BACKGROUND OF THE INVENTION

This invention relates to a deep-drawing tool for deep-drawing blanks which are punched out of sheet metal that is painted or coated with film material, in order to obtain flangeless moulded blanks, and to a corresponding method.

Deep-drawing tools of this type are used to produce container lids in particular. The blanks are punched from metal sheets and are deep drawn in a deep-drawing tool to obtain an approximately pot-shaped form. The deep-drawing tool comprises a drawing bell and a drawing core, around which the drawing bell forms the pot-shaped moulded blank in a downward movement. As used here and in the following, the term “downward movement”, is not intended to limit the invention to specific directions in space, but merely indicates a movement In the direction of a bottom dead centre of the drawing bell during the deep-drawing movement. In fact, this term coincides with a conventional arrangement in a deep-drawing machine, in which the drawing bell is lowered from above onto the drawing core. This movement of the drawing bell is usually controlled by a corresponding crank gear to ensure a sinusoidal movement of the drawing bell.

To prevent folds from forming at the edge of the moulded blank due to material compression, a so-called blank holder is provided which is pressed against the drawing bell from below in such a way that the edge portion of the blank rests between the drawing bell and the blank holder, and is clamped there. This clamping leads in turn to the problem that so-called paint hair forms on the flangeless moulded blank, i.e. hair-like structures which may contaminate the tool.

Various approaches aimed at reducing the formation of such paint hair exist. EP 2 125 264 B1, for example, shows a deep-drawing tool in which the blank holder is pressed in the direction of the drawing bell by the reaction force of a pneumatic spring. This spring acting on the blank holder, also referred to as a “drawing cushion” in the technical jargon, is formed here by a gas volume inside a chamber which is sealed by a piston that abuts against the blank holder by means of force transmission elements. During the deep-drawing process, the piston at first moves down together with the drawing bell. Once the piston reaches a pre-defined bottom position, the chamber is suddenly vented and thus the reaction force of the pneumatic spring Is set to zero. This cancels the clamping of the edge of the moulded blank. This prevents paint or film material from detaching from the sheet metal of the moulded blank and forming the undesirable paint hair as described above.

The design effort involved in the solution presented in EP 2 125 264 B1 is considerable. At every moulding cycle, the pneumatic cushions inside the chamber have to be refilled with high pressure after each venting operation. The consumption of compressed air is considerable, and hence so is the cost of generating the compressed air. Another drawback is the high level of noise at the time of venting the chamber, with the pressurised air escaping from the chamber with a bang. The sound damping systems required for compliance with occupational health and safety legislation generate further costs. In addition, the solution presented In EP 2 125 264 B1 is limited to the use of pneumatic cushions because this is the only type of cushion in which the spring force can be reset to zero by sudden venting. Nevertheless, mechanical drawing cushions are frequently also used in practice, where there Is a pre-tensioned mechanical compression spring between a fixed base plate and a mobile pressure plate. The mobile pressure plate is in contact with the blank holder via force transmission elements in the same way as the piston of the pneumatic cushion.

SUMMARY OF THE INVENTION

Hence it is a task of the present invention to provide a deep-drawing tool of the aforementioned type, offering an alternative means of inhibiting the formation of paint hair at the deep-drawing tool, which is connected with low effort, reduced cost, reduced noise and can also be used for mechanical drawing cushions.

This task is solved according to the invention by a deep-drawing tool with the features of the claims, and a corresponding deep-drawing method according to the claims.

The deep-drawing tool of the invention comprises a drive which, when the drawing bell has reached a predetermined position in its downward movement, drives the force transmission means which transmit the spring force of the drawing cushion to the blank holder to perform a movement preceding a further movement of the drawing bell. For the sake of linguistic simplicity, the position in which this further movement is triggered shall be referred to below as the “predetermined position”.

This preceding movement removes the blank holder from the drawing bell, and the clamping effect of the blank holder is cancelled. This releases the edge of the moulded blank, thereby inhibiting the formation of paint hair. The preceding movement preferably starts just before completion of the deep-drawing movement of the drawing bell, or just before the clamped flange (edge area) of the blank enters the drawing radius of the bell and is elongated. The drive used to generate the preceding movement must be capable of producing sufficient force to overcome the spring force of the drawing cushion.

