PISTON FOR THE INJECTION OF MOLTEN METAL Field of the Invention

The present invention refers to a piston provided with rings to be mounted in the interior of a hot injection chamber of an injection machine for zamak, magnesium or other metal used in the injection molding of pieces .

Background of the Invention As illustrated in figure 1 of the enclosed drawings, the prior art injection machines comprises a hot injection chamber 10 presenting an open end 10a through which is mounted a cylindrical piston 20, which travels along the interior of the injection chamber 10, to compress a charge of molten metal "Z" to the interior of a mold (not illustrated) coupled to an outlet end of the injection chamber 10. A drive rod 15 is coupled to the piston 20 to provide the latter with the force required to compress the molten metal "Z" . Piston 20 is mounted with a reduced radial gap in the injection chamber 10, to permit its displacement with reduced friction along the cylindrical side wall 11 of said injection chamber 10. In order to prevent the molten metal "Z" being compressed from passing through the radial gap between the piston 20 and the cylindrical side wall 11 of the injection chamber 10, the piston 20 is provided with a plurality of circumferential channels 20a disposed along a portion of its length, each channel receiving a sealing ring 30, whose assembly is illustrated in figures 2 and 2a.

The sealing rings 30 are split to enable them to be elastically deformed to an increased diameter, by using a tool F, such as a pincer or a device with a conical shaft, which allows said sealing rings to be mounted over the piston 20 in an axial displacement movement, until reaching their mounting position in a respective circumferential channel 20a, as illustrated

in figures 2, 2a, 2b and 3.

This prior art construction, in which the sealing rings 30 must be elastically deformed to be assembled, presents a series of inconveniences . The segmentation of the sealing rings 30, to allow the elastic deformation thereof during the assembly, causes a certain plastic deformation which impairs obtaining a perfect circularity of the ring in the mounted condition, preventing said ring from being correctly seated against the injection chamber 10, failing to provide an efficient sealing between the sealing rings 30 and the cylindrical side wall 11 of the injection chamber 10 and causing the molten metal "Z" , when compressed, to penetrate between the piston 20 and the cylindrical side wall 11 of the injection chamber 10.

Another inconvenience of this previous construction is that the sealing rings 30 are required to present a reduced cross section to allow their elastic deformation during the assembly. The operation of the system in high temperatures, in conjunction with the small cross section of the sealing rings 30, modifies the force the latter apply against the cylindrical side wall 11 of the injection chamber 10, compromising the tightness of the arrangement.

In order to minimize these problems, the known constructions of the type cited above present a substantially reduced radial gap between the piston 20 and the injection chamber 10. However, this reduction of the radial gap causes another inconvenience, as illustrated in figure 3. This reduced gap limits the capacity of the piston 20 to absorb an eventual misalignment "α" between it and the injection chamber 10, which can lead to a condition in which a binding "e" of the piston 20 occurs in the injection chamber 10 (see figure 3) . Summary of the Invention

As a result of the inconveniences mentioned above and related to the known constructive solutions, it is an object of the present invention to provide a piston for the injection of molten metal, of simple construction and which allows the assembly of sealing rings with an increased cross section, without requiring deformation to be mounted, guaranteeing the tightness of the hot injection chamber, as a function of the almost perfect circularity of the sealing rings.

It is a further object of the present invention to provide a piston which can present an increased radial gap between the piston and the injection chamber, without impairing the tightness of the latter and preventing the piston from binding in the interior of the injection chamber.

As already previously mentioned, the present piston is of the type coupled to a drive rod, to be axially displaced in the interior of a hot injection chamber, presenting a cylindrical side wall and defined in a molten metal injection machine.

According to the invention, the piston comprises: a median portion having an axial extension with an outer diameter substantially smaller than the diameter of the injection chamber; two end portions, each affixed in an end of the median portion and presenting a diameter slightly smaller than that of the injection chamber, one of said end portions being coupled to the drive rod, one of said end portions being removably affixed to the median portion; a plurality of split sealing rings mounted around the median portion, each sealing ring being dimensioned to be externally and circumferentially seated against the injection chamber; and a floating stripper ring mounted around the median portion and between each two consecutive sealing rings, each floating stripper ring having a radial thickness inferior to that of the sealing

rings, so as to maintain a radial spacing in relation to the cylindrical side wall of the injection chamber. The construction defined above allows obtaining a piston, whose sealing rings can be dimensioned with a cross section sufficient for them to exert, against the cylindrical side wall of the injection chamber, a homogeneous radial force in the whole circumferential interface region and with the required intensity for the desired tightness of the piston in the interior of the chamber, as the sealing rings are not submitted to any plastic or elastic deformation during assembly thereof.

