The present invention related to an expansion dummy block for an extruding press for hollow bodies according to the preamble of claim 1. An exemplary dummy block is known from JP-5 076927-A.

Two types of extruding presses for hollow bodies, e.g. pipes, are essentially known in the art.

Both types comprise a container, in which a billet of material to be extruded is inserted, a die against which the billet is pressed by a punch to generate the extruded hollow body.

In a first type of press (figures 1.1 and 1.2), die 1 is provided with an opening 2 from which the material from the billet placed in the container 3 exits, in the form of a hollow body 4 The element producing the internal diameter of the tube is a so-called lance 5, coupled and fixed to the fixed part of the die by wings 6.

The billet material flowing about the wings is cut passing by the side of the latter, and then beyond the wings is joined, compacted and aggregated, finally flowing through the lance. A so-called welded tube is thus obtained, because it consists of separate parts which are joined together by means of seams, or fractures, at which the pipe has weak points, whereby this type of presses manufactures pipes which are not suitable for supporting high internal pressures and strains.

A second type of press (figures 2.1 and 2.2) is provided with an element named mandrel 21. There is again the container 22 of billet 23, as well as a die 24 provided with a bottleneck, through which the material of the billet flows thus forming the external diameter of the tube. At the internal level, the mandrel 21 drives a hole into the billet, which is thus perforated, and at the end of the drilling operation it appears with the head inside the die (fig. 2.2). Once that position has been reached, it stops and the proper extrusion process starts: the material of the billet is forced to pass through the circular crown delimited by die and mandrel, thus producing a tube which in this case is homogeneous because the dividing wings are no longer present, and therefore of higher quality. On the other hand, the machine is more complex because it has the additional part of the mandrel and the controlling device associated therewith.

A type of dummy block 7 (fig. 1.2), named "fixed, expansion dummy block", is used in the presses of the first type on the head of the punch 9; the dummy block is made of elastic material so that when it receives the thrust, and thus the strength of the material to be extruded, it expands about its external circumference, thus obtaining a correct coupling with the internal hole of the container 3 in which it moves. The dummy block consists of a body, which has an external, flexible-ring expansion head; the head has an inner conical shape in which a conical pan 8 is inserted: when the conical pan receives the thrust of the resistance force of the material being extruded, it in turn pushes the external ring of the body, so as to spread it by a given amount (small, e.g. tenths of a millimeter) sufficient to obtain a coupling with the internal edge of the container so that there is no space therebetween. This avoids the leakage and backflow of material which could remain within the container. At the end of the extrusion process, the dummy block is to go back and thus the head is to return to the initial dimension of smaller diameter, otherwise the container could be scratched. Thereby, instead, the dummy block body may be removed without problems. For this purpose, the conical pan 8 has a tang which is inserted into an appropriate recess of the conical head and there is a spring which promotes the spacing apart (releasing) of the pan from the head during the return movement, to return the external ring to its initial size (a small movement is sufficient, e.g. 1 mm).

The second type of press (fig. 2.1) includes a punch 25 which pushes a movable dummy block 26, consisting of a steel disc, with the correct coupling to the internal channel of the container 22, which movies within the channel until it touches the die 24. Once the movable dummy block has reached the travel end, it is not able to go back into the channel. It is thus extracted from the front part by removing the die by means of a so-called "dummy block recirculation" system, which takes lit back to the beginning of the channel for the next extrusion. It is apparent that this system causes waste of time and increase of costs.

It is thus an object of the present invention to indicate an expansion dummy block for extruding press for hollow bodies, adapted to overcome all the aforesaid drawbacks, which may be used in a press equipped with mandrel, thus eliminating the problems of wasting of time and increased costs, and obtaining high-quality extruded hollow bodies.

It is the object of the present invention to provide an expansion dummy block for an extruding press for hollow bodies. according to claim 1.

It is the specific object of the present invention an expansion dummy block for extruding press for hollow bodies, as described in greater detail in the claims, which form an integral part of the present description.

Further objects and advantages of the present invention will be apparent from the following detailed description of an embodiment thereof (and variants thereof) and from the accompanying drawings given by way of non-limitative explanation, in which:

• figures 1 and 2 show section views of known types of dummy block;
• figures 3 shows section views of the dummy block object of the invention;
• figures from 4 to 8 show enlarged section views of the parts forming the dummy block object of the invention;
• figure 9 shows the dummy block object of the invention during its operation;
• figure 10 shows a detail of the head of the dummy block.

The same reference numbers in the figures identify the same elements.

