A METHOD FOR MANUFACTURING A FILM INCLUDING CAVITIES, WITH PROFILES DETERMINED FOR FILM FLEXIBILITY, DENSITY, THICKNESS AND / OR POROSITY

Abstract

A method for manufacturing a film (F1) includes cavities and is formed from a polymer in which a cavitation agent is dispersed. The method includes a step of extruding the polymer through an extrusion die equipped with adjustment actuators to adjust the thickness of the extruded film and a stretch stage (Str1) of the film. It also establishes a mapping function of the film based on the mass per unit area profiles of the film before and after the stretch stage, establishes a stretch profile of the stretched film based on the mapping function and the cross-sectional mass per unit area profiles, and establishes a cross-sectional profile that is characteristic of the film based on the stretch profile and a cross-sectional profile of the mass concentration of the cavitation agent in the film when stretched.This makes it possible to take into account the distribution of cavities in the film; in this method, the adjustment actuators are controlled as a function of the characteristic cross-sectional profile.

Description

A METHOD FOR MANUFACTURING A FILM INCLUDING CAVITIES, WITH PROFILES DETERMINED FOR FLEXIBILITY, DENSITY, THICKNESS AND / OR POROSITY OF THE FILM FIELD OF INVENTION The invention relates to a method for manufacturing a film in which cavities or voids are generated during one or more stretching stages, with stages for characterizing the film. BACKGROUND OF THE INVENTION A film made from a polymeric material can become porous when stretched, as a cavitation material is finely mixed with the polymer. When a film is stretched longitudinally, that is, in the direction in which the film advances, its length is increased. A film, when stretched lengthwise, can undergo shrinkage in the transverse direction and therefore may have a reduced width. When a film is stretched transversely, that is, transversely in the direction of the film's advance, its width is increased. A film is often stretched both transversely and longitudinally, either sequentially or simultaneously. A cavitation agent is a material dispersed in the polymer in which islands of the cavitation agent mechanically cause cavities or voids to appear in the volumes around them during stretching, as described in the publications of US Patent Documents 2006 / 0121259 and WO 2010 / 059448. Characterization techniques have been developed that take into account the deformation of such a film and the distribution of its component material during stretching, with a view to optimizing the manufacturing method in general and the stretching stage in particular. For example, US patent 7,813,829 describes characterizing the thickness of a film, first after it has been extruded and directed upstream from the transverse stretch and then downstream from the transverse stretch. The techniques for characterizing a film on its production line are based on sensors, whose principle is to detect the absorption of radiation or a wave and which naturally report the distribution of matter. While a film is being manufactured, the distribution of matter leads to defining a mapping function that establishes a correspondence between a transverse position of the film before stretching and a transverse position of the film after stretching. Furthermore, if the implemented sensors make it possible to accurately characterize the edges of the film, it is possible to define the mapping function by plotting the curves that represent the accumulated mass per unit area in the transverse direction based on the mass per unit area measurements. Fig. 1A shows the C1 and C2 mass per unit area profiles, respectively, of the film before and after it was stretched, with the transverse locations Xmin and Xmax corresponding to the edges of the film before stretching, and the transverse locations and Χ^αχ corresponding to the edges of the film after stretching. Fig. 1B shows the C3 and C4 mass per unit area curves that accumulate in the transverse direction and are normalized, whose curves are taken from the respective C1 and C2 curves of Fig. 1A, before and after stretching. In this example, the mapping function is defined as a correspondence between a transverse location X' of the film after stretching with an accumulated and normalized mass per unit area Y' and the transverse location X of the film before stretching with an accumulated and normalized mass per unit area, where Y is equal to Y'. With a film that has no cavities, a thickness profile can be directly deduced from a mass per unit area profile by dividing the mass per unit area values ​​(expressed, for example, in grams per square meter (g / m2)) by the density of the film (expressed, for example, in grams per cubic meter (g / m3)) which, in such a situation, is a constant. The combination of knowing the mapping function and characterizing the film thickness profile after stretching makes it possible to adjust the manufacturing parameters that act on the film thickness profile upstream from stretching in such a way as to make the film of uniform thickness after stretching as described in US Patents 5,928,580 and US 7,813,829. In this context, characterizing the film thickness profile is therefore an unavoidable step in monitoring and controlling the manufacturing method. However, with films that include cavities, such as microporous membranes, breathable films or "pearlescent" films, a thickness profile cannot be satisfactorily deduced from a mass per unit area profile. In fact, with respect to whether they are based on beta rays, X-rays, infrared rays, or ultrasound, the techniques for measuring mass per unit area in films remain in measuring a quantity of matter and do not directly provide the thickness of the film to be characterized. The thickness at a given point of the film is given by the ratio of its mass per unit area to its density at that point. Conventionally, since the local density is unknown, it is assumed to be constant. However, in practice, for a porous film, the cavities may not be uniformly distributed throughout the film, in which case the assumption that the density is constant is no longer realistic and the characterization of its thickness is subject to errors. Furthermore, even when a non-uniform density is predictively estimated based on knowledge of the machines and methods used, such an estimate remains empirical and does not make it possible to adapt to a new situation or to react when faced with unexpected changes. Thus, conventional methods for characterizing a film that includes cavities are insufficiently reliable and accurate and do not make it possible to cope with the anticipated changes in the characteristics of a film that is forming. BRIEF DESCRIPTION OF THE INVENTION An object of the invention is to improve a method for manufacturing a film that includes cavities cfrnfrnn / zznz / E / YiAi by improving the monitoring of the characteristics of such a film, by increasing the accuracy of measurements of the characteristic profiles of the film, in particular the stretch, density, thickness and / or porosity profiles. The method of the invention makes it possible to reliably characterize a film having a density that varies due to a non-uniform distribution of closed cavities and / or through cavities or gaps in the film, including porosity. To this end, the invention provides a method for manufacturing a film that includes cavities and is formed from a polymer in which a cavitation agent is dispersed. The method includes a step for extruding the polymer on a production line equipped with adjustment actuators for adjusting the characteristics of the extruded film and a step for stretching the film, as well as a step consisting of establishing a mapping function of the film, wherein: a first mass-per-unit sensor of the unstretched film, a second mass-per-unit-area sensor that measures a cross-sectional mass-per-unit-area profile of the film when stretched; and a computer unit (S50a, S150a) that calculates the mapping function of the film when stretched on the basis of the cross-sectional mass-per-unit-area profiles.The computer unit calculates a stretch profile of the film when stretched based on the mapping function and the mass per unit area cross-sectional profiles; the computer unit calculates a characteristic cross-sectional profile that is characteristic of the film based on the stretched profile and a cross-sectional profile of the mass concentration of the cavitation agent when the film is stretched, making it possible to take into account the distribution of the cavities and