N. N. KHALID et al.: INTERFACIAL BONDING OF POLYAMIDE-REINFORCED CARBON FIBRE VIA ...
587–595
INTERFACIAL BONDING OF POLYAMIDE-REINFORCED
CARBON FIBRE VIA ADDITIVE MANUFACTURING
KOHEZIJA NA MEJAH MED OGLJIKOVIMI VLAKNI IN
POLIAMIDNO MATRICO V KOMPOZITIH IZDELANIH
Z DODAJALNO TEHNOLOGIJO
Nisa Naima Khalid
1
, Nabilah Afiqah Mohd Radzuan
1,2
, Farhana Mohd Foudzi
1
,
Abu Bakar Sulong
1,2
, Nurul Najwa Abd Rahman
3
1
Advanced Manufacturing Research Group, Department of Mechanical & Manufacturing Engineering, Faculty of Engineering & Built Envi-
ronment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
2
Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
3
Division of Mechanical and Aerospace Technology, Science and Technology Research Institute for Defence (STRIDE),
Ministry of Defence, Malaysia
Prejem rokopisa – received: 2024-02-28; sprejem za objavo – accepted for publication: 2024-08-22
doi:10.17222/mit.2024.1125
Printing orientation in polymer additive manufacturing (AM) is a crucial factor that affects both printing accuracy and the me-
chanical properties of the final products. Notably, printing orientation influences the interfacial bonding within printed lay-ups,
thereby altering the mechanical properties to meet specific application requirements. This paper reviews studies on polyamide
reinforced with carbon fibre, evaluating the impact of printing orientation, where interfacial bonding affects mechanical proper-
ties. The review shows that the common three printing orientations, 0°, 45°, and 90°, are often discussed in terms of tensile
strength, fracture toughness, and electrical performance. Factors such as continuous-carbon-fibre raster angle, stacking sequence
and loading direction are believed to be the main factors affecting mechanical properties. Among these, the 0° printing orienta-
tion is often associated with the highest tensile strength and stiffness due to the strong interfacial bonding between the
polyamide and reinforcing carbon fibres in AM.
Keywords: polyamide-reinforced carbon fibre, mechanical properties, orientation, 3D printing, raster angle
Orientacija tiskanja je pri dodajalnih tehnologijah (AM; angl.: Additive Manufacturing) klju~nega pomena, ker vpliva na
natan~nost izdelave in mehanske lastnosti kon~nega izdelka. Orientacija tiskanja pomembno vpliva tudi na kohezijo med
posameznimi plastmi nanosa in s tem zagotavlja zahtevane mehanske lastnosti glede na vrsto uporabe izdelka. V ~lanku avtorji
opisujejo {tudijo polimernega kompozita oja~anega z ogljikovimi vlakni. V raziskavi so ocenjevali vpliv orientacije tiskanja na
mehanske lastnosti s tehnologijo AM izdelanega poliamidnega kompozita. V pregledu literature avtorji ugotavljajo, da so v
raziskavah vpliva natezne trdnosti, lomne `ilavosti in elektri~nih lastnosti kompozitov obi~ajno izbrane tri orientacije vlaken 0°,
45° in 90°. Faktorji, kot so rasterski kot ogljikovih vlaken, sekvenca zlaganja in smer obremenitve kompozita, raziskovalci
smatrajo kot glavne parametre, ki vplivajo na mehanske lastnosti. Med temi kot tiskanja 0° velja za tistega, ki daje najve~jo
natezno trdnost in togost z AM tehnologijo izdelanih kompozitov s polimerno matrico zaradi zelo pomembnega vpliva trdnosti
vezave med poliamidno matrico in ogljikovimi vlakni.
Klju~ne besede: z ogljikovimi vlakni oja~an poliamid, mehanske lastnosti, orientacija, 3D tiskanje, rasterski kot.
