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II International Conference on Simulation for Additive Manufacturing - Sim-AM 2019
I. Ntintakis, G.E. Stavroulakis, N. Lyroni, N. Plakia
THE PERSPECTIVE OF TOPOLOGY OPTIMIZATION ON 3D
PRINTED FURNITURE PROTOTYPES
Sim-AM 2019
I. NTINTAKIS*, G.E. STAVROULAKIS†, N. LYRONI*, N. PLAKIA†
* School of Engineer, Department of Mechanical Engineering
Hellenic Mediterranean University, Estauromenos, 71004 Heraklion, Greece
e-mail: ntintakis@hmu.gr, web page: https://www.hmu.gr
† School of Production Engineering and Management
Computational Mechanics and Optimization Laboratory, Technical University of Crete
GR-73100 Chania, Greece
email: gestavr@dpem.tuc.gr, web page:www.comeco.tuc.gr
University of Thessaly
V. Griva, 431 00 Karditsa, Greece
email: niki_plk@hotmail.com- Web page: http://www.uth.gr
Key words: Inject binder, additive manufacturing, topology optimization, compression test
Summary. A new design starts from an idea and become a final product, during design
process the evaluation stage it’s a necessity, the most useful evaluation tool is prototyping.
Conceptual models are very important in product design. Improving product quality is always
an important issue of manufacturing, even if a design study is well organized it is possible some
errors may still escape from the review of engineers and designers. The touch of the physical
objects can reveal unanticipated problems and sometimes spark a better design. With the
traditional method, developing of prototypes to validate or optimize a design is often time
consuming and costly. Inject Binder is one of the most well-developed rapid prototyping or
additive manufacturing technology, actually is a Powder-based inkjet 3D printing method.
Mainly is a powder-based RP system in which a binder solution spays onto pre-deposited
powder layers. One of the main advantage of this method is the production of fullcolor models
suitable for architecture maquets, prototypes of new products like furniture and other objects.
In prototyping process, a restriction is production cost that basically depends from the amount
of printed material. In current study furniture prototypes are printed in a inject binder printer,
the printed models have common design like a chair and a table. The raw materials used in this
study were a plaster-based powder (zp151) and an appropriate water based solution with 2-
Pyrrolidone as a binder (zb63). Three different model of each model (three chairs and three
tables) are printed, the main difference between them is wall thickness, the first model thickness
is 5mm, the second 10mm and third is 20mm. The printed parts tested in a compression tester
device to check models elasticity and compressive resistance. Then the lab results used to create
a FEA (Finite Element Analysis) study in a popular CAE program. In the final stage, the
perspective of an optimization study are presented to determine an optimized shell geometry
and wall thickness. The paper discusses useful tools for designers and engineers in order to
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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decide the appropriate wall thickness and shell geometry (pattern) of furniture pieces, so to
avoid model over dimension. And check functional and aesthetic aspects before massive
production.
1 INTRODUCTION
1.1 Design Process and Methodology
Design process is a critical point for product development. From late of 19th and early 20th
century established the principle that form follows function in architecture and industrial
design. It means that the shape of a building or object should primarily relate to its intended
function or purpose. The American architect Louis Sullivan was the foremost exponent of this
principle as it expressed in his article tiled are ‘The Tall Office Building Artistically
Considered’, although he attributes its core idea to ancient roman architect Marcus Vitruvius
Pollio (Sullivan, 1896). Before the second world war became a strong argument between the
modernist architects for this phrase because of they believed that decorative elements, which
architects call "ornament", were superfluous in modern buildings (Greenough, 1947). However,
Sullivan did not contend with this view. Besides, the buildings he designed were characterized
byart nouveau and celtic revival decoration elements. In the same periode the same argument
took place in product design, between functonal design and market demands. American auto
industry halted attemps to introduce aerodynamics forms to mass production. The same time
some car resellels thought that earodynamic shape would lead to a specific form, very similar
for all cars. That it would not be good for unit sales (Meikle's,1939).
