Methods of manufacturing gypsum board and board made therefrom

Abstract

Apparatus and method for producing gypsum board, comprising apparatus for coating a cover sheet with a relatively high density gypsum slurry, apparatus for forming edge borders on the sheet with relatively high density gypsum slurry, and apparatus for forming a core on the coating and between the edge borders, the core comprising a relatively low density gypsum. The apparatus for coating the sheet comprises a relatively soft pressure roll and a relatively hard coating roll, the two rolls being normally pressed together to form a nip between them and a sheet to be coated passing through the nip. The pressure roll is below the sheet and the coating roll, and the axis of the pressure roll is offset from the axis of the coating roll in upstream direction of the movement of the sheet. The pressure roll has a surface area which contacts and moves in the same direction as the sheet, and the coating roll has a surface area which contacts and moves in the opposite direction of the sheet. A trough is formed between the upper side of the sheet and the coating roll, and a relatively high density gypsum is poured into the trough. The rotating coating roll picks up a quantity of the slurry from the trough and wipes it onto the sheet to form a high density gypsum coating.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to gypsum board, and more specifically to apparatus and method for coating a cover sheet of gypsum board, and to the board produced thereby.

2. Description of Related Technology

Gypsum board is well known and widely used in the construction industry. A typical sheet of gypsum wallboard comprises a gypsum core, a back cover sheet on one side of the core and a face or front cover sheet on the other side of the core. The face cover sheet is folded around the long edges of the core and overlaps the side edges of the back cover sheet.

To reduce the weight of the core, it has been common practice to introduce small bubbles into the gypsum to produce a foamed gypsum core. This has been done, for example, by adding a foaming agent to the gypsum slurry. However, a core formed entirely of foamed gypsum has two disadvantages. First, the foamed gypsum core, when set or cured, is relatively fragile and tends to crack and crumble when a nail is driven through it during installation. Second, the foamed gypsum core does not always adhere to the cover sheets as well as desired.

To avoid the first problem mentioned above, it has been common practice to provide unfoamed gypsum along the long edges of the board. The unfoamed gypsum along the edges is denser and harder than the foamed gypsum, and the "hard edges" are stronger and less easily fractured. The second problem has been more difficult to resolve. Starch has been added to the gypsum slurry to produce better adherence with the cover sheets. Another solution has been to coat the cover sheets to produce better adhesion.

The White U.S. Pat. No. 4,327,146 describes a method of coating a cover sheet with a defoaming agent which acts to remove the foam bubbles from the gypsum at the core-sheet interface. The defoamed gypsum adheres well to the cover sheets.

The Brookby U.S. Pat. No. 1,511,500 describes a method of coating a cover sheet with a "normal" gypsum and water mixture, and then forming a core layer of gypsum having an expanded cellular body.

The R. Bruce U.S. Pat. Nos. 5,085,929 and 5,116,671 describe a method of producing a foamed gypsum slurry using a foaming agent which is added to the gypsum. According to these patents, the foaming agent produces a high density slurry at the interface with the cover sheets, and the high density slurry purportedly adheres well to the paper.

Other U.S. patents of possible interest are U.S. Pat. Nos. 2,954,302, 3,516,882, and 3,607,486.

It is a general object of the present invention to provide an improved apparatus and method for coating cover sheets with a relatively high density gypsum slurry, and to an improved gypsum board produced thereby.

SUMMARY OF THE INVENTION

Apparatus and method in accordance with the invention for producing gypsum board, comprises apparatus for coating a cover sheet with a relatively high density gypsum slurry, apparatus for forming edge borders on the sheet with relatively high density gypsum slurry, and apparatus for forming a core on the coating and between the edge borders, the core comprising a relatively low density gypsum.

The apparatus for coating the sheet comprises a relatively soft pressure roll and a relatively hard coating roll, the two rolls being normally pressed together to form a nip between them and a sheet to be coated passing through the nip. The pressure roll is below the sheet and the coating roll, and the axis of the pressure roll is offset from the axis of the coating roll in upstream direction of the movement of the sheet. The pressure roll has a surface area which contacts and moves in the same direction as the sheet, and the coating roll has a surface area which contacts and moves in the opposite direction of the sheet. A trough is formed between the upper side of the sheet and the coating roll, and the relatively high density gypsum is poured into the trough. The rotating coating roll picks up a quantity of the slurry from the trough and wipes it onto the sheet to form a high density gypsum coating, and the coating roll is wiped clean by the sheet which is pressed against the coating roll by the pressure roll.

