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Article

Experimental Analysis of Plastic-Based Composites Made by Composite Plastic Manufacturing

Faculty of Science and Engineering, School of Engineering and Built Environment, Anglia Ruskin University, Chelmsford CM1 1SQ, UK
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2022, 6(5), 127; https://0-doi-org.brum.beds.ac.uk/10.3390/jcs6050127
Submission received: 26 March 2022 / Revised: 17 April 2022 / Accepted: 24 April 2022 / Published: 26 April 2022

Abstract

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The significance of composites cannot be overstated in the manufacturing sector due to their unique properties and high strength-to-weight ratio. The use of thermoplastics for composites manufacturing is also gaining attention due to their availability, ease of operation, and affordability. However, the current methods for plastic-based composites are limited due to the requirements of long curing times and pre- and post-treatment, thereby resulting in longer lead times for the desired product. These methods also limit the freedom to operate with different forms of materials. Therefore, a new manufacturing process for plastic-based composites is required to overcome such limitations. This research presents a new manufacturing process to produce high-quality plastic-based composites with bespoke properties for engineering applications. The process is referred to as Composite Plastic Manufacturing (CPM) and is based on the principle of fused filament fabrication (FFF) equipped with a heat chamber. The process integrates two material extrusion additive manufacturing technologies, i.e., filament and syringe extrusion. The paper presents the principle of the process, both in theory and in practice, along with the methodology and materials used to manufacture plastic composites. Various composites have been manufactured using the CPM process with thermally activated materials and tested according to British and International standards. Polylactic Acid (PLA) has been interlaced with different thermally activated materials such as graphene-carbon hybrid paste, heat cure epoxy paste, and graphene epoxy paste. The process is validated through a comparative experimental analysis involving tests such as ultrasonic, tensile, microstructural, and hardness to demonstrate its capabilities. The results have been compared with commercially available materials (PLA and Graphene-enhanced PLA) as well as literature to establish the superiority of the CPM process. The CPM composites showed an increase of up to 10.4% in their tensile strength (54 MPa) and 8% in their hardness values (81 HD) when compared to commercially available PLA material. The composites manufactured by CPM have also shown strong bonding between the layers of PLA and thermally activated materials; thus, highlighting the effectiveness of the process. Furthermore, the composites showed a significant increase of up to 29.8% in their tensile strength and 24.6% in their hardness values when compared to commercially available Graphene-enhanced PLA material. The results show that the CPM process is capable of manufacturing superior quality plastic composites and can be used to produce products with bespoke properties.

