Mechanical and Viscoelastic Properties of Polylactic Acid (PLA) Materials Processed Through Fused Deposition Modelling (FDM) by Mst. Faujiya Afrose B.Sc. (Hons) in Mechanical Engineering A thesis submitted in fulfilment of the requirements for the degree of Master of Engineering School of Engineering Faculty of Science, Engineering and Technology Swinburne University of Technology Hawthorn, VIC 3122, Australia December 2016
97
Embed
Mechanical and viscoelastic properties of polylactic acid ......creep tests were conducted with Zwick Z010 and TA Instrument DMA 2980 machines to predict the changes of properties
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Mechanical and Viscoelastic Properties of Polylactic
Acid (PLA) Materials Processed Through Fused
Deposition Modelling (FDM)
by
Mst. Faujiya Afrose
B.Sc. (Hons) in Mechanical Engineering
A thesis submitted in fulfilment of the requirements for the degree of Master of Engineering
School of Engineering Faculty of Science, Engineering and Technology
Swinburne University of Technology Hawthorn, VIC 3122, Australia
December 2016
i
Declaration
I am declaring that the works presented in this thesis is my own work and to best
my knowledge, this work does not contain any materials which have not
previously published by any other person or submitted for the requirement of any
other degree, except where due acknowledgement and reference are provided
within the thesis. An explicit acknowledgement has been for the contribution
from any other colleagues within and outside the university.
Mst. Faujiya Afrose
05 December 2016
Declaration
ii
Abstract
Polylactic acid (PLA) is a biodegradable thermoplastic polymer and its bulk properties have
limited end use applications as the properties of materials change through its processing
methods. A great deal of attention has been paid to use PLA from packaging and biomedical
applications to structural loading applications such as in building, electrical and electronics,
furniture etc. The processing of PLA materials, mainly based on melt flow technique is time
consuming process and experienced poor surface finishing. Therefore, rapid prototyping (RP)
process has been considered for processing PLA material which fabricate physical prototype
from computer aided data (CAD) in a shorter time. The properties of PLA materials change
when processing through RP technology which has a great influence on the end use
applications.
The aim of this research is to investigate the mechanical and viscoelastic properties of PLA
material processed through fused deposition modelling (FDM) technique for end use
applications by taking account the changes of properties in three different build orientations.
In evaluating these properties, dog-boned shape and rectangular shape samples were fabricated
according to ASTM D638 and ASTM D 790 standards. To fabricate the PLA material samples
in X-, Y- and 450- build orientations, a Cube 3D FDM machine was used. Therefore, a number
of tests such as tensile, fatigue, flexural, impact, dynamic mechanical analysis (DMA) and
creep tests were conducted with Zwick Z010 and TA Instrument DMA 2980 machines to
predict the changes of properties in X-, Y- and 450- build orientations. Injection moulded
samples were also tested and to compare the results with FDM samples results.
In this study, it was found that the build orientations have great influence on mechanical
properties as well as viscoelastic properties. However, the results showed that certain build
orientation samples exhibit better properties than other build orientation samples and thus FDM
parts made in these build orientations assist in developing design guidelines for end use
applications under different loading conditions.
Abstract
iii
Acknowledgements
I would like to thank my principal coordinating supervisor, Associate Professor Pio Iovenitti
for his continuous support and guidance during my candidature. I really appreciate his help and
I was really pleased with his professionalism. He was always willing to support me despite his
extremely busy schedule.
I would also like to thank Professor Syed Masood whose guidance and suggestions were the
most valuable part parts of this work. I would not be able to publish any articles without his
support and encouragement. The help and guidance provided by Associate Professor Igor
Sbarski were helpful to get into the more insight of the research activities. I must also
acknowledge the help from Dr. Mostafa Nikzad and Mr. Warren during the experimental
activities of my research activities.
I am also grateful to my husband, Dr. Md Apel Mahmud for his continuous support. It was not
possible to start and complete the thesis without his help. I would also like to acknowledge the
help from my brother-in law for taking care of my baby during the period of my thesis writing.
At the end, I would like to thank my fellows who make my research journey enjoyable at
Swinburne University of Technology.
Acknowledgements
iv
Dedicated to my husband, my son, Faiyaz Mahmud and my parents
v
List of Publications
Journals
1. Afrose, M.F., Masood, S.H., Iovenitti, P., Nikzad, M. and Sbarski, I., 2015. Effects of part
build orientations on fatigue behaviour of FDM-processed PLA material. Progress in
Additive Manufacturing, pp.1-8.
2. Afrose, M.F., Masood, S.H., Nikzad, M. and Iovenitti, P., 2014. Effects of build
orientations on tensile properties of PLA material processed by FDM. In Advanced
Materials Research (Vol. 1044, pp. 31-34). Trans Tech Publications.
List of Publications
vi
Contents
Declaration i
Abstract ii
Acknowledgements iii
List of Publications v
List of Tables x
List of Figures xi
List of Symbols xiii
List of Abbreviations xiv
Chapter 1 Introduction 1
1.1 Overview 1
1.2 Polylactic acid (PLA) 2
1.3 Processing of Polylactic acid (PLA) 2
1.4 Properties of PLA 3
1.5 Research Project Aims 5
1.6 Contributions to New Knowledge 6
1.7 Thesis Structure 7
Chapter 2 Literature Review 9
2.1 Introduction 9
2.2 Rapid prototyping process 9
2.2.1 Stereolithography 11
Contents
vii
2.2.2 Solid base curing 11
2.2.3 Fused deposition modelling 12
2.2.4 Ballistic particle manufacturing 12
2.2.5 3D printing 13
2.2.6 Selective laser sintering 13
2.2.7 Laminated object manufacturing 14
2.3 FDM thermoplastics materials and their properties 15
2.3.1 Acrylonitrile Butadiene styrene (ABS) 15
2.3.2 Polycarbonate (PC) 17
2.3.3 Nylon 12 18
2.3.4 Acrylonitrile Styrene Acrylate (ASA) 19
2.3.5 Polyphenylsulfone (PPSF/PPSU) 20
2.3.6 ULTEM 21
2.3.7 Polylactic acid (PLA) 23
2.4 Applications of FDM thermoplastics 24
2.5 Properties of PLA 25
2.5.1 Tensile Properties 25
2.5.2 Fatigue Properties 26
2.5.3 Flexural Properties 27
2.5.4 Impact Properties 28
2.5.5 Dynamic Mechanical Properties 28
2.5.6 Creep Properties 30
2.6 Summary 30
Chapter 3 Materials and Test Methods 32
3.1 Introduction 32
Contents
viii
3.2 Materials 32
3. 2.1 Cube 2 FDM machine 33
3. 2.2 Part Fabrication by FDM 34
3. 2.3 Part Fabrication by Injection Moulding 36
3.3 Test Methods 37
3. 3.1 Tensile Test 37
3. 3.2 Fatigue Test 38
3.3.3 Flexure Test 40
3.3.4 Impact Test 40
3.3.5 DMA Test 41
3.3.6 Creep Test 43
3.4 Summary 44
Chapter 4 Mechanical Properties of FDM PLA Thermoplastic 45
4.1 Introduction 45
4.2 Tensile properties 45
4.3 Fatigue Properties 48
4.4 Impact Properties 53
4.5 Flexural Properties 55
4.6 Summary 57
Chapter 5 Viscoelastic Properties of FDM PLA Thermoplastic 58
5.1 Introduction 58
5.2 DMA Properties 58
5.3 Creep Properties 65
5.4 Summary 67
Contents
ix
Chapter 6 Conclusions and Further Research 68
6.1 Overview 68
6.2 Conclusions 68
6.3 Further Research 70
References 72
Contents
x
List of Tables
Table 1.1 Typical properties of NatureWorks PLA for extrusion and injection moulding
were fabricated using solid normal build style and three values each of raster width and raster
angle in a Stratasys FORTUS 900 mc FDM machine and tested in a DMA 2980 with
temperature sweep at three different fixed frequencies. This study concludes that the FDM
parameters (raster angle and raster width) affects the dynamic mechanical properties. Stratasys
Inc presents the data of ULTEM for design engineers for proper applications (see Table 2.6).
