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Abstract

The mechanical properties of hybrid composites prepared by 3D-printing Polylactic Acid (PLA) injected with a syringe with a compound comprising 35% walnut shells and epoxy resin are evaluated in the current research and aligned with the Sustainable Development Goals (SDGs) attentive to responsible consumption and responsible production (SDG 12) and climate action (SDG 13). The study encourages recycling of agricultural waste in order to enhance sustainability in production. There are nine samples that are formed with different design parameters, such as the printing angle (450, 50, 55, 20), infill density (10, 15, 20), and the number of wall thickness (1, 2, and 3) each wall is 0.4 mm thick. Tensile tests, flexural tests, and impact tests are mechanical tests that are conducted to test the characteristics of the synthesized samples. The results indicate that model (L5 has (angle of print 50°, Infill percentage 15%, and three walls)) attains a maximum tensile strength of 30.9 MPa, which is a 31% reduction compared to the pure PLA. Specifically, this is an improvement of 37% than the epoxy and 35% walnut shell composite. Moreover, the L5 displays an elongation at break of 4.5%. Model L7 (angle of print 55°, Infill percentage 10%, and three walls) achieves a maximum flexural strength of 42.5 MPa and elastic modulus of 2147.6 MPa. This is corresponded to reductions of 24% and a 46% if compared to pure PLA. When the maximum flexural strength is compared to the sample of the walnut and epoxy composite, this increases by almost 73%. Checking the highest impact strength of 5 kJ/m², this is possible in case of model L5, which is a 27% decrease as compared to pure PLA, yet 21% higher than the walnut and epoxy composite. The Taguchi method is employed to make a systematic evaluation of the contribution of each variable. This indicated that the wall thickness is the main factor influencing tensile and flexural strength, while printing angle is the key factor affecting impact test. The multi-criteria decision-making (MCDM) approach is incorporated to weight the relevant mechanical properties. This shows different weight factors of 33% for impact strength, 33% for flexural strength, and 34% for tensile strength. Clearly, model L5 is found to be the optimal model following this analysis, presenting the potential for sustainable manufacturing practices that support SDGs by combining waste materials into high-performance products.

Keywords

3D-printer polylactic acid walnut shell/epoxy sustainable development goals mechanical properties Taguchi technique

Introduction

Recently, 3D printing technology has attracted significant interest from researchers globally. 3D printing creates prototypes of functional parts without requiring the final assembly process of physical models. 3D printing has expanded to build faster, less expensive models and enable complex engineering models based on a digital design to reduce material waste, which promises to bring new technologies in many important industrial fields to produce composite materials such as energy, biotechnology, medical devices, and others.¹ Manufacturing engineering models using 3D printing of a matrix of polymeric materials, metals, and ceramics have excellent mechanical, thermal, and electrical properties when compared to other engineering products manufactured by conventional methods.² The combination of different materials and printing technologies have drawn the intention of many researchers, expanding the possibilities to be used in several practices.³ Hodásová et al.⁴ experimentally investigated multiple interconnected phases of materials, a matrix, and a hollow structure to create a 3D design with two connected components that exhibited exceptional topological bonding, referred to as Interpenetrating Phase Composites (IPCs). In this regard, the epoxy resins stand out among the materials used for their strength and chemical resistance, making them ideal for multiple applications.⁵ Particularly, the polymer adhesives based on Epoxy resin have a hybrid microstructure that enhances their strength and durability, as well as load bearing capacity, in addition to increasing the characteristics of the involved phases. Nevertheless, even in the case of 3D-fabricated structures, their strength and stability are not strong enough and, that is why, there is an open search of innovative solutions to eliminate the problem. An example is given by Dahmen et al.⁶ who bonded composite T-joint structures with the 3D printing technology using double-cured epoxy. The epoxy printed joints were found to have adequate bonding power to the bonding technique that was used in the past. Alarifi⁷ performed a tensile experiment on a geometrical representation of polyethylene terephthalate glycol (PETG) composite material that had been reinforced by short carbon fibers (CFs). This showed an enhanced yield strength than the traditional manufactured structures and minimized the use of material, wastage as well as processing time.

Al-Arkawazi et al.⁸ conducted an experiment to determine the influence of adding percentages of walnut shells to polycarbonate as a type of composite wood-plastic, which is produced through extrusion. The theorists showed how tensile and flexural strength of the composites with 20%, 30%, and 40% walnut shells increased. They also produced negative effects of polycarbonate on the mechanical properties as a result of stress concentration particularly the impact strength of those that had more than 20% walnut shells in the composite. Thus, it was mentioned that the mechanical properties of the wood-plastic composites can be enhanced by adding 20% of walnut shells.

