Failure investigation of fused deposition modelling parts fabricated at different raster angles under tensile and flexural loading

Abstract

Fused deposition modelling is an efficient rapid prototyping process used to rapidly fabricate three-dimensional solid objects with complicated geometries. Many process parameters affect the fused deposition modelling process and their settings influence the quality of the specimen. This article investigates the effect of raster angle on surface roughness (along and across the length direction) and mechanical properties (tensile and flexural strength) of fused deposition modelling parts built at 10 different raster angles (0°–90° at 10° interval). All parts are built using acrylonitrile butadiene styrene P430 material. Surface roughness for circular and parabolic curved surfaces is also measured when specimens are built at different raster angles. Fracture surfaces are inspected with scanning electron microscope to study the modes of failure under different loading conditions. The samples where raster angle is 0° and layers are deposited along the length of the specimen exhibited optimal mechanical strength and good surface finish (when measured along the length). Scanning electron microscope results reveal that for 0° raster angle, failures are mainly due to pulling and rupture of fibres and for 30° and 60° raster angle, failure is due to brittle shear in a direction parallel to raster orientation. Due to the presence of number of heating and cooling cycle in 90° raster orientation, interlayer cracking and distortion of raster take place leading to lower strength.

Introduction

Fused deposition modelling (FDM) process is one of the widely used rapid prototyping (RP) technologies that rapidly fabricate three-dimensional (3D) solid objects with complex geometries. In this process, the model material is deposited layer-by-layer (additive manufacturing) through a very fine nozzle that moves in X and Y directions and fabricate a physical model by extruding semi-liquid thermoplastic material in the form of layers. After depositing one layer, the worktable moves down so that the next layer can be deposited along the depth of the component. The specific pattern of deposition and the nozzle movement, that is, the raster pattern decides the tool path, deposition time and also controls most of the mechanical and physical properties of the fabricated part. The nozzle position along the tool path is controlled by the computer relative to the base which allows the fabrication of parts with enhanced geometric complexity maintaining a tight tolerance. ¹ To fabricate the part without any geometric damage/distortion, the parts are built with simultaneous deposition of support materials at the desired locations. FDM uses two separate nozzles, one for the part material (model material) deposition and another for support structure deposition. Both model and support materials are used is in the form of filaments which melt at preselected temperature and rapidly solidify in 0.1 s when adhered to the previous layer. ¹ The bonding between the neighbouring fibres takes place through a thermally driven diffusion welding. ² The second nozzle creates a support structure that can be removed easily through a wash basin once the model is completed. The entire model is built on horizontal platform which is lowered in Z direction as each layer is added. ³

In FDM process, final mechanical properties (stiffness, tensile strength, flexural strength, toughness, etc.), surface finish and geometric accuracy obtained are often influenced by number of process parameters such as raster width, raster angle, air gap and layer thickness. Many attempts have been made to determine optimum process parameters for FDM process to obtain good surface finish, dimensional accuracy and minimum built cost of the FDM built parts.^4–7 The influence of layer orientation on mechanical properties (tensile strength, flexural strength and impact resistance) was investigated for acrylonitrile butadiene styrene (ABS) P400 RP parts. ⁸ Influence of air gap, raster width, raster angle and layer thickness was investigated on elasticity of ABS flexible part using Taguchi’s method followed by analysis of variance (ANOVA) to determine the optimal performance of ABS parts. ⁹ The influence of layer thickness, part orientation, raster width, raster angle and air gap was investigated on tensile, flexural and impact strength of FDM specimens using design of experiment and statistical analysis approach. ¹⁰ Using finite element analysis and considering coupled thermal and mechanical phenomena, part distortion and residual stresses were studied. ¹¹ Parametric study to evaluate the effect of process parameters on distortion revealed that scan speed followed by layer thickness are the most significant factors affecting part distortions while residual stresses are significantly influenced by layer thickness and road width. Optimal part orientation that reduces built time and enhances part quality was proposed using swarm intelligence approach. ¹² Non-dominated sorting genetic algorithm was used to determine optimum orientation of FDM parts considering built time and average surface roughness as objective functions. ¹³ The influence of raster angle and part orientation was investigated on surface roughness and mechanical properties of ABS P430 parts. ¹⁴ The effect of raster angle on tensile strength, elastic modulus, shear modulus and Poisson’s ratio was investigated for FDM built parts using both experimental and analytical models. ¹⁵ The effect of staircase effect on surface roughness of FDM part was investigated and a hybrid machine combining FDM and hot cutter machining was proposed to overcome the staircase effect and improve the surface quality. ¹⁶ The influence of scanning speed and layer thickness was investigated on mechanical properties, surface roughness, accuracy, speed and material cost of parts fabricated by various RP processes. ¹⁷ Effect of process parameters on built time, surface roughness and mechanical behaviour was investigated for FDM parts.^18,19

