Patent Publication Number: US-9835150-B2

Title: Piston for cold chamber die-casting machines

Description:
This application is a National Stage Application of PCT/IB2012/052007, filed 20 Apr. 2012, and which application is incorporated herein by reference. To the extent appropriate, a claim of priority is made to the above disclosed application. 
     BACKGROUND OF THE INVENTION 
     In cold chamber die-casting machines the use of injection pistons with a steel or copper body and at least one outer sealing ring fitting in a seat next to the piston head are known of. 
     An example of such pistons is described in U.S. Pat. No. 5,233,912. 
     In WO2009125437, in the name of the same applicant, a piston for cold chamber die-casting machines is described comprising a body terminating at the front with a frontal surface pressing the molten metal and at least one sealing ring mounted in a respective annular seat made around said body. At least part of the bottom surface of the seat is crossed by at least two channels which extend mainly in a longitudinal direction and which come out at the front in said frontal surface of the piston for an inflow of the molten metal under the ring. 
     Preferably, said channels extend from the frontal surface of the piston almost up to the median line of the seat of the ring, so as to bring the molten metal mainly towards the barycentre of the sealing ring  16 . 
     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. 
     It has however been experimented that with the piston described above, the molten metal which penetrates the channels does reach a central zone of the ring seat, that is to say deposits mainly under the barycentre of the ring, but, in certain conditions of use, is not always successfully distributed in an even manner around the entire bottom surface of the ring. In other words, in some cases, the metal which comes out of a channel penetrating under the ring does not have sufficient thrust to continue to flow towards the adjacent channels, but tends to solidify only at the end of the channel which it came out of. Consequently, the radial thrust caused by the metal which has flowed under the ring is located mainly in some zones causing an uneven distortion of the ring. 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. 
     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. 
     To such purpose, it is to be noted that while in hot chamber die-casting machines the piston is always immersed in a bath of metal in a liquid state, in cold chamber applications, every time the piston is returned to a rearward position and the die opened, the cooling system leads to the formation of a metal riser in front of the frontal surface of the piston and, in the case of the piston described above, to the solidification of the metal which has found its way into the channels and under the ring. One of the difficulties of making a piston recovering wear for cold chamber die-casting such as that described above consists of the fact that if one wishes new metal to flow under the ring at each work cycle to progressively recover wear, then when opening the die to remove the casting the metal which has solidified in the channels must also remain attached to the metallic riser attached to the piece. It is clear that the objective of trapping the metal under the sealing ring, therefore in a rearward position of the frontal surface of the piston as evenly as possible along the circumference of the piston, contrasts with the need to remove the riser so as to liberate the inflow channels of the metal under the ring at each cycle. 
     For example, it has been seen in some cases, with the piston described above, that the metal which has solidified in the channels is not completely removed together with the metallic riser but remains inside such channels preventing a correct inflow of metal under the ring in the subsequent cycle. 
     As said, all these problems are not present in hot chamber die casting machines in that the metal which has found its way into any interstices or passages intentionally created or present in the piston, does not solidify. 
     SUMMARY OF THE INVENTION 
     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. 
     Such purposes are achieved by a piston. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features 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 
         FIG. 1  is a elevated view of a piston according to the invention; 
         FIG. 1 a    is an enlarged view of the piston part in the box C in  FIG. 1 ; 
         FIG. 1 b    is a perspective view of the piston; 
         FIG. 2  is an axial cross section of the piston along the line A-A in  FIG. 1 ; 
         FIG. 2 a    is an enlarged view of the piston part in the box B in  FIG. 2 ; 
         FIG. 3  is an axial cross-section of the piston with a sealing ring mounted next to the piston head; 
         FIG. 4  shows the piston mounted on a stem; 
         FIG. 5  is an axial cross section of the piston-stem assembly along the line A-A in  FIG. 4 ; 
         FIG. 6  shows the piston at the end of a working cycle, with metal solidified under the sealing ring in axial cross-section; 
         FIG. 6 a    is an enlarged view of the piston part in the detail B in  FIG. 6 ; 
         FIG. 7  shows the same enlarged view as  FIG. 6 a    during a subsequent cycle; 
         FIGS. 8 and 9  respectively show in exploded perspective and in axial cross-section, a piston according to the invention with sealing ring in one embodiment variation; 
         FIGS. 10 and 11  show perspective and elevated views of a piston according to the invention in a further embodiment variation; 
         FIG. 12  is an elevated view of the piston in  FIGS. 10 and 11 , fitted with a sealing ring, and 
         FIG. 13  is an axial cross section of the piston in the previous figure, along the line A-A in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings, reference numeral  10  indicates a piston having a cylindrical body  11 , preferably in steel. The body  11  terminates at the front, that is on the side pressing the molten metal, in a head  12 . The head  12  is defined by a frontal surface  13  pressing the molten metal. Said frontal surface  13  may be flat or, as for example shown in  FIGS. 8 and 9 , convex, so as to facilitate the detachment of the metallic riser. 
