Abstract:
A seal for a check valve for a metal molding machine. The seal is provided by the combination of a peripheral groove in an outer surface of the check valve and a helically wound core in the groove. The helically wound coil is expandable into sealing engagement with a cylindrical wall of the molding machine. The helically wound coil may be movable laterally in the groove between a melt channel open position and a melt channel closed position to open or seal the melt channel.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This patent application is a divisional patent application of prior U.S. patent application Ser. No. 10/876,584, filed Jun. 28, 2004. This patent application also claims the benefit and priority date of the U.S. patent application Ser. No. 10/876,584, filed Jun. 28, 2004. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates, generally, to check rings and seals for injection molding machines and more particularly, but not exclusively, the invention relates to check rings and seals for metal injection molding machines and die casting machines. 
         [0004]    2. Background Information 
         [0005]    The state of the art includes U.S. Pat. No. 3,578,803 issued May 18, 1971 to Huhn that describes the use of a spiral spring to urge a seal ring towards a counter-ring to create a seal on a shaft. 
         [0006]    U.S. Pat. No. 3,655,206 issued Apr. 11, 1972 to Durametallic Corp. describes the use of a spiral sealing ring that is pressed against a wedge shaped surface to apply a radially inward and axial compressive force to the sealing ring to form a seal around a shaft. The sealing ring is constructed of multiple layer graphite material. The sealing ring is designed to maintain a seal around the shaft. 
         [0007]    US Patent Application 2002/0100507 published Aug. 1, 2002 by Hauser et al describes a check valve for a piston pump in an automotive braking system. The check valve is formed as a single piece consisting of a helical coil with a base ring on one end and a closure disk on the other end. Movement of the base ring provides the opening and closing of the check valve. The helical spring provides the opening and closing mobility of the valve. The outer surfaces of the helical spring are not used as closing or sealing surfaces. 
         [0008]    US Patent Application 2004/0001900 published Jan. 1, 2004 by Dominka describes a check valve for an injection system. The valve includes a shut-off pin, a spring guide member and a helical spring. The helical spring is compressed by the guide member to force the pin to close the flow path and decompressed to enable the flow path to open. The surfaces of the helical spring are in contact with the flow path but do not provide any of the closing or sealing surfaces. 
         [0009]    None of the prior art suggests the use of a spiral coil to actually seal a flow channel. 
         [0010]    There is a need for a wear resistant reliable seal for sealing the flow path through check valves in injection molding machines. 
       SUMMARY OF INVENTION 
       [0011]    In the injection molding of plastics it is common to employ check valves without any seals and to rely on the comparatively large clearance and the high viscosity of the melt to create full sealing. Metals used in metal injection molding do not have the high viscosity of plastics and therefore will leak back through the clearances that are typically employed in plastic injection molding. In addition, the highly corrosive nature of the metals and the high temperatures required for injection also debilitate against using plastic injection molding sealing arrangements in metal injection molding. Accordingly, an effective seal for metal injection molding is required to have a tight clearance and tolerance and must withstand high temperatures and corrosive environments. The present invention provides such a seal using a spiral coil. 
         [0012]    The present invention provides a seal for injection molding machine that prevents back flow of melt in a check valve, reduces wear in the barrel and check valve and will operate reliably even when significant wear is present. The invention is achieved by providing a spiral coil to seal the channel. The spiral coil may also act as a check ring to open and close the melt path. 
         [0013]    The present invention provides a seal for a check valve for a metal molding machine. The seal comprises a peripheral groove in an outer surface of the check valve and a helically wound coil in the groove. The helically wound coil is expandable into sealing engagement with a cylindrical wall of the molding machine. 
         [0014]    The present invention further provides a check valve for a metal molding machine. The valve includes a helically wound coil. The coil seals the check valve and slides on a cylinder of the check valve to open and close a flow path through the valve. A first turn of the coil has a surface conforming to a mating surface on the cylinder to close the valve when in contact with the mating surface. Outer peripheral surfaces of the coil conform to a cylinder wall surrounding the check valve to provide an axial seal for the check valve. 
