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Title

Does Thin-Walled Metal Pipe Insertion Increase the Bending Strengthof 3D Printed Parts?

Authors

Kurumi Osawa, Gen Endo

Citation

Proceedings of the 2024 IEEE/SICE International Symposium on
System Integration, , , pp. 585-591

Pub. date

2024, 1

Copyright(c) 2024 IEEE. Personal use of this material is permitted. Permission
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http://dx.doi.org/10.1109/SII58957.2024.10417167

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Does Thin-Walled Metal Pipe Insertion Increase
the Bending Strength of 3D Printed Parts?
Kurumi Osawa1 and Gen Endo1
Abstract— Resin 3D printing is a promising modeling method
for Agile development, however its material strength is lower
than that of metal parts. We hypothesized that embedding athin-walled metal pipe in a 3D printed beam would increase
the strength of the part while keeping it lightweight. This
paper aims to verify the effectiveness of this method. To test
our hypothesis, specimens of potassium titanate ﬁber-reinforced
material POTICON and ABS were fabricated, and conducted
three-point bending tests. As a result, POTICON specimens
embedded with stainless steel pipes showed a strength 0.7 to 1.4
times that of the solid specimens, while ABS specimens showed astrength 1.1 to 2.0 times greater than the solid specimens. These
values were small compared to our ﬂexural rigidity calculations,and we concluded that embedding a pipe did not signiﬁcantly
increase the strength.

Fig. 1.

Specimen

IndenterI. INTRODUCTION
In recent years, 3D printers have become increasingly
popular. They are available in various types, including fused
deposition modeling (FDM), stereolithography (SLA), material jetting, and binder jetting, which mainly use resin.
There are also selective laser melting (SLM), selective laser
sintering (SLS), electron beam melting (EBM), and others
that use metallic materials [1] [2]. FDM, also known as
fused ﬁlament fabrication (FFF), is a method of fabricationin which thermoplastic resin ﬁlament material is melted byheat and extruded from a nozzle. It is the most widely usedmethod because it is cost-effective and easy to obtain materials. This type of printer is also said to be an environmentallysafe desktop prototyping facility because it produces only asmall amount of waste [3].
The widespread use of FFF 3D printers has made it
possible to fabricate parts faster and lighter than machining
metal. Previous research have developed speed reducers androbots using resin parts fabricated by 3D printers [4]–[9].
However, resin has signiﬁcantly lower strength than metal,
making it difﬁcult to use in situations where large loads
are applied. Therefore, methods to increase the strength ofresin parts have been investigated, such as reinforcementby continuous carbon ﬁber [10] [11], reinforcement by
optimizing materials [12], and reinforcement of structurally
weak parts by varying the internal density [13]. Among thesemethods, the method of inserting continuous carbon ﬁber
is easiest to attempt because 3D printers that can achieve
this are commercially available [14], however it is difﬁcult
to freely allocate the carbon ﬁber due to the limitations of
the slicer software. In addition, many previous studies have
1 All authors are with the Department of Mechanical Engineering, Tokyo
Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo 152-8552,
Japan osawa.k.af, endo.g.aa@m.titech.ac.jp

Specimen

Fulcrum

32 32
Fig. 2.

The actual condition of the experimentexperimentally evaluated the mechanical properties of resinsproduced by 3D printers. With lessons learned from thesestudies, it is possible to build parts using stronger resins
and printing parameters (number of layers, printing speed,
nozzle temperature, etc.) that can improve the strength ofthe part [15]–[17]. However, there are limits to the types ofresins and printing parameters covered above, and even if themost suitable ones are employed, it is not always possible toachieve sufﬁcient strength to meet the required speciﬁcations.
Therefore, we propose a new method of reinforcing resinby embedding a thin-walled metal pipe into a resin beamfabricated by a 3D printer. We hypothesized that the metal
would support some of the load applied to the resin, thereby
increasing the strength of the part. The purpose of thispaper is to verify the effectiveness of this method and to
experimentally measure the strength by conducting a threepoint bending test.
II. EXPERIMENTAL SETUP
A. Test Procedure
To verify the proposed method, resin specimens withembedded metal pipes (Fig. 1(a)) and solid resin specimens