The greatest difference between the technology of 3D printing and the traditional production means is the vast range of materials and their constant advancement. This makes it possible to create complicated and complex models that cannot be created through the traditional processes. Therefore, choosing the right materials is crucial for achieving the desired results.⁹ However, it should be noted that 3D printing technology has been elaborated as a complementary tool to enhance technological advancement in various industries, such as medicine, aerospace, automotive, and others.¹⁰

Several researchers were conducted successful studies in the field of 3D printing. For example, Cortés et al.¹¹ developed 3D-printed conductive circuits using epoxy ink with the support of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs). Using 3D printing, Ming et al.¹² optimized the manufacturing process for continuous fiber-reinforced thermo-setting polymer composites. The results obtained the optimal values of a number of parameters including the printing speed and curing pressure, resulting in samples of 58 wt.% fiber content with an optimal maximum flexural strength of 952.89 MPa and an optimum flexural modulus of 74.05 GPa, which expands their potential applications in the aerospace and defense industries. Wang et al.¹³ found that the flexibility of epoxy acrylate materials used in 3D printing was enhanced to meet the demands of clothing accessories. Their results elucidated that modifying the epoxy with polyethylene glycol can specifically improve flexibility, resulting in a tensile strength of 8 MPa and an elongation of 30%. A combination of the epoxy resin and polylactic acid (PLA) was introduced by Kiattipornpithak et al.¹⁴ using melt mixing. The tensile strength was raised by 12% with the process and the elongation at break was enhanced by 67 MPa. The study by Zhang et al.¹⁵ employed the use of the 3D printing technology to develop carbon fiber-reinforced epoxy composites through the epoxy resin being melted to form composite filaments. The findings showed a tensile strength of up to 1372.4 MPa which indicates the efficiency of the fiber path design as it increases the mechanical property of the material. In the study by Brunner et al.,¹⁶ 3D printing and epoxy were also employed to replace tissue in the treatment of particles. The samples exhibited the same ratios of energy being absorbed, and this supported the fact that low-cost phantoms were able to be developed to test the quality. Another study by Tosto et al.¹⁷ resulted in the 3D printed parts having better properties due to the incorporation of bio-based epoxy resins with the commercial resins and enhances performance and sustainability by the fact that non-renewable resources are reduced. Moreover, Senthooran et al.¹⁸ discovered that the incorporation of different inorganic fillers such as mica enhanced the characteristics of Digital Light Processing (DLP) printed samples. It showed that tensile strength and flexibility increased by 85% and 132% respectively using epoxy-acrylate resin.

In a recent work by Khedri et al.,¹⁹ interphase composites (IPCs) with 3D-printed polylastic acid (PLA) samples reinforced with epoxy resin were developed. This was meant to increase the durability and solve the problem of low mechanical strength of the printed materials. The findings revealed that the addition of epoxy resin enhanced both tensile and flexural strength of the samples, tensile strength increased by more than 1000% as compared to samples that were not treated. The microscopic analyses indicated that there was a good attachment between the printed PLA and the layers of epoxy, indicating the possibility of the use of this technology in high-performance 3D-printed composite fabrication.

Although this research existed before, the utilization of epoxy with walnut shell reinforcement in Fused Filament Fabrication (FFF) 3D printing had not been documented in the scientific literature. This study is a novel study because it examines the use of agricultural waste in modern manufacturing technologies, and thus enhancing the environmental sustainability and mechanical character of the printed materials. In order to assess the mechanical characteristics of the constructed models tensile, flexural, and impact tests are carried out on all samples. Subsequently, a Taguchi analysis is applied to plan the experiments and evaluate the combined effect of the different 3D printing parameters, such as the number of walls, infill density, and the printing angle on the mechanical performance of the samples. Multi-criteria decision-making (MCDM) methodology is then implemented to the optimal model that would meet high performance and efficiency, as trends of sustainable manufacturing.

The comparison of recent studies (2021–2025) on 3D-printed composites with the use of PLA, epoxy, and natural additives is provided in Table 1. It dwells on materials used, production process, parameters explored, and major findings to reveal the research gaps filled by this study.

Table 1.

Comparison of recent studies (2021–2025) on 3D-printed PLA/epoxy composites with biofillers.