Graphical Abstract

1. Introduction

Composites have revolutionized the manufacturing sector due to their ability to manufacture products with tailored properties. Composites manufactured via the Additive Manufacturing (AM) process offer design flexibility and manufacturing freedom. The use of AM technology has given rise to the development of plastic-based composites due to their ease of operation. Thermoplastic materials are widely used in the production of composites due to their availability and cost-effectiveness. They are pre-processed in the form of filament and used in AM processes such as Fused Filament Fabrication (FFF) [1,2,3,4]. This process is based on extrusion technology and is one of the most commonly used AM methods in the market. Polylactic acid (PLA) and Acrylonitrile butadiene styrene (ABS) are the most commonly used thermoplastics for the FFF process [5,6,7,8,9]. Thermoplastics are polymer materials that become pliable or moldable at a certain elevated temperature and solidify upon cooling. FFF parts are heavily influenced by their process parameters such as printing/bed/nozzle temperature, infill pattern/percentage, print speed, etc. [10,11,12,13,14]. Furthermore, FFF manufactured parts show superior mechanical properties after post-process operations such as annealing, sanding, polishing, epoxy coating, etc. However, the post-treatment processes result in longer lead times, and there is still uncertainty in achieving consistent properties for wider applications [15,16,17]. Annealing is one of the most commonly used post processes for FFF parts as they tend to show voids, air gaps, and cracks due to the nature of the process. Annealing helps in reducing these defects and relieving internal stresses to enhance mechanical strength. However, annealing is a time-consuming process and can also affect the dimensional tolerances (shrinkage and expansion) of the manufactured parts [18,19].
FFF has also been widely utilized for composite manufacturing due to its ease of operation and modifications in software/hardware. Over the years, different materials have been developed to leverage the ease of operation offered by FFF through the insertion of additives or metal powders. Such composite thermoplastic materials can offer better results. Examples include the addition of copper powder and graphene nanoplatelets to manufacture filaments for FFF [20,21,22,23,24,25]. PLA is a biodegradable thermoplastic and is a popular choice for such composite filaments. However, interlacing thermoplastics with nanomaterials involves various pre-treatment techniques with a long lead time before a filament is manufactured and can be used for functional purposes [26,27]. In this context, Ramirez et al. [28] developed a new composite material using FFF and the robocasting process to manufacture parts for the medical industry. The composites were made using PLA and epoxy adhesives and were cured at 23 °C–60 °C for 4 h, resulting in longer lead times. Maurel et al. [29] utilized Fused Deposition Modelling (trademark or Stratasys, but similar to FFF) to obtain a graphene/PLA composite using Dichloromethane (DCM) as a solvent to dissolve PLA and mix it with graphite. The process started with the formulation of a solution for tape casting and ended with the extrusion process. The obtained film was extruded as a filament and the printed composite was used as the anode material for lithium batteries with 60–70 wt.% of graphite. The process blended the materials to form a filament for fabrication, which is a time-consuming process. Jakus et al. [30] developed a 3D-printable graphene composite for biomedical and electronic applications using PLG (polylactide-co-glycolide), graphene powder, and a DCM solvent to obtain a 3D-printable graphene scaffold. The flexible 3D graphene sheets obtained from this technique involved pre-treatment processes such as solvent mixing and the storage of graphene ink for several months. Luo et al. [31] developed a conductive composite material suitable for the FFF process. PLA and multi-walled carbon nanotubes (MWCNTs) were compounded together with different compositions for 3D printing. The process involves pre-treatment processes such as hot air circulation-drying, magnetic stirring, solution mixing, and filament extrusion with a lead time of 14 h. Massara et al. [32] fabricated electronically conductive composites by compounding PLA and Polycaprolactone (PCL) with different compositions of graphene nanoplatelets (10 wt.%, 15 wt.%, 20 wt.%, and 25 wt.%). The mechanical and microstructural properties were compared with samples manufactured from an injection molding process to assess their tensile strength and bonding. The compositions of PLA, PCL, and GNPs were prepared using the melt blend technique and extruded in the form of a filament for the FFF process. The process involved a number of pre-processing techniques to achieve the composite filament, which can take several days, resulting in longer lead times. Similarly, Ferreira et al. [33] manufactured composites with PLA and Ethylene-Vinyl Acetate (EVA) materials using the blend technique. They analyzed the effect of annealing on the crystallinity, tensile strength, impact strength, and Shore D Hardness. The process involved a mixture of two materials for homogeneity, followed by vacuum drying at 60 °C for 24 h. To improve the mechanical properties of the composites, annealing was carried out at 90 °C for 5 h. These examples highlight the limitations of pre- and post-treatment processes in the current methods. Furthermore, epoxy and graphene paste are also popular choices for composite manufacturing due to their properties and ease of use [34,35,36]. However, the limitations are a long curing time and variations for different materials [28,29,30,31,32,33].
The aforementioned discussion highlights the limitations of current methods to manufacture plastic-based composites with bespoke properties. It is evident that existing processes require pre and post-treatment for composite manufacturing, thus increasing the lead time. Therefore, this paper presents a new manufacturing process that reduces such limitations and is capable of manufacturing high-quality plastic-based composites. The proposed process is termed Composite Plastic Manufacturing (CPM) and is an integration of two material extrusion AM technologies equipped with a heat chamber. PLA has been chosen to be interlaced with three different thermally activated materials (graphene-carbon hybrid paste, heat cure epoxy paste, and graphene epoxy paste) to produce composites using the CPM process. The results of CPM composites have been compared with commercially available materials (PLA and Graphene-enhanced PLA) and as well as published literature in the field. The next section (Section 2) explains the CPM process in detail. Section 3 explains the materials and methods used in this study. Section 4 provides a discussion of the results, and conclusions are outlined in Section 5.