their sizes in the film; and the adjustment actuators are controlled as a function of the characteristic cross-sectional profile. As described above, when a film formed from a polymer in which a cavitation agent is dispersed is subjected to stretching, cavities or voids form in the islands of the cavitation agent. The present invention has determined that, since the volume of the cavities formed in this way is proportional to the stretch of the film, a stretch profile calculated on the basis of the mass per unit area profiles of the film before and after stretching characterizes the film and can be obtained while the film is being formed. Combined with the distribution of a cavitation agent in the film, the stretch profile obtained in this way is an indicator of the cavity distribution and thus makes it possible to obtain a film characterization that takes into account a potentially non-uniform cavity distribution in the film. In particular, the film stretch profile makes it possible to deduce a film density profile, a film thickness profile, and a film porosity profile, each of which takes into account the distribution and sizes of the film cavities. Thus, determining the profiles of the manufacturing method of the invention is based on measurements of the mass per unit area upstream and downstream from a zone in which the film is stretched transversely and on prior knowledge of the distribution of the cavitation agent or by measurement carried out in situ on the line. cfrnfrnn / zznz / E / YiAi Since the cross-sectional stretch profile is determined on the basis of mass per unit area measurements taken on the film being formed, the stretch profile provides information based on the actual characteristics of the film and not merely on an assumed uniformity of the film or on estimates made on the basis of post-production measurements of previously manufactured films. Longitudinal stretch, that is, stretch in the direction of the machine, can be calculated from the same data. Furthermore, the invention's method for measuring film thickness is independent of the type of film formed and the type of machine used; the method automatically adapts to various manufacturing environments. The manufacturing method of the invention may have the following characteristics: - the cross-sectional mass concentration profile of the cavitation agent can be deduced from the cross-sectional mass per unit area profile of the film when stretched and from a mass per unit area profile of the cavitation agent when measured by a mass per unit area sensor of the cavitation agent; - a mass per unit area profile of the cavitation agent in the film when unstretched can be measured by a mass per unit area sensor of the cavitation agent; a cross-sectional profile of the mass concentration of the cavitation agent in the film when unstretched can be deduced from the mass per unit area profile of the cavitation agent and the mass per unit area profile of the film when unstretched; and the cross-sectional profile of the mass concentration of the cavitation agent in the film when stretched can be replaced by applying the mapping function of the cross-sectional profile of the mass concentration of the cavitation agent in the film when unstretched; - the cross-sectional mass concentration profile of the cavitation agent can be approximated to the average concentration of the cavitation agent in the film; - the film may suffer a loss of the cavitation agent between the two mass per unit area cross-sectional profile measurements; a first mass per unit area sensor of the cavitation agent can measure a mass per unit area cross-sectional profile of the cavitation agent in the film when it is not stretched; a second mass per unit area sensor of the cavitation agent can measure a mass per unit area cross-sectional profile of the cavitation agent in the film when it is stretched; the computer unit can calculate the mass per unit area cross-sectional profiles of the film for the polymer by subtracting the mass per unit area cross-sectional profiles of the cavitation agent from the mass per unit area cross-sectional profiles of the film; and the computer unit can calculate the mapping function based on the mass per unit area cross-sectional profiles of the film only for the polymer; - the manufacturing method may include a first stage for stretching the film in a first zone and a second stage for extracting the cavitation agent from the film in a second zone downstream of the first zone, the second mass per unit area sensor being able to measure the cross-sectional mass per unit area profile of the cavitation agent in the film when it is stretched downstream from the first zone and upstream from the second zone; cfrnfrnn / zznz / E / YiAi - the characteristic cross-sectional profile that is characteristic of the film can be a cross-sectional density profile of the film; - the computer unit can calculate a cross-sectional thickness profile of the film based on the density profile of the film and the cross-sectional mass per unit area profile of the film when stretched; - the adjustment actuators can be controlled in response to a deviation of the thickness profile as calculated relative to a thickness profile as expected; - the characteristic cross-sectional profile of the film can be a cross-sectional porosity profile of the film; - the adjustment actuators can be controlled in response to a deviation of the porosity profile as calculated relative to an expected thickness profile; and - The edges of the film can be trimmed and removed while the film is being formed, making it possible for the computer unit to determine the film mapping function based on the cross-positions of the trimmed film edges. The invention extends to a film obtained using the manufacturing method of the invention. BRIEF DESCRIPTION OF THE FIGURES The present invention will be better understood and other advantages will become apparent upon reading the following detailed description of a given implementation by way of non-limiting example and with reference to the accompanying figures, in which: - Fig. 1A shows cross-sectional profiles of mass per unit area or area density of a film before and after it is stretched; - Fig. 1B shows the curves of the accumulated profiles from Figs. 1A and 1B and the definition of a mapping function; - Fig. 1C shows the manufacture of a film using a method that includes longitudinal and transverse stretching on a production line; - Fig. 2A shows a mapping function of the film from Fig. 1C; - Fig. 2B shows a stretch profile derived from the mapping function of Fig. 2A; - Fig. 3A shows a density profile, i.e., mass per unit volume of the film in Fig. 1A; - Fig. 3B shows a profile for the film porosity of Fig. 1A; - Fig. 4 shows a method for manufacturing the film of Fig. 1C; - Fig. 5 shows a method for manufacturing a microporous membrane that includes a plurality of stretching stages in a production line; and - Fig. 6 shows a method for manufacturing the microporous membrane of Fig. 5. DETAILED DESCRIPTION OF THE INVENTION In this first implementation, a "pearlescent" F1 film is obtained by extruding a continuous phase polymer in which a cavitation agent is dispersed in the form of solid particles and then by simultaneous and / or sequential longitudinal and transverse stretching of the resulting film. The cavitation agent can be of an inorganic type, for example, calcium carbonate particles, or of an organic type, for example, polybutylene terephthalate (PBT), whose types of cavitation agent are not miscible in the polymeric base of the film, for example, polypropylene. In a pearlescent film, cavities form around the cavitation agent particles, giving the film a white or pearlescent appearance while also reducing its density. First, film F1 is extruded through an extrusion die D equipped with adjustment actuators to adjust the film thickness. It is produced by a method that includes transverse stretching Str1 in a zone Z0. The film moves in the MD direction from a “machine” along a production line, as shown in Fig. 1C. It should be noted that longitudinal stretching can be applied to the film simultaneously with transverse stretching Str1. As described in detail below, the volume of the cavities is a function of the film stretch and at least of first order, proportional to the film stretch, so that determining a stretch profile makes it possible to deduce a porosity profile and a density profile for the film. The stretch profile can be calculated from measurement data from scanners equipped with mass per unit area or “area density” sensors. To characterize the thickness of the film, a