Literature suggests that the quality of FDM part may be affected by different process parameters; however, once the part orientation, slice thickness and so on are decided to build a component, the variation in mechanical strength and surface roughness is dependent on the raster layer directions. For functional parts built by FDM, based on the complexity of the part raster in different sections (angle between the deposited layer and parts’ longitudinal direction) may be different with respect to the subpart of the component. During service under loading conditions, these different regions will have variation in mechanical properties. The objective of this study is to investigate the effect of raster angle on the mechanical properties of parts fabricated using ABS P430 material. The ability to retain their integrity under service condition is evaluated by studying the tensile strength and the flexural rigidity of the built specimens. A total of 10 raster orientations (0°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, 80°, 90°) are considered in this study to specifically analyse the direction of road beads (rasters) with respect to loading direction of the specimen. Surface roughness of flat surfaces is measured in both along and across the length of specimen. Moreover, the variations of surface roughness are investigated for circular as well as parabolic curved surfaces built with different raster angles. Later, rapture surfaces under tensile and flexural loading are studied by scanning electron microscope (SEM) to understand the failure mode of the specimen under different loading conditions.

Materials and methods

In this study, all the samples are built in a FDM^® machine (Mojo^™; Stratasys, Eden Prairie, MN, USA) having a maximum built size of 5″ × 5″ × 5″. All specimens are built using ABS P430^™ model material (Ivory; Stratasys), a common variant of ABS and SR30^™ support material (Stratasys). The parts are built by depositing the semi-molten material in form of layers with 0.178 mm constant layer thickness. The models for tensile and flexural testing are designed in Pro/Engineer 5.0 and exported as.STL file. The dimensions of the samples are decided as per the American Society for Testing and Materials (ASTM) standards ASTM D638-02 ²⁰ and ASTM D790-02 ²¹ for tensile and flexural testing of plastic materials. Figure 1(a) and (b) shows the detailed dimensions of the tensile and flexural sample, respectively, used for building the specimens at 10 different raster angles.

Figure 1.

Tensile and flexural testing specimen dimensions as per standards, samples built at different raster angles, their testing arrangement and part geometry with curved surfaces: (a) tensile specimen (as per ASTM D638), (b) flexural specimen (as per ASTM D790), (c) part geometry with curved surfaces, (d) tensile specimens built at different raster angles, (e) tensile testing attachment, (f) flexural specimens built at different raster angles and (g) flexural testing attachment.

The machine software sliced the model and generates the deposition tool path, that is, raster pattern which is used to fill the interior region of each layer. Position and orientations of the model for building are decided in such a way that layers are deposited in 10 different angles with respect to the length of the part. Figure 1(c), (d) and (f) shows the samples built for surface roughness measurement, tensile and flexural testing.

Tensile and flexural testing (three-point bending test) of the samples are conducted on ultimate tensile testing (UTM) machine of Zwick/Roell Z010 (Zwick Roell, Ulm, Germany) having 10 kN load capacity. Gripper arrangements on the machine for tensile and flexural testing are shown in Figure 1(e) and (g), respectively. Tensile testing are conducted at a cross-head speed of 5 mm/min (as per ASTM D638), whereas in flexural testing, specimens are held on two supports rollers (maintaining support span to depth ratio = 16) and load is applied in the middle until the specimen gets fractured. Cross-head speed for flexural test is calculated by $R=(Z{L}^2)/6t$ (as per ASTM D790) and is obtained as 1.41 mm/min, when Z = rate of straining of the outer fibre in mm-mm⁻¹-min⁻¹ and should be equal to 0.01. ²¹ From the results of maximum load at fracture, the modulus of rupture values are calculated for different samples. The flexural strength or modulus of rapture (MOR) is calculated by equation (1)

$$\mathrm{Modulus\; of\; rupture}=\frac{3FL}{2b{t}^2}$$

(1)

where F is the maximum load applied (N), L = length between rollers (mm), b = width of specimen (mm) and t = thickness or depth of specimen (mm).