1 INTRODUCTION
Additive manufacturing (AM) technologies have sig-
nificant potential in cost-saving, sustainable and complex
component manufacturing. Consequently, AM is one of
the most encouraging fields within component manufac-
turing, gaining significant attention. AM technologies
encompass numerous methods. Usually referred to as 3D
printing, AM or layer manufacturing is a progressive
method that plays a key role in advancing manufacturing
technology. The article presents potential outcomes of
using 3D printing in the development of manufacturing
technology. Carbon fibre-reinforced polymer composites
(CFRPs) have a huge advantage over metals because
they are lightweight, high in strength and stiffness, and
resistant to corrosion and fatigue.
1
Using carbon fibre in
AM can improve material properties, reduce the time
required to manufacture functional parts compared to tra-
ditional subtractive technologies, and reduce warping,
thereby enabling a larger possible build envelope. Ther-
mosetting epoxy matrices are used in most CFRPs where
a high strength and stiffness to weight ratio is necessary,
for instance in aviation applications. Carbon fibre sur-
face treatments and sizing technologies have been devel-
oped for aerospace thermosetting epoxy matrices over
the years, achieving high interfacial bonding between
fibres and the matrix and good mechanical properties.
2
The advantages of continuous carbon fibres can be
achieved using printing methods, such as 3D printing of
continuous fibre-reinforced parts, which have caught at-
tention from researchers. Like with traditional manufac-
turing techniques, a large number of printing parameters
need to be carefully considered for the AM processes.
The effects of printing parameters on mechanical proper-
Materiali in tehnologije / Materials and technology 58 (2024) 5, 587–595 587
UDK 54-126:546.26:52-334.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(5)587(2024)
*Corresponding author's e-mail:
afiqah@ukm.edu.my (Nabilah Afiqah Mohd Radzuan)

ties of 3D printed composites including continuous
fibres have been only partly studied. In literature, the
mechanical properties of 3D-printed polyamide
(PA)-based composites reinforced with continuous car-
bon fibres were examined. The mechanical behaviours of
3D printed parts with different builds in horizontal and
vertical orientations were also evaluated.
3
Fused deposi-
tion modelling (FDM) is one of the most regularly used
low-cost 3D printing technology, which utilizes the
hot-melt and adhesive properties of thermoplastic mate-
rials. As one of the main methods of designing thermo-
plastic polymer materials, PA possesses excellent com-
prehensive performance. The FDM technique used in 3D
printing has significantly advanced manufacturing tech-
nology. By joining materials layer-by-layer based on
3D-model data, FDM has garnered considerable atten-
tion in recent years for its numerous advantages over tra-
ditional subtractive methods.
4
The 3D printing technology allows us to directly cre-
ate objects with intricate geometrical features in a
cost-effective way, requiring no moulding tool and pro-
viding near-net-shape manufacturing in a relatively short
period of time
5
Likewise, 3D printing is generally help-
ful in fabricating customized parts and products with
complex and tailored designs while being capable of har-
nessing digital information for the realization of a robust
and decentralized 3D manufacturing system. So far, the
latest studies on 3D printing of continuous fibre-rein-
forced thermoplastic composites have been based on
polylactide acid (PLA),
6
thermoplastic polyimide (TPI),
7
and acrylonitrile butadiene styrene (ABS).
8
However, the
mechanical properties of PLA, TPI and ABS could not
meet the critical requirements in aviation.
9
To date, stud-
ies have utilized chopped carbon fibre and PLA as the re-
inforcing materials and a thermoplastic matrix, with dif-
ferent weight percentages of chopped carbon fibre of
(12, 15, and 20) % to optimize the performance.
10
The
results show that with a 15-% carbon fibre reinforce-
ment, 32-% tensile strength and 22-% flexural strength
enhancements were observed when compared to a pure
PLA sample.
11
This indicates that increased mechanical
properties can be achieved by adding a material as the
filler instead of using a pure sample.
12
Additionally, the
fibre arrangement hugely affects the mechanical proper-
ties of a 3D printed composite.
13
There are a few studies
on the printer head design and printing path optimiza-
tion, carried out with an endeavour to control the ar-
rangement of short fibres.
14
The quality and success of
the final print depends on various process parameters
such as slicing, building orientation and temperature. All
these parameters influence the mechanical properties of
FDM products, especially end-use parts.
