polymers

Article
Characterizations of Polymer Gears Fabricated by Differential
Pressure Vacuum Casting and Fused Deposition Modeling
Chil-Chyuan Kuo 1,2, *, Ding-Yang Li 1 , Zhe-Chi Lin 1 and Zhong-Fu Kang 1

1 Department of Mechanical Engineering, Ming Chi University of Technology, No. 84, Gungjuan Road,
New Taipei City 243, Taiwan; M10118002@mail.mcut.edu.tw (D.-Y.L.);
U09117212@mail.mcut.edu.tw (Z.-C.L.); U09117222@mail.mcut.edu.tw (Z.-F.K.)
2 Research Center for Intelligent Medical Devices, Ming Chi University of Technology, No. 84, Gungjuan Road,
New Taipei City 243, Taiwan
* Correspondence: jacksonk@mail.mcut.edu.tw

Abstract: In recent years, polymer gears have gradually become more widely employed in medium
or heavy-duty conditions based on weight reduction in transmission systems because of low costs
and low noise compared to metal gears. In the current industry, proposing a cost-effective approach
to the manufacture of polymer gears is an important research issue. This paper investigates the wear
performance of polymer gears fabricated with eight different kinds of materials using differential
pressure vacuum casting and additive manufacturing techniques. It was found that both additive





 manufacturing and differential pressure vacuum casting seem to be an effective and cost-effective
method for low-volume production of polymer gears for industrial applications. The gate number of
Citation: Kuo, C.-C.; Li, D.-Y.; Lin,
one is the optimal design to manufacture a silicone rubber mold for differential pressure vacuum cast-
Z.-C.; Kang, Z.-F. Characterizations of
ing since the weld line of the polymer is only one. Polyurethane resin, 10 wt.% glass fiber-reinforced
Polymer Gears Fabricated by
Differential Pressure Vacuum Casting
polylatic acid (PLA), or 10 wt.% carbon fiber-reinforced PLA are suggested for manufacturing gears
and Fused Deposition Modeling. for small quantity demand based on the deformation and abrasion weight percentage under process
Polymers 2021, 13, 4126. https:// conditions of 3000 rpm for 120 min; epoxy resin is not suitable for making gears because part of the
doi.org/10.3390/polym13234126 teeth will be broken during abrasion testing.

Academic Editors: Keywords: polymer gear; additive manufacturing; differential pressure vacuum casting; polyurethane
Mohammadali Shirinbayan, resin; abrasion
Nader Zirak, Khaled Benfriha,
Sedigheh Farzaneh and
Joseph Fitoussi

1. Introduction
Received: 28 October 2021
In practice, product developers need to overcome a tricky issue by making a small
Accepted: 23 November 2021
batch of prototypes for testing economy and feasibility. A gear is a rotating circular machine
Published: 26 November 2021
part, which can change the torque, speed, and direction of a power source in industrial
applications. The polymer gear has some distinct advantages compared to the metal gear,
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
including low weight, quietness of operation, and no need for external lubrication [1],
published maps and institutional affil-
and has been widely used in the automotive industry and consumer electronics. Additive
iations. manufacturing (AM) [2,3] has been defined as the process of building physical models by
joining materials layer upon layer using computer numerical control data. The application
of AM processes has increased in fabricating physical models across various industries
because of its capability in manufacturing functional parts with complex geometries. Thus,
the AM technology has been widely used to produce prototypes or physical models
Copyright: © 2021 by the authors.
since it has the capacity to manufacture components with sophisticated geometric shapes.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
Ghelloudj et al. [4] developed an engineering model to express the evolution of tooth flank
distributed under the terms and
wear in polyamide spur gears as a function of the number of cycles. It was found that a wear
conditions of the Creative Commons
correction parameter was added to compensate for the measuring errors when plotting the
Attribution (CC BY) license (https:// wear profile curves. The simulation results are in good agreement with those obtained from
creativecommons.org/licenses/by/ experimental measurements. Lu et al. [5] detected the injection molding lunker defects
4.0/). by X-ray computed tomography. Results showed that the lunker defect jeopardizes the

Polymers 2021, 13, 4126. https://doi.org/10.3390/polym13234126 https://www.mdpi.com/journal/polymers