2.3.7 Polylactic acid (PLA)
Polylactic acid (PLA) is a biodegradable thermoplastic polymer which can be produced
from lactic acid. Since PLA is an environmentally friendly polymer, it is the most successful
biodegradable polymer with a global market in excess of 200,000 tonnes per annum and
projected growth rates over 15% [55]. As it is dimensionally stable, it is ideal for RepRap FDM
technology. Typically, PLA is relatively inexpensive and harder than ABS. It possesses high
mechanical strength, good crease-retention, grease and oil resistance and excellent aroma
barrier properties [56]. It is ideally suited in high volume commodity markets such as
automotive, fibres and consumer durable goods. Although it offers superior properties
compared to the other commercial polymers, its properties for FDM technology are not listed
in detail even on the Stratasys’ materials website. Also, while there have been some studies of
mechanical properties of well-known additive manufacturing materials such as ABS and PC,
PLA has received very little attention. In the published literature, some researchers have
investigated the mechanical properties of PLA as composite materials, with many fibres added
[26], [27]. Recently the mechanical properties such as tensile strength and modulus of elasticity
of PLA materials have been investigated by changing RepRap 3D printer slicing variables [28].
It is noted that the properties of FDM processed PLA has not yet been addressed by researchers
in published work.
24
Therefore, in this present study, the objective is to investigate the mechanical and
viscoelastic behaviour of FDM processed PLA materials through the use of flat dog-bone
specimens under tensile fatigue loading. In this work, an FDM process type Cube 2nd
generation 3D printer is used to produce the dog-bone shape tensile samples in X-, Y- and 45o-
build orientations. A Zwick Z010 universal testing machine and a DMA 2980 machine are
used to conduct testing of FDM parts. The effect of build orientation is investigated to
understand the characteristics of the PLA parts to obtain data helpful in design of such parts
subjected to desire applications. As higher compressive strengths are often observed in
polymers [14], so it is not investigated in this study. However, other mechanical properties
such as fatigue, flexural, impact and viscoelastic properties are investigated experimentally in
this present work.
2.4 Applications of FDM thermoplastics
Because of light weight, ease fabrication of complex geometry and low cost thermoplastics
have been developed at a significant pace. Their application to industrial structural parts has
accelerated in the past forty years. Because of their lower mechanical properties as compared
with metals, thermoplastics were not considered as engineering materials in past two decades.
The increasing use of low cost polymeric materials in consumer and automotive industries were
introduced in early 1980s. Although the load-bearing parts in industry are common engineering
applications of thermoplastics [57], nowadays, the trend of using thermoplastics are growing
in biomedical and tissue engineering fields such as novel scaffold architectures [58] and
knotless suture anchor [59]. Recently, researchers focused on extending the applicability (such
as electromagnetic and X-ray shielding) of FDM thermoplastics by developing materials [60].
The typical applications in which thermoplastics widely used are below:
Engineering materials as used in various technical applications (e.g., seals, gaskets,
damping elements, and membranes)
25
Rapid tooling
Packaging materials
Medical models
Functional prototypes, e.g. for experimental testing, wind tunnels, etc.
Product concept models
Patterns and cores for casting processes
LEGO applications
Consumer goods
Aerospace
2.5 Properties of PLA
2.5.1 Tensile Properties
Typically, the tensile strength of polymer is lower than metals and ceramics. The tensile
strength of a material is the ability to withstand breaking when the material is subjected to
tensile load. It is important to measure the tensile properties of materials for its structural
applications. The test for thermoplastic polymer to measure tensile properties is according to
ASTM D638 or ISO 527. From the test results, one can calculate ultimate tensile strength,
tensile yield strength, ultimate elongation and tensile modulus of the material. If the tensile
modulus is high (rigid material), then that means more stress is needed to produce that amount
of strain. As PLA is a rigid polymer, its ultimate elongation often exhibits values under 5%.
Typical tensile properties like tensile strength, ultimate tensile elongation of PLA can be found
in [61]. The greater values of tensile properties lead to polymers with high toughness.
26
2.5.2 Fatigue Properties
Fatigue occurs when parts are subjected to under cyclic loading applications is the main
concern in case of designing polymeric components for structural employments. Typically, in
the testing environment polymers are more sensitive than metals and ceramics. Therefore, a
number of parameters that control the fatigue life of polymer are considered for the safe design
of the polymeric components while undergoing cyclic loading. These parameters include stress,
strain, mean stress, stress concentrations, temperature, frequency and test environment. The
total number of life prediction cycles is the main consideration to the design engineer while
designing polymeric components to perform in cyclic loading applications. If the fatigue life
cycles are less than 105, then it is considered as low fatigue material and if the fatigue life
cycles is greater than 105, then it considered as high fatigue material. In general, thermoplastics
are in the low cycle fatigue material range, and in the case of ABS, it exhibits less than 105 life
cycles [15]. Traditionally, the fatigue life prediction is based on the endurance limit which
established from S-N curves [62]. In fatigue testing, the alternating stress and mean stress are
shown in Figure 2.1 and determined by the below equations.
Stress range, ∆𝜎 = 𝜎max − 𝜎min
Alternating stress, 𝜎a =∆𝜎
2=
𝜎max − 𝜎min
2
Mean stress, 𝜎m =𝜎max + 𝜎min
2
27
2.5.3 Flexural Properties
A typical flexural test involves a rectangular shape specimen holds on a support span and load
is applied to the center of the specimen producing three-point loading mode. Actually, it
measures the force required to bend the material under three-point bending condition. The
properties of flexural tests are the same as the tensile test like ultimate flexural strength,
ultimate flexural elongation and flexural modulus. These properties are often used to select
materials for parts which will resist bending during loading. Usually flexural modulus is used
to measure the materials stiffness undergoing bending. The flexural test of polymer is
performed according to either ASTM D790 or ISO 178. In the case of ASTM D790, the
ultimate elongation occurs under values of 5% deflection and for ISO 178, the ultimate
elongation occurs under values of 3.5% deflection Typically, a PLA thermoplastic polymer
exhibits better flexural properties than polystyrene [8]. In this research, the flexural test is
relevant to ASTM D790.
Time 𝑡
Stre
ss 𝜎
𝜎m
𝜎a 2𝜎a
𝜎min
𝜎max
∆𝜎
=𝜎
ma
x−
𝜎m
in
𝐼
𝜎a Stress amplitude 2𝜎a Range of stress variation 𝜎m Mean stress 𝜎max Maximum Stress 𝜎min Minimum stress 𝐼 Stress cycle
Fig.2.1 Stress cycles in fatigue [63]
28
2.5.4 Impact Properties
Many polymers exhibit excellent characteristics in terms of impact strength. Impact strength is
determined by applying a sudden load on a material and represents the ability of the material
to withstand the loading condition. The ability of materials to withstand an impact or sudden
deformation without breaking is usually described through the toughness. There is not a single
or universal test which can predict the impact characteristics of plastic materials under different
loading conditions which a part may need to face. There are some materials whose impact
strength may be reduced if the temperature is lowered. Thermosets and reinforced
thermoplastics do not change much with changes in temperature. Notches are machined into
the specimen to standardise the impact results against stress concentrators within the plastic
and to assess the sensitivity to weakening [64].
2.5.5 Dynamic Mechanical Properties
Using Dynamic Mechanical Analysis (DMA), the mechanical properties of materials are
measured as a function of temperature, frequency and time, and it is also a thermal analytical
method where an oscillating force is usually applied to a material sample to analyse the
responses corresponding to that particular force. Actually, it determines the basic structural
properties of polymeric material. However, DMA may not be able to distinguish between a
semi-crystalline and an amorphous material, but it provides quantitative data to design new
structural prototype in order to make the best use. DMA data are used to calculate different
properties such as the viscosity which is also called the tendency to flow from the phase lag
and the stiffness as modulus from the sample recovery. These properties are then described as
damping when they have the ability to loose energy in the form of heat and as the elasticity
which represents the ability of materials to recover from deformation. In DMA analysis, when
29
an oscillatory force is applied to the sample; a sinusoidal stress is originated due to this applied
force which in turns generates a sinusoidal strain as shown is Figure 2.2.
