GB1582021A - Extruding material around an elongate core - Google Patents

Description

(54) EXTRUDING MATERIAL AROUND AN ELONGATE CORE (71) We, WESTERN ELECTRIC COMPANY, INCORPORATED, of 222 (formerly 195) Broadway, New York City, New York State, United States of America, a Corporation organized and existing under the laws of the State of New York, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to extruding material around an elongate core, e.g. insulating tinsel conductors and, to a method of and appparatus for covering tinsel conductors.

Telephone cords which connect, for example, telephone handsets to the telephone base generally comprise a polymeric core having a plurality of tinsel ribbons wrapped helically thereabout. These cords may have either a linear configuration or may be wound in a helical configuration comprising a plurality of convolutions, the latter being referred to as a retractile or spring cord.

In a recently introduced modular telephone cord design, miniature type connectors are connected to each end of a line or spring cord to facilitate attachment to telephone instruments. Such cords should not be too strongly retractile.

In the past, the tinsel conductors were covered with a nylon knit and then insulated with an extruded polyvinyl chloride (PVC) composition. Subsequently, the plurality of individually insulated conductors were jacketed with a plasticized PVC composition. With the introduction of modularity, it was necessary to use a different cord construction because of a need for a smaller cross-section. In order to reduce the size of the insulated conductor, it was necessary to eliminate the knitted nylon covering over the served tinsel. The elimination of the protective nylon knit made it necessary to develop a tough insulation material which could function as a high strength barrier to the cutting action of the tinsel ribbon, as an electrical insulation over the tinsel conductor, and as a primary component to achieve resiliency in a retractile telephone cord. A plasticized nylon insulation was found to be suitable replacement for the knitted nylon covering.

The use of nylon in insulating tinsel conductors has not been altogether satisfactory.

Occasionally, portions of one or more of the tinsel ribbons protrude outwardly and cause protruberances in the slow crystallizing nylon insulation. As a result, the nylon-insulated conductors must be rewound and passed through a die to eliminate the protruberances. The plasticized nylon also has a tendency to creep under load thereby diminishing somewhat the effectiveness of the strain release system of the modular plugs.

It has now been discovered that by insulating a tinsel conductor with polyether polyester composition, the above difficulties can be overcome. A plasticized polyvinyl chloride jacket is formed over the plurality of the individually insulated tinsel conductors.

In attempting to manufacture tinsel conductors having an insulation of polyether polyester composition, however, problems have occured in obtaining a reliable continuously concentric insulation and one which avoids the problem of tinsel protrusions into the insulation which necessitate the rewind operation.

The invention provides means and a method for extruding material around an elongate core by advancing the core, extruding the material into enclosing spaced relation with the the advancing core, expanding the extruded material outwardly from the core whilst permitting the material to cool to initiate oriented crystalline growth therein, and drawing the extruded material down about the core.

In applying the invention to the above problems, the elongate core comprises a tinsel conductor.

Information relating to extrusion can be found in Wire and Cable Coaters' Handbook as published by the E.I. duPont Company in 1968; in Plastics Extrusion Technology and Theory authorized by Gerhard Schenkel and published by the American Elsevier Publishing Co. in 1966 and by Karl Hanser in Germany in 1963; Plastics Extrusion Technology, by A.L. Griff, published by Rinehold Book Corp.; an article entitled "Crosshead Tooling for Jacket Extrusion" by Joe B. Moss on pages 25-28 of the April, 1967 issue of the Western Electric Engineer; and, Wire Extrusion Techniques, pages 5-6 by D.C. Hank as published by the B.F.

Goodrich Chemical Company.

The present invention will be more readily understood from the following detailed description of specific embodiments thereof when read in conjunction with the accompanying drawings in which: FIG. 1 is a perspective view of one end of a telephone cord having insulated tinsel conductors constructed in accordance with an embodiment of this invention and showing the cord terminated with a modular plug; FIG. 2 is a sectional view of the cord; FIG. 3 is a fragmentary view of part of the cord; FIG. 4 is a view of an overall manufacturing facility, in schematic form of the embodiment method for producing insulated tinsel conductors FIG. 5 is a graph showing curves of apparent viscosity and temperature for several compositions used as insulation for tinsel conductors; FIG. 6 is an enlarged view of a portion of apparatus shown in FIG. 2 and showing portions of an extruder crosshead and of cooling facilities; FIG. 7 is an enlarged detail view of a portion of the extruder die and core tube; and FIG. 8 is a perspective view of an apparatus for cooling insulation extruded over the tinsel conductors.

Referring now to the drawings, and particularly FIG. 1 thereof, there is shown a retractile or spring cord designated generally by the numeral 10 having a plug 25 at both ends thereof (only one end is shown). It should be understood that while the embodiment provides a spring cord, the invention is not so limited and is applicable generally to the manufacture of cordage which includes a jacketed plurality of individual conductors which may be used for either a spring or line cord.

The spring cord 10 is for use of telephone instruments which include a plurality of insulated tinsel conductors, designated generally by the numerals 11-11. Each of the insulated tinsel conductors 11-11 includes a nylon multi-filament center core, designated generally by the numeral 12 about which preferably a plurality of tinsel ribbons 13-13, made typically from a Phosphor-Bronze material are wrapped helically to form a tinsel conductor, designated generally by the numeral 14 (see FIG. 1).

An insulating covering 18 of a suitable plastic material is extrusion tubed over the tinsel conductor 14 to form one of the insulated tinsel conductors 11-11. Dimensional constraints imposed by devices to which the cord 10 is assembled necessitated a reduction in size of the cord. This necessitated the elimination of the priorly used knitted nylon covering over the individual served tinsel conductors. The elimination of the protective nylon knit produced an irregular rough conductor capable of cutting into priorly used PVC insulation. Moreover, the nylon knit component was a major contributor to the overall resilience of priorly constructed spring cords.

In summary. conductor insulation material is required to function as a high strength barrier to the cutting action of the served tinsel ribbon conductor 14, an electrical insulation over the tinsel conductor, and the primary component to achieve resiliency in a retractile cord, resiliency being a measure of the ability of a retractile cord when extended to return to its original unextended shape when the load is removed.

The basic insulation polymer 11 which is utilized is a thermoplastic material, a polyether polyester block copolymer based on short chain ester groups derived from 1, 4 butane diol terephthalate and long chain ester groups based on terephthalate esters of polytetra methylene glycol (hereinafter "PTMEG"). More particularly, one suitable insulation 18 is a composition which includes approximately the following weight percent constituents: 15.7% PTMEG, having a number average molecular weight of about 1000, 32.4% of 1, 4 butane diol and 50.7% of a terephthalic ester-containing compound such as, for example, terephthalic acid. It will be observed that the composition comprising the insulation 18 is produced by reacting two glycols, i.e., the 1, 4 butane diol and the PTMEG, with the terephthalate ester-containing compound to form an ester and ether which results in a polyether polyester copolymer.

The foregoing composition also includes about 1% catalytic residue and is preferably stabilized with about 0.2% of a long chain hindered phenolic antioxidant such as, for example, (N, N '-hexamethylene bis (3, 5-di-terbutyl-4- hydroxy-hydrocinnamide). This is a symmetrical diamide composed of 2 moles of 3, 5 ditertiary butyl, 4 hydroxy hydrocinnamic acid and 1 mole of 1, 6 hexamethylene diamine. The long chain hindered phenolic antioxidant system offers migration resistance and is non-discoloring in the presence of ultraviolet light.

An insulation composition suitable for constructing the cordage 10 is available presently from the E. I. duPont de Nemours and Company, Inc. of Wilmington, Delaware under the trade designation HYTREL f) 7246 (HYTREL is a Registered Trade Mark), which is stabilized with an antioxidant available, for example, from the Ciba-Geigy Company of Ardsley, New York under the trade designation Irganox 0 1098 (IRGANOX is a Registered Trade Mark). A color concentrate such as a polyester elastomer available commercially from duPont under the designation HYTREL (g) 4056 and having a Durometer hardness as measured on the D scale of 40 combined with Pearl Afflair pigment available from duPont may be added to the HYTREL Q 7246 composition.

