Sandwich structure production method

Abstract

This invention is a process for manufacturing a sandwich structure comprising two metal skin layers separated by a polymer layer, which comprises the following steps:\n•\tThe dimensions of the sandwich structure are determined according to the objective to be achieved by following the following steps:\n-\tDefinition of the objective to be achieved by means of target values, for example tensile strength Tc expressed in kN / mm, flexural stiffness Bc expressed in kN / mm, and surface mass Mc expressed in Kg / m²,\n-\tDefinition of a tolerance for achieving the target values,\n-\tDefinition of the sandwich structure by means of five variables, For example, the thickness Ea of the steel skin layers expressed in mm, the thickness of the polymer layer Ep expressed in mm, the intrinsic Young's modulus Yp of the polymer layer, the intrinsic density dp of the polymer layer, and the volume ratio Rp of the polymer layer expressed as a percentage of the volume of the polymer layer. The definition of the combinations Ea, Ep, Yp, dp and Rp which enables the achievement of target values ​​with defined tolerances,\n-\tDefinition, for each variable, of a performance range,\n•\tSelection of steel and polymer layers for which each variable is within the range defined in the previous step,\n•\tConstruction of a suitable sandwich structure.

Description

Sandwich structure production method The present invention relates to sandwich structures comprising a polymer layer between two steel skin layers and a process for manufacturing them. Reducing the energy consumption of motor vehicles primarily occurs by reducing the weight of these vehicles. Steel, traditionally used to produce both skin and structural components, has competitors in lighter metals such as aluminum and composites such as glass fiber reinforced polymers or carbon fiber reinforced polymers. Although sandwich structures can be offered as another option, their complexity makes it difficult to measure their dimensions. Therefore, the aim of this invention is to provide a method for producing a sandwich structure that facilitates the measurement of the sandwich structure according to the objective to be achieved. To this end, the first object of this invention is a method for manufacturing a sandwich structure comprising two layers of steel skin separated by a polymer layer, comprising the following steps: The dimensions of the sandwich structure are determined according to the goal to be achieved by following the following steps: -Definition of the target to be achieved by target values, for example tensile strength Tc expressed in kN / mm, bending stiffness Bc expressed in kN / mm, and surface mass Mc expressed in Kg / m², -Defining a tolerance for achieving target values, -Definition of the sandwich structure by five variables, for example the thickness Ea of the steel skin layers expressed in mm, the thickness of the polymer layer Ep expressed in mm, the intrinsic Young's modulus Yp of the polymer layer, the intrinsic density dp of the polymer layer, and the volume ratio Rp of the polymer layer expressed as a percentage of the polymer layer volume of the material, -Definition of the combinations of Ea, Ep, Yp, dp and Rp that enable the achievement of target values ​​with defined tolerances, -Determine, for each variable, a functional range, Selecting the steel and polymer layer for which each variable is within the range defined in the previous step, Construction of a suitable sandwich structure. The method of the present invention may also include the following optional features, either singly or in combination: -The goal to be achieved is a monolithic metallic material in addition to steel. -The goal to be achieved is aluminum. -The target to be achieved is 0.9 mm thick aluminum, -The target to be achieved is 0.8 mm thick aluminum, -The tolerance in achieving the target values ​​is 10%, -The step of determining the combinations of Ea, Ep, Yp, dp and Rp enables the production of target values ​​that have a specified tolerance, which includes a step during which the combinations