BACKGROUND OF THE INVENTION In our earlier filed Australian Patent Specification No. 673446 entitled "MEANS AND METHOD FOR THE MANUFACTURE OF BUILDING PANELS", there is described an improved moulding process for the production of a rectangular pre-cast building panel unit and which includes the steps of setting up a suitable rectangular mould, pouring into the mould a quantity of wet-mix concrete or plaster to form a first layer therein of predetermined thickness, adding a layer of reinforcement material such as glass rovings, into the first wet-mix layer, inserting a plurality of void forming plug members on said first layer so as to extend between opposite sides of the mould, with the spaces between adjacent plug members defining internal mould cavities which form webs in the finished panel, adding a further quantity of wet-mix concrete into the mould sufficient to fill the spaces between the plug members and to form a second layer of plaster, allowing the wet- pourable mix in the mould to partially set, removing the void forming plug members horizontally sideways from the mould, tilting the mould to an approximately vertical position, and thereafter removing the thus formed hollow building panel from the mould.

During the commercialisation of the invention described and illustrated in our Australian Patent No. 673446, a number of problems were encountered and these are discussed herein below but not necessarily in any order of importance.

It has been found that in order to extract the plug members from the mould when the wet pourable mix in the mould has set, a significant extraction force is required. During the extraction step, the extraction force is restrained by the top edge of the set panel. It has been found that the stronger the mix used, the greater the force required to extract the plug members. This in turn caused a limitation to the extent to which the formulation of the mix could be changed in order to achieve a panel of greater structural strength. It is desirable therefore that the plugs are designed in a manner to facilitate their withdrawal from the mould after the panel has set.

SUMMARY OF THE INVENTION In a first aspect the present invention accordingly provides an expandible/contractible void forming plug member for forming a void during the moulding of a unitary hollow building panel, said plug member including : an elongate inner core movable longitudinally of the plug by a predetermined distance between an extended position wherein the plug is in an expanded state and a retracted position for when the plug is in a contracted state; a plurality of movable outer shell portions substantially surrounding said inner core; attachment means for attaching said outer shell portions together to enclose said inner core; first and second sets of interengageable elements respectively attached to said outer shell portions and the inner core and adapted to cooperate to cause the outer shell portions to move outwardly in order to expand the plug to said expanded state when the inner core is moved to its extended position or to move inwardly so as to collapse said plug to said contracted state when the inner core is moved to its retracted position.

Preferably, the first and second sets of interengageable elements include opposing wedge elements, said wedge elements arranged to cause said shell portions to move outwardly or inwardly on movement of said inner core between said retracted position and said extended position or between said extended position and said retracted position respectively. Use of wedge elements provides an effective means to transform the longitudinal movement of the inner core to outwards and inwards movement of the outer shell portions.

Preferably, the outer shell portions include four quarter shells each arranged to extend along an edge of said inner core. This provides a convenient geometry for the insertion and retraction of the plug members.

In a second aspect the present invention accordingly provides a method of forming a void in a unitary building panel, said method including the steps of: pouring a first layer of settable wet mix in a mould to form a first layer; positioning the plug member according to a first aspect of said invention on said first layer, said plug member in said collapsed state; inserting said inner core to said extended position, thereby changing said plug member to the expanded state.; 'forming a second layer of settable wet mix over said plug member; allowing said settable, wet mix to set; retracting said inner core of said plug member to said retracted position, thereby changing said plug member to the collapsed state; and removing said plug member.

Preferably, the method incudes further the step of adding at least one reinforcement layer of reinforcing material.

Secondly, with the methodology described in our Patent No. 673446, the bonding between the first layer of concrete poured into the mould and the transverse webs formed between the void forming plug members was not entirely satisfactory.

It is desirable therefore that the bonding of the transverse webs to both major wall portions of the panel be improved so as to minimise the likelihood of web separation under loading forces.

Therefore in a third aspect the present invention accordingly provides a method of moulding a unitary hollow building panel, said method including the steps: 'pouring a first quantity of settable wet mix in a mould to form a first wet mix layer therein; 'adding a first reinforcement layer of reinforcement material to said first layer; positioning on said first reinforcement layer a first group of plug members, said first group of plug members arranged to extend transversely of said mould, said plug members being spaced apart and having gaps therebetween; adding a second reinforcement layer of reinforcement material over said first group of plug members and into the gaps between the plug members of said first group; inserting further plug members into said gaps; 'pouring a second quantity of settable wet mix to form a second wet mix layer sufficient to cover all plug members; allowing said first and second quantities of wet mix to set; removing said plug members; removing said unitary hollow building panel from said mould.