2. Composite Plastic Manufacturing (CPM)

2.1. Process Details

Composite Plastic Manufacturing (CPM) is based on the principle of Fused Filament Fabrication (FFF) and works with a combination of two material extrusion AM technologies, i.e., Filament and Syringe Extrusion. A CAD model for a machine based on the process of CPM is shown in Figure 1.
The system is capable of working with both filament-based and thermally activated materials. The two extruders work together with one depositing polymer material and the other depositing thermally activated material. There is no need for a tool head change as the system already has two independent extruders to dispense multiple materials. The system is equipped with a heat chamber that cures the thermally activated materials during the process and eliminates the post-processing steps. It is to be noted that such an integration is not commercially available, which makes the CPM process unique in its operation. Compared to other available methods for the production of plastic-based composites, this process completely eliminates the need for filament production as well as long pre- and post-processes [29,31,32,33], thus resulting in a faster lead time.

2.2. Working of CPM Process

The CPM process starts with a 3D CAD model of the part, and the flowchart of operation is shown in Figure 2. The 3D CAD model (stl format) is sliced into cross-sections using the slicing software called Ultimaker Cura [37] to obtain the number of layers according to the geometry of the part and transferred into the machine for manufacturing. Cura allows users to modify g-codes with ease and has options of different useful settings such as pause at height or layer, which are helpful features for this work. The system activates the heating chamber once the process is turned on and functions according to the defined temperature. The temperature controller displays the inside temperature where the heating element turns on if the process value is lower than the set value. If the temperature is maintained at a set value, then the heating element automatically cuts off through the Proportional–Integral–Derivative (PID) controller. The CAD file has been set up with pauses for the thermally activated material to deposit on PLA plastic at different layer numbers.
The operation starts with printing support material that can be removed after the completion of the build. After the completion of the support material, the system prints the build material in a layer-by-layer configuration based on the part’s geometry. The filament extruder deposits polymer material whereas the syringe extruder dispenses thermally activated material. The layer of thermally activated material has been programmed for set intervals (depending on the number of layers) and the system pauses the filament extruder head and moves it to the home position.
At this stage, the system activates the syringe extruder to deposit the thermally activated material. After the deposition of the thermally activated material, the heating chamber activates at a defined temperature (based on the material being used) to cure the deposited material. This is to ensure that the thermally activated material cures during the printing process thereby reducing the time and post-processing steps required for the manufacture of plastic-based composites. The heat given to the parts during the process helps in relieving internal stresses and achieving good bonding. After each layer, the system checks for layer completion and moves to the other extruder. If the layer is incomplete, then the system prints the required material to achieve the layer thickness. The system prints the build material until it reaches the total number of layers (N) according to the geometry (Figure 2). After the build is complete, the part can be removed and tested based on the user requirements.

3. Materials and Methods

3.1. Materials and 3D Printer

A prototype has been developed for the CPM process as shown in Figure 3. TENLOG® TL-D3 Pro (Shenzhen Tenlog 3D Technology Company Limited, Shenzhen, China) dual extruder desktop 3D printer has been utilized for this process with a building envelope of 300 mm (X) × 300 mm (Y) × 350 mm (Z) and an extruder diameter nozzle of 0.4 mm [38]. The Syringe Extruder has been designed to dispense thermally activated material and integrated with the TENLOG® TL-D3 Pro system. The thermally activated material will be referred to as TA material from here onwards. The two independent nozzles allow the system to manufacture plastic-based composites. The filament extruder utilizes thermoplastics with a diameter of 1.75 mm. The second extruder is equipped with a Luer-lock syringe [39] and a precision dispenser nozzle (diameter of 0.2 mm) [40]. The second extruder utilizes a syringe and motor-driven plunger to extrude TA materials. The heating chamber [41] dimensions are 750 mm (X) × 700 mm (Y) × 900 mm (Z), and it has an operating temperature of 0 °C~200 °C. The ceramic heating element 1 kW (RS Components, UK) was mounted on top of the system with the temperature probe [42]. PID Temperature Controller Unit ITC-100VH (Inkbird Technology Company Limited, Shenzhen, China) integrated with a K-type thermocouple has been used in the heating chamber [43].
Different materials have been used in this work to establish a baseline for comparative analysis. PolyMaker PolyLite™ PLA [44] and Haydale HDPlas® PLA-GNP-A [45] (3D Filaprint, Southend-on-Sea, UK) were used as examples of commercially available materials. They will be referred to as PLA and GPLA, respectively. The PLA filament has a typical density of 1.17–1.24 g/cm3, a glass transition temperature of 61 °C, and a melting temperature of 150 °C. The GPLA filament contains functionalized graphene nanoplatelets of a planar size between 0.3 and 5 μm, which helps in dispersion and bonding within the PLA polymer. The thermally activated materials interlaced with PLA include Graphene-Carbon Hybrid Paste (Dycotec Materials Limited, Swindon, UK) [46] referred to as GCHP and Heat Cure Epoxy ES566 (Permabond Engineering Adhesives Ltd., Hampshire, UK) [47] referred to as HCE. The Graphene-Carbon Hybrid Paste (GCHP) from Dycotec Material has a Viscosity of 2500–6500 cP, and the recommended print speed is 70 mm/s. Heat Cure Epoxy (HCE) ES566 from Permabond® is a single-part epoxy with a viscosity of 60,000–120,000 cP at 20 rpm and can withstand high temperatures for longer periods. Furthermore, Graphene Nanoplatelets (referred to as GNPs) [48] in powder form (Nanografi Nano Technology, Ankara, Turkey) were used to create Graphene Epoxy Paste (referred to as GEP). The GNPs from Nanaografi are 30 μm in diameter, 5 nm thick, with 99.9+ % purity.

