Z-Stitching Technique for Improved Mechanical Performance in Fused Filament Fabrication

Abstract

Fused filament fabrication (FFF) is a widely used additive manufacturing technique that enables the rapid, layer-by-layer creation of parts. However, its traditional planar deposition approach can produce strong material anisotropy in terms of moduli and strengths, especially when fiber-reinforced polymers are processed. These characteristics limit the application of FFF in high-performance fields. This study introduces a novel FFF printing technique, termed z-stitching, which incorporates interlocking stitch patterns to enhance interlayer interaction and reduce anisotropy. A z-stitching algorithm was developed to explain the toolpath and material deposition. Using polymer filaments, samples employing the z-stitching technique were produced as a proof of concept. Moreover, experiments were conducted to explore the mechanical properties of samples made using z-stitching. Test results in terms of moduli and strengths in different principal material directions, as well as an isotropy ratio, were contrasted with the mechanical properties of samples made using traditional FFF. The experiments showed an overall enhanced mechanical performance of parts made using z-stitching. A printing time analysis was also performed, revealing that z-stitching printing time is approximately 14% longer than that of the comparable traditional FFF processes. This study establishes a foundation for the further optimization of z-stitching and its adoption in industrial-scale additive manufacturing for structures in high-performance applications.

1. Introduction

Fused filament fabrication (FFF) is one of the most accessible and versatile techniques in additive manufacturing, enabling the rapid, layer-by-layer construction of complex parts. This process builds components by extruding a continuous thermoplastic filament layer by layer onto a build platform, as illustrated in Figure 1. Following a predetermined path in the x-y plane, the extruded filament forms each layer, and by repeating this process along the z-axis, the part gradually gains its intended out-of-plane dimension [1,2,3]. FFF is widely used due to its simplicity and flexibility. However, the conventional layer-stacking approach also has drawbacks; specifically, creating and stacking distinct layers in the x-y plane limits engagement between layers, resulting in mechanical anisotropy in terms of moduli and strengths, especially when fiber-reinforced polymers are processed, as the fiber reinforcement effect is limited to the x-y plane. In addition, achieving strong interlayer adhesion can be challenging. Therefore, FFF-printed parts often exhibit inferior mechanical properties in the out-of-plane direction compared to the in-plane one, which can be problematic, particularly for applications demanding reliability and high strength in all directions, such as load-bearing components in aerospace, automotive, and medical devices [4,5].

Poor layer adhesion along the z-direction creates weak points within the structure, making it vulnerable to out-of-plane forces. Consequently, strengthening the z-direction and reducing mechanical anisotropy have been recognized as essential steps toward unlocking the full potential of FFF in high-performance applications. The need to address these issues has driven research into techniques that enhance layer bonding and strength throughout printed parts [6,7,8]. Several methods have been developed to improve interlayer adhesion in FFF, each offering specific advantages, but often introducing trade-offs that limit their broader application across different materials and geometries. One commonly adopted approach involves the localized heating of the pre-deposited layer just before the next layer is added [9,10,11]. Preheating can be achieved through various methods, such as hot air streams, microwave irradiation, laser exposure, or infrared light, which help to re-melt the top surface of each layer, promoting polymer chain diffusion across layers and thereby improving adhesion strength. However, while localized heating can increase bonding strength, it requires precise temperature control to prevent part deformation and warping, adding both complexity and cost to the printing process.

Employing polymer crosslinking is another approach to strengthening interlayer adhesion [12,13,14]. This chemical process enhances bonding by creating connections between polymer chains across layers. Techniques such as ionization or microwave radiation exposure have been shown to increase fracture strength in materials like polylactic acid (PLA) by inducing crosslinking at the layer interfaces. Although crosslinking improves durability and cohesion between layers, it is often limited to specific polymer types, as not all thermoplastics respond to ionization or similar treatments. Additionally, while polymer crosslinking can reduce interlayer weaknesses, it does not fully address the issue of mechanical anisotropy, especially in load-bearing applications where strength in the z-direction may be crucial.

Post-processing methods, such as annealing, also offer ways to improve interlayer adhesion by altering the molecular structure of the printed part after fabrication [15,16,17]. Annealing involves heating the completed part to a temperature below its melting point, encouraging further crystallization and the relaxation of internal stresses, which can improve both strength and thermal stability along layer interfaces. However, annealing requires careful control of the heating and cooling rates to prevent warping and shrinkage, and its effectiveness is highly dependent on the polymer material being used. While it has the potential to enhance the overall durability and performance of a part, annealing does not eliminate the inherent anisotropy associated with FFF.