The apparatus for forming the edge portions comprises means for supplying streams of a relatively high density gypsum slurry to the borders of the sheet, the streams merging and being continuous with the gypsum coating.

The apparatus for forming the core comprises means for providing a relatively low density gypsum slurry over the coating and between the borders.

The invention further comprises a gypsum board comprising a first cover sheet, a coating of relatively high density gypsum on said first cover sheet, borders of relatively high density gypsum along the edge portions of said first cover sheet, the borders merging and being continuous with the coating, a core of relatively low density gypsum covering said coating between said borders, and a second cover sheet over said core and said borders.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of a machine constructed in accordance with the present invention;

FIG. 2 is a view taken along the line 2--2 of FIG. 1;

FIG. 3 is an enlarged fragmentary view of a portion of the machine shown in FIG. 1; and

FIG. 4 is a fragment of a machine constructed in accordance with an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The machine shown in FIGS. 1 and 2 comprises a flat deck or plate 10 mounted on a frame 11. A supply roll 12 is rotatably mounted adjacent the deck 10 on a shaft 13 and supplies a first cover sheet 14. From the roll 16, the sheet 14 moves downstream (toward the right as seen in FIG. 1) across the upper surface of the deck 10.

Spaced downstream from the deck 10 is a second frame 17 and deck 18, a deck cut-out or gap 19 being formed between the two decks 10 and 18.

A typical finished sheet of gypsum wallboard is four feet wide, and the widths of the sheet 14 and the two decks 10 and 18 are slightly wider than the finished sheet, as is well known in the industry. In the finished product, the sheet 14 forms the front or viewed face of the gypsum board.

Mounted in the gap 19, between the two decks, is apparatus 21 for coating the upper surface (as seen in FIG. 1) of the cover sheet 14, the coating apparatus 21 comprising a coating roll 22 and a pressure roll 23 (see in particular FIG. 3). The transport path for the sheet 14 leads from the deck 10, over the top of the compression roll 23, loops underneath the coating roll 22 as shown in FIGS. 1 and 3, and to the top of the deck 18. The two rolls 22 and 23 are respectively mounted on axles 24 and 25; the compression roll 23 is not motor driven and its surface speed is essentially the same as that of the cover sheet 14. The coating roll 22, however, is driven by a motor 27 (see FIG. 2) and its surface moves opposite the direction of the sheet 14, as indicated by the directional arrows 28 and 29 in FIG. 3. A power supply and motor speed controller 31 is connected to drive the motor 27.

The compression roll 23 is a relatively soft roll and may be made, for example, of sponge rubber. The coating roll 22, on the other hand, is hard and highly polished, and may, for example, be a polished chromed roll. The two rolls are pressed toward each other and form a nip 32 between them. The sheet 14 moves through the nip 32 and its upper surface wipes across the under surface of the coating roll 22. The sheet 14 is looped under the coating roll 22 and is pulled tightly across the underside of the roll 22, and since the surfaces move in opposite directions, the surface of the roll 22 is wiped clean by the sheet 14. During operation, it is important that the roll 22 not stop rotating.

Mounted above the decks 10 and 11 is a main mixer 36 (see FIG. 1) which contains a quantity of foamed gypsum slurry. The slurry may have a conventional composition which includes gypsum, water, a foaming agent, stabilizers, etc., forming a relatively low density gypsum slurry. The density, of course, is low because of the foam or air bubbles in the slurry.

A duct or conduit 37 leads from the main mixer 36 to a high-speed beater 38 which includes a vaned member 39 rotatably mounted in a housing 41. An electric motor 42 is connected by a clutch 43 to rotate the vaned member 39 at high speed. Another duct 44 leads from the beater 38 to the coating apparatus 21. Foamed slurry from the main mixer 36 flows through the duct 37 to the high speed beater 38, and the rapidly turning vanes 39 remove most of the air bubbles from the slurry by beating the slurry. Consequently, the slurry flowing through the duct 44 to the coating apparatus comprises a substantially defoamed (or relatively high density) gypsum slurry.

The high speed beater 38 may be similar to conventional beaters presently used to form a defoamed slurry that is located along the long side edges (the "hard edges") or margins of gypsum wallboard.

