DESIGN FOR ASSEMBLY (PART 1)
If the part is not used in isolation then consideration must be made of assembly with other plastic or non-plastic parts. Methods of assembly include adhesive, welding, snap-fit, etc and can be manual or by automated process. It is possible to combine several features/parts into a single moulding as well as adding threaded or other inserts. It is also necessary to consider other post-moulding operations such as painting or printing.
For assemblies there are various design features worth considering to cover instances where a mismatch is possible.
Ideal Match Mismatch
Ideal Match Less Obvious Mismatch
The aim of any project is to minimise the number of parts associated with a particular product. Fewer parts relates not only to lower costs but also easier assembly. Even eliminating a single screw can result in saving not just the cost of the screw but also the cost of assembly and handling. It could reduce the need for specialist equipment and scrap levels through damage caused by the incorrect fitting of the screw.
As previously mentioned there are a number of assembly techniques:
Press Fit
Snap Fit
Mechanical Fastening
Hot Staking
Welding
Adhesive Bonding
Solvent Bonding
We shall look at each of these in turn in this and future articles.
Press Fit assemblies rely on interference between the parts to keep them joined together. Annular parts such as gears and pulleys, attached to shafts are a common example.
Although simple there are potential problems. The degree of interference between the two parts is critical – too small and the joint is loose, too great and assembly becomes difficult and the material overstressed. Clearly, this type of assembly method is only viable where the associated manufacturing tolerances can be achieved and maintained.
It should also be noted that where different materials are involved then changes in temperature could affect the fit.
Automotive blanking plugs are a good example of a press-fit.
Snap fit is economical because it is a moulded feature and ideal for recyclability as no metal inserts or adhesives are involved. All involve the same principle – a protruding feature on one part is deflected briefly on assembly to locate into a recess in the mating part. This method is not suited to parts requiring repeated assembly operations. Snap fits can also be damaged due to incorrect handling especially if using a brittle or filled polymer.
There are three main types:
1. Cantilever
2. Cylindrical
3. Spherical
Cantilever
This works on the simple deflection of one part.
Cylindrical
A much stiffer structure than cantilever, relying on the radial expansion of the female part. They are sometimes designed with a number of radial slots to make this expansion easier. This type of snap fit is not suited to stiffer materials.
Cylindrical Snap-Fit
Snap-Fit Bottle Top
Spherical
An alternative version of the cylindrical snap-fit. The same principles apply.
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