TABLE I
M ATERIAL PROPERTY
Material
Tensile strength (MPa)
Bending strength (MPa)
Flexural modulus (GPa)Speciﬁc density (g/cm3 )Heat deﬂection temperature (℃)

POTICON
100
167
6.3
1.27
120

ABS
32.60
54.00
1.85
1.179
106.40without embedded metal pipes (Fig. 1(b)) were prepared,
and three-point bending tests were conducted using an Autograph (Shimadzu Corporation, AG-1 and AGX-20kNVD).
Referring to ISO 178:2010, the length of each specimen was
set to 80 mm, and the distance between the fulcrums of thebending test was set to 64 mm, the test speed to 2 mm/min,and the indenter radius to 5 mm. The indenter was adjusted
so that it was positioned in the center of the specimen, andthe displacement and load were measured until the specimen
fractured. The test is shown in Fig. 2. Since the dimensions of
3D printed parts may slightly differ from those on CAD, theactual dimensions were measured and the CAD dimensions
were ﬁne-tuned to achieve the dimensions shown in Fig. 1.
B. Materials
The 3D printer used was a Raise3D Pro2, which is FFF
type. Two types of resin were used for the specimens: apotassium titanate ﬁber-reinforced material called POTICON
by Otsuka Chemical Co., Ltd. [18], and Z-ULTRAT which isan ABS-based material by Zortrax. The physical propertiesof the resins are shown in TABLE I [19] [20]. To the bestof our knowledge, POTICON is the strongest short ﬁberreinforced ﬁlament material for FFF type 3D printers. The
slicer software used was ideaMaker, which is dedicated toRaise3D. The metal pipe used was SUS304 grade with anouter diameter of ϕ5 and an inner diameter of ϕ3. The
specimen with the embedded pipe was fabricated by making
a specimen with a hole of ϕ5.3 without support using a 3D
printer, and then pressing the pipe directly into the specimen.
In the case of adhesion, an adhesive (Cemedine Co., Ltd.,Super X No. 8008) with acrylic modiﬁed silicone resin as themain component was applied to the pipe and then pressed
into the specimen.
C. Printing Conditions
The inﬁll density of the resin was set to 100%. Since
the purpose of the test was to increase the strength of
the resin, inﬁll density below 100%, which would clearly
reduce the strength, were not included. On the other hand,
the lamination direction, wall thickness, and whether or not
the pipe is adhered, which may have a correlation withstrength, were veriﬁed in the tests. The lamination directions
are shown in Fig. 3, the wall thickness is the dimension
shown in Fig. 1 and is 2.4 mm and 3.6 mm, and thepipe adhesion is compared under three conditions, including
one without embedded pipes. As shown in Fig. 4, straightline inﬁll with one shell was used for lamination in the Z
direction, and concentric circle inﬁll with one shell was used

Fig. 3.

Lamination direction

Fig. 4.

Inﬁll pattern

TABLE IIS PECIMEN P ROPERTIES
Number
A
B
C
D
E
F
G
H
I

Lamination
Z
Z
Z
X
X
X
Z
Z
Z

Wall (mm)
3.6
3.6
3.6
3.6
3.6
3.6
2.4
2.4
2.4

Pipe
InsertAdhereNone
InsertAdhereNone
InsertAdhereNone

for lamination in the X direction. This is because a triangularinﬁll pattern tends to create a gap inside the specimen, which
increases the possibility of internal fractures. The specimens
fabricated to verify the above conditions are summarizedin TABLE II; three specimens were fabricated and tested
for each condition. Since the nozzle diameter and extrusion
width were set to 0.4 mm and 0.3 mm, respectively, when
the specimens were fabricated, the wall thicknesses were set
to 2.4 mm and 3.6 mm, which are multiples of these values.
The dimensions of the specimens without embedded pipes
were matched to those of the specimens with embedded pipes
for comparison.
D. Evaluation Method
We obtained load and displacement data from a 3-point
bending test. Using these values, stress σ and strain ε were
evaluated.
First, area moment of inertia is calculated. For the crosssection shown in Fig. 5, the area moment of inertia isfour times that in the region represented by y≥0 and z≥0.
To simplify the calculation, the equivalent area moment ofinertia I (mm4 ) is calculated assuming that all materials areresin. The calculation can be done as follows.

