Abstract:
There is provided an improved valve mechanism for use with a casting or injection mold which is quick acting and simple to construct. The valve includes an inlet coupled to the mold cavity and an outlet for venting gas from the mold cavity. The valve further includes a closure oriented perpendicularly to the inlet and movable between open and closed positions for closing off the outlet. The closure has a central axis and the valve is configured to move the closure into the closed position by directing the melt passing through the inlet to impinge substantially coaxially upon the closure so as to transfer a majority of the momentum of the melt to the closure.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to venting valves for casting and/or injection molds. 
     BACKGROUND OF THE INVENTION 
     In the die-casting process, molten metal (e.g.: Aluminum) is rapidly injected in a tightly closed mould under high pressures and is then allowed to solidify. Typical injection times for large castings are in the range of 0.1 seconds at pressures of 10,000 pounds per square inch. Before injection of the molten material into the mould cavity in said molding process, gas resides in the mould cavity. The mould is held tightly closed during injection of the molten material; this prevents the molten material from escaping said cavity. The tightly closed mould also prevents said resident gases from escaping the mould cavity during the injection phase. Said resident gases become trapped in the molded part; this results in porosity in the cast part reducing its quality and may cause the manufacturer to scrap the casting. 
     One commonly known method of minimizing trapped gases in the molded parts is to machine thin vent passageways leading from the perimeter of the mould cavity to the exterior of the mould. If the thickness of these passageways exceed approximately 0.01 inches, then the injected material will escape the mould cavity. Molten material escaping the mould cavity is a safety hazard and it is not practiced in the industry, further it may leave deposits on the mould parting plane which affects the subsequent sealing of the mould halves and compounds itself with subsequent casting cycles causing costly down-times to repair the mould sealing surfaces. And even if this venting method is used correctly (Thickness less than 0.01 inches), the available vent area is rarely sufficient to allow the gases to adequately escape within the short cavity fill times of 0.1 seconds. 
     Another known method of minimizing trapped gases in the molded parts is to machine a larger passageway leading from the mould cavity to a valve. This valve is also in fluid communication with the exterior of the mould. The object of this approach is to have a larger passageway to vent the resident gases more freely. The valve is kept open for as long as possible (During the injection phase) before the injected melt arrives at the valve chamber to evacuate the maximum amount of gas from the cavity of the mould. This valve must then be rapidly closed before injected material can escape through the valve exit. The longer the valve stays open before the molten material arrives, the more gas can be allowed to escape before the valve closes. There are a number of known inventions wherein a valve is used to permit evacuation of said resident gases in the injection moulding process (eg. die-casting). The valve described in U.S. Pat. No. 4,986,338 relies on a sensor triggered electrically to activate a valve mechanism pneumatically. This invention is at a disadvantage due to its slow response to close the valve. It also requires extensive trial and error to implement for each mould constructed and even in such measures, the valve may fail to close before the molten material arrives. If the valve fails to close in time, molten material will flow through the valve exit and fill the exhaust passageways with molten material which subsequently solidifies. This causes costly down time to remove the solidified material and to service the valve. 
     Another invention similar to this claim is outlined in U.S. Pat. No. 4,431,047 wherein a slidable plunger is used to permit (or block) fluid communication from the valve chamber to the mould exterior. Two symmetrical bypass passageways leading to the valve exit provide time to close the valve before molten material can flow through the valve exit. The position of said bypass passages requires a large valve that must be assembled at the exterior perimeter of the mould. The by-pass passageways are necessary to provide the time to close the valve before molten material arrives at the valve exit. There are two disadvantages to this arrangement: Firstly, a large valve limits the choices for where the valve can be installed without significant modification to the mould; this severely limits its effectiveness as it is vital to position the valve at the last-to-fill feature of the mould. Secondly, molten material (under high pressure) channeled to the outer perimeter of the mould makes it difficult to seal the passage leading to the valve from the mould cavity. This is due to thermal expansion of the mould center which severely compromises the ability to seal the passage leading to the outer perimeter of the mould. In the 6th embodiment disclosed (U.S. Pat. No. 4,431,047), the molten metal jet does not collide with the piston in an optimum angle for maximum momentum transfer. It requires a pressure buildup in the valve chamber to forcefully close the valve is yet another disadvantage; this requires more time to force to close the valve and cause the valve chamber to be filled with molten material. 