Polymers 2021, 13, 4126 2 of 21

loading capacity of the tooth root under medium or heavy loading conditions, while the
tooth flank failure is significantly influenced by the loading condition. Zhang et al. [6]
optimized the performance of 3D-printed gears using a machine learning process using
a genetic algorithm-based artificial neural network multi-parameter regression model;
the authors found that the wear performance of 3D-printed gears was increased by three
times. Vacuum casting (VC) [7,8] is a promising technique used for the production of
functional parts due to its fast production of high-quality prototypes. Oleksy et al. [9]
manufactured the gear wheels with epoxy composites using VC technology and found
that developed multi-stage homogenized hybrid-filled epoxy resin had a regular layered
morphology. Furthermore, the tensile strength was increased by up to 44 %. Kai et al. [10]
integrated VC and AM as well as rapid tooling for fabricating connectors. It was found that
a stereolithography apparatus mold cannot be used directly in the VC process since the
stereolithography apparatus mold must be broken into pieces for extracting the molded
parts. Puerta et al. [11] proposed a new approach to determine the suitability of the
usage of standard tensile test specimens fabricated by VC and fused deposition modeling
(FDM). The results revealed that the surface quality of the model used for the creation
of the silicone rubber mold is an important issue in the VC. Zhang et al. [12] proposed a
differential pressure technology to improve the quality of resin parts using VC technology
through the optimization method. The results revealed that the artificial fish-swarm
algorithm optimized the response surface model of the warpage via the optimized process
parameters. Zhao et al. [13] manufactured an accurate shark-skin surface in a large area
to overcome some difficulties in the replication process via VC technology. It was found
that process parameters played an important role in eliminating air bubbles on the surface
of the resin parts. Frankiewicz et al. [14] demonstrated the results of analyses performed
for the process of replicating mechanoscopic marks with the use of three vacuum-casting
variants, including a hybrid vacuum-pressure casting process developed in particular for
the purposes of replication. It was found that the proposed method not only allowed the
tool preparation to be simplified and shortened, but also caused the entire process time to
be shortened from 10 to 1.5 h.
Injection molding and machine cutting are normally used to fabricate polymer gears.
However, the use of plastic injection molding to manufacture polymer gears requires
a set of steel injection molds, which does not seem to be a good approach during the
research and development stage of a new polymer gear. A set of cutting tools is required
for machining polymer gears by machine cutting. Note that these methods are suitable for
mass production of polymer gears based on cost-effectiveness. Therefore, developing a cost-
effective method for batch production of polymer gears in the research and development
stage is an important research issue. In general, the integration of silicone rubber mold and
vacuum casting technology [15] is widely used for rapid manufacturing prototypes since
the silicone rubber mold has elastic and flexible characteristics. Accordingly, a prototype
with complex geometries can be fabricated easily [16]. Chu et al. [17] proposed an efficient
generation grinding method for a spur face gear along the contact trace using a disk
CBN wheel. Results demonstrated that the proposed method breaks new ground for the
engineering application of face gears.
Vacuum casting is a cost-effective method used for the low-volume production of
physical models. However, conventional vacuum casting employs the gravity of molding
material to fill the mold cavity, resulting in some common defects, such as insufficient
filling, shrink marks, or trapped air observed in the cast. Especially, these defects can
be eliminated using differential pressure vacuum casting (DPVC) [18]. Therefore, the
end-use prototypes can fundamentally be formed by silicone rubber mold using DPVC.
The advantages of manufacturing polymer gears using AM techniques include design
freedom and less waste of materials. However, not much work has been conducted to
characterize the differences in polymer gears fabricated by AM and DPVC. The goal of
this investigation is to investigate the characterizations of polymer gears fabricated by
AM and DPVC techniques using eight different kinds of polymers. In addition, in-house
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Polymers 2021, 13, 4126 3 of 21

investigation is to investigate the characterizations of polymer gears fabricated by AM

and DPVC techniques using eight different kinds of polymers. In addition, in-house abra-
sion testing
abrasion equipment
testing was designed
equipment and implemented
was designed to evaluate
and implemented spur gear
to evaluate life.gear
spur Finally,
life.
an effective
Finally, and cost-effective
an effective methodmethod
and cost-effective for the for
low-volume production
the low-volume of polymer
production gears
of polymer
was
gearsproposed.
was proposed.

Materials and
2. Materials and Methods
Methods
the research
Figure 1 shows the research process of this study. The gear type selected in this study
is aa spur
is spur gear
gear since
since this
this is
is the
the simplest
simplest type
type of
of gear.
gear. Firstly,
Firstly, two
two spur
spur gears
gears were
were designed
designed
using computer-aided
using computer-aided design
design (CAD)
(CAD) software
software (Cero,
(Cero, parametric
parametric technology
technology corporation
corporation
Inc.. Taipei,
Inc.. Taipei, Taiwan),
Taiwan), i.e.,
i.e., driving
driving gear
gear and
and passive
passive gear.
gear.

Figure
Figure 1.
1. Research
Research process
process of
of this
this study.
study.

Figure
Figure 22shows
showsa athree-dimensional
three-dimensional (3D) CAD
(3D) model
CAD modelandand
the dimensions of theofdriv-
the dimensions the
ing gear and
driving gearthe
andpassive gear. The
the passive number
gear. of teeth,of
The number pitch diameter,
teeth, tooth module,
pitch diameter, tooth pressure
module,
angle,
pressureand thickness
angle, of the gear
and thickness of are
the 30,
gear60are
mm, 30,260mm,
mm,20° ◦ and
and 520mm,
2 mm, respectively.
5 mm, respectively.
Figure 3 shows the 3D printing software interface of the driving gear and the passive
gear. Designing the runner system for the silicone rubber mold is crucial to the mold
design. Conventionally, designing the runner system significantly depends on the mold
designer’s experiences. To address these issues, the filling system of the silicone rubber
mold is investigated using numerical simulation software. To investigate the optimum
filling system of the silicone rubber mold, the 3D CAD models of spur gear, runner, and
gate were imported to the Moldex3D simulation software (R16SP3OR, CoreTech System
Inc., Hsinchu, Taiwan) via a data exchange STEP format. Table 1 shows the main numerical
modeling parameters used in the numerical analysis.
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Figure2.2.3D
Figure 3DCAD
CADmodel
modeland
anddimensions
dimensionsof
of(a)
(a)driving
drivinggear
gearand
and(b)
(b)passive
passivegear.
gear.

Figure 3 shows the 3D printing software interface of the driving gear and the passive
Table 1. Main numerical modeling parameters used in the numerical analysis.
gear. Designing the runner system for the silicone rubber mold is crucial to the mold de-
sign. Conventionally, designing the runner system significantlyValue
Properties depends on the mold de-
signer’s experiences. To address
Filling time (s) these issues, the filling system of the
10 silicone rubber mold
is investigated using numerical simulation software. To investigate the optimum filling
Material temperature (◦ C) 27
system of the silicone rubber mold, the 3D CAD models of spur gear, runner, and gate
Moldtotemperature
were imported the Moldex3D (◦ C) simulation software (R16SP3OR,27CoreTech System Inc.,
Hsinchu, Taiwan)
Maximum via a pressure
injection data exchange
(kPa) STEP format. Table 1 shows 30 the main numerical
modeling parameters used in the numerical analysis.
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Figure3.3.3D
Figure 3Dprinting
printingsoftware
softwareinterface
interfaceofof(a)
(a)driving
drivinggear
gearand
and(b)
(b)passive
passivegear.
gear.

