Abstract:
An injection molding apparatus which includes a check valve assembly mounted at the forward end of a feedscrew, the valve assembly having forward and rearward valve seat surfaces which co-act in a first position to allow a plasticized polymeric material to enter and flow through the valve into an injection chamber and which co-act in a second position to stop any additional material from entering the valve assembly. The second valve position is effected by a feedscrew injection stroke which generates a back pressure to close the valve, the back pressure moving a check ring of the valve assembly into a position to block entry into the valve. The valve seat surfaces are covered with a preformed layer of ceramic material and/or metal alloy which effectively increases the abrasion resistance of the valve seat surfaces and thus also increases the wear service life of the check valve assembly. Other wear surfaces of the valve assembly may also be coated for abrasion resistance.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the art of injection molding apparatus and, more particularly, to an improvement in injection molding apparatus check valve assemblies as applied to restrict backflow of a plasticized polymeric material during an injection stroke of the apparatus. The specific improvement comprises the addition of abrasion-resistant layers securely attached to friction wear surfaces of an injection molding check valve.  
         BACKGROUND OF THE INVENTION  
         [0002]    Injection molding of plasticized polymeric materials including plastic and/or elastomeric type materials has been known and practiced for a long time. These type of apparatus are conventionally associated with various molding machines which are adapted for receiving the plasticized polymer and forming it into many useful parts and/or products.  
           [0003]    Injection molding apparatus of the type alluded to are generally comprised of a feedscrew or auger member rotatably carried within a substantially cylindrical barrel, the screw effectively moving and plasticizing the polymeric material throughout the length of the barrel towards an exit end where it is forcefully injected into a molding machine for forming and curing of the material into the desired end product. At an exit end of the feedscrew there is conventionally mounted a check valve assembly which is designed to meter out the proper amount of plasticized material by a pressure reactive motion of the valve to effectively shut off the supply of material and to thereafter force the desired volume of plasticized polymer out of an exit nozzle and into the mold cavity of the molding machine.  
           [0004]    Many improvements in this art have been suggested and applied to the injection molding apparatus and these, to an improved configuration of the feedscrew member and/or to the check valve assembly to gain greater efficiency in the injection operation by shortening the injection cycle period. Obviously, a shorter cycle period of the injection molding apparatus will also result in an increase in the number of parts which may be produced inasmuch as the molding machines associated therewith may also be configured to accept and form more individual parts.  
           [0005]    These improvements in the injection molding apparatus have fairly coincided with advances in the polymer science and technology which has provided improved material chemistry. The improvements, however, have not been without problems as there is now a noticeable increase in wear of the various member elements which comprise the injection molding apparatus. For example, it has been determined that no natural lubricants are available in many of the polymeric materials and this lack of lubrication increases the friction and therefore also the heat generated in plasticizing and injecting the material. Furthermore, it is not possible to add a lubricating material to the process as these tend to contaminate the polymer and this affects the quality of the finished molded product.  
           [0006]    In view of the above, friction wear of critical working elements of the injection molding apparatus is a major and continuing problem in the industry as these must be replaced at regular and, in some instances, very short intervals.  
           [0007]    It is, of course, generally well-known and recognized by those knowledgeable in this art that the various working elements of the injection molding apparatus are comprised of very expensive tool and/or alloy steels, and this, because of the exceptional wear that these elements experience in this type of process. Thus, the very short service lifetimes of these elements will naturally also effect an increase in the cost of the molded parts being produced.  
           [0008]    The following prior art patents fairly represent what has been done in attempts to improve the injection molding apparatus: U.S. Pat. Nos. 3,698,694; 4,106,113; 4,105,147; 4,472,058; and 4,988,281. Further, U.S. Pat. No. 3,209,408 addresses the friction wear problem by providing a ball-bearing configured check valve assembly. Such type ball-bearing configurations are also evident in some of the above-listed prior art patents. In addition, U.S. Pat. No. 4,530,605 attempts to alleviate part of this problem by providing a rapid take-down configuration for a check valve assembly such that when worn parts need to be replaced this may be done quickly and efficiently with the least amount of down-time. From this it should be apparent that the friction wear problem of critical elements of an injection molding apparatus still exists and this, irrespective of the various advances in the art.  
           [0009]    Our prior U.S. Pat. No. 5,167,971 helped to solve the problem by reducing the amount of wear on the various wear surfaces of the valve assembly, however still further improvement in abrasion resistance has been achieved by this present invention by providing a layer covering the wear surfaces of the valve assembly which exhibits even further abrasion-resistance.  
           [0010]    It is, therefore, in accordance with a primary aspect of the present invention an object to provide an improved check valve assembly for an injection molding apparatus wherein the service lifetimes of the various working elements is increased such that many more molded products may be produced before it becomes necessary to replace the working elements of the apparatus.  