     An improved valve mechanism for venting injection molds which overcomes the disadvantages of the prior is required. Such a valve mechanism should be small, simple to construct and, most importantly, responsive to seal the mold vent as quickly as possible at the most opportune time. 
     SUMMARY OF THE INVENTION 
     The goal of the present invention is to provide a venting arrangement incorporated in a die-casting mould that is small enough to be positioned close to the mould cavity without reducing the venting area. Secondly, the valve is shaped to make full use of the momentum of the incoming melt to force the valve shut. And finally to use a simple valve mechanism of small mass to further decrease the time to slide the piston from the open position to its closed position. Therefore, in accordance with one aspect of the present invention, there is provided an improved valve mechanism for an injection mold which is quick acting and simple to construct. The valve permits gas to escape from a mold cavity when the mold cavity is injected with a melt. The valve includes an inlet coupled to the mold cavity and an outlet for venting gas from the mold cavity. The valve further includes a closure oriented substantially perpendicularly to the inlet and movable between open and closed positions for closing off the outlet. The closure has a central axis and the valve is configured to move the closure into the closed position by directing the melt passing through the inlet to impinge substantially coaxially upon the closure so as to transfer a majority of the momentum of the melt to the closure. 
     In accordance with another aspect of the present invention, there is provided a valve for permitting gas to escape from a mold cavity when the mold cavity is injected with a melt. The valve includes an inlet coupled to the mold cavity and an outlet for venting gas from the mold cavity. The valve also includes a closure which is movable between opened and closed positions for closing off the outlet. The closure has a central axis and the valve is configured to direct the melt passing through the inlet to impinge substantially coaxially upon the closure by applying a favorable pressure gradient to the melt, thereby moving the closure into the closed position. 
     In accordance with yet another aspect of the present invention, there is provided a quick acting valve for venting gas from a mold cavity when the mold cavity is injected with a melt. The valve includes an inlet coupled to the mold cavity and an outlet for venting gas from the mold cavity. The valve further includes a closure member movable between open and closed positions for closing off the outlet when the closure is in its closed position. The valve is configured to move the closure member into its closed position by transferring momentum from the melt passing through the inlet to the closure member. 
     With the foregoing in view, and other advantages as will become apparent to those skilled in the art to which this invention relates as this specification proceeds, the invention is herein described by reference to the accompanying drawings forming a part hereof, which includes a description of the preferred typical embodiment of the principles of the present invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . is a cross section of a mold incorporating the valve mechanism of the present invention and showing the mold in its closed (molding) orientation. 
         FIG. 2  is a cross section of the mold shown in  FIG. 1  showing the mold in its open position with a first mold half suspended above a second mold half. 
         FIG. 3  is a cross section of the valve portion of the present invention showing the closure member in its opened position. 
         FIG. 4  is a cross section of the valve shown in  FIG. 3  with the closure member in its closed position. 
         FIG. 5  is a bottom view of the upper valve component of the valve portion shown in  FIG. 3  at line A-A. 
         FIG. 6  is a top view of the lower valve component of the valve portion shown in  FIG. 4  at line B-B. 
         FIG. 7  is a bottom view of the upper mold half of the mold shown in  FIG. 2 . 
         FIG. 8  is a top view of the lower mold half of the mold shown in  FIG. 2 . 