As a consequence of the actuation of the sealing rings, the piston can be dimensioned with a larger radial gap in relation to the cylindrical side wall of the injection chamber, preventing the piston from binding due to misalignments in relation to the injection chamber or passage of molten metal through the sealing rings. Brief Description of the Drawings

The invention will be described below, with reference to the enclosed drawings, given by way of example of a possible embodiment of the invention and in which: Figure 1 is a longitudinal sectional view of the prior art piston actuating in the interior of an injection chamber;

Figure 2 is an exploded longitudinal sectional view of the prior art piston and of a tool for mounting the sealing ring carried thereby; Figure 2a is a top plan view of a tool carrying a sealing ring in an elastically open condition, to be mounted around the piston;

Figure 2b is a cross sectional view of the prior art piston, in the region of a sealing ring and mounted in the interior of the injection chamber, illustrating the plastic deformation of the sealing ring after being mounted;

Figure 3 is a longitudinal sectional view of the prior art piston in the binding condition "e" in the interior of the injection chamber, with a misalignment "α" in relation to the axis of the injection chamber; Figure 4 is a longitudinal sectional view of the piston of the present invention, when in the interior of the injection chamber;

Figure 5 is an exploded longitudinal sectional view of the piston of the invention, illustrating the assembly of the sealing rings;

Figure 6 is a longitudinal sectional view of the piston of the present invention, when actuating in the interior of the injection chamber and illustrating the absorption of a misalignment between the piston and the injection chamber;

Figures 7a and 7b are cross sectional views of the sealing rings of the invention, respectively in the operative condition in the hot injection chamber and in the rest condition in the cold injection chamber; Figure 7c is a plan view of the sealing ring of the invention; and

Figure 8 is a cross sectional view of the piston of the present invention, in the mounted condition in the interior of the injection chamber, illustrating the actuation of the sealing ring against the cylindrical side wall of the injection chamber. Detailed Description of the Invention

As already mentioned and illustrated in the enclosed drawings, the present piston 20 is used in the injection of molten metal, said piston 20 being coupled to a drive rod 15 and axially displaced in the interior of a hot injection chamber 10, presenting a cylindrical side wall 11 and defined in a molten metal injection machine (not illustrated) , as represented not only in figures 1-3 related to the prior art, but also in figures 4-8, which illustrate one embodiment of the invention.

According to the invention, the piston 20, to be mounted in the injection chamber 10 through an open end 10a of the latter, can be constructed, for example in DIN X20CoCrWMoIO -9, with adequate characteristics to the operationa-1 conditions of said piston 20.

In general, the present piston 20 comprises a median portion 21 having a cylindrical axial extension with an outer diameter substantially smaller than the diameter of the injection chamber 10 and presenting opposite ends.

In each of the opposite ends of the median portion 21 is affixed an end portion 22, 23, also of cylindrical contour, but presenting an outer diameter only slightly smaller than that of the injection chamber 10, to allow the piston 20 to axially slide in the interior of the injection chamber 10, without interference of the end portions 22, 23 with the cylindrical side wall 11 of said injection chamber 10. One of the end portions 22, 23 is coupled to the drive rod 15 and at least one of the end portions 22, 23 is removably affixed to the median portion 21, as described ahead.

In the illustrated embodiment, the end portion 23 opposite to the drive rod 15 incorporates an externally threaded axial projection 23a, the median portion 21 being provided, in its adjacent end, with an internally threaded axial hole 21a, in which is threaded an axial projection 23a of said end portion 23. The other end portion 22 is incorporated, in a single piece, to the median portion 21. Thus, the illustrated construction provides a piston 20 with the median portion 21 affixing, in a single piece, the end portion 22 to be coupled to the drive rod 15, and further removably carrying the end portion 23 opposite to the drive rod 15.

