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Availableatonline
at www.sciencedirect.com
ScienceDirect
ScienceDirect
ScienceDirect
ScienceDirect
Procedia CIRP 00 (2018) 000–000
ProcediaCIRP
CIRP00
00(2017)
(2018)000–000
000–000
Procedia
Procedia
CIRP 79
(2019) 113–118
www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
12th
2018,
12thCIRP
CIRPConference
ConferenceononIntelligent
IntelligentComputation
ComputationininManufacturing
ManufacturingEngineering,
Engineering,18-20
CIRPJuly
ICME
'18
12th CIRP Conference on Intelligent Computation
in Manufacturing
Engineering, CIRP ICME '18
Gulf of Naples,
Italy
Design
Conference,
2018, Nantes,
Methodology28th
forCIRP
design
process
of aMay
snap-fit
jointFrance
made by additive
Methodology for design process of a snap-fit joint made by additive
A new methodology to analyzemanufacturing
the functional and physical architecture of
manufacturing
a
a
a
a
a,
existing
products
for an assembly
oriented
product family
identification
Emilio A. Ramírez
a, Fausto Caicedoa, Jorge Hurela, Carlos G. Helgueroa, Jorge Luis Amayaa,*
Emilio A. Ramírez , Fausto Caicedo , Jorge Hurel , Carlos G. Helguero , Jorge Luis Amaya *
Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat
ESPOL Polytechnic University, Advanced Machining and Prototyping Laboratory CAMPRO, Faculty of Mechanical and Production Sciences Engineering
a
ESPOL Polytechnic University,
Advanced
and Prototyping
Laboratory
CAMPRO,
of Mechanical
andEcuador
Production Sciences Engineering
FIMCP,
Campus Machining
Gustavo Galindo
Km 30.5 Vía
Perimetral,
P.O. BoxFaculty
09-01-5863,
Guayaquil,
FIMCP, Campus Gustavo Galindo Km 30.5 Vía Perimetral, P.O. Box 09-01-5863, Guayaquil, Ecuador
Écoleauthor.
Nationale
d’Arts etE-mail
Métiers,
Arts et jorge-luis.amaya@espol.edu.ec
Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France
* Corresponding
Tel.:Supérieure
+593-42-269-295.
address:
* Corresponding author. Tel.: +593-42-269-295. E-mail address: jorge-luis.amaya@espol.edu.ec
a
* Corresponding author. Tel.: +33 3 87 37 54 30; E-mail address: paul.stief@ensam.eu
Abstract
Abstract
The use of additive manufacturing (AM) technology has been widely adopted due to its facility to produce highly complex
Abstract
The
use ofNevertheless,
additive manufacturing
(AM) technology
has been
widelylimitations
adopted due
to its facility
producecapabilities
highly complex
elements.
elements fabricated
by AM have
dimension
regarding
printerstobuilding
(e.g.
elements.
Nevertheless,
elements
fabricated
by
AM
have
dimension
limitations
regarding
printers
building
capabilities
(e.g.
more volumetric
than
printers
building
chamber).
potential
solution
to divide
elements
in sections, the
which
are
Inelements
today’s business
environment,
thethe
trend
towards
more product
varietyAand
customization
is is
unbroken.
Due
to this development,
need
of
elements
more volumetric
than
printers
building
chamber).
A potential
is to
divideTo
elements
in sections,
which
are
later
3D-printed
and joined
usingthe
snap-fits.
The
present
work
an AMsolution
design
methodology
fordesign
elements´
couplingproduction
by snapagile
and
reconfigurable
production
systems
emerged
to cope
with proposes
various
products
and
product
families.
and
optimize
later
3D-printed
joinedthe
using
snap-fits.
The
present
work
proposes
ananalyze
AM design
methodology
for elements´
coupling
by
snapsystems
as A
well
as and
todivision
choose
optimal
product
matches,
product
analysis to
methods
arethe
needed.
Indeed, design
most
ofinthe
known
methods
aim
to
fit
joints.
model
and
parts
assembly
case
study
is presented
parts
mating
terms
of
joining
features
fit
joints.