Table 1. Main
Figure numerical
4 shows themodeling
viscosityparameters usedof
as a function in the
the numerical analysis.
temperature of the epoxy molding
material. Q stands for temperature
Properties ramping rate of the mixture.
ValueFigure 5 shows the
viscosity as a function of the temperature of the polyurethane (PU) molding material. In
Filling time (s) 10
this study, a standard sprue–runner–gate system was used due to the low pressure drop
during DPVC.Material
Thus, temperature
the pouring(°C)
materials can flow directly into the27silicone rubber mold
cavity without passing through the intricate runner system. Figure 627
Mold temperature (°C) shows the relationship
betweenMaximum injection
the filling system,pressure (kPa)
cast part, and the silicone rubber mold. 30

Figure 4 shows the viscosity as a function of the temperature of the epoxy molding
material. Q stands for temperature ramping rate of the mixture. Figure 5 shows the vis-
cosity as a function of the temperature of the polyurethane (PU) molding material. In this
study, a standard sprue–runner–gate system was used due to the low pressure drop dur-
ing DPVC. Thus, the pouring materials can flow directly into the silicone rubber mold
cavity without passing through the intricate runner system. Figure 6 shows the relation-
ship between the filling system, cast part, and the silicone rubber mold.
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Figure
Figure 4.
Figure 4. Viscosity
4. Viscosity as
as aaa function
Viscosity as function of
function of the
of the temperature
the temperature of
temperature of the
of the epoxy
epoxy molding
the epoxy molding material.
molding material.
material.

Figure 5.
Figure
Figure 5. Viscosity
5. Viscosity as
Viscosity as aaa function
as function of
function of the
of the temperature
the temperature of
temperature of the
of the polyurethane
the polyurethane molding
polyurethane molding material.
molding material.
material.
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Figure
Figure6.6.Relationship
Relationshipbetween
betweenfilling
fillingsystem,
system,cast
castpart,
part,and
andsilicone
siliconerubber
rubbermold.
mold.

