GB2227444A

GB2227444A - Injection moulding hollow articles

Info

Publication number: GB2227444A
Application number: GB8902099A
Authority: GB
Inventors: Jonathan Christopher Mott
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-01-31
Filing date: 1989-01-31
Publication date: 1990-08-01
Anticipated expiration: 2009-01-31
Also published as: GB8902099D0; GB2227444B

Abstract

A method of making a racquet, for example a tennis racquet or a racquet-ball racquet, comprises injecting a molten stream of plastics material into a mould shaped to define the frame and handle of the racquet. A low density fluid, preferably nitrogen, is injected under pressure into the molten stream as it enters the mould, and flows into the mould, where it pressurises the plastics material against the walls of the mould. The pressure of the fluid is maintained until the plastics material has at least partly set, whereby the fluid renders the thicker sections of the frame and handle hollow.

Description

RACOUETS This invention relates to racquets for use in games such as tennis, squash, badminton, racquet ball and the like, and is more particularly concerned with moulded plastics racquets.

One known form of moulded plastics racquet is described in my United Kingdom Patent No. 2 148 133, and comprises a generally oval frame having a handle extending outwardly therefrom in alignment with the major axis of the oval frame. The frame and handle are moulded in one piece, with a sinuous wire string-securing member supported in the mould during the moulding process so that it becomes embedded in the frame.

Although this known form of racquet is relatively simple and economical to make and generally satisfactory in use, the frame and at least part of the handle are solid, which can result in the finished racquet being somewhat heavier and more rigid than is favoured by some racquet users.

Another known form of moulded plastics racquet is described in United Kingdom Patent Applications Nos. 2 015 886 and 2 056 864. In this form of racquet, at least the frame (or head) is moulded around a fusible core, which has a sufficiently low melting point that it can be heated to its melting point and removed after the plastics moulding has set, thus producing a hollow moulded frame. This hollow moulded frame may then typically be filled with a polyurethane foam, forming a foam core.

Although this second known form of racquet can be made lighter than the first, the process for making it involves several more steps, and is accordingly more time consuming and expensive.

It is therefore an object of the present invention to provide a moulded plastics racquet in which the drawbacks of the known forms of racquet mentioned above are alleviated.

According to the present invention, there is provided a method of making a racquet, in which the frame and at least part of the handle are integrally formed by an injection moulding process in which a molten stream of plastics material is introduced into a mould having a cavity shaped to define said frame and handle part, wherein a predetermined quantity of a fluid less dense than the plastics material is injected under pressure into the stream of the plastics material so as to flow therewith within the mould cavity and pressurise the plastics material against the surfaces of the mould cavity, the pressure of said fluid being maintained until the plastics material is self-supporting, whereby at least the thicker sections of the frame are of reduced weight.

The fluid can be introduced into the molten stream of plastics material upstream of the mould cavity (eg within one or more of the sprues leading to the cavity), or at the point of entry of the plastics material into the cavity, or even at one or more points within the cavity.

Advantageously, the fluid is an inert gas such as nitrogen, whereby to render said thicker sections of the frame hollow.

The mould can incorporate side action devices to define stringing holes passing through the frame, as generally described in United Kingdom Patent Application No. 2 175 243 (but without the requirement that the side action devices be pointed or needle-like, since no actual piercing is required of them). Surprisingly, however, experiments have shown that the moulding method of my aforementioned UK Patent No. 2 148 133, modified by the injection of an inert gas as defined hereinabove, can also be used, since, contrary to expectations, the intrusion of the string-securing loops into the mould cavity does not significantly interfere with the formation of a hollow interior to the frame: rather, the plastics material coats the portions of the string-securing loops disposed in the mould cavity, while leaving the interior of the frame hollow around these coated portions.

The plastics material may typically be nylon or polycarbonate plastics material, advantageously filled with short reinforcing fibres, selected from glass fibres, carbon fibres, aramid fibres or any combination thereof.

The invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a part-sectional plan view of a moulded racquet-ball racquet made in accordance with the present invention; Figure 2 is an enlarged sectional view on the line 2-2 of Figure 1; Figure 3 shows a mould for making the racquet of Figure 1, using a method in accordance with the present invention; and Figure 4 shows part of the mould of Figure 3 in more detail.