The composition comprising conductor insulation 18 is applied by using a tubing extrusion technique in which there is provided an air-induced space between the served tinsel conductor 14 and the tubed insulation 18. Extrusion of the polyester polyether thermoplastic composition is affected by extrusion temperatures and screw design since the insulation composition is characterized by rapid changes in melt viscosity and melt strength with slight variations of polymer temperature. Moreover, the material undergoes a rapid transition between liquid and solid phases. These characteristics could result in non-uniform wall thickness and polymer flow pulsations.

Advantageously, the effects of these characteristics are offset by the addition to the extruder charge of the lower molecular weight lower hardness, and lower melting point polyester in the form of a color concentrate. The lower melting, i.e., 334"F, polyester HYTREL (F) material resulted in stabilizing the melting point of the polymer in the extruder transition zone. In addition, the pigment portion of the color concentrate minimized variations in melt viscosity of the polymer resulting in a more uniform extrusion process with improved size control of the critical dimensions. It is believed that this is an unexpected result.

The lower hardness HYTREL t) polyester had been used as a pigment. It was unexpected that a color concentrate would also function as a processing aid.

An alternative insulation composition comprises about 19.4 percent by weight of 1, 4 butane diol, about 44.8 percent by weight of polytetramethylene glycol having a number average molecular weight of 1000, about 27.4 percent by weight of terephthalic acid and about 7.9 percent by weight of isophthalic acid.

The air-induced space between the tinsel conductor 14 and the insulation 18 allows the conductor to move within the insulation thereby reducing conductor fatigue. With an average conductor outside diameter of about twenty mils and the size limitation imposed by a modular-terminated cord 10, the tubular insulation 18 has an outside diameter typically on the order of thirty-seven mils. The criticality of the outside diameter coupled with approximately a two mil air space, necessitates a tubular wall thickness of about seven mils. This thin wall construction mandates that the polyether polyester thermoplastic insulation material possess excellent mechanical strength, such as, for example, cut-through resistance, hardness, tensile and compression strength. Insulation 18 can be drawn down about the core in an oval shaped configuration.

The polyester polyether thermoplastic insulation 18 is characterized by crystalline growth when cooled below the melt point temperature which is approximately 424" to 4280F. The crystalline growth which occurs generally within a defined temperature range below the melting point temperature of the copolymer makes possible horizontal extrusion tubing of the irregular tinsel conductor while developing the necessary strength and rigidity in order to maintain design limitations of the insulated conductor 11.

The polyether polyester insulation 18 constructed in accordance with the composition disclosed hereinbefore has a Durometer hardness of 72 as measured on the D scale and as determined in accordance with specification of the American Society of Testing Materials (ASTM D-2240), and a compression strength of 30,000 psi as determined in accordance with ASTM D-692. In comparison, priorly used plasticized nylon insulation exhibited a hardness of about 50 D. These characteristics provide crush resistance to the individual conductors 11-11, e.g. as where a telephone cord 10 would be caught in a sliding glass door or between a wall and desk. Superior cut-through resistance is also provided thereby preventing the sharp tinsel conductor from cutting through the insulation 18 of a cord 10 upon flexing thus maintaining the integrity of the cord.

The polyether polyester thermoplastic insulation 18 exhibits a modulus of rigidity of approximately 75,000 psi as determined in accordance with ASTM D-790. This characteristic alllows the unknitted insulated conductor 11 to be processed and taken up without entanglement of the insulated conductors. This is a necessary characteristic in order to remove the insulated conductor 11 for subsequent jacketing operations.

The high tensile strength of the polyether polyester thermoplastic insulation 18, i.e. 6,000 psi, as determined in accordance with ASTM D-638, in conjunction with the above hardness and compression strength achieves excellent plug-pull resistance, i.e. of the force necessary to remove an end device from the cord 10 after termination to the cord. This property is necessary to prevent an easily applied force from disconnecting the terminated cord 10 resulting in loss of electrical transmission.

A suitable conductor insulating material is a polyester polyether termoplastic composition which in some literature distributed by E. I. duPont Company and relating to a product line under the general designation HYTREL 0 is described as a polyester elastomer material.

The ASTM's Glossary of Definition's ASTM Definitions, second edition 1973, defines an elastomer as a natural or synthetic polymer which at room temperature can be stretched repeatedly to at least twice its original length and which after removal of the tensile load will immediately and forceably return to approximately its original length. Since the polyether polyester thermoplastic possesses a minimum permanent set of approximately 88%it cannot be construed as a elastomeric but should be considered as a thermoplastic material.

The polyether polyester thermoplastic composition is also disclosed generally in U.S.

Patents 3,651,014 and 3,763,109 both of which are incorporated by reference hereinto. See also U.S. Patent 3.766,146 for a composition of reduced hardness. HYTREL Q plastic materials are also well described in E.I. duPont's brochure titled HYTREL Q polyester Elastomer and having a designation A-99608. As disclosed in that brochure, HYTREL Q plastics span a range between rubber and rigid plastic materials with Durometer hardness, as measured on the A and D scales, ranging from 92 A to 63 D. Softer members of the series resemble elastomers rather than plastics while the converse is true of the harder members of the series. In fact, subsequent trade literature by duPont designated E-00862 discloses properties for a still further harder HYTREL Q plastic having a Durometer hardness of 72 D. This latter HYTREL 0 plastic is designated HYTREL 0 7245.

A plurality of the insulated tinsel conductors 11-11 (FIG. 1) are arranged in parallel, nontwisted, contiguous relationship with respect to each other and enclosed in a jacket which is comprised, for example, of a plasticized polyvinyl chloride composition. Subsequently, the cord 10 is completed by attaching to one or both ends thereof a modular plug 25.

FIG. 2 is a cross-sectional view through cord 10 and shows a plastic insulating jacket 22 of a suitable plastic material extruded over the insulated tinsel conductors 11-11 to form the jacketed cordage, designated generally by the numeral 23. A plastic material suitable for use as a jacketing material is that disclosed in U.S. Patent 3,941,908 issued March 2, 1976 in the names of M. P. Valia and W. C. Vesperman.

The jacketed cordage 23 may be made into straight cords or various lengths by cutting indefinite lengths of the cordage to a desired length. Subsequently, one of the modular plugs 25-25 (see FIG. 1) made in accordance with the disclosure of hereinbefore mentioned U.S.

Patents 3,699,498.3,761.869 or 3,860,316. is assembled to each end of the length of cordage 23.

The jacketed cordage 23 may be in straight line form or fomed into spring cords 10-10 of various lengths having different numbers of insulated conductors 11-11 therein. For example. the number of insulated conductors 11- 11 are commonly three to eight, and the nominal extended lengths of the cords are commonly 4feet. 1/2 feet, feet and 13 feet. The spring cords 10-10 are formed preferably as disclosed in U.S. Patents 2,920,351 and 3,024,497.

Not only are these properties imparted to the cordage 23 by the polyester polyether copolymer, which is significantly less expensive than nylon, but the results were unexpected.

For example. it will be recalled that a spring cord 10 is produced by winding a plurality of convolutions of cordage 23 on a mandrel (not shown) after which the wound cord is heat set by being exposed to elevated temperatures. Generally, it is assumed that the higher the melting point of the insulation the higher must be the heat-set temperature. The melting point of the plasticized nylon. i.e. about 365 F, is greater than the "melting point" of the PVC jacketing material i.e. temperature at which the plasticized PVC flows without coercion, which is about 285 F, but that of the HYTREL i) plastic is in excess of that of the nylon. The melting point of a polyether polyester copolymer insulating composition described hereinbefore is approximately 424"F. Therefore, it was expected that if the polyether polyester composition was used, a heat-set temperature substantially higher than that for the nylon would be required to set the helices of the cordage 23 wound on a mandrel (not shown) to produce a retractile cord 10.

