MXPA01013088A - Gypsum wallboard core, and method and apparatus for making the same. - Google Patents

Abstract

A gypsum wallboard core, and methods and apparatus for making the same are disclosed. Methods of making a gypsum wallboard core include extruding a gypsum slurry containing water, gypsum, slip agents, water-reducing agents, surfactants and, optional additives, through a die and onto a substantially flat, smooth, moving surface. The die has provisions at its outer sides for the introduction of slip agents into the slurry, and provisions at its lateral outer edges for the introduction of a strength-enhancing agent. Once extruded onto the conveyor belt, the slurry is chemically-activated to set and form a hardened board core which then may be easily removed from the conveyor belt and dried.

Description

NUCLEUS PRESSED PLASTER FIBER AND METHOD AND APPARATUS FOR ELABORATING THE SAME FIELD OF THE INVENTION The present invention relates generally to the production of gypsum board materials and, more specifically, the invention relates to the manufacture of pressed gypsum fiber using an extrusion technique to prepare a gypsum core. 10 • BRIEF DESCRIPTION OF THE RELATED TECHNOLOGY A common method for building walls and ceilings includes the use of panels or sheets of inorganic pressed fiber such as fiber pressed from 15 plaster, often referred to simply as "pressed fiber" or "dry wall". The pressed fiber can be formulated for interior, exterior, and wet applications. The use of pressed fiber as opposed to cartons • Conventional made from wet calcined gypsum methods, it is desirable because the installation of pressed fiber is normally It is less expensive and less problematic when compared to the installation of conventional calcined gypsum walls. Walls and ceilings made of pressed gypsum fiberboard are typically constructed by securing, for example, nails or screws, pressed fiber panels to structural members, such as steel or wood pieces oriented vertically and horizontally often called "beams". Because the pressed fiber is often supplied in sheets or panels of standard size, when a wall is formed from the sheets, there will generally be a number of joints between adjacent sheets. In most pressed fiber construction, those joints are typically filled and coated with an adhesive material called a bonding compound so that the wall will have a smooth finish similar to that obtained with conventional calcined gypsum walls. Generally, pressed fiber is produced by enclosing a core of an aqueous suspension of calcined gypsum and other materials between two large sheets of cardboard cover paper. Various types of cover paper are known in the art. After the gypsum suspension has set (ie, reacted with the water present in the aqueous suspension) and dried, the formed sheet is cut into standard sizes. Methods for the production of pressed gypsum fiber are described in general, for example by Michelsenn, T., "Building Materials (Survey)", Encyclopedia of Chemical Technology, (1992, 4th ed.), Vol. 21, pp. 621-24, TP9E.685, the description of which is incorporated herein by reference in its entirety. The pressed gypsum fiber is manufactured using commercial processes that are capable of operation under continuous high-speed conditions. A conventional process for making the composition of I? - ^ ??? .. t.? ** & .. i-l í x? The gypsum fiber core initially includes the pre-mix of dry ingredients in a high speed mixing apparatus. Dry ingredients may include calcium sulfate hemihydrate (stucco), a f? accelerator, and an anti-desiccant (for example, starch). The ingredients 5 dry are mixed together with a "wet" (aqueous) portion of the core composition in a spike mixer. The wet portion may include a first component, commonly called a "paper pulp solution," which includes a mixture of water, paper pulp, and, optionally, one or more fluidity-increasing agents, and a 10 setting retarder. The paper pulp solution provides a major portion of the water that forms the gypsum suspension of the core composition. A second wet component may include a mixture of the reinforcement agent mentioned above, foam, and other conventional additives, if desired. Gaskets, dry and moist portions 15 mentioned above comprise a slurry of aqueous gypsum which optionally forms a core of pressed gypsum fiber. A main ingredient of the pressed fiber core of gypsum is # Calcium sulfate hemihydrate, commonly called "calcined plaster", "stucco", or "Paris calcined plaster". The stucco has a number of 20 desirable physical properties, including, but not limited to, fire resistance, thermal and hydrometric dimensional stability, compressive strength, and neutral pH. Typically, the stucco is prepared by drying, grinding, and calcining natural gypsum rock (i.e., dihydrate calcium sulfate). The drying step in stucco fabrication involves passing the raw gypsum rock through a rotary kiln to remove any free moisture present in the rock from rain or snow, for example. The dry rock is then passed through a roller mill 5 (or types of impact mill sprays), in which the rock is crushed or ground to a desired fineness. The degree of spraying is determined by the end use. Dry gypsum, crushed in fine can be called "calcined gypsum" regardless of its designed use. Calcined gypsum is used as a food for calcination procedures for conversion to stucco. 10 The calcination step (or dehydration) in the manufacture of • Stucco is made by heating the calcined gypsum, and can generally be described by the following chemical equation which shows that by heating the calcium sulfate dihydrate produces calcium sulphate hemihydrate (stucco) and water vapor: 15 CaSO4 «2H2O + heat ? CaSO4"1 / 2H2O + 11 / 2H2O This step of the calcination process is carried out in a" calciner ", of which there are several types known to those skilled in the art. 20 Uncalcined calcium sulfate (ie, calcined gypsum) is the "stable" form of gypsum. However, calcined gypsum, or stucco, has the desirable property of being chemically reactive with water, and will "set" rather quickly when the two are mixed together. This reaction of The setting is in fact inverse to the chemical reaction described above carried out during the calcination step. The setting reaction proceeds according to the following chemical equation which shows that the calcium sulfate hemihydrate is rehydrated to its dihydrate state: CaSO4"1 / 2H2O + 11 / 2H2O? CaSO4 »2H20 + heat The current time required to complete the setting reaction depends in general on the type of calciner and the type of gypsum rock used to produce the gypsum, and can be controlled within certain limits by the use of additives such as retarders, setting accelerators, and / or stabilizers, for example. In general, the rehydration time period can be on a scale from about two minutes to about eight hours depending on the amount of retarders, setting accelerators, and / or stabilizers present. After the aqueous gypsum suspension is prepared, the suspension and other desired ingredients are continuously deposited to form a gypsum-pressed fiber core suspension (hereinafter "pressed fiber core" or "core") between two moving sheets of cover paper supplied continuously. The two cover sheets comprise a pre-folded face paper and a backing paper. As the suspension is deposited on the face paper, the backing paper is lowered onto the top of the deposited core suspension and attached to the pre-folded edges of the face paper. The entire assembly is then measured for thickness using a roll bar or forming plate. The deposited aqueous suspension is then allowed to set between the two cover sheets, thereby forming a cardboard. The paperboard 5 produced continuously is cut into panels of a desired length, which are stacked vertically, and then passed through a drying oven where excess water is removed from the board to form a building material strong, dry, and rigid. The cover sheets that are used in the procedure 10 typically are made of multi-ply paper made from newsprint formed back into pulp. The face paper has an inner, unmeasured layer which is contacted with the core suspension so that the gypsum crystals can grow towards (or in) the inner layer, i.e., together with the starch present in the suspension, is the main way of union 15 between the core suspension and the cover sheet. The intermediate layers are measured and an outer layer is measured heavier and treated to control the absorption of paints and sealants. The backing paper is also constructed in a similar manner of multi-layer sheet. Both layers of cover must have sufficient permeability to allow the vapor of Water passes through them during the downstream cardboard drying step (s). The standardized pressed fiber sheets (or panels) typically are approximately 1.22 m wide and approximately 2. 4 to approximately 4.9 m long. The sheets are typically available in thicknesses ranging from approximately 0.6 centimeters to approximately 2.6 centimeters. In order to provide satisfactory strength, commercially available gypsum pressed fiber generally requires a density of about 726 to about 772 kilograms per 92.9 m2 of 1.27 cm thick board. Heavy or high-density plaster press fibers are more expensive and difficult to manufacture, transport, store, and install manually at work sites, compared to lighter or lower density cartons. It is possible to formulate pressed fiber having reduced densities by means of the inclusion of fillers and lightweight foams, for example. However, often where the pressed fiber is formulated to have a density of less than about 724.8 kg / 92.9 m2 of 1.27 cm thick board, the resulting strength is unacceptable for commercial sale. Because in general the high density or heavy gypsum fiber is undesirable, several attempts have been made to reduce the weight and density of the board without sacrificing the strength of the board. However, although lighter or less dense pressed fiber products can be produced, many of the pressed fiber products may be of unsuitable quality for commercial use. Morris et al, in the patent of E.U.A. No. 5,482,551 discloses a composition that can be extruded for use in the production of articles for buildings and constructions elaborated from approximately 45-85 by 100 percent by weight of calcium sulfate dihydrate and a filler to control the. density. In view of the above, it would be desirable to produce high strength gypsum pressed fiber having weights and 5 densities generally equal to or slightly lower than those produced by conventional methods. However, cartons of reduced weight and density must comply with industry standards and have strengths similar to, or greater than, conventional pressed fiber. Said pressed fiber must also be able to be manufactured using high speed manufacturing apparatuses and not suffer from other effects • negative side effects For example, such a high strength pressed fiber must be capable of setting and drying within a reasonable period of time. In addition, the use of conventional ingredients in the preparation of the aqueous suspension may cause clogging and undesirable clogging of mixers and piping used to prepare and transport, respectively, the aqueous solution to the paper cover sheets. For example, the mixture of setting accelerators in the suspension • in an upstream mixer, such as a pin mixer, may cause the suspension to begin to set (ie, calcined or harden) 20 before being deployed on the cover sheet (s). Therefore, it would be desirable to produce pressed gypsum fiber using procedures that do not require certain ingredients in the gypsum suspension that could cause premature clogging or setting of the suspension, or impart other effects unwanted BRIEF DESCRIPTION OF THE INVENTION One aspect of the invention is a method for forming a core of gypsum board, which includes the method steps of preparing a gypsum suspension in a mixer, and introducing the suspension in an extrusion die, the die preferably having elements in it. its periphery for the introduction of at least one gypsum