Because, in the deep-drawing tool according to the invention, the cushion spring force does not have to be reset to zero to release the edge of the moulded blank, considerable costs can be avoided for compressed air generation and noise damping, and there is no longer any limitation to pneumatic drawing cushions.

There are various ways of producing this preceding movement, and a number of preferred variants are presented in the sub-claims. This presentation should not be understood as exhaustive.

According to one embodiment of the present invention, the force transmission means comprise a piston or a pressure plate driven by the spring force in the direction of the drawing bell, whilst a pull rod driven by the drive is mounted on the piston or pressure plate.

The piston is preferably lodged in a chamber and seals a gas volume inside the chamber to form a pneumatic spring. In this case, the pneumatic spring forms the drawing cushion.

According to a further embodiment, the pressure plate is driven by a mechanical spring in the direction of the drawing bell. In this case, the drawing cushion is formed by the mechanical spring.

Further preferably, the drive includes a coupling rod positioned parallel to the pull rod or in its axial extension, and, in order to transmit a pulling movement from the coupling rod to the pull rod, engages with the pull rod no later than when the drawing bell reaches its predetermined position. Before the drawing bell reaches its predetermined position, the coupling rod can move freely relative to the pull rod, i.e. there Is no movement coupling, whereas once the drawing bell reaches its predetermined position, the coupling rod engages with the pull rod and pulls the latter downward with it. The coupling between coupling rod and pull rod may be provided by, for example, a coupling sleeve which is fixedly connected to the coupling rod and runs freely on the pull rod up to a stop which determines the engaged position. At the predetermined position the sleeve abuts against a stop and pulls the pull rod downward with it. The coupling between piston rod and pull rod may also be provided by other means.

According to another preferred embodiment, the drive comprises a cam track and a cam roller which rests on the cam track and is coupled for movement with the force transmission means. Thus, the cam roller can follow the course of the cam track. The shape of the cam track is chosen so that a downward movement of the cam roller Is only transmitted to the force transmission means once the drawing bell has reached the predetermined position in which the preceding movement is supposed to start. The cam track may be formed by various suitable mechanical elements such as, for example, a rotatable cam disk on whose circumference the curved track is formed, or by a translationally movable cam rod with the cam track on a lateral surface.

According to another preferred embodiment, the drive comprises a rotating eccentric connected to the force transmission means via a connecting rod assembly. This eccentric may be a crankshaft, for example, to which the end of a connecting rod is mounted. Apart from the connecting rod itself, there may also be other bars, levers or such like in the connecting rod assembly in order to achieve the desired movement coupling.

According to a further preferred embodiment of the present invention, the drive Includes a camshaft with the cam being arranged to push the force transmission means downward as the camshaft rotates.

Further preferably, the drive comprises a cam rod with a lateral curved profile on which a cam roller rests, which is mounted such that it can be pivoted around a pivot axis offset relative to the cam roller axis, and means for converting an oscillating movement of the cam roller around the pivot axis into a translation movement of the force transmission means. In this case, a translational movement of the cam rod and pivot axis relative to each other triggers pivoting of the cam roller, which is in turn translated into linear movement of the force transmission means.

According to another embodiment of the present invention, the drive of the force transmission means is coupled for movement with the drive of the drawing bell. This Is the simplest and most cost-effective means of synchronising the preceding movement relative to the drawing bell. For large moving masses, such motion coupling is energetically less expensive than providing an independent drive. The coupling may be performed rotationally by the press main shaft using suitable chain, belt or gear transmissions, or translationally by means of suitable coupling with the press ram, the upper tool or the drawing bell.

According to another embodiment of the present invention, the drive comprises an electromagnetic drive for moving the force transmission means.

In this case, according to other preferred embodiments, the drive comprises a coil and a plunger which dips into the coil, or a linear motor whose rotor is coupled with the force transmission means.

A method for deep drawing blanks according to the invention is claimed in the claims.