Despite the promise of the above approaches, challenges remain. Many methods are material-specific or require additional equipment, complicate the FFF process, and increase energy consumption and costs. Furthermore, most techniques do not address the issue of mechanical anisotropy, as they continue to rely on traditional planar deposition in the x-y plane. This fundamental limitation of traditional FFF limits the mechanical properties along the z-axis, even if methods for promoting interlayer adhesion in situ, post-processing, and/or fiber-reinforced polymer materials are used [18,19].

In response to the above challenges, researchers have been exploring nonplanar printing techniques that move beyond conventional layer-by-layer fabrication, seeking to improve the out-of-plane properties [20]. These techniques aim to build parts in ways that inherently enhance the mechanical properties along the z-direction and reduce anisotropy. Curved Layer Fusion Deposition Modeling (CLFDM) can be mentioned in this context. In CLFDM, the tool path is modified to follow the part’s topology, enhancing surface quality and producing more naturally shaped components [21]. Nonplanar toolpath planning introducing cyclic paths, such as zig-zag or up–down motions, is an approach that promotes layer interlocking through overlapping material deposition [22]. Another technique, called z-pinning, deposits continuous filaments vertically through stacked layers, creating filament pins that anchor layers together to improve the integrity between layers [23]. While these examples have been shown to promote engagement between layers and thus mechanical performance to some extent, they are limited in reducing mechanical anisotropy across the part thickness, particularly when fiber-reinforced materials are printed. These limitations stem from a lack of denseness of interlocking features and/or reliance solely on extruded filament adhesion, rather than richly set features that provide robust linkages between layers. Therefore, there is a continued need for nonplanar printing approaches that improve mechanical performance, minimize anisotropy, and maintain fabrication simplicity [24].

In response to the above challenges, this study introduces a novel z-stitching technique designed to create robust linkages between layers, improve interlayer bonding, and thus enhance mechanical performance and promote mechanical isotropy. In short, the z-stitching method deposits material in a staircase motion to create interlocking “stitches” between layers. The proposed z-stitching technique is described in the subsequent section. To demonstrate the efficacy of this technique, an experimental campaign was conducted. The corresponding materials and methods are covered in Section 3. In Section 4, the experimental results are presented and discussed.

2. Description of Z-Stitching Technique

2.1. Overview

Interlayer and intralayer adhesion zones, as depicted in Figure 2a, are critical to the structural integrity and mechanical performance of FFF-printed parts. Interlayer adhesion refers to the bonding strength between adjacent layers, which is often the weakest point in traditional FFF due to challenges in achieving sufficient fusion of the deposited material, compounded by the rapid fabrication process. Weak bonding arises from the cooling and solidification of a layer before the next one is deposited, leading to the poor diffusion of polymer chains across the layer interface. Intralayer adhesion, on the other hand, pertains to the bonding between individual extruded filaments within a single layer. While intralayer adhesion tends to be stronger than interlayer adhesion, filament deposition, alignment, and/or material flow inconsistencies can still create localized weaknesses within the layer.

The z-stitching technique aims to mitigate potential weaknesses between layers through stitch patterns. In this manner, interface regions are extended, increasing the effective bonding area, as illustrated in Figure 2b. Moreover, the stitch pattern counteracts delamination, a common failure mode caused by weak interlayer adhesion, while also promoting robust intralayer bonding by ensuring continuous material deposition within each layer. The z-stitching method introduces a staircase-like deposition pattern alternating between right-hand and left-hand stitches, providing mechanical interlocking between the layers, as shown correspondingly in Figure 3a,b by the motion sequence of the nozzle path along the x, y, and z axes. A schematic example part with a z-stitching pattern is shown in a side and isometric perspective in Figure 4a,b. Each stitch is color-coded (red for right-hand stitches and blue for left-hand stitches) to highlight their alternating arrangement. Also shown are part contouring (black), corner supports (gray), and stitch supports (yellow). This unique printing approach ensures that adjacent layers are not merely stacked but are physically linked, which not only serves to improve interlayer adhesion, but also reduces anisotropy in the printed part.

