Method and apparatus for making a gypsum board

1 . A method for manufacturing a gypsum board comprising:

depositing a front first cover sheet, having first and second opposed surfaces, over a forming surface and moving the front first cover sheet horizontally in a machine direction along the forming surface;

optionally depositing a first aqueous gypsum slurry on the front first cover sheet moving in the machine direction to form a first aqueous gypsum slurry layer as a higher-density region in layer form contacting an upper surface of the front first cover sheet, wherein the first aqueous gypsum slurry layer has a first slurry density of 75-100 lbs/ft3;

depositing a second aqueous gypsum slurry in a foamed state on the first aqueous gypsum slurry layer, if the first aqueous gypsum slurry layer is present, or if the first aqueous gypsum slurry layer is not present depositing the second aqueous gypsum slurry in a foamed state on the front first cover sheet, to form a second aqueous gypsum slurry layer as a middle-density region in layer form contacting an upper surface of the first aqueous gypsum slurry layer, if the first aqueous gypsum slurry layer is present, or if the first aqueous gypsum slurry layer is not present contacting an upper surface of the front first cover sheet, wherein the second aqueous gypsum slurry layer has a second slurry density of 40-65 lbs/ft3, wherein if the first aqueous gypsum slurry is present the second slurry density is lower than the first slurry density, and

depositing a third aqueous gypsum slurry in a foamed state on the second aqueous gypsum slurry layer to form a third aqueous gypsum slurry layer as a lower-density region in layer form contacting an upper surface of the second aqueous gypsum slurry layer, wherein the third aqueous gypsum slurry layer has a third slurry density of 30-55 lbs/ft3 which is lower than the second aqueous slurry density;

wherein the optional first aqueous gypsum slurry, the second aqueous gypsum slurry and the third aqueous gypsum slurry comprise a respective mixture of water and stucco, wherein the stucco comprises calcium sulfate hemihydrate, wherein the optional first aqueous gypsum slurry, the second aqueous gypsum slurry and the third aqueous gypsum slurry each comprises a respective mixture of at least 60 wt. % said calcium sulfate hemihydrate on a dry (water free) basis, and the water at a weight ratio of water to the calcium sulfate hemihydrate of 0.2:1 to 1.2:1;

passing a back second cover sheet, having third and fourth opposed surfaces, over a first transition roller spaced above the front first cover sheet, and subsequently applying the back second cover sheet over the third aqueous gypsum slurry layer, to form a multilayer assembly including the front first cover sheet, optionally the first aqueous gypsum slurry layer on the first cover sheet, the second aqueous gypsum slurry layer on the first layer, if the first layer is present, or if the first layer is not present then the second layer is on the first cover sheet, the third aqueous gypsum slurry layer on the second aqueous gypsum slurry layer, and the back second cover sheet over the third aqueous gypsum slurry layer, with the third surface of the second back cover sheet facing the third aqueous gypsum slurry layer;

passing the multilayer assembly into a forming station for forming the multilayer assembly;

setting the calcium sulfate hemihydrate respectively of the optional first aqueous gypsum slurry layer, the second aqueous gypsum slurry layer, and the third aqueous gypsum slurry layer by respectively reacting the calcium sulfate hemihydrate with the water of the optional first aqueous gypsum slurry layer, the second aqueous gypsum slurry layer, and the third aqueous gypsum slurry layer to form a panel comprising a gypsum core of an optional first board layer comprising calcium sulfate dihydrate, a second board layer comprising calcium sulfate dihydrate, and a third board layer comprising calcium sulfate dihydrate between the front first cover sheet and back second cover sheet,

wherein the first board layer, if present, has a first board layer density, wherein the second board layer has a second board layer density, wherein if the first board layer is present the second board layer density is lower than the first board layer density, and wherein the third board layer has a third board layer density lower than the second board layer density; and

cutting and drying the panel into the gypsum board; and

wherein the method comprises:

A) said depositing of the third aqueous gypsum slurry to comprise directing the third aqueous gypsum slurry on the third surface of the back second cover sheet, passing the back second cover sheet with the third aqueous gypsum slurry over the first transition roller spaced above the first front cover sheet, while the fourth surface of the back second cover sheet contacts the first transition roller, so that a majority of the third aqueous gypsum slurry falls off the back second cover sheet onto the second aqueous gypsum slurry over the front first cover sheet; or

B) said depositing of the second aqueous gypsum slurry to comprise directing the second aqueous gypsum slurry on the third surface of the back second cover sheet, passing the back second cover sheet with the second aqueous gypsum slurry over the first transition roller spaced above the front first cover sheet, while the fourth surface of the back second cover sheet contacts the first transition roller, so that a majority of the second aqueous gypsum slurry falls off the back second cover sheet onto the deposited first aqueous gypsum slurry if present or, if the first aqueous gypsum slurry is not present, onto the front first cover sheet, and then depositing the third aqueous gypsum slurry onto the deposited second aqueous gypsum slurry.