         [0015]    The present invention further provides an injection unit for an injection molding machine including an injection screw, a nozzle body on one end of the injection screw and a check valve on the nozzle body. The check valve includes a sealing ring. The sealing ring comprises a helically wound coil that surrounds the nozzle body and is slidable between a first position where the nozzle is open and a second position where the nozzle is closed. A first turn of the coil sealingly engages a shoulder on the nozzle body when the coil is in the closed position. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]    Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is an end view of barrel assembly for a metal injection molding machine. 
           [0018]      FIG. 1A  illustrates a barrel assembly of a typical injection molding system on which the present invention is useful. 
           [0019]      FIG. 2  is a cross sectional view of the barrel assembly of  FIG. 1  taken along the sectional line  2 - 2  of  FIG. 1  showing the spiral seal provided by the present invention. 
           [0020]      FIG. 3  is a detailed view of a portion of  FIG. 2  showing the check valve with the spiral seal in the closed sealing position taken along sectional line  3 - 3  in  FIG. 4 . 
           [0021]      FIG. 3A  is a detailed view of circled portion A of  FIG. 3  showing the relationship between the spiral geometry and the groove more closely. 
           [0022]      FIG. 4  is an end view of the check valve of  FIG. 3 . 
           [0023]      FIG. 5  is a perspective view of the check ring of the invention. 
           [0024]      FIGS. 5A and 5B  are sectional and end views, respectively, of the check ring shown in  FIG. 5 . 
           [0025]      FIG. 6  is a perspective view of the spiral coil to be fitted on the check ring of  FIG. 5  to seal the check ring. 
           [0026]      FIGS. 6A and 6B  are sectional and end views, respectively, of the spiral coil shown in  FIG. 6 . 
           [0027]      FIG. 7  is a cross sectional view along sectional line  7 - 7  of  FIG. 8  of a check valve with a spiral coil functioning as a seal and check ring. 
           [0028]      FIG. 7A  is an enlarged view of the area A from  FIG. 7 . 
           [0029]      FIG. 8  is an end view of the check valve shown in  FIG. 7 . 
           [0030]      FIG. 9  is a further embodiment of the invention where the spiral coil combines as a check ring and seal. 
           [0031]      FIG. 10  is a cross-sectional view of a further embodiment of the invention that includes a wear ring between the spiral coil check valve and seal and is taken along sectional line  10 - 10  in  FIG. 11 . 
           [0032]      FIG. 11  is an end view of the check valve shown in  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    The structure and operation of the present invention will be explained, hereinafter, within the context of improving the function and durability of a check valve that is configured for use in a barrel assembly of an injection molding system for the molding of a metal alloy, such as those of Magnesium, in a semi-solid (i.e. thixotropic) state. A detailed description of the construction and operation of several of such injection molding systems is available with reference to U.S. Pat. Nos. 5,040,589 and 6,494,703. Notwithstanding the foregoing, no such limitation on the general utility of the check valve of the present invention is intended, or its compatibility with other metal alloys (e.g. Aluminum, Zinc, etc.). 
         [0034]    The barrel assembly of a typical injection molding system is shown with reference to  FIG. 1A . 