The racquet-ball racquet shown in Figure 1 is indicated generally at 10, and comprises a symmetrical, approximately oval- or pear-shaped, frame 12 having a handle 14 projecting from the narrower end of its pear-shape in alignment with the major axis of the pear or oval shape. The frame 12 and handle 14 are integrally moulded in a plastics material, such as nylon or polycarbonate, preferably reinforced with 10% to 30% of carbon fibre, or of glass fibre, or of aramid fibre, or of a mixture of any combination of these.

As can be seen in Figure 2, the interior of the frame 12 is hollow, as is the handle 14: the hollow interior of the frame 12 is indicated at 15, and the method of producing this hollow interior will become apparent hereinafter.

The frame 12 is provided with a one-piece wire string-securing member 16, which is moulded into the frame during the moulding process by which the frame is produced. As can be seen in Figure 1, the string-securing member 16 comprises a single length of 2mm steel or titanium wire bent into a sinuous shape resembling castellations or crenellations in plan view, and defining a plurality of U-shaped loop portions 18, of which adjacent ones are joined together at the tops of the limbs of their U-shapes by connecting portions 20.

The string-securing member 16 is moulded into the frame 12 such that the tops of the limbs of the U-shapes of the loop portions 18, and the connecting portions 20 joining them, are embedded in, and therefore firmly and irremovably secured in, the plastics material of the frame, with the remainder of each loop portion 18 projecting radially into the frame, to receive and position at least one string of the stringing (not shown).

Again as can be seen in Figure 2, although the frame 12 is hollow, and the tops of the limbs 18 of the string-securing member 16 and the connecting portions 20 joining them project into the hollow interior 15, they are nevertheless coated in the plastics material of the frame: this is shown at 22 in Figure 2, and the method of achieving it will become apparent hereinafter.

Figures 3 and 4 show the components of the mould used to make the racquet 10, this mould being designated generally by reference 30. As can be seen from Figure 3, the mould 30 comprises four principal components, specifically upper and lower complementary mould plates 32 and 34 respectively, which together define the basic mould cavity 36 in which the racquet 10 is moulded, and upper and lower locating members 38, 40 respectively, which together define a "cassette" for precisely locating and supporting the wire string-securing member 16 with respect to the mould cavity 36: for clarity, only the lower mould plate 34 is shown in detail in figure 2.

The lower mould plate 34 comprises a flat rectangular plate of steel, having a flat upper surface 44 in which is formed the lower half 36a of the cavity 36, ie the half defining the lower half of the finished racquet 10 when the racquet is lying on a flat, horizontal surface. Thus the plane of the surface 44 coincides with the plane which the stringing of the finished racquet 10 will occupy.

The half-cavity 36a has a radially inner wall 46 which is of the order of lmm thick, and which surrounds a recess 48 shaped to receive the lower locating member 40.

The lower locating member 40 has a flat upper surface 50 which is precisely flush with the upper surface 44 and the top of the wall 46 of the lower mould plate 34 when the member 40 is mounted in the recess 48. Furthermore, the locating member 40 is precisely located within the recess 48 by two upwardly extending, circular-section, locating pins 52, which project from the base of the recess into corresponding locating holes 54 passing through the locating member.

The flat upper surface 50 of the member 40 is provided with a plurality of U-shaped grooves 56 of semi-circular cross-section, each of which extends into the surface 50 from one edge of the member 40, and then back out again. The spacing and shape of the grooves 56 is such that each one receives and locates a respective loop portion 18 of the wire string-securing member 16. The tops of the limbs of the U-shapes of the loop portions 18 thus project radially outwardly of the member 40, and pass through respective semi-circular cut-outs 58 formed for that purpose in the top of the wall 46 of the lower mould plate 34. As a result, the connecting portions 20 joining the loop portions 18 of the string-securing member 16 are positioned near the centre-line of the half-cavity 36a.

The upper surface 50 of the lower locating member 40 is provided with a plurality of upwardly extending locating studs 60, which co-operate with corresponding locating holes 62 provided in the complementarily-shaped upper locating member 38.

