An investigation on impact resistance of FDM processed
Nylon-12 parts using response surface methodology
Salam Nori Kamoona1, a), Syed Hasan Masood2, b), and Omar Ahmed Mohamed 3, c)
1
Midland Refineries Company, Al-Daura refinery, Baghdad, Iraq
Department of Mechanical and Product Design Engineering, Swinburne University of Technology, Hawthorn,
Victoria 3122, Australia.
3
Department of Mechanical and Product Design Engineering, Swinburne University of Technology, Hawthorn,
Victoria 3122, Australia.
2
Corresponding author: c)[email protected]
Abstract. Fused Deposition Modelling (FDM) is one of the leading additive manufacturing processes for plastic part
manufacturing. However, engineers often face difficulties to specify the actual levels of process parameters in FDM
process to achieve the proper mechanical properties of FDM fabricated parts. The effect of large number of FDM process
parameters and the interaction among them need to be understood to achieve desired level of mechanical performance. This
paper presents a study on the influence of three FDM process parameters (air gap, raster angle, and build orientation) on
the impact strength and mechanical properties of the FDM Nylon 12 fabricated parts by Fortus 450mc FDM machine. The
Response Surface Methodology (RSM) based on face centered central composite design was used to analyse, validate, and
optimize the results. The significance of parameters was statistically validated with the analysis of variance (ANOVA)
technique. The results show that the part build Y-orientations (flat) at 0° and 45° have a significant directly proportional
influence on the impact strength, while Z-orientation (upright) at 90° has indirectly proportional effect on the impact
strength. Moreover, raster angle has a much significant directly proportional influence on the impact strength at 0° and 60°
angles and indirectly proportion influence at 30 °.
INTRODUCTION
The product design and manufacturing cycle time reduction is considered as the most important factor in
manufacturing industries because of the marketplace competition, which leads to adoption of rapid fabrication
techniques over traditional fabrication methods [1]. Additive manufacturing (AM) technology represents a novel
method that builds and creates most complicated geometry parts using layer by layer fabrication technique. It will
produce parts with low cost and without using incurring process because of the absence of tooling [2]. Moreover,
additive manufacturing provides fabrication of functional assemblies by uniting sub-assemblies directly from
computer aided design (CAD) data, which will reduce time handling, object’s parts and storage requirement.
Nevertheless, additive manufacturing applications have not got much headway to replace conventional mass
production because of the slow speed of fabrication and limited range of the available materials and their properties
[3]. There are many approaches, which have been done to increase the range of applications. Firstly, efforts have been
made to develop new materials, which will have superior properties than the conventional materials and are more
compatible with the specific AM technology. Secondly, the work has been done to adjust and optimize the process
parameters and factors of the AM process before the fabrication stage, which will secure the properties improvement.
The second approach has received more attention than the first one by the researchers. Fused Deposition Modelling
(FDM) is one such AM process, which requires a large number of process parameters to be set to fabricate a quality
part [4]. The proper adjustment of the process parameters in FDM will improve the product’s performance and
properties, so it is important to study the influence of various process parameters on the mechanical properties and
select the best setting.
There have been a number of studies done to determine the optimum parameters of the FDM process for the parts
made by FDM machines associated with improving different properties and quality characteristics. Sood et al.[1]
found that small build part orientation decreases the tensile and flexural strength associating with the number of layers.
They also found that increasing of part orientation will increase the impact strength because of increasing number of
layers and raise the heat conduction towards bottom layers. Mohamed et al.[5] concluded that dynamic flexural
modulus and complex viscosity are highly affected by layer thickness, air gap, road width and number of contours.