           [0011]    In accordance with another aspect of the invention it is an object to provide an improved injection molding apparatus check valve assembly which may be made from less expensive base metal and/or tool steel than now applied for these type elements while also providing an operational service life which is greatly for extended over what is available with presently made check valve assemblies.  
           [0012]    An even further object of this invention is to provide an abrasion-resistant layer covering the wear surfaces which may be adhered to the metal wear surfaces without the use of heat being applied to the metal surfaces which might affect the base metal hardness.  
           [0013]    Another object of the invention is to provide an abrasion-resistant layer having lower frictional heat due to density of the layer.  
           [0014]    An even further object of the invention is to provide a preformed layer which may be adhesively attached to any hardness of metal.  
           [0015]    Another object of the invention is to provide a preformed layer which may be adhesively attached to stainless steel and corrosion resistant high nickel alloys.  
         SUMMARY OF THE INVENTION  
         [0016]    This invention is a check valve assembly for an injection molding apparatus having a rotatable and axially translatable feedscrew within a barrel bore and adapted for moving a polymeric material through the valve assembly towards an exit chamber of the apparatus, the check valve assembly characterized by: a valve body member attached to the forward end of the feedscrew and moveable with the feedscrew, said body member having at least one valve seat surface thereon, an axially slidable member mounted on the body member for limited axial movement thereon, said slidable member having; at least one valve seat surface which frictionally engages a corresponding valve seat surface on the body member, and a circumferential surface at its outside diameter which frictionally engages the inner surface of the barrel bore, the improvement comprising a substantially abrasion-resistant preformed layer securely attached to at least part of the frictionally engaging surfaces of the apparatus, said preformed layer effectively reducing frictional wear between coacting frictionally engaging surfaces and between the surfaces and the plasticized material as it is moved through the check valve assembly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    These an other advantages and features of the invention will hereafter appear for purposes of illustaration, but not of limitation, in the accompanying drawings, in which like-reference numerals are used to identify like elements and wherein:  
         [0018]    [0018]FIG. 1 is a side elevational view, in cross-section and with various parts broken away, illustrating a state-of-the-art injection molding apparatus as may benefit from the concepts taught by the present invention;  
         [0019]    [0019]FIG. 2 is a greatly enlarged elevational view, in cross-section, of but a portion of the injection molding apparatus shown in FIG. 1 illustrating the application of the present invention; and  
         [0020]    [0020]FIG. 3 is a greatly enlarged elevational view, in cross-section similar to FIG. 2 but showing a slightly different modification of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    In the drawing, FIG. 1 illustrates an injection molding apparatus generally indicated by reference numeral  10 . The apparatus  10  conventionally comprises a substantially cylindrical barrel  12  having a specific longitudinal length and it will be recognized that only the exit or output end of the barrel  12  is shown in the drawing. Of course, and as is well-known and understood in this art, an input end (not shown) will include a hopper mechanism for feeding various type of materials into the barrel  12  and the manner of doing this is not an important consideration of the present invention.  
         [0022]    The barrel  12  may be characterized by a bore  14  centered on a longitudinal axis as indicated by the line Ax-Ax in the drawing. The exit end of the barrel  12  is generally indicated by reference numeral  16  and it may comprise an end cap member  18  which is affixed at  20  to the end of the barrel  12  by any of various well-known methods and/or techniques. The end cap  18  is characterized by a through-bore  22 , a partial portion of which is conically shaped as at  22   a  and it connects into an exit bore  24  of a nozzle tip  26 . The nozzle tip  26  is adapted for a mating relationship of the injection molding apparatus  10  to a molding machine (not shown) in the well-known and understood manner of such apparatus.  
         [0023]    A feedscrew member  30  is mounted co-axially within the bore  14  of the extruder barrel  12  and it is characterized by a helically oriented thread  32  having a land portion  34  exhibiting an outside diameter D 1  which is substantially but not exactly equal to the inside diameter D 2  of the bore  14 . A slight frictional engagement between the two is evident when the feedscrew  30  is rotated within the barrel bore  14 . The feedscrew  30  has a body  36  exhibiting an outside diameter D 3  which is less than the outside diameter D 1  of the thread  32  by a specific amount and it may be appreciated that a rotation of the feedscrew  30  will effect a movement of any material caught between the outside surface of the feedscrew body  36  and the inside surface of the bore  14  toward the exit end  16  of the apparatus  10 .  
