"Piston for cold chamber die-casting machines*

DESCRIPTION [ooi] The present invention relates to a die-casting machine and specifically to a piston for a cold chamber die-casting machine.

[002] In cold chamber die-casting machines the use of injection pistons with a steel or copper body and at least one outer sealing ring straddling a collar next to the piston head are known of .

[003]An example of such pistons is described in US 5 233 912. [004] In EP1197279, in the name of the same applicant, a piston for a cold chamber die-casting machine is described which comprises a steel body having a head with or without a peripheral chamfer and at least one sealing ring in copper alloy placed around the body in a respective seat made in a rearward position in relation to the head, and where on the outer surface of the piston between the head and the ring at least two channels have been made, the channels being suitable for placing the piston head in communication with the ring annular seat for an inflow of the melted metal under said ring. In such a way, the metal flowing to the seat, solidifying,

creates a continuous thickening which radially pushes the ring outwards, thus progressively recovering wear, adapting it to any deformation of the piston container and thus protecting the latter. [005] It has however been experimented that with the piston described above, the objective of recovering the wear of the ring by creating a permanent and progressively thickening layer below the ring, thanks to the flow of metal which in its molten state remains and solidifies in the seat of the ring, is achieved but not in an optimal manner, and is subject to improvement, especially for some conditions of working parameters (speed, pressure, temperature, weight of the casting) . [006] In fact, it has been ascertained that the molten metal which penetrates the channels is not always successfully distributed in an even manner under the ring. The radial pressing caused by the metal which has flowed under the ring is in fact sometimes located mainly on the front portion, with the result that the ring tends to buckle, raising itself more towards the front, that is towards the head of the piston, and less towards the back. The recovering of wear is, as a result, uneven around the ring, and the perfect adaptation of the ring itself to the inner surface of the container, which the piston slides in, is not achieved.

[007] In addition, such distortion of the ring in turn causes a counter-thrust or reaction on the solidified metal below it, which obstructs the flow of new molten metal below that already solidified. [008] The purpose of the present invention is therefore to propose a piston for cold chamber die-casting machines which makes it possible to overcome the aforesaid limitations of the pistons according to the state of the art. [009] Such purpose is achieved by a piston according to claim 1.

[ooio] Further characteristics and advantages of the piston according to the present invention will be more evident from the following description made with reference to the attached drawings, by way of an indicative and non- limiting example, wherein:

[ooii] figure 1 is an exploded perspective view of a piston according to the invention; [0012] figure 2 shows the assembled piston; [0013] figure 3 is an axial cross-section of the piston;

[0014] figure 4 shows a perspective view of a piston body in a first embodiment, without the sealing ring; [0015] figure 5 shows the piston body with a sealing ring; [0016] figure 6 shows an elevation view of the piston body of figure 4 ;

[0017] figure 6a is an enlarged, view of the particular marked in figure 6;

[0018] figure 7 shows the piston body in axial cross section; [0019] figure 7a is an enlarged view of the particular marked in figure 7;

[0020] figure 8 is an elevation view of the piston body- fitted with a sealing ring;

[0021] figure 9 shows the piston body of figure 8 in axial cross-section;

[0022] figure 9a is an enlarged view of the particular marked in figure 9;

[0023] figure 10 is an overhead view of the piston, in partial transversal cross-section; [0024] figure 11 is an elevation view of the piston body without ring in one embodiment variation;

[0025] figure 12 is a perspective view of the piston body of figure 11;

[0026] figure 13 is an overhead view of the piston of figure 11;

[0027] figure 14 is an axial cross-section of the piston body of figure 11;

[0028] figure 15 shows a perspective view of a piston body in a further embodiment, without ring; [0029] figure 16 shows the piston body with ring;

[0030] figure 17 shows an elevation view of the piston body of figure 15;

[0031] figure 17a is an enlarged view of the particular marked in figure 17; [0032] figure 18 shows the piston body in axial cross- section;

[0033] figure 18a is an enlarged view of the particular marked in figure 18;

[0034] figure 19 is an elevation view of the piston body fitted with a sealing ring;

[0035] figure 20 is an overhead view of the piston of figure 19;

[0036] figure 21 shows the piston body of figure 19 in axial cross-section; [0037] figure 21a is an enlarged view of the particular marked in figure 21;

[0038] figure 22 is a perspective view of a piston body in a further embodiment variation, without ring;

[0039] figure 23 is an elevation view of the piston body in figure 22;

[0040] figure 24 is a cross-section of the piston body along the line A-A in figure 23; and

[0041] figure 25 shows, in cross-section, a piston body according to the invention at the end of the working cycle and after having removed the metallic riser.

