# Patent

## Title
Composite door, door core and method of manufacture

## Abstract

A method for making the core of a door of autoclaved aerated concrete (AAC). A billet of AAC is provided from which slabs are cut to the approximate dimensions of the door core. A reinforcing band is secured about the perimeter of a slab or a pair of slabs sandwiching a reinforcing panel. The thickness of the slab, or the pair of slabs and reinforcing panel, is then sized for the door core as by sanding. The door is finished by applying skins to the core surfaces and trim to the reinforcing band.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of Ser. No. 10/920,871, filed Aug. 18, 2004, now Pat. No. 7,244,728, and provisional applications Ser. Nos. 60/526,036, filed Dec. 1, 2003 and 60/496,953, filed Aug. 21, 2003.


BACKGROUND OF THE INVENTION
It is known to manufacture special purpose doors, as fire doors, with a core of light weight concrete. Such doors have typically been manufactured by casting a core from raw materials and, after the core has cured, finishing the doors as with surface and edge coverings. Casting and curing are time consuming and require a large facility to accommodate raw materials and the cast cores while they cure. The lightweight concretes which have been used in the cores of doors have not provided an optimum combination of fire protection and core density.
BRIEF SUMMARY OF THE INVENTION
The composite door and method of manufacture of this invention preferably utilizes a core material of autoclaved, aerated concrete (sometimes referred to herein as AAC).
In accordance with method, a billet of AAC is provided having a length and width approximating the intended dimensions of the door and a thickness approximating the thickness of two or more doors. Slabs are cut from the billet, each having a length and width forming a perimeter and a thickness approximating the intended thickness of the door. A reinforcing band is secured about the perimeter of each slab, forming a core for a door. A door is finished with optional trim for the reinforcing band and skins covering the surfaces of the core. The banded slabs, i.e., cores, can be sold in unfinished form to a door manufacturer who performs the finishing.
Precast AAC units are commonly used as wall, floor and roof sections in building construction. Plants are located throughout the country which specialize in the manufacture of AAC units. The cured billets from which the door cores are manufactured may be obtained from such a plant. This eliminates the need for a door manufacturing operation to provide an AAC casting and curing facility.
The autoclaved, aerated concrete core material may have a cured density of the order of 30-50 lb/ft.3 and preferably about 35-40 lb/ft.3. This affords an optimum combination of fire resistance, R value, physical strength and door weight.
Further features and advantages of the door and its method of manufacture will be apparent from the drawings and the following description.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a perspective view of a billet of AAC from which a plurality of door cores are manufactured;
FIG. 2 is a perspective view of a billet of AAC showing a slab cut therefrom to manufacture a door core;
FIG. 3 is a fragmentary section of a slab showing a tool routing the slab surface at an edge;
FIG. 4 is fragmentary section of the slab of FIG. 3 with a reinforcing band secured thereto;
FIG. 5 is a fragmentary section of the slab of FIG. 4 showing the upper surface being sanded to its final dimension;
FIG. 6 is a perspective view illustrating a plurality of slab sections to be joined to form a door core;
FIG. 7 is an edge view of the slab sections of FIG. 6;
FIG. 8 is a fragmentary section showing a door core with a wooden reinforcing band;
FIG. 9 is a fragmentary section of a door core with surface skins and edge trim;
FIG. 10 is a fragmentary section of a door core with another form of reinforcing band with surface skins and edge trim;
FIG. 11 is a fragmentary section of a door core with a further form of reinforcing band, surface skins and edge trim;
FIG. 12 is a fragmentary section of a door core with yet another form of reinforcing band, surface skins and edge trim;
FIG. 13 is a fragmentary section of a door core with a reinforcing band and with a suitable adhesive securing surface skins to the door core;
FIG. 14 is an exploded perspective of the door; and
FIGS. 15 and 16 are fragmentary sections of alternate constructions for the door core embodying the present invention.