Asupport
model
division
and
partsonassembly
caselevel.
study
is presented
tofamilies,
analyze however,
the parts may
mating
design
inin
terms
features
analyze
a product
or one
product
family
the physical
Different
product
differ
largely
termsofofjoining
the number
and
resistance,
material
consumption
and printing
times.
resistance,
support
and printing
times. and choice of appropriate product family combinations for the production
nature
ofThe
components.
This factconsumption
impedes
anB.V.
efficient
comparison
© 2018
Authors.material
Published
by
Elsevier
©
2018AThe
The
Authors.
Published
by
Elsevier
B.V. committee
Peer-review
under
responsibility
ofElsevier
the scientific
of the 12th
Conference
on Intelligent
Computation
in Manufacturing
system.
new
methodology
is proposed
to analyze
existing products
in CIRP
view of
their functional
and physical
architecture.
The aim is to cluster
©
2019
Authors.
Published
by
B.V.
Peer-review
under
responsibility
of
the
scientific
committee
of
the
12th
CIRP
Conference
on
Intelligent
Computation
inof
Manufacturing
Engineering.
these
products
in
new
assembly
oriented
product
families
for
the
optimization
of
existing
assembly
lines
and
the
creation
future reconfigurable
Peer-review under responsibility of the scientific committee of the 12th CIRP Conference on Intelligent Computation in Manufacturing
Engineering.
Engineering.
assembly
systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and
method,
Additive manufacturing,
a Keywords:
functionalDesign
analysis
is performed.
Moreover, aSnap-fit
hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the
Keywords: Design method, Additive manufacturing, Snap-fit
similarity between product families by providing design support to both, production system planners and product designers. An illustrative
example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of
precision.
potential
solution is to divide the elements in
1.
Introduction
thyssenkrupp
Presta France is then carried out to give a first industrial evaluation
of theA
approach.
precision.
Aproposed
potential
solution
is to
elements
in
1.
Introduction
sections,
which
are
later
3D-printed
anddivide
joinedthe
using
snap-fits.
© 2017 The Authors. Published by Elsevier B.V.
sections,
which
are
later
3D-printed
and
joined
using
snap-fits.
Snap-fits
joints are
mechanical features for part mating
Additiveunder
manufacturing
(AM)
a layer-by-layer
fabrication
Peer-review
responsibility
of theisscientific
committee
of the 28th CIRP Design
Conference
2018.
Snap-fits
joints deformation,
are mechanical
featuresa flexible
for part element
mating
Additive used
manufacturing
(AM)
is a layer-by-layer
fabrication
based
on material
in which
technology
to construct
computer-aided
designs
(CAD)
based
on
material
deformation,
in
which
a
flexible
element
technology
used
to
construct
computer-aided
designs
(CAD)
deflects during the assembly and fixes in the mating
Keywords:
Assembly;
Design The
method;
identification
models or
prototypes.
useFamily
of AM
technology has been
deflects
duringAlthough
the assembly
and fixessnap-fit
in the mating
models
or prototypes.
Thefacility
use oftoAM
technology
been
component.
traditional
design
widely adopted
due to the
produce
highly has
complex
component.
Although
traditionalfor polymer
snap-fit injection
design
widely
adopted
due
to
the
facility
to
produce
highly
complex
methodologies
have
been
developed
elements compared to conventional fabrication processes.
methodologies
have studies
been developed
for polymer
injection
elements
compared
to
conventional
fabrication
processes.
applications,
other
have
reported
that
additiveDue to AM technology development, it is possible to
applications,
studies
have
additiveDue to AM
technology
it is
possible(e.g.
to
1.manufacture
Introduction
of
the product other
range
and
characteristics
and/or
manufactured
snap-fits
joints
have reported
a manufactured
similarthat
behavior
to
products
with adevelopment,
growing list of
materials
manufactured
snap-fits
joints
have
a
similar
behavior
to
manufacture
products
with
a
growing
list
of
materials
(e.g.
injected
features
and
that
previous
developed
design
rules
for
assembled
in
this
system.
