An experimental study of the impact of various infill parameters on the compressive strength of 3D printed PETG/CF Open Access

Additive manufacturing, often referred to as 3D printing, has become a revolutionary technology with substantial impacts across various industries. Unlike traditional manufacturing methods, which involve carving out materials from a solid block to achieve a desired shape, additive manufacturing constructs objects layer by layer from digital models.¹ This approach provides exceptional design flexibility, enabling the creation of complex geometries that were previously impossible to achieve with different materials.² One promising material in this field is PETG/CF, a composite that merges PETG (polyethylene terephthalate glycol) with carbon fiber. PETG/CF exhibits superior mechanical properties to standard plastics, making it ideal for engineering applications. To maximize their effectiveness in practical uses, it is crucial to understand how infill variables affect the compressive strength of 3D-printed PETG/CF structures.³ The mechanical properties of 3D-printed items, essential for their structural integrity and performance, are significantly impacted by infill factors such as density, pattern, and layer height.⁴ 

Carbon fiber reinforced PETG (polyethylene terephthalate glycol) composites are a significant advancement in additive manufacturing, drawing attention due to their outstanding mechanical properties.⁵ PETG is well-known for its excellent impact resistance, chemical durability, and ease of printing, making it a suitable base material for carbon fiber reinforcement. This combination leverages the high strength-to-weight ratio and rigidity of carbon fibers, enhancing the composite’s mechanical capabilities.⁶ PETG/CF is increasingly popular in additive manufacturing due to its compatibility with fused deposition modeling (FDM) techniques. One notable advantage of this material is its suitability for food contact applications. The FDA has approved it, ensuring it does not release toxic substances when in contact with food or beverages. In addition, it is widely accessible and cost-effective.⁷ PETG’s dimensional stability ensures precise printing, which is essential for complex aerospace components and medical devices.⁸ Moreover, PETG’s chemical resistance makes it suitable for use in environments exposed to harsh chemicals, such as automotive parts. These composites offer versatile solutions across various industries, from aerospace to medical, meeting the need for high-performance materials.⁹ PETG is ideal for 3D printing applications requiring intricate designs due to its easy processing capabilities. Its adoption enhances product durability and functionality and drives innovation in additive manufacturing technology. Optimizing the manufacturing process and designing functional parts with tailored properties require understanding the impact of the infill pattern and density on the mechanical properties of FDM-printed carbon fiber-reinforced PETG samples. A notable drawback of PETG is its relatively low heat resistance compared to other technical plastics such as ABS or polycarbonate. Furthermore, PETG has strong impact resistance and cannot be as durable in specific applications as nylon or ABS.¹⁰ Under proper process conditions, 3D-printed PETG reinforced with composites creates complicated structures. Optimizing the layer height and printing temperature reduces porosity, improves dimensional accuracy, and boosts yield load. High temperature, low layer height, and lowered speed provide better results than single-objective optimization, which focuses on layer height and printing temperature.¹¹ Annealing and infill patterns have an effect on the mechanical properties of composite components manufactured using FDM. Different infill schemes exhibited anisotropic behavior. Significant improvements in strength and stiffness were seen when the beads were oriented in the direction of the applied load, which was also apparent in pure resins. Although carbon fiber-filled materials demonstrated increased stiffness, there was no substantial improvement in strength, probably due to the fiber lengths failing to meet essential criteria.¹² 

The selection of infill patterns plays a vital role in establishing the internal composition and mechanical characteristics of 3D-printed components. Infill, the internal structure of a 3D-printed object, reinforces the overall structure by providing support.¹³ In contrast to solid infill, which fills the interior of an object completely, infill patterns create intentional voids or spaces within the object. This approach conserves material and reduces printing time while maintaining mechanical properties.¹⁴ In addition, infill density, which measures the percentage of the interior volume filled with material, affects the weight, stiffness, and mechanical strength of the printed part. To study the effect on the compressive strength of PETG/CF composites, various designs were selected. The research examined several infill patterns, such as line, cubic, and tri-hexagon, to identify the most efficient pattern for maximizing compressive strength while minimizing material usage and print time.¹⁵ While numerous studies have explored the impact of infill patterns on the mechanical properties of 3D-printed components, there is a need for more research focusing specifically on the effect of infill patterns on PETG/CF composites.

