as used herein , “ absorbency ” refers to the functional capacity and the rate at which absorption occurs as measured by absorption under load ( aul ) or finite volume absorption under load ( fvaul ). “ air permeability ”, as used herein , refers to the amount of air which the surface permits to pass through during a specified amount of time relative to another surface having the same total area as the first surface . as used herein , the term “ absorbent article ” refers to articles that absorb and contain exudates , and more specifically refers to articles which are placed against or in proximity to the body of the wearer to absorb and contain various exudates discharged from the body . a non - exhaustive list of examples of absorbent articles includes diapers , diaper cores , diaper covers , disposable diapers , training pants , feminine hygiene products and adult incontinence products . the term “ disposable article ” refers to absorbent articles that are intended to be discarded or partially discarded after a single use , i . e ., they are not intended to be laundered or otherwise restored or reused . the term “ unitary disposable absorbent article ” refers to a disposable absorbent article that is essentially a single structure ( i . e ., it does not require separate manipulative parts such as a diaper cover and insert ). as used herein , the term “ diaper ” refers to an absorbent article generally worn by infants and incontinent persons about the lower torso . the claims are intended to cover all of the forgoing classes of absorbent articles , without limitation , whether disposable , unitary or otherwise . these classifications are used interchangeably throughout the specification , but are not intended to limit the claimed invention . the invention will be understood to encompass , without limitation , all classes of absorbent articles , including those described above . preferably , the absorbent core is thin in order to improve the comfort and appearance of a garment . the employance of thin , comfortable garments is disclosed , for example without limitation in u . s . pat . no . 5 , 098 , 423 to pineiak et al . which is herein incorporated by reference . referring to the drawings , fig1 shows the forming surface 2 which is mounted on the drum assembly 4 . a forming chamber 8 is attached to the outlet shroud 10 and feeds an air stream to the forming surface on the drum assembly 4 . a hammermill having a blade or a plurality of blades receives a fiber board 18 at a pair of feed roles 19 . any blade or plurality of blades capable of fiberizing the fiber board are contemplated by the invention . non - limiting exemplary blades include steel , metal alloy and carbide tipped blades . preferably , the blades are carbide tipped blades . the feed roles 19 feed the fiber board to the carbide tip blades 14 which disintegrate the board into particles which are carried into the outlet shroud 10 by an air stream originating from an air inlet scoop 20 which is connected to the hammermill 12 at a hammermill screen 16 . the forming chamber 8 contains a nozzle 22 having an outlet into the interior of the forming chamber 8 through which a substance such as a super absorbent polymer (“ sap ”), for example without limitation , may be sprayed or injected into the air stream passing through the interior of forming chamber 8 where the substance will combine with the particles contained in the air stream . the drum assembly 4 has an inner vacuum chamber 6 positioned in such a manner as to create a vacuum at the forming surface on certain portion or portions of the drum assembly 4 . mounted to the exterior of the drum assembly 4 is a scarfing roll 24 which is located in close proximity to the forming surface 2 . a conveyor belt 28 is located adjacent to and beneath the drum assembly 4 and optionally contains tissue layers 26 for receiving the absorbent article formed on the forming surface when the article is released from the vacuum of the inner vacuum chamber 6 after passing through the scarfing role 24 . thus , once the drum assembly 4 rotates so that the absorbent article formed on the forming surface 2 is directly over the conveyor belt , the absorbent article is deposited onto the conveyor belt . for example , the absorbent article may be processed into a diaper , without limitation . the conveyor belt 28 transports the absorbent article for further processing . for example , the absorbent article may be processed into a diaper , without limitation . referring to fig2 a along with fig1 , a plurality of forming surfaces 2 ( fig1 ) are secured to a drum assembly 4 . one of the plurality of forming surfaces is illustrated in fig2 a and 2b . forming surface 2 is divided into four separate zones , each containing perforations 42 defining an open area through which air passes giving a certain air permeability based upon the diameter and number of holes or perforations 42 within each zone and / or the thickness of the sheet metal . a first zone 46 , which corresponds to the front of the absorbent article being prepared , has perforations 42 in a sufficient number and of a sufficient diameter to define an open area , through which air passes , of about 30 % to about 85 %, preferably about 40 % to about 60 %, more preferably about 45 % to about 55 %, and even more preferably about 49 % of the total area of the first zone . a second