15
Thus, this pa-
per reviews mechanical properties which include printing
parameters, mechanical and electrical performance of
printed samples and their effects on printing orientations
and interfacial bonding when using carbon fibre-rein-
forced polyamide.
2 MATERIAL AND FABRICATION
The basic component of plastics and elastomers is a
polymer. Since pure polymers often show poor resistance
to external factors such as weathering, mechanical stress
during their processing or end-use applications, they
need an additional material to increase their mechanical
properties including tensile and flexural strength.
16
This
research is about using polymer composites as they allow
the production of high-added-value products compared
to pure polymer. Different kinds of additives present
within a polymer result in different vulnerabilities and
strengths. Thus, this research reviews additive fillers
within a polymer that can influence the effectiveness of
polymer production. A multi-phase polymer composite
includes a reinforcing filler integrated into the polymer
matrix, resulting in synergistic mechanical properties
that cannot be achieved by either component alone.
17
A
study of fibre-reinforced composites demonstrated that
the amount of the filler material significantly affects the
properties of the composites. The interaction with adhe-
sion, dispersion in the matrix, and particle motion are
significantly dependent on the amount of the filler. Their
effectiveness is increased with a decrease in the filler
size.
18
For example, nanoscale fillers exhibit a very large
surface-to-volume ratio. As properties like catalytic reac-
tivity, electrical resistivity, adhesion, gas storage, and
chemical reactivity depend on the nature of the interface,
these properties change dramatically.
19
The effectiveness
of nanoscale fillers was demonstrated by previous re-
searchers, who observed that a unique microstructure de-
pended on the simultaneous deposition of nanoscale car-
bon and nano-sized Cu particles on carbon fibres. This
process shows great potential for tailoring the structural
and functional performance of advanced fibre compos-
ites.
20
According to the results, carbon nanoscale rein-
forcements and nano-sized copper particles deposited on
a carbon fabric through cathodic electrophoretic deposi-
tion (EPD) results in the enhancement of electrical and
mechanical properties.
21
In another research, the tensile
properties of multiwall carbon nanotube-filled PA6 com-
posites were assessed and the impact of phenyl glycidyl
ether (PGE) as an effective noncovalent functionalization
agent was verified.
22
Recent studies investigated a nano-
particle polymer for medical applications, where re-
searchers used thermally responsive and magnetic/poly-
mer composite nanoparticles (MPCNPs). MPCNPs
possess unique properties required for a combined simul-
taneous application of a magnetically induced, targeted
delivery of drugs to tumours, hyperthermia, controlled
drug discharge and magnetic resonance imaging (MRI).
The results of an in-vitro drug release under magnetic
hyperthermia conditions using MPCNPs show signifi-
cant promise for a multi-modal mode of cancer treat-
ment.
23
In conclusion, an increase in the strength of
nanoparticles in composite materials has shown an im-
provement in mechanical, electrical and thermal proper-
N. N. KHALID et al.: INTERFACIAL BONDING OF POLYAMIDE-REINFORCED CARBON FIBRE VIA ...
588 Materiali in tehnologije / Materials and technology 58 (2024) 5, 587–595

ties, observed when reviewing multifunctional polymer
composites.
In the scope of market requirements for engineering
components, there is a huge demand for polyamide ther-
moplastic matrices reinforced with carbon fibres due to
their versatile applications.
24
There are many methods of
manufacturing composites using carbon fibre-reinforced
polyamide such as 3D printing, resin transfer moulding
(RTM), twin-screw extruder and injection moulding.
25,26
As a manufacturing process of engineering composites,
advanced manufacturing technique such as 3D printing
facilitates a layer-by-layer fabrication of customized
products using a wide selection of materials. Figure 1 il-
lustrates the concept of multifunctionality, starting with a
pure polymer and progressing through additive manufac-
turing polymers and conventional composites, finally
leading to 3D-printed composites. Also, 3D printing is
often referred to as solid freeform manufacturing for
computer-assisted manufacturing (CAM) and layer-by-
layer design.
27
3D printing, depicted as AM, provides
manageability, cost efficiency, good product design, and
waste remediation including various material choices.
