There are always customers intending to start a cable production completely from the very first. Very often they only have a piece of reference cable and look just for the one and only machine being able to produce this device. But such a machine does not exist! We therefore like to specify which kind of different machines and requirements are necessary for an elementary cable production.
Individual Production Steps
The production of a cable requires many different steps. When the semi-finished products are completed they are directed to the next production facility for further processing. Usually the different stages of production take place spatially one after the other which results in a short transport distance. You will find more info about this under "Demands on infrastructure". An elementary cable is roughly produced by the following steps:
- Wire Drawing Block (optional - when buying finished wire in addition)
- Core Insulation
- Cable Construction
- Shielding or Plating (optional)
- Sheathing Extrusion
We now would like to explain these individual steps in detail:
Wire Drawing Block
Actually, a wire drawing block (bull block) is a complete production on its own. There are many companies working only on wire drawing and selling only wires resp. wire products like strands and ropes.
First of all, the cores being necessary for the cable have to be insulated. Those can be solid wires or flexible strands, i.e. a bundle of twisted, very thin wires. For this procedure you would need a so-called extrusion line. Such a production line typically consists of the following elements:
- Pay-off: a machine gradually delivering the blank wire of a bobbin or another tank into the line
- Pre-heating Unit (optional): depending on the type of cable it is necessary to pre-heat the core before insulation is applied; this causes a better sticking
- Extruder:the machine injecting the plastic on the core, in principle a long cylinder which is heated or - depending on material - cooled. Via rotating screw the material mostly is transported to a so-called crosshead. During this procedure the mostly granular plastic becomes liquid and flows round the core in order to cover it. This process is supported by appropriate tools like nipple and nozzle. The core is being drawn continuously through the crosshead and will subsequently be covered by plastic lengthwise. Certain cores demand 2 extruders to apply different colours at once (green/yellow earth wire). In this case 2 extruders, having different capacities and sizes, are connected to one crosshead
- Cooling Tank: as the plastic is hot and soft after application it has to be cooled in order to become hard again. This is mostly done in a long water tank which the newly formed core probably runs through several times until it is cooled down and the applied plastic is hard again.
- Hauling-off: the core has to move continuously through the crosshead of the extruder as well as the whole extrusion line. This pushing is done by a hauling-off device. There a several types of construction, i. e. caterpillars or capstans.
- Testing Units: before the finished core can be rewinded it has to be tested first to see if there are some insulation faults and if diameter and wall thickness of insulation are having exact size.
- Take-ups: finally the finished core has to be rewinded again, mostly on a bobbin. For this procedure there are many different types of take-ups, from simple ones with only one bobbin up to fully automatic dual take-ups with 2 bobbins changing automatically from one bobbin to the other when it is full.
As the cores to produce a cable have to be very long (for a finished electric cable with 5 cores being 100 m long you would need 500 m core length) and the production of regular cables is carried out in kilometers, core insulation lines are mostly running with very high speeds. Depending on modernity of those machines an efficient production of cores starts at a speed of 300 m/min and can reach speeds up to 2200 m/min today when using the newest machines.
After producing the single cores for a cable, they have to be assembled, to make up the final cable construction. Within the cable industry there are many different terms used for this procedure. As English is not our first language we do not know what the most common term is, however, we will use the terms "bunching" and "stranding" for this process. There are many different machines available for bunching and stranding, each specialised for a certain type of cable strand. The most common ones are:
- Single- and Double Twist Stranders
- Planetary and Rigid Cage Stranders
- S/Z Stranders
- Drumtwisters resp. Rotating Take-ups
- Tubular stranders
Depending on the numbers cores or conductors, the required arrangement of the cores, the requirement of backtwist freeness, and others, a special machine is used. Generally one can say, that the bigger the final cable construction is in diameter, the bigger the bobbin to take up the finished material becomes, and the slower the production process. While a small cable pair, which will be taken up to a bobbin of 400 mm flange diameter, can be bunched together with a linear speed of 100 m/min and more, you will get of 10 cm or more diameter high voltage cable, which is taken up to a 3 m flange diamter bobbin or even larger, only few meters per minute.
Finally the completely assembled cable will have to be covered with an outer sheath. As the amount of material required to make up the sheath is typically much more than for insulationg a smaller core, sheathing extrusion lines are usually much bigger than core insulation lines. Thsi means the used extruders have a bigger screw diameter and length and therefore a higher metrail output. Also the capstand and pay-offs and take-ups are bigger, as the bobbins used are larger in flange diameter to be able to wind up the cable with the much bigger mass. Spacial cables that will be exposed to intensive physical stress are often armoured prior to sheathing with an extra layer of protecting wires or robust steel tape prior to applying the final sheath.
We have often experienced that customers, which have not yet a working cable production, present us a piece of cable and ask for the machine, which produces this cable. We usually have to disenchant the customer and tell him, that he needs a somplete factory and not only a single machine. During our business life we have visited hundreds of cable factories. The absolute minimal requirements, you have to reach, to procude justa simple household cable we want to list as follows (however please note, that this is only our point of view, and we not actually cable producers):
- Building: The production hall or building should have a floor space size of at least 30 x 15 m. If it is less, it will become difficult to set up all production machines in a straight line, without the need to deflect the material. Also there should be some space available between the production steps, in order to be able to store pre-materials and efficiently feed the machines.
- Cooling water: Many processes, especially extrusion require cooling, resp. cooling water. So facilities for cooling should be available.
- Power supply: POwer supply is (like in almost all industrial prodctions crucial. The connection power should be sufficient to operate all machines, and it is also very important that the supply is stable! A power breakdown damages not only the current work piece, but in most unhappy case, a several kilometer long cable directly in the middle, which than has to be repaired costly and time intensively.
- Lifting equipment: As copper and other conductors are usually quite heavy, fork lifts and cranes should be available to move the bobbins and also load and unload the machines.
- Personnel: As cable production is a continuous process, which means the production line is running and creating scrap, if the parameters are set wrongly, the personnel should be familiar with the task they are asked to do. Starting a cable production without appropriate know how carriers is futile.
- Capital: As copper, which is used for most cables, is expensive and so are also a lot of the other raw materials used, a lot of money has to be invested until a company can get revenues from sales of finished cables. You can say that it can take up to 3 - 4 month until you get the money back for copper you buy and start to process into a cable. Someone starting a cable factory should have this long period of time in mind and take care that enough working capital is available.
Annotation at the End
This abstract shall give a coarse overview and claims no right to be even close to 100% complete or correct. Please have this in mind, reading this document. However it should give a good start in to the matter. If you want to go deeper into details, we want to suggest to take a look at the following very good, and very extensive book, which covers cable production theory: Power Cables Theory and their Manufacturing Procedures from Dr. Eng. Sotiros Balafutas (ISBN 978-960-89803-7-2).