plastic Waste for Advanced 3D Printing Applications’ (Repair 3D), under GA No 814588. The Commission is not responsible for any use that may be made of the information it contains.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Diagrams showing 3D printing techniques with (A): the FFF principle and (B): the DPAM principle as a representation of the conventional single-screw extruder (SSE) system.
Figure 1. Diagrams showing 3D printing techniques with (A): the FFF principle and (B): the DPAM principle as a representation of the conventional single-screw extruder (SSE) system.
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Figure 2. Optimization tests to define the optimal FFF printing parameters. (A): Printing temperature tests, (B): printing speed tests, (C): filament retraction tests, and (D): a complete printed benchmark part using predetermined settings.
Figure 2. Optimization tests to define the optimal FFF printing parameters. (A): Printing temperature tests, (B): printing speed tests, (C): filament retraction tests, and (D): a complete printed benchmark part using predetermined settings.
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Figure 3. Number average (dark colors, left scale) and mass average (light colors, right scale) molar mass of rPC/ABS + 1% PEQ with and without sCF for pellets and processed materials via DPAM and FFF as determined via GPC.
Figure 3. Number average (dark colors, left scale) and mass average (light colors, right scale) molar mass of rPC/ABS + 1% PEQ with and without sCF for pellets and processed materials via DPAM and FFF as determined via GPC.
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Figure 4. Most important peak assignments for (A): rPC/ABS + 1%PEQ, (B): rPC/ABS + 1%PEQ +2.5%sCF, (C): rPP, (D): rPP + 20% sCF, and (E): rPP + 20% sCF + 4%PPI.
Figure 4. Most important peak assignments for (A): rPC/ABS + 1%PEQ, (B): rPC/ABS + 1%PEQ +2.5%sCF, (C): rPP, (D): rPP + 20% sCF, and (E): rPP + 20% sCF + 4%PPI.
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Figure 5. (A): G′, G″ vs. angular frequency of rPC/ABS + 1% PEQ: pellets, processed through DPAM and FFF. (B): Frequency of crossover points of rPC/ABS + 1% PEQ: pellets, processed through DPAM and FFF. (C): G′, G″ vs. angular frequency of rPC/ABS + 1% PEQ with sCF: pellets, processed through DPAM and FFF. (D): Frequency of crossover points of rPC/ABS + 1% PEQ with sCF: pellets, processed through DPAM and FFF.
Figure 5. (A): G′, G″ vs. angular frequency of rPC/ABS + 1% PEQ: pellets, processed through DPAM and FFF. (B): Frequency of crossover points of rPC/ABS + 1% PEQ: pellets, processed through DPAM and FFF. (C): G′, G″ vs. angular frequency of rPC/ABS + 1% PEQ with sCF: pellets, processed through DPAM and FFF. (D): Frequency of crossover points of rPC/ABS + 1% PEQ with sCF: pellets, processed through DPAM and FFF.
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Figure 6. (A) G′, G″ vs. angular frequency of rPP: pellets, processed through DPAM and processed through FFF. (B): Frequency of crossover points of rPP: pellets, processed through DPAM and FFF. (C): G′, G″ vs. angular frequency of rPP with sCF and with sCF and PPI: pellets, processed through DPAM and processed through FFF. (D): Frequency of crossover points of rPP with sCF and with sCF and PPI: pellets, processed through DPAM and FFF.
Figure 6. (A) G′, G″ vs. angular frequency of rPP: pellets, processed through DPAM and processed through FFF. (B): Frequency of crossover points of rPP: pellets, processed through DPAM and FFF. (C): G′, G″ vs. angular frequency of rPP with sCF and with sCF and PPI: pellets, processed through DPAM and processed through FFF. (D): Frequency of crossover points of rPP with sCF and with sCF and PPI: pellets, processed through DPAM and FFF.
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Figure 7. Surface quality comparison for (A): rPC/ABS + 1%PEQ parts produced by DPAM and FFF techniques and (B): rPP parts produced by DPAM and FFF techniques (the right pictures show the surface quality in the vertical direction).
Figure 7. Surface quality comparison for (A): rPC/ABS + 1%PEQ parts produced by DPAM and FFF techniques and (B): rPP parts produced by DPAM and FFF techniques (the right pictures show the surface quality in the vertical direction).
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Figure 8. Surface quality comparison for rPC/ABS + 1%PEQ + 2.5%sCF parts produced by DPAM and FFF techniques (the right pictures show the surface quality in the vertical direction).
Figure 8. Surface quality comparison for rPC/ABS + 1%PEQ + 2.5%sCF parts produced by DPAM and FFF techniques (the right pictures show the surface quality in the vertical direction).
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Figure 9. Surface quality comparison for (A): rPP + 20%sCF + 4%PPI parts produced by DPAM and FFF techniques and (B): rPP + 20%sCF parts produced by DPAM technique (the right pictures show the surface quality in the vertical direction).
Figure 9. Surface quality comparison for (A): rPP + 20%sCF + 4%PPI parts produced by DPAM and FFF techniques and (B): rPP + 20%sCF parts produced by DPAM technique (the right pictures show the surface quality in the vertical direction).
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Figure 10. Material properties for rPC/ABS + 1% PEQ and rPC/ABS + 1% PEQ + 2.5% sCF via (A): tensile, (B): flexural, and (C): impact tests, and for rPP, rPP + 20%sCF, and rPP + 20%sCF + 4%PPI via (D): tensile, (E): flexural, and (F): impact tests.
Figure 10. Material properties for rPC/ABS + 1% PEQ and rPC/ABS + 1% PEQ + 2.5% sCF via (A): tensile, (B): flexural, and (C): impact tests, and for rPP, rPP + 20%sCF, and rPP + 20%sCF + 4%PPI via (D): tensile, (E): flexural, and (F): impact tests.
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Figure 11. (A): Power usage as a function of the type of operations and (B): printing time to produce tensile and flexural/impact parts. Only focus on the printing, not the making of the filament, for the FFF case.
Figure 11. (A): Power usage as a function of the type of operations and (B): printing time to produce tensile and flexural/impact parts. Only focus on the printing, not the making of the filament, for the FFF case.
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Table 1. Defining the printing settings for the FFF technique.
Table 1. Defining the printing settings for the FFF technique.
Matrix and Nozzle Temperature [°C]AdditiveVol% of sCFRetraction Distance
[mm]		Retraction Speed
[mm/s]		Fan Speed
[%]		Layer Thickness
[mm]
rPP, 220PPI204, 8, 1220, 40, 60500.2
rPP, 230PPI204, 8, 1220, 40, 60500.2
rPP, 240PPI204, 8, 1220, 40, 60500.2
rPP, 250PPI204, 8, 1220, 40, 60500.2
rPC/ABS, 260Hostanox P-EPQ2.54, 8, 1220, 40, 600, 12.5, 25, 50, 1000.1
rPC/ABS, 270Hostanox P-EPQ2.54, 8, 1220, 40, 600, 12.5, 25, 50, 1000.1
rPC/ABS, 280Hostanox P-EPQ2.54, 8, 1220, 40, 600, 12.5, 25, 50, 1000.1
The bed temperature was set to 110 °C for all samples; see further information in Table 2.
Table 2. Used parameters for different materials with DPAM and FFF.
Table 2. Used parameters for different materials with DPAM and FFF.
MaterialrPC/ABS + 1%PEQrPC/ABS + 1%PEQ + 2.5% sCFrPPrPP + 20%sCF
rPP + 20%sCF + 4%PPI
Used parameters for DPAMT1 (°C)230230190170
T2 (°C)250250210235
T3 (°C)270270230210
Print speed (mm/s)21.67202020
Screw frequency (RPM)0.951.1541.3/1.6
Used parameters for FFFPrinting temperature (°C)280280190220
Print speed (mm/s)40604040
Fan speed (%)10102050
Retraction length (mm)121288
Retraction velocity (mm/s)60604040
Table 3. Results of DSC measurements of rPC/ABS + 1% PEQ, rPC/ABS + 1% PEQ + 2.5% sCF, rPP, rPP + 20% sCF, and rPP + 20% sCF + 4% PPI.
Table 3. Results of DSC measurements of rPC/ABS + 1% PEQ, rPC/ABS + 1% PEQ + 2.5% sCF, rPP, rPP + 20% sCF, and rPP + 20% sCF + 4% PPI.
SamplesTm (°C)Tg (°C)Crystallinity Degree (%)
rPC/ABS + 1%PEQ/106.9 (ABS); 137.8 (PC)/
rPC/ABS + 1%PEQ + 2.5%sCF/106.8 (ABS); 137.9 (PC)/
rPP164.9/39.1
rPP + 20%sCF164.8/42.3
rPP + 20%sCF + 4%PPI164.3/47.7
Table 4. Results TGA measurements of rPC/ABS + 1% PEQ, rPC/ABS + 1% PEQ + 2.5% sCF, rPP, rPP + 20% sCF, and rPP + 20% sCF + 4% PPI.
Table 4. Results TGA measurements of rPC/ABS + 1% PEQ, rPC/ABS + 1% PEQ + 2.5% sCF, rPP, rPP + 20% sCF, and rPP + 20% sCF + 4% PPI.
SamplesT 99% (°C)Residual Mass (%)
rPC/APS + 1%PEQ (DPAM)386.613.19
rPC/APS + 1%PEQ (FFF)368.912.44
rPC/APS + 1%PEQ + 2.5%sCF (DPAM)368.812.62
rPC/APS + 1%PEQ + 2.5%sCF (FFF)381.411.06
rPP (DPAM)335.61.89
rPP (FFF)332.01.10
rPP + 20%sCF + 4%PPI (DPAM)256.122.25
rPP + 20%sCF + 4%PPI (FFF)334.524.75
rPP + 20%sCF (DPAM) *359.917.66
* rPP + 20%sCF (only DPAM) as the material was not processable with FFF.
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MDPI and ACS Style