[0042]With reference to the figures, reference numeral 10 indicates a piston having a cylindrical body 11, preferably in steel . The body terminates at the front , that is from the side pressing the molten metal, in a head 12. The head 12 is defined by a frontal surface 13 pressing the molten metal .

[0043] In a preferred embodiment, the body 11 is assembled, for example screwed on, to a support peg 120. The peg 120 has a front portion 121 coupling to the body 11, for example by means of threading 121', which defines with the interior of said body 11, a cooling chamber 140. The peg 120 is crossed axially by a channel 122 able to transport a cooling liquid inside the chamber 140. [0044]Advantageously, between the front end of the peg 120 and the head 12 of the body 11 of the piston a copper pad may be interposed which helps to increase the cooling of said head 12, which is the part of the piston that overheats most during use. [0045] On the front part of the body 11 of the piston, near the head 12, at least one sealing ring 16 is mounted, preferably in copper alloy.

[0046] The sealing ring 16 is housed in a respective annular seat 18 made around the body 11. The seat 18 comprises a cylindrical bottom surface 19. The sealing ring 16 is of the type with a longitudinal split 17,

preferably step-shaped, so as to flexibly widen during fitting to the body and during use, when pressed radially by the molten metal which has flowed under it. The step shape of the longitudinal split 17 also prevents the transit of the molten metal through such split, enabling an optimal pressure seal.

[0047.At least part of this bottom surface 19 of the seat 18 is crossed by at least two channels 20 which extend mainly in a longitudinal direction and which come out at the front in the frontal surface 13 of the piston. A mainly longitudinal direction is taken to mean that the direction along which each channel extends has a longitudinal component, that is parallel to the axis of the piston, as its main component. In other words, each channel is essentially orthogonal to the circumference along which the seat 18 of the ring 16 extends. [0048]As a result, the molten metal which strikes the frontal surface 13 enters the channels 20 and is conveyed by these downwards to the seat 18, coming out on the bottom surface 19 of said seat 18, below the ring 16.

[0049] Preferably, each channel 20 extends from the frontal surface of the piston almost up to the median line of the seat 18 of the ring 16, so as to bring the molten metal mainly towards the barycentre of the sealing ring 16. This permits the molten metal which has flowed under the

ring to press the ring radially mainly at its barycentre, thus avoiding asymmetric pressures which would lead to the buckling of the ring. In addition, the molten metal which reaches the centre of the seat, under the ring, tends to distribute itself quite evenly both frontward and rearwards in relation to the central point of the seat. This is also thanks to the fact that the ring is pressed radially at its .barycentre, and therefore, as soon as it becomes thinner due to wear, all its inner surface detaches from the bottom surface 19 of the seat . [005O]It should be noted that the radial pressing exerted by the molten metal on the ring is taken to mean both the pressing exerted directly on the internal surface of the ring, in other words the surface which the bottom surface of the seat rests on at the beginning of the working of the piston, and that exerted on a layer of metal previously solidified under the ring, after some working cycles of the piston. [0051]According to a preferred embodiment, each channel 20 has a bottom surface 22 inclined in such a way as to join the bottom surface 19 of the seat 18 of the ring 16. In other words, each channel 20 is shaped like an inclined plane which, starting from an essentially central area of the bottom surface 19 of the seat 18, extends frontward converging towards the axis of the piston.

[0052] In this manner, the molten metal pressed by the frontal surface 13 of the piston, once it has penetrated the channels 20, is conveyed directly and with a straight path against the internal surface of the ring 16 (or against a layer of metal previously solidified) , so as to maximise the thrust produced by the molten metal [0053] In addition, as shown in figure 25, the metal which solidifies and which is withheld in each inclined section of channel 20 under the ring, forms a sort of wedge 45 having a pressing surface 46 essentially orthogonal to the axis of the piston, facing and opening at the front. The melted metal pressed by the frontal surface 13 of the piston body, also acts therefore on such pressing surfaces 46 of the metal wedges 45 solidified in the channels. These wedges are then pressed by the molten metal and made to translate axially backwards, with the result that they provoke a radial thrust of the ring. This radial thrust effect caused by the translation of the metal wedges 45 solidifying in the channels adds therefore to the radial thrust effect caused by the accumulation of metal in the seat of the ring, under the ring itself.