Year	Reference	Materials/system	Manufacturing method	Key parameters investigated	Main findings	Remarks/novelty gap
2021	Kiattipornpithak et al.¹⁴	PLA/epoxy resin	Reactive blending	Epoxy ratio variation	Improved tensile and impact strength via reactive compatibilization	No use of 3D printing or geometrical factors
2021	Hodásová et al.⁴	Epoxy/ceramic scaffold	3D printing + infiltration	Porosity and infiltration density	Achieved biocompatible and strong ceramic-polymer networks	Not related to PLA or mechanical optimization
2022	Ameen et al.²⁸	Fiber reinforced polymer (FRP) for prosthetic foot	Composite lamination	Fiber type and orientation	Enhanced strength and lightweight prosthetic design	Did not use 3D printing or epoxy systems
2023	Hasanzadeh et al.²¹	PLA/aluminum composite	Fused filament fabrication (FFF)	Layer height, infill, printing speed (Taguchi)	Optimized process parameters for improved strength	Focused on metal reinforcement, not epoxy or fillers
2024	Khedri et al.¹⁹	PLA/epoxy (interpenetrating phase composite)	Additive manufacturing + epoxy infiltration	Infill density variation	Higher tensile and flexural strength with denser infill	Closest to this study; did not examine print angle or wall number
2024	Senthooran et al.¹⁸	Mica/epoxy acrylate	DLP 3D printing	Mica content	Improved mechanical and thermal stability	Used thermoset epoxy, not PLA or fused deposition
2025	Tosto et al.¹⁷	Bio-based epoxy/acrylate blend	LCD 3D printing	Blend ratio and curing	Developed recyclable, sustainable resin system	No structural or printing parameter study
2025	Al-Arkawazi et al.²⁷	PP/cow bone/silicone composite (prosthetic foot)	Injection molding + simulation	Reinforcement ratio	High stiffness and impact resistance with natural filler	Focused on PP system, not PLA/epoxy 3D printing

Material and experimental procedure

Materials

This selected materials of the current research including the matrix phase of the main compound, Sika 52 LP epoxy resin, with hardener (A + B), which were purchased from Sikadur^®-Iraq. This is specifically a thin, low-viscosity epoxy, making it ideal for filling micro-cavities. Moreover, it is also recommended for hot and tropical climates and can be readily accessed from local Iraqi markets, making it an economically viable option.

American walnut shells, used as fillers, were particularly suitable due to their natural properties, which included being free from insect worms and high availability in local markets. Walnut shells are an agro-industrial waste source with the potential to be recycled for use as a filler in resin, thereby enhancing the mechanical properties and promoting the sustainability of the manufacturing process. Moreover, PLA filaments manufactured by Creality Corporation, one of the USA manufacturers for 3D printing parts.

Design of experiments: Taguchi method

The Taguchi method is a statistical methodology for designing experiments and analyzing experimental results in a way that dramatically decreases the number of necessary experiments while retaining the maximum expected benefit from the available data. Reducing interference allows for cleaner results and improves the quality of the obtained results when isolating variables that most significantly impact model performance. This also requires less cost and time which is good for research with minimum funds. In this vein, the L9 matrix was adopted to reduce the number of experiments to be performed without compromising the precise evaluation and analysis.^20–23

The experiments were planned, and the results, as per the Taguchi method, were analyzed to improve the mechanical performance of the samples using a minimum number of experiments, using the statistical software Minitab. This experiment examined three main factors: printing angle, infill density, and number of walls, each with three levels.

The adhesion between the print model and injection material was to be enhanced by low fill ratios. Their effects on the mechanical properties of the samples were studied at different levels of count of the walls. The printing angles were also chosen, according to a previous research showing a direct influence on the performance of the samples in 3D printing.²² Table 2 provides the factors and the corresponding levels that were incorporated in the experimental design whereas Table 3 provides the Taguchi L9 orthogonal matrix that was used to structure the experiments. The sample parameters that were used to print are shown in Figure 1. The printing angle (45, 50, and 55) is defined as the build angle of the internal structure, or the angle of the printed layers with regard to the build platform when 3D printing is undertaken.

Table 2.

Main factors used in the Taguchi method.

No.	Printing angle (°)	Infill flow percentage (%)	Wall thickness
1	45	10	1
2	50	15	2
3	55	20	3

Table 3.

Number of Taguchi experiments.