     In a preferred embodiment, said body  11  is assembled, for example screwed on, to a stem  120 . The stem  120  terminates at the front with a peg  121  coupling to the body  11 , for example by screwing. Said peg  121  defines with the interior of said body  11 , a cooling chamber  140 . The stem  120  is crossed axially by a channel  122  able to transport a cooling liquid inside the chamber  140 . 
     Advantageously, the head  12  of the piston  10  has an axial aperture  12 ′, in which a copper pad  150  is inserted which helps to increase the cooling of said head  12 , which is the part of the piston that overheats most during use. 
     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. 
     The sealing ring  16  is housed in a respective ring seat  18 , having an annular extension, made around the body  11 . The seat  18  comprises a cylindrical bottom surface  19 . 
     In a preferred embodiment, the ring seat  18  is defined rearwards by a rear annular abutment shoulder  20  made on the body  11  of the piston. Even more preferably, the ring seat  18  is made in a position rearward of the frontal surface  13  of the body  11  of the piston and is defined by a rear shoulder  20  and by a front shoulder  22  made in said body  11 . In other words, the bottom surface of the ring seat  18  is lowered in relation to the outer cylindrical surface of the piston  10 . In this preferred embodiment the head of the piston  12  is the front portion of the piston extending between the frontal surface  13  and the front shoulder  22 . 
     As will be explained below however, there is nothing to prevent the ring seat  18  from extending frontwards as far as coming level with the frontal surface  13  of the piston; in this case, the piston head  12  practically coinciding with said frontal surface  13 . 
     In a preferred embodiment, 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  11  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. 
     A distribution channel  24  is made in an intermediate annular portion  19   a  of the bottom surface  19  of the ring seat  18 . Said distribution channel  24  has an annular extension, that is, extends coaxially to the piston axis X. In other words, said distribution channel identifies a bottom surface  24 ′ of the channel lowered further than the bottom surface  19  of the ring seat  18 . 
     Consequently, the bottom surface  19  of the ring seat  18  comprises a rear annular portion  19   b  for supporting a corresponding rear portion of the sealing ring  16 , said intermediate annular portion  19   a , which the distribution channel  24  is made in, and a front annular portion  19   c  for supporting a corresponding front portion of the sealing ring  16 . 
     Preferably, the rear annular portion  19   b  has a greater axial extension than the front annular portion  19   c . Preferably, in addition, the distribution channel  24  has a lesser axial width than the rear  19   b  and front  19   c  annular portions of the bottom surface  19  of the ring seat  18 . 
     Moreover, in a preferred embodiment, the distribution channel  24  is equal or inferior in depth to the ring seat  18 , that is, in relation to the depth of the rear  19   b  and front  19   c  annular portions in relation to the outer cylindrical surface of the piston. 
     Furthermore, in a preferred embodiment, the distribution channel  24  is connected to the rear annular portion  19   b  of the bottom surface  19  of the ring seat  18  by means of a conical connection surface  26 , for example having an inclination of approximately 30°. Advantageously, as will be described further below, said conical connection surface  26  terminates substantially midway of the axial width of the ring seat  18 , that is substantially below the median line of the sealing ring  16 . 
