Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
7~941 :
This application is a division of Canadian Patent
;~ Application Serial No. 218,734 filed January 27, 1975.
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Background of the Invention
This invention relates to the manufacture
of rackets, and particularly to tennis rackets although
the principles thereof are applicable to any type of
strong racket, e.g. sauash rackets.
Throughout most of the history of racket
sports, all good rackets were made primarily of wood,
generally in the form of a plurality of curved pieces
laminated together by glue or the like. These rackets
possessed many desirable qualities from the standpoint
of strength, but also were subject to certain disadvantages.
; For example, wood is of variable quality at best, and wood
of the best quality is increasingly scarce. In any event,
wood is subject to warping and to fatigue, particularly
j under the stress of tightly stretched strings, and accurate
control of weight, and especially of balance, was difficult.
Comparatively recently, the art has produced
rackets wherein the frame is constructed of steel or aluminum. -
Obviously, a racket frame of such a material does not warp
and possesses a very high degree of strength, but the initial
cost of materials, as well as the cost of the equipment
for forming the metal racket frames, is high, thereby making
the consumer cost of such a racket high. Further, metal
frames have problems of cracking of welds, and with physical
properties of density, strength and stiffness tending to
; 25 result in rackets which are too flexible.
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10799~1
The use of glass reinforced plastic materials
has become widespread during this same period of time. It
is well known that the glass reinforced plastics have a very
high strength, they have a good modulus of elacticity, the
raw materials are inexpensive, and they can be readily formed
and otherwise handled. The use of glass reinforced plastic in
a tennis racket frame was proposed as long ago as1949, in
Robinson patent 2,878,020. Yet in spite of this knowledge
and early suggestion, the art has been unable to develop
a satisfactory racket frame formed of fiber reinforced plastic
material.
_mmary of the Invention
In accordance with the invention in one aspect
there is provided the method of making a racket frame
comprising the steps of substantially filling an elastomeric
tube at least as long as the peripheral outline of the frame
. .
with a removable core to form a mandrel, supporting the
resulting mandrel in straight position, forming a tubular
shell on said mandrel by successively applying thereto a
plurality of individual layers of high tensile strength
fiber impregnated with binder, removing said core from within
said tube, placing said shell with said tube therein in a
mold cavity defining a racket shape, forming aligned pairs of
holes for strings in opposite sides of said shell by separating
the fibers in said fiber layers at predetermined opposed
positions along the head defining portion of said shell without
perforating said tube, applying fluid pressure to said tube
to press said shell into uniform contact with said mold
cavity, and hardening said binder while maintaining said
pressure supplying step and said holes in said shell.
In a further aspect of the invention said hole
; forming step is carried out by means of probe elements
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proportioned and positioned to pierce the wall of said shell
, substantially without rupturing the fibers therein or
penetrating said tube.
: Preferably said forming operation includes the
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following steps:
(a) helically winding a first said fiber layer
along substantially the full length of said assembly,
(b) helically winding a second said fiber layer
of opposite hand and substantially the same length around
said first layer,
(c) helically winding on said second layer a
, third said fiber layer of a shorter length equal only to
c~ the head portion of the racket frame,
(d) helically winding a fourth said fiber layer
of opposite hand and substantially the same length on said
third layer,
(e) applying a cover layer over said first and
second layers. -
Preferably said cover layer is helically wound.
The racket frame provided by the method of the
present invention has as its primary structural member an
elongated hollow tube, the wall of which consists essentially
of a plurality of concentric layers of glass fiber impregnated
and bonded together by binder resin. The majority of these
- fiber layers are helical windings of predetermined uni-
directional hand with respect to the longitudinal axis of
the tubular member, alternate windings being of opposite -
hand, but there should also be one or more layers wherein
the fibers run lengthwise of the tube to provide adequate -~
- 30 bending strength in the finished frame. This tubular member
is formed in a loop so that its central portion defines the
head of the racket frame, and the two end portions converge
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at the base of the head portion to define an open throat from
which they extend in parallel relation to form the frame
handle, to which a suitable grip is applied.
Special features of the frame of the racker may
include a groove molded in the outer end part of the head
portion for receiving loop portions of the racket strings in
recessed relation with the surrounding peripheral area of
t~ the frame. Special provision may also be made for rein-
- forcing the throat portion of the frame, preferably by means
of a filler member positioned between the converging parts
of the tube and secured in bridging relation therewith, in
one form by layers of fiber and binder, and in another form
by means of the racket strings.
