FIELD OF INVENTION

The present invention relates to a method and apparatus for manufacturing structures, especially for manufacturing board units having hollow channels arranged between outer board portions. The method comprises providing a fiber cement mix (typically a mix mainly comprising a fiber material, a cement based binding agent and an activating liquid (e.g. water) between outer press panel elements and a retractable inner core element defining the size of a hollow channel, subjecting said mix to a pressure by said press panel elements that compress evenly all four main sides of the hollow board unit; and after curing of the mix, the inner core element is retracted such that it can be removed from the hollow channel, the pressure of the press panel elements is released, to obtain a board unit having a hollow channel between outer board portions.

The invention is in particular intended for producing hollow building elements from composite mixes that are compressed and cured in a compressed state to obtain durable strong building elements. CFB (cement fiber board) is a preferred material, withstanding prolonged exposure to severe conditions. Such material mixes are substantially compressed in the element manufacturing process, or on the order of 2 to 5-fold, i.e. down to a volume/material thickness that is 50-20% of the original volume of the uncompressed non-cured mix.

TECHNICAL BACKGROUND AND PRIOR ART

The invention makes use of a process utilizing a mix of fiber material, e.g. wood chips, such as from waste timber or the like, and cement-like binding agent, which is formed and cured in a special press for that purpose. The process of consolidating of the mix, the system of the invention a special process more closely described e.g. in EP patent 189127- Bl incorporated herein by reference, which process uses carbon dioxide for speeding up the consolidation of the mix. Injecting excess carbon dioxide into such a mix facilitates a rapid carbonization of a part of the cement binding agent, mainly by the process CaO + CO ₂ => CaCO ₃ .

A high pressure is needed to compress sufficiently composite mixes such as CFB in order to produce a rigid and strong board structure, or on the order of about 15 to 50 bar (1500- 5000 kPa). It must also be ensured that all portions of the board structure are substantially evenly compressed.

An earlier system and method developed by the present inventor is described in European patent no. EP 796 165-B1, the whole contents of which are included herein by reference.

The prior art system comprises a special core press expander element to be located in the fiber cement mix between outer press panel elements; before the curing of said cement based binding agent, the mix located between said outer press panel elements is subjected to an outer pressure by said press panel elements as well as an additional inner pressure by expanding one or more core press expander elements essentially uniformly inside said mix between said outer press panel elements.

More recently the applicants developed an improved version of the prior apparatus and methods, by inventing a special expandable and retractable core element which is located in the fiber mix between the outer press panel elements, the new core element being based on a simplified mechanical design allowing for varying sizes of the board units, especially allowing varying width of the hollow channels in said unit, and even narrow channel widths down to about 7-8 cm. The core element is described in detail in applicant's co-pending application PCT/IS05/00001 which is incorporated herein in full by reference.

During the initial trials and continued development the applicants have found a need to simplify and improve the arrangement of the outer press panels in order to speed up the overall process and lower the cost as well as minimize the bulk and dimensions of the outer press.

SUMMARY

The object of the present invention is to provide an improved apparatus and method for producing hollow building elements, based on a new press with four-way press panels that compress a board element evenly and ensures that the element is formed properly located in the center of the press ensuring proper symmetry and shape including correct thickness of all portions of the element.

The present invention provides an improved apparatus and method for manufacturing hollow board elements. The apparatus described herein comprises at least four sets of (movement panes (such as hydraulic pistons) to move both of opposing horizontal press panel elements ('bottom' and top' elements) and both opposing vertical press panel elements.

FIGURE LEGENDS

Figure 1 shows a perspective view of a section of the apparatus (a typical apparatus is 4-6 times longer than the illustrated section).

Figure 2 shows a cross-section of the apparatus when the chamber (49) is fully expanded.

Figure 3 illustrates a cross section of the apparatus of the invention showing the outer press elements (40, 41, 42) surrounding the retractable core element (45), the space (44) in between the press elements (40, 41, 42) and the core element to be filled with the CBF mix or the like. A ready-made building element produced with the apparatus is shown in Figure 3b.

Figures 4 to 18 illustrate a typical duty cycle of a preferred apparatus and method of the invention.

Figure 19 shows an exploded view of a preferred retractable core element of the invention.

Figure 20 shows different views of the core element.