No.	Printing angle (°)	Infill flow percentage	Wall thickness
L1	45	10	1
L2	45	15	2
L3	45	20	3
L4	50°	10	2
L5	50	15	3
L6	50	20	1
L7	55	10	3
L8	55	15	1
L9	55	20	2

Figure 1.

Sample parameters utilized for printing.

The data that was acquired in the mechanical experiments were analyzed through the Taguchi method where the signal-to-noise ratio (S/N) was determined on each experiment to determine the best conditions. A principle of optimizing the mechanical properties was the adoption of the Larger is better principle, and the S/N ratio was determined with the aid of the following equation:

\frac{S}{N} = - 10 \log \frac{\sum \frac{1}{Y^{2}}}{n}

(1)

$Y$ represents the responses to a given set of factor levels, and n represents the amount of responses to the set of levels.

Samples preparation and methodology

The experiment involved two steps of sample preparation. At the first stage, a mixture of American walnut shell powder and pure epoxy resin was used to make composites in different ratios of 35%, 40%, and 45% to produce composites. The shells of the walnuts were milled in an industrial mill into fine granules which were screened to obtain a uniform size of 250 microns. Air-drying of the granules was done to remove the moisture that had been developed during the grinding process to improve the dispersion of the granules in the epoxy resin.²⁴ The Sika 52 LP epoxy with low viscosity was applied to allow flowability and easily fill small crevices in the molds. The epoxy was prepared according to the instructions of the manufacturer with two parts of resin to one part of hardener, stirred, but slowly so as to reduce the amount of air incorporated during the mixing.

The final result of mixing was then a vacuum process in a vacuum chamber at a pressure of less than 1 bar of the mixture of 5 min to form a homogeneous composition and eliminate air bubbles even more. The mixture was placed in tensile, flexural, and impact testing molds by injecting the mixture into the molds with a 50 ml implant syringe which enabled the mixture to be neatly and correctly placed in the molds. The samples were kept at room temperature in 1 week after being injected and then subjected to mechanical testing. Figure 2 demonstrates the process of a composite preparation and laboratory experiments. The first step of this study as described in Figure 2 will entail mixing of epoxy and walnut shells to form samples in mechanical tests which will include tensile, Flexural, and impact tests. Once the tests are made and the results analyzed, the most ideal combination in terms of mechanical properties is obtained and then employed in the injection process to come up with models using the 3D printing technology.

Figure 2.

Stages of preparing and manufacturing the walnut shell–epoxy composite and the testing methods.

The second phase involved printing of 3D models with 100% of fill on PLA filament. These models were further put under tensile, flexural, and impact tests. The other models were produced based on the experiment characteristics in Table 2.

The outcomes of the mechanical tests during the first phase allowed choosing the composite with 35% of the walnut shell powder as the most suitable formulation in terms of performance. The pre-printed models were then injected with a syringe with this composite.

Figure 3 illustrates the use of the 3D printing device in this research, along with the samples manufactured using this device to conduct three mechanical tests: Tensile, Flexural, and Impact testing, following the parameters listed in Table 2.

Figure 3.

Samples printed using 3D printers.

Figure 4 shows the samples after the injection process and their filling with the composite. In particular, Figure 4 shows how the first and the second phases of the research are combined with each other. The optimal composite (35% walnut shells mixed with epoxy) was chosen and injected into the samples produced at the second stage with the help of a syringe into the samples to get the final samples. The mechanical tests were done and the results analyzed to measure the performance of the composite.

Figure 4.

Injection of the composite into 3D-printed samples.

Mechanical characterization techniques

The mechanical properties of the samples were investigated through a series of standard tests, beginning with the tensile test, which was conducted in accordance with ASTM D638. This test is used to test the longitudinal resistance of plastic materials. The samples were cut in a typical shape of a dog bone with the dimensions of 165 × 13 × 3 mm as shown in Figure 5. The testing was conducted on a Gotech U60 universal testing machine (Taiwan) at the constant loading speed of 5 mm/min. The highest tensile stress as well as the elongation when breaking was recorded.

Figure 5.

The standard tensile test specimen.

Flexural test was also conducted in ASTM D790 whereby the same testing machine was used but at 2 mm/min loading speed. The samples were rectangular shaped and 127 mm in length and 3 mm in thickness (Figure 6). This test aims at examining how the material reacts to the process of bending and also evaluate its resistance to a constant obliquely distributed load.

Figure 6.

Flexural testing specimen.