         [0035]    The barrel assembly  138  is shown to include an elongate cylindrical barrel  140  with an axial cylindrical bore  148 A arranged therethrough. The barrel assembly is shown connected to a stationary platen  16  of a clamping unit (not otherwise shown). The bore  148 A is configured to cooperate with the screw  156  arranged therein, for processing and transporting metal feedstock, and as a means for accumulating and subsequently channeling a melt of molding material during injection thereof. The screw  156  includes a helical flight  158  arranged about an elongate cylindrical body portion  159 . A rear portion of the screw, not shown, is configured for coupling with a drive assembly, not shown, and a forward portion of the screw  156  is configured for receiving a check valve  160 , in accordance with an embodiment of the present invention. An operative portion of the check valve  160  is arranged in front of a forward mating face or shoulder  32  of the screw  156 . The barrel assembly  138  includes a barrel head  2 A that is positioned intermediate the machine nozzle  144  and a front end of the barrel  140 . The barrel head  2 A includes a melt passageway  10  arranged therethrough that connects the barrel bore  148 A with a complementary melt passageway  148 C arranged through the machine nozzle  144 . The melt passageway  10  through the barrel head  2 A includes an inwardly tapering portion to transition the diameter of the melt passageway to the much narrower melt passageway  148 C of the machine nozzle  144 . The central bore  148 A of the barrel  140  includes a lining  12 A made from a corrosion resistant material, such as Stellite™, to protect the barrel substrate material, commonly made from a nickel-based alloy such as Inconel™, from the corrosive properties of the high temperature metal melt. Other portions of the barrel assembly  138  that come into contact with the melt of molding material may also include similar protective linings or coatings. The barrel  140  is further configured for connection with a source of comminuted metal feedstock through a feed throat, not shown, that is located through a top-rear portion of the barrel  140 , not shown. The feed throat directs the feedstock into the bore  148 A of the barrel  140 . The feedstock is then subsequently processed into molding material by the mechanical working thereof, by the action of the screw  156  in cooperation with the barrel bore  148 A, and by controlled heating thereof. The heat is provided by a series of heaters, not shown, that are arranged along a substantial portion of the length of the barrel assembly  138  and heaters  150  along machine nozzle  144 . 
         [0036]    The injection mold includes at least one molding cavity, not shown, formed in closed cooperation between complementary molding inserts shared between a mold cold half, not shown, and a mold hot half  125 . The mold cold half includes a core plate assembly with at least one core molding insert arranged therein. The mold hot half  125  includes a cavity plate assembly  127 , with the at least one complementary cavity molding insert arranged therein, mounted to a face of a runner system  126 . The runner system  126  provides a means for connecting the melt passageway  148 C of the machine nozzle  144  with the at least one molding cavity for the filling thereof. As is commonly known, the runner system  126  may be an offset or multi-drop hot runner, a cold runner, a cold sprue, or any other commonly known melt distribution means. In operation, the core and cavity molding inserts cooperate, in a mold closed and clamped position, to form at least one mold cavity for receiving and shaping the melt of molding material received from the runner system  126 . 
         [0037]    In operation, the machine nozzle  144  of the barrel assembly  138  is engaged in a sprue bushing  55  of the injection mold whilst the melt is being injected into the mold. 
         [0038]    The molding process generally includes the steps of:
       i) establishing an inflow of metal feedstock into the rear end portion of the barrel  140 ;   ii) working (i.e. shearing) and heating the metal feedstock into a thixotropic melt of molding material by:
           a) the operation (i.e. rotation and retraction) of the screw  156  that functions to transport the feedstock/melt, through the cooperation of the screw flights  158  with the axial bore  148 A, along the length of the barrel  140 , past the check valve  160 , and into an accumulation region defined in front of the check valve  160 ;   b) heating the feedstock material as it travels along a substantial portion of the barrel assembly  138 ;   
           iii) closing and clamping of the injection mold halves;   iv) injecting the accumulated melt through the machine nozzle  144  and into the injection mold by a forward translation of the screw  156 ;   v) optionally filling any remaining voids in the at least molding cavity by the application of sustained injection pressure (i.e. packing);   vi) opening of the injection mold, once the molded part has solidified through the cooling of the injection mold;   vii) removal of the molded part from the injection mold; and   viii) optionally conditioning of the injection mold for a subsequent molding cycle (e.g. application of mold release agent).       
 
         [0049]    The steps of preparing a volume of melt for subsequent injection (i.e. steps i) and ii)) are commonly known as ‘recovery’, whereas the steps of filling and packing of the at least one mold cavity (i.e. steps iv) and v)) are commonly known as ‘injection’. 