     The distribution channel  24  communicates with the frontal surface  13  of the piston through at least two communication holes  30  made in the piston body  11 . In one embodiment shown in  FIGS. 1-7 , there are three of said communication holes  30 , angularly equidistant from each other. Such communication holes  30  permit a flow of molten metal into the distribution channel  24 , and therefore under the ring  16 , to achieve the recovering effect of the wear of the ring through the formation of successive annular layers of metal which solidify under the ring  16 . Such layers of solidified metal radially push the ring outwards, recovering the thinning ( FIG. 7 ). 
     Unlike the piston channels described above with reference to the prior art, which were radially open outwards, said communication holes  30  are made entirely inside the piston body  11 , between an inlet aperture  32  of the molten metal, made in the frontal surface  13  of the piston, and an outlet aperture  34  of the molten metal, made in or facing the distribution channel  24 . 
     The communication holes  30  are inclined in relation to the piston axis X. In other words, the axes of the inlet apertures  32  are distributed along a circumference coaxial to the piston axis X, said circumference having a smaller diameter than the circumference around which the outlet apertures  34  of said communication holes are made. For example, the communication holes  30  form an angle of about 30° with the piston axis X. For example, the inlet apertures  32  are made in the circular crown portion of the frontal surface  13  which surrounds the axial aperture  13 ′. 
     In addition, said communication holes  30  have a through section which increases towards the distribution channel  24 , that is are a conical shape. For example, the solid angle identified by the communication holes  30  is about 10°. 
     According to a preferred embodiment, the outlet apertures  34  of the communication holes  30  are made in the front annular portion  19   c  of the bottom surface  19  and are open towards the annular distribution channel  24 . Said front annular portion  19   c  is therefore interrupted by the outlet apertures  34  of the communication holes  30 . 
     More in detail, each outlet aperture  34  is connected to the distribution channel  24  by arched connection walls diverging towards said channel  24 . In a preferred embodiment, said connection walls  35  are a portion of the same front lateral wall  24 ″ which defines the distribution channel  24  at the front in relation to the front annular portion  19   c  of the bottom surface  19  of the ring seat  18 . In other words, the front lateral wall  24 ″ of the distribution channel  24  forms, at each outlet aperture  34 , a recess in the lower annular portion  19   c  of the bottom surface  19  of the ring seat  18 , for example cusp-shaped, as shown for example in  FIG. 1 a   . In such a way, each outlet aperture  34  comes out on an outlet surface coplanar with the bottom surface  24 ′ of the distribution channel  24 , but made in the front annular portion  19   c  of the bottom surface  19  of the ring seat  18 . 
     In one embodiment variation of the piston shown in  FIGS. 8 and 9 , particularly suitable for vacuum presses, the body  111  of the piston is provided with a lubrication circuit  112  coming out under the sealing ring  116 , for example at the rear portion  19   b  of the ring seat  118 . In a preferred embodiment, the sealing ring  116  is fitted with an inner circular tooth  117  which couples geometrically with a corresponding annular groove  119  made in the ring seat  118 . Preferably said annular groove  119  is made distally to the exit holes  112 ′ of the lubrication circuit  112  coming out under the sealing ring. For example said annular groove  119  is made axially between said exit holes  112 ′ and the outlet apertures  34 , in an intermediate position of the ring seat. The coupling between the tooth  117  of the ring and the annular groove  119  improves the seal between the ring and the outer surface of the piston, obstructing the passage of air between them. 
     Preferably, in addition, in the sealing ring  116  according to this embodiment, the transversal section  17 ′ of the split  17 , which identifies the step in said split  17  that is, is made along a portion of the tooth of the ring, that is where the thickness of the ring is greater. This makes it possible to avail of the greatest thickness possible between the facing transversal surfaces of the split  17 , to the advantage of an improved seal of the ring. 
     In one embodiment variation of the piston shown in  FIGS. 10-13 , the ring seat  18  is not made in a rearward position and embedded in the piston, but terminates at the front next to or flush with the frontal surface  13  of the piston. Said ring seat  18  is therefore defined only by the rear shoulder  20 . In addition, near the front end of the ring seat  18 , an annular groove  40  is made in the ring seat  18 . Said annular groove  40  in other words crosses the front portion  19   c  of the bottom wall  19  of the ring seat  18 . More specifically, said annular groove  40  is tangent to the front end of the outlet apertures  34 . The sealing ring  16  is provided with an internal annular projection  161  suitable for inserting in said annular groove by means of a shaped coupling. 
