3D Printed Hybrid Composite Structures - Design and Optimization of a Bike Saddle

Abstract

As designers and engineers continue to push the boundaries of high performance and lightweight design, the use of complex geometries and composite materials is growing. However, traditional composite manufacturing often requires the use of additional tooling and molds which can significantly increase the cost. In this study, a carbon fiber reinforced composite bike saddle is designed and manufactured to demonstrate a newly developed hybrid composite manufacturing process. Using a dual curing 3D printed epoxy to print the final part geometry and co-cure pre-impregnated carbon fiber reinforcement, the bike saddle can be optimized, designed and manufactured in less than 24 hours without tooling.

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Society of Plastics Engineers – Annual Technical Conference (ANTEC), Virtual Edition, 2020
3D PRINTED HYBRID COMPOSITE STRUCTURES – DESIGN AND
OPTIMIZATION OF A BIKE SADDLE
Hongrui Chen 1, Alec Redmann 1, Rui Zhang 2, Sue Mecham 2, and Tim A. Osswald 1
1Polymer Engineering Center, University of Wisconsin - Madison, USA
2Department of Chemistry and the Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill, USA
Abstract
As designers and engineers continue to push the
boundaries of high performance and lightweight design, the
use of complex geometries and composite materials is
growing. However, traditional composite manufacturing
often requires the use of additional tooling and molds
which can significantly increase the cost. In this study, a
carbon fiber reinforced composite bike saddle is designed
and manufactured to demonstrate a newly developed
hybrid composite manufacturing process. Using a dual
curing 3D printed epoxy to print the final part geometry and
co-cure pre-impregnated carbon fiber reinforcement, the
bike saddle can be optimized, designed and manufactured
in less than 24 hours without tooling.
Introduction
With the introduction of AM, more and more intricate
parts can be made. By combining 3D printing with
composite materials, complex structures with desirable
mechanical properties can be achieved. A relatively new
addition in AM technology is Digital Light Synthesis
(DLS) by Carbon, Inc. (Redwood, CA). DLS, shown in
Figure 1, involves a UV light source projecting through an
oxygen-permeable window. In this process, layers can be
generated continuously, producing parts with close to
isotropic mechanical property and excellent surface
properties [1]. Many of the resins used in the DLS process
are dual curing resin with a UV and thermal curing stage.
Due to the resin being used in the DLS process, a two-
stage curing process is required to produce the final part.
During the printing process, only the UV portion of the
curing is completed, resulting in a partially cured part.
Next, the part is removed from the build platform and
placed in an oven, where the heat from the oven activates
the thermally cured stage.
A new process has been developed to utilize this two-
stage curing process for hybrid composite fabrication.
First, the part is 3D printed to activate the UV cure,
resulting in a semi-rigid, but only partially cured part. This
part still has chemical potential and bonding availability
when it is then integrated with pre-impregnated fiber
reinforcement. This assembly is then heated to activate the
thermal curing reaction of both materials and co-cure the
assembly.
Previous work has been done to co-cure lattice
structure with pre-impregnated carbon fiber. The result is a
sandwich structure where carbon fiber reinforcement is
integrated on the top and bottom of the 3D printed lattice
structure. It was shown that with this co-curing process,
there is an increase in strength and stiffness and a strong
bond between the 3D printed part and fiber reinforcement
[2]. In this paper, we further develop the idea of creating
such hybrid structures and demonstrate the possibility of
incorporating such design with a more complex geometry.
In order to explore the practical application of such a
process, we have chosen to 3D print a bike saddle with the
DLS process and co-cure the saddle with pre-impregnated
carbon fiber. This demonstration shows that in addition to
simple planer surfaces, carbon fiber composites can also be
co-cured with surfaces that have more complex geometry
using this process.
Figure 1. Schematic diagram of the DLS Process [3]
Finite Element Analysis
One important reason to make the saddle a hybrid
structure is to increase the mechanical performance by
placing the carbon fiber reinforcement in high stress
regions. In order to determine the high-stress regions of a
bike saddle in typical applications, Finite Element Analysis
(FEA) is utilized.

