Improving Snapfit Design (Part 1)

By Chul Lee, BASF

ABSTRACT: This series of articles provides a basic overview of snap-fit design types and their applications, as well as traditional and improved formulas for snap-fit strength and assembly force.  The series also reviews common causes of snap-fit failure and how to overcome them, appropriate materials and processing considerations.  The articles provide links to BASF’s Snap-Fit Design Manual for a downloadable document with discussions of the issues presented here, access to an on-line tool of BASF’s improved formulas for calculating snap-fit design’s deflection and strain, enrollment information for BASF’s Plastics Seminar “Part Design - Assembly of Components,” and reprints of technical articles pertinent to various snap-fit topics.

Welcome to the first in a series of five articles on Snap-Fit design presented by BASF Engineering Plastics.  Our intent is to help design engineers achieve better results through the use of improved principles and procedures for snap-fit design, application, processing and fabrication.

IMPROVING SNAP-FIT DESIGN

PART I - IMPROVING SNAP-FIT DESIGN:  GENERAL APPLICATION AND TYPES

Introduction
Snap-fit designs are the simplest, quickest and most cost-effective method of assembling two parts.  No screws.  No rivets.  No welds.  And, after assembly, faster disassembly and servicing of components.

Thermoplastic materials possess many characteristics and features that are ideal for snap-fit designs: 

            • High flexibility
            • Integrative designs enabling the molding of complex geometries
            • Relatively high elongation
            • Low coefficient of friction
            • Strength and rigidity sufficient for most applications. 

Properly designed, thermoplastic snap-fits can be assembled, disassembled and reassembled several times – without adverse effect on the functional integrity of the snap-fit.  If materials are prone to relax over time, snap-fits can be designed to self adjust and maintain sealing capability.

From the broader perspective of sustainability, snap-fit’s ease of disassembly simplifies the recycling of components made from different materials, thereby encouraging more environmentally friendly behavior throughout the lifecycle of components and end products.

Still, various factors keep some snap-fit designs from delivering the full benefits of their potential.  BASF hopes to help overcome such obstacles through this series of articles that will provide designers a basic overview of the types of snap-fit designs, applications and principles, improved methods, and guidelines for avoiding common pitfalls in snap-fit design, processing and materials selection.

Most Common Types Most snap-fit applications use one of three cantilever designs: Straight beam cantilever U-shaped cantilever L-shaped cantilever.  The straight beam cantilever design is broadly applicable.  U- and L-shaped cantilevers, on the other hand, are specifically used in applications where space restrictions constrain beam geometry.  Cantilever structures have been around for a long time.  Design engineers know them from their common use in bridges and such architectural structures as overhangs and balconies, not to mention their popularity in single-wing aircraft design.  Applications for snap-fit cantilever designs now range anywhere from mundane tamper-proof aspirin bottle-and-cap assemblies  to highly engineered, rugged parts such as power tool housings, automotive wheel covers, air cleaner housing and door handle assemblies to name just a few. Traditional materials for cantilever structures – such as stone and metals – are strong and rigid.  Evaluation based solely on those two properties, however, overlooks important elements of cantilever design.  Use of thermoplastics introduces additional concepts to the mix of criteria.  To take advantage of the true strengths of thermoplastics for snap-fits, designers must factor in new considerations that are the subjects of the second and third articles of this series: “Principles of Classical Beam Theory and Design” and “Improved Cantilever Design” Part II will explore the traditional formulas for calculating cantilever stresses and strain.  Part III will present improved formulas that provide precise calculations applicable in the use of thermoplastics.

BASF Plastics Seminar: Part Design - Assembly of Components (recorded course)
This seminar is for engineers and designers involved in the design of injection molded components. The course provides an overview of various assembly methods of plastic-to-plastic components and plastic-to-metal components. Techniques covered are snap-fits, self-threading fasteners, welding, and press-fitting. Advantages and limitations of each will be discussed in detail, as well as several examples with actual design calculations. This seminar will help educate the engineer/designer about which assembly method to use for their particular application, allowing them to design a cost-effective and efficient joint the first time.

For additional information or to ask questions about Snapfit Design, contact:

• Chul Lee: chul.lee@basf.com
• Emile Homsie: emile.homsi@basf.com

September 19, 2007

• Part 1: Introduction and Overview of General Applications and Types
• Part 2: Principles of Classical Beam Theory and Design
• Part 3: Improved Cantilever Design and Guidelines to Avoid Common Difficulties
• Part 4: Materials Selection