With specific reference to FIG. 3, the coating roll 22 of the coating apparatus 21 has its axle 24 offset upwardly and downstream (in the direction of movement of the sheet 14) relative to the axle 25 of the pressure roll 23. In the specific example of the invention described herein, a line drawn through the two axles 24 and 25 makes an angle of substantially 45.degree. with a horizontal line, and each of the rolls 22 and 23 have diameters of six and four inches respectively. Due to the offset of the axles, an upwardly opening trough or valley 51 is formed between the upper side of the sheet 14 and the coating roll 22, and the duct 44 is located to pour a quantity of the defoamed slurry 52 into the trough 51. The slurry 52 flows laterally from the duct 44 and fills the trough over the length of the roll 22. As the roll 22 rotates clockwise (as seen in FIG. 3), the surface 53 of the roll 22 picks up a coating 54 of the slurry 52, and the coating 54 is wiped off the roll 22 by the sheet 14. The sheet 14 is pulled downstream by a conventional drive mechanism (not illustrated), and the sheet is pulled tightly against the underside of the roll 22. Further, the soft pressure roll 23 presses the sheet tightly against the roll 22. The sheet 14 moves opposite the direction of the surface 53 of the roll 22 as previously mentioned and the paper wipes clean the surface of the roll 22. As a consequence, the coating 54 is transferred to the sheet 14 and forms a substantially uniform coating or layer 56 across center area of the sheet. The thickness of the coating 56 depends upon the rate of movement of the sheet relative to the rate of rotation of the roll 22, and the controller 31 is preferably adjusted to produce a coating having a thickness of approximately 1/16" to 1/8".

As shown in FIG. 2, the axial lengths of the two rolls 22 and 23 is slightly less than the width of the sheet 14. For example, in the production of 4 by 8 feet gypsum wallboard, the rolls 22 and 23 may be 45 inches long. Consequently, a sheet 14 of conventional width paper extends beyond the ends of the roll 22, and some of the slurry 52 in the trough 51 flows around the ends of the roll 22 and onto the edge portions or borders of the sheet 14.

The coated sheet 14 is moved along the transport path onto the deck 18, and additional high density slurry is poured onto the borders of the sheet 14 to form the hard edges. Two ducts 57 and 58 (FIGS. 1 and 2) extend from the high speed beater 38 downwardly to just above the borders of the sheet 14, and they form layers of high density slurry along the borders. The slurry of these two layers has the same composition and density as the slurry forming the coating 56, and the slurry of the borders merges and is continuous with the slurry of the coating 56 and the portions of the slurry 52 which flows around the ends of the roll 22.

The gypsum core 60 of the board is formed by the foamed gypsum from the main mixer 36 which flows through a duct 59 to the center area of the coated sheet 14. As illustrated in FIG. 2, the slurry flows and spreads out across the sheet 14 on top of the coating 56 and between the layers of slurry at the borders of the sheet.

Conventional folding shoes 61 at the sides of the deck 18 along the borders of the sheet fold the borders upwardly and then down on top of the high density layers of slurry, the shoes 61 forming opposing long side edges of the finished board. A second cover sheet 62 is then laid over the core 60 and the borders of the first cover sheet 14. The second cover sheet 62 is unreeled from a supply roll 64 and passed under an idler roll 65 which guides the cover sheet 62, smooths the upper surface of the slurry, and reduces the slurry thickness to the desired value. In the finished product, the first sheet 14 normally forms the front face of the wallboard and the second sheet 62 normally forms the back face or side of the board.

Following the laying down of the second cover sheet 62, the board is processed in the customary manner. The two cover sheets and the slurry are moved along the deck 18 until the slurry has set to the point where the board can be handled. Then the board is cut to the desired length, turned over, and then moved through a kiln (not shown).

The coating of the high density gypsum slurry has excellent adhesion with the first sheet 14, and the coating adheres to the low density gypsum core; since the coating 56 and the hard edge layers merge and are formed from the same supply, the adhesion is continuous and consistent across the front face of the board.

As previously mentioned, the rate of rotation of the coating roll 22 should be carefully controlled to produce the desired thickness of the layer 56, and it is important that the roll 22 be driven with sufficient torque to prevent it from stopping during operation. It is also important that the compression roll 23 presses against the sheet 14 and the roll 22 and that the sheet 14 is pulled tightly across the underside of the coating roll 22, so that the roll 22 is cleaned by the sheet 14.