It would appear that if the polyether polyester copolymer insulation was heat-set at a higher temperature than that used for nylon, it could undesirably fuse to the PVC jacket 22.

Moreover, a fusing together of adjacent ones of the convolutions of the helically wound cord 10 could occur. On the other hand, if the insulating material is not heated to a temperature in excess of that used to heat-set the priorly used lower melting point nylon, then it would be expected that it would not be heat set and that when it was removed from the mandrel (not shown), it would return to an uncoiled configuration and exhibit poor mechanical memory, i.e. creep resistance and retractility.

Surprisingly, even though the coiled cordage 23 having conductors insulated with a HYTREL 0 plastic having a Durometer hardness of 72 measured on the D scale and a melting point of 424"F is heated to a temperature substantially less than its melting point, the resultant cordage has excellent retractility--extensility characteristics. It was found that a temperature approximately the same as that used for the nylon, e.g. 260"F, was sufficient to heat set the polyether polyester thermoplastic material. Moreover, the nonplasticized polyether polyester copolymer composition avoids the problem of plasticizer migration thereby further avoiding fusing of adjacent ones of the helices.

Upon termination, it is necessary to remove a portion of the outer jacket 22 exposing the individual conductors. Advantageously, the insulation 18 exhibits relatively high melting and softening temperatures, i.e. melting point of 424"F and softening point of 397"F, respectively. This characteristic is important when the PVC jacket 22 is extruded over the plurality of individually insulated conductor 14-14. The high melting and softening temperature of the HYTREL Q prevents the lower "melting point", i.e. about 300"F, plasticized PVC jacket 22 from adhering to the individual insulated conductors 11-11 and causing termination problems.

The extensility-retractility properties of a cord 10 having conductors 11 - 11 insulated with a polyether polyester copolymer material is also surprising. During the development of a retractile cord having improved properties, a polyether polyester thermoplastic material having a Durometer hardness of 63 measured on the D scale was used, but resulted in unfavorable creep characteristics as well as poor retractility properties, i.e., while the cord could be extended with an acceptable force, a longer time was required for the helices to retract than with the priorly used nylon insulation. In order to complete the investigation, a polyether polyester material having a Durometer hardness of 72 D was used and since the literature discloses that the resilience of such a material decreases over that of a 63 D hardness material as well as having a higher melt point, it would be reasonable to assume that the extensility-retractility properties would worsen.

It will be recalled from the ASTM definition hereinbefore that an elastomer generally fully recovers from an extension while plastics do not. Therefore, it would be expected that the cord 10 of this embodiment would not have the extensility-retractility properties of one which employed a truly elastomeric insulation.

Surprising, the cord 10 exhibited outstanding retractile-extensile properties.

That this result is surprising appears to be further strengthened from E. I. duPont's HYTREL 0 brochure HYT-SO1 and designated A-99590. There it can be seen that the lower hardness HYTREL 0 plastics exhibit classical S-shaped stress-strain curves of an elastomer. On the other hand, the harder HYTREL Q plastics exhibit pronounced yield points at 50% and 25 % strain, respectively, for 63 D and 72 D. It would hence appear that if either of those are strained beyond these limiting values. that they would develop an irreversible permanent set. Therefore, it would not appear that the retractile properties of a cord having conductors insulated with such a material would be acceptable because of poor recovery.

Further. the polyether polyester copolymer has a flexural modulus of approximately 75,000 psi as compared to 1,200 psi for vinyl and 50,000 psi for plasticized nylon. It was expected that the use of a polyester thermoplastic material having a modulus so much in excess of the plasticized nylon would result in a cord which required excessively high forces for extension thereof. This expectation seemed to be reinforced by the observation that cords having conductors insulated with lower modulus polymers. e.g.. 30.000 to 50,000, resulted in cords having excellent extensibility properties; however. these polymers did not have adequate creep, retractility and cut-through resistance. Surprisingly. the polyether polyester material having a 75,000 modulus resulted in a cord which demonstrated superior strength and extensibility properties.

The cord constructed in accordance with the embodiment exhibits superior resilient characteristics as compared to the conventionally plasticized, i.e. 60.0 parts of DOP per 100 parts of PVC resin PVC insulation material, and has a torsional resilience of one second as compared to 15 seconds for the plasticized PVC insulation. Low temperature impact characteristics of the polyester polyester thermoplastic material are maintained to -940F whereas those for the conventionally plasticized PVC insulation are maintained to -100F, in accordance with ASTM D-746.

Further benefits accrue because the insulation 18 is a non-plasticized polymer as compared with the conventional priorly used PVC or nylon insulation which require external plastification to achieve low temperature impact, resilient and acceptable flex modulus properties.

Additional plasticized levels above 50.0 parts per hundred parts of PVC resin will improve both low temperature impact and resilient properties, but will substantially reduce the softening point of the PVC insulation thereby causing excessive jacket insulation adhesion.

During heat setting, it has not been uncommon that the plasticizers from PVC or nylon plasticized insulation migrated into the PVC composition jacket 22. This causes an undue softening of the jacket 22 with attendant fusion together of the jacket and conductors 11-11 thereby reducing the ability to remove the outer PVC jacket in order to expose the individual conductors in field stripping operations. Moreover, the softer jacket 22 causes, undesirably, adjacent ones of the convolutions of the helices of the coiled cord to adhere together.

Plasticizer migration also contributes undesirably to deformation under load thereby affecting adversely stain relief systems of the plugs 25-25.

Also, the use of non-plasticized insulation allows the use of higher annealing temperatures, thus imparting improved stress relief to the mandrel-wound telephone cordage and promoting improved mechanical memory, resilience and creep resistance. Higher annealing temperatures are rcalized when using a non-plasticized insulation as opposed to using a conventionally plasticized PVC compound. Improved creep resistance, i.e., resistance to long term deformation under load of the initially set helical configuration, attributed to higher annealing temperatures. is dramatized in the utilization of wall telephone sets where the cord 10 is suspcnded and hangs from the receiver. The polyether polyester thermoplastic insulated retractile cord 10 exhibits a creep deformation of 25% as compared to 48% for a standard PVC insulated cord over a four month period.

The hardness, improved crush resistance, improved resistance to creep and the characteristic of internal plasticizing provide the cord 23 with properties which cooperate with the plugs 25 to maintain the strain relief system. In the plug 25, a free end of the cord 22 is inserted into an unipartite housing of the plug after which plastic portions of the housing are moved into engagement with the conductors 11- 11 and with the jacket 22 to provide strain relief therefor during use. Advantageously, the polyethcr polyester insulation does not exhibit appreciable creep under load and resists the tendency of the impacting portions to crush the plastic insulation. Also, externally added plasticizers in plasticized materials have a tendency to flow or shift when the material is subjected to load. This is avoided with the one of the internally plasticized polyether polyester materials which comprises the cord 10.

It was believed that the specific gravity of the polyether polyester copolymer, i.e 1.23 as compared with nylon 1.05. would add undesirably to the weight of the cord 10 and hence cause poor creep performancc. Contrary to this expectation, notwithstanding the additional weight the cord l 0 having polycther polyester copolymer insulated conductors 11 - 11 display creep resistance which is superior to that of a nylon-insulated conductor cord.

It may be observed from a table of properties that a polyether polyester material of the composition disclosed hercinbefore</

In prior tubing operations, for a given outside diameter, a specific wall thickness was obtained. It has been found that for insulated conductors 11-11 of the size contemplated for modularity, a normal tubing operation is not adequate to control the dimensions of the insulation 18. Moreover, while a HYTREL (g) plastic material is most advantageous form the standpoint of properties, its consistency tends to vary thereby necessitating a greater degree of control.

