TOOL FOR FORMING COATED SHEET-METAL BLANKS

The disclosure relates to a tool with a material layer on one or both sides, said tool having a cutting bell rigidly coupled to an upper forming punch and a drawing cushion coupled to a hold-down device, the upper forming punch and a lower forming punch being movable relative to one another to close the tool, and a first gap present between the cutting bell and the hold-down device when the tool is closed, and an end stop formed by the lower forming punch for a lower pressure piece that can be moved relative to it, that the sheet metal blank can be clamped between the upper and the lower pressure piece, and that during a relative movement between the lower forming punch and the lower pressure piece, the upper and the lower pressure piece remain in their position relative to one another clamping a sheet metal blank.

FIELD OF THE INVENTION

The invention relates to a tool for forming sheet metal blanks provided with a material layer on one or both sides, said tool having a cutting bell which is rigidly coupled to an upper forming punch and a drawing cushion which is coupled to a hold-down device, the upper forming punch and a lower forming punch being movable relative to one another to close the tool, and a first gap being present between the cutting bell and the hold-down device when the tool is closed. The material layer can be a layer of paint, a film or any coating.

BACKGROUND OF INVENTION

Such a tool is known, for example, from DE 10 2015 113 267 A1. The tool can be used, for example, to deep-draw a tin can lid or base, a cup-shaped object, in particular the blank of a screw cap for glass bottles or similar, from a painted sheet.

DE 198 42 750 A1 describes a method for producing deep-drawn hollow parts. In the tool used in this case, the sheet metal blank is clamped between the upper forming punch and the lower pressure piece and deep-drawn in successive steps.

Deep drawing is understood to mean, for example, the forming of a sheet metal blank (round blank, plate) into a tin can lid or base or generally into a hollow body or a hollow body into a hollow body with a smaller circumference with or without an intended change in the sheet thickness. During forming, segments of the blank must be folded up to the raised edge of the product and the intermediate sections displaced. Radial tensile and tangential compressive stresses occur in the process. A bend is created when the cutting bell overflows. The force-loaded hold-down device is provided to prevent the outwardly projecting seam or flange from buckling and forming folds un-der the effect of the tangential compressive stresses.

The lids and bases intended for tin cans are made from relatively thin sheets. Said sheets are painted or coated with a material, especially if they are intended for use in the food industry, and often feature advertising prints.

Sheet metal blanks are not only deep-drawn to form caps for glass bottles, but are also formed into bases or lids for tins, such as food cans.

The problem with production is that the paint or coating cracks in the axial outer area of the sheet during forming and fine threads of paint or lacquer (also known as “angel hair”) form. However, the lacquer threads not only became stuck in the tools, but also on the edge of the blank. A cotton-like consistency forms in the tools, which must be removed regularly. The threads that adhere to the formed part, especially at its edge, must also be carefully removed so that they do not come into contact with the filling product (food) during filling, which would be unacceptable.

Bases and lids for cans are mass-produced items of which large quantities are manufactured in one tool. To remove the threads, the production line that includes the tool must be shut down and blown down or cleaned, which increases production time and thus also the manufacturing costs. The threads must be blown off very carefully so as not to jeopardize the health of the personnel operating the system. The hall in which the system is installed must also be constantly cleaned of the paint threads.

In order to significantly reduce or completely eliminate the formation of threads or tails, particularly at high cycle times, EP 1 663 541 B1 proposes to carry out the punching and drawing step in a first tool while applying force and to interrupt the drawing when the seam has reached a defined width, and then to carry out the drawing in a second tool without a hold-down device and without applying force.

In the tool known from DE 10 2015 113 267 A1, when a predetermined position is reached by the cutting bell, the drawing cushion is pulled away downwards so that the lower forming punch moves faster than the upper forming punch, forming the gap between the cutting bell and the hold-down device and thereby releasing the clamping between the edge of the drawing bell and the hold-down device.

SUMMARY

The engineering effort required to move the drawing cushion and the hold-down device responsible for clamping the sheet metal blank in time in the direction of movement of the upper forming punch (downwards) is high. The invention therefore aims to simplify the design of the tool for forming the gap.

To solve the problem, a tool according to the preamble is characterized in that an end stop can be formed by the lower forming punch for a lower pressure piece that can be moved relative to it, that the sheet metal blank can be clamped between the upper forming punch and the lower pressure piece, and that during a relative movement between the lower forming punch and the lower pressure piece, the upper forming punch and the lower pressure piece remain in their position relative to one another clamping the sheet metal blank.

Due to this design, the upper forming punch and the lower pressure piece can only be moved further together when they are attached to each other, holding the sheet metal between them. The gap that has formed between the cutting bell and the hold-down device at the point when the two components strike against each other can no longer change, but remains constant. The force applied by the upper forming punch to the lower pressure piece during forming is directed into the force- or pressure-loaded drawing cushion, which deflects accordingly.

