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CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM TO PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 62/260,998, filed Nov. 30, 2015, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to doors with solid cores, preferably shaker doors with solid cores, and methods for making the same. The devices and methods provide for simplified assembly of solid core shaker doors, including fire rated doors. 
       BACKGROUND 
       [0003]    Doors having wood composite molded door facings are well known in the art. Typically, a perimeter frame is provided, which includes first and second vertically extending stiles and at least first and second horizontally extending rails attached together, frequently by an adhesive such as polyvinyl acetate, to form a rectangular frame. A lock block may also be utilized to provide further support for a door handle and/or a locking mechanism at the periphery of the door. The lock block is preferably secured to a stile and/or a rail. Door facings are adhesively secured to opposite sides of the frame, and the door facings (also known as door skins) typically are identical in appearance. 
         [0004]    The resulting door includes a void or hollow space defined by the opposing door facings and perimeter frame. This void typically causes the door to be lighter than a comparably sized solid, natural wood door, which is not as desirable for many consumers. In addition, the sound and/or heat insulation provided by such doors may not be satisfactory. Therefore, it is often desirable to use a core material (e.g., core pieces or components) to fill the hollow space. 
         [0005]    A suitable core material should provide the door with a desirable weight, for example the weight of a similarly-styled natural solid wood door. In addition, a core material should provide the door with a relatively even weight distribution. The core material should also be configured to match the dimensions of the interior space defined by the facings and frame with sufficiently close tolerances so that optimal structural integrity and insulation properties are achieved. The core material may also provide noise attenuation, thermal resistance and other properties that enhance the functionality of the door. Another function of the door core is to provide resistance to distortion. This distortion includes both distortion that might be built in to the door during assembly, and also distortion that might result later from exposure to moisture, for example. 
         [0006]    Door facings may be molded from a planar cellulosic fiber mat to include one or more interior depressions or contours, such as one or more square or rectangular depressions which extend into the hollow space of a door assembly relative to the plane of an outermost exteriorly disposed surface of the door. For example, a door facing may include molded walls having a plurality of contours that include varied curved and planar surfaces that simulate a paneled door. One type of door facing commonly referred to as shaker or shaker-styled is characterized at least one rectangular depression in the door facing. 
         [0007]    If the door facings are contoured to include one or more depressions, the interior void of the door assembly will have varying dimensions given the facings are secured to co-planar stiles and rails. When providing a core material or component within the void of a door assembly having such contoured facings, it is necessary to compensate for the varying dimensions of the void. 
         [0008]    In the past, various materials, such as wheat board, corrugated cardboards, and/or paper, have been used as the core material. However, due to the contoured door facings, the thickness of the core material varies within a door, which may result in lowered strength and stability in the thinner areas (formed by depression in the door facing). As a result, the door may be susceptible to distortion, such as when it is exposed to high humidity. This is particularly true for a shaker door, due to the relatively large panel area. 
         [0009]    Therefore, there remains a need for a shaker door that contains improved dimensional stability and reduced distortion when exposed to high humidity. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention relates to doors with solid cores, preferably shaker doors with solid cores. Preferably, the shaker door is a one-panel shaker door, as illustrated in  FIG. 1 . The present invention provides core constructions that provide solid core shaker doors with improved dimensional stability and reduced distortion when exposed to high humidity. 
         [0011]    In an aspect of the present invention, the core of the shaker door contains two different core densities. The relatively thin core material in the panel area has a higher density than the relatively thick core material in the raised peripheral region. The density of the thin core material is inversely proportional to the height difference between the thick core material and the thin core material. Preferably, the density of the thin core material is about 120 to about 250% greater than the density of the thick core material. 
         [0012]    In another aspect of the present invention, the core material in the raised peripheral region contains three layers, where the middle layer is contiguous with the core material in the panel. The middle layer is formed by a thin core material (under the panel) substantially spanning the entire area of the door inside the frame at the thickness of the panel. Additional layers of material are then used to fill in the void in the raised peripheral region. 
         [0013]    In a further aspect of the present invention, the thin core material extends slightly beyond the area under the centrally oriented panel. The remaining volume of the interior void may then be filled with at least a filler core material. 
         [0014]    Methods for making the different aspects of the present invention are also provided. 
