Material extrusion‐based additive manufacturing of polypropylene: A review on how to improve dimensional inaccuracy and warpage

Abstract

Material extrusion‐based additive manufacturing (ME‐AM) is an emerging processing technique that is characterized by the selective deposition of thermoplastic filaments in a layer‐by‐layer manner based on digital part models. Recently, it has attracted considerable attention, as this technique offers manifold benefits over conventional manufacturing technologies. However, to meet the challenges of complex industrial applications, certain shortcomings of ME‐AM still need to be overcome. A case in point is the limited amount of semicrystalline thermoplastics, which are still not established as reliable, commercial filament materials. Particularly, polypropylene (PP) offers attractive properties that are unique among the ME‐AM material portfolio. This review describes the current approaches of fabricating PP components by ME‐AM. Both commercial and scientific strategies to make PP 3D‐printable are elaborated and compared. As dimensional issues are especially problematic for PP, a comprehensive section of this review focuses on the strategies developed for mitigating warpage for PP parts fabricated by ME‐AM. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48545.

Full text

REVIEW
Material extrusion-based additive manufacturing of polypropylene:
A review on how to improve dimensional inaccuracy and warpage
Martin Spoerk , Clemens Holzer , Joamin Gonzalez-Gutierrez
Polymer Processing, Montanuniversitaet Leoben, Otto Gloeckel-Straße 2, Leoben 8700, Austria
Correspondence to: J. Gonzalez-Gutierrez (E-mail: joamin.gonzalez-gutierrez@unileoben.ac.at)
ABSTRACT: Material extrusion-based additive manufacturing (ME-AM) is an emerging processing technique that is characterized by the
selective deposition of thermoplastic filaments in a layer-by-layer manner based on digital part models. Recently, it has attracted consid-
erable attention, as this technique offers manifold benefits over conventional manufacturing technologies. However, to meet the chal-
lenges of complex industrial applications, certain shortcomings of ME-AM still need to be overcome. A case in point is the limited
amount of semicrystalline thermoplastics, which are still not established as reliable, commercial filament materials. Particularly, polypro-
pylene (PP) offers attractive properties that are unique among the ME-AM material portfolio. This review describes the current
approaches of fabricating PP components by ME-AM. Both commercial and scientific strategies to make PP 3D-printable are elaborated
and compared. As dimensional issues are especially problematic for PP, a comprehensive section of this review focuses on the strategies
developed for mitigating warpage for PP parts fabricated by ME-AM. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley
Periodicals, Inc. J. Appl. Polym. Sci. 2020,137, 48545.
KEYWORDS: 3D-printing; additive manufacturing; dimensional accuracy; polypropylene; warpage
Received 10 July 2019; accepted 2 September 2019
DOI: 10.1002/app.48545
INTRODUCTION
Polypropylene (PP) is a thermoplastic derived from propene,
which is a relatively inexpensive by-product of the oil refining
process. Besides being inexpensive, PP is a very versatile thermo-
plastic with numerous applications due to its good mechanical
and biological properties, chemical resistance, and inertness.
These properties make PP a good candidate to fabricate products
by additive manufacturing (AM) techniques, such as material
extrusion-based AM (ME-AM) and powder bed fusion.
1
How-
ever, due to the semicrystalline nature of PP, it is not so easy to
obtain specimens with excellent geometrical accuracy, as the fab-
ricated components tend to shrink and warp during the AM pro-
cess. Many research teams throughout the world have been
studying ways to improve the processability of PP for AM tech-
niques, in particular for ME-AM. In this review article, the
authors summarize the findings of these research groups on how
to prevent warpage of PP specimens by adapting the ME-AM
processing parameters, copolymerizing, blending, and adding
fillers to PP polymers.
This review is structured in the following manner:
1. A description of the ME-AM process is given including the
requirements for materials and the materials currently available.
2. The properties and advantages of using PP in ME-AM are
discussed.
3. A summary of the properties of filaments sold as PP is given.
4. The problems of processing neat PP via ME-AM are
described.
5. The strategies different research groups have used to prevent
warpage of PP during ME-AM are discussed.
6. A summary of the actual printing conditions used to process
PP in the literature is given.
7. The possible applications for parts made out of PP via ME-
AM are outlined.
8. The conclusions are drawn and an outlook for the future is
provided.
Material Extrusion
Material extrusion, also known as ME-AM, fused filament fabri-
cation, or fused layer modeling is an extrusion-based AM tech-
nique that was developed in the late 1980s by Stratasys Inc.
under the name fused deposition modeling (FDM™).
2,3
In the
© 2019 The Authors.
Journal of Applied Polymer Science
published by Wiley Periodicals, Inc.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and repro-
duction in any medium, provided the original work is properly cited.
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course of a state-of-the-art ME-AM process, a solid thermoplastic
filament is hauled off into a hot die by two counter-rotating driv-
ing wheels (Figure 1). The spooled filaments, typically prepared
by extrusion of any thermoplastic polymer, are transported
through a moving deposition unit onto a heated build platform,
resulting in a layer-by-layer fabrication of the structural element
according to CAD-defined layer contours. In order to be
extruded through the nozzle, the filament is heated in the lique-
fier and the nozzle up to a temperature, at which it can easily
flow, which is mostly above the melting temperature of semicrys-
talline thermoplastic filaments. After leaving the nozzle, the
extruded material is deposited onto a build platform or a previ-
ous layer in the horizontal plane; the deposited melt cools down
and resolidifies. Once the selective deposition of one layer is com-
pleted, the build platform is lowered by the amount of one layer
height in order to print subsequent layers.
2–5
Filament Material Requirements
The filament materials used in ME-AM need to fulfill certain
requirements in order to be flawlessly processable. The filament
needs to be a thermoplastic that can be extruded within a certain
diameter and ovality tolerance in order to be three-dimensional
(3D) printable at a constant flow rate over time.
6,7
Moreover, the
filament needs to be stiff yet flexible enough so that the filament
can be spooled during filament production and despooled during
printing.
8,9
It has been suggested that the filament should reveal a
minimum strain at yield of roughly 5% so that the filament can
be continuously spooled and despooled.
8
This can be a challeng-
ing factor for composites that contain high percentages of fillers.
The addition of small amounts of amorphous polymers such as
poly(vinyl chloride),
10
polycarbonate (PC),
11
or amorphous poly-
olefins
8,12,13
to PP-based composites can provide a remedy as it
increases the yield strain.
For a reliable transport through the drive wheels, the filament needs
to retain its shape, withstand frictional forces from the drive wheels,
and withstand buckling between the drive wheels and the liquefier.
This can only be guaranteed as long as a sufficient strength and stiff-
ness of the filament is given.
8,14
In turn, for multicomponent mate-
rials, a strong filler–matrix adhesion is a prerequisite for high filament
strength and stiffness. In order for the material to deposit in a con-
trolled manner without dripping, the viscosity of the filament material
cannot be too low. Concurrently, if the viscosity is high, the filament
needs to reveal an improved strength and stiffness to be able to pass
through the nozzle.