The method of the invention by which the
racket frames are produced, namely by applying successive
layers of binder-impregnated fiber to a removable matrix,
lends itself particularly well to the establishment of
the proper strength characteristics at stress points in
the frame, as well as proper characteristics of weight,
balance and flexibility or stiffness which are important
- for the best playing qualities. For example, the windings
of which the tubular member is composed can be varied in
number and length to provide extra wall thickness in the head
portion of the racket as compared with the handle portion.
Similarly, after the tubular member has been formed to the
basic racket shape, reinforcement can be provided where
it may be needed, in the throat and/or handle portion, by
cover layers of binder-impregnated fiber which are bonded
into the integral frame during the subsequent curing of
the binder.
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10'79941
Brief Description of_the DraWi gs
' :
Fig. 1 is a perspective view showing a complete -
racket in accordance with the invention;
Fig. 2 is a fragmentary view, partially in
side elevation and partially in vertical elevation, illustrating
a preliminary stage in the fabrication of the racket of
Fig. l;
Fig. 3 is an enlarged section on the line
3--3 of Fig. 2;
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941
Fig. 4 is a further enlarged fragmentary view
illustrating an intermediate stage in the fabrication of
;~ the tubular member which is the main structural part of
:
the racket of Fig. l;
S Fig. 5 is a partial exploded isometric view
illustrating the preform mold and the corresponding stage
in the fabrication of the racket frame;
Fig. 6 is an exploded isometric view illustrating
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~j; the operation of assembling the component parts for finally
!
molding the racket frame;
Fig. 7 is an elevational view, partially broken
. .
away, of the racket frame following the stage of Fig. 6;
Figs. 8, 9 and 10 are enlarged sections on the
lines 8--8, 9--9 and 10--10 respectively of Fig. 7;
Figs. 11 is an enlarged fragment of the head of
- the complete racket of Fig. 1, partially broken away in
section;
Figs. 12 and 13 are further enlarged sections on
the lines 12--12 and 13--13 of Fig. 11 respectively;
Fig. 14 is a fragmentary exploded view illustrat-
~ ing the assembly of the handle portion of the frame of Fig. l;
- Figs. 15 and 16 are fragmentary elevations, partially
broken away, showing modifications of the racket frame of the
invention having an opening in the throat portion thereof;
Fig. 17 is a fragmentary elevational view illus-
trating another modified frame construction in accordance with
the invention;
Fig. 18 is a further modified construction showing
the converging portions of the tubular member in the throat
region where no spacer or bridging means is utilized so that
an open throat is defined therein;
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107~1941
Fig. 19 is a fragmentary view illustrating a
,~ modification of the method of the invention wherein the
holes for the racket strings are preformed;
,; Fig. 20 is an enlarged cross sectional view
~-; 5 along the line 20--20 of Fig. 19; and
Fig. 21 is an enlarged cross sectional view along
;~ the line 21--21 of Fig. 19.
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Description of th Preferred Embodiments
Fig. 1 shows a com~lete tennis racket in
i
accordance with the invention in which the frame includes
a generally oval shaped head portion 10, a handle portiGn
11, and a throat portion 12 interconnecting the head and
handle portions. The handle 11 is provided with a grip
13, and the head 10 carries the strings 15. As shown in
Figs. 7 and 10, the head 10 is formed with a groove 16
; extending around approximately the outer end half of its
periphery, and the loop portions 15' of the strings are
recessed in this groove below the adjacent peripheral
portions of the frame for protection in use.
; The basic structural part of this racket frame
, is the unitary tubular member 20, which includes the loop
, 15 21 defining the head 0, the converging sections 22 defining
the throat 12, and the parallel end sections 23 defining
the handle 11. The only other pieces of the frame, with
- the exception of the grip 13, are a filler piece 25 between
- the throat portions 22, and a fin-like spreader member 26
extending between the handle sections 23, both of which
are optional and are made of light material such as balsa
wood or plastic foam. Also, grommets 27 and 28 are pro-
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vided as liners for the holes which receive the strings 15,
` the difference between the two sets of grommets being only
that the grommets 28 have straight sided heads proportionedto seat within the groo~e 16.