3.2. Process Parameters and Configuration of Composites

For material characterization, dog-bone samples and hardness samples were manufactured according to BS EN ISO 527-2: 2012 [49] and BS EN ISO 868:2003 [50], respectively. PLA and GPLA samples were manufactured using the traditional FFF process whereas the plastic-based composites were manufactured using the CPM process. The parts were built at two different print speeds, i.e., PLA at a speed of 60 mm/s and TA material at 30 mm/s. The layer thickness was set to 0.2 mm with a 100% infill pattern of ‘Lines’. The nozzle temperature for PLA and GPLA was kept at 210 °C and the TA material was printed at 0 °C with the build plate temperature kept at 70 °C for all the samples.
The parameters for the nozzle, build platform, and heating chamber were set according to the technical datasheets of the materials. The system automatically activated the heating chamber to cure the TA material. The heat transfer occurred through the convection process where the heat rose upward and circulated in the chamber due to the fan. The heating chamber was maintained at an ambient temperature for PLA, and upon depositing the TA material, the heating element turned on and started to heat up the chamber. The heat chamber temperature was maintained at 100 °C for all the plastic-based composites. The system was programmed to pause at set intervals where the filament extruder moved back to the home position. The syringe extruder then moved to the exact coordinates to print a single layer of TA material on PLA plastic. The thermally activated material was added to PLA without any surface treatment and a total of three samples were tested for each material configuration. Graphene-Carbon Hybrid Paste (GCHP) was used to manufacture PLA/GCHP composites. Heat Cure Epoxy ES566 (HCE) was used to manufacture PLA/HCE composites. HCE and GNPs were used to manufacture PLA/GEP composites with 0.2 wt.% GNPs. The HCE was used to manufacture the PLA/HCE composite as well as the PLA/GEP composite. A single layer of TA material was added at 50% of the overall thickness of the tensile samples. The reason for adding the layer at 50% is to allow for symmetrical load distribution during tensile testing and investigate the bonding between the plastic and TA material through the microstructural analysis. For hardness testing, a single layer of TA material was added at 90% completion of the samples. Research conducted by Butt et al. [51] on the development of Cu/PLA composites shows that the addition of copper mesh closer to the surface of the sample is more effective in assessing its hardness characteristics due to the nature of the indentation test. The same samples were manufactured as with the tensile testing, i.e., PLA/GCHP, PLA/HCE, and PLA/GEP.

3.2.1. Preparation of Graphene Epoxy Paste

GEP was prepared using GNPs (diameter of 30 μm and thickness of 5 nm) and Permabond Heat Cure Epoxy. Figure 4a illustrates the steps involved in the preparation of GEP. Preweighed GNPs (0.2 wt.%) were initially dispersed in HCE manually and mixed for 5 s at room temperature. The mixture was then sonicated using the Ultrasonic Homogenizer Sonicator (Vevor Machinery Equipment Company Limited, Taicang, China) for uniform dispersion. For the sonication process, the tip sonicator was used for homogenous mixing. The system comprises a titanium alloy probe with auto amplitude compensation as shown in Figure 4b. The mixture was sonicated for 60 min with an output power of 80% and 20 kHz frequency [52]. After sonication, the homogenous GEP was carefully loaded into the syringe extruder for composite manufacturing.