Built specimens are observed under the optical microscope to measure and verify the raster angle. Figure 2 represents optical micrographs of four samples to indicate the raster layer orientations. Figure 2(a)–(d) shows the micrographs of the samples built at raster angles 0°, 30°, 60° and 90°, respectively. Inset of each figure in Figure 2 schematically shows the raster patterns.

Figure 2.

Optical microscopic images of built specimens with four different raster angles: (a) 0°, (b) 30°, (c) 60° and (d) 90°.

The fracture surfaces are analysed by SEM (JSM 6510LV (SCM); JEOL, Tokyo, Japan) images to study the fracture behaviour of the FDM samples when built at different raster angles. The surface roughness of the parts is measured by a surface roughness tester (SJ400; Mitutoyo, Kawasaki, Japan) using cut-off length 0.8 mm. Roughness is measured twice and the average value is considered for analysis. Average surface roughness (R _a) and root mean square (rms) surface roughness (R _q) is measured in two orthogonal directions, that is, along and across the length of the specimen for each part on flat as well as two different curved surfaces.

Results and discussions

Surface roughness, tensile and flexural strength of components built at different raster angles are measured and analysed to study the effect of raster angle on these responses. Microscopic observations by SEM of tensile and flexural samples are carried out to understand the fracture behaviour of the specimens built by FDM.

Surface roughness

Surface roughness is measured on flat surfaces in directions along (parallel) and across the length (perpendicular) of the all tensile specimen, and average values are reported. Variation of R _a and R _q along and across the direction for different samples built at 10 different raster angles are shown in Figure 3.

Figure 3.

Surface roughness measurement for along and across the length of the specimen for different raster angles: (a) variation of R _a and (b) variation of R _q with raster angle.

It can be seen in Figure 3 that roughness values for flat surface increases with an increase in raster angle along the length, whereas it decreases when measured across the length direction. The best surface finish is obtained for 0° raster angle when measured along the length of the specimen. A possible explanation to this phenomenon is that with an increase in raster angle, number of raster increases with smaller rasters; therefore, more turns occur at the layer boundary and number of heating and cooling cycle increases as a part of FDM fabrication technology which causes increase in non-uniform thermal stresses and hence distortion. Moreover, due to smaller raster length, fluctuation occurs in straightness of depositing road because servo motor has to change the direction in very short time to synchronize in both X and Y directions. This fluctuation causes the void formation which increases the surface roughness of the specimen. Therefore, it can be noted that better surface finish is obtained for smaller raster angle. However, minimum roughness is observed across the length of the specimen for parts built at 90° raster angle. A moderate roughness is noted in both the directions for components formed at 40° and 50° raster angles and leads to a minimum mean roughness (average of roughness along and across the length) for 50° raster angle component.

To investigate the variation of roughness with raster angle on curved surfaces, another part with different geometric surfaces is designed and built with different raster angles (one built part is shown in Figure 1(c)). Surface roughness is measured on circular and parabolic curved surfaces of FDM model (indicated in Figure 1(c)) and variation of R _a and R _q for along and across the direction of curved surfaces built at three different raster angles (0°, 45°, 90°) is shown in Figure 4(a) and (b). From Figure 4, it can be seen that surface roughness increases with an increase in raster angle for both circular and parabolic surfaces when roughness is measured along the length of the specimen, whereas roughness decreases when measured across the length of the specimen. It may be noted that this variation is alike the variation observed for flat surfaces. The possible reason behind this phenomenon is already explained above for flat surfaces, that is, the minimum surface roughness is obtained for smaller raster angle (0° raster angle), and 90° raster specimen exhibits maximum surface roughness when measured along the length of the specimen. A moderate roughness is noted in both the directions for components formed at and around 45° raster angle for both circular and parabolic surfaces.

Figure 4.

Surface roughness measurement for (a) circular surface and (b) parabolic surface when specimens are built at different raster angles.

