Posted 17 July 2019
Part 3 - The Laying Up Process
This instalment of “What’s in a Cable” covers the process of laying up. This process is used to create the core bundle once the conductors have been insulated.
In a simple cable two cores can be laid next to each other and have a jacket extruded over them. However, in many two core cables and all multicore cables it is normal practice to twist the cores together. This has a number of benefits. Firstly, it improves flexibility with each core taking the same amount of stress as the others. Additional benefits include easier production (by keeping the cores together as they pass through the jacketing extruder) and better cable uniformity.
Multicore cables have a number of cores twisted together to form the core bundle. The ideal core bundle is circular and to achieve this, dummy or blind cores may be added. These cores contain no metallic elements and just act as fillers. In cables with 12 cores or more, the cores will be arranged in layers with each layer twisted in opposite directions. This ensures the finished cable has the best chance of remaining circular even if driven over or trodden on.
Cores can also be arranged into pairs, triples or quads. This is common for data, signal and instrumentation applications. By twisting the cores to form pairs the problem of crosstalk is reduced. Crosstalk occurs where the signal on one circuit leaks over onto another circuit. Analogue signals in particular can be affected by this.
By creating a twisted pair, the cores are in limited contact with adjacent cores and are exposed to equal levels of electromagnetic interference (EMI). The process of twisting cores into pairs is relatively fast with speeds of up to a few hundred metres a minute. The pairs are then laid up in the same way as with multicore cables. On high performance data cables the twist rate may vary, for example in Cat 5 cable the twist rate of pairs varies between 18mm and 22mm. On telephone cables this may extend to one complete twist every 80mm or so which is hardly noticeable.
Termination can be easier and faster on twisted paired cables. Core identification is normally achieved by either colouring each core differently (colour coding) or printing a number on each core. On twisted pair cables colour coding is very popular. Number coding on tapes wrapped around each pair, or a combination of colour and number coding are also sometimes used.
Having laid the cores or pairs into a bundle the next stage is to add a jacket. There is one vital element prior to extrusion and this is to create a barrier between the core bundle and the jacket. A barrier is particularly important because if the insulation and jacket are of similar compounds the jacket could stick to the insulation.
The most common barrier is talcum or French chalk which is held in a trough through which the core bundle is pulled before going into the extruder. Talcum has the advantage of being both cheap and effective.
The disadvantage of talcum is that it is difficult to detect if it has run out or fails to cover the core bundle properly. This can result in a cable that is very difficult to strip and has reduced flexibility. Also, in a small number of applications the talcum could contaminate the surrounding equipment, for example with medical equipment or in clean rooms.
Other release agents can be used, including silicone oils, but these are more expensive than talcum and can be difficult to apply. Other methods include tapes of polyester or mylar and the non-woven polyester or fleece types. The latter are ideal for constantly flexing cables as they allow movement. They are sometimes used between layers in core bundles or either side of braid screens. Popular in robotics and audio cables, they are increasingly used in more general cables where flexibility and stripability are important.