In
this
context,
the
main
challenge
in
polymers, metal powder, ceramics) depending of model’s
injected
features
and
that
previous
developed
design
rules
for
polymers,
metal
ceramics)
depending
of model’s
dimensioning
applicable
[3].not only to cope with single
Due to requirements
the powder,
fast and
development
in the
domain
of
modelling
and are
analysis
is now
functional
AM technology
capabilities.
dimensioning
are applicable
[3]. oris existing
functional
requirements
AM technology
The objective
of
this article
to present
a systematic
communication
and
an and
ongoing
trend
of capabilities.
digitization
and
products,
a limited
product
range
product
families,
Even though
recent
advances
on AM
permit process
The
objective
of
this
article
toand
present
a systematic
Even
though
recent
advances
on
AM
permit
process
procedure
forable
snap-fit
feature
elements
coupling
digitalization,
manufacturing
enterprises
are facing feasibility,
important
but
also to be
to analyze
anddesign
toiscompare
products
to
define
stability on industrial
applications
and economic
procedure
snap-fit feature design
and mating
elementsparts,
coupling
stability on
and economic
feasibility,
based
on for
operating
of the
part
challenges
inindustrial
today’s applications
market
a continuing
new
product
families. Itconditions
can be observed
that classical
existing
elements
manufactured
by AMenvironments:
technology
have
dimension
based
on printing
operating
conditions
of technology
the matingused,
parts,
part
elements
manufactured
by
AM
technology
have
dimension
geometry,
material
and
AM
in
order
limitations
regarding
printers
building
capacities times
and work
tendency
towards
reduction
of product
development
and
product families are regrouped in function of clients or features.
geometry,
printing
material and
AM technology
used,
in order
limitations
regarding
printers
building
capacities
and
work
to
avoid
the
use
of
fasteners
or
adhesives
as
joining
methods.
piece volumes
[1].lifecycles. In addition, there is an increasing
shortened
product
However, assembly oriented product families are hardly to find.
to avoid
the use of fasteners
or adhesives
as joining
methods.
piece
volumes
[1]. have proposed
Additionally,
procedure
the
Previous
studies
of parts
demand
of customization,
being at the
thedivision
same time
in a greater
global
On
the product the
familyproposed
level, products
differ considers
mainly in two
Additionally,
the
proposed
procedure
considers
the
Previous
studies
have
proposed
the
division
of
parts
greater
subdivision
of
a
single
element,
accounting
general
Design
for
than the printer
building chamber
on a voxelization
competition
with competitors
all overbased
the world.
This trend,
main
characteristics:
(i) element,
the number
of components
and
(ii) the
subdivision
of
a
single
accounting
general
Design
for
than the with
printer
building chamber
based
onmodel
a voxelization
Manufacturing and Assembly (DFMA) criteria’s, which bases
method,
an
interlocking
approach
to
final
assembly
which is inducing the development from macro to micro
type
of components
(e.g.
mechanical,
electrical,
electronical).
Manufacturing
and
Assembly
(DFMA)
criteria’s,
which
bases
method,
with an this
interlocking
to final
model
assembly
on multiple-part design for ease of manufacturing and ease of
[2].
However,
joining approach
method
have
been
reported
as
markets,
results in diminished
lot sizes
due
to augmenting
methodologies
mainly single
onClassical
multiple-part
design forconsidering
ease of manufacturing
andproducts
ease of
[2].
However,forthis
joining
method
have that
beenrequire
reported
as
assembly.
inappropriate
hollow
objects
and
models
further
product
varieties
(high-volume
to
low-volume
production)
[1].
or
solitary,
already
existing
product
families
analyze
the
assembly.
inappropriate for hollow objects and models that require further
To cope with this augmenting variety as well as to be able to
product structure on a physical level (components level) which
2212-8271possible
© 2017 The optimization
Authors. Published
by Elsevier B.V.
identify
potentials
in the existing
causes difficulties regarding an efficient definition and
2212-8271 ©under
2017responsibility
The Authors. of
Published
by Elsevier
B.V.of the 11th CIRP Conference on Intelligent Computation in Manufacturing Engineering.
