Molded In Assembly Systems .1 Snap-Fit Assembly

For high-volume production, molded-in snap-fit designs provide economical and rapid assembly. In many products, such as inexpensive housewares or hand-held appliances, snapfits are designed for only one assembly without any nondestructive means for disassembly. Where servicing is anticipated, provision is made to release the assembly with a tool. Other

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snap-fit designs, such as those used in battery compartment covers for calculators and radios, are designed for easy release and reassembly over hundreds or even thousands of cycles.

In all snap-fit designs, some portion of the molded part must flex like a spring, usually past a designed-in interference, and quickly return, or nearly return, to its original position to create an assembly between two or more parts. The key to successful snap-fit design is having sufficient holding power without exceeding the elastic or fatigue limits of the material.

Figure 9.01 shows a typical snap-fit design. Using the beam equations, we can calculate the maximum stress during assembly. If we stay below the yield point of the material, the flexing finger returns to its original position. However, for certain designs there is not enough holding power due to low forces or small deflections. With many plastic materials, the calculated bending stress can far exceed the yield point stress if the assembly occurs rapidly. In other words, the flexing finger just momentarily passes through its maximum deflection or strain, and the material does not respond as if the yield stress has been greatly exceeded. Thus, a common way to evaluate snap-fits is by calculating strain rather than stress. Compare this value with the allowable dynamic strain limit (if available) for the particular material. In designing the finger, it is extremely important to avoid any sharp corners or structural discontinuities, which can increase stress. A tapered finger provides a uniform stress distribution and is advisable where possible.

h_L h_o

L TAPERED BEAM

Y (MAX DEFLECTION)

DYNAMIC STRAIN 3Yh_o 2L²K ε = L

h_o Y (MAX DEFLECTION)

DYNAMIC STRAIN 3Yh_o

2L² ε =

STRAIGHT BEAM

0.4 0.5 0.6

0.3 0.7 0.8 0.9 1.0

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3

K

PROPORTIONED CONSTANT, K, FOR TAPERED BEAM

h_L/h_o

Fig 9.01 ·Snap-fit design for cantilever beam with rectangular cross section

Since the snap-fit generally requires an undercut, a mold with side action is frequently required, as shown in Figure 9.02A. Figure 9.02B shows an alternative that works well when an opening at the base of the flexing finger is acceptable. In certain cases, the snap-finger can simply be popped off the mold.

Another type of snap-fit assembly system, which can sometimes be molded into the part, is known as snap-on or snap-in. It is used most often snap-on round parts.

Often larger portions of the part or even the entire part flexes, but the deflections are usually very small.

Figure 9.03 shows a typical example of a snap-on assembly.

CORE

CAVITY

PROJECTION FROM CAVITY, THROUGH PART, AND MATING WITH CORE FORMS UNDERCUT WITHOUT NEED FOR SIDE CORE. IT DOES, HOWEVER, LEAVE SMALL "WINDOW" OR OPENING IN MOLDED PART.

B) MOLDING SNAP-FIT FLEXING FINGER WITHOUT SIDE CORE

CAVITY

SIDE CORE MOVES DOWN TO MOLD UNDERCUT, AND IS ACTIVATED UP TO ALLOW MOLD TO OPEN

A) MOLDING SNAP-FIT FLEXING FINGER WITH MOLD USING SIDE CORE

SIDE CORE CORE

Fig 9.02 ·Tooling for snap-fit fingers Fig 9.03 ·Snap-on / Snap-in fits

SNAP-ON FIT

PROLONGED SNAP-IN

PRONGS

FULL PERIMETER SNAP-IN

DIM A DIM B

BALL OR CYLINDER SNAP-IN

R

R

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9.1.1.2 Molded-in Threads

In this type of assembly, mating male and female threads are molded into the parts to be assembled (Figure 9.04). Molded internal threads usually require some type of unscrewing or collapsing mechanism, which complicates the tooling. Many external threads can be molded by splitting them across the parting line, as shown. The most common applications for molded threads are in containers and caps, molded plastic hardware, and liquid handling applications.

Molding very fine threads, greater than 28 pitch, is usually not practical.

Fig 9.04 ·Molded plastic threads

9.1.1.3 Press-Fits

In a press-fit assembly, parts or components of a material are assembled to a plastic part, using interference fits to maintain the assembly. The main advantage of this system is that the tooling is kept relatively simple, however the method can create very high stresses in the plastic part. The consequences of this stress depend on many factors, such as temperature during and after assembly, modulus of the mating material, type of stress, use environment, and probably most important, the type of material being used. Some materials creep or stress relax, while

others fracture or craze if the strain is too high.

Except for very light press-fits, this type of assembly can be very risky due to the hoop stress in the boss, which might already be weakened by a knit-line.

Figure 9.05 presents alternative methods of designing press-fits that have a lower risk of failure.

Fig 9.05 ·Alternative press-fit designs for metal pins in plastic hub

INTERNAL THREAD

THREADED CORE PIN IN TOOL MUST UNSCREW AS MOLD OPENS EXTERNAL THREAD

SPLIT CAVITY AT PARTING LINE MOLD CAVITY

MOLD CAVITY

METAL PIN

ALTERNATE PRESS-FIT DESIGNS FOR LOWER STRESS

STRAIGHT (INTERFERENCE) PRESS-FIT CAN PRODUCE HIGH STRAINS

INTERFERENCE PIN DIAMETER STRAIN =

ADD METAL “HOOP”

RING PREVENTING EXPANSION OF PLASTIC BOSS

USE BARBS OR SPLINES ON THE METAL PIN TO CREATE INTERFERENCE FIT AND RETENTION

CREATE INTERFERENCE PRESS-FIT BY ADDING

“CRUSH RIBS” TO THE INSIDE DIAMETER OF THE BOSS

9.1.2 Chemical Bonding Systems