     As well as acting as an axial blocking element of the sealing ring, said internal annular projection  161  forms an obstacle to the liquid metal penetrating the communication holes  30  and forces said liquid metal to direct itself mainly towards the rear zone of the outlet apertures  34 , and therefore towards the distribution channel  24 . 
     It is to be noted that, in the embodiment shown in  FIGS. 8-11 , piston and sealing ring are also provided with anti-rotation means suitable to prevent a rotation of the sealing ring  16  on the piston. For example, said anti-rotation means are in the form of radial projections  70  which extend from the bottom wall  19  of the ring seat so as to engage corresponding apertures  162  made in the ring. Clearly, said anti-rotation means may also be provided on the piston in the first embodiment described. 
     Consequently, the metal in the molten state pushed by the frontal surface  13  of the piston penetrates the communication holes  30  and, by a rectilinear path, reaches the distribution channel  24 . Such channel not being engaged by the sealing ring  16 , which rests rather on the rear  19   b  and front  19   c  annular portions of the bottom surface  19  of the ring seat  18 , the metal still in the liquid state is free to expand circumferentially in the distribution channel  2 , that is, is free to evenly occupy the entire annular extension of said channel  24 . 
     Such even distribution of the metal in the distribution channel  24  is favoured by the radial and divergent connection walls  35  which surround the outlet apertures  34  of the communication holes  30 . 
     The inclined and conically shaped communication holes  30  made in the piston body are suitable to cause the breakage of the metallic riser at the inlet apertures  32 . Unlike the longitudinal channel piston described above with reference to the prior art, in which the objective was for the metal solidifying in the channels to be completely extracted with the riser, with the piston according to the present invention the metal is left inside the communication holes  30 , forming a sort of plug. Thanks to the conical shape of the communication channels in fact, when the liquid metal is pushed by the frontal surface of the piston, said plug is heated so as to amalgamate with the liquid metal acting on the frontal surface of the piston and is pushed into the distribution channel. In other terms, the communication holes  30  are made in such a way as to favour a sort of extrusion process by means of which the metal in the liquid state MM (in  FIG. 7 ) which enters the inlet apertures  32  pushes the previously solidified metal SM into the communication holes  30  detaching it from the walls which define said holes  30  and making it enter the distribution channel  24 , where it cools and solidifies ( FIG. 7 ). In other words, at each casting cycle, when new metal in a liquid state penetrates the communication holes  30 , thanks to the conical shape of said holes and the radial and divergent walls  35 , a sort of remodelling of the deposit of metal under the sealing ring takes place, with the result that any interstice below the sealing ring is occupied by solidified metal and the sealing ring is pushed radially outward in a uniform manner. It is to be noted that the conical shape of the communication holes  30  prevents a return of the metal towards the piston head through the communication holes  30  during such phenomenon of amalgamation and remodelling of the metal under the ring. 
     When the solidified metal SM has filled said channel  24 , thereby forming a ring under the sealing ring  16 , the new metal MM coming from the communication holes tends to push said ring of metal not only in a radial direction (arrows F 1  in  FIG. 7 ) but also in an axial direction (arrow F 2  in  FIG. 7 ). Thanks to the presence of the conical connection surface  26  between the bottom surface  24 ′ of the distribution channel  24  and the rear annular portion  19   b  of the bottom surface  19  of the ring seat  18 , the metal ring in the distribution channel  24  forms rearwards a sort of wedge which, as a result of said axial thrust of the new metal coming from the communication holes, tends to cause the sealing ring  16  to rise in the desired point, in other words at its barycentre. 
     Consequently, the piston according to the present invention makes it possible to recover wear of the sealing ring in a safe, reliable and efficient manner. 
     Obviously, a person 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, while remaining within the scope of protection of the invention as defined by the following claims.