Different dynamical properties such as modulus, viscosity and damping can be calculated from
the measurement of the deformation amplitude at the peak of the sine wave as well as from the
laggings of sinusoidal stress and strain curves. In the case of amorphous glassy polymers to
semicrystalline highly crystalline, the glass transition temperature of PLA polymers ranges
from 60°C and the ranges of melting points for crystalline are from 130 to 180°C [8]. The
theoretical approaches are summarised as follows,
Storage Modulus, 𝐺′ = (𝜎°/𝛾°)𝑐𝑜𝑠𝛿
Loss Modulus, 𝐺′′ = (𝜎°/𝛾°)sin𝛿
Complex Modulus, 𝐺∗ = 𝐺′+i𝐺′′ = 𝜎°/𝛾°
Complex viscosity, 𝜂∗ = 𝜂′ − 𝑖𝜂′′
Loss Tangent, tan𝛿 = 𝐺′′/𝐺′
where
𝜎° = Maximum stress
0 2 4 6-1
-0.5
0
0.5
1
Time
Forc
e
Phase lag, δ
Strain
Stress
a)
𝐺∗
𝐺′
𝐺′′
b)
𝛿
Fig. 2.2 a) Time Response of a sample subjected to a sinusoidal oscillating stress and its strain response and b) Vectorial resolution of components of complex modulus [66]
30
γ ͦ = Maximum strain
𝜂′ =Storage viscosity
𝜂′′=Loss viscosity
𝛿 = Phase lag
2.5.6 Creep Properties
The creep property of a material is one of the most fundamental form of polymer behaviours
which is directly used to analyse the performance of products. When creep occurs in a polymer,
it is in failure mode with an indication of poor design of the materials which is a usually fact
of life [65]. The creep experiment shows a material’s response over a constant loading
condition along with its behaviour when the load is removed. The creep experiment can also
be used to collect data at very low frequencies, under long test times, or under real-time
conditions [66]. The creep behaviour of a system is shown in Figure 2.3.
2.6 Summary
This chapter highlights the previous research on the properties of materials used in the FDM
process parts that has been carried out to investigate mechanical and viscoelastic properties of
FDM thermoplastics. It was found that FDM processed parts exhibit anisotropic properties [41]
Time
Stress Strain
Stre
ss, 𝜎
Stra
in, 𝛾
Fig. 2.3 Creep behaviour in a system [66]
31
and shows lower properties as compared with injection moulded parts. Also, because of no
tooling requirement, and the capability of accurately producing complex geometry that have
been used as end use of functional parts, FDM parts play a key role in many industrial
applications.
Many researchers have studied the mechanical behaviours of uniaxial tensile specimens under
static loading and also a few studies have characterised the viscoelastic properties (dynamical
mechanical behaviours) of FDM parts. However, the majority of these studies have focused on
determining properties of new materials which need to be used in the FDM process [67]-[69].
Many researchers have carried out studies on FDM processed thermoplastics and that material
property data of these thermoplastics have been published, however, PLA has not been
researched fully and data are not available on Stratasys Inc website nor published elsewhere.
Hence, we conclude that there still remains room for research on PLA.
In subsequent chapters of this thesis, the mechanical and viscoelastic behaviours of FDM
processed PLA materials have been investigated through the use of flat dog-bone specimens
under tensile fatigue loading. The 2nd generation of a Cube 3D printer is used to produce dog-
bone shapes in X-, Y- and 45o build orientations which are then used as tensile samples. A
Zwick Z010 universal testing machine and a DMA 2980 machine are used to conduct testing.
Different build orientations are considered to understand the characteristics of the PLA parts
as well as to obtain useful data for different design purposes. In addition, this thesis also
includes the experimental investigations of other mechanical properties such as fatigue,
flexural, impact and viscoelastic properties.
32
CHAPTER 3
Materials and Test Methods
3.1 Introduction
This chapter describes the sample processing technique and the test methods applied
throughout the research. PLA cartridges were used to build test samples through the Cube-2
FDM machine in three different build orientations. In the Cube-2 FDM machine, PLA
filaments were extruded in a layer by layer manner by extruding semi-molten PLA
thermoplastic through the nozzle of cube machine. These extruded filament was solidified in
three different build orientations to process test samples for the purpose of mechanical and
rheological testing.
3. 2 Materials
In this study, Polylactic acid (PLA) cartridges were used as raw materials for FDM machine.
The PLA cube cartridges were made in USA by 3D Systems Inc. [70]. The filament material
in the cartridges that work with the Cube comes in 16 different colours and can print a
maximum 13 to 15 medium size samples from a single cartridge. In this research, white and
black colour cartridges were used to print the FDM specimens. On the other hand, in case of
IM machine the raw material was starch based PLA resin produced by BIOTEC, a subsidiary
company of Biome Technologies. It was supplied by BioPak Pty Ltd Australia in granule which
is known as Bioplast GS2189 (Biotec). This compound polymer is composed of 90% corn-
derived PLA and reinforced with 10% potato starch [71].
33
3. 2.1 Cube 2 FDM machine
Cube-2 3D printer Fused Deposition Modelling (FDM) technique was employed to
fabricate the test samples. The Cube 3D (2nd generation) printers are based on FDM type
plastic jet printing technology and made in USA by 3D Systems Inc. [70]. It comes with a
single print jet, filament material cartridge; cube tube, cube glue stick and a print pad with a
maximum build envelope size of 140 mm x140 mm x 140 mm as shown in Fig.3.1.
Fig. 3.1. Cube-2 3D FDM Machine
The Cube does not require any support when part features are not angled more than 45o in the
print platform. Also, it allows moving the print jet and the platform together in X, Y and Z
directions. The cube offers wireless network and USB connectivity. In order to build a part,
Cubify software converts the 3D STL files into printer cube files and offers three different print
modes (Solid, Strong and Sparse). The cube file contains all instructions to generate the tool
path of the deposition tip for the Cube machine. Once the cube file is imported into the Cube
via USB or wireless network, the front panel in the bottom of the Cube shows all instructions
34
in order to print the creation. The print jet print tip heats the thermoplastics at 280o C and
produces a thin flowing material of plastic creating 0.20 mm thickness of layers that adheres
to the print platform. After each layer is produced, the print platform lowers so that a new layer
can be drawn on top of the last. This process continues until the last layer on the top of the part
is jetted. Fig. 3.2 shows the schematic illustration of the FDM printing process.
Fig. 3.2. Schematic diagram of a Cube FDM Machine
3. 2.2 Part Fabrication by FDM
In this study, ‘dog-bone’ shape and flat shape samples were printed by Cube 3D machine to do
the numerous tests. In order to print ‘dog-bone’ shape specimen, typically ISO 527 and ASTM
D638 standards are followed by researcher to do tensile test for investigating the properties of
plastic materials. In order to be able to obtain significant results with the minimum number of
samples specimens, a design of experiments (DOE) was applied. The geometry of each
fabricated dog-bone shape sample was taken according to ASTM D638 to investigate tensile
Print Platform
Nozzle
Temperature Control Unit
Plastic Filament Cartridge
Roller
Fabricated Part
35
and fatigue properties in tension and size is 105 mm x 10 mm x 4 mm [72]. In order to do DMA
test, creep test, impact test and flexure test, several flat samples sized was 63 mm x 12.7 mm x
3 mm were printed according to ASTM D790. Fig. 3.3 shows the dimensions of the samples
used for this research, which could be fitted on the Cube 3D Printer. The 3D CAD model was
created using Creo Parametric software and then converted to a Stereolithography (STL) file.