of Ea, Ep, Yp, dp and Rp are created, -It includes a graphical analysis step for the Ea, Ep, Yp, dp and Rp compounds that are generated. The secondary object of this invention is a sandwich structure obtainable by the method of this invention, which is implemented for a 0.9 mm aluminum target with Tc=31.5 N / mm, Bc=10.2 N / mm, Mc=2.43 Kg / m² and a tolerance of 10%, which sandwich structure comprises: -Two layers of steel skin with a steel thickness Ea between 0.133mm and 0.165mm, -A polymer layer that is sandwiched between two layers of skin and has: A thickness Ep between (-2.5xEa+0.713) and (-2.5xEa+0.88), Density dp between 0.9 and 1.4, Volume fraction Rp greater than or equal to 0.2 and exactly less than 1, Young's modulus Yp less than 4000 MPa Where the volume fraction Rp and Young's modulus Yp satisfy the following inequality: Yp*(0.49*Rp²+0.23*Rp+1 / (Yp*(1-Rp))) ≥ 50 MPa The sandwich structure according to the invention may also include the following optional features, either singly or in combination: -Steel thickness Ea between 0.141 mm and 0.158 mm and polymer layer thickness Ep between (-2.5xEa+0.73) and (-2.5xEa+0.87), -The polymer layer comprises a combination of a polyamide and a copolymer of ethylene and non-aromatic carboxylic acid and / or its derivatives. The third object of this invention is a sandwich structure obtainable by the process according to any one of claims 1 to 9, which is performed for a 0.8 mm aluminum target with Tc=28.0 N / mm, Bc=7.2 N / mm, Mc=2.16 Kg / m² and a tolerance of 10%, which sandwich structure comprises: -Two layers of steel skin with a steel thickness Ea between 0.118 mm and 0.146 mm, -Polymer layer that is sandwiched between two skin layers and has: A thickness Ep between (-2.5xEa+0.632) and (-2.5xEa+0.75), A density dp between 0.9 and 1.4, Volume fraction Rp greater than or equal to 0.2 and exactly less than 1, A Young's modulus Yp less than 4000 MPa Where the volume fraction Rp and Young's modulus Yp satisfy the following inequality. Yp*(0.49*Rp²+0.23*Rp+1 / (Yp*(1-Rp))) ≥ 50 MPa The sandwich structure according to the invention may also include the following optional features, either singly or in combination: -Steel thickness Ea between 0.126 mm and 0.140 mm and polymer layer thickness Ep between (-2.5xEa+0.646) and (-2.5xEa+0.728), -A blend of polyamide and a polymer of ethylene and unsaturated carboxylic acid and / or its derivatives. Other features and advantages of the present invention appear in the following description. The determination of the dimensions of a sandwich structure begins with the selection of a goal to be achieved. This goal can be either a virtual goal defined individually by target values ​​or a competitive combination for which it is intended to be repeated. In the latter case, the values ​​are the values ​​of the competitive materials under consideration. Specifically, the target to be achieved is defined by three target values, for example the tensile strength Tc expressed in kN / mm, the bending stiffness Bc expressed in kN / mm, and the surface mass Mc expressed in Kg / m². As non-limiting examples, in the case of aluminum as the target, the target values ​​Tc, Bc and Mc may have the following values, respectively: -31.5 N / mm, 10.2 N / mm, 2.43 Kg / m² for 0.9 mm aluminum -28.0 N / mm, 7.2 N / mm, 2.16 Kg / m² for 0.8 mm aluminum When determining the dimensions of the sandwich structure, a tolerance margin is created in achieving the target values. This tolerance can be adjusted on a case-by-case basis by a person skilled in the art according to the sensitivity of the target value in the context. For example, a first approach could be to accept that the target values ​​are achieved within a range of plus or minus 10%, or preferably within a range of plus or minus 5%. The sandwich structure is then defined by the thickness Ea of the folded skin layers expressed in mm, the thickness Ep of the polymer layer expressed in mm, the intrinsic Young's modulus Yp of the polymer layer, the intrinsic density dp of the polymer layer expressed in MPa, and the volume fraction Rp of the