Although in early times after the second world war and until the Oxford conference on
Design Methods in 1963 design was more considered as an unitive work and less than a
scientific process. The way in which designers follow in design process has been the subject of
considerable investigation over the last six decades. In the beginning of design research, the
contention was that designers have to follow specific design process through formalized
procedures or ‘design methods’. In the beginning the designers believes that following a
specific process will eliminate the creativity and imagination. However, after the integration of
brainstorming on design process this problem will overcome. A main point of this debate was
the development of design methodology connected with computer science as requirement to
understand and define exactly what we mean by design. Alan Colquhoun focused on topology
centrality and in earlier knowledge in design thinking process (Colquhoun, 1967). Often a
design problem did not processed by reference to a good past solution or through an analysis of
the solution types that would be suitable for this specific problem. In seventies Bill Hillier
developed a new design method (Hillier, Musgrove, et al., 1972; Hillier and Leaman, 1974), in
which the knowledge from a local design problems could be useful for other design problems
in global scale. During this period, many researchers were against of Design methods, even
though in next years they change their opinion completely: Alexander: I think I should be
consistent here. I would say forget it, forget the whole thing.” (Alexander, 1971), “in the
seventies I reacted against design methods. I dislike the machine language, the behaviourism,
the continual attempt to fix the whole of life into a logical framework.” (Jones, 1977). This
period the Design methods was developed rapidly through the recognition that following linear
process, which characterized the early period of design methodology, was insufficient
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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(Checkland, 1981), furthermore the design problem was not specified clearly so on to adopt an
optional solution through definition of possible solutions for a specific design problem.
The last decade’s product development process follows a more specific process (fig.1).
Designers have to make thinks that people wants so they must be sure that new products are
well designed according to people needs. In the beginning of design process design team have
to recognize the people needs from the initial research (market research, questionnaires,
interviews etc), (Babalis et al. 2013). The second stage of product develop include concept
design process, there design team have to select an appropriate idea which is capable to give
solution in design problem. Next stage is model prototype, designers have to produce functional
physical models in order to evaluate their ideas. Since 1984 when Charles Earls invent the first
3d printing machine until today great progress have been made in rapid prototyping. The ability
to produce cheap and very accurate prototypes is a main advantage for designers so they can
easily evaluate their ideas checking products ergonomic, functionality and stability. This stage
is very critical in design process, prototypes are fully functional and end users can use these in
real world life so to give back their feedback in design team. Now, design team is able to make
all the appropriate changes so the new product to be closer to people needs.
Figure 1: Product development process
1.2 Rapid Prototyping Technology
Rapid prototyping technologies adopted as a means of manufacturing physical prototypes
and new product models that are in the design and development phase. These technologies have
enabled designers to realize their ideas in natural prototypes that approach the final product in
terms of geometry and function. One of the most well known technologies of rapid prototyping
is 3d printing. In the last decades, more and more designers and engineers are using 3D print
technology and adapted according to the new way of presenting complex structures, objects and
services are created and at the same time realizing ideas and projects personalized, which a few
years ago, looked utopian and the consumer becomes and producer (Wong & Hernandez, 2012).
However, the most important contribution of 3D printing technology is based on a radical
review of the core of scientific research, hence of knowledge (Giannatsis et al. 2015). 3D
Printers allow designers to create tangible patterns of their designs quickly, compared to two-
dimensional representations. In addition, "3D printers" allow designers to quickly create
prototypes of objects they design which can touch so to evaluate and to improve them if is
necessary.
With the evolution of technology various three-dimensional printing techniques have been
well developed, each one has strengths and weaknesses. Differences are based on how the
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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individual layers have been spread to create the various components, such as material melting,
melt deposition, or the use of liquid materials through different technological processes.
Mainly, the discussion is related to the issues of speed, cost of prototype and 3D printers, choice
and cost of materials and the ability for multicolor prototypes (Giannatsis, et al. 2015).
According to the process parameters effects, concerning the dimensional accuracy of parts has
been tried by (Kechagias et al. 2014)
One of the most well develop technologies is ‘Inject Binder’. In this technique the main
material is powder, the granules of the powder are homogeneous in size and shape, showing
only limited variation with respect to their size. The smaller the particles they form, the better
(Gibson et al. 2017). Modeling requires the use of powder as a feedstock and adhesive to
achieve the agglomeration of powder grains to produce a solid physical prototype. In particular,
the production of model is to be implemented in two distinct phases. In the first phase powder
is going to be spread in each single layer of same thickness on the print table. Then, each powder
layer be sprayed selectively with an adhesive. Selective spraying is determined by the structure
of the digital model already created in CAD software. After the first layer has been sprayed, the
second layer is deposited, the selective powder spray is repeated and the process continues until
the entire model is printed / produced. In the second phase the produced model is removed from
the container and using compressed air is cleaned from the excess powder. Then the model
sprayed with cyanoacrylate or other substances to improve their strength and surface finish.
