Manufacture And Construction Of Strings

Abstract

An integrated string of the type used for tennis rackets of the type comprising a plurality of individual thermoplastic strands, each of which is individually twisted in a first hand, the strands then being twisted together in an opposite hand, the resultant string then being coated in a thermoplastic formulation and then passed through a heating chamber to become dried, said coating and drying steps being repeated a plurality of times, after which the string is stretched under heat and then again coated and dried.

Description

BACKGROUND OF THE INVENTION

Integrated plastic strings for use in tennis rackets and the like are broadly old and well known in the art as evidenced, for example, by U. S. Pat. Nos. 2,649,833; 2,712,263; 2,735,258; 2,861,417; and 3,050,431, all previously issued to the present inventor. In all of these prior patents, however, the integrated plastic string comprised a core with some sort of a sheath positioned therearound, said core and sheath then being integrated by use of a compatible plastic coating. The resultant string in all of these cases embodied a substantially smooth cylindrical configuration on their outer surface; and, while eminently satisfactory for their intended purposes, it has been found that these prior art string constructions, when used for tennis purposes, did not permit the player to impart maximum spin to the ball. Expressed differently, the perfectly smooth, flat outer surface of these prior art strings did not permit maximum bite when the string engaged the ball, since the flat, smooth outer surface tended to minimize friction between the string and the ball during contact.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide an integrated plastic string wherein the outer surface of same, while smooth and continuous, nevertheless has gentle undulations therein which permit the string to impart maximum bite upon contact with a tennis ball, thus permitting the player to impart a more effective and pronounced spin upon the ball during the striking thereof.

In addition, it is the object of the present invention to provide an integrated plastic string which, when used for tennis purposes, will cushion the shock which results when the ball is struck and which, at the same time, is durable and wear resistant during use, thus resulting in relatively long and effective life.

In carrying out the above objectives, it has been found that an outer sheath is not required. Quite to the contrary, the string of the present invention comprises a plurality of individual strands, preferably three in number, each of which is individually twisted in a first hand. The strands are then twisted together in the opposite hand, after which the string is coated with a plastic formulation and then passed through a heating chamber for drying. The coating and drying processes are sequentially performed a plurality of times, after which the string is stretched under heat to a predetermined elongation. The coating and drying steps are then sequentially repeated to arrive at the finished string which, while thoroughly coated and integrated, nevertheless maintains on its outer surface the gentle undulations resulting from the twisting together of the individual strands.

DESCRIPTION OF THE DRAWING

In the drawing which illustrates the best mode presently contemplated for carrying out the present invention,

FIG. 1 is a perspective view of a portion of one of the individual strands used in the instant invention;

FIG. 2 illustrates a plurality of the strands shown in FIG. 1 twisted together;

FIG. 3 schematically illustrates the coating and drying means used in the instant invention;

FIG. 4 is an elevational view, on an enlarged scale, and partly in section, showing the completed string; and

FIG. 5 is a section taken on line 5--5 of FIG. 4.

DESCRIPTION OF THE INVENTION

Referring now to the drawings, there is shown at 10 in FIG. 1 an individual strand that is used in the manufacture of the present invention. The strand 10 is of thermoplastic construction, and nylon has been found to be highly preferable because of its strength. Where a tennis racket string is being manufactured, it has been found that the strand 10 may be of 3,360 deniers, although, obviously, the size of the finished string may be controlled by using individual strands of lesser or greater deniers. A plurality of individual strands 10 are provided, and preferably, three such strands are utilized in the manufacture of a tennis string in accordance with the present invention. Each of the strands 10 is individually twisted, and, preferably, it has been found that an "S" twist of 12.4 turns per inch has proven to be particularly desirable. After the individual strands 10 have been so twisted, they are laid together and then twisted in the opposite hand, as illustrated in FIG. 2. More specifically, where the individual strands have been "S" twisted, the bundle of strands, as illustrated in FIG. 2, are twisted together in a "Z" twist, preferably at the rate of 5.3 turns per inch.

The resultant string 12 is then dipped in a thermoplastic formulation 14, as illustrated in FIG. 3, it being noted that suitable pulleys 16, 18 and 20 are provided for continuously passing through the formulation 14, it being understood that any other suitable mechanical means may be employed for effectively coating the string 12. The formulation 14 is a nylon solution, preferably comprising by weight approximately 14.4 per cent nylon; 61.1 per cent Methanol; 4.9 per cent tetrahydrofurfyl alcohol; and 19.6 per cent water. The above proportions are not critical but have been found to be desirable for providing a nylon solution of suitable consistency.

After the string 12 has been coated with the solution 14, it passes through a heating chamber 22 where the string is dried. Specifically, it has been found that proper drying is effected where the chamber 22 is approximately 100 feet long, where the string passes therethrough at a speed of approximately 33 yards per minute, and where the temperature within the chamber is maintained at approximately 220.degree. F. The coating and drying of the string, as just described, is repeated a plurality of times, since it has been found desirable to apply several thin layers of coating rather than a single heavier layer. With the particular formulation hereinbefore described, it has been found that the coating and drying operation should be performed approximately twenty times in order to achieve the desired coating and integration of the string.

After the string has been coated and dried as above described, it is stretched under heat to properly orient the string and draw it down to the desired diameter. Specifically, it has been found that placing the string under 75 pounds tension in a chamber maintained at approximately 450.degree. F. will result in approximately 17 per cent elongation of the string, which has been found to be desirable. After the heat stretching operation, it has been found that minute pockets or voids sometimes exist in the string coating; and hence, in order to fill in these pockets or voids and provide a string having a smooth and uninterrupted outer surface, the aforesaid dipping and drying steps are again repeated, preferably two times.

As will be seen most clearly in FIGS. 4 and 5, the resultant string 24 is coated and integrated by the plastic 14, it being noted that the coating 14 not only surrounds the twisted strands 10, but also penetrates therebetween to effectively bond the string. Actually, the repeated dipping and drying operations performed on the string 12 will result in substantial integration of the plastic 14 with the strands 10, although, for purposes of illustration, the coating 14 has been shown in FIGS. 4 and 5 as being distinct from said strands. Also note that the coating 14 conforms to the indentations which exist between adjacent strands 10, thus resulting in indentations 26 on the outer surface of string 24, said indentations imparting a gentle undulation to the finished string, which undulation enhances the bite that the string may impart to a tennis ball, whereby a more effective spin may be imparted to the ball when it is struck. In addition, it has been found that a string manufactured in accordance with the instant invention is long wearing and durable, and, at the same time, cushions the shock that results when the string strikes a tennis ball.

Claims

What is claimed is:

1. An integrated string comprising a plurality of thermoplastic strands, each of which has an identical twist in a first direction, said strands being twisted together in the opposite direction to form the core of said string, and a coating of plastic material penetrating between said strands and covering same to integrate said string, the outer surface of said string being gently undulated.

2. In the string of claim 1, said first twist being approximately twelve turns per inch, and said second opposite twist being approximately five turns per inch.

3. In the string of claim 1, said strands and coating both being nylon.

4. In the string of claim 1, there being a total of three individual strands.