first scanner SCAN1 and a second scanner SCAN2 are used, each of which includes a respective mass per unit area Capm.surf sensor and the first and second scanners are configured to scan the film over its entire width, in the TD transverse direction upstream and downstream, respectively, of zone Z0. Mass per unit area (MPA) sensors work based on the transmission of X-rays through the film to be characterized. The transmission varies with the mass per unit area of ​​the film, and each sensor has an X-ray emission head to emit X-rays and an X-ray detection head to detect the X-rays that pass through the film to be characterized. Alternatively, the mass-per-unit-area sensor could, for example, be based on the transmission or backscattering of beta rays or some other rays and could have a beta-ray or other ray emission head to emit beta rays or other rays and a beta-ray or other ray detection head to detect the beta rays or other rays transmitted or backscattered by the film to be characterized. In general, measurements can be taken in transmission or backscatter of electromagnetic rays or ultrasound or particles. For backscattering, the emission heads and detection heads are placed on the same side of the film, optionally incorporated in the same housing or recess. Between scanners SCAN1 and SCAN2, the total mass flow of the film is constant. To quantify the distribution of the cavitation agent in the film, at least one of the SCAN1 and SCAN2 scanners can be equipped with a Caps.ag sensor capable of detecting the mass per unit area of ​​the cavitation agent contained in the extruded film. For example, a Caps.ag sensor can be based on the absorption of infrared rays by the cavitation agent. The application of the law of conservation of mass between two scanners SCAN1 and SCAN2 is shown in Fig. 1C, which shows the material flow lines in the film when subjected to transverse stretching Str1. The flow of matter between the edge of the film identified by the coordinate Xminy and any position X in the first scanner SCAN1 shall be equal to the flow of matter between the edge of the film when identified by the coordinate X^iny and the corresponding position X' in the second scanner SCAN2, which is expressed by equation (1) v. f* Ws(x)dx = v'.f5 Ws'(x')dx' (1)xmin Xmin where vy ν' represent the motion velocities of the film in the first and second scanners, respectively, and H / (x) and W / (x') represent the mass profiles per unit area (usually expressed in g / m2) of the film at the transverse positions xyx' at the levels of the first and second scanners, respectively. Equation 1 can be rewritten independently of the velocities if equation (2) is written, which is equivalent to equation (1) for the entire width of the film: v.^naxWs^dx = v'.^W^dx'Amin and if (1) is divided by (2) to obtain equation (3): fx>Wdx=(X(3) ^min ^s,ot KünW' with an integration Wstot of the mass per unit area over the entire width of the film on the first scanner using equation (4): rXW Wstot= fmaxWs^x)dxJXmm and the integration W¿tot of the mass per unit area over the entire width of the film, including the edges, in the second scanner using equation (5): x, (5) Kcot = $χ·MλWs'(x')dx'λmm where the positions X+ny X^ax are known because they are the positions of the edge of the film and the masses per unit area I4^(x) and M£(x') are measured by the scanners. Equation (3) establishes a relationship between any transverse position X of a film in the first scanner SCAN1 and its corresponding position X' in the second scanner SCAN2 by defining an unambiguous relationship between position X and position X', the mapping function which can be seen graphically by the Map curve shown in the graph of Fig. 2A in which the abscissa and ordinate axes are represented respectively of the transverse positions xyx' in the first scanner SCAN1 and the second scanner SCAN2. For any transverse position x' after the transverse stretch, the transverse stretch Stretch'!D(x') on the second scanner can be expressed by equation (6): dv (θ) Stretch'TD(x') = — dx which is the derivative of the mapping function. For any transverse position x' after stretching, the total stretch (longitudinal and transverse) Stretch'(χ1) on the second scanner can be expressed by equation (7): Stretching'(x') _ W's(x') (7) where x is the position that corresponds to ax' determined by the mapping function when solving equation (3). Fig. 2B shows the Stretch profile'(x') of the film stretch shown in Fig. 1C on the second scanner SCAN2, as deduced from the mass per unit area measurements taken by the Capm sensors and from the processing of equation (7) by conventional mathematical processing means implemented by a CALC computer unit. The density profile W¿(x') of the stretched film in a transverse position x' can be expressed by: ,=(8) vorxx'RVoí jxjwoi'Mρυι agcavwhere Wpoi(x') represents the mass of the polymer, W¿g(x') represents the mass of the cavitation agent, Vol'pol(x') represents the volume of the polymer, Vol'ag(x') represents the volume of the cavitation agent and Vol'cav(x) represents the volume of the cavities when considered in an elemental volume of the film that is located by the coordinate x' which is representative of a transverse position after stretching the film in the second scanner SCAN2. Equation (8) can be written in the form of equation (9): Vol'cav(x') and with the relative volume of the film cavities when stretched proportional with a proportionality constant aa to the concentration of the cavitation agent C¿5(x') and to the actual stretch stretch'(x') in the x' coordinate, it is possible to express the density profile in the form of equation (10): _________VolpolCrnvolqglx·)_________ 1+aC'ag(x) (Stretch'(x')-l) (10) where a represents the effects of the average film characteristics, the particle size of the cavitation agent and its interactions with the polymer and Stretch'(x') depends on the transverse position x'. The factor (Stretching'(x') -1) is introduced to be consistent with the definition of stretching provided above, due to the fact that Stretching'(x') is equal to 1 when there is no stretching and no cavity formation. Equation (10) can be written in the form of equation (11): 'ú'.i'oÍHmpníΛ) (11) + aC'ag(x) (Stretching'(x')-l) where W¿voíumm(x^ is defined by equation (12), which corresponds to a density profile of the film when it is not stretched in the transverse position x', i.e., while ignoring stretching and cavities: , w^M+w^(12)vvv.volumexΛJ , , Vol'vopnvol'agU) YC¿a(x') represents the mass concentration of the cavitation agent in the film and is defined by equation (13): C«a(x') = Ws,ag(.x) (13) where Ws'as(x') is a mass profile per unit area of ​​the cavitation agent in the second scanner SCAN2, when measured, for example, by a Caps.ag sensor that equips that scanner. From equation (11), it is possible to determine the transverse density profile W^x') and then deduce from them the transverse thickness and porosity profiles of the film being formed by determining (a) the mapping function of the film between scanners SCAN1 and SCAN2, (b) the stretching profile Stretch'(x'), (c) the profile Cag(x) of the mass concentration of the cavitation agent, (d) the profile W¿volume( x') of the density of the film when not stretched and (e) the value of the constant a. Regarding points (a) and (b), the mapping function and the stretch profile Stretch'(x') of the film can be deduced from the mass per unit area measurements taken by the sensors using the respective ones from equations (3) and (7). With respect to point (c), the profile C¿a(x') of the concentration of the cavitation agent in the film is provided by equation (13). In the particular situation in which the cavitation agent is uniformly distributed, its mass concentration profile (CágQx') no longer depends on the position, the concentration profile is flat at a constant value equal to the average concentration of the cavitation agent (Cagen) in the film, and it is no longer necessary to install a sensor to detect the mass per unit area of ​​the cavitation agent. Regarding point (d), the value W¿vo[umen( x') is determined from the densities of the polymer and the cavitation agent, both known to the person experienced in the technique and the concentration of the cavitation agent, using equation (14): W' , v.volnmen l'C'ag(x') Cag (x') (14) where Wvpoly Wvag represent respectively the densities of the polymer and the cavitation agent, whose densities are quantities known to the person experienced in the technique. Regarding point (e), the constant a is determined by inverting equation (11) and using the average concentration of the