Agnes and Volker et al. [6] explained that using negative air gap of different part orientation gave different results,
which showed that the strength for Y-direction specimen has improved over that for X-direction and Z-direction
specimens. Durgun & Ertan [7] reported that the part built in a perpendicular orientation showed lower mechanical
properties than the horizontally and vertically built samples because of the maximum stresses carried from the bonds
that stick the fibers together and give high chance to failure. Iyibilgin et al. [8] investigated the effect of sparse-build
style on rapid tooling specimens of Ultem plastics by using the FDM process. It was observed that the sparse-double
dense built coupons have higher modulus-to-mass and strength-to-mass ratios than the coupons built with sparse (no
double dense) in compression and flexure tests. Torrado and Roberson [9] showed that the anisotropic mechanical
properties in relation to the part build orientation is a characteristic of parts fabricated with three dimensions printing
technologies. Mohamed et al.[10] reported that layer thickness, air gap and number of contours have a marked effect
on dynamic mechanical properties. The previous studies have revealed that the FDM process exhibits an obvious
difference in mechanical properties in relation to the fabrication conditions. Apparently, the part properties are very
sensitive to the processing parameters because of their meso-structure effect and also fibre-to-fibre bond strength. It
can be noticed that many efforts have been made on FDM strength evaluation, which are devoted to the study of the
influence of processing conditions on built part strength. However, to no studies seem to have been conducted in the
past to investigate the effect of FDM process parameters on the impact behavior of FDM processed Nylon-12 plastic
parts. This study investigates the influence of three process parameters on the impact properties of manufactured
Nylon 12 prototypes processed by FDM technology using Response Surface Methodology. An empirical model is
derived to establish and examine relationships between the three manufacturing parameters (air gap, part orientation,
and raster angle) and the impact properties using design of experiment based on face centered composite design
(FCCD) and analysis of variances (ANOVA).
EXPERIMENTAL PROCEDURE
The face centered composite design (FCCD) is one of the commonly used response surface design experiments,
which contains imbedded factorial design with centre points. It is used to find the best set of values for a given set of
factors providing an optimal response. This design approach helps in exploring quadratic surface response,
representing each experimental response. In this study, a three-factor, three levels FCCD is used and FCCD is applied
to determine the interaction of impact strength with FDM process parameters. Statistical polynomial models with
interaction terms were derived to investigate the relative influence of the three variables. The polynomial model is
based on regression analysis of the statistically significant variables to allow the study of the effects of each process
parameter and their interaction over the considered response. For impact testing, 20 specimens were manufactured on
Stratasys FDM Fortus 450 mc at the Swinburne University of Technology. Each specimen has a rectangular shape
with dimensions 62.5 x 12 x 3 mm as shown in Fig.1a following ASTM D6110 - 10. The material of these specimens
is Nylon 12, which is associated with high fatigue resistance, high impact strength and good temperature resistance
compared to other engineering thermoplastics. This plastic is used in many applications, for instance, panels, covers,
control ducting and venting, bosses, components with high vibration/fatigue, prototyping, cutting fixture and other
applications. Izod impact test was done by using Instron testing impact machine, as shown in Fig.1b. Table 1 shows
the three FDM process parameters considered in this study, which are air gap (A), part build orientation (B) and raster
angle (R). These process parameters and their levels were selected based on practical use and according to the previous
studies [1, 10-13]. Other FDM parameters were kept constant at their levels without any changes in order to
concentrate on these particular parameters. The process parameters considered in the current study are represented
graphically in Fig.2. Regression analysis has been used to generate the model, do the optimization and validity of
results, which was also tested using graphical tool method and analysis of variance (ANOVA). Table 2 shows the
experimental design matrix based on faced centered central composite design. Each row in the matrix represents an
experimental run and each run represents a result of the response or dependent variable (impact strength). Minitab 17
was used for planning and analyzing the design of experiment. The impact strength was calculated by the equation. 1
using average values of three test results for each case, where K is impact energy in joules, and T is the dimensions of
test specimen.
