POLYETHVLKXK CABLE SHKATIU THICKNESS 573 



sheathing hue is diagrammed in Fig. 13. At the top left is the supply reel 

 of metal jacketed cable. The cable is pulled through the flood tank where 

 the hot rubber asphaltic compound is flowed over the corrugated metal 

 sheath. It then progresses through the die head of the extruder where 

 the polyethylene sheath is extruded over the flooded metal sheath. The 

 cable with plastic polyethylene then enters the cooling trough where it is 

 cooled and solidified. At the exit of the cooling trough is an air blower 

 for drying the water from the sheath surface. The test set is located after 

 the dr3'cr. The next unit is the capstan which pulls the cable. At the final 

 unit to the right, the sheathed cable is taken up on the shipping reel. 



A typical recorder graph taken along 360 feet of cable length with the 

 sensing probe held at one location on the sheath circumference is shown 

 in Fig. 14. With apparently stable conditions of extrusion the spot thick- 

 ness indications will vary as much as plus or minus 0.010 inch while the 

 lengthwise average remains stable as sho^\^l in Fig. 14. These fluctuations 

 are sheath thickness variations \\'hich result from the complex interaction 

 of the many sheathing line variables, but they may be increased or de- 

 creased b}^ response to luieven flooding distribution and/or variations 

 in surface curvature. However, it is practical to visually average this 

 graph to within ±0.001 inch. 



For die adjustment, thickness measurements are obtained visually by 

 estimating the average of the fluctuations of the recorder's visual indica- 

 tor. Measurements are taken at quadrant locations corresponding to the 

 locations of the four die adjusting screws. Opposite thicknesses give the 

 amount of eccentricity. Die adjustments can be made accurately because 

 the amount of eccentricity is known and the amount of die movement is 

 governed b}- the adjusting screw pitch. 



Adjustment to specified average sheath thickness is made by averaging 

 measurements at eight positions equally spaced around the sheath. In- 

 creasing the speed of the cable in relation to the speed of extrusion in- 

 creases the stretch of the polyethylene and decreases the average 

 thickness. Decreasing the cable speed' increases the average thickness. 



APPLICATIOX OF TEST EQUIPMENT FOR SHEATH INSPECTION 



The thickness test provides an accurate gage for the inspection organi- 

 zation to mea.sure compliance of the sheath to specified requirements. 

 Inspection possibilities with the thickness test set are many and the 

 problem becomes one of an economic procedure that will assure the re- 

 quired quality. Continuous recording of the entire cable length is prac- 

 tical but is unnecessary from a manufacturing viewpoint. Recorder chart 

 speed is one half inch per minute and cable speeds are from 20 to 100 