zone 48 corresponds to the back of the absorbent article being prepared . the second zone 48 has perforations 42 in a sufficient number and of a sufficient diameter to define an open area , through which air passes , of about 5 % to about 25 %, preferably about 10 % to about 22 %, more preferably about 15 % to about 20 %, and even more preferably about 15 %, of the total area of the second zone . a third zone 50 , which defines a gradual transition from the first zone to the second zone , contains perforations 42 of varying diameter and / or number sufficient to provide a gradually decreasing open area from the first zone to the second zone . likewise , a fourth zone 48 , which defines a gradual transition from the second zone to the first zone , contains perforations 42 of varying diameter and / or number sufficient to provide a gradually decreasing open area from the second zone to the first zone . the forming surface 2 , while being permeable to air , is substantially impermeable to the solid materials carried in the airflow . by use of the term “ air ”, it is contemplated that other vapors , gases or mixtures thereof may be used in place of air . the forming surface 2 may be in the form of a screen , a mesh , a grid , a matrix , or any selectively permeable form , and combinations thereof . the perforations 42 or openings in the forming surface 2 may be of any shape and combinations thereof . preferably , the perforations 42 or openings are circular . the perforations 42 or openings may be of a wide variety of sizes provided the desired open area or air permeability is achieved . the forming surface 2 may be in any overall size or shape and may be bent or molded in various ways to achieve a wide variety of affects , as desired . preferably the forming surface 2 is in a rectangular form , as shown in fig2 a and is bent as shown in the side view of fig2 b . it has been found that the varying air permeability of the forming surface unexpectedly results in an improved absorbent article having zones with differing absorbencies in a simple and cost effective manner while achieving a high level of precision . the forming surface 2 may be composed of any material or combination of materials which can withstand the process conditions and produce the desired effect . preferably , the forming surface is composed of sheet metal . the perforations may be of any diameter . preferably , the perforations in the sheet metal are a diameter ranging from about 0 . 020 to about 0 . 080 inches , more preferably the diameters of the holes range from about 0 . 030 to about 0 . 060 inches , even more preferably the diameters of the perforations range from about 0 . 036 to about 0 . 040 inches . the perforations may be of uniform or non - uniform sizes . preferably , the perforations are of uniform size . referring to fig3 , the drum assembly 4 includes an outer cylinder upon which the forming surface 2 is mounted . the outer cylinder is supported and rotates around a central shaft 62 . a vacuum or suction is drawn within the drum assembly 4 by means of an inner vacuum chamber 6 . the vacuum created within the drum causes the airborne particles to adhere to the forming surface 2 . as described above , the predetermined zones 46 , 48 , 50 and 52 on the forming surface 2 have different air permeability . thus , the particles are deposited in varying amounts on the forming surface corresponding to the different zones . the basis weight of the fiber build up in each zone therefore is different from the other zones . this results in an absorbent article having a desired absorbent profile . the inner vacuum chamber 6 includes a main chamber 63 and a main chamber outlet 64 . the main chamber 63 receives the air flow from the forming chamber 8 ( fig1 ) after the air passes through the forming surface . the air then travels to the main chamber outlet 64 . in this manner , a vacuum or suction is created on the forming surface 2 as described above . outer cylinder 60 upon which the forming surface 2 is mounted rotates past the main chamber 63 . after the forming surface passes the main chamber 63 it encounters the pad transfer chamber 66 . the pad transfer chamber 66 does not receive the air flowing from the forming chamber 8 . therefore , there is no vacuum or suction beneath the forming surface 2 as the forming surface 2 rotates past the pad transfer chamber 66 . accordingly , the absorbent article formed on the forming surface is released from that surface and is able to be transferred to the conveyor belt , shown in fig1 , for subsequent processing . referring to fig4 , the forming chamber 8 is composed of a main air duct 74 , an inlet opening 76 and an outlet opening 78 . a nozzle 22 may be mounted on the main air duct 74 such that the nozzle penetrates into the interior of the main air duct 74 by means of an air tight seal . the nozzle 22 provides a means for injecting or spraying or providing in some other way a substance into the interior of the main air duct . the material may be a polymer such as a sap , for example without limitation . the forming chamber 8 is mounted to the drum assembly at the outlet opening 78 . the input opening 76 is attached to the outlet shroud 10 by means of an air tight connection . referring to fig5 , optionally , a scarfing roll 24 is situated in close proximity to the drum assembly 4 . the scarfing roll 24 