suspension additive selected from the group consisting of slip agents and resistance improving agents. The method also preferably includes the steps of introducing the suspension additive into the suspension as the suspension exits the die, and extruding the gypsum suspension containing additive onto a flat moving surface. Then, the suspension can be dried in a hybrid dryer by convection heating and microwave drying techniques. Accordingly, another aspect of the invention is a core of pressed gypsum fiber prepared according to the aforementioned method. Additionally, another aspect of the invention is an apparatus for preparing a core of pressed gypsum fiber. In general, the apparatus includes a mixer that is in fluid communication with a die input of a die. The die includes the entry of the die and an exit of the die and a multiple arranged between the entrance and the exit. The apparatus also includes a substantially planar, movable surface disposed adjacent the die outlet and a dryer. The advantages of the invention will be apparent to experts in the art from a review of the following detailed description, 5 taken in conjunction with the drawings, the examples, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS For a full understanding of the invention, reference should be made to the following detailed description, to the examples, and to the appended drawings, in which: Figure 1 is a schematic diagram illustrating various processing equipment that may be useful for carrying out a method for making a pressed gypsum fiber according to the invention; and, Figure 2 is a schematic diagram illustrating in more detail characteristics of the equipment and method illustrated in Figure 1. Although the invention is susceptible to modalities in various forms, they are illustrated in the drawings, figures and hereafter 20 specific embodiments of the invention will be described, with the understanding that the description is designed to be illustrative, and is not intended to limit the invention to the specific embodiments that are described and illustrated herein.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES In general, the invention is directed to a method for forming * A core of pressed gypsum fiber, which includes the steps of the method of To prepare a gypsum suspension in a mixer, and to introduce the suspension, preferably under positive displacement in an extrusion die, the die preferably has elements at its periphery for the introduction of at least one gypsum suspension additive selected from the group consisting of of slip agents and resistance improvement agents. He The method also preferably includes the steps of introducing the additive of • suspension in the suspension as the suspension leaves the die, and soft extrusion of gypsum suspension containing additive on a movable flat surface. Then, the suspension can be dried in a hybrid dryer using convection heating and drying techniques. 15 microwaves For a general description of the invention, reference should be made to the figures of the drawings in which like reference numbers designate the same or similar structure through the various figures. Figure 1 illustrates one embodiment of a method 10 of this 20 invention. Process 10 includes a mixer 12 which mixes dry and wet ingredients into a gypsum suspension (not shown) and discharges the suspension formed through a die 14, which is described in more detail below. The suspension leaves the die 14 and Extrude directly on a uniform surface of a conveyor belt 16. An upper conveyor belt 18 can also be optionally used. Downstream of the conveyor belt 16 B is a hybrid dryer 20 which includes heating sections for 5 microwaves and convection drying (not shown). Downstream of the hybrid dryer 20 is a convection dryer 22 in which any excess water present in the extruded suspension is removed by convection drying to result in a dry pressed fiber core. He * hybrid dryer 20 and the convection dryer 22 are described in more 10 detail right away. • With continuous reference to Figure 1, downstream of hybrid dryer 20 and convection dryer 22, the dry pressed fiber core can undergo optional core breeding treatments in a suitable apparatus 24 known to those skilled in the art. To the degree in Since such optional core enhancement treatments include the application of water to the pressed fiber core, a surface treatment dryer 26, typically a small convection dryer, can be used to remove the water. The pressed fiber core, optionally, can be surface treated to include various coatings (ie, sheets of 20 paper cover), decorative coatings, and / or lamination in a suitable surface treatment apparatus 28 before being cut with cutting equipment 30. After the pressed fiber has been cut and formed, the pressed fiber can be stacked and tie yourself using known stacking equipment 32 for those skilled in the art and stored in a hold 34, or using other suitable storage means. Figure 2 illustrates a more detailed view of the procedure 10. n Specifically, the mixer 12 is shown in Figure 2 connected to 5 a discharge end of the mixer 36 to the die 14. The die 14 includes a die inlet 38, a die outlet 40, and a die manifold 42 disposed between the inlet 38 and the outlet 40. The die 14 preferably includes a plurality of conduits, such as for example, one ot plus conduits 44, to introduce various additives into the suspension of 10 download. The suspension is downloaded directly in the band • conveyor 16 carrying the suspension deposited to downstream processing equipment (which is described and shown in Figure 1) where the preparation of the pressed fiber product can be completed. A preferred embodiment of the invention includes the slurry preparation 15 in a mixer (which is described in more detail below), other than a pin mixer. The suspension includes gypsum, water, glidants, water reducing agents, surfactants and, optionally, binders, setting retarders (setting agents), paper pulp, fiberglass, ash spheres, mica, 20 granules of paraffin, and / or vermiculite. Preferably, a pre-generated foam is also introduced into the mixer in such a way that the foam is homogeneously distributed through the suspension; the foam is not injected into the suspension as the suspension leaves the mixer or die because the injection of foam does not necessarily ensure homogeneous distribution. A mixer suitable for use according to the invention should be capable of supplying a slurry of viscous gypsum to a given under positive displacement. The term "positive displacement" is designed to mean that a gypsum suspension, or ingredients comprising a gypsum suspension, is passed through a mixer that has no (or no excess) space in it. The phrases "empty spaces" and "excess empty spaces" are designed to mean spaces that exist within a mixer • during mixer operation that are not filled with a plaster suspension or ingredients that comprise a plaster suspension. Empty spaces (or excess spaces) within the mixer provide areas for portions of the suspension to be collected undesirably or 15"to hang". The collected suspension sits in these empty spaces and eventually hydrates (hardens). Over time, the hardened plaster is likely to detach from the voids and undesirably flow into a mixture of the suspension out of the mixer. Alternatively, the stripped and hardened plaster can damage and / or clog the mixer and, consequently, cause the operator to turn off the mixer for a sufficient period of time to fix the damaged mixer components and / or to remove the material. of hardened plaster. A mixer that operates to provide a low plaster suspension - ^ - iSí'-í *? l? X + l? &? . ** rß * **. t. M?! * ». The *. , ^ > ^ lth »MJt ^ -Jiak» jfe < «^: > ^ A ^ ^ * ^^ j ^ j ^^ $ j ^^ J ^ ^ positive displacement, by the definition mentioned above, has a minimum amount or no empty space in it. By way of example, a mixer suitable for use in the invention includes a continuous double screw mixer modified to 5 ensure a positive displacement. A double screw mixer (hereinafter a "TSC mixer") can be obtained from Readco Mfg. Co., of York, Pennsylvania, and is described in a product bulletin for Readco's continuous processor (titled the "Readco continuous processor", undated) and in the United States patent number 5,000,900, the description of which 10 is incorporated herein by reference. Although it is not recommended that said TSC mixer be used in the present invention without modifications (which is described below), the overall design of the TSC mixer is anyway informative. A TSC mixer usually includes adjacent axes of co 15 rotation (screws) arranged axially inside a single barrel or housing. The screws rotate in opposite directions with respect to each other. Each axis includes blades (or screw threads) that extend from a screw shaft to almost make contact or touch the inner surface of the barrel and to almost make contact or touch the other shaft and its associated blades 20 (that is, there should be no contact). During operation, the associated screws and vanes transport or push the materials (ingredients and / or suspension) from one end of the mixer to the other end of the mixer, which is preferably connected to a die. Additionally, during the operation, the close proximity between the vanes of an axis and its neighboring axis, and between the vanes and the inner surface of the barrel, serve to provide the TSC mixer with an auto-carving action which reduces the probability of failure or ßk plugging of the mixer. 5 In general, a TSC mixer is divided into multiple sections that can be used to perform various functions including, but not limited to, pre-mixing dry ingredients, transporting dry ingredients, pre-mixing wet ingredients, mixing dry ingredients and wet to form a mixed suspension, and 10 transportation of the suspension. In other words, the sections of the mixer • TSC are desirably designed to perform different functions as the mixed suspension is formed and passes through each section. The TSC mixer is described below with respect to imaginary sections or locations along the screw axis where the 15 raw materials are introduced. The sections are generally not separated by structure, and instead are defined by the locations in which the operator determines that several components will be added. For example, a first section of the TSC mixer preferably serves to introduce and transport various dry ingredients, such as stucco and, Optionally, vermiculite and glass fiber, which comprises a dry mix combination. A second downstream section of this first section preferably serves to transport the dry mix combination in admixture. A third section downstream of the first and second sections is preferably designed to introduce pulp water (preferably containing potash and pre-generated foam) into the dry mix combination. Here, the mixing / mixing action of the TSC mixer is relatively high shear stress ensuring a deep combination of all the ingredients (wet and dry) in a short time and on a screw shaft length. Additionally, a suitable setting accelerator, such as a dial mill accelerator, may be added to the mixer and combined with the mixed material to form the suspension. The suspension is preferably expelled from this mixing / combining section and discharged in a die (which is described herein in greater detail). Agree with this, the mix / mix section of the TSC mixer is also designed to transport the suspension to the die. A commercially available TSC mixer is preferably modified in order to ensure positive displacement of the gypsum suspension. One such modification includes ensuring that the cross-sectional area for the flow through the mixer remains constant. As noted previously, positive displacement is believed to be ensured where the mixer contains a small amount or no empty space. With a constant flow of raw materials towards the mixer and a corresponding consistent rotation of the screws, it is unlikely that the suspension will be harvested and hardened within a mixer having a constant cross-sectional area.