8
However, high viscosities can be counteracted by
increased nozzle temperatures
15–17
or by implementing additional
hardware like an ultrasonic transducer at the nozzle,
18
resulting in an
easier dispensability of the melt.
14
For composites, although, highly
viscous materials tend to agglomerate, which can result in clogged
printing nozzles.
19
Consequently, a homogeneous filler distribution
within the filament is a must for a reliable processability.
8
For semicrystalline thermoplastics, filaments with a low degree of
crystallinity and a slow crystallization rate are preferred, as the
material’s tendency to shrink and warp is therefore extensively
Martin Spoerk received his PhD at the Montanuniversitaet Leoben, Austria, and Ghent University, Belgium. His
doctoral dissertation dealt with the investigation of how to improve the processability of polymeric materials for
material extrusion-based additive manufacturing. He has coauthored 17 peer-reviewed journal papers and
works currently as a senior scientist at the Research Center Pharmaceutical Engineering GmbH, Graz, Austria.
Clemens Holzer is a full university professor and head of the Institute of Polymer Processing at the
Montanuniversitaet Leoben. He is an expert in polymer processing, rheology, and process and material
simulations. His current interests include material development and applications of additive manufacturing with
polymers. He is coauthor of 80 peer-reviewed journal papers and 2 book chapters.
Joamin Gonzalez-Gutierrez is a postdoctoral senior researcher at the Institute of Polymer Processing,
Montanuniversitaet Leoben. He received his BSc at the University of Manitoba, Canada, his MSc at the
Universite catholique de Louvain, Belgium, and his PhD at the University of Ljubljana, Slovenia. His current
research interests include the development and characterization of polymer-based materials for extrusion-
based additive manufacturing. He has coauthored 25 peer-reviewed journal papers and 5 book chapters.
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reduced.
20
Simultaneously, the thermal expansion is diminished,
resulting in a dimensionally more accurate 3D-printed part.
14
Parts produced by ME-AM are built onto a build platform. The adhe-
sion between the first deposited layer and the platform determines the
success of the print.
21
Therefore, each filament material needs an
appropriate build platform. If the adhesion is too weak, the deposited
material detaches from the platform.
22
As a result, the production of
the final part cannot be continued flawlessly. If the adhesion is too
good, especially at room temperature, the final product cannot be
removed from the platform without damaging the part, the platform,
or both.
23
In an ideal process, the part is processed at high adhesion
between the first layer and the platform, controllable, for example,
through the temperature of the build platform, whereas the part
removal is conducted at a state of low adhesion.
24
The type of plat-
form material and the corresponding process parameters heavily
depend on the filament material used.
Materials Used in ME-AM
In contrast to photopolymerization- and powder-based AM tech-
niques, ME-AM allows to use a wide range of thermoplastics that
are commercially available in spools, satisfy nearly all the material
requirements discussed above, and are moderately priced compared
with other AM techniques.
4,25
Until the year 2012, the materials for
ME-AM, especially those for low cost 3D printers, were mainly lim-
ited to poly(lactic acid) (PLA) and acrylonitrile butadiene styrene
(ABS),
5
due to their facile processability both in terms of filament
extrusion and ME-AM. Up to now, these two materials are still the
top sellers among the ME-AM material portfolio
5
and are two of
the few materials that can be processed nearly without distortions.
Recently, the material alternatives have increased considerably,
5
leadingtoavarietyofcommerciallyavailablethermoplastics.
Figure 2 summarizes the current availability of the most important
polymer types as filaments for ME-AM, in which the information
about the commercial availability is based on Refs. 5 and 26–30.
Many polymer types (displayed in orange in Figure 2) have already
been commercialized, as both the industry and researchers have
emphasized in widening the material portfolio for ME-AM.
31
Apart
from PLA and ABS, particularly poly(ethylene terephthalate) and
PC can nowadays be already declared as standard ME-AM mate-
rials.
5
Most of the other materials, even those that are commercially
available, though, cannot always be used trouble free, need plenty of
hands-on experience and, thus, still need improvements in terms of
part processability, stability and accuracy, as has been shown for
various investigated filament types.
22,32–36
The evolving growth of scientific studies (displayed in purple in
Figure 2) on polystyrene (PS),
38
poly(ether sulfone),
39
poly(butylene
terephthalate),
40,41
and other polyesters,
42
as well as poly
(ε-caprolactone)
43–48
represent the expanding urge of widening the
material portfolio. The fact that even niche materials, such as plant-
based polymers,
49
biopolymers,
50
silicone elastomers,
51
recycled
polymers,
52–54
or highly filled polymers for the production of
metals/ceramics,
55,56
have been under investigation for the use in
ME-AM confirms the desired rapid growth in the process’s material
variety. Nevertheless, the usability of such novel materials for ME-
AM as an everyday usable and reliable material such as PLA or ABS
will be determined in the future.
It is derivable from Figure 2 that in particular a large proportion of
the amorphous polymers are widely commercialized. Their main suc-
cess factor in terms of ME-AM is their low coefficient of thermal
expansion, which facilitates their processability, especially in terms of
shrinkage, warpage, and distortion. However, most of the amorphous
filaments reveal low toughness, a small range of service temperature,
and a very weak chemical resistance.
57
Conversely, Figure 2 presents
that only a limitedamount of semicrystalline thermoplastics are avail-
able for sale or are under scientific investigation. Especially polymers
with a high degree of crystallinity (>40%), such as the commodity
semicrystalline plastics, namely low-density polyethylene (LDPE),
linear LDPE, high-density polyethylene (HDPE), ultrahigh molecular
weight polyethylene (UHMWPE), and PP, polyoxymethylene homo-
polymers, polytetrafluoroethylene (PTFE), or certain polyamide
types, appear to be particularly challenging to be processed by means
of ME-AM. Although these materials possess outstanding and unique
properties,
58
their application in ME-AM has notyet been thoroughly
studied in the literature.
To exemplarily visualize the tremendous potential of the semicrystal-
line materials, Figure 3 represents the toughness/stiffness balance of
Figure 1. Schematic illustration of the material extrusion process adapted from
Ref. 4. The components are labeled as: (1) spooled material storage, (2) thermo-
plastic filament, (3) horizontally movable, heated deposition unit consisting of
(4) counter-rotating driving wheels, (5) a liquefier, (6) a nozzle, (7) structural
element fabricated in a layer-by-layer manner, and (8) vertically movable build
platform. [Color figure can be viewed at wileyonlinelibrary.com]
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different commercially available filament types as well as PP-based
3D-printing materials from literature. Amorphous polymers may be
easy to process, but their mechanical properties are restricted to a
small toughness (elongation at break between 3 and 9%) and stiffness
(Young’s modulus between 1900 and 2400 MPa) area. Semicrystal-
line thermoplastics, though, reveal a much wider toughness and stiff-
ness range. Their Young’s modulus can stretch between 800 and
4000 MPa and their elongation at break between 2.5 and 1600%, for
example for polyolefins,
8,13
outperforming even the very flexible ther-
moplastic elastomers that have currently been commercialized for
ME-AM. If fillers are introduced into semicrystalline polymers
(referred as semicrystalline composites in Figure 3), their stiffness can
be enhanced drastically (e.g., up to 15,000 MPa for PA filled with car-
bon fibers [CF]), surpassing that of the amorphous polymers, whereas
their toughness stays in a range comparable to that of the amorphous
polymers. Hence, semicrystalline polymers, especially when filled,
possess great potential in terms of mechanical properties for the use
as filaments in extrusion-based AM. In particular, PP reveals out-
standing and incommensurable elasticity compared with all other
semicrystalline thermoplastics. Therefore, PP-based composites
exhibit potential for a very broad range of applications in terms of
mechanical performance. If the price of the raw material is taken into
consideration, the PP-based composites clearly reveal the most prom-
ising properties. Therefore, this material class might be in the focus of
future commercializations.