Figure 21 shows cross-sectional views of the core elements and interconnected core elements, both in expanded and retracted configuration.

Figure 22 shows a cleaning element (60) comprised in one embodiment of the apparatus of the invention. Figure 22a showing an overview of the cleaning element resting on a surface to be cleaned, either a main surface (3, 4) of the core element or a surface of the outer press panels (40, 42). Figure 22b shows a cross-section of the cleaning element. Note that the gas vents (22) are not drawn to scale.

DEETAILED DESCRIPTION

Reference is made to Figure 3b showing a cross-section of a fabricated hollow building element (50) with outer wall boards (51) and connecting boards (52) surrounding a core element defining the width of a hollow channel (54) to be created. In order to produce an element such as shown in Figure Ib both the outer wall boards (51) and the connecting boards (52) must be subjected to pressure in order to compress all material of the element. In principle, it suffices to move only one main press panel (40) against either of the outer wall boards (i.e. either the 'upper' or 'lower' board) as the law of forces and counter-forces causes both outer wall boards to be compressed equally. Similarly, it would in principle suffice to move one side press panel (42) against one of the two connecting boards (52).

It is however for many practical reason advantageous to produce hollow board elements with substantially symmetrical distal end boards (53), by moving both of the two side press elements (41) each against its adjacent connecting board (52).

In order to obtain strong elements, all parts thereof must be subjected to substantially equal compression. Therefore, in order to produce elements such as shown in Figure 3b

which shows a cross-section of a fabricated hollow building element pressure must be applied both vertically to compress the wall boards (51) and horizontally to compress the connecting boards (52)

Figure 3b shows the cross-section of the fabricated building element (50) with outer wall units (51) and wall connecting units (52) and a hollow channel (54), which for indoor wall elements is typically about 7-10 cm wide but can be substantially wider if desired. The end rims (53) can be removed or shaped suitably to connect to adjacent wall elements.

Hollow board building elements described herein are interchangeably referred to as hollow board structures or hollow slabs. The system makes possible much greater flexibility with respect to the size of the board elements, in particular the width of the hollow channels (54) in the board elements as compared to prior art methods. This is realized by providing a novel method and a retractable core element (45) which is adjustable from an nominal expanded size to a retracted size. A novel method is presented in which the core element (45) of the present invention is fully expanded before it is inserted in between the outer press elements (40, 41, 42) and the gaps (44) between the press elements and the core element are filled with the mix comprising a fibre material, a binding agent and an activating agent. The force generating the pressure on the fibre cement mix is applied by the outer press elements, i.e. all the movement of the "mould" elements to compress the mix is effected by the outer press elements (40, 41, 42) and not the surfaces (3, 4, 7, 8) of the core element, which remains in its nominal expanded size during the filling of the mix and curing of the board element. After the board element (50) has been cured, typically by carbon dioxide curing, the pressure of the press elements (40, 41, 42) is released and the core element (45) is retracted to a retracted size to slacken the pressure on the inner board surfaces inside the hollow channel (44), enabling the smooth withdrawal of the core element.

Apparatus

Another aspect of the invention provides an apparatus for manufacturing hollow board building elements comprising outer press panel elements (40, 41, 42) suitably operable to compress between them a cement fibre mix (i.e. a mix comprising a fiber material, a binding agent and an activating agent), which mix in a cured state defines the board building element, and gas injection means for injecting a gas into said mix to perform rapid curing of the mix, wherein the apparatus further comprises an inner retractable core element (45) such as described above to be located in said mix between said outer press panel elements (40, 41, 42) for forming an internal channel (54) in said board element (50) manufactured between said outer press panel elements.

The apparatus of the invention for manufacturing an elongated hollow building element as described herein comprises:

. a support frame structure (80),

• a set of oppositely arranged main press panels comprising an upper main press panel (40) and a lower main press panel (42),

• a set of oppositely arranged side press panels (41), said main press panels and side press panels form an elongated chamber (49) with a substantially rectangular cross-section. The apparatus further comprises

• a set of upper moving means (75) (e.g. hydraulic pistons) operably connected with said upper press panel, • a set of lower moving means (76) (e.g. hydraulic pistons) operably connected with said lower press panel,

• two oppositely arranged sets of side moving means (77) operatively connected to said side press panels,

• a retractable core element (45) to be located in between said upper and lower press panels,

• removable distal end closing panels (78)

• gas injection means (79) for injecting gas into said mix, wherein : said upper and lower press panels can be moved to compress said composite mix above and below said core element being centered in between said side press panels, said core element being retractable from a nominal extended size to a smaller retracted size, such that after curing of the building element and withdrawal of said press panel elements, the core element can be retracted and withdrawn from within the hollow channel (54) of the obtained building element.