The typical thickness of each cross section is 0.1mm. The technique is high speed and produces
objects with a relatively harsh finish (Papathanassis, 2005).
1.3 Inject Binder Printer Characteristics
Inject binder has some advantages over other 3D printing methods. Initially, in binder inject
powder has to roles, the first is to be the main material for physical model, the second one is to
support the structure as the model is being built so the need for supports is eliminate. Another
advantage is the possibility of simultaneously producing multiple objects in the same
manufacturing process (Gibson et al. 2017). Also is the only method that does not use heat in
the production process. Other additive manufacturing techniques use a heat source that can
create residual stresses in the parts. These stresses must be relieved in secondary operation after
processing. Also has the ability to print large parts and is often more cost-effective than other
additive methods. A particular advantage of Binder Jetting is that the adhesive can sprayed from
multi-nozzle print heads. Adhesives from different nozzles can differently allowing changes in
properties in manufactured model, such as the change in color (Gibson et al., 2017). Despite
the many advantages Binder Jetting has some drawbacks, such as the fact that only rough details
can be printed with this printer, as the parts are very fragile, may break during the post-
processing. Also compared to other 3D printing processes, Binder Jetting offers a limited choice
of material (Bournias, 2018). Finally, the surface quality and dimensional accuracy achieved
by this method are in many cases lower than the corresponding 3D printing methods having
raw materials in liquid form (Gibson et al. 2017).
1.4 Computational Mechanics
Computational mechanics is the scientific area that uses numerical methods to solve
engineering problems. Traditionally, the problems of engineering were solved either
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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analytically or experimentally by making many measurements on a model of the problem that
we are interested in. Computational mechanics is the third way, the development of computers
over the last few decades has enabled engineers to approach problems that were impossible to
solve in the past either because of the large size or the large amount of computing time required.
Computational mechanics complements analytical solutions and significantly reduces the
number of experiments required. In inject binder technique the printed models are consists from
powder grain, because of the distinct grain structure of printed objects, the Finite Element
Analysis (FEA) method can be implemented as a cost-effective computational engineering tool.
Addressing the problem arises from developing specific tools based on examining the
distinctive structure of the print material. Furthermore, the need for new products has increased
rapidly. The same time products life cycle digressed. Manufacturers want to develop new
robust products rapidly. Computational mechanics can play a significant role in this way.
Shortening product development time can be achieved with method of structural optimization.
Aims to improve the form of a construction, while satisfying various constraints. This method
becomes significant in design because raw material stocks are limited, there is a need for light
structures, economic construction with emphasis on efficiency and minimization of
environmental impact. There are three different types in structural optimization: a) size
optimization, b) shape optimization and c) topology optimization (Gebisa, 2017). In size
optimization the values of the selected dimensions are defined, so on to satisfy the design
constrains. In shape optimization the optimization of the shape within a given area is performed.
In topology optimization size and shape are defined in same time (Fig.2). Topology
optimization methods can be based on simplified Optimality Criteria iterative reanalysis
methods, Heurestic and optimization techniques. (Zhou et al, 1993, Bendsoe, 2089).
Figure 2: Optimization methods
2 MATERIALS AND METHODS
The main purpose of current research is whether standard furniture designs can be
reproduced on a 3D printer such as inject binder getting with better mechanical strength
minimized the used material. The study consists from two phases, initially two different
furniture models are printed with different wall thickness and then tested in a compression tester
device. In the second phase 3d models are redesigned in a CAE software using topology
optimization tool to minimized the used raw material and improving the mechanical strength.
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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2.2 Inject Binder Printer Z450
The selected printer is Z-450 from Z-Corp company, which is a fully automated and
affordable color 3D printer. It delivers printing at a speed of up to 450 dpi, five to ten times
faster than other printers and in half cost. In addition, it is ideal for applications such as design,
education, architecture, medicine, geography and electronic art. Its automation simplifies
printing and saves valuable time. Each model produced with ZPrinter® 450 and the excess
powder after each printing is recycled in its entirety for re-use. In addition, this machine sets a
new standard for office-to-office compatibility, office-safe materials, automated absorption
systems, and no waste of material. It is also a great alternative to monochrome 3D printing and
modeling costs less than half the cost of other three-dimensional printing technologies.