Tensile strength

Results of tensile strength for all the 10 specimens are shown in Figure 5. Uniaxial tensile testing is performed for all the samples. From the experimental results, it is clear that strength of the specimens in which raster are laid along the length of the specimen (0° raster angle) is higher than the specimen in which raster is laid across the length (90° raster angle). Samples built with 30° or 60° raster angle exhibit almost similar amount of ultimate tensile stress and elongation. However, parts with raster angles 70°, 80°, 40°, 50° or 10°, 20° have somewhat lower tensile strength. These results clearly indicate that FDM samples manufactured at different raster angles exhibit anisotropic properties. These properties are sensitive to processing parameters because these affect the meso-structure and bond strength between successive fibres. For example, in 90° raster angle, number of raster increases with small raster length, thus increasing the number of heating and cooling cycles which results in increase in residual stresses causing interlayer cracking, distortion, delamination and fabrication failure. Hence, tensile strength reduces, whereas in 0° raster angle, layers are aligned in a direction parallel to the loading direction and thus produces strongest direction for raster layers.

Figure 5.

Tensile strength measurement for specimen built at different raster angles.

The physical inspections of FDM specimens are done both at macroscopic and microscopic levels. The fracture patterns are examined macroscopically to understand the weakest path for crack propagations. The specimen with 0° raster angle failed in transverse direction with some fibre pullout and delamination occurs at weak points and tearing of individual fibres took place. For 90° specimens, crack propagates in the transverse direction (perpendicular to loading axis). Specimens with 30° and 60° raster angles failed along the 30° and 60° lines mostly following the line of the deposited layers along the raster direction. These tendencies and patterns of fractures are schematically represented in Figure 6(a)–(d).

Figure 6.

Schematic illustration of fracture type under uniaxial tensile loading of FDM built specimens at different raster angles: (a) 0°, (b) 30°, (c) 60° and (d) 90°.

Microscopically, fracture surfaces are inspected with the help of SEM and results showed that the failures are mainly because of the pulling and rupturing of the raster fibres. The separation of the material occurs in a perpendicular direction to tensile stress as shown in Figure 7. The tearing/brittle failure at a direction perpendicular to the loading can be observed in Figure 7(a) for the sample built with raster angle 0°. Fractures are mainly controlled by weak interlayer bonding and porosity caused due to volumetric shrinkage of polymer fibres during solidification. This interlayer porosity reduced the load caring capacity across raster layer and hence leads to early failure. Although in FDM machine, during fabrication, no air gap (gap between two slices or layers) is considered but still air voids are inherently present as can be observed in Figure 7(a). These voids reduce the tensile strength of the FDM parts. In 30° and 60° raster angles, failure of the specimen is due to brittle shear, as each raster is pulled under tensile loading and failed at 30° and 60° relative to tensile loading as seen in Figure 7(b) and (c). For specimen with 90° raster orientation, failure occurs at weak interface between the raster layers. Due to the presence of number of heating and cooling cycle in 90° raster orientation, interlayer cracking and distortion of raster take place (Figure 7(d)) which reduces the tensile strength.

Figure 7.

SEM micrograph of tensile failure samples of four specimens built with raster angles (a) 0°, (b) 30°, (c) 60° and (d) 90°.

Flexural strength

Flexural testing or three-point bending test for all 10 samples built at different raster angles is shown in Figure 8. From the maximum load, the modulus of rupture (MOR) for each sample is calculated (using equation (1)) and is obtained as 51.96, 53.86, 53.02, 50.72, 51.76, 50.14, 50.48, 50.1, 50.5 and 47.12 MPa, respectively, for samples with 0°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, 80° and 90° raster angles. These results clearly reveal that calculated flexural strength of the FDM specimens is greater than tensile strength for each raster angle as MOR measures the maximum flexural strength at the outer most fibres of the specimens. Flexural strength is highest for 10° and 20° raster angles. This is due to the fact that sample with low raster angle (in a range around 10°–30°) exhibits strong interlayer bonding due to relatively larger length of interlayer contact as compared to other rasters, the deposited layers are almost parallel to bending planes and thus offer resistance to bending.

Figure 8.

Flexural strength measurement for specimen built at different raster angles.