In a useful embodiment said lower main press panel (42) is slidably removable from said elongated hollow chamber (49) along the longitudinal axis of said chamber, for example on a set of rollers on which the lower press panels rests when the lower moving means (76) have fully retracted.

The apparatus preferably comprises slidably detachable side plates (81) mountable on the lower press panel (42) such that a layer of fibre mix can be placed and contained on the lower press panel. Such side plates may be inserted in grooves (93) along the edges of the lower press panel.

In order to facilitate removal of the side plates (81) the apparatus preferably has vertically movable side shields (86) that are mountable in between said side plates (81) and fibre mix within said elongated chamber (49). In one embodiment, the side shields are made from flexible polymer blankets that may be kept on rollers kept in roller compartments (94) along the edges of the lower press panel.

As illustrated in the accompanying Figures, the apparatus is preferably configured having upper side panels (82) and lower side panels (83), which are operatively connected to the side press panels (42) such that when the side press panels are retracted from a tight end retracted position (84) to a final retracted position (85) said upper side panels (82) and lower side panels (83) move in concert with the side press panels (42).

The gas injection means comprise in a preferred embodiment gas tubing connected to channels (71) within the outer press panel elements (40, 41, 42) that lead to a plurality of small vents/apertures (70) substantially regularly distributed on the press panel surfaces facing the mix during the curing of the board structure.

Preferably, the core element according to the invention has corresponding gas distribution means, which suitably comprise channels/grooves (23) connecting a dot pattern of gas bores/apertures (22) on at least the main surfaces (3, 4) and preferably also the end surfaces (7, 8).

The gas distribution means in the press panel elements and the core element ensures more effective distribution of gas (e.g. CO ₂ ) for fast and even curing of the board structures.

During the operation of the apparatus, the gas channels may however become obstructed or clogged due to buildup (72) of dust from the cement mix in the gas channels. In one embodiment the apparatus therefore comprise a cleaning element (60) to clean dust and debris from the gas channels. The cleaning element preferably has a suction head (61) with one or more suction areas (62) connected with a suction tube (64) to a vacuum source (67). In one embodiment, the cleaning head comprises in addition to the suction area(s) one or more pressure areas (63) which are connected through tubing (65) to a source of pressurized gas (68). By using a cleaning head with both a suction area and a pressure area more efficient cleaning is accomplished, by establishing a circulation of gas through the gas channels (23/71) of the press panel elements (40, 42) and/or core element surfaces (3, 4). In practice, the apparatus may be cleaned at regular intervals, e.g. after the manufacture of a certain number of hollow slabs, by moving the cleaning element along the length of the press panel elements (40, 42) and core element surfaces (3, 4) respectively. The cleaning element may suitably contact the surface to be cleaned through soft lips (66), e.g. rubber edges or rollers to create a substantially tight interface between the surface to be cleaned and the cleaning element. Such lips/rollers are also useful for, simultaneously with the cleaning procedure, lubricating the surface with machining oil. In the case where the cleaning element comprises both one or more suction areas and one or more pressure areas, these are separated by rubber gaskets or rollers to ensure circulation of the gas from the pressure area through gas channels in the press panel or core element and back to the suction area(s).

The cleaning head preferably is dimensioned with a width matching substantially the width of the outer press panel elements (40, 42) such that the complete surfaces can be cleaned by sliding the head along the length of the press panel elements.

Core element

The apparatus of the invention comprises in a preferred embodiment in a retractable core element (45) to be located in said mix or mortar between said outer press panel elements for forming an internal channel (54) in said board manufactured between said outer press panel elements (40, 41, 42). The inventive method is characterized by arranging said retractable core element in a nominal expanded size configuration in the aformentioned mix between said outer press panel elements, and subjecting, before the curing of said cement based binding agent, said mix to a pressure by the outer press panel elements, and after curing of the element, the core element is retracted to a lesser width and withdrawn from the channel of the fabricated board unit.