In a tubing operation such as that described hereinbefore, problems also occur because the insulation 18 is to be drawn down on the advancing tinsel conductor 14 which is irregularly configured. The advancing tinsel conductor 14, because of its construction, is not of solid conductor design, and occasionally has thin slivers of tinsel material, metal burrs or lumps protruding outwardly from the periphery thereof. It has been found in the past that these slivers or burrs intrude into insulation drawn down thereonto causing protuberances in the outside surface of the insulated conductors. For that reason, it was sometimes necessary to rewind the insulated conductor in order to locate and correct these protuberances.

It has been found that by controlling the draw down of the insulation 18 about the conductor 14, the outside diameter of the insulated conductor 11 as well as the wall thickness of the insulation cover can be controlled and the occurrence of irregularities in the surface of the insulated conductor can be minimized. The control is to be obtained by maintaining the extrudate spaced out from the tinsel conductor 14 whilst it begins to crystalise. This is made possible by the relatively high melt viscosity and melt strength of HYTREL 0 type plastic.

Melt strength is intended to refer to a property of plastics which is analogous to a measure of ductility in metals.

The aforementioned dimension and strength control are also facilitated by the polyether polyester copolymer which comprises the insulation 18 being a crystalline material in which crystalline growth occurs in the range of about 140"F to about 420"F. In order to resist disfiguration of the outward configuration of the insulated conductor 11 by irregularities in the tinsel conductor 14, it would be most advantageous to provide for substantial crystalline growth and development of sufficient melt strength in the copolymer prior to the copolymer being caused to assume its ultimate position relative to the tinsel conductor. This may be accomplished by extruding the copolymer at a temperature slightly above but preferably as close as is conveniently possible to its melting point, and by spacing the extrudate from the tinsel conductor 14 for a time sufficient to obtain a desired crystal growth and development of melt strength. The melting point of the copolymer is approximately the maximum temperature at which crystal growth of the copolymer will occur.

In order to develop the high melt viscosity of HYTREL (E) material, the temperature of the extrudate as it flows out of the extruder 33 should be slightly greater than but as close to the melting point of the composition comprising the extrudate, i.e. 424"F to 428"F. Typically, the temperature of the melt at the extruder die opening is about 435"F which although just a few degrees more than in the extruder barrel increases the melt viscosity because of the steepness of the curve of viscosity versus temperature of the composition which comprises the insulation 18.

The E. I. duPont Company has published a number of brochures which describe the properties of the HYTREL 0 polyester elastomer materials. For example, one of the brochures published by duPont is entitled Rheology and Handling. This particular brochure is important in discussing melt viscosity of the insulation material. Melt viscosity goes to the characteristic of how viscous the material is at a specified temperature. A low viscosity material flows very freely within the extruder 33. The melt viscosity for HYTREL (g) changes rapidly with time and temperature. For example, as shown in FIG. 3 the apparent viscosity in poises for HYTREL (g) 7246 plastic drops from approximately 7 x 104 at a temperature of 430"F to approximately 1 x 104 poises at a temperature of approximately 470"F. By comparison, plasticized nylon has an apparent viscosity of approximately 7 x 104 poises at approximately 420"F and drops to about 3.75 x 104 poises at a temperature of 480"F.

Moreover, the vinyl melt viscosity does not change as drastically with respect to temperature.

Nylon and HYTREL (g) become watery. HYTREL 0 much more so than nylon.

Further, it is important with the HYTREL (E) material to obtain a uniform melt because the HYTREL 0 material is a crystalline material, with a crystallization rate about one and one half times the crystallizations rate of nylon. If the melt is uniform, then the crystallization rate becomes more uniform.

Since the temperature of the insulation composition is dropped adjacent the die opening in order to maximize the melt strength of the extrudate, it would appear that the flow path in the extruder crosshead should be as streamlined as possible. If the flow path in the extruder 33 were not streamlined, it would seem to follow that the thermoplastic material would remain for a longer period of time within the extruder crosshead and would therefore possibly degrade. See page 254 of Processing of Thermoplastic Materials, "High Density Polyethylene Wire Extrusion" by C. Lowe and W.H. Joyce on pages 862-865 of the July, 1960 issue of Wire and Wire Products, and "The Design of Dies for High-Speed Wire Coating" by L.R.

Hammond on pages 725-728 and 783-785 of the June, 1960 issue of Wire and Wire Products.

If degradation occurs, it would appear that the thermoplastic material would lose melt strength which is required in order to form the next step of the process wherein the insulation 18 is spaced substantially from the tinsel conductor 14 while the insulation crystallizes. If the melt strength decreases, then it would seem that air pockets would occur undesirably in the insulation 18.

As shown in FIGS. 4 and 6, the extruder 33 has a barrel 39 in which there is formed a cylindrical bore (not shown) in which a stock screw (not shown) of the type for example shown in U.S. Patent 3,579,608, which is incorporated by reference hereinto, is rotated by suitable source of power (not shown) for the purpose of forcing the insulating material 18 such as HYTREL (g) polyester elastomer through an extruder crosshead, designated generally by the numeral 41. The crosshead 41 comprises a body member 42 provided with an opening forming a continuation of the bore in the barrel and which communicates with a cylindrical bore 44 formed in the body member 42 transversely with respect to the barrel 39.

A cylindrical tool holder 50 having a central bore 51 which extends coaxially with respect to the bore 44 is removably mounted in the body member 42 by a back head nut 53 and an adapter nut 54. The tool holder 50 supports a die 59 in alignment with the bore 51 and mounts a core tube 61 in axial alignment with die 59. The tool holder 50 is designed to deflect insulation material 18 from a direction flowing downwardly as viewed in FIG. 4 to a direction flowing to the right around the core tube and through the die 59 to form concentrically the covering 18 around the tinsel conductor 14 being advanced therethrough.

Referring to FIGS. 6 and 7, there is shown in detail the core tube 61. The core tube 61 includes a stepped cylindrical shape having an enlarged base portion 66, a second portion 67 having a reduced diameter but with outside walls thereof being parallel to the centerline of the core tube 61 and a third cylindrical section 68 which is connected to a tapered conical portion 69 finally culminating in a cylindrical portion 71. The core tube 61 is designed to be received in the cavity 51 in the crosshead 41 supported therein such that the cylindrical portion 71 extends at least to a surface 75 of the crosshead to which the die 59 opens. In this way, the extruder 33 is designed to provide a tubing insulation over the conductor 14 such that the insulation 18 is pulled down on the conductor as it is advanced out of the extruder, thereby facilitating the implementation of an air space between the insulation and the tinsel conductor.

As can best be seen in FIG. 6, the core tube 61 is constructed with a tapered bore 72 extending through the portions 66-68. The bore 72 communicates with a cylindrical bore 73 which opens to the atmosphere (see FIG. 7).

The die 59 is constructed with a cavity 76 having a side or bearing wall 77 which has a frustoconical configuration and converges at some predetermined angle toward an opening 79 of the die 59. Typically the angle formed between a line of generation of the frustoconical wall 77 and the centerline of the core tube 61 is on the order of magnitude of 15 to 300.

Surprisingly, with the use of the HYTREL 0 insulation it was determined that an angle in the range of 60 to 90" and preferably 63" would be required in order to extrude successfully the HYTREL 3 insulation 18 about the tinsel conductor 14.

The plastic material flows between a wall 80 of die cavity 76 and the frustoconical portion 69 of the core tube 61 which are spaced apart in a diverging direction (see FIG. 7). The plastic material flows at a high velocity to the end face of the core tube 61. Just to the left of the end walls 77-77. as viewed in FIG. 7, the cross-sectional area of the flow channel is greater than that prior thereto in the diverging portions of the die and the core tube 61. The pressure of the material on all sides of the portion of the core tube 61 is balanced. The tinsel conductor 14 is guided from the core tube 61 substantially in alignment with the cylindrical passage 79 of the die 59 so that the tinsel conductor is centered generally within the extrudate as the tinsel conductor is advanced out of the cylindrical bore 73 and the insulation cover material is extruded through the cylindrical passage in the die.