3.3. Experimental Testing Methodology

A structured approach was utilized to ascertain the effectiveness of the process and the capabilities of the manufactured composites. Non-destructive testing was the first step as it allows one to assess materials (defects, cracks, and voids) without destruction. In this context, an ultrasonic test was conducted using Proceq PUNDIT® PL-200 (Screening Eagle UK Limited, London, UK) [53] comprising two transducers (54 kHz) and a measuring resolution of 0.1 μs. Couplant gel was used to facilitate the transmission of ultrasonic sound waves through the samples. The manufactured parts were tested at three different points along their length to obtain an average value. The dog-bone samples were manufactured and tested according to BS EN ISO 527-2:2012 [49] on the Universal Tensile Testing Machine with a load cell of 5 kN and a cross head speed of 1.5 mm/s. The fractured surfaces, after the tensile test, were analyzed under a tabletop Scanning Electron Microscope (JCM-5000 NeoScope™, JEOL UK Limited, Welwyn Garden City, UK) [54] to investigate the fracture mechanism, stress relief, and interaction of different TA materials with PLA. The analysis was performed to investigate the bonding between layers of polymer and other TA materials. The hardness test samples were manufactured and tested according to BS EN ISO 868:2003 [50] on the Shore D Durometer [55] to measure the hardness of the samples. Indentation hardness was carried out on the manufactured samples as it measures the resistance of material through the applied load from a sharp object. The hardness was measured on five different points to obtain the average hardness value.

4. Results and Discussions

4.1. Ultrasonic Testing

Work conducted by Butt et al. [56,57,58] has shown the effectiveness of ultrasonic testing (UT) in assessing defects in thermoplastic and composite products. The rationale for conducting UT is to assess whether the part manufactured by CPM has reduced the internal stresses and air gaps/voids and lowered the transmission time. As this is a non-destructive test, it helps in analyzing the compactness of the deposited layers and the bond quality of plastic and TA material. The time taken for sound waves to travel from one transducer to the second transducer through the test piece was measured and the results are shown in Figure 5. The results showed that the time taken for sound waves to travel through PLA was 1.9 μs and 2.0 μs for GPLA. The lowest transmission time of 1.8 μs was observed for all the plastic-based composites (PLA/GCHP, PLA/HCE, and PLA/GEP). The results clearly show that the addition of TA materials to PLA did not adversely affect the compactness of the deposited layers and helped in reducing the internal stresses and air gaps as well. This is a good indication of the effectiveness of the CPM process in manufacturing high-quality plastic-based composites.
To provide a comparison with other works in the field, we consider a popular post-process for FFF parts called annealing. Several researchers have utilized annealing to enhance the properties of FFF-printed thermoplastics [59,60,61]. Butt and Bhaskar [57] investigated the effects of annealing on the mechanical properties of FFF-printed thermoplastics at different temperatures. They used polymeric materials (PLA and ABS) and metal-infused thermoplastics, i.e., copper-enhanced PLA (Cu-PLA) and aluminum-enhanced ASA (acrylonitrile styrene acrylate) in their work. The results show that the time taken for sound waves to travel in each material differs, and lower transmission times were observed for the annealed samples when compared to un-annealed samples. Cu-PLA samples showed the lowest average values of 3.32 μs (annealed at 80 °C). Cu-PLA parts annealed at different temperatures can be used as an example of post-processed materials from their work. It is to be noted that the increase in transmission time means layers are not closely bonded and there are gaps affecting the weld zones between the printed layers. To achieve high-quality composites, the transmission times should be lower, and upon comparison of the annealed Cu-PLA samples with the composites made by CPM, it becomes evident that CPM composites exhibit lower transmission times (fewer voids/gaps) and consistent results as shown in Figure 6.