UNIQUE PROPERTIES OF PP FOR ME-AM
PP exhibits a wide range of customizable properties, and it has
been studied thoroughly over the last 70 years. During the last
decades, it has undergone great growth both in scientific studies,
in which PP often has served as the standard thermoplastic to
explain novel phenomena, and production and use, as it has the
Figure 3. Elongation at break as a function of the Young’s modulus for
commercially available ME-AM materials that are subdivided into thermo-
plastic elastomers, amorphous and semicrystalline polymers, and semicrys-
talline composites. The scientifically available PP-based materials are
highlighted by black rectangles. The mechanical properties are based on the
technical information provided by the respective material suppliers and sci-
entific publications.
8,13,27,59–63
The material abbreviations are described by
the corresponding references in the legend. [Color figure can be viewed at
wileyonlinelibrary.com]
Figure 2. Pyramid of polymeric materials as a function of the availability of the materials in the ME-AM market. Please refer to Ref. 37 for the commonly
used polymer abbreviations. The asterisk on PP refers to the unclear commercial availability (refer to Commercially Available PP Filaments section). [Color
figure can be viewed at wileyonlinelibrary.com]
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potential to substitute engineering polymers and metals. For the
low price of PP (roughly 1.2€/kg
64
), it provides satisfying mechani-
cal properties, such as a decent tensile strength (25–40 MPa
64
)and
Young’smodulus(1300–1800 MPa
64
), high toughness (elongation
at break of >50%
64
), impact and abrasion resistance, in combina-
tion with a low density of approximately 0.9 gcm
−3
,
64
which
makes PP particularly attractive for ME-AM.
As PP is an easily customizable polymer, different approaches
exist to improve/alter its mechanical properties. A variation of its
chain regularity content and distribution, its tacticity, its orienta-
tions, or its average chain length leads to a very broad property
portfolio that can be adjusted depending on the prevalent needs.
Furthermore, PP is known to be easily modified by the addition
of comonomers such as ethylene or octane into the polymer
chains or by incorporating additives such as fillers, impact modi-
fiers, fibers, or other polymers.
65,66
Additionally, its nontoxicity, applicability as a biologically inert
material and its excellent chemical resistance against various
reactants make PP an outstanding material for the ME-AM mar-
ket, which can only be outreached by far more expensive poly-
mers such as poly(ether ether ketone). Particularly, the
outstanding chemical resistance of PP to polar solvents, non-
oxidizing acids, aqueous alkalis, and aqueous salt solutions cre-
ates novel possibilities for ME-AM, as other commercially
available filament types are considerably less resistant to
chemicals, which has limited the applicability of parts produced
by ME-AM.
57,67
All polyolefins including PP inherently reveal a very low water
and moisture absorption. Compared with the standard ME-AM
materials PLA and ABS, the water absorption of PP is more than
one order of magnitude lower.
57
Consequently, vaporized water
cannot complicate the printing process, resulting in less voids
and a more appealing surface quality.
68
Moreover, the low mois-
ture absorption enables the longevity of components in demand-
ing applications in humid surroundings, which would not be
feasible for conventional ME-AM materials.
69
Finally, the low
water absorption of PP saves costs, as no additional drying steps
prior to printing are required.
57
Having a glass transition temperature of around −15 C,
70
PP
provides good thermal stability between 0 and 150 C. As it is a
semicrystalline polymer, its melting point at 165 C limits its
upper service temperature, but it is high in comparison to many
other commercially available ME-AM materials. Moreover, the
constituent monomers of this nonpolar polymer are readily avail-
able, consolidating its position as a leading thermoplastic material
also in the future.
66,71,72
COMMERCIALLY AVAILABLE PP FILAMENTS
Due to the aforementioned unique properties of PP among the
commercially available materials for ME-AM, the ever increasing
interest for PP filaments from the industry has been satisfied by
many different PP filament producers. As the term PP can be
quite variable, Figure 4 compares the differential scanning calo-
rimetry thermographs of all commercially available PP filaments
found by the authors. Interestingly, none of the commercially
available PP filaments corresponds to pure isotactic PP, which
has a characteristic melting peak of the monoclinic α-crystals at
~165 C (highlighted by the gray area and the example in
Figure 4),
73
although mainly PP containing largely α-crystals
reveal the aforementioned outstanding properties.
58
Most of the
investigated filaments reveal either small quantities of β-crystal
structures or are random PP copolymers. Some available fila-
ments are just sold as PP, although the material does not even
contain small quantities of PP (Figure 4).
Figure 4. Comparison of the second heating thermograms of all commer-
cially available PP filaments. The gray area highlights the melting area of
the monoclinic α-crystals of isotactic PP around ~165 C. Right to the gray
area, the degree of crystallinity (α
cr
) of each material is represented. For rea-
sons of comparison, at the bottom, a thermogram of a PP heterophasic
copolymer that would be a suitable material for 3D printing, but is only
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As can be derived from the PP atypical thermograms, the fila-
ment producers obviously make use of the facile modifiability of
PP. Accordingly, the degrees of crystallinity (α
cr
) of most of the
investigated PP filaments (Figure 4) are untypically low for
industrial grade PP. On the one hand, such low α
cr
results in
rather weak mechanical properties, as only isotactic PP with α
cr
up to 70% enables the aforementioned exceptional mechanical
properties.
64
On the other hand, such low α
cr
can mitigate the
main disadvantage of PP: PP is known to be vulnerable to dimen-
sional inaccuracies, especially in the form of warpage due to its
high shrinkage coefficient. Therefore, the available materials
make a compromise between dimensional accuracy and mechani-
cal performance.
PROCESSING OF NEAT PP BY ME-AM
Apart from studies dealing with PP as a base compound for
highly filled systems, in which the polymeric part is burnt away
before the sintering step,
56,74–77
only a handful of studies on neat
PP have so far been conducted for the use in ME-AM. Next, to
PP-based blends used for ME-AM
10,11
and PP blends based on
polymeric waste,
78–80
Volpato et al.
81
used neat PP for one of the
first times in an extrusion-based AM approach by feeding the
material in the shape of pellets into their self-designed piston-
driven 3D printer. As the focus on their research lay on the opti-
mization of the feeding system, the novel material’s behavior in
the 3D-printing process was not analyzed. One of the first that
recognized the significance of PP as a material for ME-AM was
Jagenteufel et al.
