US3068531A - Retractile cord and method of making same - Google Patents

Description

Dec. 18, 1962 E. L. LOVE RETRACTILE com) AND Minnow OF MAKING SAME:

2 Sheets-Sheet 1 Filed May 16, 1957 Dec. 18, 1962 E. L. LOVE RETRACTILE com: AND METHOD OF MAKING sms Filed May 16, 195'"! 2 Sheets-Sheet 2 FILEIU United States Patent G ce This invention relates to an improved retractile cord and to an improved method for making the same.

The term cord refers to one or more conductors, such as one or more lengths of flexible copper wire, which are enclosed within a jacket formed of suitable insulating material, to provide what is often referred to as a jacketed conductor. A retractile cord is a cord of helical shape and elastic, to the end that the helical cord will return to its original position after extension.

It is an object of this invention to provide a retractile cord having improved retractile properties and a smooth surface finish.

It is another object to provide an improved method for making retractile cords, and in particular for imparting a smooth surface finish thereto.

A further object is to provide an improved method for making a retractile cord having a high degree of prestress.

In the manufacture of retractile cords, the jacket is first extruded around a conductor, or around a plurality of conductors. Then, prior to vulcanization or heat curing, the jacketed conductor is arranged in a helix, and then vulcanized. This causes the material of the jacket, which is preferably an elastomer, to take a set in this helical form, to provide a structure having the desired retractile properties. Such retractile cords are used as telephone cords and with various other types of electrical apparatus.

According to the manufacturing methods now employed, the surface of the rubber jacket has a satiny finish. As a result, a considerable amount of dust collects on the cord which is undesirable from an aesthetic point of view. This is particularly noticeable with respect to certain colors such as black, and some of the lighter colors which are becoming quite popular with telephone subscribers.

To illustrate the importance which is attached to the character of the finsh, many cords are now made of a plastic material which will provide a smooth finish upon heat curing. However, the plastic materials which are used to achieve this result are elastomers only to a very small degree with the result that when such plastic cord is manufactured in coil form, its retractile properties are very poor. According to the present invention, it is possible to provide a smooth finish upon an elastomer in order to provide a cord having satisfactory retractile properties.

Another object of the present invention is to provide a low cost method of making retractile cords.

According to the present invention, the retractile cord is vulcanized in a metallic mold, as contrasted with the steam cure of the prior art. There are numerous dithculties inherent in steam curing, one of which is the necessity of protecting the finsh against abrasion or deformation during the handling operation inherent in this prior art curing method. As a result it is usually desirable to perform the curing operation in two stages, the first stage comprising a partial cure which takes place continuously as the jacket is being extruded. This first stage must be carried forward to a point wherein the conductor can be arranged in coil form around a spindle or a mandrel without causing a serious flattening of the cord section.

The further the partial cure is carried, the less effective is the final cure or vulcanization in causing the elastomer to take a helical set. According to the present invention, the partial cure is carried out to a much lesser degree 3,068,531 Patented Dec. 18, 1962 since the surface characteristics are imparted by the mold. This has the additional result of increasing the effectiveness of the final cure.

Furthermore the use of a metallic mold according to the present invention permits the fabrication of a small diameter helix in which the stress is equalized in all parts of the elastomer.

It is customary to pre-stress retractile cords by reversing the pitch after vulcanization. This is accomplished by rewinding the cord with the pitch reversed, or by pulling the cord through itself. The stress of a helix is essentially a torsional stress, and hence the smaller the diameter of the helix with respect to cord diameter, the greater the degree of pre-stress.

According to the present invention, a helix can be wound and set to any desired diameter within practical irnits.

Other object features and advantages will become apparent as the description proceeds.

With reference now to the drawings, in which like reference numerals designate like parts:

FIG. 1 is a sectional elevation taken along line 11 of FIG. 2, showing the apparatus employed in the practice of this invention, and illustrating the method of this invention;

FIG. 2 is an end view of the apparatus shown in FIG. 1;

FIG. 3 is an elevation of a section of cord after extruding and partial cure;

FIG. 4 is a view of a single convolution showing the same after it has been placed on the mandrel and prior to the final cure or vulcanization;

FIG. 4a is an enlarged section taken along line ta-4a of FIG. 4;

FIG. 5 is a view similar to FIG. 4 but showing the convolution after molding and vulcanization;

FIG. 6 is a section taken along line 6-6 of FIG. 5 showing two adjacent convolutions prior to pitch reversal;

FIG. 7 is a view similar to FIG. 6 but showing said two convolutions after pitch reversal;

. FIG. 8 is a diagram illustrating the pre-stress which is obtained by pitch reversal;

FIG. 9 is a diagrammatic view showing a modified form of the apparatus shown in FIG. 1;

FIGS. 10 and 11 are views similar to FIGS. 1 and 2 but1 showing a further modified form of the apparatus; an

FIG. 12 is a view showing the shape of the retractile cord produced by the apparatus of FIGS. 10 and 11.