The provision of at least one of the end portions 22, 23 as a removable piece in relation to the median

portion 21, is fundamental to allow the latter to receive, around its axial extension, a plurality of axially split sealing rings 30, each sealing ring 30, made of adequate metallic material, for example DIN X2 OCoCrWMoIO -9, being dimensioned to be circumferentially and elastically forced against the cylindrical side wall 11 of the injection chamber 10. The removal of at least one of the end portions 22, 23 further allows the assembly, around the median portion 21, of a floating stripper ring 40, for example, of the same material as that of the sealing rings 30, disposed between each two consecutive sealing rings 30, each floating stripper ring 40 having a radial thickness inferior to that of the sealing rings 30, so as to maintain a radial spacing in relation to the cylindrical side wall 11 of the injection chamber 10. With the proposed construction, the sealing rings 30 can be easily mounted around the median portion 21 of the piston 20, without being submitted to any elastic deformation capable to produce a plastic deformation, as it occurs in the prior art assemblies. The floating stripper rings 40 can be constructed in a closed loop and also mounted, by axial slide, around the median portion 21 of the piston 20. Although not illustrated herein, it should be understood that both end portions 22, 23 can be removably affixed in the median portion 21, or only the end portion 22, which is coupled to the drive rod 15. In another embodiment (not illustrated) , the externally threaded axial projection 23a can be provided in the median portion 21, the axial hole 21a being provided in the end portion 23 opposite to the drive rod 15. In the illustrated embodiment, the end portion 22, adjacent to the drive rod 15, is provided with an internally threaded axial hole 22a, inside which is retained a threaded end projection 15a of an adjacent end of the drive rod 15.

While not illustrated herein, it should be understood that the end portion 22, adjacent to the drive rod 15, can be provided with a threaded end projection, in which construction the threaded axial hole is provided in the adjacent end of the drive rod 15.

As illustrated, the end portion 22, adjacent to the drive rod 15, can be provided with an eccentric axial orifice 24 connecting the axial hole 22a of said end portion 22 to the axial hole 21a of the median portion 21 and receiving a locking screw 50 with an inner end 51 to be positioned against the axial projection 23a of said end portion 23 opposite to the drive rod 15. The locking screw 50 prevents the end portion 23 from releasing from the median portion 21 during the hot work in the injection chamber 10.

As it can be observed in figures 4, 5 and 6, between each two sealing rings 30 is mounted a floating stripper ring 40 in a non-split closed loop with a height substantially smaller than that of the sealing rings 30.

The sealing rings 30 have an increased cross section, guaranteeing an almost perfect circularity, which is maintained during the operation in the injection chamber, providing the tightness of the system and preventing the liquid metal from passing between the piston and the cylindrical side wall 11 of the injection chamber 10, upon the occurrence of injection pressure (see figure 8) . This increased cross section, with the consequent greater piston X injection chamber tightness, allows the piston 20 (its end portions 22, 23) to be constructed so as to present an increased radial gap in relation to the cylindrical side wall 11 of the injection chamber 10 (see figure 4) , preventing that an eventual misalignment "α" between the axis of the piston 20 and the axis of the injection chamber 10 causes the piston 20 to bind in the cylindrical side wall 11 of the injection chamber 10 (see figure 6) .

In order to allow the thermal expansion of the sealing ring 30 during the hot work in the injection chamber 10, increasing the contact force therebetween, each sealing ring 30 is split according to a section line formed by two axial portions 31, 32 extending from opposite end edges of the ring, circumferentially spaced apart and joined by a median circumferential portion 33. The axial portions 31, 32 of the section line present opposite sides spaced from each other, defining a gap for the thermal expansion of the ring, the opposite sides of the median portion 33 being maintained seated to each other upon mounting the sealing rings 30 to the piston 20, so as to guarantee that the molten metal will not pass from one axial portion to the other, which would represent a loss of tightness in the injection chamber 10.

While only one embodiment for the present invention has been illustrated herein, it should be understood that alterations can be made in the form and physical arrangement of the elements that form the assembly, without departing from the constructive concept defined in the claims that accompany the present specification.