polymer layer expressed as a percentage of the polymer layer volume of the material. Intrinsic Young's modulus of a polymer layer means the Young's modulus of the polymer or mixture constituting the polymer layer, excluding the foam of the polymer layer, if present. Intrinsic density of a polymer layer means the density of the polymer layer or polymer blend constituting the polymer layer, excluding fillers and foam, if present, in density calculations. The volume fraction Rp is a function of the foaming of the polymer layer, if any. In the presence of foaming, the volume ratio is 1. Control of the volume fraction is achieved by controlling the foaming rate. Preferably, the polymer layer is foamed, i.e. Rp must be less than 1. Foaming provides a better material for weight reduction / mechanical performance of the sandwich structure. The next step is to determine the combinations of Ea, Ep, Yp, dp and Rp that allow the achievement of target values ​​that have the specified tolerance. This step can be performed according to any method known to a person skilled in the art. As a non-limiting example, a possible method is described below. The first step of this method is to produce the compounds Ea, Ep, Yp, dp and Rp as follows: -Young's modulus Ya1 of the first steel skin layer is set to 210,000 MPa, -The size of the first metal skin layer is set to 50 mm in width Ia1 and 100 mm in length La1, -Poisson's ratio va1 of the first steel skin layer is set to 0.3, -The size of the polymer layer is set at 50 mm in width Ip and 100 mm in length Lp, -Poisson's ratio vp of the polymer layer is set to 0.3, -The thickness Ea2 of the second steel skin layer is adjusted to the size of the first skin layer, -Young's modulus Ya2 of the second steel skin layer is set to 210,000 MPa, -The size of the second steel skin layer is set at 50 mm in width La2 and 100 mm in length La2, -Poisson's ratio va2 of the second steel skin layer is set to 0.3, -The thickness of the first skin layer Ea1 varies randomly, preferably between 0.1 mm and 0.2 mm, to limit the number of iterations, -The thickness of the Ep polymer layer varies randomly, preferably between 0 and 1 mm, to limit the number of repetitions, -The intrinsic Young's modulus Yp of the polymer layer varies randomly, -The volume fraction Rp of the polymer layer varies randomly, -The intrinsic density dp of the polymer layer varies randomly. -The fourth level anchor of the first steel skin layer MM4a1 is calculated using the equation MM4a1= Ia*Ea1^3 / 12. -The shear modulus of the first steel skin layer Gca1 is calculated using the equation Gca1=Ya1 / (2*(1+ va1)). -The neutral density of the first skin layer NFa1 is calculated using the equation Gca1=Ea1 / 2 -The mass Ma1 of the first steel skin layer is calculated using the equation Ma1= Ea1*7.8, -The apparent Young's modulus Yappp of the polymer layer is calculated using the equation Yappp=Yp*(0.7^2*Rp^2+(1-0.77)*Rp+1 / (Yp*(1-Rp))) -The fourth level anchor MM4p of the polymer layer is calculated using MM4p= Ip*Ep^3 / 12, -The shear modulus Gcp of the polymer layer is calculated using the equation Gcp= Yappp / (2*(1+vp)), -The neutral density NFp of the polymer layer is calculated using the equation NFp=Ep / 2+Ea1, -The mass Mp of the polymer layer is calculated using the equation Mp=Rp*Ep*dp. -The moment of the fourth surface MM4a2 of the second steel skin layer is calculated using the equation MM4a2= Ia2*Ea2^3 / 12, -The shear modulus Gca2 of the second steel skin layer is calculated using the equation Gca2= Ya2 / (2*(1+va2)). -The neutral density NFa2 of the second skin layer is calculated using the equation NFa2=Ea2 / 2+Ep+Ea1, -The mass Ma2 of the second steel skin layer is calculated using the equation Ma2=Ea2*7.8. -The bending stiffness of the sandwich structure Bs is calculated using the following equation: Bs= 1 / (La2^3 / (48*A)+La2 / (4*D)) Where: A= (Ya1*MM4a1+Ea1*Ya1*Ia*(Gca1-C)^2) + (Yappp*MM4p+Ep* Yappp*Ip*(NFp-C)^2) + (Ya2*MM4a2+Ea2* Ya2*Ia2*(NFa2-C)^2) D = Ip*Gcp*((Ea1+Ea2) / 2+Ep)^2 / Ep Where: C=(Ea1*Ya1*Ia*Gca1+Ep*Yappp*Ip*NFp+Ea2*Ya2*Ia2*NFa2) / (Ea1*Ya1*Ia+Ep*Yappp*Ip+Ea2*Ya2*Ia2) -The