         [0015]    Other aspects of the invention, including apparatus, devices, kits, processes, and the like which constitute part of the invention, will become more apparent upon reading the following detailed description of the exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The accompanying drawings are incorporated in and constitute a part of the specification. The drawings, together with the general description given above and the detailed description of the exemplary embodiments and methods given below, serve to explain the principles of the invention. In the drawings: 
           [0017]      FIG. 1  shows a front elevational view of a one-panel shaker door; 
           [0018]      FIG. 2  shows a cross-section view at line A-A of  FIG. 1  in accordance with a first embodiment of the present invention; 
           [0019]      FIG. 3  shows steps in the construction of the shaker door in accordance with a first embodiment of the present invention (cross-section view at line A-A); 
           [0020]      FIG. 4  shows a cross-section view at line A-A of a core in accordance with a second embodiment of the present invention; 
           [0021]      FIG. 5  shows steps in the construction of the shaker door in accordance with a second embodiment of the present invention (cross-section view at line A-A); 
           [0022]      FIG. 6  shows a cross-section view at line A-A of a core in accordance with a third embodiment of the present invention; 
           [0023]      FIG. 7  shows steps in the construction of the shaker door in accordance with a third embodiment of the present invention (cross-section view at line A-A); 
           [0024]      FIG. 8  shows a cross-section view at line A-A of a core in accordance with an alternate third embodiment of the present invention; 
           [0025]      FIG. 9  shows steps in the construction of the shaker door in accordance with an alternate third embodiment of the present invention (cross-section view at line A-A); 
           [0026]      FIG. 10  shows the construction of an exemplary thin core material for a fire resistant door; and 
           [0027]      FIG. 11  show the construction of another exemplary thin core material for a fire resistant door. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    Reference will now be made in detail to exemplary embodiments and methods of the invention. It should be noted, however, that the invention in its broader aspects is not necessarily limited to the specific details, representative materials and methods, and illustrative examples shown and described in connection with the exemplary embodiments and methods. Like reference characters refer to like parts throughout the drawings. 
         [0029]    A shaker door  100 , as best shown in  FIGS. 1 and 2 , contains at least one panel  102  that is recessed from and surrounded by a raised peripheral region  104 . The door  100  preferably has a pair of opposed, identical door facings  200 , each having a panel  102  and a peripheral region  104 . The door facings  200  are secured to opposite sides of the peripheral door frame, so that the panels  102  and peripheral portions  104  are aligned with each other. The panel  102  is rectangular in shape with generally squared corners. While the transition between panel  102  and peripheral portion is illustrated as being squared, those skilled in the art will appreciate that the transition typically is formed by a contoured transition region, sometimes known as an ovolo, due to the molding process. The area within the panel  102  forms a planar surface that is recessed from the surface of the raised peripheral region  104 . Due to the recessed panels  102 , the thickness t of the door  100  at the aligned panels  102  is thinner than the thickness T at the aligned raised peripheral regions  104 . In an exemplary embodiment, the thickness t of the door at the panels  102  is about 30 to about 50% of the thickness T at the raised peripheral regions  104 , preferably about 30 to about 35%. Consequently, because the door facings  200  also have a thickness, the thickness T c  of the thick core material  206  is less than the thickness T of the door  100 , and the thickness t c  of the thin core material  204  is less than the thickness t of the door  100 . Because the thickness of the door facing  200  is relatively small, on the order of about 1.7 to about 3.2 mm, preferably about 2.8 to about 3.2 mm, the differences between T and T c  is relatively minor, but the difference between t and t c  may be significant. Preferably, for a non-fire rated door, the thickness t c  at the panels  102  is about 0.370±0.005 inches (about 9.40±0.13 mm) and the thickness T c  at the raised peripheral region is about 1.120±0.0625 inches (about 28.40±1.59 mm); and for a fire rated door, the thickness t c  at the panels  102  is about 0.750±0.005 inches (about 19.5±0.13 mm) and the thickness T c  at the raised peripheral regions  104  is about 1.75±0.0625 inches (about 44.45±1.59 mm). 
         [0030]    As typical for cored doors, the door  100  is supported by a rectangular frame  108  containing two parallel stiles  202  attached at their respective ends to two parallel rails  110 . Door facings  200  are attached to opposite sides of the frame to form a door. A core material fills the internal cavity inside the frame and between the door facings  200 . 
         [0031]    In a first embodiment of the present invention, the core is formed from materials having different densities. Referring to  FIG. 2 , a thick core material  204  fills the space under the between peripheral regions  104  and has a lower density than the thin core material  206  that fills the space beneath the recessed panels  102 . In an exemplary embodiment, the density of the thin core material  206  under the panels  102  has a density that is about 120 to about 250% greater than the density of the thick core material  204  under the raised peripheral regions  104 , preferably about 150 to about 185%, more preferably about 160 to about 180%. The thin core material  206  preferably has a density of about 25 to about 35 lbs/ft 3 , more preferably about 29 to about 31 lbs/ft 3 ; and the thick core material  206  preferably has a density of about 13 to about 23 lbs/ft 3 , more preferably about 17 to about 19 lbs/ft 3 . Without being bound by any theory, it is believed that the higher density core material in the thinner portion of the core provides additional strength and stability to the overall door and allows it to resist distortion when exposed to high humidity. 