Figs. 2-10 illustrate successive stages in
the manufacture of the racket frame of Fig. 1. The first
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11~7994~
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stage is the formation of a mandrel comprising an elastomeric
tube 30, core wires 31 and 32, and a filler member 33. The
tube 30 is preferably of a rubber material which will not
disintegrate in the curing cycle of a frame, and satisfactory
results have been obtained with a rubber tube having an
outer diameter of 7/16 inch and a wall thickness of 1/32
inch. The length of this tube should be somewhat in excess
of the final length desired for the tubular member 20.
It is possible to use only a plurality of wires
32 of small diameter, e.g. 1/16 inch, but it is auicker
and easier to use also at least one wire or rod 31 of sub-
stantially larger diameter, e.g. 1/4 inch and the rod 31
is shown as provided at one end with a drive collar 36. A
sufficient number of the wires 32 is used to fill the tube
30 completely, and preferably to expand it slightly, for
example to an outer diameter of 5/8 inch. -
':
The purpose of the filler member 33 is to
increase the peripheral dimension of the mandrel along its
portion corresponding to the part of the head in which the
groove 16 is formed. The member 33 can be placed within
the tube 30, but it is simpler to locate it on the outside,
and this is easily done by welding a section of quarter-
inch rod 33 to the middle of a carrier wire 35 of the same
length as the other wires 31-32. For a full size frame, the
; 25 filler member 33 may be 21 inches long. One end of the
wire 35 fits in a groove in the collar 36 and is held in
place by an O-ring 37. The other end is similarly held
on tabe 3~ by a similar O-ring 37.
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~07994~
The completed mandrel is then mounted in a tension-
ing and winding apparatus as shown in Fig. 2. The shaft of a
low speed drive motor 40 holds and drives the collar 36 through
a pin and bayonet slot connection 41. The other end of the rod
~- 5 31 is secured in a chuck 42 mounted for free rotation on an
adjustable tail stock comprising a screw 44 threaded in a stand
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45 and having a handle 46. Backing off of the screw 44 will
provide the necessary tensioning of rod 31 to support the
entire mandrel in essentially straight position.
The multiple layers of binder-impregnated fiber are
then successively applied to the mandrel, which is easily done
while it is being rotated by the motor 40. To some extent, the
number and sequence of application of these layers may be
varied, but it is important that the majority of the layers be
helically wound of unidirectional hand with successive such
' layers being of opposite hand, and also that there be at
least one layer wherein the fibers run lengthwise of the mandrel
and are not twisted. It is particularly important, for optimum
results from the standpoint of both proper control of weight
and the proper combination of strength and resiliency in the
` finished racket, to use tape composed of essentially continuous
parallel filaments, as distinguished from woven or braided
- tape or tubing. One reason for this is that in a woven (mesh)
- tape, the cross fibers add thickness, since the thickness of
the tape doubles at each crossover, and also weight without
comparable contribution to strength for the purposes of the
invention. In fact, the cross fibers would add no significant
strength to applicant's frame as compared with continuous
-- filament tape, but they would double the weight and effectively
double the thickness of the wall of tubular member 20 for the
same number of tape layers.
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~79941
Another aspect of this matter is that in a fiber mat
structure, wherein relatively short fibers are held together
by resin binder, load transfers are required to take place
through the resin securing adjacent fibers together, and this
is an inefficient use of the tensile strength of the fibers.
- This same deficiency would be present in helically wound mesh
tape, in that the cross fibers would be relatively short, and
would have to depend on the resin to transfer loads there-
; between. In contrast, with unidirectional continuous filament
tape wound helically and with adjacent layers of opposite hand,
the continuous filaments provide the most efficient transfer
of loads throughout the frame, and their ability in this
respect is increased when they are placed in tension in
accordance with the practice of the invention as described
hereinafter.
, In a typical example of the practice of the invention,
~ preferred results have been obtained by applying the following
- layers of binder-impregnated fiber tape one inch wide in the
specified sequence:
A helical layer 50 extending slightly in excess
of the full length desired for the tubular member 20,
e.g. 63 inches.
A straight full length layer 51 composed of two
lengths of the tape.
A second full length helical layer 52 of the opposite
- hand from laye~ 50.
Two helical layers of alternately opposite hand
extending over only the central portion corresponding
to the loop 21 and converging portions 22, e.g. 32 inches.
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i 1079941
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A full length helical layer 55 of the opposite
hand from the adjacent under layer.
Optionally, particularl~ for a heavier racket frame, two
.~ helical wraps of opposite hand may be applied before the
layer 55 along only the central portion of the assembly
- overlying the filler member 33.