82
The authors compared the standard printing
material ABS with PP by means of rheological measurements and
die swell experiments on 3D-printer nozzles. It was found that
PP could be a promising material for ME-AM, as compared with
ABS, PP was more stable over time at elevated temperatures, less
prone to oozing and revealed a higher melt stiffness. However,
tests on parts produced by ME-AM have not been conducted.
Several individual research groups discovered that the first layer
adhesion to the build platform is one of the main process limita-
tions of PP.
21,23
It was found that PP does not adhere to standard
ME-AM build platform substrates,
83
such as glass mirrors or pol-
yimide tapes,
24
mainly due to the material’s lack of surface func-
tional groups, low surface energy, and low polarity.
84
In order to
counteract a possible delamination of the first deposited layer,
most researchers recommend to deposit PP-based filaments onto
PP substrates.
23
However, special care needs to be taken in order
not to weld the first deposited layer onto the PP build platform,
which can lead to a complete damage of the 3D-printed compo-
nent during part removal. Strategies to counteract this issue
address the choice of build platforms with a slightly different sur-
face energy as well as polarity but similar chemical composition.
Recommended platform materials for successful 3D-printing PP
include random PP copolymers or UHMWPE.
13,23
In the seminal work of Hertle et al.,
85
actual 3D-printed speci-
mens of PP that were produced by means of a special ME-AM
technique, in which pellets are used instead of a filament, were
investigated. The authors elaborated the influence of different
processing conditions (varying extrusion, build platform, and
cooling temperatures) on the temperature evolution, the shear
stress, and the strand interface morphology and suggested a pro-
cess window for neat PP. They found that for semicrystalline
polymers such as PP, the build platform temperature is limited
by the material’s crystallization onset temperature, in order to
provide dimensional stability of the printed part. To achieve a
high interfacial bonding between adjacent strands, the interface
between the freshly deposited as well as the previously deposited
material, which was controlled by the build platform tempera-
ture, should reveal a contact temperature higher than the crystal-
lization onset temperature of PP. If the strands exhibit such high
temperatures for a longer period of time, an improved auto-
adhesion and interdiffusion depths, and therefore a better bond-
ing is realized. For best possible interlayer strengths, the contact
temperature should succeed the crystallization temperature up to
the melting temperature of PP, in order to enable a short-term
melting of the crystalline areas of the adjacent strands. Addition-
ally, low cooling rates were found to be essential for a homoge-
neous strand morphology, in which weld lines were hardly
detectable. The authors additionally expected a lower shrinkage
for higher cooling rates. This higher specific volume may lead to
a lower stress development at the strand interface, but also to an
increased shrinkage after the production of the part, as the poly-
mer chains tend to reach their thermodynamical equilibrium
after a certain time.
86
However, these hypotheses have not been
verified.
Recently, Wang et al.
87,88
adapted the idea of using high build plat-
form temperatures for best mechanical properties from Hertle
et al.
85
The authors studied the influence of the extrusion tempera-
ture and the layer thicknesses on the impact strength of neat PP
produced by ME-AM at a platform temperature of 130 Cand
compared the results to homogeneous specimens produced by
injection molding, similarly to Ref. 57. Elevated extrusion tempera-
tures and small layer thicknesses resulted in smaller air gaps
between adjacent strands and a higher part density due to a higher
degree of diffusion and a bigger cross flow. Additionally, the
authors discovered that the lower extrusion temperature of 200 C
in combination with the rather high platform temperature resulted
in a mixture of α-andβ-PP, whereas the settings for extrusion
temperatures of 250 C and injection-molded specimens only rev-
ealed the crystalline modification of α-PP. Due to the existence of
β-PP and more air gaps, the specimens produced at the extrusion
temperature of 200 C led to significantly higher impact strengths
than those produced at 250 C, so that the specimens processed at
200 C were comparable to those of injection-molded specimens.
Spoerk et al.
13
also found small fractions of β-crystal structures
among the dominant α-PP when fabricating PP by means of ME-
AM in an encapsulated insulated chamber with a chamber temper-
ature (T
Ch
)of55C. Due to the increased T
Ch
, the strand tempera-
tures repeatedly surpassed 100 C, which triggered the formation of
β-PP. Specimens printed at a T
Ch
of 25 C, however, did not form
β-crystals due to the considerably lower strand temperatures. The
increase of the T
Ch
of only 30 C additionally resulted in significant
crystallographic changes in terms of spherulite size. The parts
printed at the elevated T
Ch
revealed twice as large spherulites as
those processed at the lower T
Ch
, as the mean strand temperature
during printing is for the high T
Ch
close to the temperature of the
maximum crystal growth rate, whereas the nucleation rate is low.
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Consequently, few nuclei grow to a larger size at the high T
Ch
.
Accordingly, such drastic changes in the morphology and crystal-
lography altered the impact properties of 3D-printed PP, increased
the yield stress,
57
but did not influence the interlayer strength.
13
Based on these investigations, the temperature of the strands obvi-
ously not only determines the interdiffusion depth and therefore
the mechanical performance of printed PP parts but also the
growth and nucleation of different crystalline modifications. The
studies revealed a fundamental understanding of how complex the
behavior of PP in AM technologies can be. Moreover, the studies
highlight that only minor changes in the processing settings, for
example, an increase in the T
Ch
, can critically alter the morphology
of 3D-printed PP, as annealing steps or postcrystallization for PP
takes place in the typical temperature range for ME-AM.
THE BIG ISSUE OF WARPAGE FOR 3D-PRINTED PP
The main disadvantage of using PP in ME-AM is the strong sus-
ceptibility of the polymer to shrink and warp extensively. As a
polymer melt cools down, the volume of the polymer, both the
free volume between the macromolecular chains and their vibra-
tional volume, decreases, as long as the temperature is above the
glass transition temperature. This results in material shrinkage.
Amorphous polymers such as ABS exhibit a linear weakly devel-
oped decrease of specific volume until its glass transition temper-
ature during cooling (Figure 5). Semicrystalline polymers such as
PP, however, reveal a drastic change in specific volume in the
crystallization region of the polymer chains, as the formed crystal
structures are considerably denser than the amorphous structures
in the melt state.
89
Due to the considerably higher change of spe-
cific volume over a certain temperature range compared with the
amorphous materials, PP tends to shrink far more than ABS, espe-
cially if the α
Cr
is high (Figure 5), for example, for PP
homopolymers. When polymeric parts shrink differently or in an
anisotropic manner at various positions, for example, due to an
inhomogeneous or nonuniform cooling, or due to a different
amount of shrinkage in flow and transverse flow direction, which
is nearly omnipresent for 3D-printed semicrystalline polymers, the
fabricated part is prone to warpage. Thermoplastics that reveal
extensive volumetric shrinkage and high degrees of crystallinity are
particularly susceptible to warp.
86,90
Especially in the ME-AM pro-
cess, the combination of the material-dependent shrinkage upon
cooling, the anisotropic deposition of the strands,
91
the associated
introduced polymeric chain orientations,
92
and the complex tem-
perature distribution in the AM machine
93–97
causes contractile
forces within the deposited strands, which result in residual
stresses and finally in excessive warpage.