Here are several studies from previous literature on using PETG materials for fabricating components. One study investigated the effects of layer thickness, infill density, and pattern on the tensile strength and surface roughness of PETG components. Findings indicated that increased layer thickness reduced the tensile strength, while higher infill density enhanced it. Moreover, the infill pattern and construction orientation significantly influenced tensile strength, and higher infill density decreased surface roughness.¹⁶ Another study examined how infill density affects the mechanical properties of printed specimens, revealing that higher infill densities improve these properties. In addition, annealing enhanced the composite’s mechanical properties and promoted interlayer diffusion bonding. The findings suggested that using 100% infill density with annealing can be a viable alternative to traditional metallic components in automotive and aerospace structures, utilizing CFPETG materials.¹⁷ The mechanical characteristics of PETG concerning the infill pattern and density were thoroughly investigated. Response surface methods were used to determine the optimal settings to enhance these properties. Results showed that the triangle infill pattern could absorb more force and exhibit the highest compression strength and modulus. Increasing the density from 25% to 75% further improved the mechanical properties.¹⁸ Another study analyzed the mechanical characteristics of PETG material for FDM-based product production. The Taguchi approach was used to study how infill percentage, layer height, and infill pattern impact these characteristics. According to ASTM standards, the optimal PETG material parameters for FDM are 80% infill percentage, 0.3 mm layer height, and a hexagonal infill pattern, which provide superior mechanical performance.¹⁹ Another study focused on optimizing printing parameters to enhance the mechanical properties of PETG, PETG + CF, and PETG + KF materials. The Taguchi method was used to create the experimental plan, and L16 orthogonal array specimens were produced. Optimal bending properties were achieved with PETG, PETG + CF, and PETG + KF at extrusion temperatures of 265, 195, and 265 °C, respectively. Printing speeds were 20, 60, and 20 mm/s, while layer heights were set to 0.4, 0.53, and 0.35 mm, respectively. Maintaining a 100% infill density was crucial for enhancing the mechanical properties of these materials.²⁰ Another study explored the impact of various input factors on the mechanical characteristics of a PETG-based polymer composite reinforced with carbon fibers. Results showed that optimizing the layer thickness to 0.25 mm and ensuring a maximum infill percentage of 20% significantly increased the flexural and tensile strength, beneficial for solid structural configurations. In addition, compressive strength improved notably when using reinforced PETG with hexagonal and circular geometries.²¹ Research into the mechanical characteristics and process parameters of PETG and CFPETG involved tensile, hardness, and flexural tests. Findings indicated that the carbon fiber-reinforced PETG sample had a tensile strength 1.28% higher than that of the pure PETG sample.²² Another study examined the production of thin-walled carbon fiber (CF) reinforced PETG composite tubes with octagonal corrugated lattice structures using fused filament fabrication (FFF). Compressive strength and dimensional accuracy were assessed by carefully adjusting parameters such as layer height, nozzle temperature, printing speed, linewidth, and infill density. Optimal parameters for enhancing the compressive strength were identified as 0.1 mm layer height, 220 °C nozzle temperature, 20 mm/s printing speed, 0.1 mm linewidth, and 100% infill density.²³ Research into the use of PETG/carbon fiber composite for a hexagonal lattice structure with a shell wall involved experimentation with temperature and infill density. Optimization methods such as the Taguchi approach and analysis of variance found that the optimal configuration for maximum compressive strength included a 220 °C nozzle temperature, 0.1 mm layer height, 100% infill density, and 20 mm/s printing speed. Compression testing indicated uneven buckling and sliding at lower infill densities.²⁴ A composite polymer based on PETG reinforced with carbon fiber was synthesized through FDM, with an investigation into the effects of carbon fiber content and process variables on tensile strength, flexural strength, and tribological characteristics. The inclusion of 20 wt. % carbon fiber resulted in a remarkable enhancement of up to 114% in tensile strength, alongside a notable increase in bending strength of up to 25%. Tribological assessments revealed a substantial decrease in the coefficient of friction (COF) compared to pure PETG, exhibiting reductions of ∼47.3% at low and 44.79% at high velocities.²⁵ Three-dimensionally printed carbon fiber-reinforced PETG thermoplastics were tested for compressive strength under high-impact forces. Optimizations included filling patterns (rectilinear and honeycomb) and density (25%, 50%, and 75%). Higher filling densities increased the compressive strength and modulus, with the 75% honeycomb design performing best independent of impact pressure, producing 35.5–56.16 MPa compressive strengths. Honeycomb-filled samples had a 2787.8 MPa compressive modulus, substantially greater than that of rectilinear patterns.²⁶ Another study explored the influence of different infill patterns on the mechanical characteristics of thermoplastic materials and reinforced polymers. Findings indicated a substantial improvement in the tensile stress at break, witnessing an average surge of 70.87% for composite materials compared to polymers. Similarly, there was a noteworthy increase of 69.93% in the tensile stress at yield. Importantly, the configuration of fibers within the infill plays a pivotal role, particularly with the arrangement showcasing significant enhancements in both tensile stress at break (220.18 MPa) and tensile stress at yield (198.26 MPa).²⁷ Another work examined the experimental examination of CF-reinforced PETG utilizing fused filament fabrication (FFF). Tests were conducted on dumbbell-shaped specimens that have different infill densities and fill patterns. Tests on an INSTRON 5969 apparatus demonstrated elastoplastic behavior, with honeycomb patterns exhibiting superior strength and stiffness to rectilinear patterns.²⁸ Another study examined the manufacturing of lattice composites using walnut shell/PLA (WSP) materials by fused filament fabrication. It optimizes Printing-Based Factors (PBFs) such as infill density, nozzle temperature, printing speed, and layer height. By optimizing these variables, the hexagon lattice-structured WSP composites achieved a flexural strength of 4.98 MPa, a compression strength of 28.19 MPa, and a reduction in thickness error to 0.186 mm. The findings showed considerable promise for biopolymeric materials.²⁹ The research on PETG/graphene combinations demonstrated that optimal mechanical characteristics are achieved when PETG is reinforced with a minute percentage of graphene, specifically 0.04 wt. %. Unlike the unadulterated PETG, substantial improvements are noticeable in the tensile, compression, flexural, and impact strengths of the PETG/graphene composite at 0.04 wt. %, showing increases of 89.71%, 81.76%, 21.60%, and 81.25%, respectively.³⁰ In another study, a composite filament comprising polypropylene (PP) and carbon fiber (CF) underwent extrusion, serving as the material for fabricating standard test specimens with different infill densities (60%, 80%, and 100%) via a Fused Deposition Modeling (FDM) printer. Results suggested that utilizing a 100% infill density of PP with CF composite results in improved tribological characteristics attributed to the effective amalgamation of uniformly distributed particles and cohesive bonding between individual layers across the specimen. Specifically, the PP with CF composite at 100% infill density exhibited a marginally higher hardness rating of 33.9 Vickers Hardness Number (VHN), which was observed during a test where a disk slid at a velocity of 3 m/s under a maximum load of 20 N.³¹  Table I shows the previous research on the fabrication of PETG using various reinforcements.