4.2. Tensile Testing

A tensile test was carried out on the manufactured samples to investigate the fracture load values and bonding strength of plastic composites. The samples were tested, and the stress–strain curve and maximum values have been plotted for comparison in Figure 7a,b. PLA and GPLA showed an average load of 1968 N and 1674 N, respectively. PLA/GCHP composites showed an average load value of 2173 N with an increase of 10.4% compared to PLA and a 29.8% increase from GPLA samples. PLA/HCE showed an average load value of 1830 N with a decrease of 7% from PLA. However, when compared to GPLA samples, PLA/HCE showed an increase of 9.3%. PLA/GEP composites showed an average load value of 2103 N with an increase of 6.85% compared to PLA and 25.6% compared to GPLA samples. As discussed in Section 4.1, CPM plastic composites showed the lowest transmission times when compared to PLA and GPLA. This indicates the compactness of the deposited layers, also evidenced here by the increased fracture load values. Both PLA/GCHP and PLA/GEP composites showed a significant increase in their fracture load values when compared to PLA and GPLA samples due to the addition of TA materials. The composites form bonds where the TA materials are bonded strongly to PLA due to the presence of heat throughout the manufacturing process. The heat also relieves internal stresses in the PLA layers, and the cumulative effect of stress relief, as well as stronger layers of TA materials, help in enhancing the strength of the composites made by CPM. Furthermore, all the tested samples exhibited brittle failure without any deformation or necking before the fracture. The composites showed a significant increase in their tensile strength as shown in Figure 7a,b. PLA and GPLA showed a tensile strength of 49.205 MPa and 41.85 MPa, respectively. All the composites showed increased tensile strength when compared to PLA and GPLA samples. PLA/GCHP composites showed a tensile strength of 54.325 MPa with a strain value of 6.3%. PLA/GEP showed a tensile strength of 52.575 MPa with a strain value of 4.9%. PLA/HCE composites showed a lower strain value of 4.6% when compared to PLA/GCHP and PLA/GEP composites. These values are higher than the commercially available PLA and GPLA due to the presence of TA material within the composites.
To further substantiate the effectiveness of the CPM process, the tensile test results are also compared to the literature where post-processing has been undertaken on materials. Research conducted by Butt and Bhaskar [57] showed that the samples annealed under different temperatures yield higher fracture load values compared to un-annealed samples. However, annealing can affect the dimensional tolerances of FFF-manufactured parts (shrinkage or expansion) and is a time-consuming process.
A comparison between CPM composites and Cu-PLA composites (annealed at different temperatures for one hour and cooled for two hours [57]) is shown in Figure 8. It is evident that CPM plastic composites show a significant increase in their tensile strength when compared to Cu-PLA annealed samples. Cu-PLA showed an increase in tensile strength after annealing (at 70 °C, 80 °C, and 90 °C) and a maximum fracture load value of 750 N was observed at 90 °C. The CPM process eliminated the requirement of annealing (saving time) and showed a significant increase in the fracture load values of all the composites, thus highlighting its effectiveness.
Another comparison where composites were made by researchers using different materials and mixing them together to be used in FFF is provided in Figure 9. Masarra et al. [32] fabricated the PLA/PCL/GNP composites using the FFF technique to investigate the thermal, mechanical, and electrical properties. The three materials, PLA, Polycaprolactone (PCL), and GNPs (at different weight percentages; 10%, 15%, 20%, and 25%) were compounded together through various techniques to manufacture the filament. The results show that the addition of GNPs to PLA and PCL material reduced the tensile strength when compared to PLA/PCL composites. This could be due to the addition of GNPs to the PLA/PCL matrix, which initiates cracks, forms agglomerations and sharp edges during the filament extrusion process, and reduces tensile strength. The bonding or adhesion is another important factor that affects the tensile strength of composites [32,62]. CPM plastic composites do not require post-processing as they cure during the manufacturing process. GNPs were loaded carefully with HCE to obtain a GEP concoction and ultrasonicated for uniformity as shown in Figure 4a,b. The two composites of interest here are PLA/GCHP and PLA/GEP as they contain GNPs. PLA/GEP composites showed higher tensile strength when compared to PLA/PCL/GNP composites. The addition of GNPs to HCE increased the tensile strength of PLA/GEP composites. The PLA/PCL composite has the highest tensile strength of 47.4 MPa. On the other hand, both PLA/GCHP and PLA/GEP manufactured by the CPM process show an increase of 13.9% and 11.8%, respectively, in their tensile strength when compared to PLA/PCL composites. It is clear from the comparison that the CPM process can provide higher tensile strength due to superior bonding between the layers created during the process as opposed to mixing materials to create filaments.