Figure7 7shows
Figure showsthe thefive
fivestages
stagesofofthe
theVCVCandandinformation
informationabout aboutballballvalue
valueand
andintake
intake
area. In general, the VC process involves five distinct stages:
area. In general, the VC process involves five distinct stages: preliminary, vacuuming, preliminary, vacuuming,
casting,vacuum
casting, vacuumrelief,
relief,and
andpost-processing
post-processingstages. stages.The TheP1, P1,P2,
P2,andandP3 P3stand
standforformixing
mixing
chamber pressure, casting chamber pressure, and atmospheric pressure,
chamber pressure, casting chamber pressure, and atmospheric pressure, respectively. The respectively. The
preliminary stage is the preparation of the silicone rubber mold
preliminary stage is the preparation of the silicone rubber mold based on the size of the based on the size of the
gearprototype.
gear prototype.The Theradii
radiiofofball
ballvalve,
valve,ball,
ball,and
andseat
seatareare1515mm,mm,7.5 7.5mm,mm,and and6.25
6.25mm,
mm,
respectively. In this study, a room temperature vulcanization liquid
respectively. In this study, a room temperature vulcanization liquid silicone rubber (KE- silicone rubber (KE-
1310ST, Shin Etsu Inc, Hsinchu, Taiwan) was used to fabricate the
1310ST, Shin Etsu Inc, Hsinchu, Taiwan) was used to fabricate the silicone rubber mold. silicone rubber mold. The
base compound and hardener (CAT-1310S, Shin Etsu Inc.) were mixed in a weight ratio of
The base compound and hardener (CAT-1310S, Shin Etsu Inc.) were mixed in a weight
10: 1. A vacuum casting machine (F-600, Feiling Inc., Taoyuan, Taiwan) was used to remove
ratio of 10: 1. A vacuum casting machine (F-600, Feiling Inc., Taoyuan, Taiwan) was used
air bubbles in the mixture resulting from the mixing process under vacuum conditions. The
to remove air bubbles in the mixture resulting from the mixing process under vacuum
epoxy and polyurethane resins were selected as casting materials to fabricate spur gears by
conditions. The epoxy and polyurethane resins were selected as casting materials to fab-
silicone rubber mold using differential pressure vacuum casting technology. The process
ricate spur gears by silicone rubber mold using differential pressure vacuum casting tech-
parameters for manufacturing gears include a ball valve angle of 60 ◦ , a silicone rubber
nology. The process parameters for manufacturing gears include a ball valve angle of 60
mold preheating temperature of 27 ◦ C, a molding material mixing time of 30 s, a pouring
°, a silicone rubber mold preheating temperature of 27 °C, a molding material mixing time
time of 40 s, and a differential pressure time of 20 s. The spur gears were also manufactured
of 30 s, a pouring time of 40 s, and a differential pressure time of 20 s. The spur gears were
using an FDM machine (Infinity X1E, Photonier Inc., Taipei, Taiwan) with a nozzle diameter
also manufactured using an FDM machine (Infinity X1E, Photonier Inc., Taipei, Taiwan)
of 0.4 mm. In this study, the six different kinds of filaments, i.e., virgin polylactic acid (PLA)
with a nozzle
(Thunder 3Ddiameter of 0.4
Inc., Taipei, mm. In
Taiwan), PLAthisfilled
study, the10
with sixwt.%
different
glasskinds of filaments,
fiber (Thunder i.e.,
3D Inc.),
virgin polylactic acid (PLA) (Thunder 3D Inc., Taipei, Taiwan), PLA
PLA filled with 10 wt.% carbon fiber (Thunder 3D Inc.), acrylonitrile butadiene styrene filled with 10 wt.%
glass
(ABS) fiber (Thunder
(Thunder 3D 3D Inc.),
Inc.), PLA filled with
polycarbonate (PC),10andwt.% carbon fiber
polyamide (PA)(Thunder
were used 3DtoInc.),
print
acrylonitrile
polymer gears butadiene
using thestyrene
FDM(ABS) (Thunder
technique 3D Inc.),
according to polycarbonate
the standard of (PC), and polyam-
ASTM52900. The
ide (PA) parameters
process were used to forprint polymer
printing gears with
spur gears usinga PLA
the FDM technique
filament according
are printing to the
temperature
standard
of 200 ◦ C,ofhot
ASTM52900. The process
bed temperature of 60 ◦parameters
C, printing forspeedprinting spur gears
of 50 mm/s, with athickness
and layer PLA fila-of
ment are printing temperature of 200 °C, hot bed temperature
0.1 mm. The process parameters for printing spur gears with both PLA filled with of 60 °C, printing speed
10 wt.%of
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50 mm/s, and layer thickness of 0.1 mm. The process parameters for printing spur gears
with both PLA filled with 10 wt.% glass fiber and 10 wt.% carbon fiber filaments are print-
glass
ing fiber and 10ofwt.%
temperature carbon
200 °C, fibertemperature
hot bed filaments areofprinting temperature
70 °C, printing speedofof20050 ◦mm/s,
C, hot and
bed
temperature ◦
ofof700.1C,mm.
printing speed of 50 mm/s,for and layer thickness
layer thickness The process parameters printing spur gearsofwith0.1 mm.
ABS, The
PC,
process parameters for printing spur gears with ABS, PC, and PA filaments
and PA filaments are printing temperature of 100 °C, hot bed temperature of 60 °C, print- are printing
temperature ◦ C, hot bed temperature of 60 ◦ C, printing speed of 50 mm/s, and layer
ing speed of of50 100
mm/s, and layer thickness of 0.1 mm. The infill density was set as 100%.
thickness
The of 0.1
Ultimaker mm.software
Cura The infill density
(New Taipei,was set as was
Taiwan) 100%.usedThe
toUltimaker
generate the Cura software
program for
(New
the FDM Taipei, Taiwan)
machine. was used
Chemical to generateofthe
compositions sixprogram
differentfor the of
kinds FDM machine.
filaments wereChemical
charac-
compositions
terized using of six different kinds
energy-dispersive of filaments
x-ray were characterized
spectroscopy using energy-dispersive
(EDS) (D8 ADVANCE, Bruker Inc.,
x-ray spectroscopy (EDS) (D8 ADVANCE, Bruker Inc., Karlsruhe,
Karlsruhe, Germany) and field-emission-scanning electron microscopy (FE-SEM) Germany) and field-
emission-scanning electron microscopy
(JEC3000-FC, JEOL Inc., Tokyo, Japan). (FE-SEM) (JEC3000-FC, JEOL Inc., Tokyo, Japan).

Figure
Figure 7.
7. Five
Five stages
stages of
of the
the VC
VC and
and information
information about
about ball
ball value
value and
and intake
intake area.
area.

Tool
Tool wear
wear is is the
the main
mainfactor
factorcontributing
contributing to totool
toolfailure
failureinincutting
cuttingdifficult-to-machine
difficult-to-machine
materials
materials [19].
[19]. Similarly,
Similarly, thethe abrasion
abrasionrate
rate isis the
the main
main factor
factor causing
causing spur
spur gear
gear failure.
failure.
Various methods, including cylinder-on-plate [20], block-on-wheel,
Various methods, including cylinder-on-plate [20], block-on-wheel, pin-on-disk [21], block-pin-on-disk [21],
block-on-ring, pin-on-plate,
on-ring, pin-on-plate, or flat-on-flat
or flat-on-flat can
can be be used
used to investigate
to investigate the wear
the wear rate.rate. How-
However,
ever,
thesethese
methodsmethods
requirerequire several
several testing
testing conditions.
conditions. In this
In this study,
study, a simple
a simple geargear abra-
abrasion
sion testing
testing equipment
equipment was designed
was designed and implemented
and implemented for investigating
for investigating the wear
the wear perfor-
performance
of the fabricated
mance polymerpolymer
of the fabricated gears. Figure
gears.8 Figure
shows a8 gear showsabrasion
a geartesting machine
abrasion testingdeveloped
machine
in this study.
developed The study.
in this tooth flank wear flank
The tooth of spur gears
wear as a function
of spur gears as of the number
a function of number
of the cycles was
of
investigated. Corner wear evolution of gears fabricated with eight different
cycles was investigated. Corner wear evolution of gears fabricated with eight different materials was
investigated
materials wasusing an OM (M835,
investigated using anMicrotech,
OM (M835, Inc., Dresden,Inc.,
Microtech, Germany).
Dresden, The deformation
Germany). The
angles of theangles
deformation printedofspur the gears
printedwere
spurmeasured
gears were using a vision using
measured measuring system
a vision (Quick
measuring
Vision 404,
system (QuickMitutoyo
Vision Inc.,
404, Gunpo,
Mitutoyo Korea).
Inc., Gunpo, Korea).
Polymers 2021,13,
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Figure8.8.AAgear
Figure gearabrasion
abrasiontesting
testingmachine
machinedeveloped
developedininthis
thisstudy.
study.