[0054]According to a preferred embodiment, each channel 20 is defined by lateral walls 24 which at least at the seat 18 of the ring 16, diverge rearwards. For example, each

channel 20 presents, at least in proximity of the seat 18, a V or fan shape. In other words, the outlet of the channel on the bottom surface 19 of the seat 18 has a greater aperture than the inlet aperture. This way the molten metal coming in to the seat 18 under the ring is led to expand circumferentially under the ring, that is, it is invited to evenly occupy the entire annular extension of the seat 18. [0055]Advantageously moreover, the fan shape of the channel 20 allows the molten metal to enter under the ring without difficulty but hinders its exit as soon as the metal has solidified in contact with the cold piston body. This means that the point of detachment between the riser and the aluminium solidified in the channels is clean and without burrs .

[0056] It is clear that the angle formed by the lateral walls 24 of each channel 20 depends mainly on the number of channels distributed evenly along the circumference of the piston body: the greater the number of channels 20, the smaller such angle will be. In the example shown, there are six channels and the angle formed by the lateral walls 24 is approximately 15°.

[0057]According to one embodiment illustrated in figures 4-13, the seat 18 of the ring 16 is made in a rearward position in relation to the frontal surface 13 of the

piston body and is defined by a rear shoulder 26 and by a front shoulder 27 made in the body 11. In other words, the seat 18 has a U shape, or a rectangular cross- section. In this embodiment therefore the sealing ring 16 is protected by the front shoulder 27 from direct contact with the molten metal introduced into the container. In this case, the head 12 of the piston acts as a protective head for the sealing ring 16. [0058]According to one embodiment, the frontal surface 13 of the piston head is a flat surface and the front shoulder 27 joins such frontal surface 13 with a conical surface 28.

[0059] In the presence of the front shoulder 27, in each channel 20 a rear section 20' affecting only the bottom surface 19 of the seat 18 of the ring, and a front section 20", in continuation of the back section, which crosses the front shoulder 27 which defines the seat 18 and which comes out in the frontal surface 13 , can be distinguished . [0060] From a functional point of view, the front section 20" of each channel enables the molten metal to pass the barrier formed by the head 12 of the piston, fitted with a front shoulder 27, and pass below the front rim of the ring 16, as happens in the pistons according to the prior art; the rear section 20' amplifies to a surprising

extent the effect produced by the front section 20" alone, exasperating the incoming flow of molten metal and conveying such flow deep down into the seat of the ring, with a resulting improved distribution of the pressing forces in a radial direction.

[006I]In addition, while it is opportune for the metal which solidifies in the front section 20" to be removed at each cycle together with the metallic riser, it is advantageous to act so that the metal which solidifies in the rear section 20' of the channels remains there, held back so as to form that sort of wedge 45 which was spoken of above .

[0062]Advantageously, moreover, the front section 20" of each channel 20 has a bottom surface 22" with an inclination towards the axis of the piston which is more accentuated than the inclination of the bottom surface 22 of the rear section affecting the seat 18 of the ring 16. In fact, the greater inclination of the front section 20" of the channel makes the material which solidifies in this section of the channel remain attached to the riser during the expulsion of the casting, thus facilitating the detachment of the riser from the head of the piston. The reduced inclination of the rear section 20' of the channel is designed to have the opposite effect, in other words that of facilitating the detachment from the riser

of the metal which has solidified in the rear section 20' of the channel, holding it back therefore under the ring. [0063] In the embodiment with front shoulder 27 protecting the ring, the metal which solidifies immediately below the ring thanks to the cooling of the piston remains blocked in the seat 18 mainly thanks to the undercut defined by such front shoulder 27. As a result, when the metal riser is extracted from the container at the end of each working cycle of the piston (figure 25) , the pad, or biscuit 47 which is formed between the frontal surface 13 and the front end of the container breaks in proximity of the channels 20, which are the only points of interruption of the undercut created by the front shoulder 27, and specifically at the point of discontinuity between the two sections 20' and 20", leaving the metal which has solidified under the ring 16 in the seat 18. Such solidified metal thus remains "encapsulated" between the bottom surface 19 of the seat 18, the two lateral shoulders 26, 27 and the inner surface of the ring 16, or a layer of previously solidified metal.