The configuration of the die cavity 76 having an unusually large approach angle of the bevelled bearing portion 77 causes an acceptable extrusion of the insulation notwithstanding the creation of an enlarged so called "dead space" 81. It should be apparent from FIG. 7 that the "dead space" 81 would appear to result in the extrudate being retained within the die for a longer period of time.

In viewing the orientation of the core tube 61 and the die 59 as shown in FIG. 7, it would appear that because of the modification of a typical arrangement having a streamlined passageway converging toward the land 78, that the "dead space" 81 created toward the forward end of the die 59 on radially opposite sides of the core tube 61 would cause turbulent flow in the HYTREL ( ) material or act as a reservoir to avoid starving the extrudate flow.

Gencrally it would seem as though a streamlined flow would be desirable in order to extrude a perfectly concentric material about the advancing tinsel conductor 14. Surprisingly, it has been found that the use of streamlined passages between the streamlined flow passages of the core tube 61 and the walls of the die cavity 76 does not produce an acceptable product.

Rather it has been found that a die cavity 76 should be constructed having an approach angle of the wall 77 diverging from the land 78 to the innerwalls of the die cavity in the range of 1200 - 1800 to extrude a generally concentric covering about the tinsel conductor 14.

It is extremely desirable to maintain the molecular weight of the extrudate as the melt viscosity decreases. Since the molecular weight tends to decrease, it is important to attempt to extrude the thermoplastic material through the die opening as rapidly as possible, in order to maintain the strength thereof. This further strengthens the aforementioned expectation that it would be desirable to minimize the holdup time of the extrudate within the extruder 33 die.

On the other hand, if the die 59 is designed so that the approach angle or angle of attack, i.e.

the included angle or angle embraced by the beveled side wall 77 of the die cavity 76 diverging from the throat 78 of the die adjacent the die opening 79, is substantially great, it would appear that the insulation composition may undergo degradation within the extruder crosshead 41.

One explanation for the unexpected success of the die construction described hereinabove relates to the affinity of HYTREL 0 material to metal as compared to nylon for example.

The HYTREL (g) material may coat effectively the walls 77 and 80 of the die cavity with a laminar layer thereupon providing a measure of lubrication for HYTREL 0 material advanced between the laminar layer and the external face of the core tube 61. This facilitates an improved flow of the thermoplastic material within the flow path.

If the angle of attack were substantially less, e.g. the typical order of magnitude of 60 , the buildup of the layer on the walls of the die cavity could unduly restrict the flow path and lead to pulsations in the extrudate about the tinsel conductor 14. By using a larger angle of attack, there is sufficient size to the flow path after buildup of a laminar layer to obtain advantageously a uniform melt flow because of the flow of the copolymer material along a flow path bounded at least on one side thereof with a coating which may act as a lubricant.

An additional advantage accrues to the process from what is believed to be a buildup of a laminar layer of copolymer along the walls 77 and 80 of the extruder die 59. This layer may act advantageously as a heat shield to insulate the copolymer in the flow path between the die and the core tube 61 from the heat of the crosshead thereupon facilitating extruding the copolymer at a temperature only slightly or marginally above the melting point thereof.

It will be seen from viewing FIG. 6 that the initial over-spacing of the HYTREL (g) insulation from the tinsel conductor is accomplished by using the core tube 61 having the tapered cavity 72 at the upstream end thereof. In this way, a gaseous medium such as, for example, air at ambient comparative and a pressure of 20 psi may be introduced through a flow rate meter 85 to introduce air at a volume flow rate of about 6 to 8 cubic feet per hour through a tube 86 into an opening 87 which communicates with a passageway 88 through a member 89 disposed concentrically with respect to the centerline of the crosshead 41 and into the tapered cavity between the walls of the cavity and the tinsel conductor 14 being advanced centrally therethrough. The air which is driven under pressure toward the downstream or exit end of the extruder crosshead 41 enters into the cylindrical portion of the core tube 61 between the tinsel conductor 14 and the walls of the core tube (see FIG. 7) and finally exits out from the extremity of the core tube which protrudes into the die opening 79.

The effect of the construction of the core tube 61 with respect to the die opening 79 in cooperation with the air pressure causes the HYTREL 0 insulation to "balloon" or expand outwardly from the tinsel conductor as shown in FIGS. 6 and 7. Then, after the conductor 14 is advanced through a predetermined distance at a specific line speed sufficient for crystalline growth of the HYTREL 0 insulation to occur, the insulation is drawn down about the tinsel conductor.

Crystalline polymers such as, for example the HYTREL (g) copolymer exhibit improved strength and flexibility when they have been treated in a manner to orient the polymer molecules parallel to one another and parallel to a major surface of the insulation 18. Ir provisions are made for molecular orientation, then the crystalline structure obtained during a period of crystalline growth will be oriented. Moreover, it has been found that molecular orientation enhances the rate of crystal growth.

The introduction of the air into the core tube 61 to expand the insulation cover 18 for a short distance downstream of the die opening 79 stresses the insulation. This causes the desired molecular orientation to occur within the copolymer material. It should be noted, however, that the use of air to expand and molecularly orient the copolymer is made possible by extruding the copolymer at a temperature slightly or marginally above, but as close as possible to, the melting point of the copolymer extrudate. If the copolymer extrudate was extruded at a temperature substantially higher than the melting point of the copolymer, any attempt to expand the extrudate would rupture the insulation 18. Such a higher temperature would not be regarded as being close to the melting point.

The polyether polyester material exemplified by HYTREL (g) polyester elastomer has a melting point about 424"F to 428"F. Above these temperatures, crystalline growth of the polymer material does not occur. It has been found that the crystalline growth of the particular polymer material hereinbefore identified experiences crystalline growth in a temperature range of 140"F to about 420"F and that maximum growth occurs in the range of 1800F to 200"F.

The introduction of the air into the core tube 61 to expand the extrudate outwardly from the tinsel conductor 14 cools advantageously the extrudate below the melt temperature at the die opening 79. This causes the temperature of the polymer to decrease to within the aforementioned temperature range wherein crystalline growth occurs.

The crystalline insulation material will have suitable strength to overcome any upstanding tinsel slivers and to compress them toward engagement with the configuration of the tinsel conductor. Moreover, as the insulation 18 assumes its generally final positions relative to the tinsel conductor with a wall, e.g. two mil air space therebetween, it has sufficient melt strength to stretch over any burrs or lumps of metal of considerable size which may occur on the tinsel without rupture. Advantageously, this results in a continuous reliably configured insulated tinsel conductor 11 which obviates the necessity for a rewind operation.

Subsequently, the insulated tinsel conductor 11 is advanced along a path through the treating facilities 34 (see FIG. 8) whereat the insulation is treated with a fluid, e.g. water, to cause the insulation to be cooled in a predetermined manner. The cooling facility 34 typically comprises a trough 90 in order to treat and anneal the insulation covering the tinsel conductor 14.

Referring now to FIG. 8, the treating facilities are shown in detail and may be observed to include a cold water section, designated generally by the numeral 91, and a hot water quench designated generally by the numeral 92.

The cold water section 91 includes facilities for engaging the insulation 18 with chilled water having a temperature on an order of magnitude of 50"-60"F. The section 91 includes a first V-shaped trough 93 mounted for movement on a rack 94 and spaced above the overall trough 90, supplied by a line 95 and having three spaced openings 96 - 96 therein for discharging three streams of chilled water into engagement with the tinsel conductor; the most upstream one of the streams being adjacent the exit port of the extruder. The upstream end of the water trough 93 is spaced a predetermined distance, "d", from the downstream end of the extruder crosshead plug 54. Changing "d" changes the outside diameter of the insulated conductor 11, the length of the cone of drawdown, and affects the crystallization rate of the copolymer.