57
As the forces induced
by warpage counteract the adhesion forces of the first layer to the
build platform,
24
3D-printed PP components easily detach from
such platforms.
23
If sufficient first layer adhesion is guaranteed, PP
components can nonetheless suffer heavily from warpage occur-
ring after processing due to the tremendous residual stress build
up during ME-AM.
57
Carneiro et al.
21
were the first researchers to address warpage
issues during the fabrication of PP parts produced by ME-AM.
Apart from an evaluation of the entire production chain starting
from the filament and investigations on the effect of printing ori-
entation, infill degree, and layer thickness on tensile properties,
the authors argued that changes in the processing conditions can
improve the dimensional control in terms of warpage. They
found that an improved compatibility between the build platform
and the printed part and an optimal printing direction are advan-
tageous for the warpage control of printed parts. Moreover, they
incorporated glass fibers to PP and fabricated parts of this com-
pound by means of ME-AM for the first time. Although no influ-
ence on the warpage behavior was investigated for the fiber-
reinforced PP, it can be expected that due to the fillers’low ten-
dency to shrink, the warpage of the composite was improved
compared to the part produced of neat PP as it was later demon-
strated by Spoerk et al. with carbon fibers.
62
In a nutshell, Car-
neiro et al.
21
laid the cornerstone for further studies on the
warpage improvement of PP and displayed two strategies for
improving the dimensional inaccuracies of 3D-printed specimens
that are analyzed in more detail in the following paragraphs.
Process-Induced Warpage Optimization for 3D-Printed PP
Apart from material alterations, for example, by decreasing the
degree of crystallinity by incorporating ethylene monomer seg-
ments to PP
90,98
or by blending PP with other thermoplastics,
99
changes in the processing can significantly improve the warpage
behavior of 3D-printed PP components. Hämäläinen,
57
for exam-
ple, found that the degree of warpage of 3D-printed PP is highly
dependent on the geometry of the component that is processed.
Dense cylindrical specimens revealed a well-controllable warpage,
whereas cubic parts exhibited part distortion especially on the
corners. This finding was caused by strong contractile forces orig-
inating from the corners of the cubes toward the center of the
cube, the lengthwise shrinkage of the 45infill, and a higher
stress build up. A similar trend for decreased warpage for cylin-
drical parts was found for hollow specimens. However, both
Figure 5. Specific volume normalized to the specific volume at 40 Casa
function of temperature at a constant pressure obtained by an SWO
pvT100 (SWO Polymertechnik GmbH, Germany) exemplarily shown for
ABS and two PP types with different degrees of crystallinity (α
Cr
). The
shrinkage, glass transition temperature (T
G
) and crystallization temperature
(T
Cr
) are exemplarily shown. [Color figure can be viewed at
wileyonlinelibrary.com]
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2020, DOI: 10.1002/APP.48545

thin-walled parts exhibited considerably poorer dimensional sta-
bility than the dense parts. This finding was related to the fast
cooling rate of the freshly deposited material in thin walled speci-
mens, which resulted in higher residual stresses and therefore
increased warpage compared with the dense specimens.
Watanabe et al.
100
recently published a study, in which the warpage
of neat PP was investigated both by modeling/simulation of the first
two layers of 3D-printed strands and by an experimental parametric
study. The authors found that certain changes in processing condi-
tions can have detrimental effects on dimensional distortions. A
minimal amount of warpage was achieved by short stacking lengths,
lower nozzle temperatures, higher deposition speeds, and increased
layer thicknesses, which is also in agreement with warpage studies on
PLA.
101
All these factors change the temperature distribution within
the printed strands. Therefore, factors that induce a small tempera-
ture gradient in the produced part result in an improved dimensional
control. Fitzharris et al.
33
extended these findings by comparing the
modeled warpage of PP to that of PPS and by additionally simulating
the temperature and residual stress distribution in the strands.
According to the authors, the inferiority in terms of warpage of PP
over PPS can be explained by the material’s higher coefficient of ther-
mal expansion and by the poor adhesion to the investigated build
platform, which was also confirmed in the form of the distortion
ratio by Duty et al.
102
In addition, Watanabe et al.
100
simulated the
effect of the addition of fillers on the part warpage of the first two
layers of 3D-printed parts. The incorporation of the filler resulted in
an increase in thermal conductivity and Young’smodulusanda
decrease in the coefficient of thermal expansion. All these conse-
quences had in turn a positive effect on the simulated part deforma-
tion, leading to an improved warpage compared with neat PP.
Apart from studies on PP-based materials, plenty of other processing
strategies on improving the warpage of standard ME-AM polymers
have been conducted.
103
Many other independent researchers agree
on the positive effect of a short stacking length and a high amount of
layers on the warpage of 3D-printed parts.
104–106
However, recently
the consequences of the amount of layers on the distortion of parts
have been debated. At a first glance, it was expected that lower layer
numbers lead to more warpage dueto a lower bending stiffness of the
component. Armillotta et al.,
107
however, revealed in their experi-
mental results that very thin parts distort less than slightly thicker
parts due to the thermal conductivity of the deposited material and
the permanent deformation of the material under bending stresses.
Moreover, another contradicting trend was discovered for the effect
of stacking length on the warpage. Kantaros et al.
108
experimentally
found that residual strains in parts produced by ME-AM are lower
for longer stacking lengths than for shorter ones, although it is known
that a reduction in residual stresses and strains is essential to decrease
warpage in 3D-printed parts.
109
Irreversible thermal strain that is
formed during the solidification of a strand
110
and causes residual
stresses was determined to be critically dependent on the printing ori-
entation and the layer thickness.
111
Low layer thicknesses result in ele-
vated thermal strains and therefore high residual stresses and
warpage,
111–113
which is in agreement with simulation results.
104–106
Other more elaborated strategies that could be applied to PP compo-
nents to mitigate warpage deal with geometrical or external
processing parameters. A case in point is a change in the slicing
strategy, in which the warpage is decreased by reducing the stacking
section length by slicing smaller brick structures instead of the whole
geometrical feature.
114
This strategy, although, can have detrimental
effects on the mechanical properties of the printed parts. Another
possible solution is to slightly modify the CAD data. One way is to
predeform the CAD data, contrary to the expected warpage devia-
tions.
115
Another possibility is to adapt the interior design of 3D-
printed parts in a way that the interior design shrinks so that the
essential outer periphery does not shrink.
116
One elegant and practical solution that has been elaborated for amor-
phous polymers in a mathematical model is to diminish the warpage
of 3D-printed parts by rising the surrounding temperature of the
deposited strands in the printing chamber (T
Ch
).
104
For ABS, the esti-
mated warpage turns almost to zero, as soonas the temperature in the
printing chamber equals the glass transition temperature of the fila-
ment. Although this theory is not fully applicable to semicrystalline
polymers due to their low glass transition temperatures and complex
crystallization kinetics, an increased T
Ch
was shown to be beneficial in
terms of part distortion for PP produced by ME-AM.