The advantage of STL format is that the most CAD systems support it and it simplifies the part
geometry by reducing its basic components. The disadvantages of STL format is that it loses
some resolution which introduced acceptable by approximations [73]. To achieve the desired
sample, the chord height was set to 0 and angle control was set to 1 while saving as STL file
for the Cube software.
Typically, building a part using different print modes and different build directions will affect
the part strength and properties, so it is necessary to test in a variety of print orientations [14].
In this study, Solid print mode and three build orientations (X-, Y-, and 45o-) were used. Fig.
3.4 shows the three build orientations in the Cube software to make the PLA samples. Fig. 3.5
shows the deposited material build road pattern tool path for each of the three builds
orientations.
10 mm
4 mm 15 mm
20 mm
76 mm
50 mm105 mm135 mm
3 mm
63 mm
12.7 mm
a) b)
Fig. 3.3. Dimensions of the a) dog-bone shape and b) flat shape samples
36
3. 2.3 Part Fabrication by Injection Moulding
The test samples were fabricated from pellet form PLA material according to the
manufacturer’s product manual, using a Battenfeld BA 350/75 injection moulding (IM)
machine as shown in Fig 3.6. The pellets were injection moulded into standard rectangular
specimens according to ASTM D790 and tensile samples according to ASTM D638. The
temperature profile of injection from the feeding zone to the nozzle was controlled at
230/220/190/30°C and the measured injection pressure was1740 bar. Finally, the samples were
dried in a vacuum oven at 50°C.
b) Y-orientationa) X-orientation c) 45o-orientation
Fig. 3.5. Representative building layer styles of the samples
b) Y-orientation
X Y
a) X-orientation
X Y
c) 45o-orientation
X Y
Fig.3.4. Build orientations in Cube software
37
3.3 Test Methods
3. 3.1 Tensile Test
The tensile test of the PLA plastic material was conducted according to ASTM D638 using a
Zwick Z010 testing machine, which allows a maximum of 10 kN load and is controlled by
testXpert® II intelligent software. From the tensile testing results in stress-strain curve, one can
calculate ultimate strength, breaking strain and Young’s modulus which corresponds to a
polymers strength, ductility and stiffness respectively. The test was carried out at room
temperature and a strain rate of 50 mm/min. Wedge style cross-hatched grips were used for
proper griping of the specimens as shown in Fig. 3.7
Fig. 3.6. Battenfeld BA 350/75 injection moulding machine
38
3. 3.2 Fatigue Test
Fatigue tests are considered when parts are expected to perform under cyclic load applications.
In recent years, researchers have paid more attention on the fatigue behaviours of plastics as
plastics are increasingly being used in aerospace, automotive, biomedical and other leading
industries. Like all engineering materials, if plastic parts are considered under repetitive
loading then it is important to know the fatigue life of such parts. In general, thermoplastics are
more sensitive to various parameters and these parameters include stress or strain amplitude of
the loading cycle, mean stress, stress or strain rate, initial defects present in the component,
temperature, frequency and environment. These factors are to be considered when designing
the part for the fatigue life under cyclic loading and would provide a better understanding to
define materials to be used in specific applications.
Fig. 3.7. Specimen holding in a 10 kN Zwick machine
39
For fatigue testing, a Zwick Z010 universal testing machine was used, which allows a
maximum 10 KN load capacity. The machine was controlled by testXpert® II intelligent
software to control and record all test data. It was observed that a higher frequency increases
the body temperature of the sample, which results in decrease of fatigue life by enabling
material flow and increasing ductility, localized deformation at the weakest section of the
gauge length. Conversely, a lower frequency results in an increased fatigue life appearing
mostly in brittle fracture with limited deformation over the gauge length [74]. Therefore, the
tests were set at a frequency of 1 Hz at room temperature. No sample temperature control
device was supplied during the test due to the requirements defined for the test program. The
wedge style cross-hatched grips were used for proper griping of the samples as shown in Fig.
3.7.
In order to perform the fatigue tests, it was important to know the ultimate tensile strength
(UTS) of samples. So several number of trial run samples were tested under static loading, and
therefore, three close results were taken to determine the UTS for each orientation samples at
a strain rate of 50 mm/min. In the cyclic test program, the test parameters were kept unchanged
for each tested samples at various applied load conditions over the cycles. As the fatigue test
is a time consuming so one sample for each orientation was performed under cyclic load. To
set the number of cycles, three samples for the three different orientations were tested and then
set to 5000 cycles to overcome the data overflow in the test program. The applied load was
varied at 50%, 60%, 70% and 80% of UTS from sample to sample during testing. Due to time
consuming nature of cyclic loading experiment in a tensile tester, only one sample was tested
for each orientation and percentage of maximum load as the objective was to see the trend in
the fatigue behaviour of PLA parts in build orientations.
40
3.3.3 Flexure Test
In this study, the Zwick Z010 universal testing machine was used to perform flexure test of test
samples at a test speed of 10 mm/min. The selection type of testing was three-point bending
mode. According to ASTM D790, the support separation was at 40 mm, the tool separation at
start position at 6.5 mm and the preload was 5 N. The test program was controlled by
testXpert® II intelligent software which processed all data and evaluate all results in final form.
Fig 3.8 shows the three-point bending arrangement in a Zwick machine.
3.3.4 Impact Test
The experiments were carried out using a Ceast Instron Impact Tester. Notched samples for
impact tests were cut at the middle portion of the sample and were notched using a bench-top
notched machine. A minimum of 5 samples with notch length of 1 mm were tested for each
Fig. 3.8. Three point bending arrangement in a 10 kN Zwick machine
41
orientation and the results were averaged for each orientation samples. Fig. 3.9 and Fig. 3.10
shows the required testing arrangement for the Impact test.
3.3.5 DMA Test
Temperature ramp/single frequency is performed to determine storage modulus and loss
modulus and rest of all formulation by using a DMA 2980 dynamic mechanical analyser as
Fig. 3.9. Ceast Instron Impact test machine
Fig. 3.10. V-notched machine
42
shown Fig. 3.11 and tested in dual cantilever clamping mode at ramp rate 5 0C/min as shown
in Fig 3.12.
Fig. 3.11. DMA 2980 Dynamic Mechanical Analyser
Fig. 3.12. Dual cantilever arrangements in a DMA 2980 Dynamic Mechanical
43
The temperature was set to 30 0C to 110 0C. It works by supplying an oscillatory force, causing
a sinusoidal stress to be applied to the sample, which generates a sinusoidal strain. By
measuring both the amplitude of the deformation at the peak of the sine wave and the lag
between the stress and strain sine waves, quantities like the modulus, the viscosity, and the
damping can be calculated. A minimum of 7 samples of FDM PLA and injection moulded PLA
were tested and the results were compared.
3.3.6 Creep Test
Creep tests were conducted on a TA instrument Dynamic Mechanical Analyser. A tension type
clamp was used to evaluate the creep properties as shown in Fig. 3.13. Dimensions of sample
used in creep test were 63 mm in length, 12.70 mm in width and 3 mm in thick. Tensile stress
of 0.4 MPa was applied on each sample for two (2) hours which starts at room temperature
300C. A stress of 0.4 MPa was selected as it is the stress level at which PLA samples shows
linear viscoelastic properties. A total number of six (6) samples of FDM PLA were tested and
the average of each orientation samples was determined and was compared with the result of
injection moulded PLA samples.
Fig. 3.13. Specimen holding for creep test in a DMA 2980 machine
44
3.4 Summary
This chapter highlights the research specimens of PLA material which were processed through
FDM additive manufacturing process in three different build orientations and the test methods
to determine the effect of build orientations on its mechanical and viscoelastic properties. The
next chapter will develop and recommend information, appropriate for the design engineer,
about FDM PLA material and it also leads to a comparison with the injection moulded
materials.
45
CHAPTER 4
Mechanical Properties of FDM PLA Thermoplastic
4.1 Introduction
In this chapter, FDM processed PLA specimens under different build orientations were tested
and analysed to determine which orientations have most impact on maximising the mechanical
properties of PLA. The mechanical properties include tensile, flexural, fatigue strengths and
impact energy of PLA thermoplastic in three build orientations. Then, these results were
compared with the injection moulded (IM) parts which provides knowledge for design
engineers about the suitable application of FDM PLA parts in industrial use.
