DESIGN FOR ASSEMBLY (PART 2)

Following on from last month’s article, this month we shall look at mechanical fasteners and hot staking.

Mechanical Fasteners.  There is a wide variety of different fasteners – self-tapping screws, press-in and mould-over inserts, clips, studs, rivets, etc they can either allow disassembly for repair or be permanent fixings.  However, they can cause local stress in the moulding either through their insertion or the moulding of the holes to accommodate them.  If the component is to be exposed to wide temperature variations it must be remembered that the vast majority of inserts are metal and so will have different expansion properties to the plastic material.

Hot Staking  An alternative method of making a permanent joint is hot staking.  A pillar on one moulding is inserted through a hole in the part it is to be joined to.  This could be another moulding, or a plate or printed circuit board, for example.

Heat is then used to soften this pillar or stake and a tool used under pressure to form a head similar to a rivet.  Frequently hot air is used hence the term hot air staking.

Advantages are similar to snap or press fits:

a    They are an integral part of the moulding adding little or nothing to part or mould cost.

a    They eliminate the need to buy and stock fasteners.

a    Recycling is simplified as there is no need to remove foreign materials.

As thermoplastics are poor conductors of heat, when designing parts that are to be joined using this method in assembly it is better to have a greater number of small stakes rather than a few thick ones.  Although the appearance is most commonly of a rivet head it is possible to achieve a flush finish by use of a countersunk head.

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