Field of the invention

The invention relates to a method for manufacturing an engraved metal sheet, said engraved metal sheet being suitable for the direct mechanical attachment of a thermoplastic polymer. Prior art

Composite products associating a metal part and a thermoplastic polymer are used in very many fields, in particular the automobile field, housings for electronic application, domestic electrical appliances, panels for architecture, sport equipment, etc. They may be either decorative products or functional products.

Such composite products are traditionally obtained by a technology consisting of preparing the metal part using a primer (epoxy glue, polyacrylic dual component, etc.) in order to obtain good adhesion, and then adhesively bonding or injecting the polymer element.

In order to simplify and optimise the method for manufacturing such composite materials, it has been proposed to produce a metal sheet having surface depressions or protrusions and then directly applying a thermoplastic polymer onto said metal sheet, for example by injection. The polymer applied hot then insinuates itself in the surface depressions and protrusions on the metal sheet and then hardens when cooling, thus giving rise to a direct mechanical attachment between the metal sheet and the polymer, that is to say a mechanical attachment in the absence of any intermediate material or primer.

The patent application WO 2004/041533 teaches an aluminium alloy material having a surface roughness of at least 5 μιη and comprising surface depressions and protrusions 0.01 to 0.1 μιη in diameter, said material being mechanically associated with a thermoplastic resin. The rough surface of the aluminium alloy material is obtained by chemical treatment, in particular by treatment of a 2XXX alloy in a concentrated sulfuric acid bath containing chromium.

The patent applications WO 2012/161183 and EP 2 298 525 also describe a metal sheet suitable for the mechanical attachment of an injected polymer. The metal sheets have surface irregularities obtained by a chemical etching method.

The patent application FR2803946 discloses a method for the industrial production of a form of attachment of a plastic molding resin wherein at least one anchoring groove is produced on the entire length of a support by milling, and a controlled re-rolling operation is made in order to reduce the thickness of the support to the thickness considered (x) , which has the effect of provoking, n a concomitant manner, the collapse of flanks of said groove. It is advantageously used to make power electronic components .

The patent application US2015/183185 discloses a composite of metal and resin includes a metal piece and a resin piece combined with the metal piece wherein a surface of the metal piece defines a plurality of micropores including an upper portion and a lower portion, the upper portion is communicated with the lower portion, and an aperture of the lower portion is larger than an aperture of the upper portion. This document is not directed to metal sheets.

The patent application DE102007023418 discloses a method for roughening component surfaces of the internal combustion engines to improve adhesion of layers, which are thermally sprayed onto the surface, comprises roughening the surface under the formation of microscopic undercuts, and introducing obliquely running molding pockets into the surface by pulsed laser beam with inclination angles of 20-80 [deg] . This document is not directed to metal sheets. The patent application DE102013108447 discloses a method for producing a stable, non- bonding agent-free metal-plastic composite component comprising the steps of providing a metal part, processing the surface of the metal part to which the plastic part is to be connected by forming undercut clamping structures and, injection molding of plastic onto one or more machined surfaces or surface portions of the metal part in a plastic molding tool for forming the plastic part under conditions that the plasticized plastic introduced into the plastic molding tool penetrates into the undercut clamping structures and after the cooling of the plastic or the metal the plastic composite component is positively locked therein. This document is not directed to metal sheets.

However, chemical etching methods are difficult to use industrially on a large scale, may be detrimental to the environment and have a high economic cost. Moreover, in order to obtain a metal sheet one main face of which has irregularities while the other main face of which remains smooth, that is to say substantially free from any irregularity, it is necessary to use a resist on the other main face of the metal sheet in order to protect it from any chemical engraving.