3.3.Results
ResultsandandDiscussion
Discussion
The
The efficiency,yield,
efficiency, yield,ororproduct
productquality
qualityofofthe
thevacuum
vacuumcasting
castingwas
wasaffected
affectedby bythe
the
design
design of the pouring gate. The most common defects such as air-traps or short shotwill
of the pouring gate. The most common defects such as air-traps or short shot will
occur
occurdue
due toto poor filling in
poor filling in the
thevacuum
vacuumcasting.
casting.The
The shrinkage
shrinkage or or warpage
warpage of the
of the castcast
part
part will occur due to unbalanced flow. The post-processing time and costs
will occur due to unbalanced flow. The post-processing time and costs will increase due will increase
due to incorrect
to incorrect gate
gate size
size ororlocation.
location.To
Toavoid
avoid these
these disadvantages
disadvantages described
described above,
above,thethe
Moldex3D
Moldex3D molding simulation software was utilized to investigate the most suitablegate
molding simulation software was utilized to investigate the most suitable gate
for
forvacuum
vacuumcasting.
casting. There
There arearefour
fourdifferent
differentgate
gatetypes:
types:single
singlepoint,
point,two
twopoints,
points,three
three
points
points and four points. These gate types were investigated for the gear designininvacuum
and four points. These gate types were investigated for the gear design vacuum
casting. Figure 9 shows the filling results of different gate numbers. It was found that
casting. Figure 9 shows the filling results of different gate numbers. It was found that the
the gears can be filled completely for four different gate numbers. The fill times for gate
gears can be filled completely for four different gate numbers. The fill times for gate num-
numbers of one, two, three, and four are all about 10 s.
bers of one, two, three, and four are all about 10 s.
Polymers 2021,13,
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Figure9.9.Filling
Figure Fillingresults
resultsof
ofdifferent
differentgate
gatenumbers.
numbers.

Figure 10
Figure 10 shows
shows the weldweld line
line results
resultsfor fordifferent
differentgate gatenumbers.
numbers.The The weld lines
weld are
lines
formed
are formedby two
by twodifferent meltmelt
different fronts joining
fronts together
joining during
together the filling
during stage,stage,
the filling whichwhich
signif-
icantly reduces
significantly the strength
reduces of the
the strength ofmolded
the molded part.part.
Figure 11 shows
Figure the filling
11 shows maximum
the filling max-
pressures
imum for different
pressures gate numbers.
for different The filling
gate numbers. Themaximum pressures for
filling maximum gate numbers
pressures for gateof
a single point,
numbers two point,
of a single points,two
three points,
points, threeandpoints,
four points
and four arepoints
1.439 ×are
101.439
−4 MPa, 10−4 MPa,
× 1.035 × 10−4
MPa,×8.441
1.035 − 4
10 ×MPa, 10 MPa,
−5 × 10
8.441and − 5
4.272MPa,
× 10 and
−5 MPa,4.272
respectively.−
× 10 MPa, 5 The maximum
respectively. filling
Thepressure
maxi-
mum fillingwith
decreases pressure
as thedecreases
number ofwith gatesasincreases.
the number of gatesbeincreases.
It should noted thatIt the
should be notedin
differences
that thepressure
filling differencescan in
befilling
ignoredpressure
since the canmaterial
be ignoredwassince
poured theinmaterial
a vacuum was poured in a
environment.
vacuume nvironment.
Figure 12 shows the silicone rubber molds with different gate numbers. As can be
seen,Figure 12 shows
the number the silicone
of weld lines forrubber molds with
gate numbers of one,different gateand
two, three, numbers.
four areAsone,cantwo,
be
seen,
three,theandnumber of weld linesBased
four, respectively. for gate onnumbers
practicalof one, two, three,
experience, fewer and
weldfourlinesarerepresent
one, two,a
three,
betterand four, respectively.
the quality of gears. InBased on practical
addition, experience,time
the post-processing fewerandweld
costslines
of therepresent a
cast parts
better
for thethe quality
gate number of gears.
of oneInwere
addition,
less thanthe post-processing
those of the casttime partsand
madecosts
withof the
gatecast parts
numbers
for
of the
two,gate number
three, of oneAccording
and four. were less to than thethose of the
results cast parts
described madethe
above, with gate numbers
single-point gate
of
seems to be the optimal gate number to fabricate a silicone rubber mold for DPVC. gate
two, three, and four. According to the results described above, the single-point
seemsFigure
to be the optimalFE-SEM
13 shows gate number
imagestoof fabricate
10 wt.%a glass
silicone rubber mold for
fiber-reinforced PLA DPVC.
and 10 wt.%
carbon fiber-reinforced PLA. This result indicates that glass fiber or carbon fiber was ob-
served in the filaments applied to fabricate polymer gears using the FDM technique. Im-
purity was not observed, which was also confirmed by EDS element mapping analysis.
Figure 14 shows EDS analysis of PLA, ABS, 10 wt.% glass fiber-reinforced PLA, 10 wt.%
carbon fiber-reinforced PLA, PA, and PC filaments. The major compositions of PLA, ABS,
10 wt.% carbon fiber-reinforced PLA, PA, and PC filaments are C and O. In particular,
components of 10 wt.% glass fiber-reinforced PLA are Si, C, O, Ca, and Al. Figure 15 shows
the spur gears fabricated with filaments of PLA, ABS, 10 wt.% glass fiber-reinforced PLA,
10 wt.% carbon fiber-reinforced PLA, PA, and PC using the FDM technique.
 Polymers 2021, 13, x FOR PEER REVIEW 11 of 23
Polymers 2021, 13, 4126 11 of 21
Polymers 2021, 13, x FOR PEER REVIEW 11 of 23

Figure 10. Weld line results for different gate numbers of (a) single point, (b) two points, (c) three
points, and (d) four points.
Figure10.
Figure 10.Weld
Weldline
lineresults
resultsfor
fordifferent
differentgate
gatenumbers
numbersofof(a)
(a)single
singlepoint,
point,(b)
(b)two
twopoints,
points,(c)
(c)three
three
points, and (d) four points.
points, and (d) four points.