Academics suggest that this technology potentially im-
proves rapid prototyping, design production, sustain-
ability and cost minimization compared to traditional
methods. 3D printing is a broad area covering all pro-
cesses that produce three-dimensional models by adding
materials layer-by-layer, transitioning from liquid to
solid, rather than by subtracting materials.
25,28,29
2.1 Background of polymer composites
Polymer composites, also known as polymer matrix
composites (PMCs), are composite materials containing
various short or continuous fibres bound together by an
organic polymer matrix. The shape of the fibre can be
spherical, cubic, platelet, having regular or irregular ge-
ometries.
31
The reason of adding fibres to a material is an
improvement of a product that can be found in its creep,
wear, fracture toughness, and thermal stability.
32
For ex-
ample, an application of polymer composites in aircraft
and other products improves their quality and service
life. The overall ductility of all composites can be in-
creased by adding polymer reinforcement and curing
them in dry air.
33
Ductility enhancement is achieved by
adding polymer reinforcement; ductility of composites
can be shown using a visco-plastic material model to
capture the complex behaviour of the composites. The
capability of a visco-plastic model includes capturing
measured anisotropic properties, such as tension/com-
pression asymmetry and material rate dependence.
34
In
this study, PMC is used as a raw material in 3D printing
that facilitates layer-by-layer customized fabrication us-
ing different materials such as polymers, metals, ceram-
ics and composites. Using 3D printing in fabricating
polymer composites is subjected to an extensive review
in this paper.
35
During the design, the sustainability of a
polymer composite is ensured by choosing the appropri-
N. N. KHALID et al.: INTERFACIAL BONDING OF POLYAMIDE-REINFORCED CARBON FIBRE VIA ...
Materiali in tehnologije / Materials and technology 58 (2024) 5, 587–595 589
Figure 1: Graphical representation of the multifunctional concept,
showing the progression from polymers to 3D-printed composites
30
Table 1: Recent applications of polymer composites
Matrix material Reinforcement Findings Application
Acrylonitrile butadiene sty-
rene
Modified tapioca starch
Reduction in mechanical strength,
improved features
Food packaging industry
38
Thermoplastic composites Carbon fibre
Increased tensile strength and stiff-
ness, improved printing versatility
Bio-medical industry
39
Polyactic matrix composites Carbon fibre
Higher mechanical strength, im-
proved extrusion of materials
Automotive industry
40
Polycarbonate/ acrylonitrile
butadiene styrene
Carbon fibre
Minimum wear rate, higher me-
chanical strength
Additive manufacturing
industry
41
Acrylonitrile butadiene sty-
rene
Plastic fibre Increased tensile strength Aerospace industry
42
Polyactic acid
Carbon fibre with plastic as
The matrix
Increased flexural strength and flex-
ural modulus
Light structures in aviation
and aerospace industry
43

ate raw material.
36
Raw polymer materials widely used
for 3D printing have to provide for various properties of
the final product according to the specified require-
ments.
37
Various applications of polymer matrix compos-
ites can be seen in Table 1.
2.2 Polyamide and carbon fibre
In this work, the use of carbon fibre-reinforced
polyamide (PA)-based composites in 3D printing was
studied. Polyamide-6 (PA6), as a promising engineering
thermoplastic, is used in electronic components, mechan-
ical parts and automotive industry due to its excellent
abrasion resistance, heat resistance, and high mechanical
strength. In a few recent studies, the crystalline PA6 was
modified by introducing an amorphous polymer to pre-
vent severe warpage so that it could be applied in FDM
technology. PA-6 or polycaprolactam is a biodegradable
and synthetic polymeric substance with good physical
and mechanical properties.
44
Continuous carbon fibre
(CCF)-reinforced polyamide was chosen in this study as
the effective matrix with reinforcement due to superior
mechanical properties of both materials. According to
previous studies, CCF tows are not pure clusters of con-
tinuous carbon fibres. Specifically, it was found that CCF
tows include continuous carbon fibre-reinforced poly-
amide, containing 48 w/% of pure continuous carbon
fibres. As an outcome of a previous study, a composite
was produced, where the matrix phase provided higher
toughness values.