         [0032]    In an exemplary embodiment, both the core materials  204  and  206  are made from wheat board, albeit manufactured to different thicknesses and densities. Wheat board is made from wheat stalks. Essentially, wheat straws are prepared by first shredding the straw bales and milling the straw to the desired fiber size range, preferably about 1/32 to about ¼ inches (about 0.80 mm to about 6.35 mml) long. After shredding and milling, the milled fiber may be screened to remove fines and dried to a desired moisture content, preferably about 5 to about 8% moisture. Finally, the milled fiber is blended with an uncured resin binder, formed into a resin/fiber mat of a suitable thickness, and cured in a press at a suitable pressure and temperature, preferably in a press. In certain embodiments, the process may further include sanding and trimming the cured wheat board to a desired final thickness. As noted above for the present invention, the wheat boards are made with two different thicknesses, each having a different density. The thicker wheat board with lower density is used as the thick core material  204 , and the thinner wheat board with higher density is used as the thin core material  206 . Those skilled in the art recognize that density is inversely related to thickness for a mat after pressing. During formation of the wheat board, the thickness may be controlled by the distance between press platens. The desired thickness may be achieved by pressing to a hard stop, or by measuring the press platen separation during pressing. Alternatively, pressure may be used to control thickness. Further, once completed, the wheat boards may be sanded to reduce the desired thickness, although doing so does not vary the density of the pressed board. The desired density of the wheat boards may be controlled by the amount of material (milled fiber and resin binder) fed into the press for a given thickness. The more material is used the higher the density. 
         [0033]    Alternatively, the core materials  204  and  206  may be made from other bio based materials, such as particleboard, oriented strand board (OSB), plywood, medium density fiberboard (MDF), plywood, and stave core. Synthetic materials, such as polystyrene and polyurethane may also be used as the core materials  204  and  206 . The core material may be formulated to include a fire retardant, such as boric acid or a blend thereof, monoammonium phosphate, diammonium phosphate, magnesium hydroxide, zinc borate, alumina trihydrate, or combinations thereof. 
         [0034]    For fire resistant doors, the thin core material  206  may be a mineral core material. Typically, mineral core material contains a composition including a mineral and a binder. The mineral may be, for example, sodium silicate (preferably in hydrated form), gypsum, perlite, vermiculite, calcium silicate, potassium silicate, or combinations thereof. The composition may also include fiberglass or wood fibers. In an embodiment, the mineral core material may contain a center layer of hydrated sodium silicate and binder, which is coated on both major surfaces with epoxy resin and reinforced with glass fibers, textile-glass fabric, and/or woven wires. In another embodiment, the mineral core material may contain a woven panel of fiberglass that is impregnated with hydrated sodium silicate. In that case, the sodium silicate itself may also serve as the binder. The mineral core material may be laminated to one or more MDF layers to achieve the desired thickness of the thin core material  206 . For example, a ¾ inch thick thin core material  206  may be formed by laminating a ⅝ inch mineral core to a ⅛ inch MDF. More than one layer of mineral core and/or MDF may be used to produce the thin core material  206 . For example, as illustrated in  FIG. 10 , the thin core material  206  may be formed by sandwiching a mineral core  1004  between two MDF layers  1002 , or as illustrated in  FIG. 11 , by alternating layers of mineral cores  1004  and MDF layers  1002 . Although MDF layer(s) are mentioned as examples, the layers may also be formed from other bio based materials, synthetic materials, or wheat board. Further, the position of the layers, as shown in  FIGS. 10 and 11 , may be interchanged, i.e., MDF layer  1002  being substituted for a mineral core  1004  and vice versa. Mineral cores disclosed in U.S. Pat. Nos. 5,522,195, 4,811,538, 4,746,555, 7,279,437, 7,655,580, and 8,097,544, which are incorporated herein by reference, may be appropriate for the present invention. Also, commercially available fire resistant materials, such as Palusol® from BASF and/or fire door core materials from Fyrewerks, Inc., are also appropriate for the present invention. Preferably, fire resistant doors pass 20, 45, 60, or 90 minute fire rating. Standard methods of fire tests of door assemblies are found in NFPA 252 (2012), UL10C (2016), and/or CAN/ULC-S104 (2015). 