tensile strength Ts of the sandwich structure is calculated using the following equation: Ts= 50 / 100*(Ya1*Ea1+Yappp*Ep+Ya2*Ea2) / 1000 -The mass Ms of the sandwich structure is calculated using the following equation: Ms= Ma1+ Mp+ Ma2 -Ts, Bs and Ms are compared to the target values ​​for Tc, Bc and Mc with defined tolerances. The results obtained are analyzed to determine the combinations of Ea, Ep, Yp, dp and Rp that allow the target values ​​to be achieved within the defined tolerance. In particular, the results can be analyzed graphically. By way of example, the graph may show the thickness of the polymer layer Ep as a function of the thickness Ea of the steel skin layer and the intrinsic Young's modulus Yp. The graph may also show the thickness of the polymer layer Ep as a function of the thickness Ea of the steel skin layer and the volume fraction Rp of the polymer layer. During the process of determining the combinations of Ea, Ep, Yp, dp, and Rp that allow the achievement of target values ​​with defined tolerances, specific combinations can be excluded because they are defined in a way that does not work. For example, this is the case for sandwich structures with an apparent Young's modulus of the polymer layer below 50 MPa. Below this value, it has been observed that the expansion of the Young's modulus over time generally becomes sensitive to the environment. In particular, water intrusion or exposure of the sandwich structure to temperatures close to the melting point of the polymer layer may lead to shearing of the sandwich structure and reduced adhesion. Similarly, it has been found that a volume fraction of less than 0.2, for example a foamed ratio of more than 80%, does not allow the sandwich structure to achieve and / or maintain sufficient rigidity. For example, for a 0.9 mm aluminum target with Tc=31.5 N / mm, Bc=10.2 N / mm, Mc=2.43 Kg / m², and a tolerance of 10%, the performance range is determined as defined below: -Steel thickness Ea between 0.133 mm and 0.165 mm, -The thickness Ep of the polymer layer is between (-2.5xEa+0.713) and (-2.5xEa+0.88), -Density dp of the polymer layer between 0.9 and 1.4, -The volume ratio Rp of the polymer layer is between 0.2 and 1, -Young's modulus Yp of the polymer layer less than 4000 MPa, -where the volume fraction Rp, and Young's modulus Yp satisfy the following inequality: Yp*(0.49*Rp²+0.23*Rp+1 / (Yp*(1-Rp))) ≥ 50 MPa Similarly, for a 0.9 mm aluminum target with Tc=31.5 N / mm, Bc=10.2 N / mm, Mc=2.43 Kg / m² and with a tolerance of 5%, the performance range is defined as follows: -Steel thickness Ea between 0.141 mm and 0.158 mm, -The thickness Ep of the polymer layer is between (-2.5xEa+0.73) and (-2.5xEa+0.87) -Density dp of the polymer layer between 0.9 and 1.4, -The volume ratio Rp of the polymer layer is between 0.2 and 1, -Young's modulus Yp of the polymer layer less than 4000 MPa, where the volume fraction Rp and Young's modulus Yp satisfy the following inequality: Yp*(0.49*Rp²+0.23*Rp+1 / (Yp*(1-Rp))) ≥ 50 Mpa Similarly, for a 0.8 mm aluminum target with Tc=28.0 N / mm, Bc=7.2 N / mm, Mc=2.16 Kg / m² and a tolerance of 10%, the performance range is defined as follows: -Steel thickness Ea between 0.118 mm and 0.146 mm, -The thickness Ep of the polymer layer is between (-2.5xEa+0.632) and (-2.5xEa+0.75). -Density dp between the polymer layer between 0.9 and 1.4, -The volume ratio Rp of the polymer layer is between 0.2 and 1, -Young's modulus Yp of the polymer layer less than 4000 MPa, -where the volume fraction Rp and Young's modulus Yp satisfy the following inequality: Yp*(0.49*Rp²+0.23*Rp+1 / (Yp*(1-Rp))) ≥ 50 MPa Similarly, for a 0.8 mm aluminum target with Tc=28.0 N / mm, Bc=7.2 N / mm, C Mc=2.16 Kg / m² and a tolerance of 10%, the performance range is determined as follows. -Steel thickness Ea between 0.126 mm and 0.140 mm -The thickness Ep of the polymer layer is between (-2.5xEa+0.646) and (-2.5xEa+0.728). -Density dp of the polymer layer between 0.9 and 1.4, -The volume ratio Rp of the polymer layer is between 0.2 and 1, -Young's modulus Yp of the polymer layer is less