A board product produced in accordance with the present invention may in one embodiment comprise a slab element having two about 15-20 mm thick panels (51), typically about 1 m wide and in the range of 2,5-12 m high, preferably in the range of about 8-10 m high, connected by equally thick connecting panels (52), the width of which can vary depending on the application of the product. For inner wall elements the connecting panels and thus the hollow channel (54) of the unit (50) , would typically have a width of about 6-8 cm, creating an external width of the unit of about 10-12 cm. However, for outer wall elements, floor elements or other type of elements the connecting panels are wider, e.g., about 20-30 cm wide.

The mix is preferably a woodchips-cement-water mix referred to as CFB (cement flake board) mix. Due to the use of excessive carbon dioxide the complete hardening process takes only some 4 to 5 minutes while an ordinary cement consolidating process takes initially 8 to 10 hours. CFB is very advantageous material for the present invention as CFB elements may be used both indoors and outdoors, they are weather resistant, waterproof, non-frost susceptible and fairly fire-resistant.

The longitudinally extending retractable core element (45) comprises four surfaces (3, 4, 7, 8) that come in contact with the mix during the process. The surfaces form an essentially rectangular cross-section (see Figure 2a) transversal to the longitudinal axis of the element (the x-axis as defined in Figure 3). The distal end surfaces of the element (i.e. the surfaces defining the longitudinal ends) typically will not be in direct contact with the cement mix but will align with the distal ends of the hollow board structure.

The element comprises at least two oppositely arranged surface portions (I ₇ 2) with essentially flat parallel main surfaces (3, 3' 4, 4') parallel to the longitudinal axis and

facing opposite directions, wherein at least one of said main surfaces is moveabie such that the width of the element between said two main surfaces (across the z-axis) is adjustable. It will be appreciated that the retractable core element allows the manufacture of relatively thin board structure elements (board elements with thin channels), e.g., with an outer width in the range of about 8-15 cm, such as in the range of about 10-12 cm, suitable for the construction of inner walls. In such units, the hollow channel (54) has a width in the range of about 5-10 cm, such as in the range of about 6-8 cm, including about 6, 7 or 8 cm.

The retractable core element (45) further comprises at least two end surface portions (5, 6) with oppositely arranged end surfaces (7, 8) essentially perpendicular to said main surfaces (3, 4), wherein at least one of said end surfaces (7, 8) is moveabie such that the width of the element between said two end surfaces (across the /-direction) is adjustable. Thus, the element is retractable in both the x and y-direction, as defined in Figure 3 showing an exploded view of a retractable core element of the invention. The movement of the at least one of said main surfaces is accomplished with adjacent first glide planes (9, 10) that are at an angle in the range of about I ⁰ - 10°, and preferably in the range of about 2°-8°, including the range of about 2°-5°, such as in the range of about 3°-5° or 3°-4°, with respect to the plane of the main surfaces (3, 4), wherein one of said glide planes is fixed to one of said surface portions (1, 2) such that relative movement of the glide planes cause an increase or decrease in the width of the element measured between said main surfaces (3, 4). Thus, the element is expandable such that it remains in an expanded configuration when pressure is applied to its main and end surfaces, i.e., when pressure is applied by the outer press panels (40, 41), and can subsequently be retracted to a retracted smaller size.

Preferably, the retractable core element (45) is constructed such that the movement of at least one of said end surfaces (7, 8) is accomplished with adjacent second glide planes (11, 12) that are at an angle in the range of about l°-10° with respect to the plane of the end surfaces (the x-z plane), such that relative movement of said glide planes cause an increase in the width of the element measured between said end surfaces (7, 8).

In preferred embodiments, such as, e.g., described in more detail below, said first glide planes (9, 10) and second glide planes (11, 12) are operatively connected such that adjustment of the width measured between said main surfaces (3, 4) causes a corresponding adjustment of the width of the element measured between said end surfaces (7, 8).

One embodiment of the retractable core element has two pairs of first glide planes (9, 10; 9', 10'), wherein each of said opposite surface portions (1, 2) has a glide plane (9, 9') opposite its flat surface (3, 4).