cavitation agent Cago obtained by averaging equation (13), the average stretch Estremiento' obtained by averaging equation (7), the average density corresponding to the density of the film when not stretched W(volume obtained by averaging equation (14) and the average density of the film when stretched WJ measured in the laboratory, leading to equation (15). W, v.volume (O C'ag (Stretch'- 1)WV ag(15) It is possible to introduce the porosity p'^ of the film after stretching, which is an essential characteristic of the film and is defined as the ratio of the volume of the cavities to the total volume of the film and is expressed by equation (16): _______Volcad(W) Vol'cavM^Vol'ag (x) + Vol'po¡(x) which can also be derived from equations (9) and (10) by means of equation (17): P'(x') = 1 i + a C'ag(x') (Stretching'fx')-1) with the notations as defined above. The thickness profile T'^ of the film can be calculated simply by using equation (18): T(x) = W's(x) / W'v(x') (18) where W / CO represents the mass per unit area of ​​the film obtained directly from the measurements taken by the second scanner SCAN2 and Wv'(x') represents the cross-sectional density profile of the film on the second scanner deduced from the measurements taken by the two scanners located on each side of the stretching zone, as described in detail above, in particular by going through a stage of determining the local stretch of the film, not empirically, but, unlike conventional methods, from the mass per unit area measurements. Manufacturing the film includes the steps of determining the thickness profile T' <x'’ y / o el perfil de porosidad P'^1al utilizar el método 10 mostrado en la Fig. 4 descrito al seguir el procesamiento de un segmento de la película a medida que avanza. In practice, it is a method that takes place continuously in which time is taken to transport the film between the two scanners. In an S10 stage, the polymer and the cavitation agent mixed with the polymer are extruded through the D matrix to form an unstretched film that is conveyed in the MD direction of the machine. In stage S20, a first Capm.navigation mass per unit area sensor, which equips the first scanner SCAN1 and is controlled by the tracking and control unit C / C, scans over the film, measures a first cross-sectional mass per unit area profile Ws(x) of the film before such film is stretched, and stores representative data of the first mass per unit area profile of the film in a computer MEM memory, the contents of which are accessed by the computer CALC unit. In stage S30, the film is stretched in the MD direction of the MD machine and in the transverse TD direction, which is substantially perpendicular, sequentially or simultaneously in the MD direction of the machine. In stage S40, the second Capm.navigation sensor of mass per unit area and the Caps ag sensor for detecting the mass per unit area of ​​the cavitation agent, which equip the second SCAN2 scanner and are controlled by the monitoring and C / C monitoring and control unit, scan over the film, measure respectively a second cross-sectional profile of mass per unit area of ​​the film and a cross-sectional profile of mass per unit area Ws'ag(x') of cavitation agent in the film after stretching and store the representative data of these profiles in the computer MEM memory. In an S50 stage, the transverse density profile W^x') of the film is determined after the stretching S30 stage on the basis of the first transverse mass per unit area profile W / fx) and the second mass per unit area profile (x'), whose profiles are measured in the respective S20 and S40 stages by solving equation (11) by means of processing by the computer CALC unit of the transverse profiles stored in the computer MEM memory and of the parameters relating to the materials used for manufacturing (densities of the polymer Wvpoly of the cavitation agent Wvagy the average density of the film when stretched Wv), which are known or can be measured by conventional methods and the characteristics of the film in question as a whole, whose parameters are stored in the computer MEM memory and are accessible to the computer CALC unit. cfrnfrnn / zznz / E / YiAi The S50 stage includes the S50a substage to determine a table representing the mapping function based on the mass per unit area profiles !4 / (x) and M / '(x') measured by the Capm sensors, mass per unit area navigation during the S20 and S40 stages, and the solution of equation (3) by means of the CALC computer unit, whose table is recorded in the computer MEM memory. The S50 stage further includes the substages S50b, S50c, S50d and S50e to determine the individual elements of the right-hand side of equation (11) by means of the CALC computer unit and the cross-sectional profile data stored in the computer MEM memory and then the S50f substage to calculate the appropriate cross-sectional density profile WJCx'). Substage S50b consists of determining the stretching profile Stretch'(x') from the mass per unit area measurements 14 / (x) and W / '(x') taken by the Capm sensors. Mass per unit area navigation from the mapping function determined in stage S50a and from solving equation (7) using the CALC computer unit. Optionally, substage S50b may include calculating the cross-sectional stretch profile Stretch'TDQx'') based on equation (6). The S50c substage consists of determining the concentration profile (Cágí?) of the cavitation agent in the film by applying equation (13) to the measurements taken by the mass per unit area sensors in the S40 stage. Substage S50d consists of determining the density profile W¿volume( x') of the film when it is not stretched as explained above, by solving equation (14) using the concentration profile Cág(x') determined in substage S50c by means of the CALC computer unit. The sub-stage S50e consists of determining the proportionality constant a, as explained above, based on equation (15) in which the following is introduced: the stretching profile Stretch'(x') determined in stage S50b, the concentration profile C¿a(x') determined in sub-stage S50c and the density profile x') of the film when it is not stretched, whose profile is obtained in stage S50d by the CALC computer unit. Substage S50f consists of using the individual elements determined during substages S50b to S50e to determine the cross-sectional density profile 14 / ,'(x'), defined by equation (11) using the CALC computer unit. Fig. 3A shows a density profile 14 / ,'(x') of the film in Fig. 1C, with the film edges, portions of the film that are not stretched, as shown in Fig. 2B, the density being substantially equal to the density of a film that is not stretched and therefore has no cavities. In an S60A stage, the cross-sectional profile of the thickness T'^ is determined by solving equation (18) using the CALC computer unit, the mass per unit area profile I4 / '(x') of the film and the density profile 14 / ,'(x') of the film on the second scanner which is known as the result of the S10aS50 stages. As shown in Fig. 4, this thickness profile T'^1 is a feature of the film when stretched and represents a source of information about the film manufacturing method and can be used to act on the method itself by adjusting FBK1 in a setting of the equipment to form the film in response to a deviation of the profile T'^ in relation to an expected thickness profile manually by means of adjustments made by the person experienced in the art or even automatically by means of a feedback control loop that connects the CALC computer with the equipment to form the film. For example, the extrusion of the polymer that forms the film is conventionally performed by pushing between a stationary lip and an adjustable lip of the die D, the thickness of the extruded film being controllable by the actuators Act of the die D, whose actuators are distributed along the adjustable lip and whose actuator action can be individually adjusted in response to the measurement of the cross-sectional profile of the film thickness. In particular, from the film mapping that is determined from the mass profiles per unit area of ​​the film it is possible to deduce the actuators that are necessary to adjust as a function of the film thickness profile. Thus, the invention is a method for manufacturing a film that includes the method of measuring the thickness profile described above as an element of a feedback loop for monitoring and controlling the thickness profile of the film, with the advantage of continuously monitoring the manufacturing method. Furthermore, in an S60B stage, the cross-sectional profile of the porosity P'^ is determined by solving equation (17) using the