4.3. Microstructural Analysis

The microstructural analysis supported the results obtained through ultrasonic and tensile testing. The fractured samples were examined under SEM to observe the cracks and voids of the PLA and GPLA samples and the bonding quality of PLA/GCHP, PLA/HCE, and PLA/GEP samples. The microstructural analysis was conducted to investigate whether the layers are bonded tightly without any voids. As PLA is a brittle material, all the tested samples showed a brittle failure with no necking or deformation. It is to be noted that porosity and voids are common in FFF-printed parts, and they can be observed through microstructural analysis [63,64]. However, the focus here is to observe the bonding mechanism and whether voids are present between the PLA and TA materials that could adversely affect their bonding.
Figure 10 shows the SEM fracture interfaces where the voids have been highlighted for all the tested samples. PLA (Figure 10a) and GPLA (Figure 10b) showed some voids, which is to be expected in the FFF-printed parts. PLA/GCHP showed uniform bonding between the layers as evident in Figure 10c with a few voids. This is supported by ultrasonic and tensile testing where lower transmission times and higher fracture load values were observed. The voids can be clearly seen in the PLA/HCE composite (Figure 10d) where the bonding is not uniform, therefore resulting in empty areas between the layers and causing the degradation of mechanical properties. On the other hand, the PLA/GEP composite showed inconsistent bonding along the length of the layer with some voids as shown in Figure 10e. This indicates the need to optimize the process parameters. However, it still proves the concept and validates the working mechanism of the CPM process. PLA/GCHP and PLA/GEP showed good bonding between the layers and resulted in high tensile strength. Improving the bonding between the layers will reduce the voids and will be achieved in future works with the optimization of the process parameters.

4.4. Hardness Testing

Hardness testing was conducted to determine the hardness of the composites manufactured by the CPM process. The layer of TA material was added to PLA at 90% completion. The samples were tested and the results are shown in Figure 11. As evident from the results, the composites manufactured by the CPM process showed an increase when compared to PLA and GPLA samples. PLA and GPLA showed hardness values of 75 HD and 65 HD, respectively. All the composites showed increased hardness values. PLA/GCHP composites showed a hardness value of 76 HD with an increase of 1.3% from PLA and 16.9% from GPLA. PLA/HCE composites showed a hardness value of 77 HD with an increase of 2.6% from PLA and 18.4% from GPLA. The highest hardness value was observed for the PLA/GEP composite. PLA/GEP showed a hardness value of 81 HD with an increase of 8% from PLA and a 24.6% increase from GPLA. The composites showed higher hardness values when compared to PLA and GPLA. This shows that the layers are properly bonded and packed closely, thereby creating strong reaction forces during the indentation. The reason for the high hardness values can be attributed to the consistent resistance offered to the indenter from the entire structure due to the presence of the thermally activated material. A material experiences tension and compression forces upon the application of load, and subsequently, reaction forces are generated due to the material properties. In the case of plastic-based composites, the additional TA materials also experience tension and compression but provide stronger reaction forces as shown in Figure 11, which lead to higher hardness numbers [51].
To further analyze the effectiveness of the CPM process, the hardness test results are compared to the work conducted by Ferreira et al. [33] where they developed a PLA/EVA (Ethylene Vinyl Acetate) composite and investigated the effects of annealing carried out at 90 °C for 5 h. PLA and EVA material were mixed together at different compositions and extruded through a double-screw extruder as a PLA/EVA blend. The samples were then manufactured using an injection molding machine (Arburg Model Allrounder 207C Golden Edition) and annealed at 90 °C for 5 h. The hardness value was measured through Shore D Durometer for annealed and unannealed samples. The annealed PLA/EVA 80/20 composite showed the highest hardness value of 69 HD. The comparison of the yield of PLA/GCHP and PLA/HCE composites manufactured by the CPM process shows an increase of 10.14% and 11.59%, respectively. Furthermore, PLA/GEP showed a significant increase of 17.39% when compared to the annealed PLA/EVA 80/20 composite. Figure 12 shows the comparison of hardness values between PLA/EVA with CPM plastic composites. It is evident from the comparison that CPM plastic composites can provide higher hardness values and hence demonstrate the effectiveness of the process.