     
    
    
     In the drawings like characters of reference indicate corresponding parts in the different figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 and 2 , the method of die-casting occurs in a mold  10  comprise of two mould halves  12  (first or movable mold half) and  14  (second or stationary mold half) in which the mold cavity is machined in each of the die halves; this mold cavity (see item  16  in  FIG. 1 ) defines the geometry of the part that is cast. One of the mould halves is stationary (illustrated in  FIGS. 1 and 2  as the second mold half) and fixed to the die-casting machine and the other half (the movable or first mold half) is mounted to a moving platen of the die casting machine. The stationary half is connected to a cylindrical sleeve (Referred to as the “shot sleeve”)  18  with a piston  20  positioned in the sleeve. The inside of the shot sleeve is in fluid communication with the cavity  16 . The casting process begins with tightly closing the mould halves, pouring the melt  22  into the shot sleeve and rapidly accelerate the piston to force the molten material into the die cavity. Typical speeds as the melt enters the cavity is in the range of 50 m/s. The melt is then allowed to solidify and the casting is ejected. The mould cavity must be shaped in such a way to prevent any under-cuts so that the casting may be easily extracted from the mold. 
     In order to ensure that the mold cavity is completely filled with the melt, and to ensure that there is not air mixed in with the melt as the part is molded, air contained in mold cavity  16  must be vented as the melt is injected into the mold cavity. Valve  24  is formed on the mold halves as upper portion  26  and lower portion  32  and has an inlet  28  and an outlet  30 . Air from mold cavity  16  is forced through inlet  28  and out of outlet  30  as the melt is injected into the mold cavity. 
     Referring now to  FIG. 3 , valve  24  is formed of upper portion  26  and lower portion  32 . It will be appreciated that the term “upper” portion and “lower” portion is purely arbitrary as the mold may be oriented such that the mold halves are side by side, in which case portions  26  and  32  would be side to side and not one above the other. In the embodiment illustrated, inlet  28  is formed on portion  26 ; however, it may just as easily be formed one either portion  32  or partly on portion  32  and partly on portion  26 . A valve chamber  38  is formed in the valve between portions  26  and  32 . A spring biased closure member  34  is movably retained in the valve between inlet  28  and outlet  30 . Closure  34  has an axis of movement  42  and is slidingly retained in sleeve  44  formed in portion  26 . Closure  34  is movable along axis  42  between an open position as shown in  FIG. 3  and a closed position as shown in  FIG. 4 . Closure  34  has cut outs  54  which permit air to pass through the valve when the closure is in its open position. 
     Closure  34  has a concave surface  40  formed on projection  41  which projects into valve chamber  38  such that the concave surface projects into the valve chamber. Valve  24  has a first wall  36  formed on portion  32  adjacent inlet  28 . First wall  36  is shaped as a curve which is configured to direct the melt  22  passing through inlet  28  to be re-directed by 90° so as to directly impinge upon concave surface  40  of closure  34 . 
     Preferably first wall  36  is shaped in a curved fashion such that the first wall applies a favorable pressure gradient to the melt as the melt encounters the first wall. The favorable pressure gradient ensures that the melt continues to flow and prevents separation of the flow to ensure that the flow does not become chaotic. This favorable pressure gradient allows for an efficient re-direction of the melt to impinge on closure  34  coaxially so as to efficient transfer momentum from the melt to the closure, thereby compelling the closure to move quickly from its open position to its closed position. A side wall  50  is formed in the valve adjacent inlet  28  to direct melt  22  towards first wall  36  and to ensure that the melt flows in a laminar fashion and that wall  36  can apply a favorable pressure gradient to the melt. 