CALC computer unit; the necessary information is known as a result of stages S10 to S40 and S50b, S50c and S50e. Fig. 3B shows a porosity profile P'^ of the film shown in Fig. 1C, with very low porosity at the edges of the film, whose edges do not stretch and therefore have no cavities or only a few cavities. As shown in Fig. 4, this porosity profile P';x> is a source of information about the method for making the film and can be used to act on the method itself by adjusting FBK2 in a setting of the equipment to form the film in response to a deviation of the profile P'x> in relation to an expected porosity profile manually by means of adjustments made by the person experienced in the technique or even automatically by means of a feedback control loop that connects the CALC computer with the equipment to form the film. One regulation method consists of controlling the actuators to adjust the temperatures of the transverse temperature zones in the stretching device included in a production line, in response to the measurement of the transverse porosity profile of the film, in such a way that the transverse stretch SEstretch'TD(x') is modulated to obtain the desired porosity profile. Therefore, the method of the invention for manufacturing a film includes the method for measuring the porosity profile described above as an element of a feedback loop for monitoring and controlling the porosity profile of the film with the advantage of continuously monitoring the manufacturing method. Among the variations of the above method that are accessible to the person experienced in the technique, it can be mentioned that the profile of the cavitation agent Ws'ag(x') in the second cfrnfrnn / zznz / E / YiAi scanner SCAN2 can be deduced from a measurement of the profile of the cavitation agent 14 / as(x) in the first scanner SCAN1 by means of the mapping function. In another variation, there is no transverse stretching stage on the production line to produce the film, but rather only a longitudinal stretching stage. However, stretching in the machine direction can lead to transverse shrinkage of the film and thus a stretch profile with values ​​less than one. The equations defined above continue to apply in the same way, with the only difference being that in this variant the FBK2 adjustment is not relevant. Description of a second implementation of the method of the invention This second implementation constitutes a particular case of the first implementation in which the cavitation agent can be partially or totally removed from the film due to the nature of the film in question and the processing that is applied between the SCAN1 and SCAN2 scanners. Therefore, the assumption is maintained that the mass flow made in the first implementation is not valid between two scanners and it is necessary to take into account the mass loss in the method and in the calculations. Therefore, it is possible to refer to the first implementation for the procedure applied to the manufacturing method and for the type of equipment used, in particular for mass per unit area sensors and actuators. As shown in Fig. 5, as an example in this second embodiment, the film in question is a microporous polymeric membrane that can be produced from an F2 film produced continuously by extruding a polymeric solution, the film moving in the MD direction from a “machine” along a production line. One particular application of a microporous membrane is the manufacture of membranes that are to fulfill the function of physically separating the cathodes and anodes of battery cells while also allowing electric charge carriers to travel from an anode to a cathode through the pores of the membrane. In the context of this second implementation, it is explained how the following are determined: a stretch profile, a porosity profile, a density profile, and a thickness profile of a microporous membrane prepared from a polymeric solution comprising a polymeric resin and an oil used as a cavitation agent. The polymer resin can be a polyolefin such as polyethylene or polypropylene and the cavitation agent can be a paraffin oil, as described in International Patent Application WO 2008 / 016174 and in U.S. Patent 8,841,032. The film is subjected to a first stage of the method in a first zone Z1, during which it is stretched by stretching Str1 and cavities are formed in the oil inclusions, whose cavities subsequently give rise to pores in the microporous membrane. During a second stage of the method, namely an Extrde stage to extract the oil from a zone Z2 downstream from zone Z1, the film stretched by the first stretching Str1 goes through a solvent bath which dissolves the oil contained in the pores and then the pores are emptied of the oil cfrnfrnn / zznz / E / YiAi by means of a mechanism to extract the solvent and oil mixture. During this second stage of the method, and particularly during the extraction of the mixture, the film undergoes a Retr shrinkage. Optionally, a second stretching stage Str2 implemented during a third stage of the method in a Z3 zone downstream of the Z2 zone aims to correct the film shrinkage that took place during the extraction of the mixture. As described in detail below and as in the first implementation, the volume of the cavities is a function of the film stretch and at least of first order proportional to the film stretch, so that determining a stretch profile makes it possible to deduce a porosity profile, a density profile and a thickness profile for the film. In contrast, this second implementation differs from the first in that the cavitation agent (the oil in this case) is removed as completely as possible from the film in a processing zone between two mass per unit area measurements. Therefore, the conservation of mass flow, which makes it possible to establish a mapping function between a position of the film before and a position of the film after the processing zone in question, no longer applies to the mass per unit of total area, but rather only to the mass of the polymer. Therefore, equations (1) to (7) remain valid as long as the total mass per unit area profiles W;(x) and I4£(x') are replaced by the mass per unit area profiles Wspo¡(x) and Wjpol(x') of the polymer. In this mode, the SCAN1 and SCAN2 scanners located respectively upstream from the first zone Z1 and downstream from the second zone Z2 perform the same functions as the SCAN1 and SCAN2 scanners of the first implementation, by performing the function of zone Z0 in zones Z1 and Z2 of the first implementation. The Capm.navigation mass per unit area sensors measure the Ws(x) and (x) profiles of total mass per area, respectively, in the first scanner SCAN1 and in the second scanner SCAN2 and are sensitive to all detected matter, which in addition to the polymer, includes oil, most of which is removed between the first and second scanners. Therefore, these sensors measure the mass per unit area of ​​the film constituents as a whole and not just the masses per unit area of ​​the polymer that forms the extruded film. To quantify the oil loss from the film, the first scanner SCAN1 can be equipped with a Caps.oil sensor capable of detecting a WsaOil(x) mass profile per unit area of ​​the oil, only that contained in the extruded film and the second scanner SCAN2 can be optionally equipped with another Caps.oil sensor to measure in the second scanner SCAN2, the mass per unit area Wjaoil(x') of the oil residues after the stretching and extraction stages, in particular at the edges of the film. The mass per unit area profiles Ws.poi(x) and Wjpol(x') of the polymer in the first scanner and in the second scanner are obtained, respectively, by subtracting between the total mass per unit area profiles and the corresponding mass per unit area profiles Wsaceits(x) and W^aceite(x') of the oil using equations (19) and (20), respectively. cfrnfrnn / zznz / E / YiAi Ws.po¡(x) = M4W - facetW (19) KPOÍO') = ιν / (χ') - W'oil(x') (20) Alternatively and equivalently, the SCAN1 scanner and / or the SCAN2 scanner can be equipped with one or more sensors capable of detecting the mass per unit area of ​​the polymer contained only in the extruded film. A Capsaceite sensor to detect the mass per unit area of ​​oil and a sensor to detect the polymer contained only in the film can, for example, be based on the detection of infrared absorption by the oil and by the polymer, respectively. Based on these equations, the mapping function is defined by using equation (21) in a manner analogous to the first implementation with the same notations X, Xmin, X' and X,'ninrX WSpOi(x) _ rX7, j (21) Jv W— Jv' —<----min s.pol.tot inin ^s.pol.tot