EoW September 2009

technical article

A newmethod for cable gel filling By Zhenfu Zhou, Zhuang Xiong, Zhongping Luo, Yangtze Optical Fibre and Cable Co Ltd

Abstract In this paper, a new cable gel filling method is proposed for optical fibre cables with metal tape armour; the fundamentals as well as the implementation of the method are described in detail. In addition, the optimum gel parameters in production are obtained by the water penetration test, which is also performed after the temperature cycling test and the mechanical performance test of the cable. The test results exhibit that this new method is quite reliable and easy for operation. Moreover, it can effectively reduce production cost. 1 Introduction Generally, good water-blocking perfor- mance is a prerequisite for the required life expectancy and high-quality transmission characteristics of optical fibre cables. Thus, water penetration test is an indispensable item in international standards for cable performance tests [1] . In China, the water blocking performance of optical fibre cables is usually achieved by the following two methods (for cables with metal tape armour): The interstices in the cable core • are filled with ordinary gel, and a water-blocking tape is wrapped around the cable core. If corrugated steel tape is adopted, a layer of hot melt compound or water-swellable filling compound is added continuously at the overlap of the tape but, in some cases, nothing is added. If aluminium tape is employed, the melted PE coating on the tape often conglutinates the overlap The interstices in the cable core are • filled with water or heat-swellable filling compound. If corrugated steel tape is adopted, a layer of hot melt compound or water-swellable filling compound is added continuously at the tape overlap. If aluminium tape is employed the water or heat-swellable filling compound is squeezed into the overlap, or the overlap is conglutinated by the melted PE coating on the tape

For hot melt compound, if it overflows into the steel tape armour, it will crush the cable core (loose tubes) since the compound will harden after it cools down. If it overflows out of the armour, it will contaminate the forming dies for the tape. In the case of water-swellable filling compound, if it overflows out of the steel tape armour, it will reduce the adhesion strength between the cable sheath and the steel tape, sometimes resulting in defects on the surface of the cable sheath, eg lumps and neck downs. In order to solve the above-mentioned problems, a new method for cable gel filling is proposed in this paper. 2 Fundamentals and characteristics of method The new method is suitable for cables with metal tape armour and it is implemented layer by layer. The procedures of the method are illustrated as follows: First, the interstices between the • central strength member and loose tubes are filled with ordinary gel during the stranding process The cable core goes through a • compound filling pump during the sheathing process when the interstices in the cable core are also filled with ordinary gel, and superfluous gel is wiped away by an eraser with the inner diameter almost the same as the outer diameter of the cable core

In some countries, semi-dry cable structure is often used in which water-blocking yarns are stranded over the non-metallic central strength member and a water-blocking tape is wrapped around the cable core. If water penetrates into the cable, the water-blocking yarns or tape will swell immediately to block the water. For those cables with metal tape armour the overlap is sealed, mainly, by the melting of PE coating on the tape. However, this method is not reliable in practical production, especially for those cables with corrugated steel tape armour. From the above descriptions, we can conclude that the water blocking measures for the cable core are quite effective but the water blocking performance at the overlap of the metal tape cannot be guaranteed. For cables with aluminium tape armour the overlap should be sealed by the melting of the PE coating on the tape, so production speed should be relatively low to allow the coating to melt. On the other hand, if the cable sheath wall thickness is rather small, the same problem will occur due to the insufficient heat during the production. For those cables with steel tape armour, even if the hot melt compound or water-swellable filling compound is employed, the quantity and the position of the compound cannot be controlled to get the best result. In practical production, the quantity of the compound should be increased in order to ensure the good sealing at the overlap, leading to the overflow of the compound into or out of the steel tape armour.

Figure 1 ▼ ▼ : Sketch map of water-swellable gel spread intermittently on metal tape

Water swellable gel

Metal tape

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EuroWire – September 2009