EP3479919A1 - Cold forming method for sheet metal or metal foils - Google Patents

Abstract

Method for cold forming sheet metal (2) or metal foil, in which at least one side of the sheet or foil surface is at least partially acted upon by an adhesive plastic film (1) before the sheet (2) or the metal foil is cold worked. The adhesion of the plastic film (1) to the sheet metal or the metal foil must be greater than that during the forming process on the between sheet (2) and plastic film (1) acting shear forces. The plastic film (1) has a strain rate dependent on the deformation rate, in which the tensile stress increases at higher deformation rate.

Description

The present invention relates to a cold forming process for sheets or metal foils.

Known means and methods for improving the cold workability of sheets or metallic foils relate, on the one hand, to the improvement of the sheet or film properties with regard to, for example, elongation at break or solidification and, on the other hand, the tribological properties during the forming process, i. the reduction of the coefficients of friction between the interacting surfaces in relative movement, for example by suitable lubrication.

Many known efforts to reduce the frictional forces between the relatively moving surfaces, that is to say between the surface of the metal sheets to be formed or metal foils and the surface of the forming tools, concern the optimization of the lubricants or lubricants, the use of lubricious intermediate layers such as Teflon layers, or the optimization the sheet or foil surfaces using, for example, Electrical Discharge Texturing (EDT) to form micro-grease pockets on the sheet or metal foil surface. A further measure for improving the tribological properties relates to the active lubrication of the forming tools by, for example, a continuous supply of lubricant between the sheet to be formed or the metal foil to be formed and the forming tool.

The document WO 03/089507 A1 discloses, for example, a mixture for applying a polymeric, corrosion resistant and low-wear formable coating and a method of making this coating. Due to lubricious particles, the coating or the coating is suitable for a low-wear forming in the sense of less abrasion in the mass production of sheets or metal foils.

Object of the present invention is to provide a method which further improves or increases the cold workability of sheets or metal foils.

This object is solved by the features of the independent method claim. Advantageous developments of the invention are specified in the dependent claims.

The following phenomena were found in material tests for the cold workability of sheet metal: Nakajima's determination of limit forming curves using a hemispherical punch to shape sheet metal blanks of different geometry until failure surprisingly changed the cold workability of an aluminum sheet when the sheet surface was applied with a plastic film and depending on which type of plastic film was used.

Cold forming occurs below the recrystallization temperature of the metallic material. This leads to a solidification of the metallic material during the forming. As soon as the tensile stress of the material exceeds a certain limit, the hardening of the material caused by the deformation is no longer sufficient and the workpiece (sheet metal, metal foil) begins to constrict at a weakest point. This constriction counteracts the plastic film with its strain rate-dependent strength behavior. As a result, the plastic film adhering to the sheet or the metal foil during the forming process acts as a solidification aid.

According to the invention - contrary to previous efforts - directed against the forming side of the sheet metal or metal foil surface is not only applied to the friction reducing lubricant, but at least one side of the sheet or metal foil surface is at least partially acted upon by an adhesive plastic film. The at least partially on at least one side of Sheet metal applied plastic film supports at these points the reshaped sheet as a bandage.

In the forming process according to the invention, at least partially a plastic film is laminated on at least one side of the sheet or the metal foil before the cold forming of the sheet or the metal foil, wherein the plastic film or the plastic film sections on the sheet or metal foil surface so strongly liable / liable that the plastic film or whose sections do not detach from the sheet surface during the forming process, ie the adhesion must be greater than that during the forming process to the shearing forces acting between sheet metal or metal foil and plastic film.

Essential for the inventive method is now that the plastic film has a strain rate dependent on the strain rate, in which the tensile stress increases at higher deformation rate. The term expansion stress behavior is used in the present text synonymous with stress-strain behavior or strength behavior. Accordingly, the material strength of the plastic film increases with increasing deformation or forming speed.

The elongation at break of the sheet to be formed is preferably increased by more than 2 percent by the action of a plastic film adhering to the sheet or the metal foil, provided that the adhesion of the plastic film to the at least one sheet or metal foil surface is greater than that between sheet metal during the forming process or metal foil and plastic film acting shear forces and provided that the plastic film has a strain rate dependent on the tensile stress, wherein the tensile stress increases at higher deformation rate.

The inventive forming method is preferred for cold forming of sheets (2) or metal foils, in particular for deep drawing, bending or stretch drawing of sheets for the automotive industry, such as Body parts, and used for building accessories, furniture, household appliances, housings and equipment panels, or for cold forming, in particular deep drawing or stretch drawing, of packaging films of metal. In particular, pipe bending or bending of profiles, if appropriate together with a stretch bending process, are suitable as the bending process.