Peer-review
the scientific
committee
production
system,
it
is
important
to
have
a
precise
knowledge
comparison of different product families. Addressing this
Peer-review under responsibility of the scientific committee of the 11th CIRP Conference on Intelligent Computation in Manufacturing Engineering.
2212-8271©©2017
2019The
The
Authors.
Published
by Elsevier
2212-8271
Authors.
Published
by Elsevier
B.V. B.V.
Peer-reviewunder
underresponsibility
responsibility
scientific
committee
of the
CIRP
Conference
on 2018.
Intelligent Computation in Manufacturing Engineering.
Peer-review
of of
thethe
scientific
committee
of the
28th12th
CIRP
Design
Conference
10.1016/j.procir.2019.02.021
114
Emilio A. Ramírez et al. / Procedia CIRP 79 (2019) 113–118
E. A. Ramírez et al. / Procedia CIRP 00 (2018) 000–000
2.
General Snap-fit design procedures: from
traditional manufacturing to AM
Most studies in snap-fit applications focus on a feature-level
design methodology; this means them often consider the
dimensioning of the deformable beam of the joint and the
locking mechanisms for diverse snap-fit types and crosssectional areas [4]. For instance, software-based tools have
been developed for snap-fit selection and optimization and for
feature evaluation and machinery dimensioning for consumer
products applications [5]. Furthermore, industry manufacturers
have developed snap-fit feature-dimensioning guidelines;
however, these design methodologies do not consider the
location and orientation of snap-fit features nor additional
elements of the joint mechanism [6, 7].
Early studies have been made regarding a methodical
procedure for an integral snap-fit joint design [8]; which
considers the locating features for part-mating positioning, the
snap-fit dimensioning and locking features, and additional
enhancement features design for assembly and disassembly
enablers, among other characteristics.
Concerning AM technologies, recent studies have stated that
the working principles of traditional snap-fit design prove to be
independent of the manufacturing process and have proposed
additive manufactured snap-fit design guidelines in which
previous methodology needs to be adapted to the restrictions of
the new manufacturing technology [3].
Seepersad, et al. [9] states that current design guides often
focus exclusively on process limitations instead of evaluating
the possible advantages in part fabrication, and, although this
can contribute to a poor utilization of the new manufacturing
technologies capabilities, there is still the need for an in-depth
evaluation of design conditions or principles. One of the many
considerations in AM applications is the orientation of the
printed pieces relatively to the building plates, as the discrete
characteristic of the layer-by-layer fabrication process is a
source for anisotropic properties. Previous studies have
developed a systematic approach for part orientation based on
a qualitative analysis of the concept designs [1].
The available literature seems to suggest the need of further
studies in developing design guidelines for integral snap-fit
joint fabrication adapted to some of the current fabrication
characteristics of AM technology. This present study does not
propose an exhaustive analysis for process characteristics
regarding a specific AM technology, but accounts for the
general considerations and the limitations that seemed to have
the most relevance.
made separately and there has been no formal integration
between both design stages. Consequently, our study considers
the integration of the conceptual and detail design, on which
the results for the conceptual stages are used on the design
phases.
The proposed design methodology for Snap-fit systems
design process made by AM technology is shown on Fig. 1.
The flowchart starts with the input variables of performance
and manufacturing conditions labeled as Design Specifications.
The conceptual design stage includes the part mating design
and the Snap-fit systems type selection and location.
The detail design section is formed by three independent
procedures, which are material deflection limits calculation
based on the joint performance conditions, deflection
mechanism geometry dimensioning, and, retention mechanism
geometry dimensioning.
3.
Additive-manufactured Snap-fit design process
Methodology
A systematic procedure for additive-manufactured snap-fit
systems can be developed accounting for general design
guidelines. Common design methodologies are formed by four
main stages: performance conditions or design specifications,
conceptual design, detail design, and lastly a design validation
stage. Once design process has been validated, the final product
can be manufactured.