Another preferred modification to the TSC mixer available commercially includes elements (for example, conduits or ports of injection) through the mixer for the introduction of the various Sp dry and wet ingredients comprising the gypsum suspension. As 5 was previously noted, the dry and wet ingredients are preferably introduced into the mixer in a sequential manner. In order to introduce these ingredients in sequence and to ensure proper mixing of all suspension ingredients, the TSC mixer must be designed or modified to include a plurality of conduits or ports 10 of injection for the introduction of these ingredients. More preferably, the mixer must be modified to include ports of multiple injection 46 (refer to figure 1) to ensure that the water mixes properly with the dry ingredients. The water that is introduced into these ports 46 may be in the form of foamed water and / or pulp water.

Other mixers suitable for use in the present invention include modified versions of the mixers that are describe in U.S. Patent Nos. 5,304,355 and 5,607, • 233, which are assigned to Quantum Technologies, Inc., of Twinsburg, Ohio, and are incorporated herein by reference. The mixers that 20 are described in patents 355 and 233 are generally mixers of a single screw that provide a mixture of liquids, solids, and / or gases, and action of auto carving similar to that which is characteristic of the TSC mixer described above. It is believed that the mixers described in patents 355 and 233, when designed to include the aforementioned modifications with respect to the TSC mixer, will provide a slurry of viscous gypsum at a die inlet under positive displacement. jflp The use of a mixer modified appropriately in the The present invention is particularly attractive because the screw of the single-screw extruder (and the co-rotating screws of the TSC mixer) and the closed clearances between the blades of the mixer on the screw shaft (s), as well as between the blades and the barrel provide an efficient, uniform mixture to result in a viscous plaster suspension. For example, him The performance in the TSC mixer is preferably designed such that an average residence time of the material in the mixer is less than 20 seconds, more preferably less than 15 seconds, and even more preferably less than 10 seconds. The residence time, for purposes of the present invention, can be defined as the period of 15 time when the water makes contact with the "dry" stucco for the first time and ends when that water / stucco mixture (ie, gypsum suspension) comes out of the TSC mixer and discharges into the die or multiple die inlet. The most efficient mixture and short residence times prevent the accumulation of material (clogging or plugging) inside the barrel and 20 result in a self-tapping action that can reduce the cleaning time by as much as 90%. The gypsum suspension is extruded in soft on a substantially flat, smooth, movable surface, such as a conveyor belt covered with Teflon® material. A preferred die 14 is shown in Figure 2 and includes a die inlet 38 connected to a discharge end of the mixer 12, a die manifold 42, and an output • of the die 40. Within the die multiple 42, the suspension occupies and takes the 5 form of the manifold 42 and therefore, obtain its final cross-sectional shape before leaving the die 14 through the outlet of the die 40. Desirably, the die 14 and, more specifically, the die multiple 42 reform the suspension leaving the mixer 12 from a cross-sectional area round to the thickness and width of the finished product. 10 A suspension of plaster coming out of the mixer at the end • Discharge from the mixer 36 will find several passages of the die as it passes through the die 14. The cross-sectional area that increases, perpendicular to the direction of suspension flow, of the various passages of the die (i.e. given 38, output from die 40, and multiple from 15 given 42) are preferably constant (or constantly increasing or decreasing) to prevent the gypsum suspension from being collected undesirably and hardening inside the die. Additionally, the entrance • the die 38 preferably has a cross-sectional area that is substantially the same as that of the discharge end 36 of the mixer. The die specified for use according to a preferred embodiment of the invention deposits a gypsum suspension in such a way that the suspension exiting the die has cross-sectional dimensions substantially identical to that of the hardened pressed fiber core. This is achieved by using a die having a die exit (or opening), through which the suspension is discharged onto the movable conveyor belt, having dimensions in cross section «Fc substantially identical to that of the hardened pressed fiber core. By For example, a core of hardened pressed fiber (and also the output die) can be from about 0.6 to 2.54 centimeters thick (high), which includes specific cardboard core thicknesses of about 0. 79, 0.95, 1.27, 1.59 and approximately 1.90 cm, respectively. From 'additionally, a core of hardened pressed fiber (and die output 10 also) can be from about 1.22 to about 1.38 -W meters wide. In this way, the die can have a width to height ratio of approximately 48: 1 to 216: 1. In contrast, the discharge stations that are used in conventional gypsum fiber-making processes do not have cross-sectional dimensions in 15 this scale. The die is preferably designed to include a die outlet having a substantially rectangular geometry defined by top and bottom plates and side plates. These plates can be arranged to provide exit dimensions of the die that is 20 described in the previous paragraph. Preferably the bottom and side plates are fixed (i.e., do not move) and the top plate can be adjusted to provide the operator with flexibility in preparing pressed gypsum fibers of a variety of thicknesses. Depending on the downstream drying capacity (discussed in more detail below), the thickness of the desired pressed fiber and, consequently, the thickness of the exit opening of the die, will help determine the speed at which it can be applied. produce the pressed fiber (ie, line speed). In general, as the cross-sectional area of the cardboard decreases the line speed will increase, and vice versa. The yield (i.e., the residence time of the suspension ingredients) in the upstream mixer can be adjusted to accommodate variable line speeds, variable cross-sectional areas of varying press fiber, and downstream drying capacity. Preferably the die has one or more (preferably a plurality) of elements or conduits for the introduction (through injection means, for example) of setting accelerators and additional slip agents into the suspension, and elements or conduits at their edges external laterals for the introduction of an acrylic resistance-improving polymer (which is described below) capable of providing reinforcement to portions of the edge of the formed cardboard product. The dice suitable for use according to the invention can be obtained from a number of manufacturers of dice and, thereafter, modified to include the elements or conduits. For example, an appropriate die can be obtained with modifications from the Phoenix Engineering Group of the National Gypsum Company of Phoenix, Arizona. Because the gypsum suspension mixer, described above, typically discharges the suspension product in a downward manner, a transition conduit preferably connects the mixer discharge to the inlet of the die or manifold. The use of the phrases "soft extrusion" and "soft extrusion", is designed to mean that the plaster suspension comes out of a 5 given without the aid or application of high pressure, which is typically required to extrude highly viscous materials, such as thermoplastic and thermosetting resins that are used in other fields. Although the invention is not limited by any particular mechanism or theory, it is believed that the ingredients (both dry and moist) that enter the mixer help 10 to force the suspension through the mixer, and towards the manifold, and through • the output of the die. The ingredients that are introduced into the mixer together with the positive displacement of the suspension by the mixer, exert a force on the suspension passing through the die, such that the suspension leaves the die under a pressure of about 35 to About 690 kiloPascals (kPa) and preferably from about 35 to about 207 kPa, more preferably from about 69 to about 137 kPa. In general, the appearance and physical consistency of the suspension extruded in soft, it seems that of cream ice cream, as opposed to a cake or 20 pancake shape. Accordingly, the deposited gypsum suspension has a desirable viscosity of about 19,000 millipascal »seconds (mPa * s) to about 22,000 mPa» s. However, the deposited gypsum suspension preferably has a viscosity on a scale of approximately 19,500 mPa »s at approximately 21,500 mPa» s. The viscosity measurements mentioned above can be obtained using a Brookfield DV-III programmable rheometer with a number 6 spindle operating at approximately 20 revolutions per minute (RPM). 5 Once extruded on the conveyor belt, the suspension is chemically activated to set and form a hardened cardboard core which can be easily removed from the conveyor belt, dried by means of a combination of microwave heating and convection drying means , laminate, and cut to a shape and 10 desired size, as described in detail below. • According to the invention, the gypsum suspension desirably has an open time (i.e., a work time) of less than about 30 seconds, preferably less than about 20 seconds, and more preferably less than 15 approximately 10 seconds. The term "open time" as used herein refers to the time that elapses between (a) the exposure to the atmosphere of the gypsum suspension, and (b) the point at which the calcined gypsum has reacted in a manner enough with the water present in the suspension to form the dihydrate. The open time of a plaster suspension is 20 can measure by conventional procedures known to those skilled in the art. Preferred ingredients of the pressed fiber core composition will be described in more detail below. In general, a preferred method for manufacturing the core and pressed fiber composition of the invention initially includes the pre-mix of dry ingredients in a mixing apparatus (eg, the modified TSC mixer). The dry ingredients may include calcium sulfate hemihydrate (stucco), an accelerator and, optionally, an anti-decay (eg, starch), as described in more detail below. The dry ingredients are mixed together with a "wet" (aqueous) portion of the core composition in the mixing apparatus. The wet portion may include a first component (called a "paper pulp solution") that preferably includes a mixture of water, paper pulp, and, optionally, agents for increasing fluidity. A setting retarder may also be included. Most of the water present in the suspension is introduced through the pre-generated foam. Another source of water in the suspension is the paper pulp solution. Other sources for the introduction