13
As the sur-
rounding printing temperature exhibited less temperature fluctua-
tions for higher T
Ch
, the strand temperatures were found to be more
homogeneous. Additionally, it was found that the strands at the higher
T
Ch
cooled down more slowly compared with the low T
Ch
,resultingin
less residual stresses and a lower degree of volumetric shrinkage. The
exposure of the components to the high T
Ch
forlongerprintingtimes
also led to typical annealing consequences, such as promoted stress
relaxations and crystallizations. Moreover, the first layer adhesion to
the build platform was improved for high T
Ch,57
leading to less warp-
age during processing. As a consequence of all these findings, 3D-
printed PP fabricated at a T
Ch
of already 55 C revealed a far better
dimensional stability in terms of warpage than specimens processed at
room temperature, if sufficient first-layer adhesion was provided.
To sum up, changes in processing conditions clearly have the
potential to improve the dimensional stability of parts produced
by ME-AM. However, parameters that traditionally have been
elaborated and declared as effective can be used as guidelines for
a warpage reduction but cannot be universally suggested for all
geometries and parts. For instance, if the geometry of the desired
part is fixed, which is the case in most of the industrial parts such
as spare parts, a change in stacking lengths or part thickness is
not an option. Furthermore, high layer thicknesses may decrease
the warpage effectively but also completely deteriorate the
mechanical and surface properties of the produced specimen.
16
Hence, recent strategies that diminish the residual stresses or
homogenize the temperatures within the part, for example, an
increase in the T
Ch
, appear more appealing. All in all, the most
effective warpage control is expected to be achieved by such
processing adaptions in combination with a modified build mate-
rial that is optimized for a minimal degree of warpage, such as by
the inclusion of fillers.
Fillers Preventing Warpage for 3D-Printed PP
Within the last years, a myriad of studies on the incorporation of vari-
ous fillers into thermoplastics for extrusion-based AM have been
conducted.
117–119
Various fillers, such as cellulose nanofibrils,
98,120,121
thermotropic liquid crystalline fibrils,
122,123
bamboo fibers,
124
hemp
and harakeke fibers,
125
short glass fibers,
126
ash,
87
and mineral
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fibers
127
have been incorporated into 3D-printed PP to improve the
material’s mechanical, rheological, or thermal properties. However,
only a handful of studies have focused on the warpage prevention by
incorporating fillers into PP filaments.
Wang et al.
20
did a prestudy on the warpage behavior of PP com-
pounds, in which the complex crystallization behavior of PP was
assumed to be responsible for the warpage susceptibility. The authors
claimed that mainly the crystallization rate determines the degree of
shrinkage and warpage of 3D-printed PP. According to the authors,
PLA is much less prone to warpage and therefore easier to 3D print, as
the crystallization rate is nine times lower than that of PP. By adding
10 wt % spray-dried cellulose nanofibrils and a compatibilizer to PP, a
slightly lower crystallization rate compared with PP was realized.
Simultaneously, the coefficient of expansion was decreased by 11.7%,
which in turn reduced the shrinkage tendency. Although the real
warpage of 3D-printed specimens was not analyzed by the authors
and it is influenced by far more factors than discussed by Wang
et al.,
20
a trend toward a reduced part warpage due to the decreased
crystallization rate and coefficient of expansion can be expected.
Stoof et al.
128
and Pickering et al.
129
showed for the three fillers
harakeke, hemp fibers, and recycled gypsum that were incorporated
into preconsumer recycled PP a clear warpage improvement on spe-
cially designed 3D-printed specimens. A trend toward decreasing
warpage was found for increasing filler contents. The filler harakeke
exhibited the most effective warpage reduction. As soon as the fillers
were agglomerating (e.g., gypsum), the warpage reduction as well as
the mechanical property improvement were not as effective as for
theevenlydistributedfillers harakeke and hemp. A similar trend was
also found for PP filled with glass spheres and cellulose, respec-
tively.
8,130
The filaments of both materials were only processable by
ME-AM and only revealed an effective warpage reduction, when a
compatibilizer and filler coating were applied to guarantee a homo-
geneous filler distribution and a strong filler–matrix interface.
Spoerk et al.
12
analyzed the shrinkage and warpage behavior of
3D-printed PP parts filled with expanded perlite spheres as a
function of the mean filler diameter and the filler load. As the
inorganic fillers hinder the volumetric change of the polymer
chains, the volumetric shrinkage decreased with increasing filler
load. The compounds filled with smaller fillers showed a reduced
shrinkage compared with those filled with larger fillers due to the
larger interfacial surface area between the fillers and the matrix.
As the particles were evenly distributed in the matrix and
exhibited a good filler–matrix interface, both filled materials were
flawlessly printable, despite the high filler load of 30 vol. %. The
dimensional accuracy of 3D-printed components was tested by
means of optical analysis on specially designed specimens that
were particularly prone to warpage. As expected from the distinct
difference in volumetric shrinkage, both filled materials revealed
an improved warpage behavior. However, the composite filled
with the smaller particles was shown to be considerably more
dimensionally stable compared with the composite filled with the
larger spheres. The same finding was also later confirmed for 3D-
printed PP filled with glass spheres with different sizes.