45
In general, carbon fibre-reinforced
polyamide containing 30 w/% of carbon fibres achieved
a tensile strength of 250 MPa in high-performance
parts.
46
2.3 Processing parameter
In recent years, additive manufacturing has become a
well-known method. The method, which involves 3D
printing, provides possibilities for the creation of innova-
tive prototypes and technologies in manufacturing. As
several other significant parameters rely on the content in
3D printing the content is the most critical parameter.
47
There are several different methods of 3D printing, but
the most widely used is the process known as fused de-
position modelling (FDM). Using this technology, a
wide range of polymeric materials can be processed.
Nowadays, fibre-reinforced thermoplastic composites are
becoming more and more important for this technique.
With this technology, low-melting-point polymers are
transformed into a semi-liquid and extruded in a con-
trolled way through a nozzle, until the desired layers are
deposited. So far, different printing parameters including
temperature control, deposition pattern and layer thick-
ness have been found to influence the final properties of
FDM-printed parts. The extrusion of 1.75 mm carbon fi-
bre/polyamide 6 filaments from prepared pellets through
a desktop single-screw filament extruder, using 3D print-
ing is depicted in Figure 2 below.
48
Different printing
parameters such as nozzle diameter, placement of the
part in the build plate, height of the layer, printing pat-
tern and infill density can be set to vary the appearance,
quality and mechanical behaviour of a sample.
49
Rectangular plates were built in the xy plane, using
three printing architectures, shown in Figure 3, with dif-
ferent bead orientations within the stacked layers. All
samples were fabricated in accordance with the printing
parameters.
51
The specimens were FDM-printed with
infill support, and the overlap area was 12.5 × 25 mm
2
.
In addition, the re-entrant honeycomb structure was sub-
sequently manufactured, and the in-plane compression
test was conducted at a constant strain rate of
2 mm/min.
50
Previous research showed that the mechani-
cal properties were influenced by process parameters
such as type of the infill pattern, shell thickness, type of
the material, printing temperature and infill density.
N. N. KHALID et al.: INTERFACIAL BONDING OF POLYAMIDE-REINFORCED CARBON FIBRE VIA ...
590 Materiali in tehnologije / Materials and technology 58 (2024) 5, 587–595
Figure 2: Schematic illustration of 3D-printed CF/PA6 composite filament fabrication
50

Other results showed that an object had to be both light-
weight and durable so the best set of parameters was
used with the honeycomb pattern with a fill density of
about 40–50 %, and a shell thickness of 2–3 lay-
ers/lines.
52
In addition, there are other parameters that
govern the properties of a printed product. Studies have
demonstrated that anisotropy and orientation of print lay-
ers are two additional factors affecting 3D-printed prod-
ucts as they cause significant variations in the electro-
chemical activity.
53
Due to the uniqueness of 3D printing, this paper re-
views the process parameters, which influence orienta-
tion. During a previous study, specimens were produced
in accordance with the 3D-printing parameters from Ta-
ble 2, with the highest build-plate temperature of 105 °C.
The influence of the build-plate temperature on the
FDM-printed carbon fibre-reinforced polyamide speci-
mens was investigated. The result shows that with an in-
crease in the built-plate temperature, the tensile strength
was also increased. The experiment was conducted on
specimens with a 0° raster angle, while the other printing
parameters were taken from Table 2.
50
Table 2: FDM printing parameters for PACF specimens
50
Parameter Value
Printing speed 30 mm/s
Infill density 100 %
Nozzle diameter 0.4 mm
Printing raster angle 0°, 45°/-45°, 90°
Layer thickness 0.15 mm
Nozzle temperature 260 °C
Environment temperature 22 °C
Built-plate temperature (30, 55, 80, 105) °C
Other studies focusing on an increased printing speed
and layer thickness found that mechanical properties
were also affected. The layer thickness and printing
speed were crucial printing parameters, affecting the im-
pact strength. The optimal mechanical properties were
achieved when the printing speed and layer thickness
were 5 mm/s and 0.1 mm, respectively.