         [0024]    The feedscrew member  30  has an extruder check valve assembly  40  mounted to its forward end and valves of this type may comprise two or more separate but co-operating parts or elements as evidenced in various of the prior art patents. The particular check valve  40  shown in the drawing comprises a valve body  42  characterized by a conically-shaped tip end  44  and a shank end  46  which has a plurality of threads  48  for a portion of its length. The valve body  42  is affixed to the forward end of the feedscrew  30  by way of the shank end  46  being threadably engaged within a threaded bore  38  at the end of the feedscrew  30 . The conically-shaped tip end  44  is shaped to mate with the conically-shaped bore  22   a  such that any material within the forward portion of the barrel bore  14  will be forceably directed into the exit bores  22  and  24  and out of the exit orifice  28  by an axial movement of the feed-screw  30  into the end cap member  18 . It is, of course, well-recognized and understood that the feedscrew  30  is connected to a power source (not shown) which controls its rotational and/or axial motion and the particular power means, therefore, is not important to the scope of the present invention.  
         [0025]    The shank end  46  of the valve body  42  has a shoulder  50  formed between the smaller diameter threaded portion  48  and a larger diameter valve passage portion  52 , the shoulder  50  providing an axial stop for a valve seat ring  54  carried on the smaller diameter portion  48 . The valve seat ring  54  has a forwardly-facing valve seat bearing surface  56  and it is further characterized by an outside diameter which is substantially equal to the diameter D 3  of the feedscrew body  36 . As clearly evident in the drawing, the valve seat ring  54  ismaintained in position between the shoulder  50  and the terminal end of thefeedscrew  30  when the valve body  42  is threadably engaged within the bore  38  in the end of the feedscrew.  
         [0026]    The valve seat ring  54  comprises the rearward valve seat surface  56  of the check valve assembly  40  while a forward valve seat surface  58  is formed on a backside annular surface of the conically-shaped tip end  44 . The forward valve seat  58  has a number of axially oriented flute passages  60  passing therethrough and the purpose of these will become apparent as this description proceeds.  
         [0027]    The check valve assembly  40  further comprises a check ring member  70  which is mounted about the shank portion  52  of the shank end  46  and it is movable in the axial direction between the rearward valve seat  56  and the forward valve seat  58 . The check ring  70  is further characterized by frustoconical valve seat surfaces  72  and  74 , the valve seat surface  72  being in a position to sealingly engage the rearward valve seat  56  of the valve seat ring  54  while the valve seat surface  74  is in a position to sealingly engage the forward valve seat surface  58  on the valve body  42 . The valve seat surfaces  56 ,  58 ,  72 , and  74  are obviously mating surfaces and these may be disposed at an angle within the range of 0°-30° with respect to a radially oriented plane which is positioned orthogonally on the Ax axis.  
         [0028]    Further with respect to the check ring member  70 , it has an outside diameter surface  75 , which is substantially but not exactly equal to the inside diameter D 2  of the bore  14 . While a sealing type engagement is effected as between the check ring  70  and the bore wall  14  such that material moving through the bore may not pass therebetween, the check ring is movable in the axial direction so as to be alternately engageable with either of the forward valve seat surface  58  or the rearward valve seat surface  56 . The check ring  70  also has an inside bore diameter which is larger than the outside diameter of the the forward portion  52  of the shank end  46  about which it is mounted. In this configuration, an annular passage indicated at reference numeral  76  is evident and it provides a pass-through for polymeric material when the check valve the check valve assembly  40  is in the “valve-opened” position as shown in the drawing.  
         [0029]    In the operation of the injection molding apparatus  10 , it will be recognized that a material distribution chamber generally indicated by reference numeral  80  may be establish ed between the tip end  44  of the check valve assembly  40  and the conically-shaped bore  22   a  of the end cap member  18 . When the volume of the distribution chamber  80  is established for a particular molded part, the feeds crew  30  is maintained in its axial position within the barrel bore  14  but it is rotated about the Ax axis. This rotation of the feedscrew  30  effectively moves polymeric material being fed into the barrel  12  longitudinally down the bore  14  towards the exit end  16 . The movement of material effectively also moves the check ring  70  into axial engagement with the forward valve seat surface  58  as shown in the drawing. Polymeric material is thus able to move through the check valve assembly  40  by way of the open annular passage  76  and the axial flute passages  60  and then into the distribution chamber  80 . As the distribution chamber  80  is filled, an injection stroke of the feedscrew  30  causes the check ring  70  to move into axial engagement with the rearward valve seat surface  56  of the valve seat ring  54 . Initiation of this powerful injection stroke of the feedscrew  30  in the axially forward direction forces any material within the chamber  80  out of the exit orifice  28  and into a relatively positioned molding chamber (not shown).  
         [0030]    From the foregoing description of the injection molding apparatus  10 , it must be appreciated that the relative motions as between the various member elements of the apparatus generates heat which also increases the friction component as between the members. This is further aggravated by heat being generated within the polymeric material as it is processed through the apparatus and by a friction component which exists as between the material itself as it passes over the various member element surfaces. It will, of course, be recognized that the operational service life of the various members will be shortened by the amount of wear of critical surfaces and especially the valve seat surfaces of the check valve assembly  40  which actually govern the operation of the injection molding process.  