Previous studies have established methodologies for Snapfit systems design in a conceptual framework and in a detail
design framework [8]; however, these approaches have been
Fig. 1. Proposed methodology of design process for snap-fit joint.
Emilio A. Ramírez et al. / Procedia CIRP 79 (2019) 113–118
E. A. Ramírez et al. / Procedia CIRP 00 (2018) 000–000
A design validation stage is also considered to give a
feedback to both conceptual and detail design phases. This
validation is formed by a feature deformation analysis based on
material limits and deflection mechanism geometry, an
ergonomics check for ease of part mating based on assembly
and disassembly forces, and a final evaluation of the retention
features for the snap-fit joint.
The final stage of the process corresponds to the 3D printing
manufacturing of the pieces with the added designed and
validated snap-fit joints. Each node of the flowchart is detailed
on the following sections.
Before describing the methodology in detail, it is necessary
to know that the design process could be linear from an ideal
standpoint but is iterative in most cases. Considering that the
iterative condition could occur in the detail design or design
validation stages, the initial design is subject to change.
3.1.
Performance Conditions
The present study focuses on four main design
specifications or performance conditions of the snap-fit system,
which are AM technology, printing material, model geometry
and snap-fit performance conditions. These restrictions are
considered as input variables to the methodology and will not
change during the design process.
AM technology considerations refers to machinery specific
constraints. Parameters such as printing layer thickness and
building chamber dimensions affect directly to the model
partitioning in the part mating design stage. Also, it is important
to evaluate the printing procedure beforehand to know the
limitations on small features printing and the need of support
structures, as this result could impact in the posterior
dimensioning of the snap-fit system.
Regarding printing material, mechanical properties
characterization is needed for the detail design stages such as
in material deformation limits calculation and deflection and
retention mechanisms dimensioning.
Overall model geometry needs to be accounted as it can
limit the partitioning approach and the location of the snap-fit
features due to aesthetic or functional requirements. Model
wall thickness and weight influence the final joining design.
The last design specification considered in the proposed
design process corresponds to the performance condition of the
snap-fit system. This input refers to operational restraints such
as in the need of a permanent joint, or if the joint will be subject
to frequent assembly and disassembly motions. These
requirements can influence both conceptual and detail design
stages.
3.2.
Conceptual Design
After the definition of the input parameters to be used, the
conceptual design stages take place. This preliminary design
constitutes the initial framework for the mating features.
3.2.1.
Part Mating Design
This first conceptual design stage refers to the subdivision
of the initial model. As stated before, a primary partitioning
method can be selected based on the general model
configuration, and the building chamber volume of the printer.
115
Although, current 3D printing capabilities offers
manufacturing advantages over traditional manufacturing
methods, the final printing approach can be further optimized
by accounting for the additive manufacturing technology
characteristics. A possible method of optimization is to
consider the part printing orientation of the model sections.
From the primary partitioning, the designer can evaluate the
subdivision according to an efficient partitioning approach,
which depends on support material utilization and printing
times, and offers a quantitative validation. Both considerations
of the efficient partitioning approach can be competing
measures, as one orientation can use less support material but
complete in a greater printing time.
The anisotropy of the parts associated with the 3D-printer
building directions could be a considerable restriction;
therefore, the printing approach, as well as model wall
thickness and model overhangs, can also affect the selected
partitioning method and the posterior snap-fit features location.
3.2.2.
Snap-fit System Type and Location selection
The snap-fit system, as an assembly mechanism, consists on
locking features, locating features and enhancement features
[8].
Locking features are responsible for restricting the
movement between mating parts in the assembly direction (i.e.
one degree of freedom DOF). The most common types of
locking features are the cantilever, torsional and annular snapfit joints. The locking feature can be further divided in two
mechanisms: the deflection mechanism and the retention
mechanism.
The deflection mechanism is chosen based on the mating
part geometry and is deflected to couple the retention
mechanism. Depending the desired operational deflection, it
will have a different mechanical behavior and resistance. The
retention mechanism is defined by the assembly/disassembly
operation and it produces the interference between the mating
parts.