While previous research has explored the effect of infill parameters on the mechanical features of FDM printed samples, existing research hypothesizes that increasing the infill density will lead to higher compressive strength due to greater material density and interlayer adhesion. Comprehensive analyses are needed to address carbon fiber-reinforced PETG composites for varying infill parameters. Thus, this study focuses on the gap by methodically assessing the effects of various infill patterns, such as line, cubic, and tri-hexagon, and infill ratios on the compressive properties of FDM-printed carbon fiber-reinforced PETG specimens. This investigation provides valuable insights for optimizing the infill parameters to enhance the mechanical performance by conducting a thorough examination that combines experimental trials with numerical models.

This investigation utilized polyethylene terephthalate glycol (PETG), a polyester copolymer renowned for its ability to replace conventional polymers in 3D printing. When reinforced with carbon fiber, this polymer exhibits enhanced strength, resilience, and reduced susceptibility to warping. Consequently, due to its advantages, PETG + CF filaments were selected as the primary material for this research. The fabrication process involved employing the Fused Deposition Modeling (FDM) technique using 1.75 mm filaments as the raw material. Table II outlines the characteristics of PETG materials manufactured through FDM technology. The PETG + CF specimens were manufactured utilizing a Creality Ender-3 3D printer, which features a print volume of 220 mm (X) × 220 mm (Y) × 250 mm (Z) (Refer to Fig. 1). The procedure entailed designing samples using SolidWorks design software and exporting them in stereolithography file format. Subsequently, these stereolithography files underwent slicing using Make Easier 3D. The codes essential for producing samples were produced through Simplify 3D software. Within Table III, one can find the consistent printing settings that are in contrast to the alterable parameters, specifically the infill pattern and ratio, adjusted across three different tiers (refer to Table IV). The experimental design was based on the Design of Experiment (DOE) utilizing RSM central composite design through Design Expert 13 software, encompassing 13 experimental trails.