As soon as winding has been completed, the
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assembly is removed from the winding and tensioning apparatus,
and the core wires 31 and 32 are removed from within the
tube 30. The filler member 33 and its carrier wire 35
are then also easily removed, but it may be simpler to
s remove them and the tube 30, and then to replace the tube
30 in the uncured tubular shell.
It is quicker and simpler to utilize fiber
tape already impregnated with binder than to add binder
resin in the mold cavity during the final molding stage, and
this is particularly true for continuous filament tape
because the resin holds the non-woven filaments together.
The pre-impregnated tape yields more uniform products, but
it tends to be sticky at room temperature, and subsequent
handling is facilitated if the tubular member is refrigerated
after the core wires have been removed, preferably in a preform
mold 60 having a cavity 61 closely corresponding to the mold
cavity in which final curing of the frame is performed. It is
also desirable at this stage and process to insert a generally
triangular filler member 25 in the open throat area between
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the converging portions 22 of the tubular member, as well
as the divider strip 26 between the handle portions 23. ~ :
-: The final assembly and molding operations
are illustrated in Fig. 6 as carried out in a mold com-
:.
,. S prising three main parts 70, 71 and 72. The mold part
' 70 includes the bottom and sides of the cavity 75 corres-
ponding to the handle portion of the racket frame, the
': throat portion, the inside of the head portion, and that
; part of the outside of the head portion which does not
. 10 contain the groove 16. The upper mold part 71 includes
a male section 76 defining the upper wall of the cavity
ç~;~ 75 in the part 70. The part 72 is movable horizontally
:; toward and away from the parts 70-71 and includes a cavity
defining the outside of the head portion of the frame and
,s~ 15 incorporating an internal rib 77 located and proportioned
. to form the groove 16.
In the final assembling steps before closing
f ~ the mold and curing the tubular member 20, a crescent-
~; .
shaped strip 80 of binder-impregnated fiber is set in the
bottom of that portion of the cavity in mold part 70 which
underlies the filler piece 25 along the inner end of the
, head portion 10 and adjacent portions of the loop 21. A
. layer 81 of the fiber material of the same dimensions as
width of the handle and throat portions is then set in the :
. 25 bottom of the cavity. Then a strip 82 of about half the
'~ width as the handle portion of the frame is set in the cavi.y,
along with a piece 83 matching the outline of the throat
portion of the frame. Also a strip 84 is set along the
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~ side of the cavity opposite the throat portion so that it
i will overlie the exposed edge of the filler piece 25
in the finished frame.
~ The refrigerated tubular member from the mold
60 is then set in the cavity 75, with the ends of the rubbertube 30 extending to the outside through appropriate grooves
85 in the mold. The divider strip 26 can be inserted at
;~,
that time if it was not inserted when the tubular member
was placed in the preform mold 60. A second series of strips
; lO 80-83 is then laid on top of the tubular member, after which
. the mold parts 71 and 72 are moved into position to close
the mold.
.~ For efficient production, the mold parts 70-72
are maintained at the desired curing temperature, so ;
15 that as soon as the mold is closed, the refrigerated
binder begins to soften. When it is thoroughly softened,
for example after two to three minutes, air pressure is
- applied to the projecting ends of the tube 30 as indicated
at 88, at a sufficient pressure to expand the tubular ::
member 20 into firm engagement with all surfaces of the
mold cavity and thereby to maintain all the fiber layers
in tension while the binder is setting, and particularly ;~
to cause the slack fiber material opposite the rib 77 to
engage this rib evenly in order to form the groove 16.
This pressure is not critical, and satisfactory results
have been obtained with air at a pressure of 40 p.s.i.
The temperature of the mold and the time of curing are
interdependent, in accordance with standard practice
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for the curing of fiber reinforced plastics. As previously
: noted, the temperature should not exceed the level at
';
which the tube 30 would disintegrate before the end of the
; initial stage of the curing cycle.
~ 5 Satisfactory results have been obtained if the
'; initial stage of the curing cycle continues for a total
of 15 minutes at 275F, after which the pressure supply
to the tube 30 is discontinued, the mold is opened, the
,~ tubular member is ejected, and the tube 30 is withdrawn
; 10 from its interior. Any flash or other surplus material
can then be removed, after which the cure should be com-
pleted, satisfactory results having been obtained in an'
oven at a temperature of 250F for a period of three hours.