According to the snap-fit joint type, the location of the
locking feature should be oriented in order to distribute the
principal stresses along the most resistant building direction.
Locating features (e.g. locators, catches) are used to
constraint the remaining DOF, guide the locking features, and
establish a reference between the mating parts.
Enhancement features are the attributes of locking and
location features that ease the performance of the snap-fit joint
and the manufacturing (e.g. assembly or disassembly aid).
3.3.
Detail Design
With the results of the conceptual design stages, in depth
analysis of the used material and the dimensioning of the snapfit features is considered in the detail design stages. For the
following subsections, a general guideline for feature design is
proposed, the authors encourage the review of available
literature [6, 7, 10], regarding general snap-fit features design.
3.3.1.
Material Deformation Limits
For establishing material deformation limits, a primary
design strain of the joint can be considered depending on the
Emilio A. Ramírez et al. / Procedia CIRP 79 (2019) 113–118
E. A. Ramírez et al. / Procedia CIRP 00 (2018) 000–000
116
presence of a definite yield point in the printing material stressstrain curve. The need of material characterization data in this
stage is crucial as it can affect the posterior joint deformation
validation, thus compromising the previous conceptual design.
In addition, the performance conditions of the joint, such as
the number of cycles for frequent assembly and disassembly
motions, or the position of the snap-fit features relatively to the
part walls, affect as a factor to the prior design strain.
Furthermore, this factor will also vary if the material has a
flexible or brittle behavior.
The maximum permissible strain of the joint is equal to the
design strain divided by a stress concentration factor. This
stress concentration is located in the interface of the snap-fit
and the wall where it is mounted, acting in the surface subjected
to bending deformation. For diminishing the effects of stress
concentration, literature suggests the use of fillets of a value of
50% the snap-fit feature thickness, which produces a stress
concentration factor of 1.5 in these interfaces.
3.3.2.
Deflection Mechanism Dimensioning
The geometry of the deflection mechanism can be defined
as a function of the snap-fit feature thickness; however, the
available literature lacks in information for the proper selection
of this dimension.
Nevertheless, an initial design value can be selected based
on the positioning of the feature relatively to the part wall
where it is mounted. If the feature extends from a wall, the
feature thickness can equal to the wall thickness. For features
protruding from a wall, literature suggests a factor of 50% to
60% of wall thickness for feature thickness values, based on
tests for filling and flow problems, as well as cooling problems,
on injection molding applications [10].
3.3.3.
Retention Mechanism Dimensioning
The final dimensioning stage is the definition of the
retention mechanism, which is comprised of a sloped surface
that engages the part during the mating procedures and permits
or inhibits the removal of the parts. The overhang of the
retention mechanism can be defined as a function of the feature
length to thickness ratio.
Common values for insertion angles tends to be less than 45
degrees, as greater values make the joining difficult to
assembly. For the disassembly interfaces, the angles will vary
depending if the joint needs to be releasing or non-releasing.
Typical angular values of the sloped removal surfaces for nonreleasing joints are above 80 degrees.
For a releasing joint, an additional consideration is the
presence of external loads along the mating directions. If no
loads are present, the retention angle can be equal to a threshold
value that accounts for part weight effects and is a function of
the material friction coefficient. If external loads are applied,
the retention angle should have a value in between the threshold
value and 80 degrees.
3.4.
Design Validation
The design validation stages are of great importance as it
compromises a series of tests or performance conditions
evaluation to ensure the successful application of the snap-fit
joint system in the final part joining, prior to the additive
manufacturing production.
3.4.1.
Deformation Validation
The deformation validation stage compares the strain
obtained while deflecting the snap-fit feature with the
permissible feature strain defined on the material deformation
limits. The feature strain depends on the snap-fit feature type
geometry. For some feature configurations, as in cantilever
snap-fit joints, the feature deflection and strain and can be
obtained based on beam theory calculations.
Additional factors of feature positioning relatively to part
wall and corrections factors due to mating parts deflection in
finite element simulation should also be considered for a final
strain value calculation.