13
Although an anisotropic behavior in shrinkage and warpage can be
expected for fiber-reinforced thermoplastics,
131
various studies, both
on ABS reinforced with carbon fibers
132
and PP filled with high
amounts of hemp fibers,
128
glass fibers,
21
or carbon fibers,
62
showed
that also high aspect ratio fillers can reduce warp deformations effec-
tively. Especially the most recent study on PP filled with short carbon
fibers
62
demonstrated an outstanding dimensional accuracy particu-
larly in terms of isotropic warpage on highly complex and large engi-
neering parts despite the highly anisotropic filler. The reason for this
rather unexpected finding is twofold. First, the stiff fibers can
decrease warp deformations especially in corners of printed speci-
mens, as the fibers can hinder the entropically driven contraction
movement of the polymer chains as long as the fibers are stiff enough
and are aligned in the printing direction. Second, it can be explained
by the high thermal conductivity of the carbon fibers, which in turn
augment the overall thermal conductivity of the composite. As a con-
sequence, the rapid temperature fluctuations during printing
94
are
compensated by the highly conductive filler. This leads to a more
homogeneous temperature distribution and therefore in less inter-
nal/residual stresses and a decreased warpage. This trend is verified
by thermography measurements shown in Figure 6, in which the
Figure 6. The temperature evolution of a contourstrand of the third printing layer of the first Charpy specimen for neat PP and the CF-filled composite
PP/CF10 (a) along with 3D-printed specimens that are particularly prone to warpage
12
for PP (b) and PP/CF10 (c). For a detailed setup of the thermogra-
phy measurement, please refer to Ref. 13. The temperature difference between the peak maximum around 90 min and its previous minimum (peak ampli-
tude) is marked for PP (ΔT
PP
=20C) and PP/CF10 (ΔT
CF
= 5.5 C) in (a). [Color figure can be viewed at wileyonlinelibrary.com]
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Table I. Examples of the Printing Conditions Used For Producing PP-Based Parts via ME-AM
Material
Softening/melting
temperature (C)
Extrusion
temperature
(C)
Build platform
temperature
(C)
Flow
rate (%)
Printing
speed (mm/s)
Layer
thickness (mm)
Build platform
material ME-AM machine
Fabricated
specimens Reference
PP homopolymer Vicat: 153 (10 N) 165 Room N/A 8 (first layer), 60
(next layers)
0.2 and 0.35 PP scrubbed plate Prusa i3, Prusa
Research s.r.o,
Prague, Czech
Republic
Single-filament-thick
wall
box and tensile
specimens (DIN
53504-S3a)
21
PP reinforced with
glass fibers
Vicat: 135 (10 N) 185 Room on PP
plate, 80 on
blue tape
N/A 8 (first layer), 60
(next layers)
0.2 and 0.35 PP scrubbed plate
or blue tape
Prusa i3, Prusa
Research s.r.o.,
Czech Republic
Single-filament-thick
wall
box and tensile
specimens (DIN
53504-S3a)
21
PP reinforced with
glass spheres
Melting temp.: 166 230 80 60 64 0.25 PP plate Wanhao Duplicator i3
v2, Wanhao
Impact specimens
100% infill (ISO
179-1)
8
PP reinforced with
short carbon fibers
Melting temp.: 166 230 70 150 28.3 (first layer),
56.6 (next
layers)
0.25 PP plate Wanhao Duplicator i3
v2, Wanhao
Bending/Charpy
specimens 100%
infill (ISO 178
and ISO 179-1) and
different technical
parts
62
PP reinforced with
glass spheres of
different size
Melting temp.: 166 230 20 and 70 150 28.3 (first layer),
56.6 (next
layers)
0.25 PP plate Wanhao Duplicator i3
v2, Wanhao
Specimens prone to
warpage and impact
specimens 100%
infill (ISO 179-1)
13
PP copolymer Melting temp.: 151 200–240 N/A N/A 10, 20, and 30 0.1–0.3 N/A HYREL System 30,
Hyrel 3D
Five-layer specimens
20 mm in length
100
Isotactic PP with
POE-g-MAH
Melting temp.: 165 220 110 N/A 30 0.1 N/A A8 Quasi-Industrial
grade, Shenzhen JG
Aurora Technology
Co., Ltd., China
Sheet specimens 99
Isotactic PP with PA6 Melting temp.: 165
and 220
250 110 N/A 30 0.1 N/A A8 Quasi-Industrial
grade, Shenzhen JG
Aurora Technology
Co., Ltd., China
Dragon fly with shape
memory effects
99
PP block copolymer
reinforced with
cellulose nanofibrils
Melting temp.: 159
to 162
200 120 100 45 0.3 PP-based packing
tape
LulzBot TAZ 6, Aleph
Objects Inc.
Specimens for DMA
(ASTM D 648)
98
PP filament Reprap
Germany; PP
filament from
Popbit, China;
PP-ethylene random
copolymer
N/A 250 120 N/A 15 0.25 Epoxide resin plate A4, 3ntr, Italy Free-standing square
tube (5 cm ×5 cm)
with
single-filament-thick
walls
90
Isotactic PP N/A 240 120 N/A 15 0.25 Epoxide resin plate A4, 3ntr, Italy Free-standing square
tube (5 cm ×5 cm)
with
single-filament-thick
walls
90
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temperature evolution of one contour strand of a Charpy specimen
is recorded for neat PP and a carbon fiber reinforced PP (PP/CF10
62
)
during the fabrication of five Charpy specimens. The printing
sequence for PP/CF10 had to be altered compared with that of neat
PP (displayed in Ref. 13) in order to guarantee the most promising
repeatability and printability. As a result, the temperature evolution
of PP/CF10 shows double the amount of temperature peaks that
occur when the measurement position is close to the printer nozzle,
compared with that of neat PP. Nevertheless, the peak temperatures
of the two materials are still comparable.
In spite of some fluctuations at the beginning of the measure-
ment [Figure 6(a)], both materials exhibit a reduction of the
maximum of the temperature peaks with increasing printing time
due to the insulation of subsequently printed layers between the
measurement position and the nozzle. A clear difference between
the two materials can be discerned, although. As neat PP behaves
like a thermal insulator
62
(λ
axial, 90

= 0.30 0.01 W m
−1
K
−1
),
the amplitude of the peaks reduces slowly over time, which
means that the thermal equilibrium is reached very late.
133
Hence, after a printing time of more than 90 min, a rather inho-
mogeneous temperature distribution of the printed part prevails,
as still a temperature amplitude ΔT
PP
of 20 C is present. On the
contrary, the temperature amplitude of the considerably more
conductive PP/CF10
62
(λ
axial, 90

= 0.87 0.02 W m
−1
K
−1
) con-
verges much faster toward a thermal equilibrium than that of
neat PP.
133
After 90 min of printing, the observed strand in the
PP/CF10 Charpy specimen shows a roughly four times lower
temperature amplitude (ΔT
CF
= 5.5 C) than that of PP. Thus,
the higher thermal conductivity induced by the incorporation of
carbon fibers leads to a more homogeneous temperature distribu-
tion within the fabricated specimens during printing. Conse-
quently, the internal/residual stresses of the material are expected
to be reduced, too, which explains the promising dimensional
accuracy [Figure 6(c)] compared with neat PP [Figure 6(b)],
despite the high aspect ratio of the fillers.
SUMMARY OF THE PROCESS SETTINGS FOR 3D-
PRINTING PP
In the previous sections of this review, the general trends
observed when different printing parameters are changed have
been described and linked to warpage of parts produced by ME-
AM. This section is a practical summary of the actual values used
during production of parts by ME-AM when using PP-based fila-
ments. The aim of this section is to provide guidelines for print-
ing PP filaments. The values of the processing parameters that
have been found in the literature are summarized in Table I.
As can be derived from Table I, PP-based filaments have been
processed in a variety of commercially available ME-AM machines.
ME-AM processing parameters for PP-based materials vary signifi-
cantly, mainly depending on the machine used, material used, and user
preferences. For example, the extrusion temperature could be set
between 165 and 250 C; the build platform temperature varies
between 25 (room) and 130 C; the flow rate multiplier could be
between 60 and 150%, where higher flow rates can result in parts with-
out air gaps
16
; the printing speeds reported were set from 8 to
64 mm s
−1
, where low printing speeds are usually recommended for
PP random copolymer;
PP reinforced with
perlite,
compatibilizer and
amorphous
polyolefin
N/A 200 25 to 100 N/A 50 0.1 to 0.3 UHMWPE Hage 3DpA2, Hage
Sondermaschinenbau
GmbH, Austria
Single strands,
cylindrical labyrinth;
screws and nut;
specimen with
pyramid and thin
walls; and hexagonal
cup
23
PP/PC blend N/A 215 90 N/A 10 to 37 0.1 N/A N/A Tensile specimens
100% infill with
different strand
orientations
(ASTM-D638)
11
PP/PET blend Melting temp.: 164 and
250
260 100 N/A 20 and 50 0.2 PET tape LulzBot TAZ 6, Aleph
Objects Inc.
Type V tensile bars
(ASTM D638)
80
PP/PS blend Melting temp.: 161 to
166
260 100 N/A 20 and 50 0.2 Polyetherimide
surface
LulzBot TAZ 6, Aleph
Objects Inc.