Figure11.
Figure 11.Filling
Fillingmaximum
maximumpressures
pressuresfor
forgate
gatenumbers
numbersofof(a)
(a)single
singlepoint,
point,(b)
(b)two
twopoints,
points,(c)
(c)three
three
points, and (d) four points.
points, and (d) four points.
Figure 11. Filling maximum pressures for gate numbers of (a) single point, (b) two points, (c) three
points, and (d) four points.
Polymers 2021, 13, x FOR PEER REVIEW 12 of 23
Polymers 2021, 13, 4126 12 of 21

Figure 12.
Figure Silicone rubber
12. Silicone rubber molds
molds for
for gate
gate numbers
numbers of
of (a)
(a) single
single point,
point, (b)
(b) two
two points,
points, (c)
(c) three
three points,
points, and
and (d)
(d) four
four points.
points.

Figure 13 shows FE-SEM images of 10 wt.% glass fiber-reinforced PLA and 10 wt.%
carbon fiber-reinforced PLA. This result indicates that glass fiber or carbon fiber was
observed in the filaments applied to fabricate polymer gears using the FDM technique.
Impurity was not observed, which was also confirmed by EDS element mapping analysis.
Figure 14 shows EDS analysis of PLA, ABS, 10 wt.% glass fiber-reinforced PLA, 10 wt.%
carbon fiber-reinforced PLA, PA, and PC filaments. The major compositions of PLA, ABS,
10 wt.% carbon fiber-reinforced PLA, PA, and PC filaments are C and O. In particular,
components of 10 wt.% glass fiber-reinforced PLA are Si, C, O, Ca, and Al. Figure 15 shows
the spur gears fabricated with filaments of PLA, ABS, 10 wt.% glass fiber-reinforced PLA,
10 wt.% carbon fiber-reinforced PLA, PA, and PC using the FDM technique.
Figure 16 shows typical spur gears printed with PLA, ABS, PC, and PA filaments.
The distinct warpage of the printed gear was found due to uneven shrinkage [22]. It
should be noted that two phenomena were found. One is that the deformation of the
gear printed with the PC filament is the largest, followed by PA and ABS; the deformation
angles are about 5.7 ◦ , 2.2◦ , and 1.8◦ , respectively. Note that this drawback can be resolved
by mounting an auxiliary heating plate on the printing head [23]. The other phenomenon
observed is that the flatness of gears printed with PLA filament is better. Small batch
production of prototypes via vacuum casting seems to be a good solution, since the cost
of silicone rubber mold is at least ten times less than a conventional steel injection mold.
In addition, the fatigue life of the polymer gear was greatly influenced by the lunker
defects generated during the injection molding process. Note that no lunker defects were
observed, which is widely observed with the polymer gears fabricated by plastic injection
molding. Figure 17 shows the spur gears fabricated by epoxy and polyurethane resins
using the DPVC technique. The results clearly show that the gears fabricated by DPVC
have excellentf latness.
Polymers 2021, 13, 4126 13 of 21
Polymers 2021, 13, x FOR PEER REVIEW 13 of 23

Figure13.
Figure 13. FE-SEM
FE-SEM images
images of
of 10
10 wt.%
wt.% glass
glass fiber-reinforced
fiber-reinforced PLA
PLA and
and 10
10 wt.%
wt.% carbon
carbonfiber-reinforced
fiber-reinforcedPLA.
PLA.
Polymers 2021,13,
Polymers2021, 13,4126
x FOR PEER REVIEW 14 14
ofof2321

Figure14.
Figure 14.EDS
EDSanalysis
analysisofof(a)(a) PLA,
PLA, (b)(b) ABS,
ABS, (c)(c)
10 10 wt.%
wt.% glass
glass fiber-reinforced
fiber-reinforced PLA,
PLA, (d) (d) 10 wt.%
10 wt.% carbon
carbon fiber-reinforced
fiber-reinforced PLA,
PLA, (e) PA, and (f) PC filaments.
(e) PA, and (f) PC filaments.
Polymers 2021,
Polymers 13,13,
2021, 4126
x FOR PEER REVIEW 1515ofof2321

Figure15.
Figure 15.Typical
Typicalspur
spur gears
gears fabricated
fabricated with
with six
six different
different filaments
filaments of
of(a)
(a)PLA,
PLA,(b)
(b)ABS,
ABS,(c)
(c)10
10wt.%
wt.%
glass fiber-reinforced PLA, (d) 10 wt.% carbon fiber-reinforced PLA, (e) PA, and (f) PC using FDM
glass fiber-reinforced PLA, (d) 10 wt.% carbon fiber-reinforced PLA, (e) PA, and (f) PC16using
Polymers 2021, 13, x FOR PEER REVIEW of 23
technique.
FDM technique.
Figure 16 shows typical spur gears printed with PLA, ABS, PC, and PA filaments.
The distinct warpage of the printed gear was found due to uneven shrinkage [22]. It
should be noted that two phenomena were found. One is that the deformation of the gear
printed with the PC filament is the largest, followed by PA and ABS; the deformation
angles are about 5.7 °, 2.2°, and 1.8°, respectively. Note that this drawback can be resolved
by mounting an auxiliary heating plate on the printing head [23]. The other phenomenon
observed is that the flatness of gears printed with PLA filament is better. Small batch pro-
duction of prototypes via vacuum casting seems to be a good solution, since the cost of
silicone rubber mold is at least ten times less than a conventional steel injection mold. In
addition, the fatigue life of the polymer gear was greatly influenced by the lunker defects
generated during the injection molding process. Note that no lunker defects were ob-
served, which is widely observed with the polymer gears fabricated by plastic injection
molding. Figure 17 shows the spur gears fabricated by epoxy and polyurethane resins
using the DPVC technique. The results clearly show that the gears fabricated by DPVC
have excellent flatness.