2.2. Interlocking of Layers

The effectiveness of the z-stitching technique lies in its innovative interlocking mechanism, which strengthens the connections between layers and improves the overall structural integrity of the printed part. Instead of relying solely on layer-to-layer adhesion, this method introduces an interwoven pattern of stitches that physically anchor neighboring layers together. Each stitch acts as a bridge, linking adjacent layers and spreading mechanical loads across multiple regions.

In Figure 5a, the red stitch again represents a right-hand pattern, while the blue stitch represents a left-hand pattern. When a force is applied in the z-direction, the interwoven blue stitches counteract the load by redistributing it to the surrounding material, effectively resisting separation between layers. Similarly, Figure 5b illustrates how forces applied in the y-direction are resisted through the complementary arrangement of opposing stitches. This interplay ensures that the material can withstand multidirectional forces, addressing one of the weaknesses of conventional FFF-printed parts. By strategically alternating the stitch patterns, the z-stitching technique builds a robust lattice of interlocked material that also serves to reduce mechanical anisotropy.

2.3. Printing Sequence

The z-stitching printing sequence constitutes a significant change to traditional FFF processes by fundamentally altering the layer-by-layer deposition approach. In conventional FFF, an entire layer is printed before moving to the next. In contrast, the z-stitching technique adopts a staggered sequence that interweaves planar layers with stitches and supports.

The sequence begins with the deposition of a bottom layer, forming the foundation for the part, as shown in Figure 6a. This initial layer provides the base for the subsequent steps and ensures that the printed part adheres securely to the build platform. Once the base is established, a contour of the part is printed, defining the outer boundaries of the design. Simultaneously, edge supports are added to stabilize the structure, as illustrated in Figure 6b,c. These supports play a critical role in maintaining the part’s geometry and preventing deformation during the subsequent addition of stitches. The first stitch is then deposited, marking the beginning of the z-stitching pattern, as seen in Figure 6d. The nozzle moves in a staircase motion, alternating between right-hand and left-hand stitches to create the interlocking features that define the technique. Each stitch is supported by intermediate beads, or stitch supports, which are printed immediately after the stitch itself (Figure 6e). These supports serve a dual purpose: they stabilize the newly deposited stitch and create a connection with the previous layers, reinforcing the interlocking mechanism.

This process of depositing stitches and supports is repeated for each subsequent layer. As the sequence progresses (Figure 6f), the alternating stitch patterns interweave with the planar layers, forming an interconnected structure. Each new stitch connects to its neighboring layers through the support blocks, creating a continuous network of interlocked material that resists separation and delamination.

As the part takes shape, the interleaved layers of stitches, supports, and planar material ensure consistent strength throughout the structure. Figure 6g–i illustrates the progressive development of the part: the intermediate layers build upon the staggered stitches, eventually leading to the completion of the top layer. This final step encapsulates the interlocking network within an external framework, providing the finished part with a closed surface.

2.4. Stitch Dimensions and Toolpath Planning

Defining precise stitch dimensions is crucial to the effectiveness of the z-stitching technique. These dimensions dictate how the material is deposited and interlocked across layers, directly impacting the mechanical strength and consistency of the final part. Figure 7 provides a schematic of the parameters defining the stitch geometry. Along the x-axis, the nozzle’s travel distances are denoted as X1, X2, and X3, while Y1 and Y2 represent the travel distances along the y-axis. The Z parameter specifies the stitch height, which governs the depth of interlocking between adjacent layers. Together, these dimensions define the staircase-like motion of the nozzle as it deposits material to form interlocking stitches. Several factors influence these dimensions, including the nozzle diameter, layer height, and the desired overlap between the deposited material. The nozzle diameter sets the minimum resolution for the stitch width, while the layer height determines the vertical increment of each step in the staircase motion. The desired stitch overlap ensures that adjacent stitches are securely interlocked, creating a continuous network of material that resists separation under mechanical loads.

3. Materials and Experimental Methods

3.1. Materials and Printing Equipment

To evaluate the efficacy of the z-stitching technique in this proof-of-concept study, parts were fabricated and experimentally characterized. Two types of FFF printers were utilized: a Prusa i3 MMU2 printer with a multi-color unit (Prusa Research, Prague, Czech Republic) was used for producing multi-color samples. The ability to print in multiple colors allowed for the visual differentiation of the various features, thus providing insights on the quality of the toolpath and the resulting structure. For single-color parts, a Prusa i3+ printer was employed, offering consistent and precise printing performance for mechanical testing.