𝑎𝑦

TABLE IIIC ALCULATED FLEXURAL RIGIDITY OF ROD AND PIPE

𝐸𝑎

𝑧

𝑟𝑖

−𝑎
𝐸𝑏

Shape
CylinderCylinder
Pipe

𝑎 𝑧

𝑟𝑜

TABLE IV
C ALCULATED FLEXURAL RIGIDITY OF SPECIMEN−𝑎Fig. 5.

Cross section model[∫ ro {}√
Eb √ 222
2
I=4
ro − y + (a − ro − y ) y 2 dy
Ea
0
]
∫ a
∫ ri
√
Eb
(1)
+
y 2 ady −ri2 − y 2 y 2 dy
Ea0ror3 (16a − 3πro ) 4a(a3 − ro3 ) πEb (ro4 − ri4 )
= o
+
+
12
3
4Ea
where Ea (GPa) is the ﬂexural modulus of the resin, andEb (GPa) is that of the pipe. The process of deriving theequation is described in the Appendix.
Since I is the equivalent area moment of inertia of theresin, the stress σ (GPa) and strain ε at the center of thebeam and y = a are calculated as follows.
M
F La
Ea
y=
y=
ρ
I
4I

(2)

1
σ
12va
F La 48vI
y=
×
=
=
ρ
Ea
4I
F L3
L2

(3)

σ=
ε=

where F (kN) is the load, v (mm) is the displacement, ρ(mm) is the radius of curvature of displacement v, M (Nm)the moment applied to the center of the beam and L (mm)is the distance between the fulcrum points.
Since embedding the metal pipe in the resin increases
the mass of the specimen, the speciﬁc strength is also
evaluated. The speciﬁc strength (Nm/g) is calculated using
the following equation.
Speciﬁc strength =σmax
DensityFlexural rigidity (Nm2 )
0.19
0.056
5.2

Material
POTICONABS
SUS304(4)

where σmax (GPa) is the maximum stress. the density of thespecimen (g/mm3 ) can be calculated by measuring the mass(g) and volume (mm3 ) of each specimen.
III. FLEXURAL RIGIDITY CALCULATIONThe specimens with and without pipes are quantitatively
compared. First, a solid cylinder of resin and a stainless steel
pipe were compared to conﬁrm the greater strength of thepipe. The ﬂexural rigidity of the solid resin cylinder EIrod(Nm2 ) and that of the stainless steel pipe EIpipe (Nm2 ) canbe expressed as follows.

Material
POTICON
POTICON
POTICON
POTICON
ABS
ABS
ABS
ABS

Wall (mm)
3.6
3.6
2.4
2.4
3.6
3.6
2.4
2.4

Pipe
✓
✓
✓
✓

Ea πD41000 64(5)Eb π(D4 − d4 )
1000
64

(6)

EIrod =
EIpipe =Flexural rigidity (Nm2 )
16.6
11.6
9.8
4.8
8.5
3.4
6.5
1.4

where D (mm) is the outer diameter of the pipe, d (mm) is
the inner diameter of the pipe.
The ﬂexural moduli of the resins are shown in TABLE I,
and the modulus of the SUS304 used for this pipe is Eb =193 GPa. Therefore, the ﬂexural rigidity can be calculated
as shown in TABLE III. These values show that, the ﬂexural
rigidity of the stainless steel pipe is about 27 times greaterthan that of POTICON and about 93 times greater than thatof ABS, which is a signiﬁcantly large value.
On the other hand, the ﬂexural rigidity of the specimen
EIspecimen (Nm2 ) can be expressed as follows.
EIspecimen =

Ea I
1000

(7)