Society of Plastics Engineers – Annual Technical Conference (ANTEC), Virtual Edition, 2020
A previous study has demonstrated that the loading on
the saddle depends on the riding position [4]. There are two
common riding position, drops and tops. When the rider is
in tops position, the rider’s center of gravity is farther
backward. The stress on a saddle mostly concentrates
around both sides of the posterior and in the middle due to
the anatomical structure of the human pelvis. On the other
hand, when the rider is in drops position, the rider leans
more forward, resulting in a more forward center of gravity.
The forward center of gravity will increase the load on the
middle of the saddle compared to the tops position.
In order to gain a better understanding of the stress
distribution on a saddle, both the loading condition of tops
and drops were modeled. For the loading condition when
the rider is in tops position, a 200 N force is placed on both
sides of the posterior and 200 N force is placed in the
middle of the saddle (Figure 2). The model is constrained
based on fixture geometry that resembles the metal rail for
which the saddle rests on.
Figure 2. The loading condition of tops riding position
SolidWorks is used to complete the FEA simulation.
The result shown in Figure 3 indicate that there is a stress
concentration at the center of the saddle due to the load
applied to the middle. In addition, there is also stress
concentration occurring in the posterior region.
Figure 3. The FEA result of tops riding position
Next, the loading condition when the rider is in drops
position is modeled. When the rider is in drops position, the
load on the posterior of the saddle is decreased and more
load is concentrated in the middle of the saddle. Therefore,
a 600 N load is placed in the middle (Figure 4).
Figure 4. The loading condition of drops riding position
The result from drops position shown in Figure 5
indicate that there is higher stress concentration around the
center of the saddle. In addition, stress around the saddle
also experienced an increase compared to the tops position.
Figure 5. The FEA result of drops riding position
The result from drops position showed that there is
higher stress concentration around the center of the saddle.
In addition, stress around the saddle also experienced an
increase compared to the tops position.
Materials
The hybrid structure was manufactured with two
materials. The carbon fiber composite is woven epoxy
resin prepreg HexPly W3T282-F263, from Hexcel. The
material properties are listed below in Table 1 [5].

Society of Plastics Engineers – Annual Technical Conference (ANTEC), Virtual Edition, 2020
Table 1. Material data of HexPly W3T282-F263 [5]
Material Properties
Value
Unit
Nominal Ply Thickness
0.18
mm
Tensile Strength
570
MPa
Tensile Modulus
60.7
GPa
Glass Transition Temperature
188
°C
For the 3D printed base, the epoxy-based
photopolymer resin EPX 82 from Carbon was
manufactured with DLS. The UV curing process from the
printer completes the first stage of the curing process. In
typical applications without creating hybrid structures, the
printed part will then be taken into an oven to fully cure
using heat. After being fully cured, the material properties
of EPX 82 are listed in Table 2 [6].
Table 2. Material data of EPX 82 [6]
Material Properties
Value
Unit
Ultimate Tensile Strength
82
MPa
Tensile Modulus
2800
MPa
Elongation at Break
5.9
%
Manufacturing
Design
Although the DLS process can produce parts with
nearly isotropic mechanical property and high surface
quality, good model design is important to prevent printing
failure and reduce the need for excessive support. In
addition, the design of the saddle also needs to incorporate
the carbon fiber reinforcement.
The geometry of the saddle (Figure 6) is designed to
be best suited for AM. A flat area on the rear is created such
that during printing, there is a firm base for the model to
stick to the build plate. There can be significant pulling
force generated during the print due to the small gap
between the part and the oxygen permeable window. The
smooth flat area prevents the model from being pulled off
from the build plate. When printing parts with a high
overhang angle, DLS requires support to be paced at
regions with high overhang angle. Therefore, most of the
features of the model follow a smooth incline angle. The
incline angle leads into the feature smoothly, reducing the
overhang angle of the part, which eliminates the need for
supports most support material.
The goal of this design is to utilize the ability of the of
DLS printed parts to co-cure with pre-impregnated carbon
fiber. Making such hybrid structures will improve the
mechanical property, however, due to geometry and the
ability to use vacuum bagging, carbon fiber cannot be place
everywhere on the part. Therefore, the FEA results serve as
a good guideline for placing the carbon fiber at the region
where stress is the highest. According to the FEA result,
there are high stress area at the center and the posterior
region. For simplicity in this demonstration, fiber is placed
in the textured region shown in Figure 7. In addition,
grooves were designed to allow a smooth finish after
placing the reinforcement.
Figure 6. The final geometry of the saddle with the
supports (in purple) and connection to the build platform
Figure 7. The placement of carbon fiber for this
demonstration

