Selecting a Locking Feature

Locking feature selection should be a calculated decision based on an understanding of the application’s needs. The previous section described a number of locking feature styles. An important question is now ‘‘How do I know which one to use?’’ The answer to that question depends on a number of conditions in the application. They are:

The desired ‘‘Function’’ of the lock feature. See the discussion in Section 2.3.1, Chapter 2.
The ‘‘Demand’’ level of the application. See the definitions in Section 9.2 and the

discussion in Section 9.6.3 in Chapter 9.

The application’s grip length. ‘‘Grip length’’ is the distance from the retention face of the locking feature to the opposite (reacting) surface. For a cantilever hook-style lock, it is the distance from the wall or edge at the base of the beam to the retention face of the catch at the end of the beam. A short grip length will rule out use of the cantilever hook style. A panel attaching to an opening in a relatively thin wall will typically be a short grip length application.

Molding requirements and the desire to keep the mold simple. Locks that do not require die-action will be preferred over those that do. But, do not sacrifice reliability just to avoid die-action.

The need to have a low installation force (usually an ergonomic or a customer satisfaction requirement) and, at the same time, high retention strength. See the discussion of ‘‘decoupling’’ in Section 5.2, Chapter 5.

The cantilever hook is the most commonly used locking feature, yet many times it is not the best feature for the application. See the ‘‘Harmful Beliefs’’ in Section 9.3 and Figure 9.5 in Chapter 9. Designers should be aware of the other locking feature styles and avoid simply defaulting to the cantilever hook, particularly in short grip length and high demand applications. Many times, other beam style locks (loop and trap) can be used in place of a cantilever hook and provide much better retention.

Also keep in mind that the locking features on a particular application do not have to be identical. It may be convenient for purposes of die simplicity or for performance reasons to mix lock feature styles or sizes in one interface.

3.5

Summary

The purpose of this chapter has been to present a descriptive explanation of the various constraint features. From this information, a designer should understand the fundamental

differences in constraint feature behavior and be able to select the appropriate constraint feature styles when developing an application concept.

This chapter described the two major kinds of constraint features, locks and locators, which are used in the snap-fit interface to create a constraint system. Constraint features remove degrees of motion from the attachment and are the ‘‘necessary and sufficient’’ conditions for a snap-fit attachment.

3.5.1

Important Points in Chapter 3

The fundamental problem in snap-fit design is that locks must be weak in order to deflect for assembly yet strong enough to prevent part separation.

Snap-fit reliability depends on establishing and maintaining a line-to-line fit between the mating and base parts. Do not expect to get any significant or long-term clamp load in a snap-fit.

The rules for mechanical advantage and dimensional robustness that were introduced and explained with the locator features are general rules for all constraint features and they also apply to lock pairs.

Some locator pairs can constrain in as many as 5 degrees of motion, others in as few as one. Most lock pairs, planar and annular being the exceptions, can constrain in only one degree of motion (the separation direction). Designing for a lock to constrain in additional degrees of motion will leave the attachment under-constrained.

The cantilever beam, planar and trap are the most common lock styles. Torsional and annular are often special usage locks.

A profile added to both the insertion and retention faces in a lock pair can significantly improve assembly and retention performance.

Caution, even non-releasing hooks will release under sufficiently high forces.

Locators can be used as low-deflection locks, particularly when an assembly motion that involves sliding (slide, twist and pivot motions) is present.

Lock efficiency, the ratio of retention strength to assembly force, is a good indication of inherent lock effectiveness.

3.5.2

Design Rules Introduced in Chapter 3

Because of the tendency of plastic to creep, avoid long-term or sustained forces across the snap-fit interface unless these forces are low and long-term performance is indicated by analysis and verified by end-use testing.

Use locators to carry all significant transient forces across the interface and arrange locks so they do not carry transient forces in the separation direction unless they are permanent locks or have special retaining capability.

Locators should be the first constraint features added when developing the snap-fit interface and the first locators considered should be the one(s) that make first contact during assembly. This locator pair(s) should also provide the guidance function.

Because locators are strong relative to locks, the more degrees of motion that can be removed with locator pairs in a snap-fit, the stronger the attachment.