Figure16.
Figure 16.Spur
Spurgears
gearsprinted
printedwith
withPLA,
PLA,ABS,
ABS,PC,
PC,and
andPA
PAfilaments.
filaments.
 Polymers 2021, 13, 4126 16 of 21
2021, 13, x FOR PEER REVIEW 17 of 23

Figure 17.Figure 17. Spur

Spur gears gears fabricated
fabricated by and
by (a) epoxy (a) epoxy and (b) polyurethane
(b) polyurethane resins
resins using DPVC using DPVC technique.
technique. Driving gear (left) and
Driving gear (left) and passive gear (right).
passive gear (right).

Polymer gears are Polymer

usuallygears are usually
designed designed
with small toothwith smalland
modules tooth modules
operated in and
dry operated in dry
contact conditions for light loading transmissions [24]. Polymer gears involve three obvi-involve three ob-
contact conditions for light loading transmissions [24]. Polymer gears
vious
ous failure types, failuretooth
including types,root
including tooth
breakage, rootwear,
tooth breakage, tooth wear,
and tooth and tooth
flank failure. In flank failure. In
general, wear and thermal damages are widely observed in polymer gears in light loading in light loading
general, wear and thermal damages are widely observed in polymer gears
conditions.
conditions. To evaluate the To evaluate
wear the wear
resistance resistance characteristics
characteristics of gears
of gears fabricated byfabricated
DPVC by DPVC and
AM technologies, an in-house abrasion testing machine was
and AM technologies, an in-house abrasion testing machine was applied to investigate applied to investigate wear
wear loss of the gear under 3000 rpm and an operating time of 120 min. The wear losseswear losses were
loss of the gear under 3000 rpm and an operating time of 120 min. The
discovered from the changes in the weight of gears before and after abrasion testing using
were discovered from the changes in the weight of gears before and after abrasion testing
a precision electronic scale.
using a precision electronic scale.
Figure 18 shows the abrasion weight percentage of gears fabricated with eight different
Figure 18 shows the abrasion weight percentage of gears fabricated with eight differ-
materials for driving and passive gears. The average abrasion weight percentages of
ent materials for driving and passive gears. The average abrasion weight percentages of
driving gears fabricated by filaments of PLA, ABS, 10 wt.% glass fiber-reinforced PLA,
driving gears fabricated by filaments of PLA, ABS, 10 wt.% glass fiber-reinforced PLA, 10
10 wt.% carbon fiber-reinforced PLA, PA, PC, epoxy, and polyurethane resins are 0.173%,
wt.% carbon fiber-reinforced PLA, PA, PC, epoxy, and polyurethane resins are 0.173%,
0.182%, 0.192%, 0.155%, 0.485%, 0.524%, 2.379%, and 0.373%, respectively. In addition, the
0.182%, 0.192%, 0.155%, 0.485%, 0.524%, 2.379%, and 0.373%, respectively. In addition, the
average abrasion weight percentages of passive gears fabricated by filaments of PLA, ABS,
average abrasion weight percentages of passive gears fabricated by filaments of PLA, ABS,
10 wt.% glass fiber-reinforced PLA, 10 wt.% carbon fiber-reinforced PLA, PA, PC, epoxy,
10 wt.% glass fiber-reinforced PLA, 10 wt.% carbon fiber-reinforced PLA, PA, PC, epoxy,
and polyurethane resins are 0.325%, 0.302%, 0.192%, 0.287%, 0.418%, 0.696%, 5.039%, and
and polyurethane resins respectively.
0.761%, are 0.325%, 0.302%, 0.192%, 0.287%, 0.418%, 0.696%, 5.039%, and
0.761%, respectively.
Polymers 2021, 13, x FOR PEER REVIEW 18 of 23

Polymers 2021, 13, 4126 17 of 21

Figure 18. Abrasion weight percentage of gears fabricated with eight different materials (a) driving
Figure 18. Abrasion weight percentage of gears fabricated with eight different materials (a) driving
gear and (b) passive gear.
gear and (b) passive gear.
Figure 19 shows the corner wear evolution of gears fabricated with eight different
FigureIt19is shows
materials. evidentthe corner
that therewear evolutionwear
is significant of gears
of thefabricated with However,
tooth surface. eight different
some
materials. It is evident that there is significant wear of the tooth surface. However,
common defects of gears (fisheye defects, debris frosting, pitting, or moderate pitting) some
were
common defects of gears (fisheye defects, debris
not found on the surface of the failed spur gears. frosting, pitting, or moderate pitting)
were not found on the surface of the failed spur gears.
Figure 20 shows the cost of materials and manufacturing time for gears fabricated
with eight different materials. The results show that manufacturing times for gears fabri-
cated with PLA, ABS, 10 wt.% glass fiber-reinforced PLA, 10 wt.% carbon fiber-reinforced
Polymers 2021, 13, 4126 2021, 13, x FOR PEER REVIEW
Polymers 18 of 21 20 o

Figure
Figure 19. Corner
19. Corner wearwear evolution
evolution of gears
of gears fabricated
fabricated with with
eighteight different
different materials.
materials.

