DFMA guidelines for snap-fits in plastics

What are snap-fits?

Snap joints are a very simple, economical and rapid way of joining two different components. All types of snap joints have in common the principle that a protruding part of one component, e.g., a hook, stud or bead is deflected briefly during the joining operation and catches in a depression (undercut) in the mating component. 

After the joining operation, the snap-fit features should return to a stress-free condition. The joint may be separable or inseparable depending on the shape of the undercut; the force required to separate the components varies greatly according to the design. It is particularly important to bear the following factors in mind when designing snap joints:

• Mechanical load during the assembly operation 
• Force required for assembly

 

 

How are snap-fits designed? 

The most widely used snap-fit is one with a cantilever form. 

A Cantilever snap-fit is designed to be fastened with another component at the end of the protrusion which extends from the base of a component and is processed to form a hook or a bead. 

Snap-fits should not be considered in components intended to be disassembled regularly. (Cantilever snap-fits exhibit easy assembly, so disassembly is possible but rather difficult. In addition, as a snap-fit is designed on the basis of the deformation caused by assembly, plastic deformation or plastic failure may result from the disassembly causing relatively larger deformation.)

Generally, the strain of parts made of unfilled materials is allowed up to 5% and that of reinforced materials is allowed up to 1~2 %. (This strain is slightly higher than the generally recommended strain. However, strain only partially occurs at the surface and in addition a supporting wall is not completely fixed, unlike the assumption of mechanics theory, and has some flexibility. Therefore the above-mentioned strain is available since the real value can be decreased more effectively than the theoretical value.) 

 

 

 

Therefore, strain by deformation of straight beams (A) and slope/tapered beams (B) is expressed as: 

Strain beam: 

`epsilon = (3hY)/(2L^2)`L2

Slope beam: 

`epsi= (3hY)/(2L^2K)` 

Whereas, 

K = Geometrical Factor `= (h_L) /(h_O)`

 

Reducing stress concentration in a snap-fit

As for permanent assembly, there is just one deformation, so the strain cannot exceed the above-mentioned strain.

In the case of using a sloped beam, stress is reduced as it is widely distributed throughout the cantilever. Therefore, stress concentration and fastening force is relatively reduced. (An added note: the ratio of hO and  hL is recommended to be 2:1.) 

In order to reduce the stress concentration, add a round form (R) { nothing but the filets} to the edge of the bottom of a beam. 

 

 

                                      (Korean Plastic Design Engineers) 

The above image graphically represents the effect that the root radius has on stress concentration. At a first glance, it seems that an optimum reduction in stress concentration is obtained considering that the ratio R/h = 0.6, since only a marginal reduction occurs after this point. 

However, using R/h of 0.6 would result in a thick area at the intersection of the snap-fit arm and its base. Thick sections will usually result in sinks and/or voids which are signs of high residual stress. For this reason, the designer should reach a compromise between a large radius to reduce stress concentration and a small radius to reduce the potential for residual stresses due to the creation of a thick sec-tion adjacent to a thin section. Internal testing shows that the radius should not be less than 0.015 in. in any instance.

The location of assembly should be considered with flexibility of the wall of the product. This helps to reduce external stress.

To avoid shrinkage marks, it should be less than 60% of thickness of the basic wall.

Good results have been obtained by reducing the thickness (h) of the cantilever linearly so that its value at the end of the hook is equal to one-half the value at the root; alternatively, the finger width may be reduced to one-quarter of the base value.

For more interesting observations have a look at the below image with comparative charts.

 

 

                                (Korean Plastic Design Engineers) 

 

Whereas, 

y =     (permissible) deflection (=undercut) 

E =     (permissible) strain in the outer fiber at the root; 

           in formulae: E as absolute value = percentage/100 

1 =     length of arm

h =     thickness at root 

b =     width at root 

c =     distance between outer fiber and neutral fiber (center of gravity) 

Z =     section modulus = Z = I c, 

I =      axial moment of inertia 

Es =    secant modulus 

P =     (permissible) deflection force 

K =     geometric factor 

 

How are these calculations performed?

These calculations need not be done manually, with the arrival of computing tools and software that simply require you to enter the required conditional inputs. The stress concentrations and deflections are then calculated, and one can decide whether to proceed with the desired value.  

One of the widely used tools for these calculations is “KSoft”, designed in Korea.  

Here is a glimpse of the software: 

 

 

                            

                                  Software window for stress and deflection calculations 

                                         (Korean Plastic Design Engineers, KSoft)