2.3 Compression test
The machine selected to control the mechanical strength of The 3d printed models are tested
in Zwick / Roell Z020 Testing Device (Figure 3.2) The test engine is specifically designed for
tensile, compression, curvature and bending tests, as well as shearing and turning tests that meet
the highest requirements of the material and component testing industries. This device selected
because of extremely low speeds can be set, coupled with excellent speed accuracy, also offers
high head movement analysis. Additionally, test loads of up to 110% of the nominal machine
load are allowed to compensate for heavy combinations of test pieces. The movement of the
transverse head is guided with great precision through two steel columns, which allow accurate
application of the force in the sample.
2.4 Topology Optimization (TO)
Topology Optimization of is a particularly powerful tool in the field of product design, in
calculating mechanisms and elsewhere. Using topology optimization, engineers can find the
best design plan that meets design requirements. The central idea in optimizing a structure is to
use a smaller amount of material than it previously maintains the same or even better behavior
under certain operating conditions. Often the resultant structure after topological optimization
is extremely difficult or economically unprofitable to produce using abstract production
methods.
Structural optimization, in general, has tremendous potential benefits in the process product
development. Optimization of topology in particular has the following benefits in the design
(Gebisa, 2017): a) creating light structures b)generation of a ready-to-build part/assembly c)
minimize the amount of raw material d) energy saving e) less need for natural prototypes f)
reduction of physical testing g) reduced entry time. Topology optimization has to main
characteristics. The first is that elastic properties of a material compared with their density may
vary in model area. The second is that the material can be permanently removed from model
space (Querin, 2017). During the TO study models have to be applied all boundary conditions.
Two methods have been developed for TO study. The first one is truss based and second is
volume based (Wang et al. 2018).
2.4.1Truss Based TO Method
The truss-based on elements relating to a grid of beams between a set of nodes that is in a
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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given volume. The method initially detects that supports are necessary for the structure and
determine their size. Then removes the beams that not meet the study requirements. In the
results, the necessary beams are represented with bold line and dark blue color. The less
necessary beams with less dark blue colors and unnecessary beam without change in their
thickness (Fig. 3), (Perez, 2007). Extension to multiobjective optimization has been tried by
(Stavroulakis et al. 2008. 2009).
Figure 3: The initial design on the left side and the TO study results on the right
2.4.2 Volume Based TO Method
The Volume-based is known as SIMP - Solid Isotropic Material with Penalization- method
and is widespread in CAE software. The process starts by defining a linear block of voxels.
Density of each voxel is defining between zero to one. If value is equal to one then in this
specific voxel the material is completely dense. If it is zero value then in this voxel there is no
need for material. Any other value indicates that material in this voxel has not to be solid for
the enforced loads. These value are very useful in FEA models for topology optimization
analysis (Bendsoe, 2003). Volex values are useful without restrictions in additive
manufacturing rather than traditional manufacturing methods. In figure 4 is presented a typical
topology optimization volume based problem (Bendsoe, 1989).
Figure 4: Typical optimization problem
3 RESULTS
3.1 3d Modeling and Printing
Initially two typical models of furniture created in CAD program (SolidWorks), the first
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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one was a chair (Fig.2) and the second was a table (Fig.3).Three different models for each type
of furniture created. Chair 3d model basic dimensions are 450 mm length, 450 mm width and
950 mm height. Table 3d model basic dimensions are 550 mm length, 550 mm width and 450
mm height. Only wall thickness defers in each model, in the first model wall thickness is 10mm
in the second 15mm and in the third 20mm. So three chair models and three models for table
created. Each model has to export in .stl or .vrml file, printing scale is 10% for chairs and 15%
for tables, all models printed in same time (fig. 4), after printing process completed following
the post process stage where moles must clean up from additional powder and smear them with
the hardener.