To determine the impact resistance, a Charpy tester (model CIT2105) was employed to determine the capacity of the material to withstand an impulse impact according to the ASTM D256. Specimens were made with pre-established rectangular size (Figure 7). In the experiment, each specimen was hit by a pendulum at a given point and the amount of energy taken by the specimen at the breakage was determined. This test gave a valid clue of the ability of a material to survive sudden loads which is important in determining whether the material is appropriate in use where there is a need to have resistance to sudden shocks and other dynamic changes.

Figure 7.

Impact testing specimen.

Each compound was tested on three mechanical properties namely tensile strength, flexural strength and impact resistance (3 samples of each). Mean of the three samples was considered the final value and standard deviations (±SD) were determined to make sure that the results obtained were reliable. Also, the error bars have been added to all figures that indicate the value of standard deviation.

Results and discussion

The waste materials in agriculture such as grain husks, sawdust, and plant fibers are usually incorporated in the composite materials as fillers or reinforcement to make the materials more sustainable at a lower cost. As much as these materials are advantageous to the environment and economy, they may at times have adverse impacts on the mechanical characteristics of these composites especially in cases where the bond between the fibers and the polymer matrix is not strong. One of the most significant impacts is the decrease in tensile strength since plant fibers can separate or slide under stress, weakening the material’s capacity to handle loads. Additionally, the flexural and impact properties can be greatly diminished due to the brittleness that arises from weak internal bonding. These issues are typical in many composite systems that use agricultural waste as a reinforcement.^25,26 Due to the novelty of the manufacturing technology used in this research, which relies on incorporating agricultural waste into 3D-printed composite materials, no previous similar studies were found that could be directly compared to this work. Therefore, the comparison in this study was limited to the produced samples and the pure PLA sample only.

Epoxy and walnut shell

In the first phase of this research, three weight ratios of walnut shells (35%, 40%, and 45%) were blended with epoxy resin to create test specimens for tensile, flexural, and impact tests. The objective of this phase was to evaluate the mechanical performance properties and viscosity of each ratio to select the optimal blend for subsequent injection of the composite into 3D-printed samples. As shown in Table 4, the tensile test results indicated that the strength of the pure epoxy sample reached 16.15 MPa. In contrast, the tensile strength of all walnut shell-reinforced samples improved by approximately 17%. However, it was observed that elongation at break decreased as the walnut shell content increased.

Table 4.

Mechanical test results for pure epoxy and composites.

Materials	Tensile strength (MPa)	Elongation at break%	Flexural strength (MPa)	Flexural modulus (MPa)	Impact strength (kJ/m²)
Pure epoxy	16.15	8.57	18.75	339.14	7.19
Epoxy+35%WS	19.54	7.23	20.625	569.59	6.36
Epoxy+40%WS	19.50	6.49	20.625	444.52	5.54
Epoxy+45%WS	19.69	5.70	22.5	654.09	5.54

The flexural strength tests showed that all the samples had a gradual increase in strength of between 9% and 17% with an increase in the content of walnut shell. Also, the flexural modulus rose by up to 48% relative to the pure epoxy sample.

In the impact resistance test, the higher the proportion of walnut shell in the mixture, the lower the resistance; the proportion of 35% mixture resulted in the greatest amount of resistance of the tested samples. The viscosity of the blends was also subsequently measured after the results analysis to determine their appropriateness to injection into complex samples generated using 3D printing. The trials also showed that the mixture with 35% walnut shells was easily flowing, and it was capable of capturing small figures in the end product. Thus, the number of 35% was chosen to supplement the existing research project with the 3D printing technology.

Tensile test

Tensile strength is one of the important mechanical tests that determine the strength with which a material can resist forces applied on it. It provides us with valuable knowledge regarding the strength of material and the extent of its stretchability before it breaks which is important in knowing how the material will behave once stressed.²⁷

Following an experiment on 10 samples produced using 3D printers, a sample was printed in pure PLA, in 100% infill and no additives. Figure 8 presents the changes in tensile strength of all the samples that were 23.46–30.94 MPa. On this regard it can be observed that the efficiency was lost by approximately 31% when the maximum tensile strength values were compared between Sample L5 and pure PLA sample at a 100% infill density. Moreover, a tensile strength enhancement of about 37% was attained by contrasting the present findings with a hybrid sample consisting of 35% of walnut shell and epoxy resin injected into the 3D printed samples. Figure 8 indicates as well that there are considerable variations in the percentage of elongation at break in all models. The maximum elongation is 4.54% and the minimum is 3.67 with L5 compound showing the maximum elongation. Compared to the pure PLA material, efficiency is reduced by 9%. The performance is however reduced by about 37% as compared to that of the walnut-epoxy composite. Such a decrease in tensile strength and elongation at break can be specified by the fact that the interior structure of the product contains the walnut shells. These additives affect tensile strength and enhance brittleness and cause a decrease in break elongation.