In order to remedy the problems posed by such chemical treatments, a method for manufacturing metal sheets suitable for the mechanical attachment of a thermoplastic polymer comprising a step of laser irradiation of one of the main faces of an aluminium sheet was proposed in the patent application JP 2008- 087409. The aluminium sheet first undergoes a first physical working creating cavities on the surface of the sheet (see figure 1) . However, such cavities are not suitable for the mechanical attachment of a thermoplastic polymer since they have an opening with a constant cross-section over their entire depth. The metal sheet therefore undergoes a second operation for producing irregularities (protrusions and recesses 51, Figure 1) in the cavities 5. Such second operation, carried out by laser irradiation, allows the direct attachment of a thermoplastic polymer .

However, such a method for manufacturing metal sheets requires the establishment of two separate method steps for creating cavities 5 and then fine holes and micropores 51 in these cavities 5. In addition, just like chemical treatments, such a method has the major drawback of not being very reproducible, causing irregularities of heterogeneous sizes, and is also difficult to use industrially on a large scale.

There therefore exists a need for a method for manufacturing metal sheets suitable for the direct mechanical attachment of a thermoplastic polymer that is reproducible and easy to use industrially. There also exists a need for a metal sheet suitable for the direct mechanical attachment of a thermoplastic polymer where the size of the surface asperities can be determined precisely according to the application sought, in other words where the adhesion force necessary between the metal sheet and the polymer constituting the composite product can be modulated. Object of the invention

A first object of the invention consists of a method for manufacturing an engraved metal sheet having undercuts, said metal sheet being suitable for the mechanical attachment of a thermoplastic polymer, comprising:

a) a step of mechanical engraving of a metal sheet performed by rolling in a single cold rolling pass and in which a first engraving tool, being a rolling cylinder, applied by pressure against a first main surface of the metal sheet, has a pattern in relief to produce an engraved sheet carrying the negative impression of said pattern in relief;

b) a step of crushing the metal sheet thus engraved to produce one or more undercuts in said negative impression of said pattern in relief. Other subjects of the invention are an engraved metal sheet obtainable by the method according to the invention as well as a composite product comprising said engraved metal sheet and a thermoplastic polymer, said engraved metal sheet and the polymer being mechanically fixed to each other by direct application of said thermoplastic polymer to said engraved metal sheet.

Description of the figures

Figure 1 : example of surface irregularities obtained according to the prior art.

Figure 2: schematic representation in cross-section of a metal sheet subjected to the engraving step (Figures 2a and 2c) or to the engraving and then crushing steps (Figures 2b and 2d) of an embodiment of the method according to the invention.

Figure 3: photograph in surface view of a main face of a metal sheet subjected to the engraving step (Figure 3a) or to the engraving and then crushing steps (Figure 3b) and of the surface of the tool (Figure 3c) , and a view in cross-section of the metal sheet obtained (Figure 3d) of an embodiment of the method according to the invention.

Figure 4 : schematic representation of a main face of a metal sheet subjected to the engraving step (Figure 4a) and view in cross-section of the metal sheet obtained (Figure 4b) of an embodiment of the method according to the invention.

Figure 5: schematic representation in cross-section of a composite product comprising an engraved metal sheet according to the invention and a thermoplastic polymer.

Description of the invention

The subject matter of the invention is a method for manufacturing an engraved metal sheet 1 having undercuts 2, 2', said engraved metal sheet 1 being suitable for the mechanical attachment of a thermoplastic polymer 3. "Thermoplastic polymer" means any polymer that becomes pliable or moldable above a specific temperature and solidifies upon cooling, preserving the form of the object to which it was applied hot. By way of example of thermoplastic polymers, mention can be made of polyvinyl chlorides (PVCs) , polyethylenes (PEs) , polypropylenes (PPs) , polystyrenes (PSs) , polyamides (PAs) , polycarbonates (PCs) and acrylic resins. Within the meaning of the present invention, the term "thermoplastic polymer" also encompasses thermosetting polymers, that is to say polymers for which the polymerisation is irreversible even if the polymers are once again heated, such as polyepoxides (EPs) or unsaturated polyesters (UPs) .