4.2 Tensile properties
In this study, three build orientations were considered and for each build orientation, and in
each case, three specimens were tested to obtain average results [21]. Therefore, a total of nine
specimens were tested for three build orientations. Although the specimens’ surface finishing
in two build orientations (X -and 45o-) were found to be good, the specimens printed in Y-
orientations resulted in uneven surface. The output file of tensile tester was converted into
tensile stress and strain graphs by using Matlab script. Fig. 4.1 shows the average stress-strain
graphs obtained from tested specimen, and also shows the comparison of the average stress-
strain curves of the FDM specimens in three build directions and IM specimens. From the
graphs, the modulus of elasticity for each PLA sample was calculated by plotting the slope in
its elastic region. The resultant tensile properties have been published in [80]. Table 4.1 shows
the average values and standard deviations of tensile stress, tensile elongation and modulus of
elasticity for three different build orientations.
46
Table 4.1- Tensile Properties of the FDM and IM specimens
Build Direction
Tensile stress (MPa)
Tensile Elongation (%)
Tensile Modulus (MPa)
Average Standard
deviation
Average Standard
deviation
Average Standard
deviation
FDM-X 38.65 0.14 4.14 0.08 1538 2.38
FDM-Y 31.43 0.33 4.53 0.12 1246 3.74
FDM-45o 33.63 0.70 4.45 0.20 1350 1.47
IM 31.4 1.32 3.6 0.28 1223 2.48
From the results shown in Fig. 4.1 and Table 4.1, it can be observed that the X-build orientation
shows the highest tensile stress of 38.7 MPa and the highest tensile modulus of 1535 MPa
compared to the Y- and 45o- build orientations and IM specimens. This is due to the fact that
the tool path beads in the X-orientation are laid parallel to the length of the tensile sample and
offer greater resistance to fracture. The specimens in Y and 45o orientations show slightly better
ductility (tensile elongation and mention other indicators). The published value of tensile stress
Fig. 4.1. Average stress vs. strain graph of different orientations FDM specimens and IM specimens
47
based on ASTM D638 standard for generic PLA material ranges from 61 to 66 MPa [75].
Hence, the PLA parts produced by FDM possess around 60 to 64% of tensile stress of the raw
PLA material. Fig. 4.2 shows the images of specimens in X-, Y- and 45o-orientations after the
completion of the tests.
Also, it was noted from Fig. 4.2 that the fracture in specimens mostly occurred nearer the neck
of the specimens built in X- and Y-orientations, but in 45-orienation specimens, it occurred
near the middle section of the specimens built. This could be attributed to the tool path layout
patterns in the specimens built in these orientations for FDM technology. Fig 4.3 shows an
overview of IM specimen.
Fig. 4.2. Tested PLA specimens in different build orientations: (a) X-orientation,
(b) Y-orientation and (c) 45o-orientation
(c)
(b) (a)
(a) (b) Fig. 4.3. Overview of an IM specimen: a) before test and b) after test
48
4.3 Fatigue Properties
In this study, the same ‘dog-bone’ specimens as used in tensile test according to same standard
were used to investigate fatigue strength at 80, 60, 70 and 50 percent of their respective ultimate
tensile strength [15]. All specimens were subjected to uniaxial tension while conducting static
and fatigue tests. The pull-out and retraction were controlled within its maximum and minimum
load for each specimen. Tensile tests were conducted for five samples for each build orientation
with a single pull until failure to determine ultimate tensile stresses and to verify tensile results.
However, the ultimate tensile stresses of the FDM specimens for the three different orientations
were found to be different since it depended on build orientation styles (solid, strong and
sparse) and orientations, rather than the material itself. Though the tensile strength of the raw
material was more consistent and higher than the FDM specimens, it was observed that the
tensile stress was around 60 to 64% of the generic raw PLA materials [75]. The output excel
file of tensile tester consisted of four columns representing maximum stress (MPa), elongation
at maximum stress (mm), stress at break (MPa) and elongation at break (mm) respectively.
The data were post-processed into stress, strain and number of cycles using Matlab script. The
results of fatigue tests have been published in [81]. Fig. 4.4 shows the typical pull history while
cyclically loading at 50% of UTS for the specimens of three orientations named PLA-X, PLA-
Y and PLA-45o respectively. Table 4.2 shows the average values of UTS and average values
of modulus of elasticity obtained by static testing and the amount of applied load at 80%, 70%,
60% and 50% of UTS used during fatigue testing for each of these three orientations.
49
Table 4.2: Data outlining the average ultimate tensile stress (σu), average modulus of
elasticity and applied load in percentage of UTS
Orientation of Specimen
Ultimate Tensile Stress (UTS)
σu (MPa)
(Table 4.1)
Modulus of
Elasticity
(MPa)
(Table 4.1)
Applied load (%UTS)
(MPa)
80% 70% 60% 50%
PLA-X 38.7 1538 30.96 27.09 23.22 19.35
PLA-Y 31.1 1246 24.88 21.77 18.66 15.55
PLA-45o 33.6 1350 26.88 23.52 20.16 16.8
0 1 2 3 40
5
10
15
20
Strain (%)
Stre
ss (M
Pa)
a) X-orientation
0 1 2 30
5
10
15
Strain (%)
Stre
ss (M
Pa)
b) Y-orientation
0 1 2 3 40
5
10
15
20
Strain (%)
Stre
ss (M
Pa)
c) 45o- orientation
Fig. 4.4. PLA specimens showing the pull history at 50% of the ultimate tensile stress
50
Generally, parts fail in high stress concentration areas under cyclic loading applications. For
homogeneous parts, a failure should appear directly in the middle of the part. Fig. 4.5 shows
the failure profile of tested specimens for three build orientations according to 50%, 60%, 70%
and 80% of their UTS respectively. The failure profile of the specimens appears in different
locations due to the different build style road pattern in each build orientations, which affect
the material properties. From Fig.4.5, it can be seen that the fatigue failure location for the X-
orientation specimens appeared consistently at the same location across the neck as the build
pattern roads are along the length of the tensile sample. In the Y- and 45o -orientations
specimens, the build pattern roads were either perpendicular or at 45o to the length of the
sample, and this resulted in failure at different locations where the parts were highly stressed.
It is noted that at 50% of UTS, the specimen in Y-orientation has lower applied tensile stress
than X- and 45o-orientations, but it has displayed better ductility as the failure occurs middle
of the specimen as compared to two other orientations (See Table 4.2 and Fig. 4.5). Note that
from stress-strain graphs, the modulus of elasticity of PLA for three distinct orientations were
calculated by plotting slope on its elastic region while specimens were tested to determine the
UTS for each orientation.
a) X-orientation b) Y-orientation
c) 45o-orientation Fig. 4.5. Images of fatigue tested specimens at their 50%, 60%, 70% and 80% of UTS
respectively
51
Because of the sensitivity in many factors, the fatigue test conditions must closely mimic the
service conditions of the thermoplastic part and the S-N approach is widely accepted in the
engineering community for design applications when considering cyclic loading. Fig.4.6 shows
the stress vs. numbers of cycles to failure curves (S-N curves) for X-, Y- and 45o-orientations
specimens subjected to static stress at their 50%, 60%, 70% and 80% of UTS. Despite the
inevitable scatter, the pattern of behaviour appears to be similar for all three build orientation
parts and each point shows the failure point of each specimen when they are cyclically loaded
at a certain percentage of their UTS value. Note that the average values of UTS for X-, Y- and
45o -orientation specimens were 38.7 MPa, 31.1 MPa and 33.6 MPa respectively as in Table
4.2. From Fig. 4.6, it can be seen that although the X-orientation specimens experienced highest
UTS, it generated lower fatigue life cycle than other two orientation specimens. However, the
specimen in 45o-orientation had a lower UTS than the X- orientation specimen, but it showed
a higher number of fatigue life cycles than X- and Y-orientation specimens. This trend is due
to build orientations of printed specimens and build pattern road in relation to build direction.