54
According to
previous results, nozzle temperature (T
n
), platform tem-
perature (T
p
), printing speed (v), and layer thickness (d)
were selected as the main FDM 3D-printing parameters
to be investigated.
55
3 EFFECTS OF FIBRE ORIENTATIONS
A printing parameter can directly or indirectly affect
mechanical properties of printed parts during fused de-
position modelling (FDM). Thus, this building and orien-
tation of 3D-FDM were studied. A previous study
showed that with an increase in the nozzle temperature,
the density of the produced parts was improved as air
pores were partially expelled. In addition, the interlayer
gap between the two materials weakened the bending re-
sistance.
56
AM or 3D printing is economical because it
neither requires rotary tools nor produces wasted raw
materials; AM also enables a simultaneous manufacture
of multiple products. Besides that, AM improves
reproducibility with the development of related technol-
ogy. Printing layer thickness, angle, orientation, laser in-
tensity and speed are critical factors in AM, along with
proper parameter setting, to achieve optimal results in
3D printing. Printing orientation is crucial because it can
affect the mechanical properties such as tensile and flex-
ural strength.
57,58
Previous research showed that a reduc-
tion in the UTS of 3D-printing materials become smaller
with the printing angle decreasing from 90° to 0°, as can
be seen in Table 3.
59
The reduction was calculated using
the UTSRC Equation (1) shown below:
UTS
RC
= UTS / UTS (90°) (1)
UTS – 3D printing materials at all printing angles
UTS (90°) – 3D printing materials at a 90° printing an-
gle
UTS
RC
– UTS reduction coefficient
N. N. KHALID et al.: INTERFACIAL BONDING OF POLYAMIDE-REINFORCED CARBON FIBRE VIA ...
Materiali in tehnologije / Materials and technology 58 (2024) 5, 587–595 591
Figure 3: (A) Conceptual sketch of the FDM manufacturing process and (B) schematic representation of 3D-printed specimens with different
printing architectures
51

3.1 Tensile strength
Excellent mechanical properties of PACF composites
can be acquired by controlling the effect of deposition
path during 3D printing on the tensile strength.
60
Since
the fibre orientation is directly related to the molten
polymer of polyamide fluid during printing, three-direc-
tional angles of 0°, 45°/–45° and 90° were adopted as il-
lustrated in Figure 4. The interface adhesion between the
carbon fibre and polyamide was improved, which was
beneficial for the stress transfer from the matrix to the fi-
bre. In addition, the tensile strength of discontinuous fi-
bre-reinforced thermoplastics is also linked with the dis-
tribution of fibre orientations.
61
When the tensile loading
direction is parallel to the deposition direction, fibres can
bear more loading, achieving a higher tensile strength
and modulus. When the printing direction is at 90°,
fibres are still oriented along the printing direction but
are perpendicular to the tensile loading direction, thus
failing at load carrying and resulting in decreased tensile
strength.
Tensile strength, stiffness and Poisson’s ratio of con-
tinuous carbon fibre-reinforced thermoplastic polyamide
with longitudinal and transverse directions can be mea-
sured using tensile testing. Effects on the mechanical
properties of 3D-printed polymer composites filled with
continuous carbon fibres with different orientations can
be seen in Table 4.
62
The maximum tensile strength and
stiffness achieved for the fibres in the loading direction
were 524.66 MPa and 73 GPa. These results can be
strongly supported with those of the other studies, which
show that specimens with a 0° printing direction aligned
with the tensile stress exhibit the maximal Young’s
modulus and tensile strength, followed by 45°/–45° and
90° orientations, as shown in Figure 5.
50
However, the parameters used in the FDM process
present a conflicting effect on the tensile properties. The
air gap, raster angle, contour width, raster width, and
contour number are the five process parameters used in
the FDM process.
64
Among these parameters, the raster
angle has the highest influence. It is important and has
the strongest effect on the tensile strength as seen in Fig-
ure 6. The sample with an angle of 0° is stronger than
that with a 90º raster angle. The sample with the 0° raster
angle is oriented in the longitudinal direction, which is
parallel to the tensile load direction. This results in an in-
crease in the applied tensile load, thus improving the ten-
sile strength of the material.
