Patent Publication Number: US-7900552-B2

Title: Piston for cold chamber die-casting machine

Description:
This application is a continuation of PCT/IT2007/000255, filed Apr. 4, 2007, which claims priority from Italian Patent Application No. BS2006A000087, filed Apr. 12, 2006. 
     FIELD OF THE INVENTION 
     The present invention relates to die-casting machines and in particular it refers to a piston of a press for cold chamber die-casting. 
     BACKGROUND ART 
     In cold chamber die-casting machines it is known to use injection pistons with a steel or copper body and at least one outer sealing band arranged astride of a collar at the piston head. 
     An example of such pistons is described in U.S. Pat. No. 5,233,912. 
     EP1197279, by the same applicant, describes a piston for cold chamber die-casting machines which comprises a steel body having a head with or without peripheral chamfer and at least one copper alloy sealing band arranged about the body in a respective seat obtained in retracted position relative to the head, and wherein on the outer piston surface comprised between the head and the band there are obtained at least two channels intended for placing the piston head in communication with the annular seat of the band for metal inflow underneath the band itself. In this way, by solidifying, the metal that flows into the seat creates a continuous thickening that radially pushes the band outwards, progressively recovering the wear thereof, adapting it to any deformations of the piston container and therefore protecting the latter. 
     However, it has been found that in all the known embodiments thereof, the sealing band tends to rotate on the piston making the operation thereof less effective. 
     In fact, all sealing bands have a split or interruption shaped as a step along the circumference that allows assembling the band on the piston and ensures certain radial expansion thereof. Especially in the assembly step, when the band is released, the elasticity thereof could cause settling in an unsuitable position that impairs the sealing thereof. For example, the band split should not be:
         facing the top side of the container, where fused aluminium exhibits greater fluidity (in fact, aluminium lays by gravity on the bottom portion, remaining in contact with the surface of the container and of the piston head for longer, losing a few temperature degrees);   at the starting points of the casting branches, where metal at the liquid state has greater fluidity,   at the mechanical interference points that could occur at the container openings (liquid metal loading inlet and casting channel starting points).       

     In an attempt to obviating such disadvantage, a piston having a radial pin and a sealing band has been proposed, which at the step-wise split or interruption exhibits a seat suitable for receiving said pin. 
     Such solution however makes the assembly of the band on the piston more difficult and in any case requires separate means for the axial and angular locking of the band. 
     SUMMARY OF THE INVENTION 
     The object of the present invention therefore is to propose a piston for cold chamber die-casting machines, which should allow obviating the disadvantage mentioned above in a safe and effective manner. 
     Another object of the invention is to provide a piston provided with means suitable for locking the sealing band in axial and angular direction at the same time. 
     Such objects are achieved with a piston according to claim  1 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the piston according to the present invention will in any case appear more clearly from the following description, made with reference to the annexed indicative and non-limiting drawings, wherein: 
         FIG. 1  shows an exploded view of the piston according to the invention; 
         FIG. 2  shows the assembled piston; 
         FIG. 3  shows a partial axial section view of the piston; 
         FIG. 4  shows an embodiment variation of the piston; 
         FIG. 5  shows a perspective view of the piston of  FIG. 4  in a retracted position relative to the container of a press; 
         FIG. 6  shows the piston of  FIG. 4  in a forward position in the container; 
         FIG. 7  shows an axial section view of the piston in another embodiment; 
         FIG. 7   a  shows an enlarged view of the circled detail in  FIG. 7 ; 
         FIG. 7   b  shows an exploded perspective view of the piston of  FIG. 7 ; 
         FIGS. 8 and 8   a  show an axial section and an exploded perspective view of a piston in another embodiment; 
         FIGS. 9 and 9   a  show an axial section and an exploded perspective view of a piston in another embodiment; 
         FIGS. 10 ,  10   a  and  10   b  show an axial section, a front view and an exploded perspective view of a piston in another embodiment; 
         FIGS. 11 and 11   a  show an axial section and an exploded perspective view of a piston in another embodiment; 
         FIGS. 12 and 12   a  show an axial section and an exploded perspective view of a piston in a further embodiment; 
         FIGS. 13 and 13   a  show an axial section and an exploded perspective view of a piston in another embodiment; 
         FIGS. 14 and 14   a  show an axial section and an exploded perspective view of a piston in a further embodiment; 
         FIGS. 15 and 15   b  show an axial section and an exploded perspective view of a piston in another embodiment; 
         FIG. 15   a  shows an enlarged view of the circled detail in  FIG. 15 ; 
         FIGS. 16 and 16   b  show an axial section and an exploded perspective view of a piston in another embodiment; and 
         FIG. 16   a  shows an enlarged view of the circled detail in  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIGS. 1-4 , reference numerals  10 ,  110  denote a piston having a cylindrical body  11 ,  111 , preferably of steel. The body ends at the front, or on the side pushing the fused metal, with a head  11 ′,  111 ′. 