In the case of no pipe, (1) holds if D = 0, d = 0. Therefore,
without considering the lamination direction, the ﬂexural
rigidity can be calculated as in TABLE IV. This indicatesthat the ﬂexural rigidity with pipe is approximately 1.4 to
4.6 times greater than without pipe.
Based on the above calculations, we expect that theﬂexural rigidity and strength would be different, but that theaddition of the pipe would increase the strength.
IV. TEST RESULTS
A. Stress-Strain Diagram
The results for POTICON are shown in Fig. 6 and theresults for ABS are shown in Fig. 7. The avg. shown in Fig. 6
and 7 represents the average value of the maximum stressat the moment the specimen fractured in each of the threetests. In the case of ABS D in Fig. 6, the maximum stress
was reached after the resin part of the specimen fractured
due to the presence of the pipe, but the maximum stress wasA (avg. 117.8[MPa])

B (avg. 128.3[MPa])

C (avg. 161.8[MPa])

D (avg. 57.1[MPa])

E (avg. 57.4[MPa])

F (avg. 40.7[MPa])

G (avg. 107.8[MPa])

H (avg. 117.5[MPa])

I (avg. 171.6[MPa])

Fig. 6.

Experimental results of POTICON

A (avg. 76.9[MPa])

B (avg. 77.4[MPa])

C (avg. 58.4[MPa])

D (avg. 27.1[MPa])

E (avg. 28.8[MPa])

F (avg. 14.2[MPa])

G (avg. 73.8[MPa])

H (avg. 77.6[MPa])

I (avg. 68.9[MPa])

Fig. 7.

Experimental results of ABS

TABLE V
S PECIFIC STRENGTH OF SPECIMEN OBTAINED FROM EXPERIMENT
Material
POTICON
POTICON
POTICON
POTICON
POTICON
POTICON
POTICON
POTICON
POTICON
ABS
ABS
ABS
ABS
ABS
ABS
ABS
ABS
ABS

Number
A
B
C
D
E
F
G
H
I
A
B
C
D
E
F
G
H
I

Weight (g)
19.4
19.6
13.6
19.8
19.9
14.3
14.9
15.0
9.5
16.4
16.4
10.6
17.2
17.2
11.1
12.6
12.7
6.9

Speciﬁc strength (Nm/g)
60.4
65.4
135.8
29.3
29.0
33.7
44.8
48.5
144.2
82.3
83.6
62.0
28.4
30.3
15.2
80.7
85.4
73.1

taken as the value at the moment the specimen fractured: an
avg. 27.1 MPa.
To compare the strength of the specimens with and without
embedded pipes, A-C, D-F, and G-I were compared respectively. Considering the averages of the specimens where thepipes were inserted or adhered, in the case of POTICON,
A and B were about 0.8 times the value of C, D and E
were about 1.4 times the value of F, and G and H wereabout 0.7 times the value of I. In the case of ABS, A and
B were about 1.3 times the value of C, D and E wereabout 2.0 times the value of F, and G and H were about
1.1 times the value of I. These experimental values differ
from those calculated in TABLE IV. Especially, the strengthof POTICON specimens was reduced in some cases. The
strength of ABS specimens increased, although not as muchas compared with TABLE IV. Moreover, it was found thata signiﬁcant increase in strength, on the order of several
tens of times, could not be expected as in the TABLE III
comparison.

A
POTICON B
C

D
E
F

G
H
I

A
B
C

D
E

G
H

F

I

ABS

Fig. 8.

Fracture specimens

fractured. POTICON in particular is a highly brittle material,and it is thought that the resin portion of the specimen
fractured before the pipe was loaded and bent. In fact, at
the moment the specimen fractured, shards of resin wereobserved ﬂying away from the fulcrum with large force. On
the other hand, since ABS is a relatively ductile material,
the specimen bent and the load was distributed to the pipe,
resulting in a larger increase in strength than POTICON. In
this case, the specimens fractured cleanly on the fulcrum.
It should be noted that, specimens C and I ‒ laminatedin the Z direction and lacking embedded pipes ‒ , bent
slowly and eventually formed fractures, whereas some of thespecimens with embedded pipes fractured instantaneously.
Therefore, it is thought that the reduced wall thickness was
the primary cause of fracturing.
B. Difference in Resin Density
The density of the resin portion of each specimen, basedon the measured mass and volume, was 92-96% for POTICON and 78-80% for ABS. The reason for the lower densityof ABS is thought to be due to the lower ﬁlament ﬂow
rate setting to prevent resin overﬂow and unevenness during
fabrication of specimens.
C. Analysis of Specimen Properties

B. Speciﬁc Strength

The specimen properties are discussed below.

