extendable near its free end whereas in the region 79 the other arm of the u will have this property the region 80 will have properties similar to the region 73 but because the strips 90 are narrower than the strips 83 it will not be quite so stiff one of the reasons for incorporating some degradable warps in the wire carriers disclosed in fig1 3 or even in the construction of fig4 is to control stretch and to prevent twisting of the wire during manufacture of an extrusion incorporating the carrier it is also possible to achieve the same degree of control in slotted metal carriers referring now to fig7 a this shows a modification of the regions 75 and 79 of the stamped or slotted metal or plastics carrier of fig7 when the fig7 carrier is subjected to tension forces for example when the carrier is being pulled with an extruded cover out of an extruder there is a tendency for the carrier to stretch at lateral locations spaced from the laterally offset connecting pieces 87 and 89 this would result in a twisted i e non straight end product this phenomenon can be prevented by providing frangible connecting straps 91 as shown in fig7 a these straps 91 have reduced width control regions and are located adjacent the edge of the strip opposite the connecting pieces 89 or 87 this means the carrier.