In ironing, the sheet or metal foil is clamped between the die and a blank holder, and the between the clamped portions or at least part of that portion of the sheet or metal foil is reshaped by means of a punch.

During deep drawing, the metal sheet or the metal foil between the die and a hold-down is held, but not clamped, so that the sheet metal or metal foil material located between the die and hold down during the thermoforming process can follow, with no sheet or foil thickness change as possible.

The sheets suitable for cold forming preferably have a sheet thickness of between 0.2 and 3 mm. Metal foils suitable for the cold-forming process according to the invention typically have a film thickness of 8 μm to 40 μm.

Preferably, only the sheet metal or metal foil surface (s) which is / are directed against the forming tool during the forming process is / are at least partially acted upon by an adhering plastic film. In this case, that part of the forming device which causes the holding or clamping of the sheet or the metal foil does not count toward the forming tool.

In a preferred embodiment of the cold forming process, both sides of the sheet or the metal foil are at least partially provided with an adhesive plastic film.

The shaping method according to the invention is particularly suitable for deep-drawing, stretch-drawing, bending or swaging. The bending can For example, concern the bending of pipes or profiles, in particular, the areas which are held in a holding or clamping device, provided with a pipe or profile cavity reinforcing mandrel.

Preference is also given to a forming process, in which only one side of the movable part of the sheet or the metal foil comes into contact with a forming tool. Here, the movable part of the sheet or metal foil is understood to mean that part which is not fixed by the holding or clamping device of the forming device. In this case, the plastic film is either on the directed against the forming tool side of the sheet or metal foil, or the plastic film is located on the remote from the forming surface of the sheet or metal foil.

In a bending process, which causes sheet metal or metal foil sections with small radii, the plastic film is preferably laminated to the bending tool opposite side of the sheet or metal foil, otherwise - due to the small radii - the plastic film is very strongly compressed at these points and so that there is a risk of delamination of the plastic film.

In a preferred variant of the method, the plastic film (s) or their sections are / are glued to the sheet metal or metal foil surface (s).

The plastic film may also be a plastic layer sprayed or sprayed onto at least one side of the sheet or metal foil.

More preferably, only the portion of the sheet or the metal foil to be formed on at least one side of the sheet or the metal foil is at least partially acted upon by the plastic film.

Preferred materials for the plastic film are oriented polyamide (oPA), polyvinyl chloride (PVC), polypropylene (PP) or high density polyethylene (HDPE).

The sheet or metal foil is preferably made of steel or of an aluminum and / or magnesium alloy.

The thickness of the plastic film is preferably 20% to 500%, in particular 140% to 300%, of the sheet metal or metal foil thickness.

• Figur 1 zeigt einen Querschnitt eines mit einer Kunststofffolie beaufschlagten Blechs während einem Kalttiefziehverfahren.
• Figur 2 zeigt einerseits eine Grenzformänderungskurve nach ISO 12004-2 und andererseits eine entsprechende Grenzformänderungskurve mit einer dem Blech anhaftenden Kunststofffolie.

Further details, features, applications and advantages of the subject of the invention will become apparent from the figures and the description.

• FIG. 1 shows a cross section of a sheet applied with a plastic film during a cold deep drawing process.
• FIG. 2 on the one hand shows a limit shape change curve according to ISO 12004-2 and on the other hand, a corresponding Grenzformänderungungskurve with the sheet adhering plastic film.

When in FIG. 1 illustrated Kalttiefziehverfahren a sheet 2 is held between a die 5 and a blank holder 4 and by means of a punch 3, which presses from top to bottom in the die 5, formed. In contrast to ironing, the sheet 2 is not clamped by the hold-down 4 on the die 5 here during deep drawing. As a result, the sheet metal 2 located between the hold-down device 4 and the die 5 can slip during the deep-drawing process or when the stamp 3 is pressed downwards, with as little as possible a change in the thickness of the sheet 2. A plastic film 1 is laminated on the side of the sheet 2, which faces upward or which abuts a stamp surface of the punch 3. However, the plastic film 1 could also be laminated on one side of the sheet 2, which faces downwards or which faces a die surface of the die 5, or the sheet 2 is laminated on both sides with a plastic film 1.

In FIG. 2 Boundary shape change curves 10, 15 are shown. Such curves 10, 15 are used to determine the forming limits of a material and serve to predict the feasibility of a sheet metal forming task. In the boundary shape diagram, the minor change in shape as a minor change in the sheet plane as a function of the larger change in shape as the main shape change on the ordinate 25 is plotted on the abscissa 20. To determine the in FIG. 2 Nakajima limit-forming curves 10, 15 shown were converted to failure with a hemispherical punch aluminum sheet metal plates of an AlMg ₃ alloy with different geometries and a sheet thickness of 1.0 mm. By varying the width of the sample or by different waisting of the samples are very different deep and stretch drawing conditions from a uniform biaxial deformation to an almost pure tensile load on the sheet surface.

