560 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



cable core "will not produce concentric extruded sheath. This is caused 

 by an unbalance in the plastic flow in the extruder. The flow makes a 

 ninety degree turn from the extruder cylinder into the die head, and to 

 reach the far side of the die, must flow around the core tube. The flow 

 resistance also varies with changes in the temperature of the plastic and 

 of the extruder screw speed. 



The core tube is fixed in position in the extruder head. The die is 

 located around the core tube and can be moved in any direction eccentric 

 to it. Fig. 3 shows a core tube and die mounted in the extruder head and 

 indicates the location of the four die adjustmg screws by which move- 

 ment of the die in relation to the fixed core tube is accomplished. The 

 die must be located at some one eccentric position in relation to the core 

 tube to compensate for the differences in flow resistances in the head. 



To set the die for concentric sheath and to adjust for specified thick- 

 ness the ijrevailing practice of the cable art of measuring the wall thick- 

 ness of a sample taken from the lead or finish ends of the sheathed cable 

 Avas of necessity resorted to because it was the best technique available. 

 The cutting of a ring of sheath and the micrometer gage are shoAMi in 

 Fig. 4. These end samples only approximate sheath conditions because 



CONDUCTORS 



CORE WRAP 



— ALUMINUM SHEATH 



SOLDERED STEEL SHEATH- 

 SEAM CEMENT 



FLOODING COMPOUND 



POLYETHYLENE 

 JACKET 



ALPETH DESIGN STALPETH DESIGN 



Fig. 1 — (Left) Telephone exchange cable of Alpeth design; (right) Stalpeth 

 design. 



