snap fit design by neeraj kumar jha

Seminar -1presentation

on

Snap Fit DesignPreparedBy:

Neeraj Kumar Jha(www.erway.in)

My Presentation

1. Introduction: - what is snap fit - types - applications2. Design features: - Theory - design - material selection3. Modifications

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

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

1.2 Types

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

lip

Types.......

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

...........Types

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

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

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

2.1 Theory

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 Fa required to assemble Fa = Fb (µ +tan a1)/( 1- µ tan a1)Where: Fb = deflection force µ= coefficient of friction a1= lead angleThe same formula is used for the separation force Fd required to disassemble, but

then with the return angle a2 instead of a1.

2.2 Design

• Cantilever beam with constant rectangular cross section:

• Deflection, Y = 2L2× (max. allowable strain)/3t

• deflection force, Fb= w t2 Es (max. allowable strain)/ 6L

Design...........

• Tapered beams with a variable height:

Y = c.2L2× (max. allowable strain)/3t1

Fb= w t12Es (max. allowable strain)/ 6L

• L = length of the beam Es =secant modulus

w = width of the beam c = multiplier t1 = height of cross section at fixed end of beam

The formula for the deflection y contains a multiplier c that depends on value t1/t2.

2.3 Material SelectionThe 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.

3. Modifications

Modifications...

Modifications...

Modifications...

Modifications...

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.

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Thank you.(www.erway.in)