than 4000 MPa -where the volume fraction Rp and Young's modulus Yp satisfy the following inequality: Yp*(0.49*Rp²+0.23*Rp+1 / (Yp*(1-Rp))) ≥ 50 Mpa The next step is to select the steel and polymer layer for each variable that is within the range defined in the previous step. A person skilled in this field, someone who is knowledgeable about the specifications of steel and polymers, can easily make this choice. In particular, steel selection is guided by the following considerations. Preferably, the skin layer is made of steel sheet with a thickness Ea between 0.1 mm and 0.2 mm. Below this range, it will be difficult to achieve sufficient bending stiffness of the sandwich structure. Above this range, the weight of the steel sheets does not allow the steel structure to be sufficiently light. Preferably, the steel sheets have a thickness between 0.118 mm and 0.165 mm. Even more preferably, the steel sheets have a thickness between 0.126 mm and 0.158 mm. The choice of steel grade depends on the intended application. For automotive applications, the grades commonly used are ES grades (EN DC01 to DC06) and HLE grades (EN H240LA to H400LA). Preferably, sheets are selected from IF-Ti grades. These grades have the advantage of being insensitive to post-treatment hardening (cooking hardening), which allows them to withstand the sandwich construction stage without damage, during which the sandwich structure is heated to temperatures in the range of 240°C -250°C. These grades also have the advantage of being able to withstand deep drawing. These skin layers can be bare. Alternatively, they can be coated to improve performance, especially corrosion resistance. In the case of coated steel, the steel thickness Ea means the thickness of the bare steel skin layer, regardless of the coating thickness. The plating may be a metallic coating obtained by hot-dip galvanizing, electroplating, or vacuum deposition, such as vapor deposition or sonic vapor jet deposition. The plating may include one or more metals such as zinc, aluminum, magnesium, or silicon. As non-limiting examples, we can mention zinc plating (GI), zinc alloy plating (GA), zinc alloy with 0.1 wt-% to 10 wt-% magnesium (ZnMg), zinc alloy with 0.1 wt-% to 10 wt-% magnesium and 5 wt-% to 11 wt-% aluminum (ZnAlMg), zinc alloy with 5 wt-% aluminum (Galfan®), zinc alloy with 55 wt-% aluminum, about 1.5 wt-% silicon, residual zinc and unavoidable impurities due to the process (Aluzinc®, Galvalume®), aluminum alloy containing 8 wt-% to 11 wt-% silicon and 2 wt-% to 4 wt-% iron, residual aluminum and unavoidable impurities due to the process (Alusi®), and aluminum plating (Alupur®). Coating may also include a surface treatment, polishing, or coating with paint or oil. These coatings are known to the person skilled in the art, who uses them and knows how to adjust them on a case-by-case basis. The skilled person is also guided in selecting a polymer layer by the following considerations. The polymer layer can consist of a single polymer or a mixture of at least two polymers (polymer blend below). The choice of polymer or polymer blend is primarily guided by the conditions available for the polymer or polymer bending during the manufacture and use of the sandwich structure. For this reason, a polymer or polymer blend should preferably be selected that: -It has a melting point that does not exceed 220°C -240°C so that it can be used in sandwich structure production lines without excessive heating. -Resistant to the cataphoresis stage, a stage in the manufacture of a motor vehicle during which the paint applied to the vehicle is cured for approximately 45 minutes at temperatures up to 210°C. Preferably, the polymer layer having sufficient adhesion to the metal should be bonded as a single layer directly to the steel skin layers during the construction of the sandwich structure. Alternatively, the polymer layer can be bonded to the steel skin layers by an adhesive at the steel skin layer / polymer layer interface. Alternatively, the polymer layer may be multilayered, the outer layers having good adhesion to the metal. According to