5. Conclusions

The paper has presented a new manufacturing process for plastic-based composites called Composite Plastic Manufacturing (CPM). A detailed description of the CPM process, materials, configuration of composites, and experimental methods utilized in this work have been provided in Section 2 and Section 3 followed by the results and discussions of the composites in Section 4. Using the principles of the CPM process, different plastic-based composites were manufactured to prove their effectiveness and capabilities. All the samples were manufactured and tested according to British and International Standards. The experimental results show that the CPM process is capable of manufacturing high-quality plastic composites with bespoke mechanical properties. The manufacturing parameters and heat chamber temperature play critical roles to ensure bonding integrity and enhance mechanical properties. In future works, multiple layers of thermally activated materials will be interlaced with plastic at set intervals for further analysis.
The following conclusions were drawn from this research work to prove the effectiveness and capabilities of the CPM process:
  • The ultrasonic test showed that the addition of GCHP, HCE, and GEP to PLA did not adversely affect the compactness of the deposited layers.
  • The tensile test showed positive results for the PLA/GCHP, PLA/HCE, and PLA/GEP samples and exhibited strong bonding between PLA and TA materials. This shows that the CPM process is capable of manufacturing plastic-based composites with superior mechanical properties using different TA materials consistently.
  • The microstructural analysis also supported the ultrasonic and tensile testing results by showing fewer voids in samples that showed lower transmission times and higher tensile strengths.
  • The hardness test showed positive results on composites manufactured by the CPM process. All the composites showed increased hardness values when compared to PLA and GPLA samples. This shows that the layers are uniformly packed with each other and create strong reaction forces, thereby resulting in higher hardness values.

Author Contributions

Conceptualization, R.B. and J.B.; methodology, R.B. and J.B.; validation, R.B., J.B. and H.S.; formal analysis, R.B. and J.B.; investigation, R.B. and J.B.; resources, R.B. and H.S.; data curation, R.B.; writing—original draft preparation, R.B.; writing—review and editing, R.B. and J.B.; visualization, R.B.; project administration, R.B., J.B. and H.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research did not receive any external funding.

Data Availability Statement

The data used in this research work can be made available upon request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. CAD model of composite plastic manufacturing process.
Figure 1. CAD model of composite plastic manufacturing process.
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Figure 2. Flowchart of CPM process.
Figure 2. Flowchart of CPM process.
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Figure 3. Composite plastic manufacturing process.
Figure 3. Composite plastic manufacturing process.
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Figure 4. Preparation of graphene epoxy paste: (a) Steps involved in preparation of GEP; (b)ultrasonic processor.
Figure 4. Preparation of graphene epoxy paste: (a) Steps involved in preparation of GEP; (b)ultrasonic processor.
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Figure 5. Results from ultrasonic testing.
Figure 5. Results from ultrasonic testing.
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Figure 6. Results of annealed Cu-PLA with CPM plastic composites.
Figure 6. Results of annealed Cu-PLA with CPM plastic composites.
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Figure 7. Comparative tensile test results: (a) Results from tensile testing with standard deviation as error bars; (b) stress–strain curve.
Figure 7. Comparative tensile test results: (a) Results from tensile testing with standard deviation as error bars; (b) stress–strain curve.
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Figure 8. Results of annealed Cu-PLA with CPM plastic composites.
Figure 8. Results of annealed Cu-PLA with CPM plastic composites.
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Figure 9. Results of PLA/PCL/GNP composites with CPM plastic composites.
Figure 9. Results of PLA/PCL/GNP composites with CPM plastic composites.
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Figure 10. SEM fracture interfaces: (a) PLA; (b) GPLA; (c) PLA/GCHP; (d) PLA/HCE; (e) PLA/GEP.
Figure 10. SEM fracture interfaces: (a) PLA; (b) GPLA; (c) PLA/GCHP; (d) PLA/HCE; (e) PLA/GEP.
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Figure 11. Results from hardness testing.
Figure 11. Results from hardness testing.
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Figure 12. Results of PLA/EVA composites with CPM plastic composites.
Figure 12. Results of PLA/EVA composites with CPM plastic composites.
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Bhaskar, R.; Butt, J.; Shirvani, H. Experimental Analysis of Plastic-Based Composites Made by Composite Plastic Manufacturing. J. Compos. Sci. 2022, 6, 127. https://0-doi-org.brum.beds.ac.uk/10.3390/jcs6050127

AMA Style

Bhaskar R, Butt J, Shirvani H. Experimental Analysis of Plastic-Based Composites Made by Composite Plastic Manufacturing. Journal of Composites Science. 2022; 6(5):127. https://0-doi-org.brum.beds.ac.uk/10.3390/jcs6050127

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Bhaskar, Raghunath, Javaid Butt, and Hassan Shirvani. 2022. "Experimental Analysis of Plastic-Based Composites Made by Composite Plastic Manufacturing" Journal of Composites Science 6, no. 5: 127. https://0-doi-org.brum.beds.ac.uk/10.3390/jcs6050127

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