The assembly motion selected for an application will determine the potential strength of a snap-fit attachment. This is because locator pair selection for a given application is a function of the assembly motion.

The potential for degrees of motion removed by locators is highest with the tip, slide, twist and pivot motions so they are generally preferred over the push motion. Of the four preferred motions, tip is usually the most practical.

When a beam lock is being considered, design to use loops or traps whenever possible. In general, the hook style locks will have the lowest lock efficiency. Cantilever beam loop style locks have much better efficiency than the hook, and the traps have the highest.

A loop or trap lock used with a tip assembly motion is a highly effective snap-fit attachment concept and should always be considered as a design alternative.

Where two natural locators make up a position-critical pair and fine-tuning may be necessary, consider adding a discrete locator as one member of the pair.

Maximize part stability and minimize sensitivity to dimensional variation by placing constraint pairs constraining the same rotation or translational movement as far apart from each other as possible.

Maximize mechanical advantage against rotational forces by placing constraint pairs acting as a couple so that their (parallel) lines-of-action are as far apart as possible.
Specify a radius on all interior and exterior corners of constraint features. This applies to

the feature intersection with the parent material and to all corners within the feature itself.

Non-releasing trap locks must be protected from over-deflection and damage.
Design compensation for knitline weakness into loop style locks.

References

1. Loops were described as a unique lock feature in Integral Fastener Design, Dave Reiff, Motorola Inc., Fort Lauderdale, FL.

2. Plastic Part Design for Economical Injection Molding, 1998, Glenn L. Beall, Libertyville, IL. Test data reproduced from LNP Cloud, McDowell & Gerakaris, Plastic Technology, Aug. 1976.

3. Luscher, Dr. A.F., Design and Analysis of Snap-fit Features, from the Integral Attachment Program at the Ohio State University, 1999.

4

Enhancements

Enhancements were introduced in Chapter 2 as the second of two groups of physical elements used in snap-fit attachments. They were also referred to several times during the discussion of constraint features in Chapter 3. In this chapter all the enhancements are presented and described in detail.

4.1

Introduction

Enhancements may be distinct physical features of an interface or they can be attributes of other (physical) interface features. They improve the snap-fit’s robustness to variables and unknown conditions in manufacturing, assembly and usage. In other words, they make a snap-fit more ‘‘user-friendly’’. Most enhancements do not directly affect reliability and strength but, by improving the snap-fit’s robustness to many conditions, they can have very important indirect effects on reliability. They are a big part of the ‘‘attention to detail’’ aspects of good snap-fit design.

Enhancements are often tricks-of-the-trade that experienced snap-fit designers have learned to use. Meanwhile, the inexperienced designer must learn their value through trial- and-error. Read this chapter thoroughly. Enhancements will do more for your application than you can imagine.

A snap-fit application does not require enhancements. Only constraint features are absolutely necessary in a snap-fit attachment. But, as we will see, enhancements are required if a snap-fit is to be ‘‘world-class’’. If you have examined some snap-fits and found features you could not identify, or maybe wondered, ‘‘Why did they do that?’’ you may have been looking at an enhancement. If you have assembled and disassembled similar snap-fit applications from different sources and marveled at how such similar applications could behave and feel so different, credit the difference to enhancements. As consumers and users of plastic products, we regularly use enhancements. If you have been frustrated by a snap-fit, chances are it was because of improper use or lack of enhancements in the product.

Certain enhancements should be considered as requirements in every application. Others are required depending on the nature of the application. Still others can be thought of as ‘‘nice-to-have’’ but not essential.

Benchmarking is an important part of the creative process for snap-fits and the subject of enhancements and benchmarking deserves special comment. As you conduct technical benchmarking studies of products, many of the best ideas and creative hints will not be dramatic or highly interesting product features. They will be subtle and rather mundane details in the parts; much like those described in this section. By studying the enhancements on parts, you can find important clues to the problems the product designers had to

overcome and how they did it. You can then predict and avoid problems of your own. Benchmarking is discussed in more detail in Chapter 7 as a part of the snap-fit development process.

Enhancements are grouped into four categories according to their effects on the attachment: assembly, activation, performance, and manufacturing.