A second V-shaped trough 101 is spaced below the first trough 93 and downstream thereof a sufficient distance to permit the discharge streams from the first trough to engage the conductor unobstructed. The second trough 101 is supplied by a pipe 102 to cause a stream of water at a temperature of about 50"F to 600F to be moved in a direction counter to that of the conductor being advanced therethrough.

Subsequently. the insulated conductor 11 is advanced through the section 92, wherein it is immersed in and treated with water having a temperature of about 150"F to 1600F. The section 92 includes a conduit 103, spaced above the trough 90, which serves as a catch basin, and an inlet 104 connected to a supply (not shown).

The embodiment apparatus optimizes the crystalline growth and moreover insures that a substantial percentage of the total growth occurs prior to takeup in order to avoid the material acquiring a permanent set corresponding to the configuration of the takeup barrel 37. The insulation material is extruded at a melt temperature of about 432"F in order to increase the melt strength thereof to facilitate the use of the air injection. The air injection is effective to cool the copolymer and promote crystalline growth as well as to orient the molecular structure of the copolymer by subjecting it to strain, which enhances the crystalline growth. If the extrudate were higher in temperature, expansion by air would rupture the insulation 18. Moreover. without the use of air injection, the extrudate at a temperature substantially in excess of the melting point of the copolymer would collapse on the tinsel with no air space; but possibly with bubble structures having formed and with non-uniform circular configurations. Thcn the cold water quench causes a rapid cooling which causes further crystallization to develop in the polymer material. Subsequently, in the section 92 the insulation is cooled more slowly to continue the growth of the oriented crystallization structure.

EXAMPLE I In a preferred embodiment for constructing an insulated tinsel conductor 11 having an outside diameter of 37 mils. an insulation wall thickness of 5.5 mils, and an air space between the insulation and the tinsel conductor of 2 mils. a tinsel conductor 14 was advanced at a line speed of 2,500 feet per minute through the extruder 33 having an L/D ratio of 24 to 1. In the extruder barrel, the feed zone temperature was about 400"F, the transition zone temperature was about 430"F, the metering zone temperature was about 440"F and that of the head about 455"F. The temperature of the extrudate at the die opening 78 was about 432"F. Air flow into the core tube was at a rate of about 8 cubic feed per hour. The extruder amperage was about 15 amps while the screw speed was about 32 RPM. The die had an opening of 0.141 inch with an approach angle of 126 , while the core tube 61 had an outside diameter of 0.090 inch. The composition of the extrudate included about 3 pounds of color concentrate per 100 pounds of HYTREL t) 7246 polyester elastomer. The depth of the channel in the feed zone was about 0.425 inch, of the metering zone about 0.150 inch, and the transition zone had a taper of from 0.425 inch to 0.150 inch. The cooling system 34 was arranged so that the upstream-most one of the cold water streams 96 - 96 of water at about 60"F was spaced 1.25 inches from the surface 75 of the die 59, with one inch between each of the three stream openings 96 - 96. The second trough 101 with water at 600F was spaced 4.5" from the downstream-most one of the streams 96 - 96. The entrance to the conduit 103 was spaced 4.5 feet from the die opening 79 and the water therein was at a temperature of about 155"F.

Insulated tinsel conductors 11-11 constructed in accordance with this example exhibited a "blip frequency" of 45,000 feet, which is to say that tinsel slivers or burrs manifested in unacceptable protrusion from the conductor profile occurred at approximately every 45,000 feet.

EXAMPLE 11 Same as Example I except that the feed zone temperature was 372"F, transition zone 444"F, metering zone 465"F and head temperature of 450"F. The polymer was extruded at a temperature of 445"F. The air flow was 7 cubic feet per hour. The feed zone depth was 0.348 inch, metering depth 0.120 inch, and transition zone 0.348 inch to 0.120 inch. Extruder amperage was 12.0 amps, and screw speed was 37.0 RPM. The "blip frequency" was determined to be 30,000 feet.

To illustrate the improvement obtained by the invention two examples, Example III and Example IV, are given of processes not in accordance with the invention. In Examples III and IV the step of expanding the extruded insulation outwardly from the tinsel conductor, is omitted. The blip frequency is much increased.

EXAMPLE III Same as Example I except that the feed zone temperature in the extruder was 410"F, transition zone temperature was 450"F, metering zone 450"F, head temperature 450"F and the temperature of the polymer extrudate at the die opening was 450"F. No air was introduced into the core tube 61. The extruder amperage was 13 amps while the screw speed was 40 RPM. Only a hot wash quench of 160"F water was used. The insulated conductor 11 exhibited no space between the tinsel conductor 14 and the insulation cover, the wall being 7 mils thick. The "blip frequency" was determined to be 4,000 feet.

EXAMPLE IV Same as Example I except that the feed zone temperature in the extruder was 410"F, the transition, metering and head temperatures each 450"F and the polymer temperature at the die opening was 450"F. No air was introduced, the extruder amperage was 13 amps and the screw speed was 40 RPM. The insulated tinsel conductor 11 had no air space and had a 7 mil thick wall. The water bath included a cold water quench at a temperature of about 60"F and a hot water anneal at a temperature of about 1600F. The "blip frequency" was determined to be 8,000 feet.

EXAMPLE V Same as Example I except that the water treatment included only a 1600 hot water anneal.

The "blip frequency" was determined to be 37,000 feet.

A suitable extrusion apparatus for the following examples is illustrated in U.S. Patent 3,579,608.

EXAMPLE VI An extrusion apparatus having a length to diameter ratio of 24 to 1 and a barrel diameter of 2 inches was charged with a segmented thermoplastic copolyester to obtain an output of about 75 Ibs/hr. The polyester comprises a plurality of recurring ester linkages such as, for example, esters of dicarboxylic acids and diols joined together into chain type molecules. At least 70% of the esters in the polyesters are derived from terephthalic acid, the remaining esters being essentially isophthalic or phthalic acid esters. The diol oxide) glycol having a molecular weight less than 250 and which comprises at least 70 percent of 1, 4 butane diol units. The proportion of the polymer which includes the poly (alkalene oxide) glycol having a molecular weight less than 250 is at least 66% by weight of the copolyester. The sum of the percentages of diacids present in the polyester which are not terephthalic acid, and the number of the short chain diol groups present which are not 1, 4 butane diol does not exceed about 30 percent. The segmented thermoplastic copolyester is HYTREL (ff) 7246.

The feed zone temperature was 450"F, the transition zone temperature was measured at 430"F, the metering zone temperature at 395"F, and the head temperature at 450"F. A tinsel conductor 14 was advanced through the extrusion head at a line speed of about 2500 feet per minute. Air was introduced into the core tube to expand the extrudate which was drawn down to form an insulative cover 18 having a wall thickness of about 7 mils and an outside diameter of about 40 mils.

EXAMPLE Vll This example was conducted under the same conditions as Example VI except that the extruder charge comprised approximately 97% of a first segmented thermoplastic copolyester comprising a plurality of recurring ester linkages such as, for example, esters of dicarboxylic acids and diols joined together into chain type molecules. At least 70 percent of the esters in the polyester are derived from terephthalic acid, the remaining esters being essentially isophthalic or phthalic acid esters. The diols from which the polyester chain is derived are comprised of(1) a poly (alkalene oxide) glycol having a molecular weight of 400 to 4000 and a carbon-to-oxygen ratio in the range of 2.0 to 4.3 and (2) a poly (alkalene oxide) glycol having a molecular weight less than 250 and which comprises at least 70 percent of 1, 4 butane diol units. The proportion of the polymer which includes the poly (alkalene oxide) glycol having a molecular weight less than 250 is at least 66% by weight of the copolyester.