65
Previous research shows
how limitations are overcome when using other materi-
als, such as polyetheretherketone (PEEK), owing to the
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592 Materiali in tehnologije / Materials and technology 58 (2024) 5, 587–595
Table 3: UTS
RC
(TAD) for 3D printing materials
59
Layer thickness (mm) 0° 15° 30° 45° 60° 75° 90°
0.1 47.71% 51.36% 56.36% 57.73% 63.71% 76.67% 100.00%
0.2 48.15% 53.01% 54.71% 57.80% 70.55% 84.50% 100.00%
0.3 52.54% 59.83% 61.80% 64.43% 76.54% 85.22% 100.00%
Table 4: Tensile strength, Young’s modulus and Poison’s ratio for different 3D-printed composite specimens
63
Test type Property Mean value Standard error
Zero-degree orientation tensile
samples
TS1 524.66 MPa 1.80
E1 73.20 GPa 1.41
õ12 0.33 0.01
Ninety-degree orientation tensile
samples
TS2 38.66 MPa 2.77
E2 4.1 GPa 0.17
õ21 0.19 0.02
Quasi-isotropic orientation tensile
samples
TS 273.6 MPa 12.46
E 50.83 GPa 1.13
Figure 4: PACF specimen with different directional angles
50
Figure 5: Tensile properties and Young’s modulus at different printing
directions
50

difficulty of preparing composites with a high melting
temperature and high viscosity in the FDM process.
66
Tensile strength, tensile modulus and flexural strength of
the carbon fibre-reinforced PA6 composites increase as
the carbon fibre content is increased. However, the use of
continuous carbon fibres provides for a higher mechani-
cal strength than that of short carbon fibre-reinforced
PA6.
67
The melting temperature (T
m
) and thermal degra-
dation temperature are affected by the addition of carbon
fibres.
68
Thus, this paper reviews the mechanical proper-
ties related to the orientation and the use of parameters in
processing carbon fibre-reinforced polyamide.
3.2 Fracture and toughness
Fracture is one of the most widely recognized rea-
sons of failure in engineering structures. Consequently,
studying the behaviour of materials with cracks and de-
fects has always been of utmost importance.
70
To im-
prove the inter-line interfacial bonding performance of
3D-printed continuous fibre-reinforced composites, en-
hancing fracture and toughness strength was proposed.
Delamination failure is the most significant and detri-
mental type of damage in carbon fibre- reinforced com-
posites because the load-bearing capability of the com-
posite may be severely decreased without showing
visible damage.
71
Numerical methods such as finite ele-
ment analyses have been extensively employed to cap-
ture the fracture properties of carbon fibre-reinforced
polyamide.
72,73
The effect of orientation during 3D printing results in
fracture toughness. Composites printed at 0° and 90° ras-
ter angle allow similar fracture toughness, while the
weakest composite is the one with the 45° direction. This
is because 3D printing induces anisotropy.
74
For 0° and
90° samples, the 90° layers are pulled along their strong
axes. However, the 0° layers depend on inter-filament fu-
sion for their strength.
75
Anisotropy is the property of
substances that exhibit variations in physical properties
along various molecular axes. Besides, the material
strength is highly anisotropic, generally much weaker
along the printing direction.
69
Previous studies found that
the toughness, stiffness, and strength of unidirectional
lay-ups are higher when fibres are parallel or aligned
with the loading direction. For lay-ups with the layer ori-
entation shifted by 90°, which is the standard in FDM
printing, we found that the material is quite isotropic in
terms of stiffness and strength, but not in terms of tough-
ness.
76
Some studies show that the arrangement of fibres
can force cracks to turn into the direction of the fibre
path. As the deflection of a crack occurs under the force
of continuous fibres, specimens exhibit a longer plateau
before failing completely and can absorb more fracture
energy.