Samples for the compression assessment were fabricated to adhere to the dimensions recommended by the ASTM D695 standard, illustrated in Fig. 2. The specimens for evaluating compression strength measured 25.40 × 6.35 mm² in size. Compression testing stands as one of the fundamental mechanical testing methodologies. Such tests elucidate a material’s response under applied crushing loads. They are commonly executed by subjecting a test specimen to compressive pressure via platens or specialized fixtures on a universal testing machine. Diverse material properties are computed and graphed throughout the testing process as a stress–strain diagram. This pictorial representation assists in identifying properties such as the elastic threshold, proportional threshold, material’s point of yielding, yield potency, and, in specific scenarios, compressive potency of the substance.

The CF/PETG fabricated specimens underwent mechanical testing through a compression assessment using a Universal Testing Machine (UTM) manufactured by DAK in India, with a maximum load capacity of 50 kN (see Fig. 3). The compression evaluation adhered to ASTM D695 standards, employing a strain rate of 1 mm/min. Throughout the compression trial, the integrated structure of CF/PETG samples was accurately positioned between the two loading plates, and compression loads were applied to the respective composite samples.

Table V provides an elaborate overview of experiments regarding compression characteristics, where 13 different combinations of infill parameter experimental runs are conducted to find the maximum compressive strength of carbon-reinforced PETG samples and investigate potential variables that impact the changes in compressive strength across various infill percentages. These aspects include the density and arrangement of reinforcing fibers inside the PETG matrix, which influence how the load is distributed and how stress is transferred. In addition, the bonding between layers and the development of voids inside the printed objects contribute to the overall analysis. To enhance understanding of the variations in compressive strength and provide crucial insights for future research and practical applications of 3D-printed PETG/CF composites, this interpretation is invaluable. The findings can guide the optimization of printing parameters to achieve superior material properties in practical use.

Figure 4 depicts the printed sample intended for compression testing using 3D printing FDM with carbon-reinforced PETG material. The fabrication of compression test samples followed the guidelines stipulated in the ASTM D695 standard.³⁷ The compression strength measurements were conducted on samples measuring 25.40*6.35 mm, encompassing three distinct infill patterns and ratios. In Fig. 4(a), the tri-hexagon infill pattern with an 80% infill ratio is depicted, (b) illustrates the line infill pattern with a 60% infill ratio, and (c) showcases the cubic infill pattern with a 40% infill ratio. The compression evaluation adhered strictly to ASTM D695 standards, with a strain rate set at 1 mm/min. Throughout the compression testing, the composite samples of CF/PETG were accurately positioned between two loading plates, and compression loads were applied accordingly. Figure 5 displays the fabricated sample after compression testing. In Fig. 5(a), a tri-hexagon infill pattern with an 80% infill ratio is depicted, (b) illustrates a line infill pattern with a 60% infill ratio, and (c) illustrates a cubic infill pattern with a 40% infill ratio.

Table VI displays the outcomes of the variance analysis performed on compressive strength measurements. The objective of utilizing ANOVA is to break the total variability in empirical data into distinct variables and assess their statistical importance. A quadratic regression model was chosen to illustrate the correlation relating compressive strength and CF-reinforced PETG-fabricated materials. Rigorous testing through ANOVA was conducted on the constructed model. An assessment of lack-of-fit was interpreted to determine the significance of the regression model.

The F-value for the model stands at 53.05, suggesting its significance, with an extremely low likelihood of 0.01% for such a high F-value to occur by chance. Model terms with p-values below 0.0500, such as B and A², are deemed significant in this context. Conversely, values above 0.1000 indicate insignificance in model terms. Streamlining the model, except for essential hierarchical terms, could potentially enhance its performance if numerous insignificant terms exist. The lack of fit F-value at 1.52 suggests insignificance compared to pure error, with a 33.96% chance of occurring due to randomness. An insignificant lack of fit is favorable as it signifies a well-fitting model. The R² value of 0.974, alongside the predicted R² of 0.8839 and the adjusted R² of 0.9559, displays reasonable consistency, differing by less than 0.2. Adeq Precision assesses the signal-to-noise ratio, where a ratio above 4 is preferred. A ratio of 22.319 indicates a satisfactory level of signal. Consequently, this model proves to be beneficial for exploring the design space.

