DFMA for Snap Fits In Plastics

Snap joints are a really simple, economical and rapid way of joining two different components. every kind of snap joint 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 during a depression (undercut) within the mating component. After the joining operation, the snap-fit features should return to a stress-free condition. The joint could also be separable or inseparable counting on the form of the undercut; the force required to separate the components varies greatly in line with the planning. it's particularly important to contact the subsequent factors in mind when designing snap joints:

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

 

 

A Snap-Fit type with a cantilever form and most generally used.

 

Designing your Snap Fit

Essentially, a Cantilever Snap-Fit is intended to be fastened with another component at the tip of the protrusion which extends from the bottom of a component and is processed to make a hook or a bead.

Snap-fits shouldn't be considered in components intended to be disassembled regularly. (Cantilever Snap-fits exhibit easy assembly, so disassembly is feasible but rather difficult. additionally, as a snap-fit is meant on the premise of the deformation caused by the assembly, plastic deformation or plastic failure may result from the disassembly causing relatively larger deformation.)

Generally, the strain of parts fabricated from unfilled materials is allowed up to five which of reinforced materials is allowed up to 1~2 %. (This strain is slightly more than the widely recommended strain. However, strain only partially occurs at the surface and adding a supporting wall isn't completely fixed, unlike the belief of mechanics theory, and has some flexibility. Therefore the above-mentioned strain is offered since the important value is often decreased more effectively than the theoretical value.)

 

 

∴ Strain by deformation of Straight Beams (A) and Slope/Tapered Beams (B)  

Strain beam: 

 `epsilon = (3hY)/(2L^2)`L2 

Slope beam: 

`epsi= (3hY)/(2L^2K)`

Where as, 

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

 

Deformation in Permanent Assembly

As for permanent assembly, there's only one deformation, therefore the strain cannot exceed the above-mentioned strain.

In the case of employing a sloped beam, stress is reduced because it is cosmopolitan throughout the cantilever. Therefore, stress concentration and fastening force is comparatively reduced. (An added note: the ratio of hO and hL is suggested to be 2:1.)

In order to cut back the strain concentration, add a round form (R) to the sting of the underside of a beam.

 

 

Fig 3 graphically represents the effect the basis radius has on stress concentration. initially glance, it seems that an optimum reduction in stress concentration is obtained using the ratio R/h as 0.6 since only a marginal reduction occurs after now. However, using R/h of 0.6 would end in a thick area at the intersection of the snap-fit arm and its base. Thick sections will usually end in sinks and/or voids which are signs of high residual stress. For this reason, the designer should reach a compromise between an oversized radius to cut back stress concentration and atiny low radius to cut back the potential for residual stresses thanks to the creation of a thick sec-tion adjacent to a skinny section. Internal testing shows that the radius shouldn't be but 0.015 in. in any instance.

The location of assembly should be considered with the flexibility of the wall of the merchandise. This helps to cut back external stress. To avoid shrinkage marks, it should be but 60% of the thickness of the essential wall

 

Significance of the Thickness of the Cantilever

Good results are obtained by reducing the thickness (h) of the cantilever linearly in order that its value at the tip of the hook is adequate to one-half the worth at the root; alternatively, the finger width is also reduced to one-quarter of the bottom value.

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

 

 

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 

Nowadays because of the event of technology one needn't try and do these calculations by hand. There are much software available within the market where you only have to enter the desired conditional inputs and they will calculate the full stress concentration and deflections and you'll be able to see the results and make a choice stating good to proceed or not with the specified value. Technology is saving lots of time, must say, isn't it?

One of the widely used software for these calculations is Ksoft, which could be Korean Software.

Take a glance at the window attached below from the software, how easy it is!