of water in the suspension include, but not limited to, mixtures of the reinforcement agent mentioned above, and other conventional additives, when used. A main ingredient of the pressed fiber core composition of the invention is calcium sulfate hemihydrate, or stucco (CaSO4-1 / 2H20). Calcium sulfate hemihydrate is generally described by petersen, D.J. et al, "Calcium Compounds (Calcium Sulfate))", Encyclopedia of Chemical Technology, (1992, 4th ed.), vol 4, pp 812-26, TP9.E685, the disclosure of which is incorporated herein by reference, and It can produce by the methods described above. As is known to those skilled in the art, there are two types of calcium sulfate hemihydrate, an α-hemihydrate form and a β-hemihydrate form. These two forms are typically produced by different types of calcination procedures and differ structurally to some degree. Any type of calcium sulfate hemihydrate, however, is suitable for use in the present invention. The stucco is preferably present in the extruded gypsum suspension composition from the die outlet in an amount of about 59 to about 64% by weight based on the total weight of the gypsum suspension composition, more preferably from about 60 to about 63% by weight, for example from about 61 to about 62% by weight. An optional anti-drying agent, such as starch, can also be included to provide increased resistance to the finished pressed fiber product, however, it is not necessary for the success of the invention. Accordingly, the anti-drying agent can be present in an amount of less than about 2.1 kg / m2. In some products, optional light weight aggregates (eg, perlite or vermiculite) may also be included. An aqueous suspension or paper pulp solution can also be included in the core composition. The pulp solution comprises water and paper fibers ("paper pulp"), and may also include an optional binder, a setting retarder (or setting agent), corn starch, and / or potash. Optional binders that can be used include, but are not limited to, inorganic binders such as, for example, colloidal silica and colloidal alumina. It may be desirable to use a suitable setting retarder when an inorganic binder is present in order to provide moisture resistance and, therefore, avoid sacrificing strength due to the presence of moisture. The setting retarder can be used to tailor the setting time of the core composition. Setting retarders are typically used in the invention at very low rates (if they are used), for example at about 0.0007% by weight, based on the weight of the core composition. The terms "setting retarder" and "setting agent" are used herein to include any substance that will react with the stucco to form an insoluble complex. A class of such setting agents that can be used in the present invention comprises divalent or trivalent metal compounds, such as magnesium oxide, zinc oxide, calcium carbonate, magnesium carbonate, zinc sulfate, and zinc stearate. The paper pulp solution may also include one or more of a number of additives that increase the fluidity of the suspension and / or reduce the water requirements of the suspension. Materials that are used as agents to improve fluidity and / or reduce water include "lignosulfonates" which are commercially available in liquid form k? .A A f ** JflÍ * l - * ..- ¿AuiSÍü? ? - * - t * t. * Í. or in powder. The agents for improving the fluidity and / or reducing the water supplied in liquid form can be incorporated into the solution or added directly to the mixing operation. The paper pulp solution can be prepared by combining or mixing the above ingredients with water in a mixing apparatus. Alternatively, a concentrated pulp solution can be produced using only a small volume of water. In this case, the rest of the water requirements of the core mixture is formed with water from a separate water source, such as a pre-generated foam, for example. In contrast to conventional gypsum suspensions, a large amount of water is preferably not included with respect to the rehydration reaction described above. Typically, about 15 to about 20 parts by weight of pulp water are used per 100 parts by weight of stucco. In contrast, conventional stuccoed fiber press manufacturing processes utilize a large excess of more than 40 parts by weight pulp water per 100 parts by weight stucco. Preferably, high shear mixing "pulps" the material, forming a homogeneous solution or suspension. The pulp solution can be transferred to a containment vessel, from which it can be added continuously to the mixture of the core composition. The paper fibers in the pulp solution can serve to improve the flexibility of the pressed gypsum fiber. The pressed gypsum fiber made without fibers can be very brittle and more susceptible to fracture during handling. Paper fibers also aid in the uniformity of drying during manufacturing, as well as improving the ability of the final pressed fiber product to accept and hold nails during installation. As indicated above, the wet portion of the core composition may also include a component that incorporates a pre-generated foam (eg, air entrapped in soap bubbles). The foam introduces air voids in the core through the use of a foam that contains very little solid material, but which is sufficiently flexible to withstand substantial decomposition in the mixing operation. In this way, the density of the core can be controlled. Known foaming agents can be supplied in liquid or flaked (powder) form, and can be produced from soaps known in the art. Methods for preparing a pre-generated foam are generally known to those skilled in the art. It is believed that the foam can be generated in situ within a modified version of mixers which are described in the U.S. Patents. Nos. 5,304,355 and 5,607,233, described above, if desired, by the suitable addition of soaps and surfactants. One or more slip agents are preferably included to reduce drag as the suspension is discharged into the die and extruded therefrom. The slip agent is added to the core mixture in an amount of about 0.4% by weight at about 1.5% by weight, based on the weight of stucco present in the core mixture. Preferably, the slip agent is added to the core mixture in an amount of about 0.4% by weight to about 1.0% by weight, based on the weight of the stucco present in the core mixture. Even more preferably, the slip agent is added to the core mixture in an amount of about 0.7% to about 0.8% by weight, based on the weight of the stucco present in the core mixture. The slip agent can be introduced into the suspension as it passes through the die by means of injection parts or ducts of the die. A suitable slip agent for use in the present invention is Zelec NE which is commercially available from Stepan Chemical Co., of Fieldsboro, New Jersey. A strength improving agent is preferably added to the gypsum suspension as it passes through the die outlet. Preferably, the strength improving agent is added to the suspension portion that will eventually form the edge portions of the pressed fiber. The strength improving agent of the invention preferably includes, and may consist essentially of, an acrylic polymer emulsion having certain preferred properties, as described below. When added to the pressed gypsum fiber core composition, the acrylic polymer emulsion can provide significantly increased core strength, paper-to-core bonding, and other physical properties. Accordingly, the cardboard density can be reduced while still maintaining other desirable physical cardboard properties. Additionally, when added in concentrated amounts to edge portions of the cardboard core, the agent can serve to greatly improve the strength of said edge portions. Strong edge portions are particularly desirable since it is at these edge portions that the pressed fibers are secured to struts by nails or screws, for example. Although the invention is not limited by any particular mechanism and the mechanisms that achieve the benefits of the invention are not currently clearly understood, it is believed that the acrylic polymer deposits itself in the contact areas between calcium sulfate dihydrate crystals. . Suitable additives and methods for using same are described in the US patent. No. 5,879,825 assigned to the assignee of the present application, the description of which is incorporated herein by reference. An important factor in selecting the acrylic polymer emulsion is the glass transition temperature, or Tg "of the acrylic polymer emulsion.The glass transition temperature is the temperature at which an amorphous material changes from a brittle vitreous state to a plastic state Many polymers such as acrylics and their derivatives have this transition point, which may be related, at least in some cases, to the number of carbon atoms in their ester groups.The polymer emulsion must have a temperature glass transition (Tg) of about 15 ° C or larger, and i *, .3 J, preferably on a scale of about 15 ° C to about 60 ° C, more preferably on a scale of about 20 ° C to about 60 ° C, and more preferably on a scale of about 35 ° C C at approximately 60 ° C. It has been found that polymer emulsions having Tg values substantially below about 15 ° C undesirably provide a core that forms a vapor moisture transmission barrier in the evaporation plane. The evaporation plane is the location at or below the core surface where the water drawn to it evaporates during the drying process. A moisture transmission barrier will form if the polymer forms a film that prevents water inside the pressed gypsum fiber from evaporating in a reasonable period of time. Said film would make it substantially more difficult to dry the pressed fiber cores of gypsum, causing greater energy and cost requirements for the drying process. Therefore, it is not desirable to form a film in the pressed fiber core. The invention therefore allows the use of commercially manufactured apparatuses and installations. Certain acrylic polymer emulsions are more stable than others in an aqueous calcium sulfate environment encountered during the production process of pressed gypsum fiber. Because the divalent calcium ions in the aqueous suspension can affect the adverse to the performance of some polymer emulsions, polymer emulsions must be formulated to be stable to calcium ions. By way of example only, the acrylic polymer may have a molecular weight on the scale of about 300,000 to about 700,000, although it is believed that this scale is variable. Acrylic polymers having other molecular weights are useful with the invention. The acrylic polymer can be interlaced or non-interlaced. The polymers which are used with the invention are preferably neutralized with sodium hydroxide (NaOH) or other non-volatile neutralizing agent, and more preferably neutralized with an agent consisting of a non-volatile neutralizing agent. The ammonium hydroxide is preferably not included in any substantial amount in the neutralizing agent for the acrylic polymer, because substantial amounts may adversely affect the product. More