         [0050]    The check valve  160  functions to allow the forward transport of melt into the accumulation region at the front of the barrel  140  but otherwise prevents the backflow thereof during the injection of the melt. The proper functioning of the check valve  160  relies on a pressure difference between the melt on either side thereof (i.e. higher behind the valve during recovery, and higher in front during injection). The structure and operation of a typical check valve, for use in metal injection molding, is described in U.S. Pat. No. 5,680,894. 
         [0051]    Referring to  FIGS. 1 and 2 , a spiral coil used in accordance with a preferred embodiment of the present invention is generally shown.  FIG. 1  shows the use of the coil as a seal. 
         [0052]    In  FIG. 2 , barrel  2  with barrel liner  4  supports a screw (not shown) that has check valve  20  attached to it by means of threads  28 . Bolts (not shown) connect barrel head  6  to barrel  2  through bolt holes  8 . A nozzle (not shown) or the like is attached to the barrel head  6  by means of bolt holes  9 . When check valve  20  is in the open position shown in  FIG. 2 , the screw is rotating and melt is fed through the check valve into a melt passageway  10  in front of the check valve  20  in a manner well understood in the metal molding art. 
         [0053]    When the melt passageway  10  is filling the melt applies a force to inclined surface  32  to move check ring  24  forward and open a flow path between the inclined surfaces  32  and  34 . Surface  40  arrests the forward movement of ring  24 . During forward movement the spiral coil is only under a slight pressure from the melt and will create little resistance to the forward movement of the ring. 
         [0054]    When melt passageway  10  is filled with melt, rotation of the screw is stopped and an injection of melt into a mold cavity (not shown) is initiated. The forward movement of the screw during injection causes a force to be applied to a forward surface of the check ring to move it back so that the inclined surfaces  32  and  34  are in contact and thereby seal the melt path. 
         [0055]    In addition, openings  12  (shown in  FIG. 3 ) in the side wall of ring  24  permit melt to press against the inner walls of the spiral coil and force it into sealing contact with barrel liner  4  to thereby seal against leakage along the length of the barrel during the injection cycle. 
         [0056]    As shown in  FIG. 3 , check valve  20  consists of main stem  22 , check ring  24  and spiral coil  26 . Stem  22  is attached to the end of an injection screw by means of threads  28 . A shoulder  30  is fixed to the end of the injection screw. 
         [0057]    In the closed position shown in  FIG. 3 , the inclined surface  32  on check valve  20  and the inclined surface  34  on shoulder  30  are pressed into sealing engagement by the back pressure exerted on ring  24  by the melt in the melt channel  36  in a manner well understood in the art. 
         [0058]    The outside diameter of the spiral coil  26  has ample clearance to enable ease of assembly. Openings  12  permit melt to flow into the space  14  adjacent the inner circumference of the spiral coil  26 . During injection, the melt in space  14  subjects the coil  26  to injection forces in an outwardly radial direction that causes the highly compliant structure of the spiral coil  26  to easily expand radially until all of the clearances are eliminated and a seal is created. Upon the dissipation of injection pressure the forces that cause the compression and expansion are no longer present and the spiral coil  26  relaxes. When the plasticizing screw (not shown) begins to turn in order to convey new material to the front of the screw any contact between the check ring  24  and the spiral coil  26  will result in an applied torque that causes the spiral coil  26  to twist such that the outside sealing diameter becomes smaller and forces a disengagement of the sealing diameter from the wall of the barrel liner thus reducing wear. 
         [0059]    The end of main stem  22  is furcated to form fingers  38  creating slots  42  in the melt channel  36  as shown in  FIG. 4 . When the injection screw is withdrawn and rotated in a manner understood in the art, the screw provides melt that moves the check ring  24  forward to open the valve  20  and permit the melt channel  36  to receive melt from the rotating screw. As the melt channel  36  fills with melt the pressure in the channel slowly moves the plasticizing screw back to its full shot position. When an injection stroke begins the closed volume of melt in front of the check ring moves the check ring  24  back to the closed position shown in  FIG. 3 . When the check ring  24  reaches the sealing position shown in  FIG. 3 , sufficient melt is provided in the melt channel  36  to enable a next injection of melt into the cavity. Rotation of the screw is stopped and the screw is translated forwardly to force melt into the mold cavity. The translational movement of the screw increases the pressure created by the melt to ensure that the melt path  36  is sealed at the inclined surfaces  32  and  34  and along the barrel surface adjacent the coil  26 . 