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Slicing and Printing
Common to all 3D printing technologies, DLS also
requires slicing the model into layers and printing based on
the sliced model. The Carbon cloud based slicing software
is used to prepare the model. The model is printed with
Carbon M1 3D printer. The print took approximately 11
hours to complete. After printing, the part is washed with
solvent to remove excess resin and the support was
removed.
Post Processing
Making the hybrid structure with DLS printed epoxy
and pre-impregnated carbon fiber involves several steps.
The goal is to achieve a part with improved mechanical
properties by co-curing the pre-impregnated carbon fiber
with DLS printed epoxy. The first step is to prepare the
carbon fiber. A rectangle segment is cut from a large roll of
carbon fiber and trimmed to the proper dimension. Finally,
the pre-impregnated carbon fiber is placed onto the part.
The next step is to place the assembly into a vacuum
bag shown in Figure 8. The use of vacuum is to ensure there
is enough compaction force for the carbon fiber to co-cure
with the epoxy while reducing voids and ensuring the fully
listed mechanical properties. When preparing for vacuum
bagging, a layer of peel ply is place at the top of the part to
prevent the carbon fiber from adhering onto the vacuum
bag. Next, a layer of breather fabric is placed on top of the
peel ply. Finally, the vacuum bag is used to fully cover both
the top and bottom of the part and a vacuum pump
connection is placed inside. After the vacuum bag is
applied, a vacuum pump is connected via the connector.
The final step is to co-cure the epoxy and carbon fiber in an
oven.
Figure 8. Arrangement for co-curing under vacuum
Finally, after the curing cycle is complete, the vacuum
bag is removed, and excess resin residue is trimmed off.
The steel rail is attached though the holes designed on the
bottom of the saddle. The final part is shown in Figure 9.
Figure 9. The finished bike saddle after co-curing
Discussion and Conclusion
In the pursuit of manufacturing an epoxy-carbon fiber
hybrid structure with complex geometry, a bike saddle is
designed and manufactured. During the designing phase,
FEA is utilized to determine the best placement for carbon
fiber. In addition, optimization to the geometry of the
saddle reduces the need of support materials and improves
the success rate of printing. Finally, the carbon fiber is
integrated with the printed part and co-cured in an oven.
This demonstration shows the possibility of co-curing
carbon fiber and epoxy for parts with complex geometry.
There are many benefits of this process. One such instance
is that it can be used to reduce the cost of some carbon fiber
parts. Even in full carbon fiber constructed parts, there are
regions that are experiencing less stress and loading, and
such region can be replaced with DLS printed parts.
Furthermore, DLS printed part can be used as a base or
mold for more complex geometry. As some hollow carbon
fiber structure require intricate mold design, a DLS printed
base can be used as a mold to reduce the cost of developing
and machining molds. In addition, it can also potentially
reduce the development time of composite parts as
machining and production time for the mold can be
reduced.
In this paper, we demonstrated the possibility of using
3D printed EPX 82 epoxy resin to manufacture end-use
parts by co-curing with pre-impregnated carbon fiber.
There are also many future improvements and research can
be done for this process. The design used for this
demonstration was very simple and it should be clear that
generative design would produce a more optimized
geometry by removing material from low-stress regions
and reducing weight. Further, different designs can be
made for specific riders or specific riding positions with
more detailed FEA analysis.

Society of Plastics Engineers – Annual Technical Conference (ANTEC), Virtual Edition, 2020
Also, while previous work shows a strong cohesive
bond between the fiber reinforcement and 3D printed part,
after producing more examples of the bike saddle, loading
tests can be conducted to determine the mechanical
properties. Another aspect that is worth investigating is to
explore the possibility of scale up production of such
process. As it requires fewer man labor compared to
traditional carbon fiber composite structure manufacturing.
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