         [0035]      FIG. 3  illustrates a process for assembling the shaker door  100  of the present invention. The frame  108 , including stiles  202 , is first attached to the interior side of a first door facing  200 , e.g. by an adhesive. The thick core material  204  is then placed over the interior side of the raised peripheral region  104  of the first door facing  200  and attached thereto, e.g. by an adhesive; and the thin core material  206  is place over the interior side of the recessed panel  102  and attached thereto, e.g. by an adhesive. Adhesive is then placed over the exposed side of the core materials  204 ,  206  and the frame. A second door facing  200  is then placed on top of the exposed adhesive, pressed against the frame and the core material  204 ,  206 , and allowed to cure. In exemplary embodiments, curing may take place in a press to allow for proper bonding between the components of the shaker door  100 . The processes disclosed in U.S. Pat. No. 7,819,163 and U.S. Patent Application publication No. 2014/0261991, which are incorporated herein by reference, may appropriately be used to assemble the shaker door  100  of the present invention. 
         [0036]    A second embodiment of the present invention is illustrated in  FIG. 4 . That construction is particularly advantageous when the thickness T c  is about three times the thickness t c . The advantages of this embodiment include simplification of both production and inventory management as the same sheets of material may be used, and simplification of processing as the thickness setting on the glue machine, preferably a direct roll coater, does not need to be changed. As illustrated in  FIG. 4 , the thick section of the core which fills the space between the raised peripheral regions  104  is composed of three different layers of core materials  400 ,  206 ,  402 . The core materials  400 ,  206 ,  402  may be the same or different. In the embodiment shown in  FIG. 4 , the thin core material  206  spans the entire area of the door  100  inside the frame  108 , but only at the thickness t c . The additional layers of material  400  (first filler layer),  402  (second filler layer) are used to fill in the cavity between the raised peripheral regions  104 . Thus, the embodiment of  FIG. 4  essentially replaces the single layer thick core material  204  of the embodiment of  FIG. 2  with three layers of materials  400 ,  206 ,  402 , where one of the layers (the middle layer  206 ) is an extension of and contiguous with the thin core material  206 . The first and second filler layers  400  and  402  may be the same or different material as the thin core material  206 . Other commonly used door support materials may be used for the thin core material  206 , the first filler layer  400 , and the second filler layer  402 . In the embodiment of  FIG. 4 , the material of the filler layers  400  and  402  may be the same as that of the thin core material  206  but cut into strips to fit the into the space between the raised peripheral regions  104 . One advantage of this embodiment is that small strips of the thin core material  206  may be reused to form the first and second filler layers  400  and  402 . Another advantage is that broken or scrap pieces of materials may be trimmed into strips and salvaged to form the first and second filler layers  400  and  402 . In certain embodiments, the filler layers  400  and  402  may not be as critical for bending resistance as the thin core material  206 , so a weaker material may be used. 
         [0037]    The assembly of the second embodiment, as shown in  FIG. 5 , may be accomplished as similarly described above and in  FIG. 3 , with some modification. The assembly of the core first involves laying strips of the first filler layer  400  on the raised peripheral region  104  and adjacent to the frame  108 . The first filler layer  400  should be sufficiently thick so that it its exposed surface  500  is flush with the inner surface  502  of the recessed panel  102  of the door facing  200 . Next, the thin core material  206 , cut to fit the entire area within the frame, is laid onto the first filler layer  400  and the inner surface  502  of the recessed panel  102 . Strips of the second filler layer  402  are then laid onto the thin core material  206 , in the areas of the raised peripheral regions  104 . Finally, another door facing  200  is then placed on top to complete the door  100 . As noted above, the door facing  200  may be secured to the core and the frame, e.g., by an adhesive. Further, the components of the core itself, i.e. the first filler layer  400 , the thin core material  206 , and the second filler layer  402  may also be secured together, e.g., by using an adhesive. A preferable adhesive is polyvinyl acetate. In certain embodiments, it is desirable for the material of the filler layers  400 ,  402  to be compressible to accommodate the stacked tolerances of the three layers  400 ,  206 ,  402  in the raised peripheral region  104 . Here each of the three layers,  400 ,  206 , and  402  in the raised peripheral region  104  may contribute, for example, a tolerance of about ±0.005″, which makes the total tolerance in that area about ±0.015″, which is greater than the thin core material  206  (±0.005″) in the recessed panel  102 . The use of compressible materials for the filler layers  400 ,  402  allows the door to be designed with a single, lower tolerance. The compressible materials may be, for example, open cell foam, low density fiberboard, low density wheat straw board, or combinations thereof. 