Impact strength (J/m) =
K
T
(1)
FIGURE 1. (a) Nylon 12 specimen dimensions, and (b) sample mounted in Zwick Roell machine
FIGURE 2. FDM process parameters
TABLE 1. Parameters and their levels
Parameters
Air Gap
Raster Angle
Part Build Orientation
Symbol
A
R
B
Level 1
0 mm
0°
0°
Y-orientation (flat)
Level 2
0.1 mm
30°
45°
Y-orientation (flat)
Level 3
0.2 mm
60°
90°
Z-orientation (upright)
RESULTS AND DISCUSSION
The results of impact strength were collected and calculated to produce the final readings which were used directly
in Minitab 17 software to analyse and optimize the process parameters. Experimental design matrix along with the
measured test values of impact strength is presented in Table 2. In this study, the quadratic polynomial model that
represents the response surface is expressed as:
𝑘
𝑘
𝑌 = 𝛽0 + ∑ 𝛽𝑖 𝑋𝑖 +
𝑖=1
𝑘
∑ 𝛽𝑖𝑖 𝑋𝑖2
𝑖=1
+ ∑ 𝛽𝑖𝑗 𝑋𝑖 𝑋𝑗 + 𝜀
(2)
𝑖<𝑗
where 𝑌 denotes the predicted response, 𝑋𝑖 and 𝑋𝑗 are the coded input variables, 𝑘 is the total number of variables, β0
is the constant term of the regression equation, 𝛽𝑖 is the linear regression coefficient, 𝛽𝑖𝑖 is the regression coefficient
for quadratic terms, 𝛽𝑖𝑗 is the regression coefficient for interaction terms, and 𝜀 is the random error, which contains
measurement error and other variability.
TABLE 2. Face centered central composite design matrix along with measured impact strength
Run
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Air gap
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0
0.1
0
0.2
0.2
0.1
0
0.1
0.2
0.1
0.2
0
0
Raster angle
30
60
30
30
30
0
30
30
30
0
60
0
30
60
30
60
30
0
60
0
Part build orientation
45
45
45
45
90
45
0
45
45
90
90
0
45
0
45
0
45
90
90
0
Impact strength )J/m(
203.6
206.1
146.1
173
28.2
324.8
232.1
128.2
193
38.5
34.5
215.2
186.7
207.9
165.8
254.2
195.2
31.2
41.5
125.5
By applying face centered composite design (FCCD) method, ANOVA results for full quadratic analysis response
surface model are shown in Table 3. The quadratic model is relying on the linear, square and interaction relationship
of parameters’ influences on the impact strength in order to validate the results. In order to improve the regression
model, the insignificant terms with high probability values were removed using backward elimination. This will
estimate the new parameters’ relation in term of their effects and coefficients for response. The final reduced quadratic
polynomial model is presented by the equation. 3. Throughout ANOVA, it can be noticed that there is a significant
individual contribution of factor B (part build orientation) with 0.000 P-value at 46.45 F-value and the square
contribution of the same factor B*B (part build orientation)2 with 0.06 P-value at 10.78 F-value. The square value
refers to a curvature polynomial relation in the response surface between the air gap and the impact strength.
𝐼𝑚𝑝𝑎𝑐𝑡 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ (𝐽/𝑚) = 188.9 + 1064 𝐴 − 3.72 𝑅 + 1.60 𝐵 − 4334 𝐴 ∗ 𝐴 + 0.0625 𝑅 ∗ 𝑅 −
0.0391 𝐵 ∗ 𝐵
(3)
TABLE 3. ANOVA for reduced quadratic polynomial model
Source
Model
Linear
A
R
B
Square
A*A
R*R
B*B
Error
Lack-of-Fit
Pure Error
Total
Degree of freedom
6
3
1
1
1
3
1
1
1
13
10
3
19
Adj sum of squares
113768
78025
3885
8
74132
35743
5166
8689
17202
20748
19935
814
134516
Adj mean squares
18961.3
26008.3
3884.8
8.1
74132.1
11914.3
5165.7
8688.5
17202.3
1596.0
1993.5
271.3
F-Value
11.88
16.30
2.43
0.01
46.45
7.46
3.24
5.44
10.78
P-Value
0.000
0.000
0.143
0.944
0.000
0.004
0.095
0.036
0.006
7.35
0.064
Fig.3 shows the main effect plot for the effect of processing parameters on impact strength. The significance of
increasing air gap underlies the effect of the long and few layers that lead to increase the heat conduction towards
bottom layers and causes rising in the temperature at bonding interface. Thus, a good diffusion will take place among
the adjacent rasters, as well as, it reduces the interior stress among the adjacent beads as a result of few voids and
porosities. It can be noticed that the impact strength will be higher at Y-orientation (flat) part build orientation between
0° and 45°, while it will decrease at Z-orientation (upright) at 90°. The highest impact strength was represented by
specimen No. 6 with air gap (0.1), raster angle (0°) and part build orientation (45°). Fig.4a shows the cross section of