is one or more rollers which comes into contact with the forming surface 2 and redistributes the material deposited on the forming surface 2 in a designated manner . optionally , a return duct may recycle scarfed material by returning the material to the forming chamber 8 . use of any conventional scarfing roll or recycling means is contemplated by the invention . the use of scarfing rolls and recycling means in this manner is well within the skill of the art . fig6 shows a cross - sectional view of the various components of the absorbent article on the forming surface 2 after the forming surface passes through the scarfing roll 24 . as the figure shows , the scarfed particles 102 are redistributed by the scarfing roll 24 from the back to the front zone , thereby further reducing the basis weight ( or absorbency ) of the back section while increasing the basic weight ( absorbency ) of the front zone . in this manner , the combination of the scarfing roll 24 with the forming surface 2 of the invention is a synergistic combination , as described below and illustrated in graph 7 . fig7 is a graph , which corresponds to table 1 below , illustrating the absorbency profile of the absorbent article when prepared in accordance with various preferred embodiments of the present invention . as shown in table 1 and fig7 , when the process of the present invention is employed with no scarfing rolls and no sap , an absorbent article is produced having a higher basis weight in the front than in the back through the use of the forming surface having different air permeability at corresponding predetermined zones . this demonstrates the effectiveness of the forming surface in selectively placing different amounts of the particles in different zones on the absorbent article . in particular , the basis weight in the front of the article was found to be 600 gsm while the basis weight in the back of the article was measured to be 500 gsm . when a sap is applied during the process , again without use of the scarfing roll , an even greater difference between the front and back zones is achieved . in particular , the basis weight in the front zone was measured to be 950 gsm , whereas the basis weight in the back zone was found to be 750 gsm , or a 1 . 28 : 1 ratio between the front and back zones . when scarfing is used in the process along with a sap , an even more dramatic result can be seen . as shown in the figure and the table , the front zone has a basis weight of 950 gsm , whereas the back zone has a basis weight of 500 gsm . this difference in basis weights represents a 1 . 86 : 1 ratio of the front basis weight to the back basis weight . accordingly , the combination of the scarfing process with the use of the zoned forming surface of the invention represents a synergistic combination . table i transition front transition back weight ratio zone 1 zone zone 2 zone of front zone ( gsm ) ( gsm ) ( gsm ) ( gsm ) to back zone 1 ) no 625 600 480 500 1 . 20 : 1 scarf □ no sap 2 ) no scarf 840 950 910 750 1 . 28 : 1 □ with sap 3 ) with scarf 860 950 880 500 1 . 86 : 1 □ with sap fig8 is a plan view of an absorbent article prepared in accordance with a preferred embodiment of the present invention . referring to fig8 , the absorbent article 200 is composed of a front zone 202 and a back zone 204 . the absorbent article 200 is shown as one component of a disposable absorbent garment 300 according to one preferred embodiment of the present invention . due to the wide variety of materials which may be incorporated into the absorbent articles of the present invention , the invention is not intended to be limited to any specific materials . the particles may contain one or more fibers , one or more polymers or combinations thereof . non - limiting exemplary fibers which may be used in the process of the present invention include , without limitation , cellulose fibers , cellulose acetate fibers , rayon fibers , courtauld &# 39 ; s lyocel fibers , polyacrylonitrile fibers , surface modified ( hydrophilic ) polyester fibers , surface modified polyolophin / polyester by component fibers , surface modified polyester / polyester bicomponent fibers , cotton fibers or blends thereof . preferably cellulose acetate , rayon , courtauld &# 39 ; s lyocel , polyacrylonitrile , cotton fibers and cotton linters or combinations thereof are used in the process of the present invention . more preferably , cellulose fibers are used as the fiber material in the present invention . other materials may be added to the fiber or pulp material which is processed in a fiberizing apparatus , such as a hammermill . the additives may be added at any point in the process . preferably , the additives are sprayed or injected into the airborne fibers prior to the depositing of the fibers on the forming surface 2 . non - limiting exemplary additives which may be incorporated into the process of the present invention include a polymer such as a super absorbent polymer ( sap ), hydrophilic polymers , potato starch , corn starch , wheat starch or rice starch , or combinations thereof . various different combinations of materials may be used as are known to persons of ordinary skill in the art and which are described in u . s . pat . no . 