Preferably, the retractable core element comprises a center portion (13) with oppositely arranged central glide planes (10, 10') symmetrical about a central plane (the center x-y plane of the core press element), wherein the center portion forms a wedge-shaped cross- section as illustrated in Figure 5a. Each of said pairs of first glide planes (9, 10; 9', 10') is formed by one of said central glide planes (9, 9') and one of said surface portion glide planes (10, 10'), the two glide planes of each of said pair facing each other.

To accommodate for the end surface expansion across the y-axis, the center portion suitably comprises oppositely arranged central end-facing glide planes (11, 11') at an angle of about 1-10° with respect to the plane of the end surfaces, wherein each end- facing glide plane is in contact with an adjacent glide plane (12, 12') each fixed with respect to the end surface elements (7, T) forming a pair of second glide planes (11, 12; 11', 12').

The retractable core element is preferably constructed such that it is connectable to at least one further retractable core element to create an interconnected element comprising two or more longitudinally connected (along the x-axis) retractable core elements. Said connection is suitably formed between a lip (29) and rim/groove (30) on the distal (x- direction) ends of center portions (13) of adjacent elements. Such a rim/groove connection can connect the center portions of two or more core elements to provide interconnected core elements that can be expanded and retracted in concert as one long core element.

In other embodiments, the core element is connectable to at least one further retractable core element through a plurality of male/female plug and socket connections on a distal end surface of said core element and an opposite distal end surface of said at least one further retractable core element. In this fashion, two or three core elements can be connected to produce up to 8-10 m long hollow board structures, this type of interconnection allows retraction and the withdrawal of core elements from both ends of the fabricated board structure, thus allowing for longer board structures to be produced. It would be inconvenient and technically difficult to withdraw a 8-10 m long core element in one piece from inside a board structure due to friction between the core element and board structure and space demands adjacent to the core press.

In a further aspect, the invention provides a method for manufacturing a building element (50), having two adjacent outer wall units (51) and wall connecting units (52) forming an longitudinal hollow channel (54) therein between, said method comprising providing a mix mainly comprising a fiber material, a binding agent, e.g., cement based, and a first activating agent, and providing outer press panel elements comprising parallel main panel elements (40, 42) and parallel side panel elements (41) that are essentially perpendicular to said main panel elements, the press panel elements forming a channel (44) of substantially rectangular cross-section, providing a retractable core panel element (45) that fits in said hollow channel (44), creating a filling space in-between said core panel

element and said outer press panel elements, wherein said method further comprises: adjusting said core panel element in an expanded position in-between said outer press panel elements, arranging said mix in said filling space in-between said outer press panel elements and core panel element, moving at least one of said main panel press panel elements (40) and one of said side panel elements (41) against said mix and oppositely arranged core panel elements creating a pressure subjecting said mixture to a compression between said outer press panel elements and said core panel element, curing said mixture during compression by subjecting said mix to the influence of a second activating agent, typically a gas which preferably comprises carbon dioxide, releasing said pressure by retracting said press panel elements, and retracting said core panel element to an retracted size and withdrawing said core element, and thereafter having an elongated hollow building element (50) in compressed and cured form.

Thus, for production according to the method of the invention, an expansion and subsequent retraction of the core press element (45) in the range of 5-20 mm in each direction (y, z), such as in the range of about 5-10 mm is sufficient to allow the retractable core element to be smoothly withdrawn from the hollow channel (44) of the fabricated hollow building element (50).

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A preferred embodiment of the invention is described herein with reference to the accompanying Figures.

In Figure 1 the press panels are shown in the outermost position, when the chamber is fully expanded. The core element is shown schematically, lying on the lower main press panel (42). Note that the surrounding square frames (101) form part of the support frame structure (80) which is not shown in full. The moving means in this embodiment comprise hydraulic pistons enclosed in piston housings. The upper moving means (75) pistons are inside upper piston housings (90), the side moving means (77) are inside side piston housings (91). The lower piston housings (98) enclose the lower moving means (76) hydraulic pistons.