Figure 6 compares the compressive strength results obtained from experiments and those predicted by the model run order. The bar chart in the figure highlights the compressive strength results of various infill patterns and ratios, showcasing which parameter combinations attain maximum compression strength. Furthermore, it indicates a close alignment between the measured compressive strength values and the predicted ones. All experimental procedures were thoroughly followed, resulting in consistently high compressive strength values. The highest recorded compressive strength was 39.16 MPa, while the predicted value was 37.26 MPa.

Conversely, the minimum evaluated value was 11.52 MPa; the predicted lowest was 9.48 MPa. This trend mirrors findings in Ref. 38, which explored carbon-reinforced PETG material. Thirteen runs revealed significant impacts from two parameters across three levels. Each run represents varying ranges of compressive strength values, spanning from 11.52 MPa to a maximum of 39.16 MPa, consistent with observations in a previous study.³⁹ 

In Fig. 7, the contour and surface plots depict the link involving two parameters concerning the anticipated compressive strength values. The contour plot is a 2D depiction of the response plotted against numeric factors and mixed component combinations. At the same time, the surface plot is a 3D depiction of the response plotted against numeric factors and mixed component combinations. It can illustrate the link between responses, mixture components, and numerical variables. These plots distinctly demonstrate how two variables impact the predicted compressive strength outcomes. They reveal the connection between the infill pattern and ratio and their consequent influence on the compressive strength attribute. Notably, a peak compressive strength of 39.16 MPa is attained with the tri-hexagon infill pattern at an 80% infill ratio.

The optimal parameter values for optimizing the response were attained using a desirability function to identify the most favorable outcome. A selection is made from seven iterations, considering its desirability score of 0.97. The resulting total desirability is 0.97, nearing the ideal value of 1, which is deemed satisfactory. A validation trial was conducted to validate the precision of regression and optimization results. This involved executing experiments with randomized and optimized parameter settings.

This study aims to examine how various infill patterns and ratios affect the compressive strength of FDM processing with carbon-reinforced PETG material. Samples for compressive testing were prepared following ASTM D695 guidelines. The study employs Design of Experiment principles, particularly Response Surface Methodology (RSM), with a central composite design facilitated by Design Expert 13 software. Thirteen investigational trials were carried out as part of the investigation.

The optimal compressive strength of 39.16 MPa was attained with an infill pattern characterized as tri-hexagon, paired with an infill ratio of 80%. Conversely, the lowest compressive strength of 11.52 MPa was recorded with a cubic infill pattern at a 40% infill ratio. An optimal parameter for FDM processing of PETG material enhanced with carbon reinforcement was established and verified through practical experimentation. These tests were conducted to validate the theoretical aspects of the research. By understanding the interaction between composite material properties and printing parameters, this study aids in designing more efficient and resilient 3D printing processes for high-performance components across various applications. The findings have significant implications for industries such as automotive and aerospace. However, higher print temperatures might reduce printer compatibility, and increased brittleness and layer adhesion require careful handling and orientation adjustments, affecting both usability and structural integrity.

This research opens avenues for further exploration. First, examining how different infill settings impact the compressive strength of 3D-printed PETG/CF composites would be beneficial. Employing advanced modeling techniques or experimental setups could enhance result accuracy. Second, investigating other additive manufacturing methods or post-processing techniques might deepen our understanding and optimize PETG/CF composites for diverse applications, thereby expanding the field.

The authors extend their appreciation to Taif University, Saudi Arabia, for supporting this work through Project No. TU-DSPP-2024-133.

The authors have no conflicts to disclose.

Shashwath Patil: Conceptualization (equal); Data curation (equal); Investigation (equal); Methodology (equal); Resources (equal); Software (equal); Supervision (equal); Visualization (equal); Writing – review & editing (equal). T. Sathish: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Jayant Giri: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Visualization (equal); Writing – review & editing (equal). Bassem F. Felemban: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Software (equal); Validation (equal); Writing – review & editing (equal).

The data that have been used are confidential.