6 , 068 , 620 which is herein incorporated by reference . preferably , the mixtures incorporated in the invention are substantially homogenous mixtures or uniformly distributed mixtures . although the invention preferably uses a hammermill , the invention contemplates use of any conventional fiberizing apparatus which accomplishes the disintegration of the fiber board into discreet particles of fiber . such conventional means are well known and readily available to persons of ordinary skill in the art . referring again to fig8 , the absorbent article of the present invention has one or more predetermined zones of a specified absorbency wherein at least two of these zones have a different absorbency . preferably , the ratio of the front absorbency to the back absorbency as measured by aul or fvaul , is about 1 . 25 : 1 to about 5 : 1 , more preferably the ratio of the absorbency of the front zone to the absorbency of the back zone is about 1 . 5 : 1 to about 2 . 5 : 1 , and even more preferably the ratio is about 2 : 1 . alternatively , the weight ratio of the basis weight of the front zone to the basis weight of the second zone is about 1 . 5 : 1 to about 3 : 1 ; more preferably , that ratio is about 1 . 6 : 1 to about 2 . 5 : 1 ; and most preferably , that ratio is about 2 : 1 . the absorbent article optionally has a third and a fourth predetermined area . each of the third and fourth predetermined areas has a gradually increasing or decreasing absorbency or basis weight in a longitudinal direction . this gradually increasing or decreasing absorbency , as measured in aul or fvaul , or basis weight may be in the form of a gradual linear progression or a gradual curved progression , as desired . an absorbent article having such an absorbent profile may be prepared in accordance with the process of the present invention or any other process which achieves these same results . the absorbent article may be composed of any material which achieves the desired absorbency . preferably , the absorbent article contains 50 to 95 % by weight particulate or fibrous sap , and about 5 % to about 50 % by weight of one other fibrous or particulate material . preferably , the absorbent article comprises a laminate . for example , without limitation , the laminate can be formed by sandwiching the absorbent article between two tissue layers of laminated material to encase the absorbent article therein . the use of aul as a measurement of absorbency is well known in the art . a person of ordinary skill in the art would readily understand how to use aul as a measurement of absorbency , as described herein . fig1 shows an apparatus used to measure finite volume absorbency under load ( fvaul ), while fig2 shows a close up view of a weight 32 used in the fvaul testing . the apparatus includes balance 34 and a sample holder 36 positioned on the balance , with the weight 32 configured for positioning on a test sample held by the sample holder . an lvdt ( linear variable differential transducer ) measuring system 38 is positioned to engage the weight 32 and measure its movement as a finite volume of liquid is introduced into the sample holder for absorption by a test sample . a lucas schaevitz type 2000 hpa lvdt system was employed , which employed lucas schaevitz system 96 oftware . since this software only provides lvdt measurements , additional software was provided to obtain readouts of values from balance 34 , and of time . as shown in fig2 , the weight 32 includes a stainless steel tube 40 and a bottom stainless steel screen 42 , with stainless steel slot 44 held within the tube and screen . liquid to be introduced into a test sample is poured through the steel slot so that it passes through the screen 42 into the sample holder 36 . a computer software program that can run the lvdt ( linear variable differential transducer ) system was booted . the lvdt system was calibrated , and the computer program to run the test was booted . 300 data sets were taken at two second intervals . a data set consists of time to the nearest hundredth second , balance reading to the nearest hundredth gram , and the lvdt reading to the nearest hundredth inch . the sample holder and a 0 . 16 psi porous weight were cleaned and then the holder was placed on a balance and the weight was put into place . the lvdt rods were then placed on the weights and the lvdt was zeroed . the lvdt and the weight were removed and weighed and then the sample was placed into the holder ( baby side up ). the weight and lvdt were replaced and the computer program calculated the sample &# 39 ; s thickness . the computer program asked for the sample weight and the ratio of superabsorbent particles ( sap ) to sample weight . this information was used to determine the total volume being taken up by the sap and pulp in the sample . the densities of 1 . 5 for sap and 1 . 7 for pulp are used by the program . the computer the “ calculates the free volume of the sample when dry . ( if this value is known to be incorrect because of pad construction , it is possible to re - enter the free volume .) an air shield was placed around the sample tester and the balance was zeroed ( tared ). 