one embodiment of the invention, the polymer blend comprises a polyamide such as PA6, PA6-6, PA11, PA12, PA4-6, PA6-10 or PA6-12, the density and Young's modulus of which are known. Preferably, the polymer blend comprises a polyamide and a copolymer of ethylene and unsaturated carboxylic acid and / or derivatives thereof. Details of the characteristics of the components of such a blend and the process for obtaining this blend can be found in the application WO2005 / 0142278, which is incorporated herein by reference. These polyamide-based blends have the advantage of being resistant to the cataphoresis step, having a good surface appearance after shaping, and having good adhesion to metal. According to an embodiment of this invention, the polymer mixture is biphasic and comprises: -Polyamide with a melting point not exceeding 210°C; -Modified polyolefin with carboxy groups, Their rheological behavior in the molten state is characterized by the presence of threshold stress. Details of the characteristics of the components of such a mixture and the process for obtaining this mixture can be found in the application WO2012 / 076763, which is incorporated by reference. Such polymer mixtures have the advantage of being resistant to the cataphoresis stage at 210°C while having a melting point that does not exceed 210°C, which facilitates its implementation and therefore the construction of the sandwich structure. The polymer layer can be foamed. The presence of gas bubbles in the mixture may be due either to the participation of a blowing agent in the polymer mixture or to the physical introduction of gas bubbles into the mixture during the process. If a blowing agent is used, it should preferably be incorporated during the manufacture of the polymer or polymer blend. During the manufacture of the sandwich panel, heating of the polymer layer activates the blowing agent, which releases gas into the polymer. The foaming rate, and therefore the volume ratio Rp, is controlled by the amount of blowing agent added to the polymer layer. Among the blowing agents that can be used, for example, Expensel microsprays can be mentioned, which are added at a very low percentage by weight of the polymer layer. The fabrication itself, for example, the assembly of the skin layers and the polymer layer, can be performed according to any process known to a person skilled in the art.

Claims

CLAIMS 1) Method for manufacturing a sandwich structure comprising two steel skin layers separated by a polymeric layer comprising steps of: - Dimensioning the sandwich structure according to a target to be attained by following the sub-steps of: o defining the target to be attained by three target values, i.e., tensile strength Tc expressed in kN / mm, bending stiffness Bc expressed in kN / mm, and surface mass Mc expressed in Kg / m², o defining a tolerance for the attainment of target values, o defining the sandwich structure by five variables, i.e., the thickness Ea of the steel skin layers expressed in mm, the polymeric layer thickness Ep expressed in mm, the intrinsic Young’s modulus Yp of the polymeric layer, the intrinsic density dp of the polymeric layer, and the volume ratio Rp of the polymeric layer expressed as a volume percentage of the polymeric layer of the material, o identifying the Ea , Ep , Yp , dp , and Rp combinations enabling attainment of target values having the defined tolerance, o determining, for each variable, an operating range, - selecting the steel and the polymeric layer for which each variable is within the range defined in the previous step, - manufacturing the corresponding sandwich structure.2) Manufacturing method according to Claim 1 for which the target to be attained is a monolithic metallic material other than steel.3) Manufacturing method according to Claim 2 for which the target to be attained is aluminum.4) Manufacturing method according to Claim 3 for which the target to be attained is aluminum with a thickness of 0.9 mm.5) Manufacturing method according to Claim 3 for which the target to be attained is aluminum with a thickness of 0.8 mm.6) Manufacturing method