DETAILED DESCRIPTION OF THE INVENTION
The manufacture of a concrete core 18 for a door is expedited by initially providing a billet 20, FIG. 1, of cured, aerated or lightweight concrete. Billet 20 has length and width dimensions which approximate but are slightly greater than the intended height and width of the door core 18. The thickness of the billet 20 approximates and is slightly greater than the thickness of a plurality of door cores as indicated by the dashed lines. Preferably, the billet 20 is of an autoclaved, aerated lightweight concrete (AAC). Conveniently, a door manufacturer may secure the billet 20 from a manufacturer of AAC. This avoids the need for the door manufacturer to invest money, facilities and time in the production of the AAC billets. Rather, the door manufacturer concentrates its efforts and assets on the manufacture of door cores and finished doors.
A concrete slab 22 is cut from billet 20 as with a band saw 24, FIG. 2. Slab 22 has a length and width which approximate but are greater than the height and width of the door core 18. The thickness of slab 22 approximates but is greater than the thickness of the door core 18.
Handling of the door core 18 of cured aerated concrete, without damage to the concrete, is facilitated by application of a peripheral reinforcing band to the perimeter of slab 22. A preferred reinforcing band is a steel strip, as of 18, 20 or 22 gauge steel. The strip is roll formed with a U-shaped channel cross section having two parallel legs with a length of 1⁄4′′-1⁄2′′. The steel reinforcing band is typically constructed of two stile sections for the edges of the core 18 which will be the vertical edges of the door and two rail sections for the edges of the core 18 which will be the top and bottom of the door. The stiles are shown at 26, 28, FIG. 14 and the rails at 30, 32. The reinforcing band sections are riveted together and are glued to the edges of the core 18. The stiles in FIG. 14 have short tabs (not shown) which mate with the adjoining rails and receive rivets 34.
The slab 22, after being cut from billet 20, is sized and configured for the internal dimensions of the reinforcing band. To accomplish this, one or both of the vertical edges may be cut or sanded to achieve the desired width of the banded core 18. Similarly, one or both of the horizontal edges is cut or sanded to achieve the desired height of the banded core 18.
The edges of the slab surfaces are routed as shown in FIG. 3 to receive the legs 36, 38 of band 40. Band leg 36 is flush with the lower slab surface 42 and the upper slab surface 44 intentionally extends above leg 38. The thickness of the slab is then sized to match the upper leg 38 of band 40 as by sanding the upper surface with a belt or drum sander. Belt sander 46 is shown in FIG. 5. This completes the manufacture of the door core 18. The banded core 18 may be finished or sold to a door finisher.
A billet 20 sized for a door may be too large and heavy to handle conveniently. Accordingly, the billet may have a length which is a fraction, as 1⁄3, the height of the intended door. Three slab sections 48 are cut from one or more billets and joined together as shown in FIGS. 6 and 7. The mating edges 50, 52 of the slab sections 48 preferably have interlocking surfaces, as the tongue and groove configuration 54, 56 shown in FIG. 7. Other interlocking surfaces, as a tenon and mortise, for example, may be used. The slab sections 48 are secured together by applying an adhesive, as a glue mortar, to the mating surfaces 50, 52.
An alternate reinforcing band 58 of wood is shown in FIG. 8. The alternate band might also be plastic or a wood-plastic composite or cement bonded particle board. However, where fire resistance of the door is an important consideration, the steel band 40 is preferred.
The banded core 18 may be finished as by applying skins 60 to the core surface and, if desired, trim strip 62 to reinforcing band 40. The skins may be of steel, plastic, wood or a plastic and wood fiber composite and are typically secured to the core surface by glue. The glue may be applied with a roller coater. Alternately, the reinforcing band may be finished by painting. The skins shown are flat. However, profiled skins may be used. The surfaces of the core may be routed to mate with the skins.
Alternate reinforcing bands and edge treatments are shown in FIGS. 10-12. The reinforcing band 64, FIG. 10, has edges 66 with a negative recess. The skins 68 have edges 70 which are interlocked with band edges 66. Trim strip 72 overlies reinforcing band 64 and the negatively recessed band and skin edges. Reinforcing band 74, FIG. 11, has edges 76 with perpendicular recesses. Skins 78 have edges 80 received in and interlocked with the perpendicular recesses. Reinforcing band 82, FIG. 12, has a longitudinal recess 84. Skins 86 have edges 88 which overlap the reinforcing band 82. Trim strip 90 has a longitudinal rib 92 received in recess 84.