The experimental speciﬁc strengths shown in TABLE V
are based on the maximum stresses shown in Fig. 6, 7. In
the case of POTICON, the values were larger without theembedded pipe, while in the case of ABS, the opposite wastrue. Although the pipe is relatively heavy at approximately
7.6 g and the mass of the specimen in which it is embedded
is also large, the speciﬁc strength of ABS is considered tohave been increased due to the sufﬁcient increase in strength.

•

V. DISCUSSION
A. Observations About Fractures
The test results showed that the addition of the pipe
effective by a factor of 0.7 to 2.0, well below the expectedfactor of several tens of times. One possible reason for thisis that the thickness of the resin is thinner in specimens witha pipe, causing a stress concentration at the point where theindenter presses it. Fig. 8 shows the specimens after they

The lamination direction was compared between specimens A-C and D-F. The results show that the strength of
specimens laminated in the Z direction is greater thanthat of those laminated in the X direction. Since the3D printed part is anisotropic and the layers tend topeel off in the lamination direction, it is reasonable thatthe strength of specimens laminated in the Z direction,
which is stacked in the direction perpendicular to theload, is greater than those laminated in the X direction.
• Comparing specimens A-C and G-I, in case of no pipe,
it was found that specimens with a wall thickness of 2.4
mm had greater strength. The maximum load obtainedin the bending test was greater for the specimen witha wall thickness of 3.6 mm, but the experimental stress(strength) for the specimen with a wall thickness of 3.6mm was superior because of its greater cross-sectional
secondary moment.

On the other hand, in the case with pipes, the strengthof POTICON specimens with 3.6 mm wall were greater.
The difference in strength between the two wall thicknesses was slight for ABS, and some specimens witha wall thickness of 2.4 mm had greater strength.The
resin portion of 2.4 mm is thin and easily cracked, but
ABS is more ductile than POTICON, so the thin-walled
specimens bend more easily and deform together withthe pipe, thus showing the greater effect of the pipe.
• Whether the pipe is adhered or not can be compared
between specimens A and B, C and D, and G and H.
The strength was greater when the pipe was adhered
than when it was pressed in, however the difference isnegligible. This may be due to the fact that there is onlya small gap between the pipe and the specimen, and theadhesive did not penetrate sufﬁciently.
D. Experimental Procedure Limitations
In this test, as shown in TABLE IV, the ﬂexural rigidity of
the resin portion alone was high, and was only several times
greater at best when a pipe was inserted. It is speculated that,by shortening the short side of the specimen (thereby reducing the area moment of inertia), the ﬂexural rigidity could
have approached the calculated values in TABLE III and theeffect could have been several tens of times larger insted.
However, as discussed in Section V-C, the reduction in the
thickness of the resin portion can be a cause of fracturing,so experimental veriﬁcation is necessary for specimens withsmaller dimensions.
In addition, the pipe is placed in the neutral axis of thespecimen in this test, but the stress is greater in the upperand lower portions of the specimen.Therefore, there is apossibility that the strength can be improved by shifting thepipe from the neutral axis and placing it in a position where
the stress is greater.
In this paper, only bending tests were conducted toevaluate the strength. Therefore, it can only be said thatthe embedded pipe method is effective against bending.
However, different results may be obtained from tensile orcompression tests. Creep tests should also be considered, as
they may be effective in reducing creep deformation.
VI. CONCLUSION
In this paper, we hypothesized that embedding a thinwalled metal pipe in a 3D printed beam fabricated by a 3D
printer would increase the strength of the part, and conducteda veriﬁcation test to conﬁrm the effectiveness. Specimens
were fabricated from POTICON (a potassium titanate ﬁberreinforced material) and ABS resin and subjected to threepoint bending tests. As a result, POTICON specimens embedded with stainless steel pipes showed a strength 0.7 to
1.4 times that of the solid specimens, while ABS specimens
showed a strength 1.1 to 2.0 times greater than the solid
specimens. These values were small compared to our ﬂexural
rigidity calculations, and we concluded that embedding apipe did not signiﬁcantly increase the strength.

