Snap Fit Design by Neeraj Kumar Jha

1. Seminar -1
presentation
on
Snap Fit Design
PreparedBy:
Neeraj Kumar Jha
(www.erway.in)

2. My Presentation
1. Introduction:
- what is snap fit
- types
- applications
2. Design features:
- Theory
- design
- material selection
3. Modifications

3. 1. Introduction
Assembly techniques for Plastic parts are:
•Allowing disassembly (Mechanical Fasteners, Plastic
Threads, Press-Fittings and Snap-Fits)
•Creating a permanent joint i.e. welding

4. 1.1 what is snap fit
•Snap-fits are the simplest, quickest and most
cost effective method of assembling two
parts.
•Snap fitting provides a simple, inexpensive
and rapid means of assembling plastic parts.
In such assembly technique basically, a
molded undercut on one part engages a
mating lip on the other

5. 1.2 Types
Types of snap -fits are:
1. Annular Snap-Fit:
Those with a full cylindrical undercut and mating
lip

6. Types.......
2. Cantilever Snap-Fit:
Those with flexible cantilevered lugs

7. ...........Types
3.Ball and socket joint Snap-Fit:
Those with spherical undercut

8. 1.3 Applications
•Toys ,Small Appliances, Automotive, Electronic Fields

9. 2. Design features
Designing a snap-fit is rather complex due to a
combination of factors:
•Functional requirements of the product.
•Assembly requirements.
•Mechanical properties of the thermoplastic.
•Design of the mold and notably part ejection.
•Integrated Snap Fits
•No extra part
•Simple & reliable
•Design freedom

10. 2.1 Theory

11. Factors………..
•Maximum allowable short-term strain (During assembling):
•Creep and stress relaxation:
•Stress concentrations
•Coefficient of friction:
•Lead angle and return angle:
•Mating force and separation force:

The mating force F
a
required to assemble
F
a
= F
b
(µ +tan a
1
)/( 1- µ tan a
1
)
Where:
F
b
= deflection force
µ= coefficient of friction
a
1= lead angle
The same formula is used for the separation force F
d
required to disassemble, but
then with the return angle a
2
instead of a
1.

12. 2.2 Design
•Cantilever beam with constant rectangular cross section:
•Deflection, Y = 2L
2
× (max. allowable strain)/3t

•deflection force, F
b= w t
2
E
s (max. allowable strain)/ 6L

13. Design...........
•Tapered beams with a variable height:
Y = c.2L
2
× (max. allowable strain)/3t
1
F
b
= w t
1
2
E
s
(max. allowable strain)/ 6L
•L = length of the beam
E
s =secant modulus
w = width of the beam
c = multiplier
t
1 = height of cross section at fixed end of beam
The formula for the deflection y contains a multiplier c that depends on value
t
1
/t
2.

14. 2.3 Material Selection
The ultimate success of any design also depends on selecting the
material that best fulfills all requirements of a specific application.
the ideal material is thermoplastic because of
high flexibility
relatively high elongation
low coefficient of friction
sufficient strength and rigidity and
its ability to be easily and inexpensively molded into complex
geometries.
Thermoplastics are ideal for
 integrative designs,
permitting one-piece molding of complex geometries, which
consolidates multiple parts into one.

15. 3. Modifications

16. Modifications...

17. Modifications...

18. Modifications...

19. Modifications...

20. Take care of…………..
Aim for uniform wall thickness.Aim for uniform wall thickness.
Design wall thickness as thin as possible and only as Design wall thickness as thin as possible and only as
thick as necessary.thick as necessary.
Use ribbing instead of greater wall thickness.Use ribbing instead of greater wall thickness.
Provide radiousing.Provide radiousing.
Provide demoulding tapers.Provide demoulding tapers.
Avoid undercuts.Avoid undercuts.
Do not design to greater precision than required.Do not design to greater precision than required.
Design multi-functional components.Design multi-functional components.
Use economic assembly techniques.Use economic assembly techniques.
Gate moulding on the thickest wall.Gate moulding on the thickest wall.

21. .
Thank you.
(www.erway.in)