preferably, the neutralizing agent is substantially free of ammonium salts or other source of ammonia. Various acrylic polymer emulsions suitable for use with the invention are commercially available. For example, suitable polymer emulsions are available from Rohn & Haas Company of Philadelphia, Pennsylvania under the trade name Rhoplex (for example Rhoplex 55-521, Rhoplex E-2409, and Rhoplex B-1162). Other polymer emulsions in the Rhoplex line have been designated by Rohm & Haas as RG 2718, RG2719, RG2721, and KAK 1868. Other suitable polymer emulsions are available from The Dow Chemical Company, of Midland, Michigan. The polymer emulsions can include from about 20 to about 80% by weight of an acrylic polymer, 5 about 20 to about 80% by weight of water, about 0.3% by weight or less of aqueous ammonia, and less than about 0.1% by weight of residual monomers. The emulsions can have a pH on the scale of about 2.1 to about 11.0, and a specific gravity on a scale of about 1.0 to about 10 1.2. • The strength improving agent of the invention is preferably included in a ratio on a scale from about 0.25 to about 2.5% solids, more preferably from about 0.5 to about 2.0% solids, and more preferably from about 0.5 to about 1.0% solids, based on the weight of rehydrated gypsum in the final product. An advantage of the invention is that the suspension can be prepared in the mixer without incorporation of a setting accelerator in the suspension until the mixing process is substantially complete. The setting accelerator is introduced into the gypsum suspension composition substantially close to the discharge end of the mixer (eg, after all other suspension ingredients have been added to the mixer). The setting accelerator is used to control, within certain limits, the speed of crystal growth and the setting time of the stucco. Examples of suitable accelerators include dial mill accelerators ("BMA") and potassium sulfate, although many others are known to those skilled in the art. In some cases, the invention may require increasing amounts of accelerator due to the retarding effect of some strength enhancing additives. The addition of the setting accelerator downstream of the mixing apparatus eliminates the likelihood that the setting reaction of the gypsum 10 occurs prematurely within a portion upstream of the • mixing device. Additionally, the probability that the processing equipment upstream of the extrusion die will clog and / or become clogged with the suspension is also greatly reduced by the addition of the setting accelerator to the discharge end of the extrusion die. 15 mixer. Another advantage of the invention is that the gypsum suspension prepared within the mixing apparatus can be supplemented by the addition of additional gypsum suspension additives via the periphery of the die. These agents include, but are not limited to, the 20 slip agents mentioned above and the acrylic polymer for strength improvement. Additional advantages of the invention include, but are not limited to, the elimination of suspension discharge means. ttt. and. jan-1-.tHÍft-. ^, Mkt? * T. * Conventional, such as unloading boxes, hard edge mixers, forming plates, rolling bearings, bars (spatulas), straighteners, and edge tape. Additionally, because the suspension can be extruded in soft directly on the conveyor belt, it does not exist 5 the typical need for roofing paper sheets to wrap the suspension, which are required when using conventional, less viscous plaster suspensions (plasta type). According to the invention, the use of those sheets of cover paper can be deferred to a point in the * pressed fiber manufacturing process where the pressed fiber already 10 has dried. The produced core composition (ie the aqueous gypsum suspension) is deposited directly on a smooth, movable conveyor belt in a continuous manner. The core composition is allowed to cure or set, whereby calcium sulfate hemihydrate is converted to dihydrate 15 of calcium sulfate. In one embodiment, as the core suspension is deposited on the smooth, movable surface conveyor belt, the upper surface of the exposed core suspension can be sprayed with a chemical activator (or accelerator). The chemical activator accelerates the setting reaction in a manner similar to that used with calcined gypsum 20 to "brown" plaster. The chemical activator acts preferably fast enough to accelerate the setting of the deposited suspension mass and must provide an induction time before the suspension begins to form crystals, however allowing the crystallization to occur after the induction time. Next, the product is preferably dried to remove water that has not been consumed in the setting reaction (ie, the reaction that forms the calcium sulfate dihydrate). When the board is removed from the smooth surface conveyor belt, the board preferably has a dense, glassy surface, which, when dried, provides an excellent surface for lamination of desired surface materials as described in more detail below. In contrast to traditional methods, the present invention preferably does not require, and preferably does not use, high levels of processing water to lower the viscosity of the suspension during production. Because the present invention utilizes a soft extrusion step, the need for high water levels is eliminated, since fluidity and viscosity are not as much a problem as in a conventional process. Additionally, the present invention eliminates the use of cover paper sheets to wrap the gypsum suspension and differs the application of these sheets, if they are used, until after the suspension has dried to form a pressed fiber core. hardened. Conventional press fiber forming processes typically require removal of approximately 3900 kilograms per 92.9 square meters (kg / m2) to approximately 4150 kg / 92.9m2 of water from a suspension wrapped by sheets of cover paper. This water has to be evaporated through the paper cover sheets in expensive, high energy drying steps, which require from about 36 to about 40 minutes to remove the water. Conversely, in a preferred embodiment of the invention, less than 3417 kg / 92.9m2 of water must be removed. More preferably, less than about 2925 kg / 92.9m2, and even more preferably less than 1950kg / 92.9m2, should be removed. Accordingly, the invention provides a reduction in the use of processing water from about 12.5% to about 53%, preferably from about 12.5% to about 25%, when compared to conventional gypsum pressing fiber manufacturing processes. With less water to be evaporated, costly, high-energy dryers are no longer necessary. On the other hand, it has been discovered that with only 2.925 kg of water / 92.9 m2 to be evaporated, a hybrid dryer using techniques of / f * microwave heating combined with convection drying can be used. This advanced hybrid dryer provides substantial cost and energy savings over conventional dryers that are used to evaporate large quantities of water. The hybrid dryer includes concurrent convection drying and microwave drying drying capabilities. He i? Microwave heating serves to heat the water present in the pressed fiber core to a temperature of about the water vaporization temperature, and preferably causes the heated water to migrate from the core to the outer surfaces of the core of the water? pressed fiber. As water migrates to exterior surfaces, water can be more easily removed by convection drying in the hybrid dryer, and when the pressed fiber core passes through the convection dryer downstream. These hybrid dryers are typical in cereal manufacturing processes where they are used to dry wet cereals. Hybrid dryers can not be used in general in conventional gypsum pressing fiber manufacturing processes because microwave heating would likely cause the paper cover sheets to fly out of the core. However, in the present invention, such paper cover sheets are preferably not used to wrap the viscous gypsum slurry and, therefore, the use of a hybrid dryer is possible. A downstream convection dryer can also be used in-line to dry the pressed fiber core of uncut gypsum. The convection dryer preferably includes a number of drying zones each of which can provide heat at different temperatures depending on the amount of moisture present in the pressed fiber core. The infrared humidity detectors can be placed inside several areas of the convection dryer to control the temperatures found in each zone. Preferably, the drying of the board is controlled by computer. The conventional gypsum fiber press manufacturing processes use convection drying ovens which are responsible for the simultaneous drying of a number of pressed fibers of 5 plaster pre cut and stacked vertically. Because the pressed fiber core prepared in accordance with the present invention is dried in the absence of conventional paper cover and prior to cutting, better drying control can be achieved. According to the present invention, using the dryer 10 hybrid mentioned above and convection dryer, time • required to remove approximately 2.925 kg water / 92.9m2 can be achieved in less than about 15 minutes, preferably less than about 12 minutes, more preferably in about 10 to about 12 minutes, and even more preferably about 6 minutes. 15 to about 7 minutes. The lack of paper cover sheets and the use of the hybrid dryer allow such short drying times to be achieved. The pressed gypsum fiber can be adapted for wet and outdoor applications, in addition to being used in the construction of interior walls and ceilings. In the production of outer covers and cores 20 of moisture-resistant cardboard, various materials can be incorporated into the core to impart to the board, for example, fire resistance and / or increased resistance to water absorption. Useful materials include silicone water repellents, mica, paraffin granules, ash spheres flying, perlite, vermiculite, waxes and asphalt emulsions. These materials are typically supplied as water emulsions to facilitate incorporation into the cardboard core. These materials can be added directly into the mixing apparatus or incorporated into the pulp solution 5 before the addition to the mixing apparatus. Additionally, some core treatments, such as, for example, silicone water treatments, can be applied to the pressed fiber core downstream of the hybrid dryer and the convection dryer. To the degree that these core treatments introduce water into the pressed fiber core, 10 can use a convection dryer running below the station • treatment to remove the unsuitable water. The soft and dry extruded gypsum core comprises calcium sulfate dihydrate in an amount in the scale of at least about 90 weight percent (% w.), Based on the total weight of the product. 15 core. Preferably, the dry core comprises at least about 95% by weight of calcium sulfate dihydrate, and more preferably at least about 99% by weight based on the weight ^ F total of the core, according to this, the dry density of the core, which can be determined by standard techniques known to those skilled in the art.