cfrnfrnn / zznz / E / YiAi where Wspol toty W'poltotson respectively the integrals of the profiles M4.poí(x) and Ws'po¡(x') of mass per unit area over the entire width of the film in the first scanner and in the second scanner. To obtain the expression for stretching, equation (7) can be adapted to this second implementation to obtain equation (22). „ . . Tx'1ws.poiM (22) Stretching and > =--------W' ir n s.polfx J Analogously to the first implementation, it is possible to consider that the volume of the cavities is proportional to the concentration of oil before stretching. However, since the oil is extracted as much as possible, measuring its concentration in the second SCAN2 scanner does not provide the information needed to calculate the distribution of the cavities. Conversely, this information can be obtained from the oil concentration measured in the first scanner SCAN1 and from the mapping function using equation (21). Equation (23) expresses the oil concentration profile Caceite(x) in SCAN1, before stretching: and the mapping function (21) makes it possible to express the distribution profile C¡.iti0(x') for the distribution of the cavity sites generated by the presence of oil and by the stretching Str1 upstream of the stage of extracting the oil from zone Z2, expressed by equation (24). Csitio(x') = Caceite(x) (24) This quantity plays a role equivalent to C'ag(x') in the first implementation. Equation (11) can be rewritten to give equation (25) which expresses the density profile of the film with its cavities but without the residual oil W'(x') = -------v1 + a C'ag(x') (Estrelaje'(x')-l) cfrnfrnn / zznz / E / YiAi where, therefore, the polymer density Wvpol, of known value, replaces W(V0Íumen( x'~). In this situation, the proportionality constant a is expressed by equation (26), which is equivalent to equation (15) of the first implementation: wv.poi-w¿ cc —----------------------Csite (Stretching '-l) Wj (26) where the mean concentration of cavity sites Ositiose is determined by averaging equation (24), the mean stretch Stretching' is obtained by averaging equation (22) and the mean density of the stretched film W( is as measured in the laboratory. Analogously to the first implementation, the thickness profile T'^ and the porosity profile P'^ in the second scanner SCAN2 are expressed respectively by equations (27) and (28). ρ / ζχ') _ ^7^W'v(x'J (28) Ρ'(χ') = 1--:------i----------------1+a C'ag(x') (Stretch'(x')-1) Fig. 5 shows the third zone Z3 of the method downstream from the second zone Z2, with a stage of removing the edges E1 and E2 of the film F2 by trimming them after passing them through the second scanner SCAN2 and before the second stretching Str2. Fig. 5 shows a third SCAN3 scanner equipped with a third mass-per-unit-area navigation sensor configured to scan the cropped film over its entire width in the TD transverse direction downstream of the third Z3 zone of the film. Downstream of the second scanner, the blades (not shown in the figure) trim and remove the outer edges E1 and E2 of the film, so that the edges of the trimmed film identified by the coordinates Xmin.cut and Xmax.cut correspond to a reduction in the width of the film. Because the edges of the film are trimmed and removed, the conservation of the mass flow of the film only affects the area that lies between the positions Xmin.Cut and Xmax.Cut, leading to equation (1) being transformed into equation (29) v'.f5 M4'(x')d;c'= vf* Ws(x)dx (29^ mineartemin where Xmin, v, W4 (x)> YX represent respectively a transverse position of a first edge of the film, the speed of movement of the film, the mass profile per unit area and any transverse position in the film, each of them in the third scanner SCAN3, the other elements of the equation being as defined above. The integration Wjcortede of the mass per unit area over the entire width of the film after trimming and before stretching Str2 is expressed by equation (30): Acorte = ^'^orte N^χ'^χ'(30)xmin.corte and the integration Wjtotde the unit area mass over the entire width of the film in the third scanner gives equation (31): = »Í(>W<31)where Xmax represents the transverse position of the second edge of the film in the third scanner SCAN3. The equation that makes it possible to define the mapping function between the second scanner SCAN2 and the third scanner SCAN3 can be extrapolated from equation (3) by means of equation (32). xmin.cut VKxminW The mapping function defined by equation (32) makes it possible to make any transverse position x in the third scanner SCAN3 correspond to a transverse position x' in the second scanner SCAN2 after shrinkage Reír, and thus define the porosity profile Ρίχ”) in the third scanner on the basis of the porosity profile in the second scanner. Local porosity remains unchanged due to the absence of new cavity formation and because existing cavities and the entire film volume deform in the same way, which gives equation (33): P(x) = P'(x') (33) where P(x”) is the porosity profile of the film on the third scanner at a transverse position x. Similar to porosity, local density remains unchanged to a first approximation and is thus expressed by equation (34): <(*) = Wv'(x') (34) where 14ζ'(χ) is the density profile on the third scanner. The thickness profile Γ'(χ”) on the third scanner is then deduced from that density profile cfrnfrnn / zznz / E / YiAi by equation (35). =wÁClI(35)v}Wv(x) For explanatory reasons, the solvent loss is considered to be zero or imperceptible in the Z3 stretching zone, but naturally it is possible to take such loss into account, in a manner analogous to what was proposed for the first Z1 stretching zone. The method for determining a film thickness profile on the third scanner is similar to the method for determining the thickness profile on the second scanner, but also includes a step of storing in the computer's MEM memory the transverse coordinates X'mtn.cut and Xmax.cut of the film edges after trimming and before the second stretch Str2 for use by the computer's CALC unit. Any cutting of the film edges could be taken into account by the person experienced in the technique by means of the method described above, for example, combined with the first implementation or with the processing of the F2 film of the Z1 and / or Z2 zones of the second implementation. Manufacturing the film includes the steps of determining the T profile (xj thickness) and following the method 100 shown by Fig. 6, described by following the processing of the moving film. In practice, it is a method that takes place continuously in which the time taken to transport the film between the two scanners is taken into account. In an S110 stage, the polymer and the oil mixed with the polymer are extruded through the die D to form an unstretched film that is conveyed in the MD direction of the machine. In stage S120, a first Caps.oil mass per unit area sensor and a Caps.oil mass per unit area sensor of oil, which equip the first scanner SCAN1 and are controlled by the C / C monitoring and control unit on the film, respectively measure a first profile 14 / (x) of mass per unit area of ​​the film and a first profile Wsaceíte(x) of mass per unit area of ​​oil of the oil in the film, before the film is stretched Str1 and store the representative data of the first profiles in a computer MEM memory. In an S130 stage, the film is stretched in the transverse TD direction, which is substantially perpendicular to the MD direction of the machine, and then the oil is extracted from the film in an S135 stage as completely as possible. In a stage S140 after stage S130, a second Capm.navigation sensor of mass per unit area of ​​oil and a Caps.oil sensor of mass per unit area of ​​oil that equip the second scanner SCAN2 and are controlled by the C / C monitoring and control unit scan over the film, measure respectively a second I4 / '(x') transverse total mass per unit area profile of the film and a W'oil(¿j) profile of mass per unit area of ​​oil, of the oil in the film after the film is stretched Str1 and store representative data of these profiles in the computer MEM memory. In stage S145, the CALC computer unit calculates (1) the cross-sectional mass per unit area profile WspoÍ(x) of the polymer only at the first scanner SCAN1 by subtracting the first cross-sectional mass per unit area profile of oil from the first total mass per unit area profile of the film and (2) the cross-sectional mass per unit area profile Wjpo¡(x') of the polymer only at the second scanner SCAN2 by subtracting the second cross-sectional mass per unit area profile Wú'ace¿te(x') of the oil in the film from the second cross-sectional