     Peripheral walls  48 ,  46  and  52  are formed in the valve to re-direct the melt which has impinged on concave surface  40  back towards concave surface  40 . It will be appreciated that surface  40 , being concave, will redirect any melt which impinges upon it towards peripheral walls  48  and  46 . Wall  48  is curved to ensure that the melt which has been re-directed by concave surface  40  is further re-directed towards peripheral wall  52 , which further re-directs the melt back towards concave surface  40 . Peripheral wall  46  is curved to ensure that any re-directed melt which impinges upon it from concave surface  40  is re-directed towards first wall  36  which again re-directs the melt back towards concave surface  40 . Peripheral walls  48 ,  46  and  52  thereby ensure that a more complete transfer of momentum between melt  22  and closure member  34  is achieved. 
     Referring now to  FIG. 4 , closure  34 , when in its closed position, closes off valve  24  to prevent the flow of melt out of outlet  30 . As mentioned above, when melt  22  passes into chamber  38  from inlet  28 , it is redirected by first wall  36 . First wall  36  has a terminal edge  37  which is oriented to point towards axis  42  and direct the melt to travel coaxially with axis  42 . Redirected melt  22   a  then impinges upon concave surface  40  substantially coaxially with axis of movement  42 . Axis of movement  42  is coaxial with the axis of closure  34 ; therefore, the transfer of momentum between melt  22   a  and closure  34  is mostly coaxial to axis  42  permitting the closure to efficiently move towards its closed position. It will be appreciated that the melt  22  is traveling very quickly when it enters chamber  38 , and the favorable pressure gradient applied by first wall  36  ensures that melt  22   a  remains moving as quickly as possible without slowing down and losing its momentum. When melt  22   a  hits concave surface  40 , a majority of its momentum is transferred to closure  34 , the remaining momentum being used to cause the melt which splashes off surface  40 , namely melt  22   b  and  22   d , to travel towards peripheral walls  48  and  46 , respectively. Peripheral wall  48  redirects melt  22   b  (now  22   c ) back towards concave surface  40 . Likewise, peripheral wall  46  redirects melt  22   d  towards first wall  36 , which in turn redirects it back towards surface  40 . Therefore, most of the remaining momentum of melts  22   b ,  22   c  and  22   d  are transferred to closure  34 . Hence, very little melt  22  is required to cause closure  34  to move into its closed position and, therefore, very little melt will have the opportunity to escape into sleeve  44  and out of outlet  30 . Biasing spring  35  is selected to apply sufficient biasing force to closure  34  to keep it in its open position until, but to permit the closure to move into its closed position when the first bit of melt  22   a  transfers its momentum to the closure. The inertia of closure  34  is overcome by the transfer of momentum from melt  22  to the closure. However, when closure  34  is in its closed position, the inertia of closure  34  keeps it in its closed position for a brief interval of time despite the biasing force of spring  35  (approximately 4 ms). During this brief interval of time, additional melt  22  enters chamber  38  which eventually fills the chamber and results in the melt applying positive pressure onto closure  34  keeping it in its closed position. The filling of chamber  38  with melt  22  requires time; therefore, biasing spring  35  must be carefully selected to ensure that the biasing force it exerts onto closure  34  is sufficient to ensure that the closure&#39;s inertia keeps the closure in its closed position long enough for the chamber to fill with melt. 
     Referring now to  FIGS. 5 and 6 , walls  48 ,  46  and ramp  50  are formed on the “underside” of portion  26  while walls  52  and  36  are formed on the “top side” of portion  32 . As seen in  FIGS. 7 and 8  respectively, portion  26  is formed on mold half  12  while portion  32  is formed on mold half  14 . As a result, the valve is quite simple and requires only two moving parts, permitting the valve components to be machined as the mold halves are machined. 
     The present invention has several advantages over the prior art. Firstly, it is very simple to construct and requires only two moving parts, namely the closure and the biasing spring. The valve is also very fast acting since it closes off by transferring momentum from the fast moving melt to the closure rather than relying on the build up of pressure. The valve is also less prone to leakage and clogging as a result of melt working its way into the moving parts of the valve. 
     A specific embodiment of the present invention has been disclosed; however, several variations of the disclosed embodiment could be envisioned as within the scope of this invention. It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.