The present invention relates to an innovatory method of variable-thickness extrusion and to an extrusion unit applying said method.

Known in the prior art- is an extrusion method identified as "impact extrusion", in which the slug or blank to be extruded is placed in a closed-bottom die and a punch is forced to penetrate into the blank so as to compress the material and cause it to flow into the annular space existing between the die and punch, thereby generating a piece having a cylindrical wall of constant thickness. To make collapsible casings of very soft metal such as lead, for producing toothpaste tubes for example, in US 1,702,278 the die bottom has been proposed to be made movable. The relative movement of the punch and die bottom (the latter embodied as a counter-punch) enables very elongated products like toothpaste tubes to be extruded. In this case too, the obtained article has a constant diameter and walls of constant thickness. The general object of the present invention is to provide a method and an extrusion unit applying said method which enable articles having walls of variable thickness to be obtained.

This is particularly advantageous for making cartridge cases where the area which is the closest to the bottom undergoes the strongest stresses on bursting of the

charge .

In the known art there was nothing suggesting the possibility of obtaining articles having walls of variable thickness, by extrusion. By the method in accordance with the invention thickness variations can be achieved both inwardly and outwardly of the extruded body. In addition, it has been possible to ascertain that the invention enables extrusion of materials traditionally considered as unable to be extruded (non-extrusile materials), such as ferrous materials for example. In view of the above objects, in accordance with the invention, a method of cold impact extrusion for hollow metal elements, in particular cartridge cases, has been provided which comprises the steps of placing a blank in a die, pressing the blank within the die between a punch and a counter-punch facing each other, during compression moving the punch forward in the die at a first velocity and moving the counter-punch backward at a second velocity slower than the first one, so as to reduce the distance between the punch and counter-punch, during said movement causing the punch head to pass through die areas having different distances from the punch sides so as to identify an extrusion annulus of variable width between the die and punch. in accordance with this method, a cold impact extrusion unit for hollow metal elements, in particular cartridge cases, has been devised, which comprises a punch and a counter-punch facing each other and coaxially movable

within a die to identify an extrusion chamber therebetween, characterized in that the extrusion chamber has a varying transverse size along the movement axis of the punch and counter-punch. For better explaining the innovatory principles of the present invention and the advantages it offers over the known art, possible embodiments of the invention applying said principles will be described hereinafter by way of example, with the aid of the accompanying drawings. In the drawings:

- Fig. 1 diagrammatically shows extrusion steps with a first extrusion unit in accordance with the invention;

- Fig. 2 diagrammatically shows extrusion steps with a second extrusion unit in accordance with the invention; - Fig. 3 diagrammatically shows extrusion steps with a third extrusion unit in accordance with the invention;

- Fig. 4 is an enlarged view of a punch head.

With reference to the drawings, shown in Fig. la is a first embodiment of an impact extrusion unit, generally identified by 10 and made in accordance with the invention.

Unit 10 comprises a punch 11 and a counter-punch 12, facing each other and axially movable within a die 13, so as to identify an extrusion chamber 14 therebetween. In an innovatory manner, the chamber within the die has a varying transverse size along the movement axis of the punch and counter-punch, unlike the known art in which the transverse size of the chamber is constant. For example,

shown in Fig. 1 is a barrel-shaped chamber of a maximum transverse size intermediate the ends.

As shown in the drawings, the solution of making the punch with a head of greater diameter than the shank and a cone- shaped point has been found advantageous. This is clearly shown in Fig. 4, where a punch 11 is shown with a head 18 having a cone-shaped end 19 and a rear union length 20 for connection to a shank 21 of reduced section. For example, for a head of a 7.53 mm diameter the shank can have a 7.37 mm diameter, with a connecting conicity of 10° and a 0.50 mm thickness of the cylindrical portion of the head. The cone-shaped head enables accomplishment of a cone-shaped bottom which is advantageous in the case of cartridge cases. According to the method of the invention, as shown in Fig. la, first the counter-punch is close to one entrance end of the punch into the die. A metal blank 15 to be extruded is introduced into the die and subsequently the punch is forced towards the counter-punch, while the latter is caused to move backward, as shown in Fig. lb. The punch and couter-punch movements take place in the same direction but at different velocities (in particular the punch moves at a first velocity and the counter-punch at a second velocity smaller than the first one), so as to reduce the space between the respective faced heads and have an extrusion action between die and punch, whereas the relative flowing between the blank outside and the die is in favour of the extrusion operation. In particular,

the relative velocity between the die side wal] and the extruded material between die and punch must be substantially zero. This enables extrusion of metal materials that traditionally are unable to be extruded, such as iron for example. It is also possible to use lower extrusion pressures, thereby the unit being subjected to less wear. Obviously, the absolute movement velocities of the punch and counter-punch relative to the die will depend on the type of the extruded material and the mechanical features of the extrusion unit. In accordance with the method, the extrusion annulus formed between the die and punch, has a varying thickness during the extrusion operation. In other words, the punch head is caused to pass through areas of the die having different radial widths.