The lower forming punch can have a pocket-shaped recess in which the lower pressure piece is accommodated. The base of the pocket-shaped recess then forms the end stop for the lower pressure piece.

Preferably, the sheet metal blank can be clamped between the upper forming punch and the lower pressure piece in such a way that a second gap is formed between the sheet metal blank and the cutting bell. The width of the first gap is the sum of the thickness of the sheet metal blank and the width of the second gap.

Preferably, the width of the first gap is up to 200%, preferably between 10 and 50%, greater than the thickness of the sheet metal blank. The width must always be selected depending on the material used for the sheet metal blank.

The width of the gap can also be 0.1 mm greater than the thickness of the sheet metal blank.

Preferably, the drawing cushion is rigidly coupled to the hold-down device and the lower pressure piece. For this purpose, the hold-down device and/or the lower pressure piece can be connected to the drawing cushion via at least one drawing pin. Preferably, three or more drawing pins are provided.

An upper pressure piece can be integrated into the upper forming punch. The width of the first gap can be easily varied or readjusted by replacing the upper, lower or both pressure pieces and by adjusting the drawing pins.

An ejector is preferably provided for ejecting the deep-drawn sheet. The ejector and the drawing cushion can be subjected to a permanent force. The force can be constant or variable.

DETAILED DESCRIPTION

As shown inFIG.1a, the deep-drawing tool is essentially composed of the cutting bell1, the ejector2, the upper forming punch3with the inserted upper pressure piece4, the cutting ring5, the hold-down device6, the lower forming punch9with the inserted lower pressure piece10and the drawing cushion12. The hold-down device6and the lower pressure piece10are connected to the drawing cushion12via drawing pins7,11. A force P2acts on the drawing cushion12, which can be applied pneumatically or hydraulically, e.g. by compression springs. The upper pressure piece4is provided on an optional basis, as demonstrated inFIG.1b.

The sheet20to be deep-drawn is between the upper forming punch3and the lower forming punch9. The sheet20is painted or coated with a material layer (e.g. laminated). The coating material can be chosen at will.FIGS.2aand2bdepict the initial position of the tool before forming.FIGS.3aand3bshow the start of the deep drawing. The ejector2is in contact with the sheet20. The upper pressure piece4rests on the upper side and the lower pressure piece10, which is inserted into a pocket-shaped recess9.1in the lower forming punch9, rests on the lower side of the sheet20. The dimensions of the pressure pieces4,10are selected so that a first gap S forms in this position between the cutting bell1and the hold-down device6. The width W of the gap S, which is obtained from the sum of the thickness D of the sheet metal blank20and the set gap D1, can be adjusted for the respective application by altering the gap S1.

FIGS.4aand4bshow the start of the deep drawing process. The upper forming punch3and the lower forming punch9continue to move towards each other without the distance between the cutting bell1and the hold-down device6changing, as the upper pressure piece4and the lower pressure piece10remain in their position relative to each other. The force that occurs when the tool is closed and that is generated by the upper forming punch3and the lower forming punch9is transferred from the lower pressure piece10via the drawing pins11to the drawing cushion12, which is loaded with the force P2. The lower pressure piece10moves in the pocket-shaped recess9.1relative to the lower forming punch3until it comes to rest in the base of the recess9.1. The distance between the cutting bell1and the hold-down device6corresponds to the sum of the thickness D of the sheet metal blank20and the set second gap S2. As a result, the sheet20can move radially inwards in the annular space between the cutting bell1and the hold-down device6during deep drawing and is not clamped, thereby stopping any threads from forming at the outer edge of the sheet metal blank20during deep drawing.

FIGS.5aandbshow the final position of the tool. The sheet20is completely formed, the annular space between the cutting bell1and the hold-down device6is still the sum of the sheet thickness D and the set second gap S2. If the inner edge1.1of the cutting bell1is rounded, it makes it easier to slide the sheet20along and minimizes frictional forces acting on the sheet20.

The forces acting on the drawing cushion12and the ejector2can be constant. These forces move the drawing cushion12and the ejector2in the direction of the sheet20. When mechanical contact is made between the upper pressure piece4and the lower pressure piece10, the sheet20is clamped and the force is transferred through the sheet. This results in a certain degree of automatic thickness compensation when using different types of sheet metal. The distance between the cutting bell1and the hold-down device6required to reduce fuzz adjusts accordingly to the thickness D of the sheet metal blank20. No additional drive other than the tool or pressing movement itself is needed to create the gap S. As a result, the mechanism for creating the gap S is completely integrated into the tool and no separate attachments are required underneath, next to or outside of the tool.

The mechanism for generating the first gap S ensures a reliable force cut-off below a definable minimum distance between the cutting bell1and the hold-down device6. This force cut-off restricts the clamping force for the sheet20. The adjustable minimum gap S to the sheet can be maintained very precisely and almost instantaneously thanks to the mechanical end stop with thickness compensation. The tolerances of the resulting gap S are in the range of <0.1 mm.