2 . The method of claim 1 , wherein the back second cover sheet moves along a downstream portion of a first path segment with movement along a slope defining an angle of incidence “A”, relative to the first cover sheet traveling on the forming surface below the first transition roller, in a range having a minimum of 0 degrees and a maximum of 20 degrees.

3 . The method of claim 1 , wherein the foamed second aqueous gypsum slurry or the foamed third aqueous gypsum slurry discharges in a direction of concurrent flow with the direction of movement of the back second cover sheet to contact the third surface of the back second cover sheet in a first path segment wherein the contact is at an angle of incidence “B” in a range of 0 to 90 degrees relative to the third surface of the back second cover sheet.

4 . The method of claim 1 , wherein the first transition roller changes the direction of movement of the back cover sheet to movement along a first portion of a second path segment that has a second horizontal movement component in the machine direction and a second vertical movement component, the second vertical movement component being upwards, or downwards, or neutral to provide movement only along the second horizontal movement component.

5 . The method of claim 1 , wherein a second path segment has a first portion and a second portion, wherein at a downstream end of the first portion of the second path segment the back cover sheet passes over a second transition roller to feed the back cover sheet to the second portion of the second path segment and change the direction of movement of the back cover sheet.

6 . The method of claim 5 , wherein the second transition roller is at a sufficiently raised position relative to the face cover sheet moving along the forming surface for a head of slurry at the downstream end of a third path segment to be visible to an operator of the method.

7 . The method of claim 1 , wherein at the downstream end of a second path segment depositing the back cover sheet on the second aqueous gypsum slurry on the face cover sheet to form the multilayer assembly to include the back cover sheet on the second aqueous gypsum slurry.

8 . The method of claim 1 , wherein the foamed second aqueous gypsum slurry or the foamed third aqueous gypsum slurry discharges onto a portion of the back-cover sheet supported by a back-plate.

9 . The method of claim 1 , wherein the front first cover sheet comprises at least one of a glass mat facer sheet or a paper facer sheet.

10 . The method of claim 1 , wherein the lower-density region resulting from the set gypsum lower-density region slurry has a thickness of 0.2 inches to 1.0 inches.

11 . The method of claim 1 , wherein the middle-density region resulting from the set gypsum middle-density region slurry has a thickness of 0.05 inches to 1.0 inches.

12 . The method of claim 1 , wherein the higher-density region resulting from the set gypsum higher-density region slurry has a thickness of about 0.02 inches to about 0.2 inches,

wherein the thickness of the lower-density region layer is greater than the thickness of the higher-density region layer,

wherein the thickness of the middle-density region layer is greater than the thickness of the higher-density region layer.

13 . The method of claim 1 , wherein the middle density slurry and the lower-density slurry are foamed and comprise air bubbles, wherein when foamed, the gypsum lower-density region layer and the gypsum middle-density region layer resulting from the set foamed gypsum slurry have a total void volume of 10 to 92 volume percent.

14 . The method of claim 13 , wherein the air bubbles have an average cross-section diameter of less than 1.5 mm.

15 . The method of claim 1 , wherein the higher density region layer is present and has a total void volume of less than 30 volume percent.

16 . The method of claim 1 , further comprising adding air to the second aqueous gypsum slurry prior to depositing the second aqueous gypsum slurry, and adding air to the third aqueous slurry prior to depositing the third aqueous slurry.

17 . The method of claim 1 , wherein said depositing of the third aqueous gypsum slurry comprises directing the third aqueous gypsum slurry on the third surface of the back second cover sheet, passing the back second cover sheet with the third aqueous gypsum slurry over the first transition roller spaced above the first front cover sheet, while the fourth surface of the back second cover sheet contacts the first transition roller, so that a majority of the third aqueous gypsum slurry falls off the back second cover sheet onto the second aqueous gypsum slurry on the front first cover sheet.

18 . The method of claim 1 , wherein said depositing of the second aqueous gypsum slurry comprises directing the second aqueous gypsum slurry on the third surface of the back second cover sheet, passing the back second cover sheet with the second aqueous gypsum slurry over the first transition roller spaced above the front first cover sheet, while the fourth surface of the back second cover sheet contacts the first transition roller, so that a majority of the second aqueous gypsum slurry falls off the back second cover sheet onto the deposited first aqueous gypsum slurry if present or, if the first aqueous gypsum slurry is not present, onto the front first cover sheet, and then depositing the third aqueous gypsum slurry onto the deposited second aqueous gypsum slurry.