Their fracture tendency of specimen with 0° raster angle is shown in Figure 9(a). Moreover, as raster angle changes from 30° to 90°, their inclination with respect to bending plane changes, thus producing raster of smaller length under flexural loading wherein resistance to bending decreases. This effect is shown in Figure 9(d) where 90° raster orientation specimen shows little resistance to bending and catastrophic failure is noticed. Investigation of the fracture specimen also reveals that failure in specimen is initiated at tensile side but pieces are held together by the unbroken fibres of compression side. Crack propagation along the load direction is seen and it is almost straight for specimen built at 0° raster angle, whereas it is erratic and non-uniform for raster angles other than 0°. Figure 9 reveals that raster fail catastrophically under brittle fracture.

Figure 9.

SEM micrograph for specimen under flexural failure with raster angles (a) 0°, (b) 30°, (c) 60° and (d) 90°.

The summary of research objectives, materials, methodology and key findings of this study are summarized and depicted in Table 1.

Table 1.

Summary of research objectives and key findings of this study.

Target	Improved mechanical properties and good surface finish
Process parameters	Raster angle, layer thickness, air gap, road width, part orientation, part geometry
Parameters investigated	Raster angle (0°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, 80° and 90°) Part geometry (Flat surfaces, circular and parabolic curved surfaces)
Other process parameters	Model material: ABS P430 Support material: SR30 Layer thickness: 0.178 mm Air gap: 0 mm Part orientation: along Y axis
Testing conditions	Tensile and flexural specimen dimensions (ASTM D638, ASTM D790), refer Figure 1(a) and (b). Tensile testing: loading rate 5 mm/min (ASTM D638) Flexural testing: support span to depth ratio = 16, cross-head speed 1.41 mm/min (calculated as per ASTM D790)
Response measured/studied	Tensile strength (sample dimensions and testing conditions as per ASTM D638) Flexural strength (sample dimensions and testing conditions as per ASTM D790) Surface roughness (R a, R q along and across direction) Fracture behaviour by SEM observations
Key findings	The specimen with 0° raster angle offer more mechanical strength (tensile and flexural) as compared to 90° raster specimen. The specimen with 30° and 60° raster angles, failure of specimen is due to brittle failure as each raster is pulled and rapture under tensile loading. In 90°, raster specimen due to weak interlayer bonding between rasters reduces the mechanical strength and layer separation provides easy failure. Surface roughness for sample with 0° raster angle is minimum and maximum for 90° raster specimen when measured along the length of the specimen. Minimum roughness is observed for 90° raster angle and maximum for 0° raster angle when measured across the length of the specimen. Surface roughness of circular and parabolic surfaces is minimum for 0° raster angle and maximum for 90° raster angle when measured along the length. Moderate roughness is noted for curved surfaces formed at 45° raster angle when measured in both the direction.

Conclusion

In this study, functional relationship between raster angle and mechanical properties (tensile and flexural) and surface roughness of FDM manufactured parts are investigated for ABS material. From the results the following conclusions are noted:

• Tensile test indicated that tensile strength is the highest for samples built with 0° raster angle as compared to those built with raster angle of 90°. Tensile fractures are affected by the strength of each layer as well a direction of the raster. The longitudinal specimens (0° raster angle) offer more tensile strength as compared to 90° raster specimen as the alignment of raster is parallel to the loading direction.

• Flexural test indicated that MOR was highest for raster angle less than 30°, whereas specimen with 90° raster orientation exhibits minimum flexural strength. The fractures in flexural testing for all samples are catastrophic with brittle failure.

• Surface roughness results exhibited that average surface roughness for sample with 0° raster angle is minimum when measured along the length of the specimen. As the raster angle increases, the number of raster increases which causes more non-uniform thermal stresses and causes distortion and poor surface quality.

• Similarly, the minimum roughness is obtained for sample with 90° raster angle when roughness is measured in a direction across the length. However, minimum mean roughness (average of roughness along and across the length) is obtained for 50° raster angle component.

• Surface roughness results for circular and parabolic surfaces exhibit almost similar trend as flat surfaces. The minimum roughness is noted for 0° raster angle, whereas 90° raster specimen exhibit maximum surface roughness when measured along the length of the specimen. A moderate roughness is noted in both the directions for components formed at 45° raster angle.

• The optimum raster angle to achieve higher mechanical strength at lower surface roughness part built with 0° raster angle can be selected.