This results in a much stronger bond between the first layer and the web members and subsequently ensures that the reinforcing material adequately connects the first or bottom layer of the wet-mix through the web members with the second or top layer of the wet-mix.

Preferably, the method further includes the step of pushing said second reinforcement layer into said first wet mix layer in said gaps before said step of inserting of a second group of plug members. This acts to further strengthen the building panel as bonding between the first layer and the reinforcing material is improved.

Preferably, the method further includes adding a third reinforcement layer of reinforcement material covering said first and second group of plug members further acting to strengthen the building panel formed using this method.

Preferably, the reinforcing material is chopped fibre glass providing reinforcement in multiple directions.

BRIEF DESCRIPTION OF THE DRAWINGS An preferred embodiment of the present invention will be discussed with reference to the accompanying drawings wherein: Figure 1 illustrates the method of placement of the reinforcing material in the mould according to the methodology of our Patent No. 673446; Figure 2 is a schematic representation of the methodology of the present invention where the first layer of wet-mix and reinforcing material are laid, following which alternate plug members are inserted and thereafter the laying of the second layer of reinforcing material; Figure 3 is a schematic representation showing the manner in which the second layer of reinforcing material shown in Figure 2 is pushed into the first layer of the wet-mix by means of a tamping tool; Figure 4 is a cross-sectional view taken through the mould'showing the complete set of plug members inserted in position and the reinforcing material laid in the final panel; Figure 5 is a side elevational view of an expandable plug member in its expanded position ; Figure 6 is a view similar to Figure 5 showing the expandable plug member in its collapsed position; Figure 7 is a plan view of the central core of an expandable plug member unit; Figure 8 is a fragmentary side elevational view of the expanded plug member shown in Figure 5; Figure 9 is a cross-sectional view of the expanded plug member depicting the cooperating wedge elements; Figure 10 is a view similar to Figure 8 showing the expandable plug member in its collapsed position; Figure 11 is a view similar to Figure 9 showing the plug member in its collapsed position; Figure 12 is a view similar to Figure 9 showing an expanded plug made according to another embodiment of the invention; Figure 13 is a view similar to Figure 11 showing the plug in its collapsed position; while Figure 14 is another cross-sectional view of the plug member illustrated in Figure 8 taken across a region between wedge elements showing inter shell springs.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT In order to assist the understanding of the following description of the preferred embodiment of this invention, the contents of our earlier Australian Patent Specification No. 673446 are incorporated herein by reference.

Figure 1 of the drawings illustrates the methodology of our earlier Patent No.

673446, in particular the manner in which the upper and lower layers of reinforcing material, in the form of glass fibres, are laid in the mould, as well as the positioning of the glass fibre in the web forming spaces formed between the removable plugs 20.

It will be appreciated that, in the finished panel, the reinforcing material in the web members will have limited bonding or connection with the glass fibre reinforcing material embedded in the first layer 15 of the moulded panel.

Referring to Figures 2 to 4 of the accompanying drawings, which illustrates the methodology of the present invention, the rectangular mould has been filled with a first layer 15 of the wet-mix into which has been rolled by means of a manual roller (not shown) a first layer of fibre-glass reinforcing material 19. Selected ones of void forming removable plugs 20 are inserted into the mould by means of a plug insertion/extraction unit (not shown) horizontally sideways through apertures in the side walls of the mould. A multi-head glass-cutter/oscillator feeder (not shown) feeds glass rovings into the mould as an upper reinforcement material layer 77.

During the moulding process, the reinforcing material is rolled into the wet-mix preferably by means of a mesh roller.

Thereafter, a vertically moveable tamping tool pushes its attached tamper basket 69 down into the wet-mix between each of the previously positioned plugs 20 so that substantial portions of the layer 77 are pressed against lengthwise portions of the first fibre-glass layer 19. A second group of plug members 20'are then inserted into the mould in a similar manner to that of the previously positioned plugs 20, so as to lie in the spaces or gaps between adjacent plugs 20, following which a further quantity of wet-mix is poured into the mould so as to embed the plugs 20, 20'and fill the spaces therebetween until the mould is filled. In addition a third layer of reinforcing material 30 may be added over both groups of plug members. The liquid mix is then screeded evenly across the open top of the mould, and thereafter the wet- mix is allowed to set. After setting of the wet-mix, the plug insertion and extraction unit is used to remove the internal plugs 20, 20'from the mould, as will be described hereinafter.