In a specific example of a machine in accordance with this invention, the rolls 22 and 23 are about 6 and 4 inches respectively in diameter, the space 19 between the two decks 10 and 18 is about 27 inches, the bottom of the coating roll 22 is about 1 inch below the surface of the deck 18, the rolls 22 and 25 are about 45 inches long, and the hose 37 has a diameter of about 1.25 inches. The compression roll 23 is preferably sufficiently soft that it will allow any lumps to pass because a lump could break in the cover sheet 14. The coating roll 22 is preferably mounted such that it will automatically move away from the compression roll 23 in case a person has his/her fingers caught between the rolls. For example, the roll 22 may be mounted on pivotable arms which are counterweighted to move the roll 22 up if it meets an obstruction.

FIG. 4 shows an alternative arrangement wherein separate high speed mixers or beaters are provided for feeding the coating apparatus and the hard edge forming apparatus. A hose or duct 37a conveys low density slurry from a main mixer 36a to a high speed beater 41a, and a duct 44a conveys the resulting high density slurry to the coating apparatus (see FIGS. 1 to 3). A separate duct 37b conveys low density slurry to a second high speed beater 41b, and two ducts 57b and 58b convey the high density slurry to the borders of the sheet. In other respects, the system of FIG. 4 is similar to that shown in FIGS. 1 to 3.

In the systems shown in FIGS. 1 to 4, a central mixer 36 (or 36a) forms a single source for the slurry used for the coating 56, the hard edge portions, and the core. Instead, separate supplies could be provided for the three functions, or, for example, one supply could be provided for the core and a separate supply could be provided for the coating and the hard edges.

The foregoing detailed description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications within the scope of the invention will be apparent to those skilled in the art.

Claims

1. A method of manufacturing gypsum board having a cover sheet and a core comprising gypsum, the method comprising the steps of:

(a) providing a machine for manufacturing gypsum board, the machine having a transport path for movement of a cover sheet in a direction toward a downstream end of the machine, the machine including a coating roll having an axis, the coating roll rotatably mounted adjacent the transport path, and a pressure device having an axis, the pressure device also mounted adjacent the transport path, the pressure device axis being disposed parallel to and upstream of the coating roll axis, the coating roll and the pressure device forming a nip for passage of the cover sheet therethrough;

(b) providing a supply of coating slurry comprising gypsum and providing a supply of core slurry for forming a gypsum core; and

(c) coating a side of a cover sheet with the coating slurry and then depositing the core slurry on the coated sheet to form the gypsum core, said coating step comprising:

(1) moving the cover sheet in the downstream direction, around a portion of the coating roll, and between the coating roll and the pressure device, the coating roll and cover sheet defining a trough, the trough disposed upstream from the coating roll axis with respect to the direction of movement of the cover sheet;

(2) pressing the cover sheet tightly against the coating roll with the pressure device;

(3) pouring a quantity of coating slurry into the trough and into contact with the cover sheet and the coating roll; and

(4) rotating the coating roll in a direction of rotation opposed to the direction of movement of the cover sheet causing transfer of the coating slurry from the coating roll to the cover sheet at a location downstream of the coating roll axis to result in a coated cover sheet.

2. The method of claim 1 wherein the pressure device is a pressure roll freely rotatable about the pressure device axis, the pressure roll rotated by the moving cover sheet.

3. The method of claim 1 wherein the coating slurry has a first density and the core slurry has a second density, the first density being greater than the second density.

4. The method of claim 3 wherein the core slurry comprises foam and further comprising the step of beating an amount of the core slurry to remove air bubbles therefrom to produce the coating slurry.

5. The method of claim 1 wherein said core slurry is a relatively low density gypsum slurry.

6. The method of claim 1 wherein the coating slurry is a relatively high density gypsum slurry.

7. The method of claim 1 further comprising the step of depositing a second cover sheet on the core slurry.

8. The method of claim 1 comprising the step of depositing the core slurry onto a central portion of the coated cover sheet and depositing coating slurry on the coated cover sheet laterally of the core slurry.

9. The method of claim 1 further comprising the step of adjusting the rate of rotation of the coating roll relative to the rate of movement of the cover sheet to form the coating having a thickness of between about 1/16 inch and about 1/8 inch.