It was observed that for 45o-orientation specimen at approximately 50% of UTS, the number
of cycles is roughly 1380 until its failure.
100 102 1040
10
20
30
40
Number of Cycles (N)
Stre
ss (M
Pa)
PLA-XPLA-YPLA-45
Fig. 4.6. S-N curves for three (X, Y and 45o) orientation samples
52
The area under the stress-strain curve is the modulus of toughness or total strain energy per
unit volume consumed by the material until failure. The strain energy can be calculated by
using the following formula [76].
Total Strain Energy (𝑈) = ∫ 𝜎 𝑑 ∈∈
0 (1)
where 𝜎 is the stress and ∈ is the strain.
Therefore, if the stress-strain curve is integrated numerically, the total strain energy can easily
be calculated. In this study, the total strain energy was calculated by using a Matlab function
“trapz” which numerically calculates the total area under the stress-strain curve, i.e., the total
strain energy. Fig. 4.7 shows the strain energy vs. cyclic load for 50%, 60%, 70% and 80% of
UTS for specimens in all three build orientations. From Fig. 4.7, it can be observed that the
45o-orientation specimen experienced higher strain energy as compared to other build
orientations with a value of 2048.9 kJ m-3 until it failed at 1380 cycles. On the other hand, the
specimens in X- and Y- orientations experienced strain energy of 466.69 kJ m-3 and 1421.69
kJ m-3 respectively, and the numbers of cycle until failure for X- and Y-orientations were 175
and 708, respectively. These three trends were consistently presented for all other tested
specimens subjected to loading of 60%, 70% and 80% of UTS.
Typically, the strain energy decreases while testing at higher tensile stress. From Fig. 4.7, it
can be observed that the specimen in 45o-orientation experienced highest strain energy with
respect to the percentage of cyclic loading conditions from other orientations. Thus, this study
reveals that the PLA specimens printed in 45o-orientations have higher modulus of toughness,
absorb more energy and last longer till failure under fatigue loading conditions compared to
the PLA specimens built in the X- and Y-orientations specimens. This aspect is to be
considered when designing FDM built parts for cyclic loading applications.
53
4.4 Impact Properties
The impact test measures the impact resistance of thermoplastics. The energy absorbed by the
tested part is the difference in potential energy of the hammer before and after the impact [77].
Each specimen with dimension (63 mm x 12.7 mm x 3) mm were notched before impact testing.
In this test, 5 FDM specimens in three (X-, Y- and 45o-) orientations and 4 injection moulded
(IM) specimens were tested, and from these, 3 tested specimens were taken to obtain an average
result for the FDM and IM specimens. All data were processed by the CeastVIEW software
and collected in an excel file to plot the graphs. Table 4.3 shows the average results from impact
tests. Due to small size of the specimens, it was noted that the surface finish was good in each
orientation and the impact test results were different for each orientation. Fig. 4.8 shows the
overview of tested specimens and Fig. 4.9 shows the impact energy absorbed by the specimens
and resilience. The resilience is the energy absorbed by the area so it can be calculated by the
formula
Resilience = Impact Energy
Notched Area (2)
50 60 70 800
500
1000
1500
2000
2500
Cyclic Load (% of UTS)
Stra
in E
nerg
y (k
J/m
3)
PLA-XPLA-YPLA-45
Fig. 4.7. Strain energy vs. percentage of UTS for specimens in different (X, Y and 45o) orientations
54
Specimens Impact Energy
(J)
Resilience
(kJ/m2)
FDM-X 0.15 4.80
FDM-Y 0.11 3.36
FDM-45o 0.14 4.35
IM 0.15 4.54
It was observed that the specimens in X- orientation have the higher impact energy as well as
higher impact resistance than specimens in Y- and 45o- orientations. The average values of
impact energy and energy absorbed over the area, that is resilience for X- orientation
specimens, were 0.155 J and 4.80 kJ/m2 which was greater than average value of IM specimens
result. On the other hand, the resultant value in Y- and 45o- orientations specimens were less
Table 4.3- Average impact energy and resilience of the FDM and IM specimens
a) FDM-X b) FDM-Y
c) FDM-450 d) FDM-IM
Fig. 4.8. Images of impact tested specimens
55
than the value of IM specimen. From the analysis, the X-orientation specimens experienced
good ductility than the Y- and 45o- orientations specimen
4.5 Flexural Properties
Flexural test was completed at room temperature via a Zwick Z010 universal testing machine.
Flexural testing was conducted according to ASTM D790 and the testXpert® II intelligent
software run the tester and finally evaluated the results. All data shown was taken from FDM
processed specimens in three different orientations and injection moulded specimens. Each
orientation had five specimens that were tested and from these tested result, three required
closer data were processed. Figure 4.9 shows all resultant data that were plotted in flexural
strength (MPa) vs. deformation (%). The flexural strength was maximum for FDM processed
specimens in the case of 450 build orientation as shown in Figure. 4.9 and Table 4.4. In Table
4.4 , although the standard deviation in the case of 450 build orientation specimens were higher
than the other two build orientations specimens and IM specimens, the resultant average values
of all FDM processed PLA specimens in three build orientations were similar, and higher than
the injection moulded (IM) specimens. The maximum flexural strength was 84.71 MPa at 4.84
percentage of deformation for FDM processed in 450 build orientation.
Fig.4.9. Average flexural strength vs. deformation curve for FDM and Injection moulded specimens.
56
Table 4.4 Flexural Properties of the FDM and IM specimens
Specimens Maximum Flexural Strength (MPa)
Deformation at maximum flexural
strength (%)
Flexural Modulus (MPa)
Average Standard deviation
Average Standard deviation
Average Standard deviation
FDM-X 82.57 1.29 4.54 0.20 2480 1.41
FDM-Y 83.08 2.73 4.20 0.08 2479 0.82
FDM-45o 84.71 5.97 4.84 0.32 2485 2.94
IM 53.48 2.85 3.18 1.15 2253 2.16
b) a)
c) d)
Fig.4.10. Specimens after testing
57
From the experiments, it was noticed that all FDM specimens did not break after testing, but
in case of IM specimens, most of the specimens experienced breakage into two parts (See
Figure. 4.10). Therefore, the injection moulded specimens showed lower ductility than the
FDM processed specimens.
4.6 Summary
From all completed tests, the FDM processed PLA materials showed much more favourable
qualities in terms of their orientations. It was clear that PLA processed through FDM method
has some desirable properties in regard to mechanical properties. Though it was experienced
that the specimens in X build orientation have higher ultimate tensile strength and higher
percentage elongation at failure than Y and 450 build orientations. However, for fatigue testing,
it is possible to say specimens of 450 build orientation last longer (higher number of cycles)
before they fail. Also, the impact energy and the flexural strength were higher in the 450 build
orientation than X and Y. It was also noticed that the impact and flexural properties were better
than the injection moulded specimens. The next chapter covers the viscoelastic properties of
FDM processed PLA specimens and injection moulded specimens as a comparison.
58
CHAPTER 5
Viscoelastic Properties of FDM PLA Thermoplastic
5.1 Introduction
Fused Deposition Modelling (FDM) has achieved its popularity as compared to conventional
machining due to its low cost and require shorter time while rapidly fabricating real parts from
Computer Aided Design (CAD) data. This chapter sheds light on the effect of build orientations
on viscoelastic properties as well as the dynamic mechanical properties and creep properties of
Polylactic acid (PLA) material that fabricated by FDM additive manufacturing process. The
considered build orientations are in the X, Y and 450 directions. The dynamic mechanical
analysis (DMA) and creep analysis were carried out by a TA Instrument DMA 2980 machine.
The experimental results from the tests were evaluated and compared with the Injection
Moulding (IM) samples results to analyse the effects of build orientations on its dynamic
mechanical and creep properties. Figure 5.1 shows an overview of FDM and IM sample.