The thermoplastic polymer 3 can be applied to the engraved metal sheet 1 by any method affording good direct adhesion between the thermoplastic polymer 3 and the engraved metal sheet 1. In particular, it may be an extrusion-coating method or a method using a plastic injection press. According to a preferred embodiment, the composite material may be produced by placing an engraved metal sheet 1 according to the invention in an injection mould and then injecting a thermoplastic polymer 3, heated and under pressure, directly in contact with said engraved metal sheet 1. The pressurised thermoplastic polymer 3 is heated so that it has the consistency of a viscous liquid, and said thermoplastic polymer 3 is thus able to insinuate itself into the surface roughness of the engraved metal sheet 1, the roughness corresponding to impressions of patterns in relief comprising undercuts 2, 2'. After cooling of the thermoplastic polymer 3, the engraved metal sheet 1 and the thermoplastic polymer 3 are then mechanically fixed to each other without the need for anything intermediate such as a primer (chemical attachment) . Within the meaning of the present invention, "mechanical attachment" means such a mechanical fixing directly between the engraved metal sheet 1 and the thermoplastic polymer 3, without using a primer. The term "undercut" is well known to persons skilled in the art. Unlike a tapered cavity, a cavity having undercuts is such that, when a thermoplastic polymer is moulded hot in this cavity, said moulded thermoplastic polymer plasticised after cooling cannot be withdrawn without deformation and therefore without damage to said cavity and/or the plasticised thermoplastic polymer. To improve the mechanical attachment between the engraved metal sheet 1 and the thermoplastic polymer 3, the surface area of the undercuts must be as large as possible and/or the number of undercuts must be as great as possible.

The method for manufacturing an engraved metal sheet 1 comprises successively a step a) of mechanical engraving of a metal sheet (fabrication of negative impressions 4, 4' on the metal sheet) , followed by a step b) of crushing of the metal sheet thus engraved (manufacture of undercuts 2, 2 ' in the negative impressions 4, 4') · Such a method therefore has the advantage of allowing precise control of the number and size of the impressions 4, 4' and undercuts 2, 2 ' in the engraved metal sheet 1. Moreover, the manufacturing method according to the invention is particularly reproducible from one metal sheet to another .

The metal sheet used for the present manufacturing method can be obtained by any conventional method known to persons skilled in the art. It may be a case in particular of a sheet made from steel, aluminium or aluminium alloy. According to a preferred embodiment, the metal sheet is a IXXX, 3XXX, 5XXX or 6XXX aluminium alloy and preferentially IXXX or 5XXX alloy in the 0 temper such as preferably a IXXX alloy comprising no more than 0.5% addition elements, the addition elements preferably being Cu and/or Mg, or the 5505 alloy or 5657 alloy in the 0 temper. Aluminium alloys are designated in conformity with the rules of The Aluminium Association, known to those skilled in the art. The definitions of the metallurgical tempers are indicated in European standard EN 515. The step of mechanical engraving of the metal sheet is performed by rolling in a single cold rolling pass and carried out by means of a first engraving tool, being a rolling cylinder, applied by pressure against a first main surface of said metal sheet. Said engraving tool has a pattern in relief to produce an engraved metal sheet carrying the negative impression 4, 4' of said pattern in relief. Preferably said pattern in relief is substantially perpendicular to said first main surface, with advantageously an angle between said pattern walls and said first main surface less than 20°, preferably less than 10°, and preferentially less than 4°, such an angle enables better closing during the step of crushing.

The pattern in relief may consist of cavities 4 as illustrated by Figure 2a or protuberances 4' as illustrated by Figure 2c or a combination of cavities and protuberances. The undercuts 2 as illustrated by Figure 2b result from the crushing of the cavities 4 and the undercuts 2 ' as illustrated by Figure 2d result from the crushing of the protuberances 4' . In an advantageous embodiment, the pattern in relief consists of cavities 4 and the undercuts 2 result from the crushing of these cavities 4.