Figure 2: Chair 3d model Figure 3: Table 3d model
Figure 4: Printing Process
3.2 COMPRESSION TEST
Tests were performed with the Zwick / Roell Z020 tester machine. In total, three tests
performed for table model and three for chair model to calculate their mechanical strength in
compression. As previously mentioned, chair prototypes print scale was 10% and wall thickness
was 10, 15 and 20mm. Tables prototypes print scale was 15%, and wall thickness was 10, 15
and 20 mm. The speed, at which the test engine piston moved down, in all six tests, was
2mm/min. The specimens kept at the center of crosshead so that a uniform compressive loading
on the specimens (Fig. 5). The table below (Table 1) shows the imposed force on each sample
measured in Newton (N), time that the piston took to stop measured in seconds (s) and the
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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distance that the plunger pulled to touch the surface of the model measured in millimeters (mm).
Figure 5: Compression Tests, chair and table specimens
Table 1: Experiment Results
Sample type
Force (N)
Time to stop (s)
Piston Distance (mm)
Chair 10mm
216
14
0,5
Chair 15mm
188
15
0,6
Chair 20mm
216
15
1,1
Table 10mm
84
38
1,3
Table 15mm
156
36
1,2
Table 20mm
244
25
1,5
3.3 TOPOLOGY OTIMIZATION FOR TABLE PROTOTYPE
Topology optimization study running in Siemens NX software. The material which used in
digital model is similar with powder in Z-450 printer (Bibb et al. 2010, Pilipovi´c et al. 2009).
According to optimization scenario includes table legs form will remain the same and the main
table surface will change. As design space determined the whole model but only the upper
surface was ‘keep in’. The selected design constrains are a) Void Fill and b) Material Spreading
in 35%. The load case is the same as in compression test results. At the center of the main
surface forced with 244N. In fig. 6 & 7 presented the results of optimization study.
Figure 6: The 3d model before and after the optimization process
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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Figure 7: The bottom optimized surface and a perspective view of the whole model
Figure 8 : Maximum Displacement and Maximum Stress
The total mass of the upper surface is reduced about 86 %. The same time the optimized
model is stiffer than before as we can see in fig. 8.
4 CONCLUSIONS
Additive manufacturing is the technology to produce a physical prototype from a 3d model
through a fast and easy process. The 3d model is analyzed in a number of two-dimensional (2D)
sections of specific thickness. Each section is a layer for AM machines. In current study 3d
printing process has been carried out in an inject binder jetting printer. In this type of printer, a
big advantage is the ability to print more than one model simultaneously saving time. Another
advantage is the fully automatic printing processing. In addition, the amount of nonuse powder
is recycled. After printing processing follows a post processing stage in order to remove powder
from the objects. Then to ingrain the model in ‘color bond’ liquid in order to increase it
durability.
According to compression tests of initial chair prototypes we come to the following
conclusions. The first chair (10mm wall thickness) withstand the largest load than the other
two. The piston took less time to stop until specimen to break. During this time specimen had
high elasticity. The piston took less time to stop that means barely endured the pressure exerted
on it, but it held the largest load compared to its thickness. The second chair with a 15mm shell
thickness withstood the smallest load, it was less durable than the other two. The third 20mm
shell thickness resisted the same power as the first and piston take the same time as the second.
The piston moved almost twice the distance from the other two until to break the specimen. The
Ioannis Ntintakis, George Eleutherios Stavroulakis, Nikoletta Lyroni and Niki Plakia
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third chair is more durable than the second one. Generally noticed that regardless of chairs shell
thickness, there were not very large differences in the time required for the piston and the force
exerted. According to the distance that piston passed, there is big difference that shows the third
chair with a 20 mm shell thickness is more durable. However, it was also noticed that after the
tests, there was not much damage to the chairs, which is attributed to their design structure.
About compression tests in table prototypes we come to following conclusions.
The first 10mm wall thick table prototype withstand the smallest force in its mass. But it took
more time and a very close distance to the other two tables until to break. It observed that it had
the smallest strength but the greatest elasticity, while at the same time it had the smallest
thickness than all. The second table prototype of 15mm shell thickness was approximately two
times stronger than the first one, but it took less time and traveled a minimum length from the
piston until to brake. So it is more durable than the first one. The third prototype of 20mm shell
thickness withstand higher force about three times the first. But it destroyed in a much shorter
period. The piston passed higher distance than the other two specimen.
Topology optimization combined with additive manufacturing can change the way that
designers create products. Further research can be done in this interesting research area. There
are so many possibilities and so many open means so lots of interesting research and
development to be done.
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