The material used for all experiments was PLA filament, obtained from Prusa Research. The filament had a diameter of 1.75 ± 0.02 mm, which is standard for most FFF printers. For the experiments, the nozzle temperature was set to 220 °C and the bed temperature was maintained at 60 °C, which resulted in good adhesion and consistent extrusion quality during printing. A summary of the printing parameters employed for the fabrication process is provided in

Table 1. Parameters employed for the 3D printing of parts.

Parameter	Value and Unit
PLA filament diameter	1.75 ± 0.02 mm
Nozzle diameter	0.4 mm
Nozzle temperature	220 °C
Bed temperature	60 °C
Layer height	0.3 mm
Printing speeds:
● Rapid motion	60 mm/s
● Contouring	20 mm/s
● Stitch support	15 mm/s
● Stitch	10 mm/s

. These parameters were adjusted to achieve compatibility with the algorithm while maintaining high-quality prints. The careful selection of materials, printers, and printing settings ensured that the experimental setup was robust and capable of demonstrating the effectiveness of the z-stitching technique.

3.2. Experimental Methods

A series of experiments were conducted using multi-color and single-color 3D-printed samples. These experiments aimed to evaluate the printability of the algorithm, adjust the printing parameters for high performance, and compare the mechanical properties of z-stitched samples with those printed using traditional infill patterns.

3.2.1. Comparative Analysis of Infill Volume Ratio

To evaluate the structure density of the z-stitched parts, akin to an infill density, a series of 10 mm cubes was printed. Besides the z-stitching pattern, cubes were made with popular infill patterns, including rectilinear infills. These cubes were carefully weighed, and their masses were compared to those samples printed using the z-stitching algorithm. This process revealed that the z-stitching technique corresponded to an infill volume ratio of approximately 60% when compared to various infill patterns. Therefore, this value was adopted to provide a benchmark for assessing the z-stitching method against conventional infill strategies, enabling direct comparisons of mechanical properties and material efficiency. By matching the infill volume ratio, the study ensures that any observed differences in performance are attributable to the structural characteristics of the z-stitching technique rather than variations in material usage.

3.2.2. Multi-Color Sample Printing and Analysis

To examine the internal structure’s stitch patterns, a set of multi-color samples measuring 50 mm × 20 mm × 10 mm was printed. The samples were sectioned using cryogenic fracturing to inspect the print quality and ensure that the printed patterns matched the designed toolpath. The sectioned samples were analyzed using an AmScope simul-focal stereo optical microscope equipped with an OMAX A3590U 9.0MP USB Digital Camera (United Scope, Irvine, CA, USA). Microscopy images were captured and edited using OMAX ToupView 3.7 software, facilitating detailed inspection of the printed structures.

3.2.3. Microcomputed X-Ray Tomography

To further study the morphology of the structures made using the z-stitching technique, high-resolution microcomputed X-ray tomography (micro-CT) was employed. Samples were analyzed using a ZEISS Xradia 620 Versa X-ray μCT (Oberkochen, Germany) at a resolution of 30 µm, with a voltage of 60 kV, power of 6.5 W, and exposure time of 1 s. The reconstructed micro-CT images provided detailed visualizations of the internal structures, serving to verify the accuracy and consistency of the z-stitching patterns. Post-processing of the CT data was performed using the Dragonfly 21 software (Comet Technologies, Montreal, QC, Canada), which facilitated the dimensional analysis of the z-stitching features.

3.2.4. Mechanical Testing

To compare the performance of the specimens made via the z-stitching technique, specimens were also printed using a standard rectilinear infill pattern with a density of 60%, as this pattern was reported to achieve higher tensile strength compared to other infill structures like honeycomb or linear [25]. In addition, samples with 90% density rectilinear infill were fabricated to approximate near-solid parts and serve as an upper benchmark for comparison with z-stitched samples. For illustration, Figure 8a,b depicts rectangular infill patterns with 60% and 90% infill densities. Note that while the infill patterns are identical in the x and y-direction, the contour layers are always oriented only in the x-direction for all specimens.