5.2 DMA Properties
Dynamic mechanical analysis experiments were carried out for FDM fabricated and IM
fabricated samples. All rectangular samples were tested by a TA Instrument DMA 2980
machine which allows various shape of geometries to fit in its clamping system. It offers
(a) (b)
Fig. 5.1. Sample for DMA and creep test: a) FDM sample and b) IM sample
59
various clamping modes including single/dual cantilever, three-point bend, shear sandwich,
compression and tension modes. Typically, DMA can sweep over frequency or temperature
range. In this study, the dual cantilever clamping mode and temperature sweep of DMA 2980
were employed to do all tests. For DMA experiments, all samples were tested using the
temperature ramp/single frequency method at ramp rate 5 0C/min, and the temperature ranged
from 30 0C to 140 0C according to the melting point of PLA material. The frequency scan was
done at three different temperatures for all samples. For all PLA samples the chosen
temperatures were 40 0C, 60 0C and 70 0C. After all experiments were completed, the
experimental results were evaluated by the Thermal Advantage software. Thermal Advantage
software is a Universal Analysis software which offers a great convenience for users to plot
custom graphs [23]. This software stored all value of storage modulus, loss modulus, tan delta
and complex viscosity across temperatures after completing tests. All necessary data were
collected for post-processing and finally plotted all graphs and bar charts in Matlab. The
temperature is set in the X-axis as it was the temperature sweep method, while storage modulus,
loss modulus, tan delta and complex viscosity were set in the Y-axis. Three different types of
Fig. 5.2. Temperature scan graph of loss modulus and tan delta of PLA FDM and IM samples
60
data were plotted in an overlapping manner. Figure 5.2 shows loss modulus (LM) and tan delta
(TD) of FDM and IM samples against the variation of temperature. From Figure 5.2, it can be
seen that the nature of loss modulus and tan delta is quite similar though their values are
different. The values of glass transition temperatures were taken out from the peak of tan delta
curves corresponding to the temperatures.
Similarly, Fig 5.3 shows such overlaid graph of storage modulus (SM) and complex viscosity
(CV) of FDM samples in three build orientations and IM samples. In Fig. 5.2 to 5.3, in all
cases, the values of loss modulus increases with the increase of temperature and the values of
storage modulus decreases with the increase of temperature at constant frequency. This is due
to the relaxation in the polymer chain because in polymers, the tendency to store energy
decreases with the increase in temperature and the tendency of loss energy increases with the
increase in temperature. The reductions in storage modulus and the glass transition temperature
are typical consequences [78]. As it is a temperature sweep method, from the graphs the
maximum property values of FDM and IM samples are taken at 40 0C, 60 0C and 70 0C
Fig. 5.3. Temperature scan graph of storage modulus and complex modulus of PLA FDM and IM samples
61
temperature and shown in Table 5.1. These values were selected to investigate the effects of
orientations in case of FDM samples and to compare with the IM samples.
Table 5.1. Property values of FDM and IM samples for solid normal build style
Samples Temper-ature (0C)
Max storage
modulus (MPa)
Max loss modulus (MPa)
Peak of tan delta
Tg (0C)
Max complex modulus (MPa)
Max complex viscosity
(MPa*sec)
PLA-X
40 60 70
1286 1060 68.84
24.32 143
100.29
1.46 75 1286.23 1069.60 121.63
204.81 170.32 19.37
PLA-Y
40 60 70
1431 1212.5 90.14
24.91 132.18 132.33
1.50 70
1431.22 1219.68 160.11
227.9 194.17 25.50
PLA-450
40 60 70
1300 1042.55 73.40
21.74 133.13 101.92
1.44 70 1300.18 1051.02 125.60
207.04 167.36
20
PLA-IM 40 60 70
2024 1612 97.57
60.8 130.9 101.7
1.04 70 2024.91 1617.31 140.94
322.44 257.53 22.44
As all property values were obtained from the experimental runs, some property values like
storage modulus, loss modulus and tan delta were collected and compared in bar charts. Figure
5.4 shows such charts plotted with temperature in the X axis and maximum storage modulus
values in the Y axis. These maximum values were obtained against temperatures for three
different orientations. This figure shows clearly the decrease in storage modulus with increase
in the temperature. This phenomena is due to the relaxation in polymer chain and such
relaxations with a large tan delta peak are responsible for the greater strength of the material
[79]. The values of storage modulus of FDM and IM samples are higher at 40 0C temperature
and deceases gradually with increase in temperature. From the Table 5.1 and Figure 5.4, it can
be seen that the value of storage modulus is higher for IM samples and then for Y build
62
orientation. The sample build in Y orientations obtained highest value of 1431 MPa, then X
and 450 build orientations, and attained around 71% strength (SM) of IM material when
subjected to single frequency/ temperature ramp method. It can be concluded that the parts in
Y build orientations have the greater toughness as compared to other two build orientations.
Figure 5.5 shows the effects of temperature on loss modulus properties. Loss modulus is the
tendency to loss energy of the material which is opposite of storage modulus. Typically loss
Fig 5.4. Effect of temperature on storage modulus properties
Fig 5.5. Effects of temperature on loss modulus properties
63
modulus increases with increase in temperature. But from this figure, it is clearly noted that
loss modulus of all FDM and IM samples have higher values at 60 0C temperature, and samples
made in X build orientation have high loss modulus value of 143 MPa, which is greater than
the IM samples. At 70 0C temperature, the values of loss modulus have decreased due to
transition of the material. Therefore, parts made in the X- build orientation is stronger than
other two build orientations.
The graph between tan delta and temperature is shown in Figure 5.6. Typically, tan delta is
directly proportional to loss modulus. Therefore, if the loss modulus increases with the
temperature, then the tan delta also increases. As mentioned earlier, the peaks of tan delta
increase with increase in temperatures and at the higher temperatures, the molecular energy
dispersion mechanism operates which indicates toughness of the material. From the Figure 5.6,
it can be seen the peak of tan delta is higher for Y build orientation samples than others. When
compared with other samples values including X build orientation, 45 build orientation and IM
samples, the values of peak tan delta are close enough for FDM samples and even better than
IM samples. Therefore, parts built in the Y orientation have the highest tan delta peak which
means the material is tougher.
.
Fig 5.6. Effect of temperature on tan delta properties
64
Figure 5.7 shows the complex modulus plot as a function of temperature. Typically, the
complex modulus is nearly equivalent to storage modulus. Figure 5.6 provides the effect of
temperature on complex modulus and glass transition temperature. It can be seen that at 40 0C
the complex modulus of FDM and IM samples are higher and gradually decreases at 60 0C and
then at 70 0C temperature. Among the three different build orientations samples, the samples
made in Y build orientation have experienced high complex modulus value of 1431.22, which
is about 71% complex modulus value attained by IM samples than X- and 45- build orientations
samples.
Figure 5.8 shows the results complex viscosity properties when subjected to temperature sweep
method. The complex viscosity property values of FDM and IM tested samples are shown on
the complex viscosity curves at temperature of 40 0C, 60 0C and 70 0C. Typically, Newtonian
fluids such as liquids and oils exhibit viscous behaviour. But in case of a material when
subjected to applied stress, and resulting strain is not recoverable, that increases proportionally
with time until the stress is removed. Thus, some energy losses in the system and materials
exhibits some viscous properties. In the case of PLA thermoplastic, the complex viscosity
Fig 5.7. Effect of temperature on complex modulus properties
65
decreases with increase in the temperature, and the sample made in Y build orientations possess
higher value when compared to other two build orientations samples.
5.3 Creep Properties
The experiments to evaluate creep properties were carried out by DMA 2980 for all FDM and
IM samples. In this study, a tension film type clamp was engaged to hold the samples. The
rectangular samples sized (63 mm x 12.7 mm x 3 mm) according to ASTM D790 were tested
for creep at isothermal 30 0C. A constant stress 0.4 MPa was applied resulting in strain for 120
mins duration. Once all experiments were done by DMA 2980, the resulting data were stored
automatically in Thermal Advantage software. All data was stored as strain and creep
compliance over time, and were post processed to plot graphs. The values of strain and creep
compliance were plotted in the Y axis and time was plotted in the X axis. These experiments
show obvious differences between these samples. Such graph showing the effects of strain over
time are in Figure 5.9.