The sum of the percentages of diacids present in the polyester which are not terephthalic acid, and the number of the short chain diol groups present which are not 1, 4 butane diol does not exceed about 30 percent. The charge comprised about 97% of the first segmented polyester combined with (b) approximately 1.5% of a second segmented thermoplastic copolyester comprising a plurality of recurring ester linkages such as, for example, esters of dicarboxylic acids and diols joined together into chain type molecules. At least 70 percent of the esters present in the second polyester are terephthalic acid, the remaining esters being essentially isophthalic or phthalic acid esters. The diols from which the second polyester chain is derived are comprised of (1) a poly (alkalene oxide) glycol having a melting point of less than about 60"C, a molecular weight of 400 to 4000, and a carbon-to-oxygen ratio in the range of 2.0 to 4.3, and (2) a poly (alkalene oxide) glycol having a molecular weight less than 250 and which comprises at least 70 percent of 1, 4 butane diol units. The proportion of the polymer which includes the poly (alkalene oxide) glycol having a molecular weight less than 250 is about 48 to 65 percent by weight of the copolyester. The sum of the percentages of diacids present in the polyester which are not terephthalic acid and the number of the short chain diol groups present which are not 1 4 butane diol does not exceed about 20 percent. The second segmented polyester is available commercially from E.I. duPont de Nemours under the designation HYTREL ( 4056 Polyester Elastomer, for example. Further, the charge comprises (c) approximately 1.5% of a silver pigment such as, for example, Pearl Afflair, available commercially from E. I. duPont de Nemours. The amperage of the current required to drive the extruder screw was measured to be in the range of 19 to 20.5 which seemed to indicate that the composition of Example VII is characterized by greater stability than that of Example VI and results in a more uniform product. The HYTREL (E) 4056 material is of the type disclosed in U.S. Patent 3,766.146. incorporated by reference hereinto.

A length of the cordage may be helically wound, heat set and reverse wound, and terminated with suitable connecting devices to provide a retractile telephone cord.

WHAT WE CLAIM IS: I. A method of forming insulation around a tinsel conductor comprising tinsel wrapped about a core, said method comprising advancing the tinsel conductor, extruding the insulation at controlled temperature into enclosing spaced relation with the advancing tinsel conductor, expanding the extruded insulation outwardly from the tinsel conductor whilst permitting the insulation to cool to initiate oriented crystalline growth thereof, and drawing the extruded insulation down about the tinsel conductor.

2. A method according to claim 1, wherein the insulation is a polyether-polyester thermoplastic copolymer.

3. A method according to claim 2, wherein the copolymer is obtained by reacting 1, 4 butane diol terephthalate with tcrephthalate esters of polytetramethylene glycol.

4. A method according to claim 3, wherein the composition of the insulative cover

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (35)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   oxide) glycol having a molecular weight less than 250 and which comprises at least 70 percent of 1, 4 butane diol units. The proportion of the polymer which includes the poly (alkalene oxide) glycol having a molecular weight less than 250 is at least 66% by weight of the copolyester. The sum of the percentages of diacids present in the polyester which are not terephthalic acid, and the number of the short chain diol groups present which are not 1, 4 butane diol does not exceed about 30 percent. The segmented thermoplastic copolyester is HYTREL (ff) 7246.

   The feed zone temperature was 450"F, the transition zone temperature was measured at 430"F, the metering zone temperature at 395"F, and the head temperature at 450"F. A tinsel conductor 14 was advanced through the extrusion head at a line speed of about 2500 feet per minute. Air was introduced into the core tube to expand the extrudate which was drawn down to form an insulative cover 18 having a wall thickness of about 7 mils and an outside diameter of about 40 mils.

   EXAMPLE Vll This example was conducted under the same conditions as Example VI except that the extruder charge comprised approximately 97% of a first segmented thermoplastic copolyester comprising a plurality of recurring ester linkages such as, for example, esters of dicarboxylic acids and diols joined together into chain type molecules. At least 70 percent of the esters in the polyester are derived from terephthalic acid, the remaining esters being essentially isophthalic or phthalic acid esters. The diols from which the polyester chain is derived are comprised of(1) a poly (alkalene oxide) glycol having a molecular weight of 400 to 4000 and a carbon-to-oxygen ratio in the range of 2.0 to 4.3 and (2) a poly (alkalene oxide) glycol having a molecular weight less than 250 and which comprises at least 70 percent of 1, 4 butane diol units. The proportion of the polymer which includes the poly (alkalene oxide) glycol having a molecular weight less than 250 is at least 66% by weight of the copolyester.

   The sum of the percentages of diacids present in the polyester which are not terephthalic acid, and the number of the short chain diol groups present which are not 1, 4 butane diol does not exceed about 30 percent. The charge comprised about 97% of the first segmented polyester combined with (b) approximately 1.5% of a second segmented thermoplastic copolyester comprising a plurality of recurring ester linkages such as, for example, esters of dicarboxylic acids and diols joined together into chain type molecules. At least 70 percent of the esters present in the second polyester are terephthalic acid, the remaining esters being essentially isophthalic or phthalic acid esters. The diols from which the second polyester chain is derived are comprised of (1) a poly (alkalene oxide) glycol having a melting point of less than about 60"C, a molecular weight of 400 to 4000, and a carbon-to-oxygen ratio in the range of 2.0 to 4.3, and (2) a poly (alkalene oxide) glycol having a molecular weight less than 250 and which comprises at least 70 percent of 1, 4 butane diol units. The proportion of the polymer which includes the poly (alkalene oxide) glycol having a molecular weight less than 250 is about 48 to 65 percent by weight of the copolyester. The sum of the percentages of diacids present in the polyester which are not terephthalic acid and the number of the short chain diol groups present which are not 1 4 butane diol does not exceed about 20 percent. The second segmented polyester is available commercially from E.I. duPont de Nemours under the designation HYTREL ( 4056 Polyester Elastomer, for example. Further, the charge comprises (c) approximately 1.5% of a silver pigment such as, for example, Pearl Afflair, available commercially from E. I. duPont de Nemours. The amperage of the current required to drive the extruder screw was measured to be in the range of 19 to 20.5 which seemed to indicate that the composition of Example VII is characterized by greater stability than that of Example VI and results in a more uniform product. The HYTREL (E) 4056 material is of the type disclosed in U.S. Patent 3,766.146. incorporated by reference hereinto.

   A length of the cordage may be helically wound, heat set and reverse wound, and terminated with suitable connecting devices to provide a retractile telephone cord.

   WHAT WE CLAIM IS: I. A method of forming insulation around a tinsel conductor comprising tinsel wrapped about a core, said method comprising advancing the tinsel conductor, extruding the insulation at controlled temperature into enclosing spaced relation with the advancing tinsel conductor, expanding the extruded insulation outwardly from the tinsel conductor whilst permitting the insulation to cool to initiate oriented crystalline growth thereof, and drawing the extruded insulation down about the tinsel conductor.
2. 2. A method according to claim 1, wherein the insulation is a polyether-polyester thermoplastic copolymer.
3. 3. A method according to claim 2, wherein the copolymer is obtained by reacting 1, 4 butane diol terephthalate with tcrephthalate esters of polytetramethylene glycol.
4. 4. A method according to claim 3, wherein the composition of the insulative cover