Figure 8.

The outcomes for tensile strength and the percentage of elongation at break.

Figure 9 reveals the variation of the stress-strain curves of the nine models, comparing them with a pure PLA specimen and epoxy composite with 35% walnut shells. The epoxy composite that is printed on walnut shells has a greater strain magnitude, and the pure PLA sample has a greater stress.

Figure 9.

Stress-strain curves.

Figure 10 shows failure modes of tensile specimens after testing which displays the fracture behavior and nature of the material when tensile loads are applied to it.

Figure 10.

Tensile specimens after the test and failure.

After conducting a Taguchi analysis, Signal-to-Noise Ratios, and the “Larger is Better” approach, the tensile strength, it was found that the number of walls is the most influential factor among the various parameters studied (Figure 11). Specifically, using three walls produced the best results. Conversely, an apparent negative effect has been revealed by the gradual and linear decrease in the mechanical performance with the reduction in the walls to two or one. The second factor was found to have the most impact and that is the fill ratio and the ratio of 15% produced the highest tensile strength. Comparatively, ratios of 10% and 20% led to poor performance. In terms of the printing angle, it was least affected on tensile strength. The best results were attained with the 50° angle when compared to the 55° angle as there was no significant effect. On the other hand, tensile strength was adversely affected by the 45° angle.

Figure 11.

SN ratio for tensile strength.

Flexural test

The outcome of flexural tests showed that flexural strength and flexural modulus of PLA reduced significantly after the addition of walnut shells and epoxy. This is an indication of a lower stiffness and a lower capacity of the material to resist loads without deforming or breaking. Figure 12 shows the flexibility strength and flexural modulus of the samples experimented. The flexural strength was detected to lie within the range of 32.5–42.5 MPa and this implies that the efficiency dropped by nearly 24% relative to pure PLA. It however increases by about 52 when compared to the walnut shell and epoxy composite. In the meantime, the flexural modulus was in the range of 1470.59–2147.53 MPa which showed that the efficiency had been decreased by 46% in comparison with the PLA pure sample. On the contrary, it demonstrates a growth of about 73% in comparison to the walnut and epoxy composite.

Figure 12.

Results of flexural strength and flexural modulus.

The common failure modes of flexural specimens after testing are demonstrated in Figure 13, and they also reflect the fracture characteristics brought about by loads exerted in the process of bending. These pictures are offered to facilitate the discussion and assist one to visualize the mechanical failure that each form of test is linked with thereby contributing to the knowledge of the fracture mechanisms in the composites that were studied.

Figure 13.

Bending samples after the experiment and failure.

Based on the analysis of the concomitant data by the Taguchi method, the Signal-to-Noise Ratios, and the Larger is better methodology, one can note that the wall thickness is the most significant of them all (Figure 14). Samples that were three-walled performed better compared to those with two or one wall. The impact of the wall thickness reduced progressively with the reduction in the number of walls and the single wall design exhibited a negative influence. The second factor that had the most influence was the printing angles with 55° effective. Conversely, 50° and 45° had an adverse impact on performance. Regarding fill ratio, the best fill ratio was 10% then 20%. Nonetheless, a fill ratio of 15% was showing negative effect.

Figure 14.

SN ratio for flexural strength.

Impact test

Impact testing is a significant mechanical test that is to examine the aptitude of a material to take in impulse in the occasion of abrupt loads, which is an indication of strength and capacity to break fragmentation. Here, the findings between PLA with filler of walnut shell and epoxy revealed a drastic drop in impact resistance than when using pure PLA which signifies the influence of the additives on lowering the impact resistance of the composite to face abrupt loads. As shown in Figure 15, pure PLA shows a high impact resistance than models with walnut shells and epoxy as internal fillers. The values of impact resistance were 5.25–3.46 kJ/m². It means that performance reduced by approximately 27% in comparison to the pure material without additives. Comparatively, the values of the impact resistance were however enhanced by about 21% as compared to the walnut and epoxy composite.

Figure 15.

