Basic Extrusion Line Lay-out in Cable Industry
An Extrusion line is the heart cable prodction. An extrusion line may have various different configurations and different units. Whether it is used for bare wire insulation, core insulation or jacketing extrusion line will have different units installed. Here we will discuss about a basic line-up extrusion in general terms with main units. A basic extrusion line either for insulating , coating or jacketing is consist of 5( five ) different main units. In order from starting point ; Pay-off , Extruder , cooling/curing shaft, capstan ( pulling ) and take-up units are synchronized. Eventhough a simple lay-out of an extrusion line with these 5 ( five ) units as above is enough and capable of coating desired wire with plastic, additional auxiliary units will be added to existing extrusion line in order to maintain good quality and higher output volume. Preheater, feeding accumulator, dancer, powdering unit, in-line diameter gauge, tape applicator, spark tester, take-up accumulator, co-extruder are common auxiliary units. Some of these units such as spark tester and diameter gauge would be ensuring the quality of final product while others increasing output of extrusion line per minute.
Payoff : Starting point of extrusion line. Pay-off unit is loaded with either wire or cable core to be coated. It feeds the Extrusion line. It can be as simple as a flyer or a motor driven pay-off with accumulator for high speed extrusion. Bobbin size also determines the type of the pay-off unit. Selection of pay-off unit based on final product type and estimated line speed. Bobin size is very important since bigger payload means less downtime which results increased output. On the other hand, smaller reel/drum size mean less weight and less tension on wire. Flyer type payoff unit is used for only solid single wire insulation. Wire would be fed to extrusion line with torsion. Speed is limited. Bobbin size is not versatile. No additional tension would be applied to paid-off wire. Single shaft payoff pay-off unit is used for solid or stranded wire as well as insulated cores. Reel size is more versatile in compare to flyer type. Usually, designed to load a unique type of bobin size but could be used other sizes within a range of flange diameter and barrel length. If motor driven could be compatible with high speed extrusion line. Fork type payoff unit is versatile when it comes to bobbin size. Motor driven samples are compatibel with high speed extrusion line. Tension control unit , an accumulator or dancer should be intalled. Portal type payoff unit is used for bigger drum sizes. It could be motor driven or not.
Extruder : Main Unit of Extrusion line where plastic is formed and coated over bare conductor or cable core. Thermoplastic material would be melted to be formed while thermoset plastics would be formed cold to be cured later on.
Screw & Barrel couple is the heart of extruder machine. By means of rotating screw in the barrel; plastic is pushed to extrusion head where it would be coated and formed. There are many variations of screw and barrel. Common aim is to produce screw and barrel according to physical aspects of extruded plastic. The angle of flyers , diameter over length ratio and zones of screw determined by extruded plastic , expected outcome, line speed, minimum and maximum diameter of final product.
Cooling Shaft : Thermoplastic coated cable should be cooled after extrusion process. A cooling shaft is full of water where extruded cable will go through. Water in cooling shaft is would be warmed up. In order to maintain a stable temperature; water needs to be cooled. Dimensions of cooling shaft would be determined by minimum and maximum cable size, line speed , ambient tempeature and water tank capacity.
Curing Shaft: Thermoset plastics extruded as cold forming. After extrusion process, plastic needs to be cured to be finished product. Curing heat and using of gases vary according to the type of plastic extruded. Cross linked therrmoplastics - XLPE for example- also cured and vulcanized as thermoset plastics. Dimensions of the curing unit would be determined by line- speed , desired temperature and Physical characteristics of the plastic material.
Capstan : The role of Capstan on an extrusion line is pulling extrudate through extrusion process and cooling/curing shaft then feeding take-up unit. Final dimensions of the product would be determined by adjusting capstan speed over extruder speed ratio. Capstan's pulling speed and pulling force determines the line speed. A capstan is simply accoupled pulleys. Pulleys can be positioned vertical , horizontal or belted. Position of accoupled pulleys depends on line speed and maximum dimesions of desired prdocution. Belted type capstans allow no slipping and higher pulling force can be applied for larger size production. On the other hand line speed of a belted capstan is limited in compare to accoupled pulleys. Therefore line speed would be reduced if a belted capstan is used. A simple belted capstan, also known as "caterpillar" is shown in picture.
Take-UP: It is at the end of an extrusion line. Various types possible with various names attached. Starting from a single bobbin/reel fork type. It can be double with manual or automatic change. For bigger size reels it can be self traversing tower type. Size of the reel which is also defined by the size of the cable extruded and the line speed would determine the type and size of the unit. Traverse unit is a must. A dancer or accumulator unit may be required in front of Take-up unit as line speed increases.
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