   encompassing each conductor comprises substantially 15.7 percent by weight of polytetramethylene glycol having a number average molecular weight of about 1000, about 32.4 percent by weight of 1, 4 butane diol, and about 50.7 percent by weight of a terephthalate ester-containing compound.
5. 5. A method according to claim 4, wherein the insulation composition has a Durometer hardness on the D scale of approximately 72 and a modulus of rigidity of approximately 75,000 psi.
6. 6. A method according to claim 3, wherein the insulation cover comprises about 19.4 percent by weight of 1, 4 butane diol, about 44.8 percent by weight of polytetramethylene glycol having a number average molecular weight of 1000, about 27.4 percent by weight of terephthalic acid and about 7.9 percent by weight of isophthalic acid.
7. 7. A method according to claim 4, wherein the composition also comprises about 0.2 percent by weight of a long chain hindered phenolic antioxidant.
8. 8. A method according to claim 4, wherein the composition has a melting point of about 424"F, and a specific gravity of about 1.25.
9. 9. A method according to any one preceding claim, wherein the expanding of the extruded insulation is accomplished by moving air between the extruded insulation and the advancing tinsel conductor.
10. 10. A method according to claim 9, wherein the copolymer is advanced along a path between a tube through which the tinsel conductor is advanced and a die cavity, and wherein the downstream end of the said tube extends at least to the face of the die opening to facilitate extrusion of the conductor insulation.
11. 11. A method according to claim 10, wherein the path along which the copolymer is advanced is such as to minimize the pressure drop through the extruder to facilitate extrusion of the copolymer at a temperature greater than but close to the melting point of the copolymer.
12. 12. A method according to any one preceding claim, wherein the extruded insulation is cooled by spaced streams of water having temperatures in the range of about 50"F to 600F.
13. 13. A method according to claim 12, wherein the insulation if further cooled by advancing the insulated tinsel conductor through a bath of water having temperature(s) of 50 to 60"F; and then contacts for a predetermined time water having temperature(s) in the range of 150 to 1600F.
14. 14. Apparatus for forming insulation around a tinsel conductor comprising tinsel wrapped around a polymeric core, said apparatus comprising means for advancing the tinsel conductor; means for extruding the insulation into enclosing spaced relation with the advancing tinsel conductor with the temperature of the extrudate being greater than but close to the melting point of the insulation; means for expanding the extruded insulation outwardly from the tinsel conductor whilst permitting the insulation to cool to initiate crystalline growth of the insulation, the expansion of the extruded insulation being effective to orient the direction of the crystalline growth; means controlling the advancing means for drawing down the insulation about the tinsel conductor; and means for cooling further the insulation to continue the crystalline growth of the insulation.
15. 15. Apparatus according to claim 14, comprising an extrusion head having a passage therethrough; the advancing means serving to advance the tinsel conductor along a path aligned with the passage through the extrusion head; an extrusion die mounted at one end of the passage, the die having a cavity formed therein and a cylindrical exit port which communicates with a portion of the die cavity, the die cavity and the exit port having colinear axes which are coincidental with the path through the extruder, the portion of the die cavity which communicates with the exit port having a generally frustoconical shape with the converging wall portion thereof having a configuration which permits extrusion of the insulation such that the temperature of the extrudate is slightly greater than the maximum temperature at which crystalline growth will occur in the insulation; a core tube positioned in the passage in axial alignment with the die for guiding the tinsel conductor toward the exit port of the die, the core tube having a cylindrical portion disposed concentrically within at least a portion of the exit port of the die and having a frustoconical portion connected thereto and extending inwardly of the die cavity, the walls of the die cavity adjacent the generally frustoconical portion thereof and of the core tube forming a gradually increasing flow passage for the insulation, the core tube having a cylindrical passage through the cylindrical portion thereof and a frustoconical passage through the frustoconical portion thereof; in use the insulation extruded through the exit port of the die being spaced from the conductor by the cylindrical portion of the core tube which extends into the exit port; the insulation being polyether polyester thermoplastic copolymer.
16. 16. Apparatus according to claim 15, wherein the means for expanding the thermoplastic extrudate spaced from the tinsel conductor includes means for introducing a gas into the core tube at the upstream end thereof such that the gas is caused to flow through the core tube and which is controlled relative to the line speed at which the conductor is advanced to space apart the extrudate from the tinsel conductor sufficient to stress orient the copolymer.
17. 17. Apparatus according to claim 15, or 16, including means for cooling the extruded insulation a predetermined time after the expansion thereof.
18. 18. Apparatus according to claim 17, wherein the frustoconical portion of the die cavity is such that twice the angle formed between a line of generation of the diverging wall thereof and the axis of the die cavity is in the range of from about 1200 to about 1800.
19. 19. Apparatus according to claim 18, wherein twice the said angle is equal to 1260.
20. 20. Apparatus according to claim 15, wherein the downstream end of the core tube is aligned with the external surface of the die with which the exit port communicates.
21. 21. Apparatus according to claim 16, wherein, in use, the gas is introduced at ambient temperature.
22. 22. Apparatus according to claim 15, including cooling means spaced downstream of the exit port for treating the insulation of the insulated tinsel conductor in such a way as to control the crystalline growth of the copolymer.
23. 23. Apparatus according to claim 22, wherein the cooling means includes means for causing water in the range of 50 to 600F to engage the insulated tinsel conductor in spaced streams whereafter the insulated conductor is immersed in water at a similar temperature.
24. 24. Apparatus according tp claim 23, which further includes means for causing the advancing tinsel conductor subsequent to the water treatment to be treated with water having a temperature of substantially 150"F.
25. 25. A cord comprising: a plurality of individually insulated tinsel conductors, and a plasticized polyvinyl chloride jacket covering the individually insulated tinsel conductors, the insulated tinsel conductors each manufactured by the method of claim 1 and comprising a polymeric core with electrically conductive, flexible tinsel wrapped helically about the core; an insulation cover encompassing each individual tinsel conductor, the insulation cover comprising a copolyether-ester composition obtained by reacting 1 , 4 butane diol terephthalate with terephthalate esters of polytetramethylene glycol.
26. 26. A cord according to claim 25, wherein the polyether polyester thermoplastic composition comprises about 15.7 percent by weight of polytetramethylene glycol having a number average molecular weight of about 1000, about 32.4 percent by weight of 1, 4 butane diol, and about 50.7 percent by weight of a compound containing a terephthalate ester group, the insulative cover having a Durometer hardness measured on the D scale of about 72 and a flexural modulus of about 75,000 pounds per square inch.
27. 27. A cord according to claim 26, wherein the melting point of the insulative composition is about 424"F and the polyvinyl chloride composition which comprises the jacket comprises about 60 parts of a plasticizer per 100 parts by weight of polyvinyl chloride, the melting point of the insulative cover composition being substantially greater than the temperature at which the composition comprising the jacket will flow without cocrcion.
28. 28. A cord according to claim 25, 26 or 27, wherein the cord comprises an electrical connector assembled to each end thereof. the electrical connector including a housing having moveable portions which comprise a strain relief system. the strain relief system in an operative position engaging the jacket.
29. 29. A cord according to claim 23,26, 27 or 28, wherein the composition of the insulation is stabilized with long chain hindered phenolic antioxidant.
30. 30. A cord according to claim 29, wherein the insulation further comprises about 1 percent by weight of a catalytic residue and about 0.2 percent by weight of the antioxidant.
31. 31. A cord according to claim 28, wherein the housing is an unipartite housing and the moveable portions thereof are moveable into engagement with the conductors and with the jacket after the cord is inscrted therein.
32. 32. A method of extruding material around an elongate core, comprising advancing the core. extruding the material into enclosing spaced relation with the advancing core, expanding the extruded material outwardly from the core whilst permitting the material to cool to initiate oriented crystalline growth therein, and drawing the extruded material down about the core.
33. 33. Apparatus for extruding material around an elongate core, comprising means for advancing the core. means for extruding the material into enclosing spaced relation with the advancing core. means for expanding the extruded material outwardly from the core whilst permitting the material to cool to initiate oriented crystalline growth therein, and means for drawing the extrudcd material down about the core.
34. 34. A method of forming insulation around a tinsel conductor, substantially as hereinbefore described with reference to FIGS. 4, 6. 7 and 8 of the accompanying drawings.
35. 35. An insulated conductor prepared by the method according to any one of claims 1 to 13 or claim 33.