Advantageously, the pattern in relief of the first engraving tool has a height of 0.05 to 3.0 mm, preferentially from 0.2 to 1.0 mm. Consequently, and in accordance with the above advantageous embodiment, the negative impression of the pattern in relief 4, 4' on the metal sheet has a height of 0.03 to 3.0 mm, preferentially from 0.1 to 1.0 mm and still more preferably from 0.3 to 0.8 mm. Preferentially, the negative impression of the pattern in relief 4, 4' has a height that may range up to half the thickness of the metal sheet.

According to a preferred embodiment, the pattern in relief of the engraving tool has a main dimension L (corresponding to the main dimension L' of the negative impression of said pattern 4, 4' on the metal sheet) of between 0.2 and 10.0 mm, preferentially between 0.5 and 5.0 mm, more preferentially still from 1.0 to 4.0 mm. "Main dimension 1/ " of the impression of the pattern means the maximum length of the impression of the pattern as illustrated in Figures 3 and 4a. According to an advantageous embodiment, in particular when the crushing step b) is carried out by rolling in a single direction RD, said main dimension L' of the impression of the pattern in relief is perpendicular to the direction RD.

The pattern in relief may be disposed regularly or not on the surface of the engraving tool. According to an advantageous embodiment, said pattern in relief and consequently the negative impressions of the pattern in relief are disposed regularly on the surface of the engraving tool and on the metal sheet respectively, such that the pattern in relief, just like the unit cell of a crystal lattice, is repeated periodically by translation in two directions of the plane of the sheet in orderly fashion. In such an embodiment, the distance M between the negative impressions of the pattern in relief 4, 4' is between 2 and 20.0 mm, preferentially between 3.0 and 15.0 mm. In the present invention, the distance M between the negative impressions of the pattern in relief 4, 4' corresponds to the minimum distance between the centre of the axes of symmetry of a first impression and the centre of the axes of symmetry of a second impression that is adjacent to it.

According to an advantageous embodiment, the proportion of the initial surface area of the sheet having negative impressions of the pattern in relief 4, 4', obtained directly after the mechanical engraving step a) , is from 5% to 40%, preferentially from 10% to 30% of the total surface area of said first main face of the engraved metal sheet.

According to an advantageous embodiment, the mechanical engraving step a) is performed by means of a first engraving tool having a pattern in relief and facing the first main surface of the metal sheet and a second tool, being a second rolling cylinder, having a shiny surface of Rz roughness of less than 1 μιη or having a decorative pattern or having a brushed surface of Rz roughness of between 1 and 15 μιη and facing the second main surface of the metal sheet. According to the invention, the Rz roughness is assessed in accordance with NF EN ISO 4287. According to a first preferred embodiment, the second tool facing the second main surface of the metal sheet have an Rz roughness of less than or equal to 1 μη. Alternatively, the second tool has a decorative pattern, generally having an Rz roughness greater than 10 μιη and giving to said second main surface of the metal sheet a special decorative surface appearance. In this advantageous embodiment, the metal sheets have, after the step of engraving the first main surface, a second main surface, substantially free from cracks and other surface defects and shiny or decorative, which makes them suitable for decorative uses. Alternatively, the second tool has a brushed surface of Rz roughness of between 1 and 15 μη.

Preferably, the engraved metal sheet has, directly after the mechanical engraving step a) , a thickness, measured in a non-engraved zone, that is to say free from any impression, of 0.2 to 6 mm, preferentially from 0.5 to 2.5 mm. According to an advantageous embodiment, the engraved metal sheet undergoes a reduction in thickness during the mechanical engraving step a) , measured in a non-engraved zone, of less than 25%, preferentially from 1% to 20%, preferentially again from 2% to 15%.

According to the method of the invention, the mechanical engraving step a) is followed, directly or not, by a step b) of crushing the engraved metal sheet to produce one or more undercuts 2, 2 ' in the impression 4, 4' of the pattern in relief.