77
3.3 Electrical properties
There are two fundamental strategies for FDM 3D
printing of functional devices. The first method allows us
to fabricate a structural component using 3D printing,
while the second one allows for the prefabrication of
electronic functional devices with conventional manufac-
turing and their integration into structural components,
such as electrical components or electrical conducting
fluid.
78–81
If a reduced mass of a part becomes important
in most areas, then carbon fibre-reinforced composites
(CFRPs) are the best choice. Besides its structural role,
carbon fibre can be used, based on its electrical proper-
ties, for several secondary functions such as welding,
sensing, crosslinking and facilitating self-healing. Car-
bon fibres can be used for different tasks based on their
electrical properties. Long carbon fibre-reinforced ep-
oxy-matrix composites are used, however, to improve the
efficiency of their electrical properties, nanosized carbon
fibres, for example, carbon black or carbon nanotubes,
are added to composites.
82
New nano-engineered appli-
cations such as multi-scale and multi-functional carbon
nanotube composites can be used for system health mon-
itoring, with changes in thermal resistance and fire resis-
tance induced by damage, along with other multi-
functional attributes.
83
Previous studies focused on the electrical properties
related to the raster angle in 3D printing. The electrical
conductivity of samples was measured with a Keithley
N. N. KHALID et al.: INTERFACIAL BONDING OF POLYAMIDE-REINFORCED CARBON FIBRE VIA ...
Materiali in tehnologije / Materials and technology 58 (2024) 5, 587–595 593
Figure 6: Plot of the main effects showing the influence of process parameters on the tensile strength
69

2400 SourceMeter, used at four different points during a
tensile test with different point-to-point spaces. The sam-
ples were prepared at different loading percentages and
different raster angles. The results showed that samples
with raster angles of 0° and 90° exhibit conductive prop-
erties at a lower reinforcement ratio compared to those
with a raster angle of –45°/45°. This is because the traces
in the raster angle of –45°/+45° are discontinuous due to
the cross-layer pattern. As printing continuity is provided
by raster angles of 0° and 90°, electrical conductivity is
provided at a lower reinforcement rate.
84
Significantly
higher electrical properties have been demonstrated in
polymeric composite materials with higher filler load-
ings.
85
Using carbon fibres in 3D printing is a good op-
tion when studying the effects of different variables on
the anisotropic electrical properties of the carbon fibre
content. At a carbon fibre content of 0.3 w/%, the sensors
exhibited the maximal anisotropy.
59
Additionally, an in-
crease in the electrical conductivity of PA6/carbon
nanotube (PA6/CNT) microparts is firmly related to
CNT loading concentrations and the development of in-
ternal microstructure. The formation of three-dimen-
sional (3D) conductive pathways is found to be essential
for the improvement of electrical conductivity of
as-moulded microparts.
54
4 CONCLUSIONS
In our study, the printing orientation at different ras-
ter angles including 0°, 45° and 90° is investigated as it
affects the 3D printing of additively manufactured
polyamide reinforced with carbon fibres. The major find-
ings are listed below:
At a printing orientation of 0°, the tensile strength in-
creased to its maximum, showing an improvement by
 72 %. Notably, the 0° printing orientation resulted in
an increased resistance to applied tensile loadings, thus
improving the tensile strength of the material.
Investigation of the printing parameters for 3D print-
ing showed that the raster angle influences most of me-
chanical properties compared to air gap, raster width,
contour number and contour width. By adjusting the ras-
ter angle, better interfacial bonding within the lay-ups is
achieved, improving the load transfer through the printed
samples.
In contrast to the excellent mechanical properties at
the 0° printing orientation, electrical conductivity is
higher at the 45° raster angle. This is due to the discon-
tinuous traces in the raster angle of –45°/45° at the print-
ing layers. Therefore, it is strongly recommended that
the printing process is tailored to the printing orientation
based on the targeted performance.
Acknowledgment
The authors acknowledge the support by Geran
Universiti Penyelidikan (GUP), grant number
GUP-2022-012, funded by the Centre for Research and
Instrumentation Management (CRIM), Universiti
Kebangsaan Malaysia. A part of this research is sup-
ported by the Faculty of Engineering and Build Environ-
ment, Dana Pecutan Penerbitan.
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