Thickness Measurement and Control in 



the Manufacture of Polyethylene 



Cable Sheath 



By W. T. EPPLER 



(Manuscript received October 22, 1953) 



'The manufacture of multiple sheath for Alpeth and Stalpeth cables re- 

 quires the application of a sheath of polyethylene over a sheath of corrugated 

 metal which is flooded with a rubber asphaltic compound. For high quality 

 and minimum cost, this outer sheath must be of uniform thickness through- 

 out its length. One of the problems in cable sheath manufacture is to maintain 

 the concentricity and average thickness of the extruded polyethylene sheath 

 to close limits during manufacture. This article reports on: (1 ) The applica- 

 tion of a capacitance sensitive bridge to the measurement of the eccentricity 

 and average thickness of the sheath on cables moving at speeds of 20 to 100 

 feet per mimde; {2) The method of thickness calibration; and (S) The use of 

 the thickness measurements in maintaining the sheath concentricity and 

 average thickness within close limits during the sheathing operation. 



mSTOEY 



In the manufacture of multiple sheath for Alpeth and Stalpeth cables, 

 an outer sheath of polyethylene is applied. It is desirable for high quality 

 and low cost to make this outer sheath of a uniform thickness throughout. 

 The construction of these cables is shown in Fig. 1. In both designs, the 

 outer sheath is polyethjdene extnided onto a corrugated metal under- 

 sheath which has been flooded -with a i*ubber asphaltic compound. 



The extrusion art had been unable to obtain a high degree of control, 

 primarily because measurements of the thickness could not be obtained 

 until after the sheath was applied to the cable core. Eccentric sheath 

 must have a greater average thickness than concentric sheath, if the 

 thickness of the thin side is not to fall below a required minimum thick- 

 ness. 



The s^Tnmetrical design of a tj'pical core tube and die for sheathing is 

 shown in Fig. 2. Concentric set-up of these extrusion tools around the 



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