If the final strain on the features is greater than the
permissible strain limits, corrections should be made on the
conceptual design stages or on the snap-fit feature geometry
values, depending on the amount of the difference.
For small deviations, a simple feature dimension correction
can be sufficient, but for greater values a more in-depth
analysis of the feature location and/or part mating orientation
is needed. In the flowchart, the unacceptable condition is
connected to the part mating design as it corresponds to the
worst-case scenario.
3.4.2.
Assembly and Disassembly forces calculation
The force needed to deflect the snap-fit features depends on
feature geometry and type. As this deflection force acts on the
sloped surfaces of the joint during the union or removal of the
parts, the assembly and disassembly forces are calculated as the
projected force along the mating or release direction.
The assembly and disassembly forces are a function of the
deflection force, material friction coefficient and the effective
angle of the slope considering mating simulations correction
factors. This correction factor accounts for actual part
deformation, as the general equations assumes that the only
deformable element is the snap-fit feature. The available
literature [6, 7, 10] have additional information regarding
calculation formulas.
3.4.3.
Ergonomics Validation
The ergonomics validation is made with the results from the
assembly forces calculation. This validation consists on the
comparison of the force needed to join the parts and acceptable
forces for manual assembly.
Lee and Gu [11] reported a mean value of roughly 81 N for
acceptable insertion forces in manual assembly of small
connectors, and a mean maximum force of 141 N. It is also
noted that acceptable and maximum coupling forces depend on
the posture and size of the mating parts.
If the assembly forces surpass the acceptable insertion
forces, the proposed methodology suggest the evaluation of the
predefined snap-fit feature dimensions. A possible correction
approach is to vary the feature width as it does not affect the
feature deflection results but lowers the required deflection
forces.
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E. A. Ramírez et al. / Procedia CIRP 00 (2018) 000–000
3.4.4.
117
Retention Performance Evaluation
The final design evaluation corresponds to the comparison
of the calculated disassembly forces to the expected joint
feature resistance.
If the disassembly forces are not greater enough to ensure a
reliable joint, the retention angle should be increased thus
increasing the retention capability. This correction does not
interfere with the general feature dimensioning. An additional
consideration in this phase is the resistance of the sloped area,
as it can fail by shear forces during part releasing.
3.5.
Additive Manufacturing Test
Once the detail design has been validated, the final stage of
the proposed methodology is the 3D-printing of the model parts
and physical assembly procedures.
4.
Case Study: A 1-gallon plastic container
The printing of a 1-gallon plastic container is considered for
evaluation of the design methodology. The design
specifications for the manufacturing of the model are based on
MultiJet Printing technology using ABS (Acrylonitrile
Butadiene Styrene) plastic for model part construction, with the
utilization of permanent joints that does not affect the overall
outward appearance.
The 3D-printer used for the construction of the elements was
a 3DSystems ProJet 3510 SD, with a building chamber of
298.45x185.42x208.78 mm for the X, Y and Z directions
respectively. The model gross dimensions are 277x115x206
mm, but the selected printing equipment needs an additional
5.48 mm for a removable base in the Z building direction,
therefore, the model does not fit in the building chamber and
needs to be printed in sections. For the part mating design, three
alternatives for model division were tested: a 2-part division, a
3-part division and a 6-part division, which are shown on Fig.
2. These approaches were evaluated in terms of support
material consumption and estimated printing time, giving more
importance on material economy.
The results for support material consumption and estimated
printing times of each alternative are reported on Table 1, the
percentages shown for support material reduction are compared
to the original model support requirements, obtained from the
printer software.
The first division approach (2-part division) was discarded
due to the relatively low performance in terms on material
economy. Although the 3-part division have a better overall
performance in printing times and support material reduction,
further analysis of the part geometries indicate that the snap-fit
joints could not be located perpendicular to the building
direction, resulting in poor mechanical performance of the
deflection mechanisms. Additionally, this type of division
affected the overall model aesthetic with divisions along
symmetry planes.
Even though the estimated printing times for the 6-part
division were longer that the other alternatives, it accounts for
a better snap-fit joints location compared to the previous
alternatives, with a relatively good support material economy.