Type V tensile bars
(ASTM D638)
80
PP homopolymer Melting temp.: 159 180, 210 and
230
25, 85, and
105
N/A 36 N/A PP specimen Screw extruder on a
gantry
Test specimen
based on DIN EN
ISO 3167
85
Nucleated PP
homopolymer
Melting temp.: 165 200 and 250 130 N/A 45 and 90 0.4 Office packing tape Makerbot Replicator
2X, Maker-Bot
Industries, LLC
Tensile specimens
Type I (ASTM
D790-10)
87
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first layers in order to increase the adhesion between layers and later
the printing speed is increased as high as for standard filaments in order
to increase productivity; and the layer thickness reported in the litera-
ture vary between 0.1 and 0.35 mm, but this depends on the geometry
to be produced and the accuracy needed. The printing surfaces used to
print PP-based filaments include PP plates or PP tapes, blue tape (tesa
56 250), packing tape (OfficeMax # 24767995), UHMWPE, and epox-
ide resins, as the use of glass results in poor adhesion.
23
POTENTIAL APPLICATIONS FOR 3D-PRINTED PP
Although PP has been reported as difficult to use thermoplastic for
ME-AM
134
due to theaforementioned warpage issues, 3D-printedPP
has been widely used in specific applications such as inapplied chem-
istry. For instance, Gordeev et al.
135
investigated the wall permeability
of 3D-printed objects with different geometries for chemical reaction
vessels. PP tubes processed by ME-AM were found to yield compara-
ble chemical transformation to the traditional glass tubes, even for
complex chemical reactions such as the Suzuki–Miyaura or the Heck
reaction. Other studies that revealed PP’s potential as an engineering
material with excellent stability against chemicals include the produc-
tion of chemical process laboratories
136
or chemical reaction vessels,
for example, for the synthesis of bicyclic and tetracyclic
heterocycles,
137
the anti-inflammatory drug ibuprofen,
138
or multi-
step organic syntheses,
139
the production of microfluidic devices for
the medical and chemical industry,
140–142
the fabrication of tailored
reactordevicesformassspectrometry,
143,144
or reactionware devices
for continuous-flow organic reactions.
145
Additionally, various
researchers recommended the use of 3D-printed PP for various
promising applications, such as for a chemically resistant laboratory
equipment for the processing of semiconductors,
146,147
for terahertz
devices,
134
or for a cheap and light weight alternative for the electroly-
sis of water.
148
Moreover, 3D-printed PP was shown to exhibit the ideal properties
for ankle foot orthoses
149
or for a cranial bone substitution,
150
as it
reveals similar strength to bone, is inert, nondegradable, nonmagnetic,
and inexpensive. In addition, 3D-printed PP was found to reveal tre-
mendous potential in terms of thermal stability in the course of mani-
fold consecutive extrusions.
127
PP-based filaments were the only
commercially available filaments that did not suffer from aging mech-
anisms such as chain scission or crosslinking, but revealed outstanding
stability over time. Consequently, both stabilized and unstabilized PP-
based composites could be used for up to 15 consecutive filament
extrusions and still remained unaltered mechanical properties. The
proposed strategy of remanufacturing unsatisfying 3D-printed com-
ponents could lead the ME-AM technique toward a cleaner produc-
tion, particularly when using PP as the raw material. All these studies
prove the potential of PP as a promising material for ME-AM.
SUMMARY AND OUTLOOK
The ME-AM technology of thermoplastic polymers has received
considerable attention over the last decade due to its simple use,
low cost, and possibility of generating large parts with rather high
throughput. From the many thermoplastic polymers, PP has
many attractive properties and low cost, which makes it a very
desirable material for the fabrication of unique products with
complex geometries via AM, particularly ME-AM. However,
processing PP via ME-AM is not so simple due to the
semicrystalline nature of PP. Currently, several PP filaments are
available on the market that have been chemically modified in
order to improve the quality of the products fabricated by ME-
AM. In this review, it was observed that none of the commer-
cially available filaments are isotactic PP homopolymers, as such
homopolymers have a great tendency to warp during the ME-
AM process, leading to specimens with poor geometrical accu-
racy. Besides the chemical modification of PP by blending with
other polymers or by copolymerizing, other strategies to improve
the geometrical accuracy of PP specimens include: (1) use of dif-
ferent build platform materials to guarantee ideal adhesion dur-
ing the printing process; (2) insulate the printing chamber and
maintain the chamber temperature relatively high to reduce ther-
mal stresses through a process similar to annealing; and finally
(3) the addition of fillers, specially thermally conductive ones in
order to prevent shrinkage during the fabrication process and to
decrease the time to reach thermal equilibrium in the deposited
layers.
Even though several methods have been devised to be able to process
PP-based materials, there are still some further investigations that
should be undertaken in the future to provide a better understanding
of the crystallization process during the ME-AM processing of PP:
1. One case in point is investigating the addition of nucleating
agents to change the crystallization kinetics during ME-AM of
PP parts. The addition of nucleating agents to PP could
decrease and homogenize the size of their spherulites. There-
fore, the mechanical properties, especially the toughness of the
parts, could be maximized.
2. More engineering investigations should also be carried out to
analyze different methods to locally heat the deposited strands
and/or the printing chamber. Infrared or microwave sources
and even low power lasers could lead to a maximization of
mechanical properties of the weld lines,
151,152
but also can
homogenize the temperature distribution within the printed
parts to locally prevent part distortion. It would be especially
important to understand the consequences of having addi-
tional heat sources on the thermal, crystallographic, mechani-
cal, and dimensional properties of fabricated PP components.
Independent of the heating method used, an investigation on
further increasing the chamber temperature, so that the mean
strand temperatures are slightly below the crystallization tem-
perature of PP, could lead to particularly fascinating results, as
no distinct fusion zone between the strands is expected to be
discerned.
85
Consequently, a comparable spherulite size is
predicted for both the bulk of the deposited strands and their
interfaces, which could reduce the heterogeneousness of the
mechanical properties of the produced components.
3. Moreover, studies to analyze whether higher amounts of β-PP
can be formed by increasing the chamber temperature could
additionally maximize the impact toughness while simulta-
neously mitigating the warpage.
4. Finally, in an effort to establish a circular economy, the use of
recycled and upcycled PP as a feedstock material for ME-AM
should be further investigated, as well as the recycling process
of parts produced by ME-AM should continue in order to
mitigate the environmental impact of the production of PP
components for numerous applications.
80
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48545 (12 of 16)
J. APPL. POLYM. SCI.
2020, DOI: 10.1002/APP.48545

ACKNOWLEDGMENTS
This work was supported by the European Union’s Horizon 2020
research and innovation program as part of the INEX-ADAM
project (grant agreement 810708) and by the Austrian Research
Promotion Agency (FFG) as part of the COMET K-project
CAMed (Clinical additive manufacturing for medical applica-
tions, grant agreement 871132). Special thanks go to Gerhard
Traxler for help with the thermography measurements.
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2020, DOI: 10.1002/APP.48545