**1. Introduction**

Layered manufacturing (LM) methods have traditionally been used for rapid prototyping (RP) purposes, with the primary intention of fabricating models for visualization, design verification, and kinematic functionality testing of developing assemblies during the product realization process (Caulfield et al., 2007). Without any need for tooling or fixturing, LM allows for the computer-controlled fabrication of parts in a single setup directly from a computerized solid model. These characteristics have proven beneficial in regard to the objective of reducing the time needed to complete the product development cycle (Chua et al., 2005).

There are numerous LM processes available in the market today, including stereolithography (SLA), fused deposition modeling (FDM), selective laser sintering (SLS), and three-dimensional printing (3DP), all of which are additive processes sharing important commonalities (Upcraft & Fletcher, 2003). For each of these processes, the object design is first represented as a solid model within a computer aided design (CAD) software package and then exported into tessellated format as an STL file. This faceted model is then imported into the relevant LM machine software where it is mathematically sliced into a series of parallel cross-sections or layers. The software creates a machine traverse path for each slice, including instructions for the creation of any necessary scaffolding to support overhanging slice portions. The physical part is then fabricated, starting with the bottom-most layer, by incrementally building one model slice on top of the previously built layer. This additive layering process is thus capable of fabricating components with complex geometrical shapes in a single setup without the need for tooling or human intervention or monitoring.

In recent years, layered manufacturing processes have begun to progress from rapid prototyping techniques towards rapid manufacturing methods, where the objective is now to produce finished components for potential end use in a product (Caulfield et al., 2007). LM is especially promising for the fabrication of specific need, low volume products such as replacement parts for larger systems. This trend accentuates the need, however, for a thorough understanding of the associated mechanical properties and the resulting behaviour of parts produced by layered methods. Not only must the base material be durable, but the mechanical properties of the layered components must be sufficient to meet in-service loading and operational requirements, and be reasonably comparable to parts produced by more traditional manufacturing techniques.

Anisotropic Mechanical Properties of ABS Parts Fabricated by Fused Deposition Modelling 161

orientation, air gap, and ABS color on both tensile and compressive strengths of FDM parts. It was determined that both air gap and raster orientation had significant effects on the resulting tensile strength, while compressive strength was not affected by these factors. Their results include a set of recommended build rules for designing FDM parts. A similar study was completed by Sood et al. (2010), with varying factors of layer thickness, build orientation, raster angle, raster width, and air gap. These researchers implemented a central composite experiment design and analyzed the functional relationship between process parameters and specimen strength using response surface methodology. Their results indicate that the tested factors influence the mesostructural configuration of the built part as well as the bonding and distortion within the part. On the basis of this work, Sood et al. (2011) further examined the effect of the same five process parameters on the subsequent compressive strength of test specimens. Their work provides insight into the complex dependency of compressive stress on these parameters and develops a statistically validated predictive equation. Results display the importance of fiber-to-fiber bond strength and the control of distortion during the build process. Lee et al. (2005) concluded that layer thickness, raster angle, and air gap influence the elastic performance of compliant ABS prototypes manufactured by fused deposition. A study conducted by Es Said et al. (2000) analyzed the effect of raster orientation and the subsequent alignment of polymer molecules along the direction of deposition during fabrication. These researchers considered the issue of volumetric shrinkage and raster orientation with respect to tensile, flexural and impact strengths. Lee et al. (2007) focused on the compressive strength of layered parts as a function of build direction. They determined that the compressive strength is greater for the axial FDM specimens than for the transverse. The foregoing studies reveal the directional dependence or anisotropy of the mechanical properties of FDM parts as a result of mesostructure and fiber-to-fiber bond strength, and provide numerous insights and recommendations regarding significant process parameters and the development of

The goal of this project is to quantitatively analyze the potential of fused deposition modeling to fully evolve into a rapid manufacturing tool. The project objective is to develop an understanding of the dependence of the mechanical properties of FDM parts on raster orientation and to assess whether these parts are capable of maintaining their integrity while under service loading. The study utilizes the insights provided by previous researchers and further examines the effect of fiber orientation on a variety of important mechanical properties of ABS components fabricated by fused deposition modeling. This study uses FDM build recommendations provided in previous work, as well as the defined machine default values, in order to focus analysis specifically on the significant issue of fiber or raster orientation, i.e. the direction of the polymer beads (roads) relative to the loading direction of the part. Tensile, compressive, flexural, impact, and fatigue strength properties of FDM specimens are examined, evaluated, and placed in context in comparison with the properties

All of the FDM specimens tested and analyzed in this study were acrylonitrile butadiene styrene (ABS). ABS is a carbon chain copolymer belonging to styrene ter-polymer chemical

component build rules.

of injection molded ABS parts.