Advantageously, the step b) of crushing the engraved metal sheet is carried out by rolling, and/or by means of a press. Crushing by means of a press has the advantage of excellent control of the formation of the undercuts 2, 2', the engraved metal sheet undergoing homogeneous crushing over its entire surface at a given moment.

According to a preferred embodiment, the crushing step b) is carried out by rolling. Rolling allows in particular manufacture of engraved metal sheets at less cost and at high rate and, moreover, requires reduced handling compared with the other embodiments. The rolling can just as well be carried out in a single unidirectional pass or in a plurality of passes. In the case of rolling in a plurality of passes, said passes may be carried out in one or more directions.

According to one embodiment, the engraved metal sheet undergoes a reduction in thickness during step b) , measured in a non-engraved zone, of from 1% to 60%, preferentially from 3% to 30%. Such a reduction in thickness allows the production of undercuts 2, 2' providing a particularly satisfactory mechanical attachment between the engraved metal sheet 1 and the thermoplastic polymer 3.

Advantageously, step b) of crushing the engraved metal sheet is performed by means of crushing tools in contact with the first main surface of the engraved metal sheet having an Rz roughness of less than or equal to 10 μιη, preferentially less than or equal to 2 μτ . In an embodiment of the invention making it possible to obtain a decorative pattern on the second main surface of the metal sheet, the step b) of crushing of the engraved metal sheet is performed by means of crushing tools in contact with the second main surface of the metal sheet having a shiny surface of Rz roughness less than 1 μιη or having a decorative pattern or having a brushed surface of Rz roughness of between 1 and 15 μη . The crushing tools are preferably rolling cylinders.

Another subject matter of the invention is an engraved metal sheet 1 obtainable by one of the embodiments of the method described above as well as a composite product comprising an engraved metal sheet 1 and a thermoplastic polymer 3, said engraved metal sheet 1 and said thermoplastic polymer 3 being mechanically fixed to each other.

Without this being limitative, the composite product of the invention finds in particular its application in fields such as the automobile, aerospace, aeronautical, railway, architectural, sporting, nautical, domestic electrical equipment, etc. fields. It may be either a decorative product or a functional product. More particularly, the composite product may be used for producing, in the automobile field, dashboard panels, interior linings of doors, central consoles, makers' plates, emblems, structural parts without any design requirement, architecture panels, panels comprising for example two thin sheets of aluminium of around 0.55 mm associated with each other by means of a thermoplastic polymer such as polyethylene, battery blocks or wheel trims. The composite product may also find particular applications as housings for electronic applications (computers, tablets, notebooks, mobile telephones, etc.), heat exchangers, air conditioning panels for interior architectural equipment (floor and ceiling panels), electrical and/or thermal and/or acoustic insulation or conduction panels, machine parts (bearings, shafts, gears, etc.), magnesium/aluminium injection parts, or plastic/aluminium/carbon fibre reinforcement parts.

These aspects of the invention, as well as others, are explained in more detail with the help of the following illustrative and non-limitative examples.

Examples

Example 1 : Embodiment of the method for manufacturing an engraved metal sheet according to Figure 3

A metal sheet made from AA99.7MgO.5Cu alloy in the 0 temper with an initial thickness of 2 mm was subjected to a step of mechanical engraving by rolling and then to a second crushing step also carried out by rolling. During the first mechanical engraving step, an engraved cylinder having a pattern in relief in rings regularly distributed over the cylinder was used (distance M between the patterns = 12 mm; main dimension L of the pattern in relief corresponding to the main dimension L' of the negative impression, here outside diameter of a negative impression = 8 mm; height of the pattern in relief 0.8 mm) . The engraved metal sheet has, at the end of the first rolling step, a relief with a central "button" with an inside diameter of 3 mm and an outside diameter of 8 mm.