Further holes 64 are provided in the member 38 to accommodate the locating pins 52 of the lower mould plate 32.

The upper locating member 38 has a flat lower surface 64, in which are formed U-shaped grooves 66.

The grooves 66 are precisely aligned with and complementary to the grooves 56 in the lower locating member 40, so that when the members 38, 40 are correctly located with respect to each other by way of the studs 60 and holes 62, and fastened together with their respective surfaces 50, 64 in contact with each other, the string-securing member 16 is firmly entrapped between them. To permit the members 38, 40 to be fastened together, several of the studs 60 are elongated so that they project through the member 38, and threaded as shown at 60a: the fastening is then completed by knurled nuts 68.

In use, the two locating members 38 and 40 are secured together with the string-securing member 16 entrapped therebetween, as described above. The cassette defined by the fastened-together locating members 38, 40 is then located in the recess 48 in the lower mould plate 34 by means of the locating pins 52, such that the connecting portions 20 of the string-securing member 16 are disposed in the half-cavity 36a. The mould cavity 36 is then closed by means of the upper mould plate 32, which is basically complementary to the lower mould plate 34.

In particular, the upper mould plate 32 has a flat lower surface (not shown) which mates with the upper surface 44 of the mould plate 34, this flat lower surface containing a half-cavity 36b which is complementary to the half-cavity 36a in the mould plate 34, so as to define therewith the cavity 36.

Additionally the half-cavity 36b has a radially inner wall (not shown) which is complementary to and mates with the inner wall 36 of the half-cavity 36a.

Finally, the upper mould plate 32 is provided with holes (not shown) which co-operate with the locating pins 52, to precisely locate the mould plate 32 with respect to the mould plate 34.

Once the mould plates 32 are secured together as described above, a predetermined amount of the aforementioned plastics material, loaded with the selected proportion(s) of the selected reinforcing fibre(s), is injected under pressure, along with a predetermined amount of inert gas such as nitrogen at a pressure of about 400 p.s.i, into the mould cavity 36, until the cavity is full. To facilitate this, the cavity 36 typically has at least two suitably positioned feed-in points or sprues, e.g. one on each side of the portion which defines the lower part (as viewed in Figure 1) of the internal periphery of the frame 12. Each such feed-in point or sprue comprises, as shown in Figure 4, an inlet duct 70 for the plastics material, coaxially surrounding a further inlet duct 72 for the inert gas.The effect of injecting the inert gas under a predetermined pressure into the middle of the flow of the plastics material, which is indicated at 74 in Figure 4 and is flowing in the direction of the arrow 76, is to compress the plastics material against all the internal surfaces of the mould cavity 36 (which surfaces effectively include the external surfaces of the portions of the string-securing member 16 disposed within the cavity, hence creating the coating effect mentioned earlier), thus producing the hollow interior 15. The pressure of the inert gas is then maintained until the injected plastics material has set.

Once the plastics material has set, the mould plates 32, 34 and the locating members 38, 40 are separated, exposing the moulded racquet 10 with the wire string-supporting member 16 firmly and irremovably embedded in the frame 12. All that remains to complete the racquet 10 is to remove any flash, to wrap and bond leather or other material suitable for gripping around the handle 14, and to string the racquet with nylon, gut or the like.

It will be appreciated that the racquet 10 has very few components and is therefore relatively simple and inexpensive to make. However, its strength/weight ratio is much improved by virtue of its hollow interior 15, so that for a given weight, it is stronger, and so less subject to twist, and therefore plays more accurately. Additionally, the pressurising of the plastics material against the internal surfaces of the mould cavity 36 significantly improves the surface finish of the finished racquet 10, inter alia by preventing the formation of sink holes in the surface of the racquet.

Several modifications can be made to the described embodiments of the invention. In particular, the cassette defined by the locating members 38, 40 can be modified so that one of the members, e.g. the member 38, is fully grooved or recessed to the full depth of the diameter of the string-securing member 16, so that the member 16 is wholly received in said one member 38. This makes it easier to insert the member 16 in the member 38. In this case, the lower surface 64 of the upper locating member 38 can be completely flat, with the result that the mating surfaces 50, 64 of the locating members 38, 40 are no longer flush with the upper surface 44 of the lower mould plate 34, but displaced parallel to and lmm above the surface 44.