As shown in Fig. lc, once the extrusion stroke has been completed, punch and counter-punch are retracted and the die is opened in a transverse direction so as to free the extruded piece or extrusion. Shown in Fig. lc by way of example is an extrusion 16 the form of which is of variable diameter, which had been hitherto considered as unfeasible by extrusion. A second embodiment 110 of an extrusion unit is shown in Fig. 2. The extrusion unit 110, in addition to the above mentioned steps of the claimed method also carries out a further step.

In the same manner as in the embodiment in Fig. 1, unit 110 is comprised of a punch 113 and a counter-punch 112,

facing each other and axially movable within a die 113, so as to identify an extrusion chamber 114 therebetween. The chamber has varying transverse sizes along its axis. For instance, a chamber is shown which has a shape similar to the one already described above.

After introduction of a blank 115 into the die, the extrusion operation (Fig. 2b) takes place in the same manner as above disclosed.

Unlike the embodiment in Fig. 1, the die 113 has an end opening 117, from which the punch enters and which embodies the aperture for drawing the blank out. If opening 117 has a smaller width than the maximum width of the extrusion chamber, this opening constitutes the die for a second extrusion or forming operation, which is carried out on the blank when the punch is retracted and the counter-punch forces the blank out of the die (Fig. 2c). In other words, the method according to the invention may comprise the further step of drawing the punch out of the die and causing advance of the counter-punch so as to extrude the blank through a die end, which end therefore constitutes the forming end for a further extrusion operation of the forward type.

This further step makes it possible to reproduce within the blank the variation which was present in the outer transverse size of the blank itself, whereas the outer transverse size of the drawn blank will be constant and equal to the extrusion opening 117. In conclusion, an extruded piece 116 is obtained which has

an inward variation in the wall thickness. Such a shaping has been hitherto deemed still more unfeasible by extrusion than shaping shown in Fig. 1. Obviously, the chamber shape and consequently the resulting extrusion shape is not limited to the one shown by way of example.

For instance, a further embodiment of an extrusion unit 210 in accordance with the invention is shown in Fig. 3. The extrusion unit 210 comprises a punch 211, a counter-punch 212 and a die 213 identifying an extrusion chamber 214. The chamber has a varying transverse size along its axis getting bigger in a direction opposite to that of the punch, so as to form a truncated cone, for example. After completion of the extrusion operation as above described, the punch is drawn out and the counter-punch moves forward to extrude the blank through a drawing opening 217. Thus an extruded piece of cylindrical outer form is obtained, the wall of which becomes thicker towards the bottom.

At this point it is apparent that the intended purposes have been achieved by providing an extrusion method and extrusion units enabling an easy manufacture of extruded pieces the shapes of which have been hitherto considered as unfeasible.

It is also to point out that surprisingly with the method of the invention extrusions can be obtained even with metal materials traditionally judged as unable to be extruded, ferrous alloys for example.

It is obvious that the method of the invention is very advantageous for making cartridge cases. Actually, by a single working, cartridge cases with walls of varying thicknesses along the case axis are produced. Thus, as shown in Fig. 3, manufacture of cartridge cases in which a stronger wall is provided close to the area near their bottom, i.e. the blast-initiating area, is made possible. Similarly, particular conformations of the detonation chamber can be obtained, as shown in Fig. 2. Since the method is carried into effect by a mere backward and forward movement of the punch and counter-punch, accomplishment of simple machinery having high production rates can be envisaged, so as to obtain a perfectly formed cartridge case during each machine cycle. Obviously, the above description of embodiments applying the innovatory principles of the present invention is given by way of example only and therefore must not be considered as a limitation of the scope of the invention as herein claimed. For example, movements between die, punch and counter-punch are obviously to be intended as relative movements, no matter which element is actually moved and which is maintained really stationary. In addition, the die may have a shape different from that shown and movements producing articles having an outwardly thickened portion and an inwardly thickened portion can be easily conceived.