In figure 3, the expansion dummy block in accordance with the present invention is applied to the head of a hollow punch 304, only the end part of which is shown, actuated in a known manner within a press container (not shown).

The expansion dummy block has a hollow shape so as to allow a mandrel to slide therein, and comprises the following elements, the peculiarities of which are also highlighted in the following figures, in addition to fig. 3.

There is a hollow body part 303 (fig. 5), connected to the punch 304 by means of a hollow connector 305.

The hollow connector 305 (fig. 4) has a double quick-screwing system with a bayonet threading: it is inserted into the punch 304, and then turned to lock itself inside the latter by means of a pin 315 (figures 3 e 8); the hollow body 303 is then inserted into the other end with a similar bayonet system and a similar locking system by means of a pin 316 (fig. 5).

The hollow connector (fig. 4) also acts as a bush for guiding the mandrel in its forward and backward movement therein, so that it is always centered and does not slip to the side, thus ensuring a precise, constant thickness of the hollow body. An expansion wear ring 302 (fig. 6) is further present, the external surface of which is adapted to come in contact with the interior of the container to perform the function of globally sealing the container itself. More in detail, this function is carried out by the end part 312 of the external edge, slightly broader than the rest of the external surface 313. The ring is fitted into the end part of the body of the dummy block 303 (fig. 3).

Finally, a hollow conical expansion pan 301 is present (fig. 7). It has a tang 317 which fits into the internal end part of the hollow body 303 and is locked by means of a bayonet attachment (see section C-C in fig. 3, and the bayonet locking protuberance 320 in fig. 7). At the other end, the pan has a broader end 318, the substantially truncated-cone shaped, external surface of which is complimentary with the internal surface of the ring 302. Furthermore, the middle wall 319 is adapted to be engaged in abutment against the external end of the body of the dummy block 303. A spring 306 (fig. 3) is placed between the ends of the tang 317 of the ring and the threaded container 305.

In operation (fig. 9), when the punch 304 pushes the dummy block, the material 323 of the billet within the container is pressed against the die 324 and causes a thrust reaction force against the conical pan 301: the latter slightly retracts (by approximately 1 mm) against the body of the dummy block 303, thus placing its middle abutment 319 in close contact against the dummy block body. The contrast spring 306 is compressed by the pressure of the material.

The retraction causes a friction on the external surface of the broader part 318 of the pan against the internal surface of the end part 312 of the ring, thus achieving an expansion of controlled type of the external part of the conical pan, and thus of the ring, due to the retraction entity of the conical pan. The opening is thus limited, e.g. less than 1 mm, but sufficient.

At the same time, the retraction of the pan also causes an inward expansion of the tip 325 of the internal part of the pan itself, because of the under load deformation of the contrast surface upon the thrust of the material. This surface has a concave shape 326.

The inward expansion of the pan edge causes a scratching effect on the mandrel 327 and cleans it to avoid the waste material of the extrusion from flowing back. As mentioned above, at this step of the extrusion process, the mandrel 327 has already completely perforated the billet 323, and its head 328 stopped in the die 324 thus allowing to extrude the round shape 329.

This contrivance allows to avoid the use of the "dummy block recirculation" system and thus to save cycle time, reduce the need for maintenance, cut both machine costs and required energy - the extrusion is thus performed with fixed dummy block which does not need to be recirculated.

At the end of extrusion, the dummy block is at the travel end against the die. Under this condition, in the absence of reaction force of the material, the external part of the dummy block retracts, under the bias of the spring which is no longer loaded and thus extends, while the internal part opens to return to its initial retracted position.

The concave shape 326 of the face surface of the hollow conical pan 301 (fig. 10) is important because when the material pushes against it, it primarily creates a longitudinal force component, but also creates transversal components, which spread the external tooth-shaped end 312 of the ring and the internal end 325 of the conical pan.

This should all be calculated to conveniently dose the force components, e.g. by means of FEM (finite element method) calculations, to create the desired elastic deformation of the ring. The following parameters should be established: shape and size of the ring, thickness, length of the tooth, material, entity of the thrust force of the material, etc.

For example, the ring may be made of H13 tempered steel, which ensures a given degree of elasticity, and a high resistance to wear. In general, the material is tool steel chosen from those available on the market.

Other possible constructional variants of the described non-limitative example are possible, without therefore departing from the scope of protection of the present claims, comprising all the equivalent implementations for a person skilled in the art.

From the description above, a person skilled in the art will be able to implement the object of the invention without introducing further constructional details.