         [0060]    As more clearly shown in  FIG. 3A , the coil  26  is substantially rectangular in cross section. The outer circumferential surfaces of the coil are machined to a high tolerance so that they will tightly interface with the wall of an associated barrel liner. The inner circumferential surfaces could be other shapes such as convex or concave. The only limitation on the shape of the inner circumferential surfaces is that they have sufficient surface to ensure the transmission of adequate force to move the coils into sealing engagement with the barrel liner surface. The radial surfaces of each turn of the coil are also machined to a high tolerance to ensure that adjacent turns of the coil seal effectively against one another. The outer radial surfaces of the outer coils and the surfaces they contact on the check ring should also be machined to a high tolerance to ensure good sealing. 
         [0061]    Check ring  24  is shown more explicitly in  FIGS. 5 ,  5 A and  5 B. Ring  24  has a circular slot  44  on its periphery. The slot  44  is shown located near the middle of the ring  24  but could be located nearer either end if desired. The only limitation is that the wall sections  46  and  48  adjacent the slot should have sufficient strength to withstand pressures exerted by the coil  26  when mounted in the slot  44 . 
         [0062]    Spiral coil  26  is shown more explicitly in  FIGS. 6 ,  6 A and  6 B. As shown in these FIGs., outer circumferential surfaces  66  are machined to a high tolerance. Radial surfaces  68  are also machined to a high tolerance. Inner circumferential surfaces  70  need not be made to a high tolerance as they contact the melt during operation. 
         [0063]      FIG. 7  shows a check ring coil  50  that combines the actions of opening and closing the check valve  52  and sealing the melt channel  54 . In this embodiment, the surface  56  of the outer coil of coil  50  engages the inclined surface  34  to close the valve as shown. The circumferential surfaces of the turns of the coil  50  engage the walls of the barrel to seal the walls against any back flow of the melt. The flexibility in the turns of the coil  50  ensure that even with wear in the barrel the coil  50  will continue to provide a reliable seal as the pressure of the melt against the inner walls of the coil  50  will force the outer walls of the coil against the barrel. Accordingly, the seal along the wall will only start to erode when the barrel is so worn that the expansion of the coils is insufficient to cover the wear gap. 
         [0064]    For metal molding, the spiral coil must be made of material that is stable at high operating temperatures, such as 600 Degrees C. for magnesium molding, and inert to corrosion. For example, when molding magnesium, nickel should not be present. 
         [0065]    The stem  22  shown in  FIG. 7  is essentially the same as stem  22  shown in  FIG. 3  so like reference numerals have been used to identify the same parts of the stem. Stem  22  need not be further described here. 
         [0066]      FIG. 7A  shows more clearly the machined surfaces of the coil  50 . 
         [0067]      FIG. 8  is an end view of the check valve  52  shown in  FIG. 7  and includes slots  42  for permitting the flow of melt into an injection cavity. 
         [0068]      FIG. 9  illustrates a further embodiment of the invention. In this embodiment, a melt flow channel  60  extends from the periphery of the check valve toward the interior of a barrel shown schematically at  64 . Spiral coil  66  acts as a check ring and seal for the check valve in a manner similar to that described hereinbefore with reference to  FIGS. 7 and 8 . 
         [0069]      FIGS. 10 and 11  show a further embodiment of the invention. In this embodiment, a ring  72  is situated between a seat  74  on a screw (not shown) and a spiral coil  76 . Ring  72  permits the use of a thinner coil  76  while maintaining the required flow path. The ring  72  moves back and forth with the coil  76   
         [0070]    It will, of course, be understood that the above description has been given by way of example only and that modifications in detail may be made within the scope of the present invention.