EoW March 2013

Technical article

as applicable. Typically the lower the melt flow rate, the better the burn performance (i.e. less smoke generation). The higher the melt flow rate, the more suitable the resin is for thinner insulation walls and smaller cable designs. Table 2 provides some general guidelines for resin selection.

Nominal expansion by wall thickness

% volume air

Process parameter and effects – foam expansion rates

Wall (in.)

▲ ▲ Figure 4 : Nominal expansion rates

It is common for cable engineers to design cables using calculated expansion rates yielding the lowest theoretical cost. However, there are other important factors that impact cost, such as process ability, overall electrical performance and cable damage and compression from subsequent operations after extrusion. Neglecting these design factors could mistakenly result in higher cost and significant scrap generation. Consider a typical video coax cable designed using a 59 per cent expansion rate versus the same cable designed with a 54 per cent expansion rate. The cable with 59 per cent expansion may push the process to its limits, subsequently increasing start-up scrap and causing greater process variation. From an electrical standpoint, higher void content typically results in larger cells and higher formation of cells around the centre conductor, which can have a major impact on cable return loss. Alternatively, the same cable can be made at a 54 per cent expansion rate with a weight increase of only 0.28lb/1,000ft. This small change will provide a robust, repeatable product with improved cable return loss, less scrap and higher productivity with the same cable impedance. Figure 4 provides general guidelines for foam expansion rates based on the dielectric wall thickness. Actual maximum expansion rates will vary based on resin selection and processing methods. High-pressure nitrogen gas injection Foaming is achieved by injecting high- pressure nitrogen gas into the molten polymer during the extrusion process. The rate of foaming is determined by the flow rate of the gas in proportion to the resin output at the operating RPMs of the extruder. The higher the gas flow to the resin output, the higher the expansion rate.

Variation (6 SDEV)

Nitrogen flow (cc/m in)

Actual capacitance (pf/ft)

Injector type

Predicted capacitance

▲ ▲ Figure 5 : Gas flow and capacitance variation

Selecting the gas injector for the product When sizing an injector, the extruder barrel pressure and the nitrogen flow rate for the desired expansion rate versus the product run speed need to be considered. The flow rate of the gas is controlled by the injector orifice size and the nitrogen gas pressure. The orifice needs to be sized so that the gas pressure is higher than the barrel pressure for the desired gas flow. Suppose a given cable construction requires a flow rate of 50cc/minute of nitrogen for a line speed of 600 feet per minute and creates an extruder barrel pressure of 1,000psig. The selected injector for this process needs to have the orifice sized no larger than to deliver a gas flow rate of 50cc/ minute at pressure greater than the barrel pressure. 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.

The consistency of this gas flow is critical to maintaining a uniform expansion rate, which is needed to maintain low variations in cable capacitance and signal time delay for the cable. Measuring gas flow Ensuring that a constant, correct gas flow is injected into the melt is one of the most important foaming process variables. Undetected variation of gas flow will result in capacitance variation, leading to process instability and significant scrap. Off-line injector flow measurements (such as water displacement) will determine the average injector flow rate at room temperature. However, it will not determine the actual process flow rate or flow variation as injector flows can change radically once heated to processing temperatures. Consequently, an in-line flow meter is recommended when utilising the gas injection foaming process. With a flow meter, the gas pressure can accurately be set to obtain the calculated flow rate required for the desired nominal capacitance. In addition, variations in flow rate can be monitored.

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March 2013

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