[0064] In one embodiment illustrated in figures 11-14, the frontal surface 13 of the piston is in the form of a spherical cap . [0065]According to a preferred embodiment, for example

like that illustrated in figures 11-14, the front section 20" of the channels 20 has lateral walls 24" which converge towards the back (or radially in the opposite direction to the axis of the piston) , to then diverge (lateral walls 24) where the rear section 20' begins. In this case too the convergent shape of the front section 20" of the channels 20 is designed to facilitate the detachment of the riser at the point of discontinuity between the two sections of channel, while the divergent shape of the rear sections 20' enhances the introduction of the metal below the ring preventing its subsequent leaking out .

[0066]According to an embodiment variation illustrated in figures 15-24, the seat 18 of the ring 16 is defined rearwards by an annular stop shoulder 26 made on the piston body and extending as far as the frontal surface 13 of the piston. In other words, in this embodiment a front shoulder 27 protecting the ring 16 is not provided. Advantageously however, the frontal surface 13 has a central zone 30 protruding frontward in relation to the front rim of the ring 16, joining for example to said rim by means of a conical surface.

[0067] Such an embodiment solution with a protruding central portion 30 makes it possible to lighten the riser, facilitate detachment of the riser itself from the

piston and protect the ring from the heat produced by the metal in its molten state. [0068] In this embodiment without front shoulder 27, the channels 20 coincide with the rear section 20' of the form of embodiment with protective head.

[0069] In the absence of a front shoulder 27 which holds back the metal solidified in the seat, other means may be provided to prevent the metal below the ring from being extracted with the riser ^' . [0070] For example, in the bottom surface 19 of the seat 18 of the sealing ring 16 a plurality of cavities 40 are made for accommodating molten metal in such a way that when solidified in such cavities, a plurality of teeth are formed which block the entire layer of metal solidified under the ring.

[0071] The cavities 40 are preferably evenly distributed along the cylindrical surface of the seat 18 , and extend mainly in a circumferential direction, that is presenting an elongated form or slot. [0072] It should be noted that in this embodiment lacking the front shoulder 27, axial blocking devices of the sealing ring 16 to the piston body are also provided. More specifically, advantageously, the sealing ring 16 and the piston have coupling means suitable for producing contemporary axial and angular blocking of each ring to

the body 11 of the piston.

[0073]According to a preferred embodiment, at least two apertures 170 are made in the ring 16 into which two respective radial lugs 180 engage and which extend from the bottom surface 19 of the seat 18. Advantageously, said apertures 170 are slot-shaped and each radial lug 180 is shaped in a circular sector. Preferred examples of embodiment of such axial and radial blocking devices of the sealing ring are described in WO2007116426, in the name of the same applicant. Specifically, the forms of embodiment of axial/angular coupling of the ring and piston described in such document WO2007116426 with reference to figures 7-16 are considered as forming part of the present invention. [0074] It is clear that, as illustrated in figures 4-14, the coupling means 170, 180 between the sealing ring 16 and piston body 11 may advantageously also be provided in the forms of embodiment with annular front shoulder 27, in this case to achieve the angular blocking of the ring 16.

[0075]According to one embodiment illustrated in figures 22-24, the channels 20 form an angle α different from zero in relation to the generatrices of the cylindrical piston body. This embodiment permits once again the creation of an obstacle to the pulling out of the

solidified metal from the seat of the ring. [0076] In addition, in the specific case in which all the channels have the same inclination, a further advantageous technical effect is produced: the molten metal penetrating such channels creates a twisting moment which prevents the unscrewing of the body 11 of the piston from the peg and in turn of the peg from the rod it is screwed onto, a phenomenon frequently occurring in die-casting machines. [0077] Obviously, a man skilled in the art may make further modifications and variations to the piston according to the present invention so as to satisfy contingent and specific requirements, all moreover contained within the scope of protection of the invention as defined by the following claims .