It is a feature of the present invention that the void forming plug members are in a collapsed state prior to their insertion into the mould and are expanded during their insertion by the plug insertion and extraction unit. The construction of one of the expandable plug members is shown in Figures 5 to 11. Each plug member 20,20' comprises an elongate inner core 70 and four outer shell portions 71 longitudinally movable with respect to the core 70. A sleeve 72 made from flexible resilient material, eg rubber, completely encapsulates the plug, thereby making it suitable for low viscosity mixes such as plaster. As shown in Figures 9 and 11, the central core 70 and the outer shells 71 are interconnected by sets of pairs of co-acting internal wedge elements 73,74 ; 75,76 spaced longitudinally along the plug. Wedge elements are attached to the inner core 70 and outer shell portions 71 respectively using a dove tail arrangement secured by spot welds.

The plug members 20, 20'are inserted by passing same through the apertures in both side walls of the mould with their outer shells 71 retained against the side walls of the mould and the inner cores 70 are then moved further inwards by a predetermined distance by the plug insertion and extraction unit. While this is occurring, the outer wedge 74 and outer side wedge 76 which are attached to the outer shells 71, slide in an opposing angle against the inner wedge 73 and the inner side wedge 75 respectively which are attached to the inner core 70, thereby expanding the shells 71 away from the main inner core 70. At the same time, the outer rubber sleeve 72 also expands outwardly and bridges the resultant gaps formed between adjacent margins of adjacent outer shells 71. This is clearly shown in Figure 9.

After the wet-mix has hardened or set in the mould, the main inner core 70 is initially retracted a predetermined distance by the plug extraction unit and during which time the wedges 73,75 on the main inner core 70 slide away from the wedges 74,76 on the outer shells 71, thus collapsing the plug member 20,20'. When in their collapsed condition, the plug members 20, 20'can be quite easily retracted from the mould, without any major force being applied to the finished panel. A square block (40 mm x 40 mm of 8 mm steel) is attached to main inner core 70 approximately 250 mm from the extraction end providing an abutment surface to prevent the core 70 being extracted separately to the outer shells 71.

In lieu of the rubber sleeve 72, the sleeve can be made from flexible resilient plastics sheet, eg canvas, with elastic bands at each end to assist in the contraction of the plug assembly.

In another variation shown in Figures 12 and 13, the sleeve is omitted and the four outer shell segments 71 are retained together by strips of canvas 80 each of which is attached along opposite margins thereof to adjacent edges of adjacent shells 71. The collapsing and expansion of the plug is the same as that described previously.

In the plug collapsed position, the joining strips 80 are folded inwardly, while in the expanded position, they are taut. Referring to Figure 14, this shows a cross-sectional view through a plug member at a region intermediate to wedge elements 73,75, 74, 76 which incorporate springs 90 to retain outer shell portions 71 to enclose the inner core 70. Springs 90 aid the inward movement of shell portions 71 on retraction of inner core 70 further facilitating removal of the plug member.

In practice a plug insertion and extraction unit suitable for 48 plug members is employed. This unit is divided into 4 separate modules consisting of 12 plugs. Each module includes an independent hydraulic power pack, electric motor and gearbox along with all associated electrical controls. Plug members for each module are divided into two groups A and B which are separately controlled in order to implement the inventive manufacturing method set out herein. In addition each module includes a plug lock arm comprised of a hydraulically activated toggle clamp which releasably attaches to the main inner core 70 of respective plug members.

In operation, after positioning of the desired collapsed plug members into the mould the entire insertion and extraction unit is moved forward to further insert the inner core a final 150 mm into an extended position thereby expanding the plug members into an expanded state. Movement of the entire unit provides the required inertial force to fully expand the plug members. On removal of the plug members, separate hydraulic breakaway units located on each module extract the main inner cores 70 an initial 150 mm thus collapsing the plug member. Following this the plug member in entirety is then removed from the mould using the electric motor incorporated into each module.

A brief consideration of the above described embodiment will indicate that the present invention provides significant improvements to the invention described and illustrated in our earlier Patent Specification No. 673446, improvements which result in the production of a pre-cast building panel having superior structural strength, due to the stronger bonding between the opposed wall portions of the panel and the web members interconnecting same, and which enable the void forming plug members to be extracted from the set mould with significantly reduced withdrawal forces and in turn less likelihood of damage being done to the finished panel.