Figure 3a shows a simplified view of the cross-section of the apparatus to illustrate the cross-section of a preferred core element (45) where outer press panel elements (40, 41, 42) surround the core press element (45) with external main surfaces (3, 4) and end surfaces (7, 8). The cross-section of the core press element is in the γ-z plane as defined in Figure 19. The cement fibre mix is inserted in the filling space (44) in between the outer press panels and the core element.

Figure 3b shows the cross-section of the fabricated building element (50) with outer wall boards (51) and wall connecting boards (52) and a hollow channel (54), which for inner wall elements is typically about 7-10 cm wide but can be substantially wider if desired. The end rims (53) can be removed or shaped suitably to connect to adjacent wall elements, but should preferably have the same thickness as the main and connecting board sections.

Figures 4-17 illustrate a typical cycle in the method/apparatus of the invention.

In Figure 4, a ready-made building element (50) is shown fully compressed in between the press panels (40, 41, 42) in their fully extended state, i.e. when the chamber (49) is fully retracted (minimum horizontal and vertical width), enclosing the building element.

In Figure 5, the side press panels (41) are retracted to a tight end retracted position (84) by use of hydraulic pistons that function as side moving means (77), enclosed in piston houses (91). In this position (849 the upper and lower side panels (82, 83) are in contact with the distal ends of the end rims (53) of the building element.

In Figure 6, the side press panels (41) are fully retracted to a final retracted position and the upper and lower side panels (82, 83) move in concert with the side press panels, as the side panel back sections (95) are pushed back by the side press panels (41).

In Figure 7 the upper main press panel has been moved to its uppermost position and the lower main press panel to its lowermost position where it rests on rollers (97) and not on the piston heads of the lower moving means (76).

In Figure 8, the lower press panel with the finished board element and enclosed core element has been removed from the chamber by rolling it out on rollers on the rollers (97).

In Figure 9 the chamber is fully expanded and empty; the lower press panel is not in place.

Figure 10 shows the finished building element resting on the lower press panel outside the press chamber ready to be lifted off the press panel

In Figure 11, the core element has been withdrawn from within the hollow channel of the building element which is being lifted from the press panel.

In Figure 12, the slidably detachable side plates (81) have been mounted in grooves (93) along the edges of the lower press panel, the vertically movable side shields (86) (not shown) are subsequently placed on the inside of the side plates.

In Figure 13, a bottom layer of fresh composite mix has been placed on the lower press panel which forms an elongated "tray" also closed at the distal ends (not shown) such that the mix is contained on the lower press panel. The core element is resting on the bottom mix layer.

In Figure 14 sufficient mix to complete the building element has been placed in the "tray" surrounding the core element.

Figure 15 shows the lower press panel "tray" unit being placed within the press chamber which is fully expanded.

In Figure 16 the lower moving means (hydraulic pistons) have moved the lower press panel to a pre-determined height such that the core element is centered, i.e. such that the horizontal center axis of the core element is aligned with the horizontal center axis of the side press panels. The upper press panel is moved downwardly to reach contact with the upper surface of the mix.

In Figure 17, side press panels have been moved from the final retracted position to a tight end retracted position (a movement of about 20 mm on each side), the upper side panels (82) and lower side panels (83) move together with the side press panel by way of spring coils (96) which are fully extended in the tight end retracted position such that the upper and lower side panels (82, 83) will not move further towards the center of the chamber.

In Figure 18, the side press panels have been moved to their fully extended position compressing the connecting boards of the building element to full extent. In the next step the upper main press panel is moved further downwardly and the lower main press panel is moved upwardly to their final fully extended position as shown in Figure 4. In the final position, the board building element is fully compressed and subjected to curing by CO ₂ through the gas distribution means.

It should be noted that although described as a stepwise process, the above process is in practice performed as a continuous process, typically operated to a large extent by an automated computer control system, which per se is well known in the field of mechanical process control.