Embedding Pipes
Fig. 9.

Application example

As a future prospect, it is possible that the beneﬁt frompipe insertion can be enhanced by reducing the size of thespecimen or placing the pipe at the point of high stress inthe specimen. In addition to bending, it is also important
to conduct tensile, compression, and creep tests to conﬁrm
whether the method of embedding thin-walled metal pipes
in resin is also effective for these purposes.
We believe that the method proposed in this paper canbe applied to reinforcement of robot frames, as shown in
Fig. 9. The robot frame is hollow because many parts areplaced inside, and it is designed to be strong with columns
and walls. However, the wires and timing belts used to movethe robot apply large forces to the frame, so thin-walled
pipes were placed in these areas to reinforce them. If themethod proposed in this paper is effective not only in bending
but also in creep deformation, it will be possible to prevent
fracture and deformation of the resin and develop robots thatare more resistant to breakage.
APPENDIX
The area moment of inertia in Fig. 5 is four times that in
the region represented by y≥0 and z≥0. Therefore, we consider the range of y≥0 and z≥0. To simplify the calculation,
the equivalent area moment of inertia is calculated assumingthat all materials are resin. In ro ≤y≤a, the material is only
resin with ﬂexural modulus Ea , and in 0≤y≤ro , the beam
is a combination of resin with Ea and a pipe with Eb . In
the composite beam, the pipe and the outer resin are in close
contact, and the short interval dy of the beam is assumed to
be uniaxial tension in the x-axis direction (shear strain in the
beam is ignored).
First, the area moment of inertia I0 at ro ≤y≤a is calculated as follows.
∫ a∫ ay 2 ady(A.1)
y 2 dA =
I0 =
ro

ro

Next, 0≤y≤ro is divided into ri ≤y≤ro and 0≤y≤ri .
The vertical force dP1 at ri ≤y≤ro is obtained by thefollowing equation.
dP1 =

Ea
Eb
yba1 dy +ybb1 dyρρ(A.2)

where ba1 is the Z-axis width of the ﬂexural modulus Ea
part, and bb1 is that of the Eb part. These can be calculated
as follows.

√ro2 − y 2√ba1 = a − ro2 − y 2
bb1 =

(A.3)
(A.4)

Similarly to (A.2), the vertical force dP2 at 0≤y≤ri is asfollows.
Ea
Eb
yba2 dy +ybb2 dy(A.5)ρρ
where ba2 is the Z-axis width of the ﬂexural modulus Ea
part, and bb2 is that of the Eb part. These can be calculated
as follows.
√√
bb2 = ro2 − y 2 − ri2 − y 2(A.6)√
ba2 = a − ro2 − y 2
(A.7)
dP2 =

Using the above, the bending moment M acting on the
cross section is calculated using the following equation.
M=

Ea
I0 +
ρ

∫ ro

∫ ri
ydP1 +

ri

ydP2 =
0

Ea ′
I
ρ

(A.8)

Here, the third term of the equation is expressed using theﬂexural modulus Ea of the resin to derive the equivalent area
moment of inertia.
Substituting (A.2)-(A.7) for (A.8), the following equation
can be derived.
∫ ro {}√
Eb √ 2222
I = I0 +ro − y + (a − ro − y ) y 2 dy+
Ea
ri
}
∫ ri{
√
√
Eb √ 222
2
2
2( ro − y − ri − y ) + (a − ro − y ) y 2 dy
Ea
0
(A.9)
′

Finally, substitute (A.1) into (A.9) to transform the equation. Calculating I = 4I ′ yields (1).
ACKNOWLEDGMENT
We thank Otsuka Chemical Co., Ltd. for providing materials and information and for their advice in writing thispaper. We thank Prof. Naoyuki Takesue (Tokyo Metropolitan University), and Prof. Yusuke Ohta (Chiba Institute ofTechonology), and Prof. Takeshi Takaki (Hiroshima University) for their valuable comments and discussion. We thankProf. Akira Todoroki (Tokyo Institute of Technology) for his
help in deriving Section II-D.
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