mass per unit area profile of the film and these profiles are stored in the computer MEM memory. In stage S150, the CALC computer unit calculates the Wv'(x') cross-sectional density profile of the film after the S130 stretching stage Str1, based on the first Ws(x) cross-sectional mass per unit area profile and the second ¡47(0 mass per area profile measured in the respective stages S120 and S140, by solving equation (25) through processing by the CALC computer unit of the cross-sectional profiles stored in the computer MEM memory and of the parameters that are known to be measurable by conventional methods and that relate to the materials used for manufacturing and characteristics of the film considered as a whole, whose parameters are stored in the computer MEM memory and are accessible to the CALC computer unit. Step S150 includes substep S150a to determine a table representing the mapping function based on the cross-sectional mass per unit area profiles Wspol(x) and Ws'poi(x') of the polymer, only obtained in step S145 and from solving equation (21) using the CALC computer unit. The S150 stage also includes the substages S150b, S150c, S150d and S150e to determine the individual elements of the right-hand member of equation (25) by means of the CALC computer unit and the cross-section data stored in the computer MEM memory and then the S150f substage to calculate the appropriate cross-section density profile 144'(x'). The sub-step consists of determining the stretch profile Stretch '(*') from the cross-sectional mass per area profiles of the polymer only that are obtained in step S145, from the mapping function determined in step 150a and from the solution of equation (22) by means of the CALC computer unit. The S150c substage consists of determining the C'siti0(x'>) distribution profile for the distribution of the cavity sites generated in the film by applying equation (23) to the measurements taken by the mass per unit area sensors in the S120 stage. In this implementation, the S50d step of the method of the first implementation has no equivalent in terms of calculation since the density WvpoÍ of the polymer is a known quantity. For the CALC computer unit, the S150d stage of the second implementation consists of retrieving the Wvpoi density value of the polymer that is stored in the computer MEM memory. The sub-step S150e consists of determining the proportionality constant on the basis of equation (26) in which the average values ​​Stretch' and C'siti0se are introduced for the profiles established in the respective steps S150b and S150c and on the basis of a value W¿ for the density of the film, whose value is obtained by measurements in the laboratory using the CALC computer unit, the polymer density Wvpoí is known. The S150f substage consists of using the individual elements determined during the S150b, S150b and S150e substages to determine the W^x'j cross-sectional density profile by using equation (25) by means of the CALC computer unit. In an S160A stage, the cross-sectional thickness profile T'^ is determined by solving equation (29) using the CALC computer unit, the mass per unit area WjpolQx'j of the film and the density Wj(x') of the film in the second scanner known as a result of the S110 to S150 stages. Furthermore, in an S160B stage, the cross-sectional profile of the porosity P'^ is determined by solving equation (28) using the CALC computer unit, the necessary information being known as a result of stages S110 to S150. As shown in Fig. 6 and in the same way as the thickness profile T'{χ'> and the porosity profile θηθ| of the second scanner SCAN2 in the first implementation, the thickness profile T'^ and the porosity profile P'^ of the second implementation can be used to act on the manufacturing method by adjusting the equipment settings to form the film in response to deviations of the profiles from the expected profiles, FBK1 and FBK2 respectively. In stage S170, the edges of the film are trimmed and the positions of the new edges of the film after trimming are recorded in the MEM memory by the monitoring and control C / C unit; the positions of the cutting blades are known. In an S180 stage, the second Str2 stretch of the film is performed in the Z3 zone. In stage S190, a third Capm.navigation mass per unit area sensor, which equips the third scanner SCAN3 and is controlled by a C / C monitoring and control unit, is detected on the film, measures a first cross-sectional W¡¡'(x) mass per unit area profile of the film after the second stretch, and stores representative data of the third mass per unit area profile of the film in computer MEM memory, using a method analogous to the method in stages S120 and S140. In an S200 stage, a table is determined that represents the mapping function on the basis of the second I4 / '(x') cross-sectional mass per unit area profile and the third M / (χ”) cross-sectional mass per unit area profile that are measured in the respective S140 and S190 stages and on the basis of solving equation (32) by means of the CALC computer unit. In an S210 stage, the Wv'(x") cross-sectional density profile of the film after the S180 stretching stage Str2 is determined on the basis of the W„(x') density profile in the second scanner as determined in the S150 stage and from the table representing the mapping function determined in the S200 stage, by solving equation (34) by means of the CALC computer unit processing. In a stage S220, the transverse porosity profile P (x”) of the film in the third scanner is determined on the basis of the porosity profile P'^ of the film in the second scanner as determined in stage S160B and on the basis of the mapping function determined in stage S200, by solving equation (33) by means of processing by the CALC computer unit. In an S230 stage, the cross-sectional thickness profile T”(x) is determined by solving equation (35) using the CALC computer unit, the mass per unit area Ws”(x) of the film and the density WvXx) of the film in the third scanner, which are known respectively as a result of stages S190 to S210. As shown in Fig. 6 and in the same way as the thickness profile T'^ and the porosity profile in the second scanner SCAN2, the thickness profile T(x) and the porosity profile P(x) can be used to act on the method itself by the FB3K4 and FBK4 adjustments of the equipment settings to form the film in response to the respective deviations of these profiles in relation to the expected thickness and porosity profiles. Optionally and in a simplified way, certain situations allow an approximation that consists of considering that the mass content of oil in the extruded film is independent of the transverse position, which allows replacing equation (19) with equation (36). (36) Similarly, it is also possible to consider that the residual oil content after the extraction stage of zone Z2 is zero, which allows Ws'pol(x') to be replaced with W / fx'). Therefore, a variation of the second implementation is obtained in which density, thickness and porosity are obtained without using the Caps.aceite sensors. Optionally, in addition to a fourth scanner SCAN2' equipped with a Capm.navigation mass per unit area sensor and a Caps.oil mass per unit area sensor, these can be placed between zones Z1 and Z2 to characterize the first stretch Str1 and the oil loss during such stretch. The use of this fourth scanner can provide the following advantages. By applying the set of equations (21) to (28) between the SCAN1 and SCAN21 scanners, it is possible to calculate a first approximation of the porosity profile and the thickness profile much more quickly, as the extraction method in zone Z2 adds a time lag after the method in zone Z1. This allows for faster feedback on the method than if the results from the SCAN2 or SCAN3 scanner were used. In addition, the SCAN2' scanner makes it possible to determine the oil loss during the method in the Z1 zone and to characterize the quality of the Str1 stretch. The cavitation agent is oil in this implementation, but the invention is not limited to that material. If a cavitation agent other than oil is used, the person experienced in the technique uses adapted mass per unit area sensors. The implementations described above apply to a category of films that are normally referred to as "pearlescent" films and battery separator films or "BSF", but the invention can also be applied to any other type of porous membrane such as, for example, breathable membranes or fuel cell membranes. Naturally, the present invention is not limited in any way to the implementations described above, which may be modified without going beyond the scope of the invention.

