Method of Manufacturing an Elongated Structural Member

1. Field of the Invention

The present invention relates to a method of manufacturing an elongated structural member of substantially uniform cross-section.

2. Description of the Prior Art

Elongated structural members can be used for constituting or construction of different types of frameworks, for example frameworks for rackets for ballgames, bicycles, wheel-chairs and furniture. Typical structural members of this kind are manufactured from metal tubes or fibre reinforced plastic tubes. Metal tube structural members can easily be manufactured by means of bending but have the drawback of being relatively heavy.

Elongated structural members consisting of reinforced plastic tubes have low weight but can not be manufactured by means of bending. Thus, such plastic tube structural members require the use of premanufactured bends to be connected with straight tube sections by means of socket joints or an adhesive making the manufacturing of the structural members expensive and cumbersome.

It is an object of the present invention to provide a simple method of manufacturing an elongated structural member of substantially uniform cross-section which provides a structural member having favorable properties, especially with regard to its weight and strength.

The foregoing and other objects and advantages of the present invention are accomplished by a new and improved method comprising the steps of providing by pultrusion a multicomponent member comprising a porous core and a shell enclosing the core and including a layer of fibres impregnated with an uncured thermosetting plastic, positioning the multicomponent member in a mould and curing said rawling. In a preferred embodiment of the method according to the invention the shell of the multicomponent member is formed from four different layers, a first layer enclosing the core and consisting of an air-tight skin, a second layer enclosing the skin and consisting of peripherally extending fibres, a third layer enclosing the second layer and consisting of axially extending fibres impregnated with an uncured thermosetting plastic, and a fourth layer enclosing the third layer and consisting of peripherally extending fibres.

A modified embodiment of the method according to the invention comprises the steps of providing two the multicomponent members, each multicomponent member comprising a compressible core and a shell enclosing the core and including a layer of fibres impregnated with an uncured thermosetting plastic, positioning said rawlings in separate mould cavities of two separated mould parts, each multicomponent member having a substantially circular section with an outer diameter D, the section of each mould cavity comprising a semi-circular portion having a diameter D and a rectangular portion having long sides D, one of the long sides being positioned adjacent the diameter of the semi-circular portion, and short sides of a length less than D/2 but greater than D/4, closing the mould halves while compressing and deforming the cores of the multicomponent members and curing said the multicomponent members.

For a better understanding of the present invention reference should be had to the following detailed description taken in conjunction with the drawings, in which:

FIG. 1 is a longitudinal section of a rawling to be used for conducting the method according to the present invention into effect;

FIG. 2 is a cross-sectional view of a multicomponent member according to FIG. 1;

FIG. 3 illustrates a section of a lower portion of a mould for conducting an embodiment of the method according to the invention into effect;

FIG. 4 illustrates a section of the mould provided with the multicomponent members prior to the closing of the mould; and

FIG. 5 illustrates a section of the mould of FIG. 4 in closed position.

In FIGS. 1 and 2 reference numeral 11 designs a core consisting of foamed plastic. This core is enclosed by a first layer 12, either consisting of a gas-tight skin unitary with the core itself or consisting of a thin plastic foil. The purpose of the skin 12 is to prevent air from being expelled from the core when the multicomponent member cured for providing an elongated structural member. Such expelling of air from the core may weaken the structural member. The skin shall also prevent the core from absorbing thermosetting plastic. Such absorption of thermosetting plastic causes an increased weight without any corresponding improvement of the strength.

A second layer 13 encloses the core 11 and the skin 12 and consists of fibres peripherally wound around the skin 12. Preferably the fibres are in the form of a glass-fibre roving. This layer 13 does not have to be separately impregnated with thermosetting plastic.

A third layer 14 consists of axially extending glass-fibre threads 15 preferably also in the form of a roving and impregnated with an excess of uncured thermosetting plastic. The glass-fibre threads 15 are positioned so as to form a covering layer around the first and second layers 12 and 13, respectively.

Around the layers 12, 13 and 14 there are finally wound unimpregnated fibres, preferably in the form of a roving, constituting a fourth outer layer 16. Because of the fact that the third layer 14 comprises an excess of thermosetting plastic, the thermosetting plastic is transferred also to the layers 13 and 16 which are thereby also impregnated. A multicomponent member as described can be bent and twisted to substantially any shape and figure. This is provided by carefully positioning the multicomponent member in the cavity of a mould, whereupon the multicomponent member is cured, preferably in heat, for constituting an elongated structural member. The specific construction of the multicomponent member makes the final product extremely strong also in the bends thereof. The principle reason thereto is that the layer 13 consisting of peripherally wound roving provides a vault effect preventing the axial reinforcement layer 15 to bend inwards and collapse when being subjected to strong loads. The fourth outer layer 16 contributes to an improvement of the vault effect. Tests have shown that the strength of the structural member is reduced if the second layer 13 is excluded and the reinforcement thereof is added to the fourth outer layer 16. It has empirically been found that the relationship between the weights of the reinforced layers 13, 14 and 16 should be 8:100:10. This is true when the total thickness of the layers 12, 13, 14 and 16 is 1 mm in sporting products such as tennis rackets and ski poles of the curved type used in downhill race.

In FIGS. 3, 4 and 5 reference numerals 21 and 22 design a lower and an upper portion of a mould. The mould portions 21 and 22 have the same section, i.e. the mould cavities of the moulds are of identical size and shape. As shown in FIG. 3 the sections of the cavities comprise a semi-circular surface having a diameter D and a rectangular surface postitioned on the semi-circular surface and having long sides D and short sides which are less than D/2 but greater than D/4, preferably having a length of about D/3. Two multicomponent members 23 are positioned in separate mould cavities, each multicomponent member consisting of a porous, compressible and deformable core 24 enclosed by a fibre layer, usually a glass-fibre layer, impregnated with a thermosetting plastic. Preferably the core consists of a relatively hard foamed plastic, for example urethane. The fibre layer 25 comprises axially extending as well as peripherally extending fibres. As appears from FIG. 4 the depths of the cavities are less than D. Thus, the multicomponent members 23 extend outside the mould cavities. On the other hand the multicomponent members do not completely fill up the cavities. Thus, there are provided ditches 27 between the multicomponent members and the sides 26 of the mould cavities. When the mould is closed the initial contact is provided between the outermost portions 28 of the two multicomponent members while the ditches 27 are still unfilled. The continued closing of the mold portions provide a draining of the thermosetting plastic from the fibre layers 25 due to a higher specific surface pressure while the axially extending fibres are at the same time partially forced out into the ditches together with thermosetting plastic. As the surface pressure is lower within the ditches than at the surface subjected to compression under the influence of the cores 24 positioned inside thereof the ditches are filled up with more fibres as well as with a higher proportion of thermosetting plastic contributing to a substantial reinforcing of the section in the area of the joint.

This method produces structural members constituting products of dramatically improved strength and life. According to prior art methods there is provided in the transition surface between the fibres of the multicomponent members and the thermosetting plastic such a compression over the entire surface that there is established direct contact between the fibres of the multicomponent members providing a disastrous strength of the product. The binding in the contact surface is so weak that a splitting takes place also at small loads.

The above method is especially favorable in manufacturing structural members for forming products in which an element is positioned between the two multicomponent members for example in ball-game rackets of the type having a tensioned net firmly connected with the structural members.

The invention can be modified within the scope of the following claims.