We are all familiar with curly cords, but most of us never give them a second thought. Yet these babies have a backstory of their own that borders on alchemy. All solids have a “memory” of what shape they are supposed to be, and if deformed, their molecular structure will endeavour to return to the original shape. This memory can be changed or adapted by applying certain principles, and I will attempt to explain them here.
The molecules in an extruded thermoplastic insulated jacket will be connected to each other by so called bonds. When a force is applied, stretching these bonds, like elastic bands, they will pull themselves back together. In making a curly cord, we rearrange these bonds to suit our design. To do this, the cable is first warmed to make it more pliable, and then wrapped around a mandrel. The number of turns, and the diameter of the mandrel chosen will be defined by the requirements of the finished product. The tails, as specified, will be cut to the required length. The whole assembly will then be placed into a warming oven for a few hours, and cooked. This causes the bonds in the material to break down. Once removed, the assembly is allowed to cool gently, during which process new bonds will be formed at a molecular level, creating a new memory. This is one time when the thickness of a cable jacket is defined by a factor other than voltage. The more material we have, the stronger the memory will be.
There are limits. If we overstretch the cable, we will exceed the elastic limit, and cause the internal bonds to break, and the cable will not return to its original shape. Hence a stretch of 4:1 is usually stated as the norm. Some materials are better than others, polyurethane is usually the best. The size of the mandrels and of the ovens employed are another limiting factor, 2.5m for the coil in its relaxed state is usually the norm, but this can be exceeded in exceptional circumstances by feed the assembly slowly through the oven, by hand, in one side and out through the other.
The important thing to remember here is that each cord has to be manufactured individually, with the relaxed length, the coil diameter, the number of turns, and the length of the tails being specified at the time of order. You cannot supply curly cable by the yard.
(by David Cairns)
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