WCA May 2013

By keeping the water quench point distance as far from the crosshead as possible will yield the best product. This is because a long air-cooling distance gives time for the resin to shrink down onto the conductor providing a consistent, tight interface with the conductor without the use of excessive preheat. This consistent conductor interface provides a uniform insulation strip force even after the initial bond is broken. The advantage is improved structural return loss and resistance to stresses of subsequent processing operations. Sometimes a long air-cooling distance is not an option because of total available cooling distance. If this is the case cold water should be avoided in the first cooling section as excessive ovality of the insulation and low conductor adhesion can result. Tempered cooling is recommended as it reduces the initial shock on the insulation improving insulation ovality and conductor adhesion. Skinning Extruding an outer layer of solid material or skinning provides additional benefits, such as: • An easy and efficient way of colouring the insulation • Improved dielectric strength, which is useful on thinner wall cable designs • Higher foam expansion rates • Greater resistance to insulation damage during subsequent processing such as twinning or braiding Applying a solid skin coat requires an initial equipment investment (an auxiliary extruder and special crosshead) but provides payback in reduced scrap and product cost. Both the foam and solid layer are achieved at the same time through a single cross-head using standard processing methods. Figure 6 illustrates a foam core with a coloured solid outer layer. Conclusions There are various foamable fluoropolymer resin options available, each having unique capabilities and limitations. Selecting the correct resin for the application is important for cost, ease of processing and desired electrical performance. Designing and processing cables within the materials’ capabilities will produce quality products with high yields. Processing equipment selection and process conditions are critical to ensure a stable process, maintain minimum product variation and achieve the lowest cost operation. Special techniques, such as the addition of solid skin layer(s) to foam constructions, can provide additional improvements to processing and performance.

With a flow rate greater than 50cc/minute @1,000psig, the gas pressure would need to be adjusted lower than the barrel pressure and doing so would result in the injector plugging leading to the product going solid. Increasing the gas pressure higher than 1,000 psig will cause too high a gas flow resulting in over foaming. This over foaming condition is often misunderstood as to be a material or processing problem. Conversely, if the injector orifice is too small there may not be enough available gas pressure to obtain the gas flow needed. This will result in an inability to achieve the desired expansion rate and product capacitance. For this reason it is typical to have several injectors with different flow rates available over a wide range of pressures. The number of different sizes needed varies with the product mix and available gas pressure. Utilising a high-pressure nitrogen pump increases the gas pressure range over that produced from a high-pressure cylinder. The use of a pump can then help reduce the number of injector sizes needed for an operation leading to lower overall costs. The injector design can also impact performance. Figure 5 shows the result of trials comparing four commercially available injector styles as quantified by the gas flow variation and the resultant capacitance variation. A 50-ohm core with a 23-gauge conductor foamed to approximately 50 per cent was used for these trials. The gas flow rate six sigma variation (± 3 standard deviations) ranged from 4cc/min to 27cc/min with a resultant capacitance variation of 0.3 to 3.8pf/ft. These results demonstrate that performance issues often attributed to the fluoropolymer material are typically a processing issue related to the equipment. Use of the wrong-sized injector or an unstable design can mask the true performance benefit of certain materials. Product cooling The cooling medium for the extruded core is usually a combination of ambient air and water. The distance required for each of these is dependent on product size and line speed. Having the correct distances is critical for cooling prior to wire take-up to avoid flattening of the insulation on the reel and impacting the electrical performance. ❍ ❍ Figure 6 : Cross section of foam core with solid skin outer layer

DuPont Chemicals and Fluoroproducts 1007 North Market Street Wilmington, Delaware USA Tel : +1 302 774 1000 Website : www.dupont.com Email : www.info@dupont.com

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