Results of the impact strength.

Figure 16 shows the specimens after impact testing as a way of identifying the fractured areas and deformations that result after energy applied in the impact test. These pictures are provided to demonstrate the mechanical failure of the test specimens, and thus, improve the visual perception of the process of the fracture in the composites of interest.

Figure 16.

Impact samples after testing and their subsequent failure.

It is possible to note the effects of different parameters on the impact resistance of the tested models after analyzing the data aided by the Taguchi technique, signal-to-noise ratios, and the bigger is better method (Figure 17). The printing angle was the parameter that had the greatest impact over the rest of the parameters with the best results being achieved at a printing angle of 55°. This was followed by the 50° and the negative effect was the 45°. Also, it shows a linear relationship, with a decline in the effect with a decrease in the angle. The infill percentage that was the second most influential parameter. The infill percentage of 10% gave the most favorable results followed by 15% whereas an infill percentage of 20% had the opposite impact on the results. Lastly, the impact resistance was least affected by the third parameter, which is the wall thickness. Two walls appeared to be the best configuration of the experiment compared to one wall, and three walls affected the results negatively.

Figure 17.

SN ratio for impact strength.

Multiple-criteria decision-making (MCDM)

The analyzed findings have revealed that the Multi-Criteria Decision Making (MCDM) method is a reliable and objective technique to classify composite models and select the best model with the consideration of three important mechanical properties in the context of 3D printing of PLA flexural strength, tensile strength, and impact resistance. All criteria were based on the principle of larger is better, and the data were normalized using equation (2) to ensure a consistent scale ranging from 0 to 1.

N_{i} = \frac{X_{i} - X_{\min}}{X_{\max} - X_{\min}}

(2)

$N_{i}$ indicates the normalized value for criterion $i$ ,

$X_{i}$ denotes the original value for criterion $i$ ,

$X_{\max}$ and $X_{\min}$ show the dataset’s minimum and maximum values for criterion $i$ .

To ensure a fair evaluation that considers the significance of each property, relative weights to each criterion were assigned as follows: 34% for tensile strength, 33% for flexural strength, and 33% for impact strength. These weights can be adjusted in the future based on specific applications. The overall score for each model was then calculated using equation (3),

S_{i} = \sum_{j = 1}^{n} W_{j} N_{ij}

(3)

$S_{i}$ denotes the final score for model i,

$W_{j}$ indicates the weight assigned to criterion j,

$N_{ij}$ shows the normalized value for criterion j in model i.

Evaluation results show that Model L5 had the highest scores among all the models on three mechanical performance criteria. The achievement serves as a proof-of-concept that supports the use of agricultural waste, such as walnut shells, as an environmentally friendly filler. This would significantly reduce manufacturing costs, but more importantly, it would enable an environmentally sustainable solution while maintaining the same possible nature of the base material, PLA. Consequently, their application potential certainly remains in several engineering sectors. The results obtained and the analysis done using the MCDM method are presented in Table 5.

Table 5.

MCDM techniques results.

Model	Flexural strength (MPa)	Impact strength (MPa)	Tensile strength (MPa)	Normalized flexural strength	Normalized impact strength	Normalized tensile strength	Final score
L1	33.750	4.656	24.808	0.794	0.887	0.802	0.821
L2	35.000	3.463	26.538	0.824	0.659	0.858	0.750
L3	36.875	3.660	27.404	0.868	0.697	0.886	0.787
L4	35.625	4.255	26.178	0.838	0.810	0.846	0.826
L5	38.750	5.058	30.938	0.912	0.963	1.000	0.959
L6	33.750	3.463	23.462	0.794	0.659	0.758	0.718
L7	42.500	3.864	27.380	1.000	0.736	0.885	0.839
L8	32.500	4.255	25.817	0.765	0.810	0.834	0.805
L9	39.375	5.253	26.322	0.926	1.000	0.851	0.944