Claims

CLAIMS 1. A method for manufacturing a film (F1; F2) comprising cavities and formed from a polymer in which a cavitation agent is dispersed, the method comprising a step of extruding the polymer on a production line equipped with adjustment actuators (Act) for adjusting the characteristics of the extruded film and a step (S30; S130) of stretching (Str1; Str2) the film, as well as a step (S50a; S150a) comprising establishing a mapping function of the film, the method characterized in that: - a first mass-per-unit-area sensor (Capm.navigation) measuring (S20; S120) a cross-sectional mass-per-unit-area profile of the film when unstretched; - a second mass-per-unit-area sensor (Capmnavegation) measuring (S40; S140) a cross-sectional mass-per-unit-area profile of the film when stretched by the stretching (Str1; Str2); - a computer unit (CALC) that calculates (S50a;S150a) the film mapping function when stretched based on the mass per unit area cross-sectional profiles; - the computer unit (CALC) that calculates (S50b; S150b) a stretching profile of the film when stretched based on the mapping function and the mass per unit area cross-sectional profiles; - the computer unit (CALC) that calculates (S50f; S60B; S150f; S50B) a characteristic cross-sectional profile that is characteristic of the film based on the stretching profile and a cross-sectional profile of the mass concentration of cavitation agent in the film when stretched, which makes it possible to take into account the distribution of cavities in the film; and - the adjustment actuators (FBK1, FBK2, FBK3, FBK4) are controlled as a function of the characteristic cross-sectional profile.

2. The method of manufacturing a film according to claim 1, further characterized in that the cross-sectional mass concentration profile of the cavitation agent is deduced (S50c) from the cross-sectional mass per unit area profile of the film when stretched and from a mass per unit area profile of the cavitation agent when measured by a mass per unit area sensor (Capsag; Caps, oil) of the cavitation agent.

3. The method of manufacturing a film according to claim 1, further characterized in that: - a mass per unit area profile of the cavitation agent in the film when unstretched is measured by a mass per unit area sensor (Caps ag; Caps.aceite) of the cavitation agent; - a cross-sectional mass concentration profile of the cavitation agent in the film when unstretched is deduced from the mass per unit area profile of the cavitation agent and the mass per unit area profile of the film when unstretched; and - the cross-sectional mass concentration profile of the cavitation agent in the film when stretched is replaced by applying the mapping function to the cross-sectional mass concentration profile of the cavitation agent in the film when unstretched.

4. The method of manufacturing a film according to claim 1, further characterized in that the cross-sectional mass concentration profile of the cavitation agent is similar to the average concentration of the cavitation agent in the film.

5. The method of manufacturing a film according to any of claims 1, 3 and 4, further characterized in that: - the film undergoes a loss of the cavitation agent between the two cross-sectional mass per unit area profile measurements; - a first mass per unit area cavitation agent sensor (Capsaceite) measures (S120) a cross-sectional mass per unit area profile of the cavitation agent in the film when unstretched; - a second mass per unit area cavitation agent sensor (Capsaceite) measures (S140) a cross-sectional mass per unit area profile of the cavitation agent in the film when stretched; - the computer unit (CALC) calculates (S145) the cross-sectional mass per unit area profiles of the film for the polymer only by subtracting the cross-sectional mass per unit area profiles of the cavitation agent from the cross-sectional mass per unit area profiles;and - the computer unit (CALC) calculates the mapping function (S150a) based on the cross-sectional mass per unit area profiles of the film for the polymer only.; 6. The method of manufacturing a film according to claim 5, further characterized in that such a film stretching step (S130) (Str1) takes place in a first zone (Z1) and is followed by a step (S135) of extracting (Extr) the cavitation agent from the film in a second zone (Z2) downstream of the first zone (Z1), the second mass per unit area sensor (oil caps) measuring the cross-sectional mass per unit area profile of the cavitation agent in the film when it is stretched downstream of the first zone (Z1) and downstream of the second zone (Z2).

7. The method for manufacturing a film according to any of claims 1 to 6, further characterized in that the characteristic cross-sectional profile that is a feature of the film is a cross-sectional density profile of the film.

8. The method for manufacturing a film according to claim 7, further characterized in that the computer unit (CALC) calculates (S60A, S160A) a cross-sectional thickness profile of the film (F1; F2) on the basis of the density profile of the film and the cross-sectional mass per unit area profile of the film when stretched.

9. The method for manufacturing a film according to claim 8, further characterized in that such adjusting actuators (FBK1, FBK3) are controlled in response to a deviation in the thickness profile when calculated relative to an expected thickness profile.

10. The method for manufacturing a film according to any of claims 1 to 6, further characterized in that the characteristic cross-sectional profile that is a feature of the film is a cross-porosity profile of the film.

11. The method for manufacturing a film according to claim 10, further characterized in that the adjustment actuators (FBK2, FBK4) are controlled in response to a deviation in the porosity profile when calculated relative to an expected thickness profile.

12. The method for manufacturing a film according to any of claims 1 to 11, further characterized in that the edges (E1, E2) of the film (F2) are trimmed and removed while the film is being formed, the computer unit determines (S200) the film mapping function of the transverse positions (Xmm.cut· ^max.cut) of the trimmed film edges.