The technique is about 800.9 g / liter or less, preferably about 720.8 g / liter or less, and more preferably about 640.7 g / liter (ie, 595.2 kg / m2) or less.

The high speed lamination equipment can be used to adhere the desired surface materials to the dry, hardened core. This can be achieved simultaneously in line to both surfaces of the cardboard. The ?. - Suitable surface materials can include a variety of films and 5 decorative papers depending on the final use. Additionally, prior to lamination, the core may be impregnated with strength improving agents, water repellent agents, and / or other treatments. As noted above, to the extent that those agents and / or treatments introduce water into the core, a convection dryer 10 downstream can be used to remove the water prior to rolling and / or cutting / trimming.

• Modern trimming equipment that typically could not be used in prior paperboard forming processes due to the presence of cover sheets and the associated problems of (cover sheet) and release, are suitable for use in the present invention. Additionally, modern trimming equipment is preferred for cutting and trimming cartons made by the method of the present invention because the cartons do not have burned (ie over calcined) edges that can damage the equipment. The use of conventional cover sheets to wrap the The gypsum suspension core is removed by the present invention. However, these cover sheets can be applied with a suitable adhesive (eg, polyvinylacetate binder) to a dry pressed fiber core. When the cardboard is going to be covered by a vinyl finish or metallic, the paper cover sheets are not necessary and the vinyl or metallic finish can be applied directly on the surface of the dry pressed fiber core. Accordingly, the present invention reduces material costs by not requiring the use of cover sheets in certain instances. In addition to cost savings, there are other procedural advantages that can be realized by reducing or eliminating the need for paper cover sheets. For example, conventional continuous procedures often encounter periods in which the procedure must be temporarily turned off to meet sheets of 10 cover jammed or improperly deployed, without tip, blasted, of poor porosity, and unpaired papers for drying. Because these cover sheets are no longer needed in certain applications, such shutdown periods are not likely to occur. Even when the paper cover sheets are desired, their application to a core 15 of pressed fiber is deferred until after the core has dried, thereby avoiding the complexities associated with the application of paper cover sheets to a wet, lump-like suspension. Additional advantages of the invention include, but are not limited to, the elimination of suspension discharge means. 20 conventional, such as discharge boxes, hard edge mixers, forming plates, energy bearing or smooth bars, and edge tape. Without the need for such conventional suspension discharge means, the present invention provides a reduction in capital costs associated with the manufacture of pressed gypsum fiber.

• EXAMPLES 5 The following examples are provided to illustrate the invention but are not designed to limit the scope of the invention. Examples 1-3 describe viscous gypsum suspensions which can be prepared in a suitable mixer and supplied to a die under positive displacement for extrusion on a conveyor belt Movable surface of smooth surface according to the present invention. Additionally, Examples 1-3 are illustrative of various pressed fiber cores and core weights that can be achieved, and the low amount of water present in the gypsum suspensions. In contrast, Comparative Example 4 describes a low viscosity gypsum slurry prepared according to conventional gypsum pressing fiber manufacturing processes. Example 5 illustrates acceptable viscosities for gypsum suspensions.

EXAMPLE 1 This example is directed to a viscous gypsum suspension composition that can be prepared in a suitable mixer and to the water content therein. This example illustrates one embodiment of the invention that Ii *. ^: ¡- r, f ». *. - - * - * ... - ..- .. ...... -., -:. *. i. *:. ,. .-. .. t.-t. *. -...,. & *. A .. - .., - ... .¿ ** ..- * -.- * Ía? ¡> í uses a high stucco level (ie 63 08% stucco, based on the weight of the gypsum suspension). Gypsum rock is used that has a purity of 89.0 to • prepare a pressed fiber having a core weight of approximately 5 8.070 kg / m2 of 1.19 cm thick cardboard. The molecular weight (gram) of the calcined gypsum corresponding to a gypsum rock purity of 89.0 is 193.45. Table I below provides the composition of a gypsum rock (ie, the molecular weights (grams) for each of the dihydrate, hemihydrate, and calcium sulfate anhydrate forms, and also for 10 any inerts present in the mixture). • TABLE I fifteen F twenty Based on the gypsum rock purity mentioned above and the target weight of the pressed fiber core, a gypsum suspension is prepared containing the ingredients listed in Table II below: TABLE II 10 fifteen = weight based on 453.0 kg of stucco. = weight after conversion to plaster based on 453.0 kg 20 stucco. The above formulation is calcined to convert all the dihydrate to hemihydrate with slight overcalcination and minimal sub calcination. According to this, a combined moisture level rehydrated from the AJ'ÍÍ ?? * ÁÍ? ¡Í. ~ I- ±. ? á á .. - ?. The suspension is reduced from 18.63% to a combined stucco humidity level of approximately 4.91%, which corresponds to a water loss of approximately 262.2 kg / 92.9 m2, of a 1.19 cm paperboard of ^^^^^^^^^^^^^^ thick. This water was removed according to the invention in a dryer by 5 convection in less than about 12 minutes. This water can be removed according to the invention in a hybrid dryer (containing microwave heating and convection drying means) followed by a multi-zone convection dryer is less than about 10 minutes. 10 • EXAMPLE 2 This example is directed to a viscous gypsum suspension composition that can be prepared in a suitable mixer and to the content 15 of water in it. This example illustrates another embodiment of the invention using a parlarly preferred amount of stucco (i.e., 61.62% stucco, based on the weight of the gypsum suspension). A gypsum rock having a purity of 89.0 is used to prepare a pressed fiber having a core weight of about 20 713.4 kg / 92.9 m2 of cardboard of 1.19 cm thick. The molecular weight (gram) of the calcined gypsum corresponding to a gypsum rock purity of 89.0 is 193.45. Table I in Example I, above, the composition of the gypsum rock.