During the second rolling step, a smooth cylinder (Rz roughness < 10 μιη) was used. The engraved metal sheet, with a thickness of 1.6 mm directly after the first step of engraving by rolling, undergoes a reduction in thickness during the second rolling step, measured in a non-engraved zone, of 12%. The photographs of the metal sheet are respectively presented in Figures 3a and 3b. The photograph of the engraved cylinder is presented in Figure 3c. The proportion of the initial surface of the metal sheet having negative impressions of the pattern in relief obtained directly after the mechanical engraving step was 21%. A photograph in cross-section of the negative impression obtained after the rolling and crushing steps showing the undercut 2 is presented in Figure 3d. The depth of the negative impression obtained was 0.4 mm. Example 2 : Embodiment of the method for manufacturing an engraved metal sheet according to Figure 4

A metal sheet made from AA99.7MgO.5Cu alloy in the 0 temper with an initial thickness of 2 mm was subjected to a step of mechanical engraving by rolling and then to a second crushing step also carried out by rolling. During the first mechanical engraving step, an engraved cylinder having a pattern in relief regularly distributed over the cylinder was used (distance M between the patterns = 1.2 mm; main dimension L of the pattern in relief corresponding to the main dimension L' of the negative impression = 3 mm; height of the pattern in relief 0.8 mm) . The pattern is schematically represented on Figure 4a.

During the second rolling step, a smooth cylinder (Rz roughness < 10 μιη) was used. The engraved metal sheet, with a thickness of 1.6 mm directly after the first step of engraving by rolling, undergoes a reduction in thickness during the second rolling step, measured in a non-engraved zone, of 10%. The proportion of the initial surface of the metal sheet having negative impressions of the pattern in relief obtained directly after the mechanical engraving step was 55%. A photograph in cross-section of the negative impression obtained after the rolling and crushing step showing the undercut 2 is presented in Figure 4b. The maximum achievable depth of the negative impression was 0.2 mm. The high proportion of the initial surface of the metal sheet having negative impressions is less advantageous than in Example 1, the negative depth obtained, which is lower, being disadvantageous for the adhesion of the polymer . Example 3: Embodiment of a composite product

A composite product is produced by injecting a thermoplastic polymer directly on the surface of a metal sheet as obtained at Example 1. The composite product thus obtained is subjected to a mechanical pull-off test in order to characterise the adhesion force between the thermoplastic polymer and the engraved metal sheet .

A sample of the metal sheet as obtained at Example 1 (202 mm in length and 52 mm in width) is cut, inserted and fixed in a plastic injection moulding press. The press is then closed and held at a force of 800 kN in order to inject the molten thermoplastic polymer into a rectangular mould with a volume of 33 cm ³ at a rate of between 10 and 20 cm ³ /s. The thermoplastic polymer used is a PA6GF polyamide. It is introduced at the middle of the sample and flows over the whole of the surface of the engraved metal sheet. The oil circuit of the press then cools the composite product thus obtained for 40 seconds before removing the composite from the press. The cooling of the tool by oil allows a slow cooling of the thermoplastic polymer, which reaches 40°C at the end of the cycle.

The composite product is next cut along the length in sections of 29 x 25 mm corresponding to a surface in contact of 7.25 cm ² . The pull-away test consists of piercing a 6 mm hole in the layer of thermoplastic polymer in order to introduce a screw for the pull-away test. The sample is then held in the pull-away machine by the edges of the metal sheet that are not coated with plastic and the screw is subjected to a tensile force. The tensile force necessary for pulling the thermoplastic polymer away from the engraved metal sheet was between 0.18 and 0.21 N/mm ² .

A comparative test carried out on a metal sheet that had not undergone the process according to the invention was carried out, and the tensile force necessary for pulling the thermoplastic polymer away from the engraved metal sheet was less than 0.1 N/mm ² .

A measurement of the shear strength of the composite was made. The composite was produced on similar samples. The aluminium part is cut on the one hand and the plastic part on the other hand, in order to preserve a common surface of the composite of 25*30 mm, that is to say 7.5 cm ² .

The shear force necessary for pulling the thermoplastic polymer away from the engraved metal sheet was between 0.65 and 1.4 N/mm ² .