Figure 20 shows the cost of materials and manufacturing time for gears fabricated with
eight different materials. The results show that manufacturing times for gears fabricated
with PLA, ABS, 10 wt.% glass fiber-reinforced PLA, 10 wt.% carbon fiber-reinforced PLA,
PA, PC, epoxy resin, and polyurethane resin are 169, 208, 173, 185, 212, 206, 305, and
134 min, respectively. The costs of materials for gears fabricated with PLA, ABS, 10 wt.%
glass fiber-reinforced PLA, 10 wt.% carbon fiber-reinforced PLA, PA, PC, epoxy resin, and
polyurethane resin are 4.16, 12.13, 22.64, 23.75, 18.75, 31.62, 19.28, and 37.5 in new Taiwan
dollars (NTD), respectively.
Based on wear resistance, flatness, production time, and the materials cost of gears,
four suggestions are proposed: (a) epoxy resin is not suitable for making gears since part
of the teeth will be broken during abrasion test. The underlying reason for gear failure
is that the material of polymer gears is fragile; (b) 10 wt.% glass fiber-reinforced PLA or
10 wt.% carbon fiber-reinforced PLA are recommended for making a small batch of gears
for functional testing; (c) ABS, PA, or PC are not suitable for making gears because of the
larger amount of deformation produced, and (d) polyurethane resin is also suitable to make
gears for small quantity demand based on the inconspicuous deformation and abrasion
weight percentage. In addition, the wear resistance of gears fabricated with polyurethane
resin can be further enhanced by adding reinforcing fillers into base materials.
According to the aforementioned results, the findings of this study are very prac-
tical and provide potential applications in consumer electronics, automotive, aerospace
engineering, medical, or architectural industries because this technique can be used to
fabricate small batch production of polymer gears for functional testing at the research
and development stage. The fabricated polymer gears can be further machined, such as
by polishing, grinding, cutting, tapping, or drilling. In practice, pressure and temperature
are the most significant variables in the differential pressure vacuum casting process. To
Polymers 2021, 13, 4126 19 of 21

achieve intelligent manufacturing during mass production of transmission components

using VC technology, it is recommended that both pressure and temperature sensors are
embedded in the cavity of the silicone rubber mold to monitor operational parameters
during the differential pressure vacuum casting process. In this study, both epoxy resin
and polyurethane resin were employed to manufacture polymer gears. Alternative poly-
mers, such as polycarbonate, nylon, acrylonitrile butadiene styrene, or polypropylene were
recommended for the manufacture of polymer gears. In addition, the mechanical proper-
ties of the fabricated polymer gears were dramatically affected by the intrinsic material
properties of the molding material. Hence, the mechanical properties of the fabricated
polymer gears can be further improved by adding reinforcing fillers, such as bentonite [25],
silsesquioxanes, silica, alumina [26], zirconium dioxide, silicon dioxide [27], silicon car-
Polymers 2021, 13, x FOR PEER REVIEW 21 of 23
bide [28], silicon nitride [29], or molybdenum disulfide [30] into the matrix materials. These
issues are currently being investigated and the results will be presented in a later study.

Figure 20. Materials

Materials cost
cost and
and manufacturing
manufacturing time for gears
gears fabricated with eight different materials.

4. Conclusions
4. Conclusions
Polymer gears
Polymer gears have
have been
been widely
widely applied
applied in
in transmission systems due
transmission systems due to
to low
low noise
noise
and low costs compared to metal gears. The main purpose of this study was to characterize
and low costs compared to metal gears. The main purpose of this study was to character-
polymer
ize gears
polymer fabricated
gears by both
fabricated DPVC
by both and AM.
DPVC and The
AM.filling systemsystem
The filling of the silicone rubber
of the silicone
mold was optimized by utilizing the numerical simulation software. Abrasion
rubber mold was optimized by utilizing the numerical simulation software. Abrasion test equip-
test
equipment for evaluating spur gear life was designed and implemented. The main con-
clusions from the experimental work in this study are as follows:
1. The remarkable findings in this study are very practical and provide potential appli-
cations in the research and development stage because this technique can be used to
fabricate small batch production of polymer gears for functional testing.
Polymers 2021, 13, 4126 20 of 21

ment for evaluating spur gear life was designed and implemented. The main conclusions
from the experimental work in this study are as follows:
1. The remarkable findings in this study are very practical and provide potential appli-
cations in the research and development stage because this technique can be used to
fabricate small batch production of polymer gears for functional testing.
2. Notably, 10 wt.% glass fiber-reinforced PLA or 10 wt.% carbon fiber-reinforced PLA
are suggested for the small batch production of gears for functional testing. ABS,
PA, or PC are not suitable for making gears because they produce a larger amount
of deformation.
3. Polyurethane resin is suitable for manufacturing polymer gears for small quantity
demand based on the inconspicuous deformation and abrasion weight percentage.
In addition, the wear resistance of gears fabricated with polyurethane resin can be
further enhanced by adding reinforcing fillers into base materials.

Author Contributions: C.-C.K.; wrote the paper/conceived and designed the analysis/performed
the analysis/devised the conceptualization; D.-Y.L., Z.-C.L., Z.-F.K.; collected the data/contributed
data or analysis tools. All authors have read and agreed to the published version of the manuscript.
Funding: This study received financial support by the Ministry of Science and Technology of Taiwan
under contract nos. MOST 110-2221-E-131-023 and MOST 109-2637-E-131-004.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.

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