**2.1 Materials**

160 Mechanical Engineering

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

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

nozzle

ABS filament

*Temperature-controlled environment*

liquefier

z

x y

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

anisotropic characteristics associated with the inherent layering.

rollers

build platform

FDM head

layer is deposited on top of it.

Fig. 1. Schematic of the FDM process

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

Anisotropic Mechanical Properties of ABS Parts Fabricated by Fused Deposition Modelling 163

(a) (b)

(c) (d)

All FDM specimens were built while holding all other machine process settings at the

Factor Value/Level Air gap 0.0 mm Nozzle T12 Road width 0.3048 mm Slice height 0.1778 mm Part interior fill style Solid normal Part fill style Perimeter/raster

Liquefier temperature 320 ºC Envelope temperature 80 ºC

In order to measure the reference strength and behaviour of the ABS filament material, for comparisons with the layered parts, additional specimens were fabricated by injection molding for the same five tests. Aluminium molds for each of the three previously described geometries (Figure 2) were designed using Pro/Engineer® software, and manufactured on a Haas VF-1 CNC machining center. Mold cavity dimensions were the same as those described for the FDM specimens, with slight increases to compensate for the shrinkage of molded ABS at approximately 0.005 cm/cm. Parting lines and runners were located with an effort to avoid potential stress concentrations or anomalies in the resulting specimens that might affect test results. All molded specimens were fabricated from the same material as the layered models by feeding the FDM-ABS filament into a polymer granulator and cutting it into pellets of 3-5 mm in length. The pellets were then fed into the hopper of a Morgan Press G-100T injection molding machine. Molding parameters were set to the recommended values for ABS plastic, including nozzle temperature of 270 °C, mold preheat temperature of 120° C, clamping force of 71 kN (16,000 lb), and injection pressure of 41 MPa (6000 psi). Ten

Tensile, compressive, flexural, and tension-fatigue tests were performed on an Instron model 3366 dual column uniaxial material testing with .057 micron displacement precision and up to 0.001 N force accuracy. The machine has a 10kN load force capacity. Impact strength was studied on a TMI impact tester. Resulting fracture surfaces were subsequently prepared by gold sputtering and analyzed with a JSM 500-type JEOL Scanning Electron

Fig. 3. Four different raster orientations investigated

recommended or default values displayed in Table 1.

replicate specimens were molded for each of the five tests.

Table 1. Fixed FDM process settings

Microscope (SEM).

family. It is a common thermoplastic that is formed by dissolving butadiene-styrene copolymer in a mixture of acrylonitrile and styrene monomers, and then polymerizing the monomers with free radial initiators (Odian, 2004). The result is a long chain of polybutadiene crisscrossed with shorter chains of poly(styrene-co-acrylonitrile). The advantage of ABS is that it combines the strength and rigidity of the acrylonitrile and styrene polymers with the toughness of the polybutadiene rubber. The proportions can vary from 15 to 35% acrylonitrile, 5 to 30% butadiene, and 40 to 60% styrene. In this study, the resulting composition was 90-100% acrylonitrile/butadiene/styrene resin, with 0-2% mineral oil, 0-2% tallow, and 0-2% wax.