Although in the described embodiment the inert gas is injected into the flowing plastics material in the sprues upstream of the mould cavity, it can instead be injected into the flowing plastics material within the mould cavity or at the point of entry thereto.

Another important modification which can be made to the invention is to use a plastics-coated braided fibre tube as the basis for the frame 12.

This tube is preformed, has a diameter close to that required of the frame-section shown in Figure 2, and is substantially self-supporting. It is laid in the mould cavity 36 prior to closing the mould, and the moulding process is carried out by injecting the molten plastics material and entrained inert gas into the mould cavity within the tube. The molten plastics material injected softens the plastic coating of the tube so that the two plastics materials (which are preferably the same) fuse together, the pressure of the inert gas assisting in this as it pressurises the injected plastics material outwardly into the braided fibres of the tube. In this case, the wire string-supporting member 16 is preferably omitted, and stringing holes are formed in the frame 12 either after moulding by drilling, or prior to moulding as described in United Kingdom Patent Application No.

2 175 243.

Finally, although the invention has been described with reference to a racquet-ball racquet, it can also be used in tennis racquets, squash racquets, badminton racquets and the like.

Claims

1. A method of making a racquet, in which the frame and at least part of the handle are integrally formed by an injection moulding process in which a molten stream of plastics material is introduced into a mould having a cavity shaped to define said frame and handle part, wherein a predetermined quantity of a fluid less dense than the plastics material is injected under pressure into the stream of the plastics material so as to flow therewith within the mould cavity and pressurise the plastics material against the surfaces of the mould cavity, the pressure of said fluid being maintained until the plastics material is self-supporting, whereby at least the thicker sections of the frame are of reduced weight.

2. A method as claimed in claim 1, wherein the fluid is an inert gas such as nitrogen, whereby to render said thicker sections of the frame hollow.

3. A method as claimed in claim 1 or claim 2, wherein the plastics material is introduced into the mould via at least one annular duct, which surrounds another duct via which the fluid is injected into the flow of the plastics material.

4. A method as claimed in any preceding claim, wherein the plastics material is nylon.

5. A method as claimed in any of claims 1 to 4, wherein the plastics material is a polycarbonate plastics material.

6. A method as claimed in any preceding claim, wherein the plastics material is filled with short reinforcing fibres comprising glass fibres, carbon fibres, aramid fibres or any combination thereof.

7. A method as claimed in any preceding claim, wherein the mould includes retractable side action devices arranged to extend across the mould cavity so as to define stringing holes in the moulded frame.

8. A method as claimed in any of claims 1 to 6, including the steps of forming a plurality of string-securing members each comprising a generally U-shaped loop having the upper ends of the limbs of its U-shape bent over, and supporting the string-securing members such that said upper ends of said limbs are disposed inside the mould cavity while the loops project out of the mould cavity into the area which will be surrounded by the moulded frame, whereby after moulding, said upper ends of said limbs are coated with the plastics material so as to embed them in the moulded frame, while the loops each project radially inwardly of the moulded frame to receive and position at least one string of the stringing of the racquet.

9. A method as claimed in claim 9, wherein the string-securing members are made from metal.

10. A method as claimed in claim 10, wherein the string-securing members are made from wire.

11. A method as claimed in claim 11, wherein a plurality of said string-securing members are integrally formed by suitably bending a single length of wire.

12. A method as claimed in any one of claims 8 to 11, wherein the step of supporting the string-securing members comprises sandwiching the loops of the string-securing members between first and second separable and cooperating locating members such that the bent over upper ends of the limbs of the string-securing members and the immediately adjacent portions of said limbs project from the periphery of the locating members substantially in the plane of their mating surfaces, the locating members being shaped and dimensioned to fit closely adjacent said mould such that the portions of said string-securing members which project from said locating members are disposed within said mould.

13. A method of making a racquet, the method being substantially as herein described with reference to the accompanying drawings.