     Piston  10 ,  110  slides inside a container  60  of a press of a die-casting machine. Considering piston and container arranged horizontally, container  60  has, obtained in a top portion  2  and at an end thereof, an inlet  1  for loading the metal at the liquid state, for example aluminium. At the opposite end relative to inlet  1 , container  60  has splits  3  corresponding to starting points of the casting branches. 
     In a preferred embodiment, said body  11  is mounted on a support plug  12 . Plug  12  exhibits a front portion  13  of smaller diameter so that between it and body  11  there is defined a cooling chamber  14 . Plug  12  is axially crossed by a duct  15  for a cooling fluid. Said duct  15  leads into chamber  14  through radial channels  15 ′. 
     Advantageously, between the front end of plug  12  and head  11 ′ of body  11  of the piston there is arranged a copper pad  50  that contributes to increasing the cooling of said head  11 ′, which is the piston portion that overheats more during use. 
     On the front portion of the piston body  11 , in the proximity of head  11 ′, there is mounted at least one sealing band  16 , preferably of copper alloy. 
     According to the invention, each sealing band  16  and the piston comprise connecting means suitable for concurrently making an axial and angular locking of each band to the piston body  11 . 
     In accordance with a preferred embodiment, at least two openings  17  are obtained in band  16  wherein respective radial projections  18  that extend from the piston body  11  engage. Advantageously, said openings  17  are shaped as slots and each radial projection  18  is configured as a circular sector. 
     Preferably, openings  17 , and as a consequence the piston projections  18 , are arranged symmetrically relative to the main axis X of the band and of the piston. Advantageously, if N is the number (greater than or equal to, two) of the openings and of the relevant projections, openings  17  and projections  18  are spaced from each other by equal angles α=360°/N. 
     Preferably, openings  17  are through openings. However, projections  18  have a height not more than the band thickness, so as to not protrude from said openings and thus damage the container wherein piston  10  slides. 
     In accordance with a preferred embodiment, each opening  17  has a greater passage section than the cross section of projections  18 . On the one side this facilitates the assembly and the expansion of the band following the penetration of metal thereunder; on the other side, it allows the excess liquid metal to escape through the openings of the band preventing the latter from being removed. 
     In particular, slots  17  have curved end portions  17 ′ not occupied by the respective projections  18 , which on the other hand have a substantially rectangular section. 
     In one embodiment illustrated in  FIGS. 1-3 , piston  10  is provided with a single sealing band  16  that extends starting from an annular stop shoulder  20  up to head  11 ′ of body  11 . Said stop shoulder  20  axially constrains band  16  during the forward movement of the piston. The axial locking of the band during the piston roll-back on the other hand, is guaranteed by the interaction between projections  18  and openings  17 . 
     In one embodiment illustrated in  FIG. 4 , piston  110  is provided with two sealing bands  116 ,  116 ′ located in respective seats  120 ,  120 ′ obtained around body  111 . As in the version described hereinbefore, bands  116 ,  116 ′ are axially and angularly locked to body  111  by the coupling between radial projections  18  and openings  17 . 
     Head  111 ′ of body  111  has a peripheral chamfer. 