In the lower 10 of the two limit-forming curves 10, 15 shown in the diagram, which are drawn in dashed lines, a lubrication package was placed between the punch of the measuring machine and the respective aluminum sheet metal blanks in order to determine the measuring points according to the standard ISO 12004-2. The lubrication package is composed of different layers of material, which are defined in the above-mentioned standard, namely a directed against the sheet to be deformed lanolin layer with subsequent layers of teflon, lanolin, PVC, lanolin, Teflon and another lanolin layer, which is directed against the stamp surface. According to this standard, the top of the material layers of the lubrication package, which is pressed directly onto the sheet metal blanks sample, consists of lanolin.

When determining the measuring points of the boundary shape change curve 15, which runs in the diagram over the dashed curve 10, a PVC layer with a layer thickness of 2 mm was arranged as the uppermost of the lubricating material layers, instead of the directed against the metal sheet to be formed lanolin layer. Further layers following the PVC layer consist of lanolin, teflon, lanolin, teflon and another lanolin layer, which is directed against the stamp surface. Since the PVC layer sticks on the surface of the sheet metal blank during forming, or when the stamp of the measuring machine presses the lubricating package against the sheet metal blank sample, there is no lubricating effect. Nevertheless, however, the formability of the sheet metal blanks improves, which is confirmed by the measuring points of the limit forming curve 15 which are displaced upward on the ordinate 25 of the diagram and run parallel over the dashed curve 10. A boundary deformation curve 15 shifted parallel upwards means that the deformability improves substantially equally in all tested geometries, or in other words that the PVC layer glued to the surface of the sheet metal blanks improves the formability of the sheet metal blanks irrespective of which geometry Sheet metal plate has. The phenomenon of in FIG. 2 15 shows that the formability of the sheet metal blanks samples improves, although the lubricating effect achieved by the lubricating package with an uppermost PVC layer deteriorates.

Claims (12)

A method for cold forming sheet metal (2) or metal foil, characterized in that before the cold forming of the sheet (2) or the metal foil at least one side of the sheet metal or metal foil surface at least partially with an adhesive plastic film (1) is applied, wherein the adhesion of Plastic film (1) on the metal sheet or metal foil is greater than that during the forming process on the between sheet (2) or metal foil and plastic film (1) acting shear forces, and the plastic film (1) has a deformation rate-dependent tensile stress behavior, wherein the Dehnspannung at higher deformation rate increases. A method according to claim 1, characterized in that only the sheet metal or metal foil surface / s, which is / are directed against the forming tool during the forming process, at least partially with an adhesive plastic film (1) is acted upon / be. A method according to claim 1, characterized in that both sides of the sheet or the metal foil are at least partially provided with an adherent plastic film (1). Method according to one of claims 1 to 3, characterized in that the forming process comprises a deep drawing, stretch drawing, bending or swaging method. A method according to claim 1, characterized in that during the forming process only one side of the movable part, ie the non-clamped area, the sheet or the metal foil comes into contact with a forming tool, wherein the plastic film (1) either on the against the forming tool directed side of the sheet or the metal foil or the plastic film (1) is located on the surface of the sheet or the metal foil remote from the forming tool. Method according to one of claims 1 to 5, characterized in that the plastic film / s (1) is glued to the sheet metal or metal foil surface / s. Method according to one of claims 1 to 5, characterized in that the plastic film (1) is a sprayed on at least one side of the sheet or the metal foil or sprayed plastic layer. Method according to one of claims 1 to 7, characterized in that only the reshaped portion of the sheet or the metal foil on at least one side of the sheet or the metal foil is at least partially acted upon by the plastic film (1). Method according to one of claims 1 to 8, characterized in that the plastic film (1) consists of oriented polyamide (oPA), polyvinyl chloride (PVC), polypropylene (PP), or high density polyethylene (HDPE). Method according to one of claims 1 to 9, characterized in that the sheet (2) or the metal foil made of steel or of an aluminum and / or magnesium alloy. Method according to one of claims 1 to 10, characterized in that the plastic film (1) has a film thickness which corresponds to 20% to 500%, preferably 140% to 300%, the sheet metal or metal foil thickness. Use of the method according to one or more of the preceding claims for cold forming of metal sheets (2) or metal foils, in particular for deep drawing, bending or stretch drawing of sheets for the automotive industry, such as body parts, and for building accessories, furniture, household appliances, housings and equipment panels, or for Cold forming, in particular deep drawing or stretch drawing, of packaging films of metal.

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