Flat plate samples measuring 170 mm × 170 mm × 3.2 mm were printed using a custom G-code to evaluate the mechanical properties of the z-stitching algorithm. These plates were sectioned into ASTM D638-22 Type I tensile test specimens using a waterjet cutter [26]. The samples were cut from a large plate to minimize the contour effects, which also maximized the distribution of z-stitch features throughout the corresponding samples. The tensile specimens were cut in four distinct orientations: along the x-axis, representing the raster direction (i.e., longitudinal); along the y-axis, perpendicular to the raster orientation (i.e., transverse); at a 45° angle in the x-y plane, representing an intermediate orientation; and along the z-axis, perpendicular to the build plate (i.e., vertical or out-of-plane). Figure 9a–c schematically depicts the specimens with the different raster directions.

The specimen dimensions are shown in Figure 10a. Tensile tests were performed in quintuplicate or more on an Instron 5966 universal testing machine (Norwood, MA, USA) equipped with a 10 kN load cell, as shown in Figure 10b. The tests were conducted at a strain rate of 5 mm/min in accordance with the ASTM D638-22 standards. An Epsilon ONE optical extensometer (Epsilon Technology, Jackson, WY, USA) with a gauge length of 50 mm was used to directly measure strain during testing. This setup ensured the accurate measurement of the specimens’ responses to tensile forces and allowed for consistent comparison between the z-stitched and traditional infill specimens.

In addition to tensile testing, the flexural properties of z-stitched parts were also examined. For this purpose, ASTM D790-17 three-point bending test samples were fabricated [27]. Samples with z-stitching and 60% or 90% rectilinear infill were tested to compare their flexural characteristics. A custom three-point bending jig, as shown in Figure 10c, was manufactured using a 25 mm-thick mild steel plate via waterjet cutting. The bending tests were conducted on the Instron 5966 universal testing machine, as seen in Figure 10d. The loading rate (R, rate of crosshead motion) is defined using Equation (1). Flexural stress, σ_f, and strain, ε_f, were calculated using Equations (2) and (3), respectively. Five samples were tested for each configuration to ensure repeatability and provide a statistical analysis of the results.

$$R={\displaystyle \frac{Z{L}^2}{6d}}$$

(1)

$${\sigma }_{\mathrm{f}}={\displaystyle \frac{3PL}{2b{d}^2}}$$

(2)

$${\epsilon }_{\mathrm{f}}={\displaystyle \frac{Dd}{L^2}}$$

(3)

where Z, L, d, P, D, and b are the rate of straining of the outer fiber of the bending test sample (0.01 min⁻¹), the span length, the specimen thickness, the applied load, the maximum deflection at the center of the beam, and the specimen width, respectively.

4. Results and Discussion

4.1. Morphology of Z-Stitching Structures

Figure 11 provides a comparison between the as-designed and as-printed z-stitching structure. The cross-section view of the y-z plane of a printed sample in Figure 11a closely resembles the designed pattern in Figure 11b. Hence, visual inspection confirms that the design intent and the programmed toolpath successfully produced actual physical structures featuring the desired interlocking stitch pattern, albeit with some deviations from the as-designed geometry. Figure 11c offers a top-down view of the printed sample, showcasing the stitch in red, edge support in light gray, and bottom layer in dark gray/black. This visualization highlights the spatial arrangement of the components, providing a representation of how the stitch interacts with the surrounding structures. The consistent placement of the stitch and its integration with the edge supports and bottom layer can be confirmed.

To further explore the congruence between the designed and fabricated z-stitching features, micro-CT imaging was employed. Figure 12 presents cross-section views of the printed samples from multiple perspectives: the y-z plane (front view), x-y plane (top view), and x-z plane (side view). Micro-CT imaging allowed for a higher inspection and assessment quality, as this technique avoided the sectioning of samples and the associated disturbances of the polymer structure on the fracture surface. Hence, the CAD models in Figure 12a,c,e could be directly compared and matched to assess the fidelity of the printed parts in Figure 12b,d,f. The sectional front views in Figure 12a,b reveal the uniformity of the stitches across layers, confirming the accurate execution of the interlocking pattern as designed. Similarly, the top view in Figure 12c,d highlights the alignment of the stitch and support structures, promoting proper load distribution and interlayer bonding. The side view in Figure 12e,f provides further evidence of the consistent stitch depth and height, which are critical parameters for achieving robust mechanical interlocking. The results demonstrate a high degree of congruence, with only slight deviations, between the CAD models and the physical samples.