The apparatus, as shown in FIGS. 1 and 2 comprises a mandrel 10 and two molds 11 and 12 which surround the mandrel and completely enclose the same. One or more heating elements 13 may be provided in each of t elements 10 and 11 and 12 for heating the same.

The mandrel 10 is provided with a helical groove 14, the cross sectional area of which is half a circle. The upper mold 11 is provided with a similar groove 15 and the lower mold 12 is provided with a similar groove 16.

The mold may be made of any suitable material such as steel, and preferably the surfaces of the grooves 14, 15 and 16 are polished. Thus, the grooves cooperate with each other to provide a helical passageway, the cross sectional area of which at every point is a full circle. The grooves in each element are separated from each other by separating ridges 17, and 17', the surfaces 18 and 18' of which are cylindrical. The ridges 17 and surface 18 pertain to the mandrel 10, and the ridges 17 and 18' pertain to the molds 11 and 12. These surfaces are preferably machined to provide the meeting faces between the mandrel 10 and each of the molds 11 and 12 to the end that the parting line on the finished products will be as inconspicuous as possible.

The method of manufacture is illustrated in FIGS. 3

to 5. As previously pointed out, the cord comprises a conductor 20 and a jacket 21 which is continuously extruded, and which may then pats from the extruding die to a high pressure steam vulcanizing chamber which, according to the prior art, has been of a'length of from 100 to 200 feet depending upon the steam temperature and the rate of travel of the jacketed conductor. The jacket is given its partial cure in this chamber to prevent deformation in subsequent handling, this type of apparatus being illustrated by way of example in the L. F. Lamplough Patent No. 1,689,205 dated October 30, 1928. However, according to the present invention the partial cure is carried forward to a lesser degree than in the prior art, or not at all, because the cord receives little handling before completion, with the result that the vulcanizing chamber may be of less length, or the other parameters varied accordingly. FIG. 3 illustrates a section of cord at this stage, namely after extruding and partial cure, but prior to Winding the same on the mandrel 10.

The second step of the operation comprises the cutting of the cord into discrete lengths, and Winding each length on the mandrel 10. This involves a flexing of the cord, as in the direction indicated by the arrow in FIG. 3, to provide a series of convolutions, one of which is shown at 22 in FIG. 4, although the mandrel is omitted for the sake of clarity. It will be observed, that in this initial forming process the jacket material 23, along the inner portion of the toroidal surface of the convolution 22, will be compressed, and the jacket material 24, along the outer toroidal surface of the convolution will be tensioned or drawn out. When the cord is formed into a helix of small diameter with respect to the cord diameter, the inner toroidal surface of the jacket will become wrinkled, as indicated by the lines 25 in FIG. 4, due to the compression of'the jacket material 23.

Although any particular, convolution, such as convolution 22, is not a toroid, it can be considered for explanatory purposes as being made up of a series of toroidal eler' ents, each one of which has an inner toroidal surface 23 and an outer toroidal surface 24.

After the cord 22 has been wound on the mandrel 19, the molds 11 and 12 are brought into operative position, and heat is supplied by the heating elements 13 in order to vulcanize the cord 22 or to effect the final cure. By virtue of the fact that the final cure is done in a mold, and the fact that the partial cure has been carried forward to a degree only 'sufiicient to eliminate surface tackiness and to permit handling without deformation, the condition of the jacket material 21, is such that it will flow in the direction indicated by the arrows in FIG. 4a from the inner to the outer toroidal surface of each convolution. Thus, the volume distribution or density of the jacket material will be equalized right .at the beginning of the vulcanization or final cure, to the end that in the finally vulcanized convolution all portions of the jacket will have taken a neutral set, which is to say a set in which any difierences in stress between the inner and outer surfaces 23 and 24, will have been equalized.

A close control of the diameter of the jacket 21, when first extruded and with respect to the diameter of the helical passageway formed by grooves 14 to 16, is maintained in order that the volume of the jacket material is sufiicient to completely fill the mold passageways, without interfering with the complete closure of the mold, on the one hand, and without creating voids within the mold on the other hand, which voids would detract from the smooth surface finish which is desired on the finished product. This control can be exercised to a certain degree by regulating the speed of the cord at the point of extrusion, as well as the pres-sure on the jacket material back of the extruding die. In other words, the closing of the molds upon the mandrel develops pressure of a degree sufficient to establish the flow of material as indicated in FIG. 4a, and otherwise to form the cord into 4 helical shape, and to impart the desired surface finish thereto.

