Snaps allow an easy method of assembly and disassembly of plastic parts. Snaps consist of a cantilever beam with a bump that deflects and snaps into a groove or a slot in the mating part.
Snaps can have a uniform cross-section or a tapered cross section (with decreasing section height). The tapered cross-section results in a smaller strain compared to the uniform cross-section. Here we consider the general case of a beam tapering in both directions.
The disassembly force is a function of the coefficient of friction, which ranges from 0.3 to 0.6 for most plastics. The coefficient of friction also varies with the surface roughness. The rougher the surface, the higher the coefficient of friction.
There is an angle at which the mating parts cannot be pulled apart. This is known as the self-locking angle. If the angle of the snap is less than this angle, then the assembly can be disassembled by a certain force given by the above formula. The self-locking angle = tan-1(1/µ)
where µ is the coefficient of friction which ranges from 0.3 to 0.6 for most plastics. This computes to angles ranging from 73° for low coefficient of friction plastics to 59° for high coefficient of friction plastics.
If this angle is exceeded then the snaps will not pull apart unless the snap beam is deflected by some other means such as a release tool.
This property can be used to advantage depending on the objective of using the snaps. If the snaps are to be used in the factory for assembly only (never to be disassembled by the end user), then the ramp angle the self-locking angle should be exceeded. If the user is expected to disassemble (to change batteries in a toy for example), then the angle should not be exceeded.
Tooling for snaps is often expensive and long lead time due to
The iterations required achieving the proper fit in terms of over travel. The amount of over travel is a design issue. This will control how easy it is to assemble, and how much the mated parts can rattle in assembly. This rattle can be minimized by reducing the over travel or designing in a preload to use the plastic’s elastic properties. However, plastics tend to creep under load, so preloading is to be avoided unless there is no other option.
Snap fits (also known as Clip Snaps) are moulded plastic features of products which clip two parts together. You have probably seen them, in various plastic products before, but you may not have realised how much thought and effort goes into getting these right. Plastic Snap fits are used to assemble plastic parts together with the need for additional (and sometimes costly) fasteners, like screws and bolts. Essentially, snap fits are a hook shaped protrusion that indexes with a corresponding depression, usually being able to come undone if you can flex the arm again. We’ll run through the 4 basic types of snap fits, and the advantages and disadvantages of snap fits in your injection moulded product.
There are four basic types of snap fits, and these are Cantilever snap fits, U-Shaped snap fits, Torsion snap fits, and finally Annular snap fits.
Cantilever Snap Fits are essentially locking lugs on arms that can flex. These fit into corresponding holes, which have a retaining lip to prevent the arm from sliding back out again. Cantilever snap fits are used just about everywhere in plastic design though you might not notice it. Chances are that there is a cantilever snap fit cover in your car’s engine bay covering up the fuse boxes or clipping a electrical terminal together.
U shaped snap fits are very similar to cantilever snap fit, just imagine it bending back on itself. It does this to provide and in built hinge point rather than rely on a cantilever pivoting point. These types of snap fits are usually find in applications where the parts need to be pulled apart again and again or where the contents of the two parts don’t require a lot of force to stay in position. For example, a lot of remote controls have a lid on the battery section, and this lid is often held in place with a U-shaped snap fit.
Torsion snap fits use shear stresses to move the arms in and out of position, so that the clips are on the side of the opening. Think of the shape of the clip being like a clothes peg. When you depress the levers the pegs open, but when you let go the peg closes again.
Annular (meaning ring shaped) clips snaps are a clip snap that goes around the perimeter of the product and locks into place. Annular snap fits are very common too, but are again often over looked. A pen with a lid that clips onto it has a very small annular snap fit, which keeps the lid in places. In addition to this, a lot of tubing and piping products have annular snap fits , to seal the entire perimeter of the tube with ease.
So know that you know the common snap fits, what are the pros and cons of using snap fits on your part?
The perks of having snap fits are:
Easy assembly – the parts just clip together.
Less unit cost (as you don’t have to buy, over mould or insert fasteners).
Improved aesthetic
Fewer Moving parts – reducing overall product complexity
Cheaper than other forms of fastening.
The downsides are:
If they break, you must replace the whole part.
Can be difficult to disengage.
Are perceived as a cheap product feature.
May cause undercuts in your plastic parts.
They are difficult to prototype (most prototypes aren’t made in the material they are moulded in).
Snaps allow an easy method of assembly and disassembly of plastic parts.Snaps consist of a cantilever beam with a bump that deflects and snaps into a groove or a slot in the mating part.
Snaps can have a uniform cross-section or a tapered cross section (with decreasing section height). The tapered cross-section results in a smaller strain compared to the uniform cross-section. Here we consider the general case of a beam tapering in both directions.
This property can be used to advantage depending on the objective of using the snaps. If the snaps are to be used in the factory for assembly only (never to be disassembled by the end user), then the ramp angle the self-locking angle should be exceeded. If the user is expected to disassemble (to change batteries in a toy for example), then the angle should not be exceeded.
Tooling for snaps is often expensive and long lead time due to
The iterations required achieving the proper fit in terms of over travel. The amount of over travel is a design issue. This will control how easy it is to assemble, and how much the mated parts can rattle in assembly. This rattle can be minimized by reducing the over travel or designing in a preload to use the plastic’s elastic properties. However, plastics tend to creep under load, so preloading is to be avoided unless there is no other option.
Some common problems of using snaps:
Too high a deflection causing plastic deformation (set) of the latch (the moving member). Care has to be taken that the latch does not take a set. Otherwise, the amount of latch engagement could reduce, reducing the force to disassemble. If the set is bad enough the engagement might even fail.
The moving arm could break at the pivot point due to too high a bending stress. This can be avoided by adhering to the design principles and not exceed the yield strength of the material-in fact it should be kept well below the yield strength depending on the safety factor used.
Too much over travel leads to a sloppy fit between mating parts resulting in loose assemblies that can rattle.
Good snap design practices
Design the latch taking into account the maximum strain encountered at maximum deflection.
In general, long latches are more forgiving of design errors than short latches for the same amount of deflection, because of the reduced bending strain.
Build mold tooling with “tool safe condition”. By this we mean that the deflection or over travel, or length of engagement can be changed easily by machining away mold tooling, rather than add material to mold tooling, which is more expensive and not good mold practice. This “safe” condition allows for a couple of tooling iterations of the latch, until the snap action is considered acceptable.
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