**2. Experimental procedure** 

**2.1 Materials** 

Fused deposition modeling (FDM) by Stratasys Inc. is one such layered manufacturing technology that produces parts with complex geometries by the layering of extruded materials, such as durable acrylonitrile butadiene styrene (ABS) plastic (Figure 1). In this process, the build material is initially in the raw form of a flexible filament. The feedstock filament is then partially melted and extruded though a heated nozzle within a temperature controlled build environment. The material is extruded in a thin layer onto the previously built model layer on the build platform in the form of a prescribed two-dimensional (x-y) layer pattern (Sun et al., 2008). The deposited material cools, solidifies, and bonds with adjoining material. After an entire layer is deposited, the build platform moves downward along the z-axis by an increment equal to the filament height (layer thickness) and the next layer is deposited on top of it.

Fig. 1. Schematic of the FDM process

If the model requires structural support for any overhanging geometry, a second nozzle simultaneously extrudes layers of a water soluble support material in this same manner. Once the build process is completed, the support material is dissolved and the FDM part can be viewed as a laminate composite structure with vertically stacked layers of bonded fibers or rasters (Sood et al., 2011). Consequently, the mechanical properties of FDM parts are not solely controlled by the build material of the original filament, but are also significantly influenced by a directionally-dependent production process that fabricates components with anisotropic characteristics associated with the inherent layering.

Several researchers have specifically considered the anisotropic characteristics of FDM parts in recent years. Rodriguez et al. (2001) investigated the tensile strength and elastic modulus of FDM specimens with varying mesostructures in comparison with the properties of the ABS monofilament feedstock. They determined that the tensile strength was the greatest for parts with fibers aligned with the axis of the tension force. Ahn et al. (2002) designed a factorial experiment to quantify the effects of model temperature, bead width, raster orientation, air gap, and ABS color on both tensile and compressive strengths of FDM parts. It was determined that both air gap and raster orientation had significant effects on the resulting tensile strength, while compressive strength was not affected by these factors. Their results include a set of recommended build rules for designing FDM parts. A similar study was completed by Sood et al. (2010), with varying factors of layer thickness, build orientation, raster angle, raster width, and air gap. These researchers implemented a central composite experiment design and analyzed the functional relationship between process parameters and specimen strength using response surface methodology. Their results indicate that the tested factors influence the mesostructural configuration of the built part as well as the bonding and distortion within the part. On the basis of this work, Sood et al. (2011) further examined the effect of the same five process parameters on the subsequent compressive strength of test specimens. Their work provides insight into the complex dependency of compressive stress on these parameters and develops a statistically validated predictive equation. Results display the importance of fiber-to-fiber bond strength and the control of distortion during the build process. Lee et al. (2005) concluded that layer thickness, raster angle, and air gap influence the elastic performance of compliant ABS prototypes manufactured by fused deposition. A study conducted by Es Said et al. (2000) analyzed the effect of raster orientation and the subsequent alignment of polymer molecules along the direction of deposition during fabrication. These researchers considered the issue of volumetric shrinkage and raster orientation with respect to tensile, flexural and impact strengths. Lee et al. (2007) focused on the compressive strength of layered parts as a function of build direction. They determined that the compressive strength is greater for the axial FDM specimens than for the transverse. The foregoing studies reveal the directional dependence or anisotropy of the mechanical properties of FDM parts as a result of mesostructure and fiber-to-fiber bond strength, and provide numerous insights and recommendations regarding significant process parameters and the development of component build rules.

The goal of this project is to quantitatively analyze the potential of fused deposition modeling to fully evolve into a rapid manufacturing tool. The project objective is to develop an understanding of the dependence of the mechanical properties of FDM parts on raster orientation and to assess whether these parts are capable of maintaining their integrity while under service loading. The study utilizes the insights provided by previous researchers and further examines the effect of fiber orientation on a variety of important mechanical properties of ABS components fabricated by fused deposition modeling. This study uses FDM build recommendations provided in previous work, as well as the defined machine default values, in order to focus analysis specifically on the significant issue of fiber or raster orientation, i.e. the direction of the polymer beads (roads) relative to the loading direction of the part. Tensile, compressive, flexural, impact, and fatigue strength properties of FDM specimens are examined, evaluated, and placed in context in comparison with the properties of injection molded ABS parts.