         [0038]    A third embodiment of the present invention is illustrated in  FIG. 6 . In this embodiment, the thin core material  206  extends beyond the area under the recessed panel  102 ; however, that extension (overhang)  602  does not go as far as the frame  108 . The overhang  602  (the portion of the thin core material  206  that extends beyond the area under the recess panel  102 ) may extend about 25 to about 50% of the distance between the recessed panel  102  and the frame  108 . The amount of overhang depends on the final door requirements and may add ease of assembly and/or strength to the final door construction. Without being bound by any particular theory, it is believed that the overhang provides more bending stiffness to resist distortion. The remaining portion of the core, at the raised peripheral region  104 , may be filled with at least a filler core material  600 , which may be a conventional core material, such as particle board and/or non-expandable corrugated cardboard. In an exemplary embodiment, the filler core material  600  immediately above and below the thin core material  206  is preferably a paper honeycomb material or corrugate cardboard material, while the remaining volume is filled with particle board. The use of two different materials may be desirable to achieve weight reduction (e.g. by using a lower density material as the filler core material) or cost reduction (by using a less expensive material). In certain embodiments, other materials may be used, such as particle board and medium density fiberboard (MDF). In addition, the filler core material  600  may be the same material as described above for the thick core material  204  or any of the filler layers  400 ,  402 . Similarly to the second embodiment, compressible materials may be desirably used here. 
         [0039]    The assembly of the third embodiment, as shown in  FIG. 7 , may be accomplished as similarly described above and in  FIG. 3 , with some minor modification to the assembly of the core. The frame  108 , including stiles  202 , is first attached to the interior side of a first door facing  200 , e.g. by an adhesive. The thin core material  206  and the filler core material  600  are placed over the interior side of the door facing  200  and attached thereto, e.g. by an adhesive. As illustrated in  FIG. 7 , the thin core material  206  is placed over the recessed panel  102 . such that the thin core material  206  extends beyond the recessed panel  102  and interlocks with the filler core material  600 , such as through a tongue and groove fitting. The filler core material  600  may be formed, e.g. by routing, to provide a channel  604  in the filler core material  600  to accommodate the overhang  602  of the thin core material  206 , Here, the overhang  602  fits into the channel  604  in the filler core material  600  to lock the thin core and filler core materials  206 ,  600  together. Adhesive is then placed over the exposed side of the core materials  206 ,  600  and the frame. A second door facing  200  is then placed on top of the exposed adhesive and allowed to cure as previously herein described. 
         [0040]    In certain applications, the formation of the channel in the filler core material  600  may be costly and inefficient. Instead of a channel  604 , the filler core material  600  may alternatively be formed to have an L-shaped cross-section, as best shown in  FIG. 8 . In this alternate third embodiment, the L-shaped cross-section, which may be formed, e.g. by routing, contains a ledge  800  onto which the overhang  602  of the thin core material  206  sits. With the shaped cross-section, however, when the door is assembled, a small void space  900  is formed under one side of the door facing  200 , as best illustrated in  FIG. 9 . 
         [0041]    The assembly and materials for this alternate third embodiment is essentially the same as those disclosed above for the third embodiment. In assembly, as best illustrated in  FIG. 9 , the frame  108 , including stiles  202 , is first attached to the interior side of a first door facing  200 . e.g. by an adhesive. The filler core materials  600  are then placed over the raised peripheral regions  104  of interior side of the door facing  200  and attached thereto, e.g. by an adhesive. Next, the thin core material  206  is placed over the recessed panel  102 , such that its overhangs  602  fit neatly on the ledges  800  of the filler core materials  600 . The thin core material  206  may also be attached to the door facing  200  and the filler core material  600 , e.g. by an adhesive. Adhesive is then placed over the exposed side of the core materials  206 ,  600  and the frame. A second door facing  200  is then placed on top of the exposed adhesive and allowed to cure as previously herein described. 
         [0042]    Although the drawings and their description above pertains to one-panel shaker doors, the present invention is applicable to shaker door having multiple panels. Shaker doors may also contain more than one panel, for example, two or three panels. 
         [0043]    It will be apparent to one of ordinary skill in the art that various modifications and variations can be made in construction or configuration of the present invention without departing from the scope or spirit of the invention. Thus, it is intended that the present invention cover all such modifications and variations, and as may be applied to the central features set forth above, provided they come within the scope of the following claims and their equivalents.

Summary:
The present invention relates to shaker doors with solid cores and methods for making the same. The shaker doors contain different core materials at the recessed panel than the raise peripheral region to provide dimensional stability and reduced distortion when the doors are exposed to high humidity. The devices and methods also provide for easy assembly of solid core shaker doors, including fire rated doors.