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DSM Engineering PlasticsTechnical Guide
Date:23 February, 2005
All information supplied by or on behalf of DSM in relation to
its products, whether in the nature of data, recommendations or
otherwise, is supported by research and, in good faith, believed
reliable, but DSM assumes noliability and makes no warranties of
any kind, express or implied, including, but not limited to, those
of title, merchantability, fitness for a particular purpose or
non-infringement or any warranty arising from a course ofdealing,
usage, or trade practice whatsoever in respect of application,
processing or use made of the aforementioned information or
product. The user assumes all responsibility for the use of all
information provided andshall verify quality and other properties
or any consequence from the use of all such information.
1
Snap fit theoryA snap-fit is an effective method to design the
fastening system into the product design itself. A snap-fit canbe
designed to allow parts to be either permanently fastened (or
pre-determined to be broken off) or forfrequent assembly and
disassembly.
In combination with O-rings or proper seals, even gas and fluid
tight connections can be made.
Designing a snap-fit is rather complex due to a combination of
factors:
- the functional requirements of the product- the assembly
requirements- the mechanical properties of the thermoplastic- the
design of the mold and notably part ejection.
Snap-fits can be found in a wide variety of shapes. Three
examples of typical snap-fit geometries are thecantilever beam
type, the cylindrical type and the spherical type.
Snap-fit cantilever beam type
a b
Snap-fit cylindrical type
a b
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DSM Engineering PlasticsTechnical Guide
Date:23 February, 2005
All information supplied by or on behalf of DSM in relation to
its products, whether in the nature of data, recommendations or
otherwise, is supported by research and, in good faith, believed
reliable, but DSM assumes noliability and makes no warranties of
any kind, express or implied, including, but not limited to, those
of title, merchantability, fitness for a particular purpose or
non-infringement or any warranty arising from a course ofdealing,
usage, or trade practice whatsoever in respect of application,
processing or use made of the aforementioned information or
product. The user assumes all responsibility for the use of all
information provided andshall verify quality and other properties
or any consequence from the use of all such information.
2
Snap-fit spherical type
Theory
The force-deflection diagram
In the general case, both parts will be deformed during
assembling, as shown in the figure below. Part 1 isbent downwards
over a distance y1, part 2 is bent upwards over a distance y2 and a
deflection force Fb acts
between the two mating parts.
Both parts are deformed
A force-deflection diagram as shown in the figure on next page
can be a useful aid for the engineer todetermine how the total
deflection will be distributed over the two parts.
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DSM Engineering PlasticsTechnical Guide
Date:23 February, 2005
All information supplied by or on behalf of DSM in relation to
its products, whether in the nature of data, recommendations or
otherwise, is supported by research and, in good faith, believed
reliable, but DSM assumes noliability and makes no warranties of
any kind, express or implied, including, but not limited to, those
of title, merchantability, fitness for a particular purpose or
non-infringement or any warranty arising from a course ofdealing,
usage, or trade practice whatsoever in respect of application,
processing or use made of the aforementioned information or
product. The user assumes all responsibility for the use of all
information provided andshall verify quality and other properties
or any consequence from the use of all such information.
3
Force-deflection diagram
The undercut h of the snap-fit determines the total deformation
y1 + y2 in this diagram and the springcharacteristic (stiffness) of
both parts determines the deflection force Fb.
Secant modulus
The spring characteristic of the parts must be calculated from
the part dimensions and the material stiffness E.
Young's modulus E0 may be used as long as the strains remain in
the proportionality range of the stress-straincurve, but for larger
strains the secant modulus Es should be used. The figure on page 4
shows the definitionof Es.
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DSM Engineering PlasticsTechnical Guide
Date:23 February, 2005
All information supplied by or on behalf of DSM in relation to
its products, whether in the nature of data, recommendations or
otherwise, is supported by research and, in good faith, believed
reliable, but DSM assumes noliability and makes no warranties of
any kind, express or implied, including, but not limited to, those
of title, merchantability, fitness for a particular purpose or
non-infringement or any warranty arising from a course ofdealing,
usage, or trade practice whatsoever in respect of application,
processing or use made of the aforementioned information or
product. The user assumes all responsibility for the use of all
information provided andshall verify quality and other properties
or any consequence from the use of all such information.
4
The definition of the secant modulus Es
The strain will vary from place to place, so that the
calculation should in fact be done using several secantmoduli. This
is not feasible for a hand calculation, in that case the engineer
will normally use an averagesecant modulus. One of the advantages
of a finite element calculation is that the complete stress-strain
curveof a material can be used as input, with the computer
determining the strain and modulus for every point ofthe
construction.