according to any of the preceding claims for which the tolerance in attaining target values is 10%.7) Manufacturing method according to any of the preceding claims for which the step of identifying the Ea , Ep , Yp , dp , and Rp combinations enabling attainment of the target values having the defined tolerance includes a step during which the Ea , Ep , Yp , dp , and Rp combinations are generated.8) Manufacturing method according to Claim 7 including a graphical analysis step for the Ea , Ep , Yp , dp , and Rp combinations generated.9) Sandwich structure obtainable by the process according to any of Claims 1 through 8 implemented for a 0.9 mm aluminum target with Tc =31.5 N / mm, Bc =10.2 N / mm, Mc =2.43 Kg / m² and a tolerance of 10%, for a sandwich structure including: - Two steel skin layers with steel thickness Ea between 0.133 mm and 0.165 mm, - A polymeric layer intercalated between the two skin layers and having: o A thickness Ep between (-2.5xEa+0.713) and (-2.5xEa+0.88), o A density dp between 0.9 and 1.4, o A volume fraction Rp greater than or equal to 0.2 and strictly less than 1, o A Young’s modulus Yp lower than 4000 MPa, o Where the volume fraction Rp and the Young’s modulus Yp satisfy the inequality: Yp *(0.49*Rp ²+0.23*Rp +1 / (Yp *(1-Rp ))) ≥ 50 MPa10) Sandwich structure according to Claim 9 for which the steel thickness Ea is between 0.141 mm and 0.158 mm and the thickness Ep of polymeric layer is between (-2.5x Ea +0.73) and (-2.5x Ea +0.87).11) Sandwich structure according to any of Claims 9 through 10 for which the polymeric layer comprises a mixture of a polyamide and a copolymer of ethylene and unsaturated carboxylic acid and / or its derivative.12) Sandwich structure obtainable by the process according to any of Claims 1 through 8 implemented for a 0.8 mm aluminum target with Tc =28.0 N / mm, Bc =7.2 N / mm, Mc =2.16 Kg / m² and a tolerance of 10%, with the sandwich structure comprising: - Two steel skin layers with steel thickness Ea between 0.118 mm and 0.146 mm, - A polymeric layer intercalated between two skin layers and having: o A thickness Ep between (-2.5x Ea +0.632) and (-2.5x Ea +0.75), o A density dp between 0.9 and 1.4, o A volume fraction Rp greater than or equal to 0.2 and strictly less than 1, o A Young’s modulus Yp lower than 4000 MPa, o Where the volume fraction Rp and the Young’s modulus Yp satisfy the inequality: Yp *(0.49*Rp ²+0.23*Rp +1 / (Yp *(1-Rp ))) ≥ 50 MPa13) Sandwich structure according to Claim 12 for which the steel thickness Ea is between 0.126 mm and 0.14 0mm and the polymeric layer thickness Ep is between (-2.5xEa+0.646) and (-2.5xEa+0.728).14) Sandwich structure according to any of Claims 12 through 13 wherein the polymeric layer comprises a mixture of a polyamide and a copolymer of ethylene and unsaturated carboxylic acid and / or its derivative. Abstract The invention relates to a process for manufacturing a sandwich structure comprising two steel skin layers separated by a polymeric layer comprising the steps of: - Dimensioning the sandwich structure according to a target to be attained by following the sub-steps of: o defining the target to be attained by three target values, i.e., tensile strength Tc expressed in kN / mm, bending stiffness Bc expressed in kN / mm, and surface mass Mc expressed in Kg / m², o defining a tolerance for the attainment of target values, o defining the sandwich structure by five variables, i.e., the thickness Ea of the steel skin layers expressed in mm, the polymeric layer thickness Ep expressed in mm, the intrinsic Young’s modulus Yp of the polymeric layer, the intrinsic density dp of the polymeric layer, and the volume ratio Rp of the polymeric layer expressed as a volume percentage of the polymeric layer of the material, o identifying the Ea , Ep , Yp , dp , and Rp combinations enabling attainment of the target values having the defined tolerance, o determining, for each variable, an operating range, - selecting the steel and the polymeric layer for which each variable is within the range defined in the previous step, - manufacturing the corresponding sandwich structure.