15 ml of test solution of 1 percent sodium chloride in water was prepared and placed in a graduated cylinder . the computer was then activated to start taking data sets and was allowed to take two data points before the solution was added . these two data sets are used to calculate the initial volume of the sample in the dry state . the 15 ml solution was quickly poured into the weight and was absorbed through the screen in the bottom of the weight into the sample . after the computer had taken 300 data sets , the computer generates the desired data such as dry free volume ( the amount of air in the sample ), the sample volume and sample mass as a function of time . the volume of the parts of the sample is calculated by taking the dry sample volume and subtracting the free volume from it and then adding the volume of liquid added . vd = volume of dry sample vf = free volume of air l = weight of the liquid 1 . 01 = density of 1 % nacl solution the sample volume and the volume by parts at 60 seconds and at 600 seconds was recorded . the computer program that reads information from the lvdt system and the balance calculates the free volume for the dry sample and records that as the first record in the computer file . the calculation is based on three pieces of information : the sample weight , the ratio of superabsorbent to sample weight , and the sample thickness . the samples are all assumed to be two inches in diameter . the following equation shows how the calculation is done . a s = ( 2 · 2 . 54 . 2 ) 2 · π = v s = volume of the sample ( cm 3 ) v s = a s · t s a s = area of the sample ( cm 2 ) fv s = v s − v sap − v pulp fv s = free volume of the sample v sap = m sap / ρ sap v sap = volume of sap in the v pulp = m pulp / ρ pulp v pulp = volume of pulp in the m sap = r · w ρ sap = density of the sap m pulp = ( 1 − ρ pulp = density of the pulp w = the mass of the sample ( g ) r = the ration of sap to sample t s = the thickness of the sample the following is the complete equation . 1 . 5 g / cc is used for the density of the superabsorbent 1 . 7 g / cc is used for the density of the pulp . the following examples are illustrative of preferred embodiments of the inventive subject matter and are not to be construed as limiting the inventive subject matter thereto . the following table shows the parameters for the design of forming screens used in accordance with various preferred embodiments of the present invention . the forming screens having the parameters described below are made of a sheet metal material . the arrangement of the zones is as shown in fig2 a and the overall configuration of the forming screens is as shown in fig2 b . table ii t1 zone * front zone t2 zone * back zone % open area □ about 5 % to about 30 % to about 5 % to about 5 % to about 79 % □ about 79 % □ about 79 % about 50 % hole diameter about 0 . 125 about 0 . 125 about 0 . 125 to about 0 . 125 to ( inches ) to about to □ about 0 . 010 about 0 . 010 0 . 010 about 0 . 010 holes per about 4 about 24 to about 4 to about 4 to square inch to about □ about 10 , 000 □ about 10 , 000 about 6 , 300 10 , 000 □ thickness about 0 . 005 - 0 . 250 about 0 . 005 - 0 . 250 about 0 . 005 - 0 . 250 about 0 . 005 - 0 . 250 ( inches ) * indicates a non - uniform transitional zone which gradually increases or decreases in open area in the longitudinal direction . this gradual increase or decrease in open area may be accomplish by increasing / decreasing hole diameter , increasing / decreasing number of holes or a combination thereof . the following table shows the parameters for the design of a forming screen used in accordance with a preferred embodiment of the present invention . the forming screen having the parameters described below is made of a sheet metal material . the arrangement of the zones is as shown in fig2 a and the overall configuration of the forming screens is as shown in fig2 b . table iii t1 zone * front zone t2 zone * back zone % open area about 32 % about 49 % about 32 % about 15 % hole diameter about 0 . 036 about 0 . 036 about 0 . 036 about 0 . 036 ( inches ) holes per about 481 about 481 about 147 about 147 square inch gradually gradually decreasing increasing to to about about 481 147 directionally directionally from back from zone to front front zone zone to back zone thickness about 0 . 015 about 0 . 015 about 0 . 015 about 0 . 015 ( inches ) * indicates a non - uniform transitional zone which gradually increases or decreases in open area in the longitudinal direction . this gradual increase or decrease in open area may be accomplish by increasing / decreasing hole diameter , increasing / decreasing number of holes or a combination thereof . the invention has been described in connection with the preferred embodiments . these embodiments , however , are merely for example and the invention is not restricted thereto . it will be understood by those skilled in the art that other variations and modifications can easily be made within the scope of the invention as defined by the appended claims .
an apparatus and a method for preparing an absorbent article is disclosed . the apparatus has at least two forming surfaces which differ in terms of air permeability to allow the selective placement of fibrous material . the forming surface comprises a perforated forming screen . the forming screen has multiple zones with different amounts of open area through which air may pass . particles carried in the air which flows through the open area are deposited on different zones of the forming screen resulting in different basis weights corresponding to the different air permeabilities . the apparatus and method thus provide an article having optimal zoned absorbency in a cost - effective manner .