the fracture surfaces of specimen 6, which indicates the close bead formation for layers, leading to high values of
impact strength. On the other hand, a sharp degradation took place at 90 ° Z-orientation (upright) part build orientation
and the impact strength decreased as shown in Fig.4b. The reason for that observation is that the high number of short
layers at high temperature gradient across the bottom of part will increase the number of cycle of heating/cooling and
will cause the increase of residual stresses inside the bonds that stuck the fibres together. These reasons could lead to
interlayer cracking, distortion, and part de-lamination or fabrication failure. Moreover, too many voids and porosity
will cause brittle and weak structure resulting in decrease of the impact strength. The lowest value was represented by
specimen No. 5 with air gap (0.1), raster angle (30°) and part build orientation (90°).
FIGURE 3. Individual influence of the parameters on impact strength
FIGURE 4. Micrographs for cross section area of the fractured surface for (a) sample No.6, and (b) sample No.5
The rest of parameters in term of linear, square, and interaction terms have insignificant influence on the impact
strength. However, they have some influences on the impact strength in case of individual and interactional terms.
Fig.5 shows the interaction plots of parameters for the impact strength. Individually, Fig.5 illustrates that the impact
strength has a lower value at raster angle of 30°, while it has a higher value between the angles 0° and 60°. Moreover,
the impact strength has a higher value at 0.1 mm air gap than 0 mm and 0.2 mm air gap. In term of interaction models
the fitted means plots illustrate that the parameters’ interactions have considerable influence on the mean of impact
strength (Fig.5). The impact strength will be of high values at the interactions of:



Air gap 0.1 mm and 60° raster angle
Air gap 0.1 mm and 0°-Y-orientation (flat) part build orientation
Raster angle 0°and 45°-Y-orientation (flat) part build orientation
FIGURE 5. Interaction plot of impact strength among the parameters
The validation of reduced regression models can be inspected graphically as shown in the residual analysis plots
for impact strength in Fig.6. It can be noticed that residual plots after elimination of insignificant parameters are much
better than before. The normal probability plot has very close blue dots around the red line except one. The
experimental data points thus follow the straight line demonstrating that the residuals have a normal distribution. The
linear pattern of these plots means that the predicted values are well fitted with actual values. Moreover, the shape of
histogram is also normal distribution. On the other hand, the residuals versus fitted values has random scatter, but all
the dots are inside the limits.
FIGURE 6 Residual analysis for reduced model for impact strength
CONCLUSION
In this investigation, the functional relationship between FDM process parameters and impact strength were
examined for twenty runs using response surface methodology based on FCCD. The observed results can be
interpreted to show the following conclusions in relation to the effect of process parameters on the impact strength.



Increasing part build orientation in the Y-orientation from 0° to 45° will increase the impact strength as a
result of increasing the number of long layers that leads to increase the heat conduction towards bottom layers
and causes raising the temperature at bonding interface. Then good diffusion will take place among the
adjacent rasters.
Part build orientation at Z-orientation (upright) at 90° will reduce the impact strength because of high number
of short layers with high temperature gradient located across the bottom of part, which will increase the cycle
of heating/cooling numbers and will cause increase of residual stresses. Then it will cause interlayer cracking,
distortion, and part de-lamination/fabrication failure.
Raster angle at 0° and 60 gives more significant effect and have directly proportional influence on the impact
strength especially at over 30° because of the position direction of diffused bead towards the impact direction.
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