.TO _. tfejtá.l Based on the purity of the gypsum rock mentioned above and the target weight of the pressed fiber core, a suspension is prepared which contains the ingredients listed in Table III below.

• TABLE III 10 fifteen = weight based on 453.0 kg of stucco. 20 * = weight after conversion to plaster based on 453.0 kg of stucco. The above formulation is calcined to convert all the dihydrate to hemihydrate with slight overcalcination and minimal sub calcination. From i, -. llllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll - .. * -. *?. * .- * ^,.? i? - -. .- *, * * -, Jaffa * l According to this, a combined moisture level of the slurry is reduced from 18.63% to a combined stucco moisture level of approximately 4.91%, which corresponds to a water loss of approximately 272.2 kg / 92.9 m2, of a cardboard of 1.19 cm of 5 thickness. This water was removed according to the invention in a convection dryer in less than about 12 minutes. This water can be removed according to the invention in a hybrid dryer (containing means of microwave heating and convection drying) followed by a multi-zone convection dryer of less than about 10 minutes. • EXAMPLE 3 This example is directed to a suspension composition of 15 viscous plaster that was prepared and the water content in it. This example illustrates yet another embodiment of the invention using a low amount of stucco (ie, 59.39% stucco, based on the weight of the gypsum suspension). A gypsum rock having a purity of 90.0 to 20 is used to prepare a pressed fiber having a core weight of approximately 660.9 kg / 92.9 m2 of 1.19 cm thick board. The molecular weight (gram) of the calcined gypsum corresponding to a gypsum rock purity of 90.0 is 191. 3. Table IV, next, provides the composition of the gypsum rock (is i? ¡Ai-ír *? bí * ^ & * * .- x-.í ^ .- i¡- .í-. ~ ** ..--. . - ... J -, * • -, ... * & t *. * - .., ...: .. **. • .. - ,, .. ^., -? * -....% .. * - .. *. £,. ** ,, -... ** * mé..t¡i ~ say, the molecular weights (grams) for each of the forms of dihydrate, hemihydrate, calcium sulfate anhydrate, and also for any inerts present in the mixture).

• TABLE IV 10 • Based on the purity of the gypsum rock mentioned above and the target weight of the pressed fiber core, a suspension containing the ingredients listed in Table V below was prepared. í -? AÁjk.-i • - * - & &? -? -.- jmMivL ± i. -..., i ^ *? **. * -; -. S- * .Í1 & - Í A pA TABLE V * = weight based on 453.0 kg of stucco. ** = weight after conversion to plaster based on 453.0 kg of stucco. The above formulation is calcined to convert all the dihydrate to hemihydrate with slight overcalcination and minimal sub calcination. Accordingly, a rehydrated combined moisture level of the suspension is reduced from 18.83% to a combined stucco moisture level of approximately 4.68%, which corresponds to a water loss of approximately 282.2 kg / 92.9 m2, of a 1.19 cm carton . ^ -, thick. This water was removed according to the invention in a convection dryer in less than about 12 minutes. This water can be removed according to the invention in a hybrid dryer (containing • means of microwave heating and convection drying) 5 followed by a multi-zone convection dryer is less than about 10 minutes.

COMPARATIVE EXAMPLE 4 p 10 This example is directed to a viscous gypsum suspension composition which was prepared in a spike mixer in accordance with conventional gypsum press fiber manufacturing practices and the water content therein. A gypsum rock having a purity of 89.0 was used to prepare a pressed fiber having a core weight of approximately 753.7 kg / 92.9 m2 of 1.19 cm thick board. Each of the two sheets of paper covering the suspension of wet gypsum EPH before setting has a thickness of about 0.034 cm. Agree with this, a finished pressed fiber of this example had a thickness of approximately 1.27 cm. Based on the purity of the gypsum rock mentioned above and the target weight of the pressed fiber core, a suspension containing the ingredients listed in Table VI below is prepared. The gypsum composition had too low a viscosity to be used in a positive displacement mixer Mljt 'i - * tiakÍ? 1, Í, -S- J Íá * at & .íí *?, .-: ..: ^ ítl, -.í ... ** .- * »and ~ ** r ** .í. Wm.?? I ?, -. Jt ** t * ^!? * »F * J. «T *. ** íía * .ü. Tlflfc-i ^ A- of the type described herein and not suitable for extrusion by means of a die.

TABLE VI = weight based on 453.0 kg of stucco. = weight after conversion to plaster based on 453.0 kg of stucco. The above formulation is calcined to convert all the dihydrate to hemihydrate with slight overcalcination and minimal sub calcination. According to this, a combined moisture level rehydrated from the suspension was reduced from 18.63% to a combined stucco moisture level of approximately 4.91%, which corresponds to a water loss of approximately 384.5 kg / 92.9 m2, from a pressed fiber of 1.27 cm thick in this case. This water was removed according to the invention in a convection dryer in less than about 12 minutes. In order to remove this amount of water using a conventional method of making pressed gypsum fiber (i.e., a conventional convection drying oven) the drying time was from about 36 to about 40 minutes. The following Table VII summarizes the composition of the various suspensions prepared in each of Examples 1-4, and Comparative Example 5, including the weight of the cardboard core and the water content.

TABLE VII • 10 fifteen ^ Weight includes 453.0 kg / 92.9m2 of paper cover sheets that are required in conventional fiber-forming operations 20 pressed. Based on Examples 1-3 and Comparative Example 4, it is evident that the pressed gypsum fiber cores made according to the present invention contain much less water (for example, from aaitíAa *, ¿A.i L approximately 262.2 to approximately 282.2 kg / 92.9m2) compared with pressed gypsum fibers made according to conventional procedures (e.g., approximately 384.5 kg / 92.9m2). He • Reduced water content is an advantage that aids downstream drying operations and improves the overall efficiency of the pressed fiber manufacturing process.

EXAMPLE 5 10 This example demonstrates the viscous nature of a • gypsum suspension that can be prepared according to the present invention using a suitable mixture and low water content and compares the viscosity of the suspensions with those of gypsum suspensions unsuitable for use in the present invention. 15 Settling tests were performed to determine the viscosity of gypsum suspensions capable of being prepared in a suitable mixer operating under conditions sufficient to provide the suspension at a die inlet under positive displacement. The tests were conducted after submerging a cylinder of 20 bronze having a wall thickness of approximately 0.17 cm, a height of approximately 10.16 cm, and an inner diameter of approximately 5.08 cm, in a bath of low viscosity lubricating oil. The cylinder was removed and excess oil drained from the surfaces of the cylinder. He i¡Í: lt it ¿.. Í * - **** íj í. t. - The cylinder was then placed erect on the central portion of a clean dry glass plate (it is dedr, without scratches), which has the following dimensions: approximately 25.4 cm long, approximately 25.4 cm • wide and approximately 0.47 cm thick. 5 The mixed suspension was poured into the cylinder in such a way that the cylinder was completely filled with a slight excess. The spoon can be a clean spoon of metal or plastic of suitable size or it can be formed from disposable plaster core paper. The excess was capped to a level with the top of the cylinder without draining any of the suspension on the surface of the glass plate. Within about 10 seconds of removing the excess suspension, the cylinder was raised vertically with a smooth and even movement, and the suspension contained within the cylinder was allowed to settle in a circular cake on the surface of the glass plate. After the suspension solidified, the glass plate was turned over and the diameter of the suspension in contact with the glass plate was measured at the nearest 1.25 cm. p A Brookfield DV-lll Programmable Rheometer with a spindle No. 6 was used to determine the viscosity of the settlements of the various gypsum suspensions. The viscosity of the various settlements is reported in the following Table VIII, based on the revolutions per minute (RPM) of the spindle and the% torque: ii? i TABLE VIII • In general, gypsum suspension compositions suitable for use in the present invention include those having a viscosity of at least about 19,000 mPa »s, preferably from about 19,000 to about 22,000. 15 mPa «s, and more preferably about 20,000 mPa * s, as measured above 20 RPM. Additionally, gypsum suspension compositions suitable for use in the present invention include those having a viscosity of at least about 29,300 mPa »s, as measured at 10 RPM, at least about 8,800 mPa» s, as 20 measured at 40 RPM, and at least approximately 6,200 mPa »s, as measured at 60 RPM. The above description is given only for clarity of understanding, and unnecessary limitations should not be understood from the same, since modifications within the scope of the invention may be apparent to those skilled in the art. '^ F ---Mr LÍJÍAÜ '.i.iii * -.- -. . ... "."to, *..*.*. * ??? Í ¿i