Also, this approach took advantage on the model surfaces and
details to have a clean division.
Fig. 2. Mating Part Design: Model division alternatives.
Table 1. Support Material consumption for model division alternatives
Division alternatives
Support
Material
Reduction*
Estimated
Printing Time
(hours)
Alternative A: 2-part Division
66%
66
Alternative B: 3-part Division
74%
62
Alternative C: 6-part Division
73%
84
* Compared to original model support material requirements.
The part mating sequence for the 6-part division starts with
the union of the parts C2 and C3 (Fig. 2). Both parts then lock
to the part C1, followed by the part C4. The remaining parts
(C5 and C6) are first joined together, and the locked to the main
body.
Loop-style cantilever locks were considered for the snap-fit
systems, and they were located in the parts perpendicular to the
printing building directions, in order to distribute the deflection
forces along the construction layers. Different sets of locators
and catches were designed to restrict additional DOF for all the
mating parts. An example of the designed snap-fit system,
regarding locks (deflection mechanism and retention
mechanisms), locators and catches, can be seen on Fig. 3, for
the mating interfaces of parts C2 and C5.
The deflection mechanism design was based on a special
case of snap-fit extended from a wall. Instead of extending the
features from the wall, they were designed to extend parallel to
the wall, thus preserving the outside model appearance and
locating all mating features inside the body.
From the required performance conditions of the snap-fit
system, and building material, the maximum allowable strain
was calculated and compared to the design strain obtained by
the feature dimensions, in order to make a first validation of the
deflection mechanism geometry.
The assembly and disassembly forces were calculated based
on the available literature [6, 7, 10]. A value of nearly 3.3 N per
feature for insertion forces was determined, which is less than
the acceptable insertion forces [11], thus proving to be
acceptable for the ergonomics validation stage. For the final
design validation stage, the retention mechanism was evaluated
mainly for shear stresses.
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E. A. Ramírez et al. / Procedia CIRP 00 (2018) 000–000
118
Fig. 3. Examples for locks, locators and catches features.
By accounting for part surfaces and geometries, the final
part mating and division approach resulted in a clean division
which does not affect the final model aesthetics and can ease
the location of the snap-fit system features.
Although there was a substantial reduction in support
material consumption compared to the original model, the
printing times augmented in the final part division iteration.
Even though the economic analysis is out of the scope of this
study, the final mating design iteration yielded an additional
reduction in printing costs. Further analysis for manufacturing
cost regarding cost of material and cost of equipment utilization
is needed.
The inclusion of the snap-fit system to the model parts had
an increase of 2.1% of part material utilization, which had little
effect on the overall part weight. Additionally, in some cases
these features served as stiffeners for vertical extended plates
such as in part C2, as shown on Fig. 3.
Future works aims to include a more in-depth analysis
regarding design specifications for the proposed methodology.
Additionally, the inclusion of a more detailed process for detail
design stages is needed in order to formally include finite
element analysis for deflection corrections and final assembly
and disassembly forces calculation.
Acknowledgements
The authors would like to thank the Advanced Machining
and Prototype Laboratory CAMPRO, from ESPOL
Polytechnic University.
References
Fig. 4. 1-gallon plastic container made by AM: (a) Printed parts, Final
model assembly (b) Isometric view and (c) Lateral view, Joints and locators
for parts (d) C2 and C5, and (e) C1 and C2.
A final analysis for the model parts was to evaluate the
impact of the addition of the additional features of locks,
locators and catches. The inclusion of the snap-fit system
represents an increase of 2.1% for part material utilization, an
increase of 1.8% for support material utilization, and an
increase of 3.5% of the overall estimated printing time.
With the validated design, the parts were 3D-printed and
then assembled following the joining procedure described.
Images of the printed parts and model assembly are shown on
Fig. 4.
5.
Conclusions
The employment of the proposed methodology for snap-fit
systems design permitted to 3D-print and assemble a model of
a 1-gallon plastic container that could not be printed in a single
job due to 3D-printing building chamber limitations.
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