After the cord has been completely vulcanized, the molds 11 and 12 are separated from the mandrel, and the cord 21, is removed from the mandrel. The surface finish of a completely finished vulcanized convolution 26, as shown in PEG. 5, is smooth, the wrinkles 25 having disappeared at this stage of the operation. The convolutions are separated from each other as shown in FIG. 6, and are free from helical stress. Although the cord as shown in FIG. 6 is retractile to a certain extent, the retractile properties can be greatly improved by reversing the pitch in order that the jacket material may be prestressed. This reversal of pitch is illustrated in FIGS. 6 to 8, in which one convolution A is shown together with a portion of an adjacent convolution B.

The reversal of pitch as illustrated by comparison of FIGS. 6 and 7 places convolution B to the left of convolution A, which is opposite to their normal position as shown in FIG. 6. The diagram of FIG. 8 shows the magnitude of the extension of the cord, which is effected by pitch reversal.

When a helix is extended, the stress is essentially a torsional stress of the section of coiled element. Therefore, the smaller the diameter of the helix, with respect to cord diameter, the greater will be the extension of the cord, as measured by the angle theta in FIG. 8. Pitch reversal necessarily results in an extension of the helix, since the adjacent convolutions are in contact with or hug each other. In other words, the helix locks itself in an extended position. As a result of this extension, the jacket material is torsionally stressed especially at its periphery, as indicated by reference numeral 27 in FIG. 8. Thus, a comparatively high degree of pre-stress is provided when the pitch of a small diameter helix is reversed.

According to the prior art processes, if a small diameter cord is given its final cure in a steam chamber, and without mold pressure, the resultant product would ap- 1 pear to be substantially identical to the convolution 22 the prior art processes, a small diameter helix will not necessarily result in an improved pre-stress.

A modified form of apparatus is shown in FIG. 9 in which the mandrel 10 and the molds 11 and 12 are heated by resistive heating, the conductors being suitably secured to the opposite ends of the elzments 10, 11, 12 and being powered from a suitable source of direct current. The mandrel 10 will require a lighter current than the molds, since it is of smaller cross-section area.

For instance, flexible conductors 30 and 31 may be employed which may be removably secured to the opposite ends of the mandrel 10 by clips 32. The conductors 3t; and 31 are connected to a power line 34 and 35. The opposite ends of molds 11 and 12 are connected to branched flexible conductors 36 and 37 either permanently, as shown, or by clips as in the case of the mandrel. A rheostat 33 is interposed in conductor 31, and a rheostat 38 is interposed in conductor 37 so that the respective currents through the mandrel 10 on the one hand, and the molds 11 and 12, on the other hand, may

i be independently regulated to develop the same degree of heat in each.

FIGS. 10 and 11 show a modified form of mandrel and molds, indicated as mandrel 40, upper mold 41, and lower mold 42. The man-drel 40 is provided with a helical groove 43 which is considerably deeper than the helical groove 14 of the mandrel 10. Adjacent convolutions of the grooves are separated from each other by a separating ridge 44, having a cylindrical surface 45. The molds 41 and 42 are not provided with grooves, but with a smooth inner cylindrical surface 46. Thus the meeting surfaces between the mandrel 40 and the molds 41 and 42 comprise the surfaces 45 and 46. One advantage to this construction is that the lateral positioning of the parts is not as critical as in the case of the elements shown in FIGS. 1 and 2.

The shape of the rectractile cord produced by the el ments 40, 41 and 42, prior to pitch reversal, is shown in FIG. 12 at 47. The outstanding characteristic is the cylindrical outside surface 48. When the pitch is reversed, and the separate convolutions hug each other, the retractile cord has the appearance of a single cylindrical body when in relaxed position.

As a specific example, the jacket composition may comprise the following:

' Pounds Neoprene 45.00- Carbon black 35.00

Mineral filler 7.00

Wax 1.50 Zinc oxide 3.00 Magnesium oxide 1.50 Mineral oil 6.25 Stearic acid 0.50 Phenyl beta naphthylamine 0.25

Total 100.00

The partial cure is carried out in saturated steam at a temperature of 366 F. for 30 seconds. For the final cure, the mandrel and the molds 11 and 12 are heated to from 390 F. to 400 F., and the molds are closed for a period of 60 seconds. The final cure will vary with cord size, but the above illustration applies to a cord having four copper conductors, of which the overall jacket diameter is .205 inch.