Claims (39)

1. NOVELTY OF THE INVENTION CLAIMS 1. A method for forming a pressed fiber core of gypsum, characterized in that it comprises the steps of: (a) preparing a gypsum suspension in a mixer; (b) introducing the suspension in an extrusion die, the die has elements in its periphery for the introduction of at least one gypsum suspension additive selected from the group consisting of • 10 slip agents and resistance improving agents; (c) introducing the suspension additive into the suspension as the suspension leaves the die; and (d) extruding the gypsum suspension containing additive from the die onto a substantially smooth, movable surface, to form a core of pressed gypsum fiber. 2. The method according to claim 1, further characterized in that it further comprises the step of: (e) drying the suspension obtained from step (d) in a hybrid dryer by means of convection heating and microwave drying techniques. to form a dry core of pressed gypsum fiber. 3. The method according to claim 1, further characterized in that in step (d) the suspension is extruded in soft from the die at a pressure of about 0.35 kg / cm2 to about 7.03 kg / cm2. ... &? £? R -. 4. - The method according to claim 3, further characterized in that the pressure is from about 0.35 kg / cm2 to about 2.10 kg / cm2. 5. The method according to claim 4, further characterized in that the pressure is from about 0.70 kg / cm2 to about 1.40 kg / cm2. 6. The method according to claim 1, further characterized in that the suspension extruded in step (d) has a viscosity of about 19,000 millipascal-seconds (Pa »s) at about 22,000 mPa» s as determined by a rheometer Brookfield DV-lll Programmable using a spindle number six at 20 RPM. 7. The method according to claim 6, further characterized in that the suspension extruded in step (d) has a viscosity of about 19,500 mPa »s about 21,500 mPa« s as determined by a Brookfield DV rheometer. -lll Programmable using a spindle number six at 20 RPM. 8. The method according to claim 7, further characterized in that the suspension extruded in step (d) has a viscosity of about 19,500 mPa »s about 21,000 mPa« s as determined by a Brookfield DV rheometer. -lll Programmable using a spindle number six at 20 RPM. i.jj 9. - The method according to claim 1, further characterized in that the gypsum suspension has an open time of less than about 30 seconds. 10. The method according to claim 9, further characterized in that the open time is less than about 20 seconds. 11. The method according to claim 10, further characterized in that the open time is less than about 10 seconds. 12. The method according to claim 1, further characterized in that the suspension that is prepared in step (c) comprises about 59% by weight to about 64% by weight of calcium sulfate dihydrate, based on the weight total of the suspension. 13. The method according to claim 12, further characterized in that the suspension that is prepared in step (c) comprises about 60% by weight to about 63% by weight of calcium sulfate dihydrate, based on the weight total of the suspension. 14. The method according to claim 13, further characterized in that the suspension that is prepared in step (c) comprises from about 61% by weight to about 62% by weight of calcium sulfate dihydrate, based on the weight total of the suspension. 15. The method according to claim 1, further characterized in that the suspension that is prepared in step (a) comprises a material selected from the group consisting of: calcium sulfate hemihydrate, antidesecants, gypsum setting accelerators, water, foam, paper pulp, increase agents of fluidity, binders, slip agents, setting retarders, and pre-generated foam. 16. The method according to claim 1, further characterized in that the suspension that is prepared in step (a) comprises from about 35% by weight to about 40% by weight of water, based on the total weight of the suspension . 17. The method according to claim 2, further characterized in that less than about 317.1 kg per 92.9 m2 of a 1.27 cm board is removed from the suspension water during the drying step (e) to form a dry core of pressed gypsum fiber. 18. The method according to claim 17, further characterized in that less than about 271.8 kg per 92.9 m2 of a 1.27 cm board is removed from the suspension water during the drying step (e) to form a dry core of pressed gypsum fiber. 19. The method according to claim 18, further characterized in that less than about 181.2 kg per 92.9 m2 of a 1.27 cm cardboard are removed from the suspension water during the drying step (e) to form a dry core of pressed gypsum fiber. 20. The method according to claim 17, further characterized in that the drying step (e) is carried out in a time of less than about 15 minutes. 21. The method according to claim 20, further characterized in that the drying step (e) is carried out in a time from about 10 minutes to about 12 minutes. 22. The method according to claim 21, further characterized in that the drying step (e) is carried out in a time from about 6 minutes to about 7 minutes. 23. The method according to claim 2, further characterized in that the dry core of pressed gypsum fiber comprises at least about 90% by weight of calcium sulfate dihydrate, based on the total weight of the core. 24. The method according to claim 23, further characterized in that the dry core of pressed gypsum fiber comprises at least about 95% by weight of calcium sulfate dihydrate, based on the total weight of the core. 25. The method according to claim 24, further characterized in that the dry core of pressed gypsum fiber comprises at least about 99% by weight of calcium sulfate dihydrate, based on the total weight of the core. -, r * .- * fcjfol tfeMÍ 26. - The method according to claim 2, further characterized in that the dry core of pressed gypsum fiber has a density of about 800.9 g / l or less. 27. The method according to claim 26, further characterized in that the dry core of pressed gypsum fiber has a density of about 720.8 g / l or less. 28.- The method according to claim 27, further characterized in that the dry core of pressed gypsum fiber has a density of approximately 640.7 g / l or less r 10 29.- The method according to claim 1, further characterized because the gypsum suspension that is prepared in step (d) has a rectangular cross section and has a width to thickness ratio of about 48: 1 to about 216: 1. 30.- A core of pressed gypsum fiber, characterized because 15 is made by a method comprising the steps of: (a) preparing a gypsum suspension in a mixer; (b) introducing the suspension in an extrusion die, the die having elements at its periphery for the introduction of at least one gypsum suspension additive selected from the group consisting of slip agents and resistance improving agents; 20 (c) introducing the suspension additive into the suspension as the suspension leaves the die; (d) extruding the gypsum suspension containing additive from the die onto a substantially smooth, movable surface; and (e) drying the suspension that was obtained in step (d) in a hybrid dryer f? tá¿ U¡. using convection heating and microwave drying techniques to form a dry core of pressed gypsum fiber. 31. The core of pressed gypsum fiber according to claim 30, further characterized in that the suspension that is extruded 5 in step (d) has a viscosity of about 19,000 millipascakseconds (Pa »s) at about 22,000 mPa» s as determined by a Brookfield DV-III programmable rheometer using a number six spindle at 20 RPM. «. 32. The core of pressed gypsum fiber according to claim 10, further characterized in that the suspension that is prepared in step (c) comprises from about 59% by weight to about 64% by weight of sulphate dihydrate. calcium, based on the total weight of the suspension. 33.- The core of pressed gypsum fiber in accordance with the 15 claim 30, further characterized in that the core of pressed gypsum fiber comprises at least about 90% by weight of dihydrate -ff of calcium sulfate, based on the total weight of the core. 34. The core of pressed gypsum fiber according to claim 33, further characterized in that the pressed fiber core 20 of gypsum comprises at least about 99% by weight of calcium sulfate dihydrate, based on the total weight of the core. 35. - The pressed gypsum fiber core according to claim 30, further characterized in that the pressed fiber core of gypsum has a density of about 800.9 g / liter or less. 36.- The gypsum pressed fiber core according to claim 35, further characterized in that the core of pressed gypsum fiber has a density of about 640.7 g / l or less. 37.- The gypsum pressed fiber core according to claim 30, further characterized in that the core has a width to thickness ratio of about 48: 1 to about 216: 1. f 10 38.- An apparatus for producing a core of pressed gypsum fiber, characterized in that it comprises: (a) a mixer; (b) an extrusion die comprising a die input, a die output, and a manifold disposed between the input and the output, characterized in that the die input is in fluid communication with the mixer and in which the die 15 has secondary entries for the introduction of gypsum ingredients; (c) a movable, substantially smooth surface disposed adjacent the die outlet f; and (d) a dryer. 39.- The apparatus according to claim 38, further characterized in that the die outlet has a rectangular cross-section 20 and has a width to thickness ratio of about 48: 1 to about 216: 1.