Fig 5.8 Effect of temperature on complex viscosity properties
66
The assumption is that the higher value of strain of the material indicates a greater strength of
the material. However, the percentage strain of Y-build orientation sample is lower than IM
sample but greater than X- and 45o- build orientations samples as shown in Figure 5.9. Among
three build orientations samples, it is clearly seen that the curve of 45o-build orientation sample
is lower than the X- build orientation sample, which is less than Y-build orientation sample.
Figure 5.10 shows a plot of creep compliance Vs time graph. Typically, creep compliance is
the inverse of modulus for an elastic material which is the willingness of the material to deform
[66]. Similar curves are shown in Figure 5.10 where samples made in build orientations X and
Y have progressively a higher value of compliance than 45o- build orientation sample.
Fig 5.9. Plot of percent strain against time
67
Fig 5.10. Plot of creep compliance against time
5.4. Summary
This chapter has focussed on the effects of build orientations on dynamic mechanical properties
and creep properties of PLA samples processed through FDM technology. The DMA
experiments were carried out according to temperature sweep method at constant frequency
and the properties were investigated at three different temperatures. Result have shown that the
loss modulus increases with increase in temperature, but the storage modulus and complex
viscosity decrease with decrease in temperature. From the results of the dynamic mechanical
properties of FDM samples, it can be seen samples made in Y-build orientation gives best
values compared to X- and 45o- build orientations samples and achieved around 71% strength
of the IM parts. The samples made in Y- build orientation have more strength. Similarly, creep
test samples made in Y- build orientation have more strength than other two build orientations
samples. This understanding will help in the material selection process and assist design
engineers to optimize the cost of the PLA material. Therefore, the Y- build orientation is
considered best choice for design engineers when PLA materials parts are fabricated through
FDM technique
68
CHAPTER 6
Conclusion and Further Research
6.1 Overview
Polylactic acid (PLA), made from renewable sources is a compostable as well as a
biodegradable thermoplastic polymer. As it is environment friendly and cheaper than
petroleum based polymers, it became popular for producing consumer products. Now
researchers have considered PLA to use in building, agriculture, transportations, electrical
appliances and houseware. Therefore, the study of PLA properties becomes an important issue
now-a-days. Current industrial processing practices based on melt-flow techniques are time
consuming and costly. To overcome such difficulties fused deposition method (FDM), which
is a rapid prototyping process, has extensively been used to produce such industrial purpose
physical objects from computer aided data (CAD) with a shorter time. Therefore, the thesis
was focused to investigate as well as analyse the effects of different FDM build parameters on
PLA properties and find out the best build parameter in which processed PLA parts shows best
properties. In this chapter, the resulting effects of three different build orientations are
summarised to analyse the properties of different PLA parts which are processed through FDM
technique and further research in this area has been discussed.
6.2 Conclusions
The literature review showed that the FDM techniques and thermoplastic polymers that are
processed through FDM have been studied. However, it was found that the properties of PLA
had not previously been investigated through the FDM process. Hence, this study involved a
Cube 3D FDM machine to fabricate samples and investigation of the build parameters on
69
material properties during fabrications. In this study, the injection moulded samples were
prepared and tested in order to compare with the FDM sample results.
First the samples were fabricated according to ASTM D 638 and ASTM D 790 by a Cube 3D
printer machine by using X-, Y- and 450-build orientations and solid normal build style was
used as it was experienced that Solid normal build style gives best properties during
fabrications through FDM. Also, a number of samples were fabricated to compare with the
FDM results by using a Battenfeld BA 350/75 injection moulding (IM) machine. The FDM
and IM samples were tested for tensile, fatigue, impact, flexural, dynamic mechanical analysis
and creep properties.
The second study attempted to characterize the mechanical properties and viscoelastic
properties of tested samples. The build orientations have a great influence on the PLA samples.
For tensile loading applications, it was experienced that material parts should be fabricated in
X- build orientation as it showed higher tensile strength and tensile modulus than Y- , 450-build
orientations and IM samples. It was observed that for samples made in 45o- build orientation
last longer under cyclic loading conditions until its failure than X- and Y-orientations samples
and have highest number of cycles such as for 50% of UTS, the number of cycles is roughly
1380 until its failure. In case of impact test, the samples made in X- orientation have the higher
impact energy as well as higher impact resistance than samples made in Y- and 45o-
orientations. However, the values of the maximum flexural strength were close enough among
in the three build orientations samples, but it was found slightly higher maximum flexural
strength as well as the flexural modulus in case of 450 build orientation samples.
For dynamic mechanical analysis (DMA) experiments, the results showed that the properties
such as storage modulus, complex modulus and complex viscosity were decreased with an
increase in temperature. On the other hand, the properties like loss modulus and tan delta were
increased with an increase in temperature. The samples made in Y- build orientation showed
70
higher values of storage modulus, complex modulus and complex viscosity than samples made
in other two build orientations and attained around 71% strength of IM material samples.
However, the value of loss modulus was higher in X- build orientations samples, but in case of
tan delta property, it was observed that samples made in Y- build orientation attained higher
values of tan delta than samples made in X- and 450 build orientations. For creep test, the results
obtained from the experiments showed that the greater value of strain as well as the creep
compliance for Y-build orientation samples increase the strength of the PLA material, thus
increasing the flowability of the material. So parts should be fabricated in Y- build orientation
while performing long term loading applications as the Y- build orientation samples have
higher creep properties than other two build orientations.
In conclusion, this understanding will improve the material selection process and assist in
optimizing the cost/ performance balance to the design engineers. The research activities
presented in this thesis will also assist the designers for developing guidelines where FDM built
parts are applicable in different orientations as well as in different loading conditions.
6.3 Further Research
Further recommended work in the area of analysing material properties as presented in this
thesis will include:
Compression test in different build orientations, which is required when parts are under
compression loading applications.
Compressive and flexural fatigue testing which enable to gather more knowledge on
fatigue behaviours of different parts processed through FDM.
The investigation of thermal properties in different build orientations, which may be
the required criteria for some of the applications.
71
Creep recovery and stress relaxation tests, which includes viscoelastic behaviours of
FDM processed parts.
Such investigations would be extremely useful to help design parts as more and more additive
manufactured parts and materials are being applied to various engineering applications in
different loading conditions.
72
References
1. Freinkel, S 2011, A Brief History of Plastics’ Conquest of the world, Scientific American,
viewed 21 July 2016, <http://www.scientificamerican.com/article/a-brief-history-of-plastic-
world-conquest/>
2. Jamshidian, M, Tehrany, E A, Imran, M, Jacquot, M and Desobry, S 2010, ‘Poly‐Lactic
Acid: production, applications, nanocomposites, and release studies’, Comprehensive Reviews
in Food Science and Food Safety, vol. 9, no. 5, pp. 552-571.
3 Lim, J Y, Kim, S H, Lim, S and Kim, Y H 2003, ‘Improvement of Flexural Strengths of Poly
(L‐lactic acid) by Solid‐State Extrusion, 2. Extrusion through Rectangular
Die’, Macromolecular Materials and Engineering, vol. 288, no. 1, pp. 50-57.
4. Furukawa, T, Sato, H, Murakami, R, Zhang, J, Duan, Y X, Noda, I, Ochiai, S and Ozaki, Y
2005, ‘Structure, dispersibility, and crystallinity of poly (hydroxybutyrate)/poly (l-lactic acid)
blends studied by FT-IR microspectroscopy and differential scanning
calorimetry’ Macromolecules, vol. 38, no. 15, pp. 6445-6454.
5. Mills, C A, Navarro, M, Engel, E, Martinez, E, Ginebra, M P, Planell, J, Errachid, A and