The foregoing example is illustrative only as it has been found that numerous types of compositions can be used for the jacket material, and in general the partial cure is effected to eliminate surface tackiness and to permit the partially cured cord to be wound on reels. However, where the intermediate stage of winding the cord on reels is eliminated, the partial cure can be correspondingly decreased, or eliminated altogether.

For instance, in the case of a jacket material having a natural rubber base, the time-temperature vulcanizing curve is so steep that it is not practical to attempt a partial cure. Therefore, in such instances, the extruded cord is directly wound on the mandrel and cured, and this involves the exercise of somewhat more care in the handling of the completely uncured cord.

An additional advantage of the present application is that the final cure takes only about a minute, as contrasted with 15 minutes where the cure is effected by high pressure steam. This decrease in required time is due, in large degree, to the greatly increased rate of heat transfer by the use of a heated mold.

Additionally, the use of a polished mold provides the desired smooth finish which increases the commercial acceptability of the retractile cord verymaterially, and without the sacrifice of retractile Properties.

It is contemplated that the cord shown in FIGS. 3 to 7 be fabricated with a plurality of conductors 20, although only a single conductor has been shown for purposes of illustration. In making up a multi-conductor retractile cord, it is frequently desirable to jacket each conductor individually, as by extrusion, and then to extrude and jacket the several conductors, this last stage of extrusion being followed by the molding operation herein described.

Although only preferred embodiments of this invention have been shown and described herein, it will be appreciated that various modifications and changes may be made in the retractile cord shown herein and in the method herein described without departing from the spirit of this invention, as pointed out in the appended claims.

I claim:

1. The method of manufacturing a retractile cord which comprises the steps of extruding a length of cord of which the jacket is substantially uncured, arranging the same in helically coiled form upon a mandrel, completely surrounding each convolution thereof by said mandrel and by cooperating mold elements, heating said mandrel and said mold elements to vulcanize said jacket, and removing said mold elements from said mandrel and said cord from said mandrel.

2. The method of manufacturing a retractile cord which comprises the steps of extruding a length of cord of which the jacket is substantially uncured, arranging same in the form of a helix, surrounding the helically arranged cord by a heated metallic molding element, subjecting said jacket to sufficient pressure as to provide a heat transferring contact between said heated molding element and said jacket in order to vulcanize said jacket and to cause the same to take a helical set, regulating the diameter of said extruded cord so that the volume of the jacket is sufficient to completely fill the cavities of said molding elements, removing said heated molding elements, and reversing the pitch of the convolutions of said coil.

3. The method of manufacturing a retractile cord from a length of jacketed conductor, of which the jacket comprises an extruded elastomer in substantially uncured form, which comprises the Step of molding said jacketed conductor in a helical form and simultaneously vulcanizing the same so that said vulcanized length of cord will be given a helical set.

4. In the art of manufacturing retractile cords the method of making from a length of straight jacketed conductor of which the jacket comprises a substantially uncured elastomer a helix of comparatively small diameter with respect to the diameter of said jacketed conductor to improve the retractile properties of the cord when the pitch of the helix is reversed, which comprises the steps of arranging said conductor in a helical form, and subjecting the same to heat and molding pressure so that the excess material of the jacket at the inner surface of any toroidal element of any particular convolution will be caused to flow toward the outer surface of said element in order to equalize the distribution of material and to cause said jacket to take a helical set as it is being vulcanized.

5. The method of manufacturing a retractile cord, having a smooth surface which comprises the steps of arranging in the form of a helix a length of extruded cord of which the jacket is substantially uncured, contacting the surface of said helically arranged cord at all points by a heated metallic molding element having polished mold surfaces, subjecting said jacket to suflicient pressure as to provide a heat transferring contact between said polished mold surfaces and said jacket in order to vulcanize said jacket and to cause the same to take a helical set, and then removing said heated molding elements.

6. A retractile cord as made by the process of claim 1.

References Cited in the tile of this patent UNITED STATES PATENTS 299,802 Kipper June 3, 1884 645,452 Bolte Mar. 13, 1900 646,413 Collet Apr. 3, 1900 2,039,475 Campbell May 5, 1936 2,173,096 Campbell Sept. 19, 1939 2,206,934 Barrans et a1. July 9, 1940 2,268,703 Dickey Jan. 6, 1942 2,375,357 Friedman May 8, 1945 2,394,762 Geraty Feb. 12, 1946 2,575,747 Cook Nov. 20, 1951

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