     Advantageously, on the outer surface of the piston body  111  comprised between head  111 ′ and the band  116  that is closer thereto there are obtained at least two channels  160  that connect the bottom of seat  120  of the band to head  111 ′ of the piston body. Through such channels, the fused metal can flow into seat  120  under band  116 , so as to create a continuous thickening that compensates the thinning of the band due to the wear thereof as a consequence of the piston use and of the thermal deformations of the container thereof. 
     The piston therefore, keeps the seal and the efficiency longer irrespective of the level of wear of the copper alloy band. 
     Bands  16 ,  116 ,  116 ′ have respective splits or interruptions  19 , for example step-wise, suitable for allowing the band assembly on the piston body and, especially in the case of the embodiment described in  FIG. 4 , the expansion thereof subsequent to the inflow of the liquid metal into the seat under the band. 
     In accordance with one embodiment illustrated in  FIGS. 7 ,  7   a  and  7   b , the piston body  11  has, in the proximity of head  11 ′, an annular channel  30 . Such annular channel  30  is covered by the sealing band  16  when this is not fitted on body  11 . The effect of channel  30  is to hold any metal that could introduce between band  16  and body  11 , creating a thickening under band  16  when the metal solidifies that contributes to keeping the band seal on the walls of container  60  unchanged. 
     According to one embodiment illustrated in  FIGS. 8 and 8   a , the sealing band  16  does not end at the level of head  11 ′ of the piston body  11 , but remains in retracted position. In particular, band  16  links to head  11 ′ by a conical surface  33 . For example, such conical surface  33  is inclined by 45° relative to the piston axis. 
     Such embodiment solution allows lightening the feedhead thanks to the presence of a head  11 ′- 33  of the piston that protrudes from the sealing band  16 ; moreover, the presence of such protruding head favours the detachment of the feedhead itself from the piston and protects the band from the heat produced by the metal at the liquid state. 
     It should be noted, always in the embodiment illustrated in  FIGS. 8 and 8   a , that band  16  has a symmetry relative to a cross axis, besides the main axis. In other words, openings  17  are obtained in intermediate position relative to the longitudinal extension of the band. This allows mounting the band on body  11  without distinction on both ends and, advantageously, reversing the band orientation when the portion thereof facing the piston head is worn. 
     Of course such device can be extended to all the embodiments of the piston described. 
     In one embodiment shown in  FIGS. 9 and 9   a , the piston body  11  has a concave head  11 ′. Such configuration of the head in the first place allows relieving the pressures exerted by the metal in the container towards the piston axis, thereby relieving the pressure on the peripheral zone where the sealing band is; in the second place, it has been proved that the feedhead separation is facilitated. 
     The piston illustrated in  FIGS. 10 ,  10   a  and  10   b  is similar to that of  FIG. 4 , since it comprises a band  16  retracted relative to a truncated-cone head  111 ′ of the piston body  111 , and a plurality of radial channels  160  obtained on the side surface of said head  111 ′. In this case, however, band  16  is not seated in a special seat communicating with channels  160 , but is held into position only by the coupling between projections  18  and slots  17 . To make the liquid metal penetrate, through channels  160 , underneath band  16 , such channels  160  do not extend to the truncated-cone head  111 ′ only but end, at the back, in the cylindrical zone of body  111  on which band  16  is mounted. Also in this case, therefore, the metal that solidifies under the band tends to compensate the wear thereof radially pushing it and keeping it always adhering to the container wall. 
     In one embodiment shown in  FIGS. 11 and 11   a , the sealing band  216  and the piston body  211  have, along the axial direction, two orders of slots  17  and projections  18  for further increasing the axial and angular locking of the band. 
     The piston of  FIGS. 12 and 12   a  comprises, like that shown in  FIG. 4 , two parallel sealing bands  16 ,  316 , front and back respectively. In this case, the back band  316  is mounted in a relevant seat  316 ′ obtained in the support plug  12 , rather than on body  11 . This allows further spacing the two bands and thereby increasing the piston coaxiality relative to the container chamber and the guide effect of the bands. Such configuration is especially suitable for vacuum die-casting. 