Manufacturing process based on optimized parameters

3D printing technologies are widely used in education, engineering, and prosthetics, as printed models effectively communicate theoretical concepts to students.²⁸ In this paper, a human foot model was used as an example for illustration, education, and demonstration. The Taguchi analysis results identified the optimal parameters (as L5 Model), which were ultimately selected for the final manufacturing step. These parameters were set to a 15% fill rate, a 50° printing angle, and a three-layer wall thickness. The upper and lower foot parts are sourced from a foot model specifically selected for use in manufacturing processes. Both parts were then impregnated with a 35% epoxy-walnut shell powder mixture. With these parameters, it took about 53 min to print both parts, used 6.24 m of filament, and weighed 18 g of plastic. Comparatively, the identical model with a fill ratio of 100% took 2.5 h to print covering 15.63 m of filament with a total weight of plastic used of 46 g. The findings were that, with an optimum parameter, material wastage was reduced and time taken in the production process of the model shortened without affecting the outer look of the model produced. This strategy facilitated the shift to sustainable production by decreasing waste, time, and energy with significant losses to the structural performance of the prototype. The development of a foot model to use in education is depicted in Figure 18. Step (a) entails printing the model in two sections. On stage (b), the walnut shell powder/epoxy solution is injected in every part of the part. Finally, stage (c) took a combination of the two to produce a complete foot model to solidify ideas among engineering and medical students.

Figure 18.

Stages of foot manufacturing: (a) printing the upper and lower foot parts in two sections, (b) impregnating each part with the walnut shell powder/epoxy mixture, and (c) assembling the two parts to obtain the complete foot model.

Conclusions

In this experiment, 35% walnut shells were used as filler and epoxy resin was combined and the mixture injected into 3D-printed models made of PLA. The objective of using agricultural waste products in modern-day production to replace low-cost and eco-friendly habits is this approach. Models were injected with these filler ratios, which yielded a sturdy external matrix with a cavity laden with filler. A series of mechanical tests were conducted, and the results were compared with a pure PLA sample with a 100% fill ratio and a hybrid sample consisting of 35% walnut shells with epoxy resin and without 3D printing. The most notable results introduced that the tensile strength values ranged between 23.46 and 30.94 MPa, with the L5 sample (angle of print 50°, Infill percentage 15%, and three walls) attained the greatest tensile value of 30.94 MPa, a decrease of approximately 31% compared to the pure PLA sample. In contrast, this value showed a 37% improvement compared to the hand-injected walnut and epoxy sample. Referring to the elongation at break, the results ascertained the range between 3.67% and 4.54%, with sample L5 having the highest elongation. Even though its efficiency was reduced by 9% as compared to that of pure PLA, it was much better than the epoxy and walnut samples, 37%. Moreover, the bending test has value of 32.5–42.5 MPa flexural strength and 1470.59–2147.53 MPa flexural modulus. The flexural strength and flexural modulus dropped by 24% and 46% respectively compared to pure PLA, but were 52 times and 73 times higher than the hand-injected composite. The values of impact strength were 3.46–5.25 kJ/m² which is 27% less than pure PLA and 21% more than the walnut and epoxy sample. The values in the associated variances in mechanical performance may be linked to the presence of walnut shells in the material structure that lower the layer cohesion in 3D printing and heighten brittleness. Nevertheless, the enhanced performance of printed models relative to hand-injected composites played the significant role of the combination of additive manufacturing methods and processed materials. In addition, a comparison of the impact of 3D printing parameters through Taguchi method indicated that the number of walls was the most significant parameter in tensile and flexural experiments. At the same time, the most significant parameter to consider in the impact test was the print angle. The multi-criteria decision-making (MCDM) method was also used to perform a comprehensive evaluation process that made the L5 sample the best model in terms of overall performance, due to 50° print angle, fill ratio of 15% and three walls. In this regard, it can be promised that the use of walnut shells, in 3D-printed models is a potential opportunity to create sustainable and environment-friendly materials. This method is useful in minimizing production expenses and minimizing environmental effects, as well as supporting an acceptable degree of mechanical performance, and thus is an efficient substitute to conventional processes of producing composite materials.

Footnotes

Acknowledgements

The researchers express their sincere gratitude to Engineer Saad Abdul Rahman and Ali Jaddoa for their technical support and valuable assistance in practical matters.

Handling Editor: Aarthy Esakkiappan

ORCID iD

Abdellatif M. Sadeq

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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Sustainable manufacturing of 3D-printed polylactic acid parts reinforced with walnut shell/epoxy composites

Abstract

Keywords

Introduction

Material and experimental procedure

Materials

Design of experiments: Taguchi method

Samples preparation and methodology

Mechanical characterization techniques

Results and discussion

Epoxy and walnut shell

Tensile test

Flexural test

Impact test

Multiple-criteria decision-making (MCDM)

Manufacturing process based on optimized parameters

Conclusions

Footnotes

Acknowledgements

ORCID iD

Funding

Declaration of conflicting interests

References