, ~,
The overall configuration of the frame after
trimming and curing is shown in Figs. 7-10 which illustrate
that the fiber layers and binder effectively combine to
~` form a solid wall in which all of the fibers are sub-
; stantially uniformly tensioned for maximum strength. As
~.
best shown in Figs. 12-13, the head portion 10 is sym-
;:.
' 20 metrical in sectional outline along both the grooved and
,:
-~ non-grooved parts thereof, so that the grooved part would
, fit the peripheral outline of the ungrooved part, but the
grooved part has a greater peripheral dimension, by reason
- of the extra fiber wall which defines the groove 16. Also,
the thickness of the tubular member will vary in these parts
of the head portion, depending upon how many layers of
fabric were wound therein.
The handle portion 11 is illustrated as having
a decorative groove 90 along the portion not covered by
the grip 13 in the finished racket, such groove being
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l~9g41
imparted by appropriate complementary configuration of
the mold parts 70-71 as desired. The outer end of the
handle portion, however, is molded to a rectangular section
for easy mounting of the grip 13, which is shown as formed in
; 5 two complementary molded plastic parts 91-92 held in place
by two bookbinder's screws 93 extending through holes
drilled in the handle portion 11, and this mounting may be
reinforced by adhesive. The grip is finished conventionally
by a winding 95 of leather or plastic as shown in Fig. 1,
~: 10 and it is apparent that other grip means can also be used,
r such as grips formed by molding a suitable foam material
around the end of handle part 11. Otherwise completion
' of the racket from the stage shown in Fig. 7 is conventional,
involving drilling of the necessary holes for the strings
15, insertion of the grommets 27-28, and painting as desired.
Some modifications of the basic frame configura-
tion shown in Fig. 1 are illustrated in Figs. 15-17. Thus
Fig. 15 shows a portion of a racket frame having an opening
100 through its throat portion, and in this case, a filler
piece 101 of generally crescent shape is positioned between
opposed locations on the converging sections 102 of the
tubular member. The fabrication of this frame follows the
same steps already described, but the strips of fiber which
are applied in the mold are of appropriate configuration
for the final design.
The filler piece 101 in Fig. 15 may be of balsa
wood or plastic foam, since it serves merely as a support
in the mold for the strips of resin-impregnated fiber which -
carry the load in the finished racket. A strip 103 of tape
- 30 should be applied in the mold to cover the inner edge
surface of filler piece 101 in the same manner as the strip
- 104, which corresponds to strip 84 in Fig. 6. An alternative
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11:)799~1
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construction is shown in Fig. 16, wherein the filler piece
';j, 101 is covered by a pair of windings 105 and 106 of tape
of opposite hand, the ends of which also wrap around the
.~ joining portions of piece 101 and sections 102 to provide
" S extra strength at those joints. One or more wraps of tape
can similarly be provided around the joining portions of
piece 101 and sections 102 in the construction shown in
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Fig. 15.
,,, Fig. 17 shows a racket frame in which the handle
,~ 10 sections 110 of the tubular member are exposed in spaced
- relation between the throat portion of the racket and
the grip 111. 'rhis arrangement is readily established
~,~ by appropriate complementary configuration of the preform
r
, mold and of the curing mold parts, and of course no divider
: lS 26 is used in this racket. The filler piece in the throat
':
portion of this racket frame may be of essentially the
~i~ same configuration as in Fig. 5, in which event it is
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bonded into position by overlving layers of fiber in the
same manner already described, but it is shown as a separate
molded plastic piece 112 held in place by the grommets which
line the holes for the racket strings and by the strings them-
selves. As shown, this piece 112 has integral channels 113
molded therein for the racket strings. These channels could
- also be molded on radii of appropriate lengths such that
the string in each channel 113 would leave the channel
, tangent thereto and thus minimize possible abrasion between
the string and the end of the channel. Otherwise, this
, racket frame is of essentially the same construction already
described in connection with Figs. 1-14.
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1079941
;~ Fig. 18 shows another modified construction
wherein the converging portions 115 of the tubular mem-
ber have no spacer or bridging means therebetween and
thus define an open throat lnto which the strings 15'
extend, the converging portions 115 having enough holes
for strings to extend across most of the open throat, and
~ all of these holes being provided with grommets 27'.
~:r; Thorough testing has established that this frame construc-
tion has all necessary strength, since overlying layers
of resin-impregnated fiber corresponding to the layers 81
and 82 in Fig. 6 are also used in this construction to
assure bonding of the handle portions of the tubular member
together up to the point at which they begin to diverge
at the apex 116 of the open throat.