Maximum allowable short-term strain during assembling
If a snap-fit fails during assembly, the maximum deflection of
the cantilever beam most likely exceeded thedeflection limit of the
thermoplastic used. The maximum strain that occurs during
assembling can becalculated for both parts if the force-deflection
diagram on page 3 is known.Since the snap-fit is only a small part
of a product, it is better to design snap-fit dimensions based on
athermoplastic chosen than to choose the thermoplastic to make a
specific snap-fit work.
Creep and stress relaxation
Internal loads in the snap-fit connection after assembly should
be avoided if possible, due to possible creepand stress relaxation.
A graph with isochronous stress-strain curves gives information
about the creep and
stress relaxation that will take place, as shown in the figure
on page 5.
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DSM Engineering PlasticsTechnical Guide
Date:23 February, 2005
All information supplied by or on behalf of DSM in relation to
its products, whether in the nature of data, recommendations or
otherwise, is supported by research and, in good faith, believed
reliable, but DSM assumes noliability and makes no warranties of
any kind, express or implied, including, but not limited to, those
of title, merchantability, fitness for a particular purpose or
non-infringement or any warranty arising from a course ofdealing,
usage, or trade practice whatsoever in respect of application,
processing or use made of the aforementioned information or
product. The user assumes all responsibility for the use of all
information provided andshall verify quality and other properties
or any consequence from the use of all such information.
5
Creep and stress relaxation
The isochronous stress-strain curves can be found in the DSM
material database. Select a material grade firstby clicking on the
grade name, then click on "PROPERTIES" and "Fct" (functions). If
the desired curves arenot available for a material grade, the
curves of a comparable grade can be used. Click here to go to
thematerial database.
If a certain pre-stress cannot be avoided, as the connection has
to resist an external load, this pre-stressshould be minimized. The
designer should be aware that both the possibility of breakage and
the requiredforce to (dis)assemble can be dealt with independently.
In most cases the number of snap-fits can bechanged.
Stress concentrations
A common factor causing failure of a snap-fit is the inside
radius r (see figure on page 2) in transitions or lackthereof. An
inside radius which is too small will induce stress-concentrations.
These sections with highstresses are often weak because the strain
limit is reached sooner. A radius r = 0.5 mm is satisfactory in
mostcases.
Coefficient of friction
The mating force Fa required to assemble and the separation
force Fd required to disassemble the snap-fit aredetermined by
several parameters. One of them is the coefficient of friction ,
which characterises the frictionforces which must be overcome.
For information about this coefficient please click here.
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DSM Engineering PlasticsTechnical Guide
Date:23 February, 2005
All information supplied by or on behalf of DSM in relation to
its products, whether in the nature of data, recommendations or
otherwise, is supported by research and, in good faith, believed
reliable, but DSM assumes noliability and makes no warranties of
any kind, express or implied, including, but not limited to, those
of title, merchantability, fitness for a particular purpose or
non-infringement or any warranty arising from a course ofdealing,
usage, or trade practice whatsoever in respect of application,
processing or use made of the aforementioned information or
product. The user assumes all responsibility for the use of all
information provided andshall verify quality and other properties
or any consequence from the use of all such information.
6
Poissons ratio
Poissons ratio must be known to calculate the surface pressure
and the stresses in a cylindrical snap-fit. Click
here for information about this ratio.
Lead angle and return angle
The lead angle 1 and the return angle 2 determine the required
mating force respectively the requiredseparation force, besides the
dimensions of the snap-fit, the material stiffness and the friction
coefficient.The lead angle 1 is normally between 15 and 30.The
return angle 2 determines the maximum load that the snap-fit can
take up. The maximum load bearingcapacity is reached for a return
angle of 90. The return angle determines if the connection will be
separableor inseparable.
Separable and inseparable joints
2 + < 90: separable joint2 + > 90: inseparable joint
= tan = coefficient of friction
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7/29/2019 Snapfit Theory PLASTIC
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DSM Engineering PlasticsTechnical Guide
Date:23 February, 2005
All information supplied by or on behalf of DSM in relation to
its products, whether in the nature of data, recommendations or
otherwise, is supported by research and, in good faith, believed
reliable, but DSM assumes noliability and makes no warranties of
any kind, express or implied, including, but not limited to, those
of title, merchantability, fitness for a particular purpose or
non-infringement or any warranty arising from a course ofdealing,
usage, or trade practice whatsoever in respect of application,
processing or use made of the aforementioned information or
product. The user assumes all responsibility for the use of all
information provided andshall verify quality and other properties
or any consequence from the use of all such information.
7
Mating force and separation force
The mating force Fa required to assemble can be calculated with
the following formula.
+ tan 1Fa = Fb . -----------------
1 . tan 1
where
Fb = deflection force = coefficient of friction1 = lead
angle
The same formula is used for the separation force Fd required to
disassemble, but then with the return angle2 instead of1.
Fb can be calculated as explained in the bulletin on design of
snap fits.