4.2. Tensile Testing

Figure 13 displays the mechanical response from the tensile testing of specimens fabricated using the z-stitching technique, along with those having the traditional rectilinear infill pattern with a 60% or 90% density. The test data were reduced to the parameters summarized in

Table 2. Reduced data from tensile testing.

Loading Direction	Specimen Type	Stiffness (GPa)		Yield Strength (MPa)		Ultimate Strength (MPa)
		Mean	SD	Mean	SD	Mean	SD
x-axis	60%	2.12	0.14	22.68	4.54	31.26	1.48
	90%	2.95	0.10	31.27	5.22	44.12	1.11
	z-stitch	2.22	0.11	21.64	0.92	29.29	0.91
y-axis	60%	1.05	0.05	10.02	1.16	13.73	0.6
	90%	2.05	0.15	11.19	1.38	14.93	2.35
	z-stitch	1.68	0.28	13.31	1.21	22.08	0.41
45°	60%	1.25	0.14	10.26	1.27	15.74	0.27
	90%	2.53	0.05	16.22	0.38	28.14	0.75
	z-stitch	1.86	0.17	16.64	1.02	23.56	0.45
	60%	0.47	0.13	4.52	0.68	6.77	0.05
z-axis	90%	0.86	0.23	5.85	0.34	10.65	0.24
	z-stitch	2.02	0.54	14.99	1.55	24.65	0.2

and depicted in Figure 14. Note that the yield strength was computed by the 0.2% strain offset method.

Inspection of the stress–strain curves in Figure 13 reveals that compared to the specimens featuring the rectilinear infill with a 60% density, the stiffness of z-stitching specimens is approximately equal (x-direction), see Figure 13a, or greater (y, z, and 45°), see Figure 13b–d, but lower compared to the specimens with a 90% infill density (except for the z-direction). All specimens exhibited significant variations in ductility (strain to failure), with z-stitching specimens offering ductility levels that were at least comparable to the specimens with the rectilinear infills. In general, as shown in Figure 13c,e, all 3D-printed structures sustained substantially higher strains to failure when loaded in the off-axis (45°) direction.

The reduced test data in Table 2 and Figure 14 indicate that the specimens with the rectilinear infill pattern of 90% density achieved the highest stiffness in the in-plane orientations, which was to be expected. When comparing these results for the rectilinear infill pattern with 60% density and the z-stitching structure, it is interesting to note that the latter regularly achieved a 4% to 59% high stiffness for the different loading scenarios. In the out-of-plane orientation (z-axis), the z-stitching structure provided notably greater stiffness by a factor of 4.3 and 2.3 compared to the 60% and 90% rectilinear infill patterns, respectively.

Referring again to Table 2 and Figure 14, the specimens with a rectilinear infill pattern of 90% density performed as one would expect in terms of yield and ultimate strength in the x-direction, achieving the highest values (broadly 40% greater than the other specimens), while the specimens with the z-stitching structure exhibited comparable strengths, albeit slightly lower, than the specimen with 60% rectilinear infill. The strength results for the y-axis and z-axis are peculiar, where both the yield and ultimate strength values for the specimens with rectilinear infill patterns are notably lower than for the x-direction. Recall that the contour for all specimens was printed exclusively in the x-direction. The loading being transverse to the contour filament orientation is seen as the reason for the observed reductions in strength for the y- and z-direction loading scenarios, presumably due to interlayer and interlayer failure in the contour that then propagated to the remainder of the specimen body. The z-stitching specimens performed remarkably well in these loading scenarios (see Figure 14b,c), as they significantly exceeded the strengths of the other specimens by 19% to 264%. It can be speculated that the reason for this behavior is rooted in the more complex internal structure of the z-stitching specimens, thus their greater resilience to failure. For the loading at a 45° angle in the raster direction, the specimens with the 90% infill again achieved the highest strengths. Of the specimens with 60% density, the z-stitching specimens were remarkably closer in performance to the specimens with the 90% infill than the rectilinear 60% infill. Again, the latter indicates the positive effect that the z-stitching pattern has on specimen strength. To further investigate the benefits that z-stitching has on mechanical performance, a numerical modeling approach is considered useful (e.g., finite element modeling), yet such work is beyond the scope of the present proof-of-concept study.