Baddour, M.; Fiorillo, C.; Trossaert, L.; Verberckmoes, A.; Ghekiere, A.; D’hooge, D.R.; Cardon, L.; Edeleva, M. Comparing Degradation Mechanisms, Quality, and Energy Usage for Pellet- and Filament-Based Material Extrusion for Short Carbon Fiber-Reinforced Composites with Recycled Polymer Matrices. J. Compos. Sci. 2024, 8, 222. https://doi.org/10.3390/jcs8060222

AMA Style

Baddour M, Fiorillo C, Trossaert L, Verberckmoes A, Ghekiere A, D’hooge DR, Cardon L, Edeleva M. Comparing Degradation Mechanisms, Quality, and Energy Usage for Pellet- and Filament-Based Material Extrusion for Short Carbon Fiber-Reinforced Composites with Recycled Polymer Matrices. Journal of Composites Science. 2024; 8(6):222. https://doi.org/10.3390/jcs8060222

Chicago/Turabian Style

Baddour, Marah, Chiara Fiorillo, Lynn Trossaert, Annabelle Verberckmoes, Arthur Ghekiere, Dagmar R. D’hooge, Ludwig Cardon, and Mariya Edeleva. 2024. "Comparing Degradation Mechanisms, Quality, and Energy Usage for Pellet- and Filament-Based Material Extrusion for Short Carbon Fiber-Reinforced Composites with Recycled Polymer Matrices" Journal of Composites Science 8, no. 6: 222. https://doi.org/10.3390/jcs8060222

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