It can be readily appreciated from the pre- -
ceding description that the invention provides a high degree
'~ of versatility in the control of the strength, weight and bal-
ance of a racket frame. Thus for a heavyweight frame, the
number of windings in its tubular member can be appropriately
' 20 selected, and its balance can be established, by applying
extra windings where needed, or by flller material at
appropriate locations within the tubular member, for example
~-- within the outer end of its head loop. Similarly, the
overall design is subject to wide modification exemplified
by Figs. 15-18.
It is also possible to add to the method of
invention the step of preforming the holes in the frame
for the racket strings, instead of drilling them after
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~079941
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molding is completed. This feature of the invention is
illustrated in Figs. 19-21, wherein the tubular shell
120 corresponds to the shell 20 in Figs. 5 and 6 at the
stage when it is ready for insertion in the oreform
mold 60 and is therefore relatively pliable and with the
fibers in the successive layers relatively loosely held
together.
A forming member 121 composed of a strip of
flexible material includes multiple probe elements 122
~-- 10 arranged thereon in appropriately spaced relation corres-
ponding to the spacing of the holes along the outer
:-.
periphery of the head and throat section of the racket
frame for receiving the racket strings. In addition, the
forming member 121 includes a rib portion 123 corresponding
~ 15 in dimensions to the rib 77 in Fig. 6 which forms the groove
: 16 in the racket frame of Figs. 1-13. As shown, each of the
probe elements 122 is provided with a relatively sharp point,
and in section, its sides are curved to produce a rounded
edge for the hole formed thereby in the wall of shell 120
but its length should be such that it will penetrate only the
fiber layers and not tend to puncture the tube 30 while the
latter is pressurized.
A similar flexible forming member 125 is
proportioned to extend around the inner periphery of the
head portion of the frame and incorporates probe elements
122 in appropriately spaced relation corresponding to
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1079941
the spacing of the string holes on the inner side of the
racket head. An additional forming member 126 is configured
to extend across the inside of the throat portion of the
frame, and its proportions will vary in accordance with the
design of the throat portion of the finished frame. The
forming member 126 is shown as designed for use in producing
a throat portion as illustrated in Figs. 5-7, with probe
elements 127 designed to penetrate through the foam filler
piece 25 to the interior of the throat portions of the shell
~ 10 120.
;, It will be apparent that with a racket frame
of the open throat type, the member 126 could have probe
elements 122 if an additional forming member is provided
for insertion in the open throat portion opposite the member
126. Similarly, if the filler piece 25 is made of wood,
it would be easier to utilize a forming member 126 having
probe elements 122 only long enough to penetrate the fibers
overlying such filler piece. In this case, the holes formed
; thereby would have to be connected with their mating holes
in the shell 120 by drilling through the filler piece, and
the holes in the inner wall portion of the shell could also
be drilled or could be preformed by special probe elements
like elements 127.
In the practice of the invention as illustrated :
in Figs. 19-21, the members 121, 125 and 126 are preferably
applied to the shell 120 before or in connection with insertion
in the preform mold as illustrated in Fig. 5. This can be
done relatively easily, with the probe elements 122 being
1079 9 41
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caused to pierce the shell wall by separating the fibers
with minimum tendency to rupture any of the fibers. The
forming memhers will then remain in place throughout the
~` molding operation illustrated in Fig. 6, and they can be
provided with appropriate locating means such as holes 130
positioned for engagement by locating pins in the mold or
., .
r'~ ~ molds. During the application of heat and pressure to
~ the shell as described, the pressure will cause the wall
. -
of the shell to conform smoothly to the forming members
- 10 and their probe elements in the initial stage of the final
molding operation when the binder softens preliminarily
to hardening.
One advantage of this procedure is that the
fibers are not cut or otherwise ruptured to form the holes
for the racket strings, as they necessarily are when they
are formed by drilling. This procedure also eliminates the
separate drilling operation, since the only drilling nec-
-~ essary in the practice of the invention as illustrated in
; Figs. 19-21 would be to connect opposed pairs of holes on
opposite sides of a wood filler piece in the racket throat
- as noted above.
, While the methods and articles herein described
constitute preferred embodiments of thé invention, it is to
- be understood that the invention is not limited to these
precise methods and articles, and that shanges may be made
therein without departing from the scope of the invention.
.,
-20-
.