To provide some greater context for the present findings, ultimate tensile strength data from the technical literature are contrasted with the present results in

Table 3. Ultimate tensile strength (UTS) data from present testing and taken from the technical literature.

Material	Infill Type and Density	UTS in Raster Direction (MPa)	UTS Transverse to Raster Direction (MPa)	Isotropy Ratio (/)	Reference
PLA	Rectilinear 60%	25.6	17.4	0.67	[28]
PLA	Concentric 60%	28.2	18.3	0.64	[28]
PLA	Hilbert curve 60%	10	8.5	0.85	[28]
PLA	Grid 60%	25.2			[29]
PLA	Triangle 60%	23.4			[29]
PLA	Gyroid 60%	24.5			[29]
PLA	Rectilinear 60%	31.3	13.7	0.44	present
PLA	Z-stitch 60%	29.3	22.8	0.78	present

. The data from the literature relates to a variety of infill patterns. Note that the table also shows the isotropy ratio for the various testing campaigns, which is defined as the ratio between the minimum and the maximum value. It can be ascertained that the specimens with a rectilinear infill pattern of 60% density tested in the present study performed well compared to the published data, yet the ultimate tensile strength transverse to the raster direction (y-axis) is somewhat lower than the other rectilinear pattern in the list. Again, it can be speculated that this behavior stems from the contour, as applied to the specimens in the present work. Contrasting the results for the z-stitching specimens, the results are favorable in terms of strength for both loading directions, as well as for the isotropy ratio. Only the Hilbert curve infill pattern exhibited an isotropy ratio closer to unity than the z-stitching pattern, but for inferior strengths.

To further explore the z-stitching structure with respect to anisotropy, isotropy ratios (IR) were calculated according to Equation (4) for stiffness, yield, and ultimate strength for all specimen types, as presented in

Table 4. Isotropy ratios for stiffness, tensile yield, and ultimate strength.

Specimen Type	Isotropy Ratio with Respect to Stiffness (/)	Isotropy Ratio with Respect to Yield Strength (/)	Isotropy Ratio with Respect to Ultimate Strength (/)
Rectilinear 90%	0.29	0.19	0.24
Rectilinear 60%	0.22	0.22	0.22
Z-stitch 60%	0.76	0.62	0.75

and Figure 15. As shown in this table, the z-stitching structure achieved isotropy ratios of 0.62 or greater, with the values being notably higher than those for the tested rectilinear infill patterns.

$$IR={\displaystyle \frac{\min (P_x,P_y,P_z)}{\max (P_x,P_y,P_z)}}$$

(4)

where P are the orthogonal properties, i.e., stiffness, yield, and ultimate strength, for the respective coordinate direction.

4.3. Three-Point Bending Tests

Delamination, a failure mode controlled by interlayer adhesion, is one of the most common failures in 3D-printed parts, particularly in load-bearing applications [30]. To evaluate the effectiveness of the z-stitching technique in terms of mitigating delamination and flexural properties, three-point bending tests were conducted. The results are plotted in Figure 16.

Flexural stiffness, a measure of the material’s resistance to bending, reflects a structure’s ability to maintain its structural integrity under applied loads. Referring to Figure 16a, z-stitching specimens exhibited superior flexural stiffness and strength compared to specimens with the rectilinear 60% infill, and similar performance to the 90% infill. Notably, strains to failure were the highest for all z-stitching specimens. The higher capacity of z-stitching specimens to bear loads in this scenario is attributed to the stitch pattern’s ability to distribute forces more evenly across layers. It is hypothesized that the interlocking geometry enhances the load transfer between adjacent layers, reducing the likelihood of delamination. The situation is somewhat different for loading in the y-direction, as depicted in Figure 16b. While z-stitching specimens were still able to bear the highest strains to failure, their stiffness was notably lower than for the specimens with the rectilinear infill patterns. In terms of strength, the z-stitching specimens performed similarly to the specimens with the 60% rectilinear infill, but lower than those with the 90% infill. Presumably, the z-stitching pattern still provides a high level of resilience to failure, hence the superior strain to failure, but the pattern does not lend itself to providing a high bending stiffness in this direction.