         [0031]    Referring now to FIG. 2 of the drawings, a greatly enlarged elevational view of a portion of the apparatus  10  of FIG. 1 is illustrated. In this figure, like-reference numerals are used to designate like elements of FIG. 1 and the primed reference numerals are used to indicate the improved elements of the apparatus in accordance with the concepts of the present invention.  
         [0032]    The showing of FIG. 2 is of the forward end of the feedscrew member  30  which carries the check valve assembly  40  in axial position at it forward end. The check valve assembly  40 ′ shown in the figure is an improved design wherein various of the element surfaces which exhibit exceptional wear and which are critical to the operation of the injection molding apparatus are covered with a hard and substantially abrasion-resistant preformed layer securely attached to the wear surfaces which dramatically increases the operational service life of the check valve assembly  40 ′.  
         [0033]    The preformed layers are shown in the drawing by the stippled areas and, very clearly, these are the valve seat surfaces of (a) the check valve body at  58 ′, (b) the valve seat ring at  56 ′, (c) the check ring valve seat surfaces  72 ′ and  74 ′, and the check ring outside diameter surface  75 ′.  
         [0034]    It will be recognized that various types of ceramic materials may provide the desired abrasion resistance and these may also be applied in a similar manner to the valve seat and other frictionally engaging wear surfaces wear surfaces. For example, ceramic materials taken from the group comprising the ceramic oxides may be fired to the desired hardness and preformed into a layer of a shape and size similar to the wear surface to be covered and adhesively attached to the wear surface.  
         [0035]    After the layers are attached to the selected wear surfaces of the valve parts, the layers can then be machined to the desired final gauge thickness. For such ceramic layers, a final gage of not less that 0.010 inch, (0.254 mm) is preferable.  
         [0036]    One of the preferred ceramic materials which provides an excellent abrasion resistance is a high alumina aluminum oxide. Another ceramic oxide which is also a good choice for abrasion resistance is zirconia and in particular Cerium Oxide Partially Stabilized Tetragonal Zirconia Polycrystal.  
         [0037]    The ceramic oxides are considered preferable for use on the forward valve seat surface  58 ′ on the back side of the conically shaped tip end  44 , the rearward valve seat surface  56  on the valve seat ring  54 , and the forward and rearward valve seat surfaces  72  and  74  on the check ring  70 . The ceramic layers described previously are attached to the wear surfaces of the valve by a high temperature adhesive having the required physical properties to withstand the environment in which it is to used within the valve assembly.  
         [0038]    The flat ceramic oxide layers previously described are not suitable for use on the OD surface of the check ring  70  and therefore a carbide layer which may be curved to encircle the outside diameter surface  75  of the check ring  70  and welded to the surface  75 . It is also desireable to use a carbid layer on the rearward valve seat surface  56  on the valve seat ring  54 . When carbide layers are used it is preferable that the layers exhibit a final gauge thickness of not less than 0.005 inch, (0.127 mm).  
         [0039]    This invention, therefore, is not limited to a particular ceramic, metal, and/or metal alloy layer but, in the broades sense covers any high abrasion-resistant material which may be adhesively attached or if carbide layers are used, they are attached to the wear surfaces by welding or brazing.  
         [0040]    Finally, it will also be recognized that when such abrasion resistant layers are used, the underlying base metal may comprise a less expensive metal and/or metal alloy than presently being used for these parts. For example, the very expensive tool and alloy steels presently being used for the valve seat ring  54  and the check ring  70  may be replaced with a number 4150 steel which costs ninety-five percent less. Obviously, a great savings in materials may be realized by the application of the present invention.  
         [0041]    [0041]FIG. 3 shows a different embodiment from that shown in FIG. 2 in which similar parts are shown with a double prime (″) instead of a single prime (′) as used in FIG. 2. In FIG. 3, the rear end of the conical shaped tip end  44 ″ is divided into two parts with a bead seat ring  45  positioned against the rear end of the tip end  44 ″. A valve seat ring  54 ″ has a forward tubular extension  55  which bears against a rear annular surface  45   a  on the ring  45  and holds it in position. All the remainder of the layers  56 ″,  58 ″,  72 ″,  74 ″ and  75 ″ are similar to those similar numbers described in FIG. 2. Except for the addition of ring  45  and tubular extension  55 , all the rest of the parts of the assembly shown in FIG. 3 are the same as in FIG. 2 and the same layer material of ceramic oxide or carbide material can be used.  
         [0042]    While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and/or modifications may be made therein without departing from the spirit or scope of the invention.