Figure 19 shows an exploded view of a core expanding element of the invention. It should be noted that the element is symmetrical both about its central x-y plane as well as the central y-z plane. The two oppositely arranged main surfaces (3, 4) are each split in two equal parts (3,3'; 4,4') to accommodate for the /-directional expansion. Similarly, the two end surfaces (7,8) are also split in two (7,7'; 8,8'). The end surface elements (7, 7'; 8,8') are fixed to the main surface elements by end fixing screws (25) and the main surface

elements (20) comprising the main surfaces are fixed to main glide elements (21) that comprise glide planes (9, 9') that are in contact with cooperating glide planes (10, 10') that are surfaces of the center portion (13). A center ridge (31) is fixed to each side ("top", "bottom") of the central portion, the center ridge comprising second glide planes (11, 11') that are in contact with second glide planes (12, 12') on the inside end-faces of the main glide plane elements (21). Relative movement of the glide planes (11,12; ll',12') cause an extension of the core element in the /-direction. A sync bar (27) is fixed in a sync bar seat (28) to each of the four side-halves of the center portion (13). The sync bars protrude through a sync groove (26) on the main glide plane element (21), the sync bar and sync groove lie in parallel with the end-facing surfaces (12, 12') of the main glide plane elements (21) such that when the main glide plane element is moved relative to the center portion in the x-direction causing an increase in the z-width of the element, a corresponding synchronized increase in the /-width is realized by the synchronized relative movement of the end-facing glide planes (11,12; ll',12'). A central groove cover (18) covers the center ridge (31) and has a ridge extending between the main glide plane elements (21). A central rim shield (15) closes the grooves formed between the main glide plane elements and is fixed to the ridge of the central groove cover (18). This ensures that the main surfaces (3, 3') form a substantially continuous surface even though the two surface portions are moved away from each other. The end surfaces (7, 8) are arranged in a similar fashion, with an end groove cover (19) fixed to the side of the center portion (13) and an end rim shield (17) is screwed to the end groove cover (19) with rim shield screws (16).

Both the main surfaces (3,4) as well as the end surfaces (7,8) are perforated on the outside with gas bores (22), on the inside of the surface elements, the bores are connected by gas grooves (23); this allows the distribution of gas (typically carbon dioxide) through the core element to the surrounding mix for curing of the mix during compression. The gas is typically inserted to the mix by gas injection means in the press panel elements (40, 41, 42).

In this embodiment of the core element, the angle of the main glide planes with respect to the main surfaces is 3,7°, as is the angle of the end-facing glide planes with respect to the end surfaces. This relatively small angle ensures that only about 5% of the force acting on the main surfaces (in z-direction) acts in the plane of the glide planes. By enclosing the top (55) and bottom (56) of the core element (distal ends in x-direction) the extended core element will not be retracted by the exerted pressure during use when a building element is being manufactured and the core element subjected to high pressure by outer press panel elements acting through the mix being compressed. Typically, the top and bottom ends (i.e. the distal ends) of the elements are kept from moving by having the flat ends in contact with closing end units enclosing the ends of the press (not shown).

4

The change in width (in both x and y direction) between the nominal fully expanded size and the retracted size is about 8 mm in each direction.

Multiple core elements can be combined (in the x-direction) to provide an extended element for the production of longer hollow board units. Typically 5 to 6 units of the dimensions as of the embodiment herein will be combined for the production of hollow interior wall units. The core elements can be combined by means of a lip and rim arrangement as shown in this embodiment with a lip (29) and rim on opposite distal ends on the center portion (13). In a multi-element unit the center portions (13) of the top and bottom elements are altered such that the top and bottom distal ends of the distal core elements are substantially flat and will contact closing end units provided for in the outer press. Figure 20c and 2Od illustrate the combination of two core elements. Note that the upper central unit (56) has been shaped (the top portion removed) such that when the core elements is in the extended configuration (20c) the top distal end (55) of the joined element is essentially flat. An extra center piece (57) is provided in this embodiment to fit the bottom distal end of the back end (right-hand side) central element (13) to provide an essentially flat surface on the back distal end (56). After curing and releasing of the pressure of the outer press elements (40, 41) the core is retracted, by pushing the top end (55) or pulling the back end (56).

In useful embodiments, the core elements are even larger to provide for long hollow board structures, e.g. 8-12 m long such as 8-10 m long, which could be used in three-floor high buildings. Such long core elements are preferably comprised of a plurality of core elements connected with distal male/female plug and socket connections, e.g. pegs, bosses or fingers that engage with inverse shaped recesses or holes. As an alternative, two or more primary core elements can be interconnected with a clutching connection, e.g. a rim and groove arrangement as described above to provide intermediate core elements, one or more of which are interconnected with male/female plug and socket connections as described herein.