4.4. Fabrication Time Analysis

Printing time was assessed for the fabrication of 50 mm cubes. Compared to the 60% rectilinear infill pattern, the printing time of the z-stitching samples was 14% longer, yet 13% shorter than for the 90% infill pattern. Compared to 60% rectilinear infill, the increase in printing time for the z-stitching pattern is considered moderate, considering the overall improvements demonstrated in mechanical performance. Clearly, the 90% infill produced a near-solid structure, with mechanical properties approaching the properties of the bulk material, but at the expense of increased printing time and greater material and energy usage to create a part. As such, one can ascertain an advantage of the z-stitching technique in terms of fabrication over the employed rectilinear infill patterns.

4.5. Limitations of the Z-Stitching Technique

While the z-stitching technique demonstrates notable improvements in mechanical performance and reduced anisotropy, certain limitations must be acknowledged, particularly concerning its practical implementation. These limitations relate primarily to challenges in printing parts with small curvatures and narrow dimensions, as well as complexities in toolpath planning and print speed optimization.

One of the anticipated challenges with the z-stitching technique is the difficulty in accurately printing parts with small curvatures or small dimensions. The staircase-like motion required to create the interlocking stitches inherently demands a certain minimum feature size to accommodate the nozzle movement and material deposition. When applied to geometries with tight curvatures or thin walls, it may not be possible to accommodate the required toolpath.

Implementing the z-stitching algorithm requires advanced toolpath planning to ensure consistent stitch placement and accurate layer alignment. Unlike conventional FFF, which follows a linear or cyclic toolpath, z-stitching involves a staggered deposition sequence that interweaves planar layers with interlocking stitches and supports. This complex motion necessitates precise coordination between the nozzle’s movement along all the axes of motion. For parts with complex part geometries, the algorithm must dynamically adjust the stitch dimensions and overlap ratios to maintain consistency in the interlocking pattern. This requires sophisticated computational processing, increasing the complexity of G-code generation and potentially extending the pre-processing time.

Nevertheless, the primary goal of this study was to advance 3D printing for large-scale industrial applications, where parts typically feature larger dimensions and less intricate details. Hence, the aforementioned challenges are considered manageable in the given context.

5. Conclusions

This study introduced the z-stitching technique, a novel approach to 3D printing designed to improve upon traditional FFF processes, particularly addressing issues related to interlayer integrity and mechanical anisotropy. The latter is seen as a challenge, especially when fiber-reinforced polymer filaments are being processed, which is the end purpose of this research. Tensile testing demonstrated that z-stitching resulted overall in advantages in terms of stiffness, ductility, yield, and ultimate strength over specimens with traditional infill patterns of the same density. In particular, z-stitching specimens were shown to improve ultimate strength when loaded transversely, out-of-plane, and 45° in the raster direction by 61%, 264%, and 50% over the rectilinear infill pattern with equivalent density, respectively. Similar results were observed for stiffness, i.e., 59%, 325%, and 49% improvements correspondingly. In addition, the z-stitching technique was effective in reducing anisotropy. Isotropy ratios, defined by the ratio between the minimum and maximum values for the principal in-plane directions, ranged between 0.62 and 0.76 for the tested loading scenarios. For comparison, isotropy ratios for the specimens with a 60% rectilinear infill pattern were 0.22. Flexural testing revealed further advantages of z-stitching, that is, greater resistance to failure in terms of attainable deflection before breaking compared to the rectilinear 60% infill pattern.

With regard to fabrication effort, the analysis revealed that the z-stitching technique moderately increased printing time over the rectilinear infill pattern with equivalent density. Hence, increased fabrication efforts, including greater complexity for toolpath planning, should be seen as a trade-off between performance and efficiency.

Overall, this proof-of-concept study exposed the z-stitching technique as an attractive option for advancing FFF technology. By promoting interlayer integrity and reducing structure anisotropy, while also maintaining or even enhancing mechanical performance compared to traditional printing strategies, z-stitching was shown to be an attractive technique that warrants further study. Future work may involve additional fabrication and testing campaigns, especially when employing fiber-reinforced polymers. It should be mentioned here that z-stitching has already successfully been employed in a pilot study by the present researchers, using an Anisoprint Composer A3 printer (Esch-sur-Alzette, Luxembourg) with a polyethylene terephthalate glycol (PETG) filament with continuous carbon fiber. Other envisioned research is high-fidelity numerical modeling using, e.g., finite element software to explore and optimize the z-stitching parameter space and to advance the understanding of the mechanical and failure behavior inside the complex z-stitching architecture.