Where the coefficient of thermal expansion of core 18 differs substantially from the coefficient of thermal expansion of the skins, the construction of FIG. 13 may be used. The skins 94 are secured to the surface of core 18 by a suitable adhesive, such as a polyurethane foam adhesive or polyvinyl acetate (PVA) adhesive 96. The foam adhesive accommodates the differential expansion and contraction of core 18 and skins 94. However, manufacture of this door may require use of a clamping fixture to hold the skins in position as the foam adhesive cures.
The preferred AAC core material is superior to other aerated or lightweight concretes in that for a given density it is stronger and therefore easier to work with during manufacture and installation. For most door applications, an AAC material with a density of 37.5 lbs. per cubic foot is suitable. This material has a compressive strength of the order of 580 lbs. per square inch.
FIGS. 15 and 16 show an alternate embodiment for the core 18 wherein two of the AAC slabs 22 sandwich a fire resistant reinforcing panel 100 to form the thickness for the core 18, rather than utilizing a single slab 22 of the AAC material as in the embodiments of FIGS. 1-14. Preferably, the panel 100 has a height and width that are the same or about the same as the height and width of the slabs 22. The slabs 22 can be prepped or processed as previously described for the embodiments of FIGS. 1-14. The panel 100 is preferably bonded to the slabs 22 using a suitable fire rated adhesive, many of which are known. After the slabs 22 and panel 100 are laminated together to form a core blank, they can be processed as previously described for the embodiments of FIGS. 1-14.
As shown in FIG. 15, the reinforcing panel 100 can be formed from a sheet of cement bonded particle board 102. One example of a suitable form of cement bonded particle board is the VERSAROC® cement bonded particle board provided by U.S. Architectural Products, Inc. which is composed of mineralized wood particles and portland cement and has passed the requirements of ASTM E136. Alternatively, as shown in FIG. 16, the reinforcing panel 100 can be formed from a sheet of steel 104. In preferred embodiments, the sheet of steel is 26 gage steel.
As also shown in both FIGS. 15 and 16, the reinforcing band can also be provided in the form of a cement bonded particle board reinforcing band 106 which is preferred in connection with the panels 102 and 104. Preferably, the band 106 would include a groove 108 that receives a perimeter of the reinforcing panel 100 that extends beyond the perimeters of the slabs 22, preferably in the range of 0.25′′ to 0.75′′. However, it should be understood that any of the reinforcing bands described in connection with FIGS. 1-14 can be utilized with the constructions of FIGS. 15 and 16. Similarly, the other features described for the embodiments of FIGS. 1-14, such as the use of skins or trim strips, can also be utilized with the embodiments of FIGS. 15 and 16.

## Claims

1. The method of making a concrete core for a door, comprising:
providing a core blank of cured, aerated concrete having length and width dimensions forming a perimeter and approximating the intended height, width and thickness dimensions of the door core;
sizing the core blank prior to securing a reinforcing band about the perimeter thereof, the reinforcing band being a channel having two legs, the perimeter of the core blank being sized and configured for the internal dimensions and configuration of the band, including the step of providing for inletting both legs into the faces of the core blank with the face of one leg flush with the adjacent core blank face;
securing the reinforcing band about the perimeter of the core blank; and
sizing the thickness of the core blank by treating the core blank face opposite said adjacent core blank face to be flush with the other leg of said reinforcing band after securing the band about the perimeter thereof.

2. The method of claim 1 in which the thickness of the core blank is sized by sanding.

3. The method of claim 1 in which the step of providing a core blank comprises:
providing a billet of cured aerated concrete having length and width dimensions approximating the intended height and width dimensions of the door core and a thickness approximating the intended thickness of a plurality of door cores;
cutting two slabs from said billet, each slab having a length and width forming a perimeter and a thickness less than one-half the intended thickness of the door core; and
laminating the two slabs on either side of a fire resistant reinforcing panel to form said core blank.