     According to one embodiment illustrated in  FIGS. 13 and 13   a , the sealing band  16  is seated in a seat  420  obtained in retracted position relative to head  411 ′ of the piston body  411 . In other words, body  411  has a cylindrical end portion  411 ″ that along with the most retracted band  16 , contributes to sealing on the wall of the chamber of container  60 . This can be obtained by making such cylindrical end portion  411 ″ with a tolerance lower than the fluidity of metal at the liquid state (that in the case of aluminium is about one tenth). 
     According to one embodiment illustrated in FIGS.  14  and  14   a , the piston body  11  is mounted on a plug  112  with simplified structure, without the reduced diameter portion  13 . The plug portion  112  is crossed by the cooling duct  15  that extends up in the proximity of the piston head. Such plug structure  112  can be used in particular for reduced diameters, where there are no particular cooling problems. 
     In accordance with one embodiment illustrated in  FIGS. 15 ,  15   a ,  15   b ,  16 ,  16   a  and  16   b , projections  518 ,  618  of the piston body  511 ,  611  are shaped as fitted blocks fixed to the body, not integral therewith. For example ( FIGS. 15 ,  15   a  and  15   b ), blocks  518  are removably connected to body  511  by screws  518 ′. Each block  518  has a seating recess for the screw head  518 ′, so that this does not protrude at the level of the sealing band  16 . Moreover, each block  518  is seated in a respective seat  518 ″ obtained in the piston body  511 . At least at the level of the holes for screws  518 ′, the latter may have an increased thickness zone. 
     In this case, if the blocks become excessively worn impairing the locking of band  16 , they can be removed and replaced. 
     In the example of  FIGS. 16 ,  16   a  and  16   b , blocks  618  are inserted in special seats  618 ′ obtained in body  611 , passing through openings  617  obtained in band  616 . Afterwards, the edge of openings  617  that protrudes in height relative to the blocks is riveted or caulked on top of the block, locking it into position. This embodiment solution has the advantage that it is not necessary to provide increased thickness zones in the piston body. 
     In the case of fitted blocks  518 ,  618 , they can have a circular or square section. 
     According to a further aspect of the invention, again illustrated in  FIGS. 16 ,  16   b , the sealing band  616  is shaped as a cap that covers not only the side cylindrical surface of body  611 , but also head  611 ′. In the practice, if made of a copper alloy like the sealing bands, the cap simulates the effect of a copper piston that for certain applications and in certain conditions, is preferable to a steel piston with copper sealing band. 
     Advantageously, cap  616  is obtained by spinning so as to be more resistant than a cap made by extrusion. 
     The embodiment with fitted blocks  518 ,  618  is particularly suitable for the use of the closed cap band  616  since it is possible to first axially fit band  616  on the piston body, and then the locking blocks are applied. 
     It is clear that the cap band  616  may be applied to the piston also in a different way, for example by screwing, mechanical deformation, etc. 
     It should be noted that the system for locking the sealing bands on the piston proposed herein allows choosing and maintaining the optimum position of the band based on the features of the machine and of the die. 
     For example, as said hereinbefore, when the points wherein the liquid metal exhibits the greater fluidity are known, for example zones  1 ,  2  and  3  of container  60  at the inlet, at the top side and at the starting points of the casting branches, respectively, it is possible to arrange the band so that the step-wise split thereof  19  is far from said points, for example in zones  4  and  5  indicated in  FIGS. 5 and 6 . 
     Advantageously, moreover, the sealing band width is at least equal to the width, measured in axial direction, of splits  3  of the casting branches. In this way, in fact, the metal into the feedhead does not reach the piston portion upstream of the band and this is therefore prevented from getting damaged or even removed from the piston. 
     Moreover, the axial and angular locking of the band is not localised in a single point but is distributed along the piston and the band circumference, and is therefore solid and safe, and is obtained by the same coupling means between the piston body and the band. 
     It is evident that a man skilled in the art may make several changes and adjustments to the piston according to the present invention in order to meet specific and incidental needs, all falling within the scope of protection of the invention as defined in the following claims. 
     For example, combinations of the described embodiments may be envisaged. In particular, the presence of the annular channel  30  in the proximity of the piston head and of the copper pad  50 , and the totally symmetric shape of the sealing band, may be elements common to all embodiments (of course where possible).