Abstract:
An apparatus to inject molten material into and evacuate gasses from a mold cavity, including a pair of ganged pistons in communication with the mold cavity. An injection piston injects molten material into the cavity from the bottom, and a vacuum piston pulls a vacuum in the cavity from the top through a vacuum line. A filter in the vacuum line catches any material that escapes from the mold cavity. A vacuum regulator may selectively regulate the vacuum in the vacuum line. During the injection sequence&#39;s first phase, a vacuum is created at a rate proportional to the rate of injection. Part-way through the piston stroke, a second phase is entered and causes a vacuum break valve to open and control the amount of vacuum. The injection sequence has a step function velocity, with the second phase being at a much higher velocity than the first.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO COMPACT DISC(S) 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates, in general, to injection molding apparatus, and in particular, to means for evacuating gasses from the mold cavity of injection molding apparatus. 
     2. Information Disclosure Statement 
     The die casting process is well-known for making parts by injecting a specific amount of molten material into a mold cavity having a specific area and shape. The problem with the die-casting process is that any air trapped in the mold cavity during the injection process will cause voids or porosity in the molded part, thereby reducing the structural integrity of the final product. 
     Well known solutions to this problem include attaching a vacuum pump to the mold by means of a valving system as described in Ozeki, U.S. Pat. No. 4,997,026. The invention disclosed by Ozeki and other similar devices create a constant uncontrolled vacuum throughout the injection sequence and rely on a mechanical device, such as a valve, to control the repeated application of the vacuum. There are many shortcomings associated with this method of gas evacuation. One such problem occurs when the valve is opened too soon, causing the vacuum to draw the molten material into the mold cavity. If this happens and the material solidifies prior to completion of the injection sequence, the part produced will lack the required structural integrity. Another problem with the prior art is that the rate of evacuation of the vacuum-evacuated air is uncorrelated with the rate of injection of molten material injected into the mold cavity. 
     Still another problem exists in the prior art due to the lack of coordination between the operation of the vacuum breaking valve and the injection sequence. Over a period of time and after a large number of repetitions, molten material may be pulled into the mechanical workings of the valve and impede its performance unless the opening and closing of the valve is perfectly coordinated with the injection sequence. 
     It is therefore desirable to have an injection molding apparatus that controls the amount of vacuum present in the system and coordinates the operation of the valve with the steps in the injection sequence. 
     A preliminary patentability search in Class 425, subclasses 546, 812, and 420, produced the following patents, some of which may be relevant to the present invention: Siggers, U.S. Pat. No. 5,277,570, issued Jan. 11, 1994; Ozeki et al., U.S. Pat. No. 4,997,026, issued Mar. 5, 1991; Hoschele et al., U.S. Pat. No. 3,804,570, issued Apr. 16, 1974; Fritsch, U.S. Pat. No. 3,477,101, issued Nov. 11, 1969; Hodler, U.S. Pat. No. 3,433,291, issued Mar. 18, 1969; Crandall, U.S. Pat. No. 2,991,506, issued Jul. 11, 1961; Cherry et al. U.S. Pat. No. 2,415,462, issued Feb. 11, 1947; and Brunner et al., U.S. Pat. No. 2,243,835, issued Jun. 3, 1941. 
     None of these references, either singly or in combination, discloses or suggests the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a vacuum device for removing gas from the mold cavity of a injection molding apparatus such as a die-casting machine. Like the prior art, the invention has a mold consisting of first and second mold portions, and, when the first and second mold portions are pressed together, they form the mold cavity where the molded part is formed. Different from the prior art, the rate of gas evacuation from the mold cavity is directly proportional to the rate of injection of molten material into the mold cavity, accomplished by a pair of ganged pistons, both in communication with the interior of the mold cavity, that are linked together for simultaneous operation. One of the ganged pistons is an injection piston that injects molten material from an injection piston cylinder into the mold cavity. The other of the ganged pistons is a vacuum piston that pulls a vacuum in the mold cavity to eliminate gases which, if present, would cause porosity of the final product. This vacuum is created through a vacuum line that is connected on one end to the mold cavity and on the other end to the vacuum cylinder. The vacuum line contains a filter interposed between the mold and the vacuum cylinder to prevent any molten material that has escaped into the vacuum lines from going into the valve of the regulator and clogging the valve mechanism. 
     The injection sequence is divided into two phases. During the first phase, the injection and vacuum pistons concurrently move at a slow pace with the injection piston of the injection cylinder slowly moving the molten material toward the mold cavity and with the ganged vacuum piston pulling a vacuum in the mold cavity at a rate proportional to the reciprocation speed of the injection piston. When the ganged pistons pass a certain point in their stroke, an electrical signal from the die-casting apparatus triggers the high-velocity second phase of the injection sequence. During this second phase, both the injection and vacuum pistons move at a high rate of speed with the injection piston rapidly forcing the molten material into the mold cavity. The electrical signal that triggers the second phase also opens a vacuum break in the vacuum cylinder. The vacuum break has a diaphragm that can be adjusted to control the amount of vacuum in the vacuum line, thus allowing a pre-selected constant vacuum during the second phase of the injection sequence. 
     It is an object of the present invention to provide a gas-evacuating device that, during the first stage of injection, develops a vacuum at a rate proportional to the rate molten material is injected into the mold cavity. 
     It is a further object of the invention to provide such a device that requires no additional energy for operation over already existing devices. 
     It is an object of the invention to provide a vacuum device with very few moving parts, none of which come into contact with the molten material, thereby decreasing repair time and increasing efficiency. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a side sectional view of the prior art showing a well-known die-casting machine. 
     FIG. 2 is a side sectional view of the present invention showing the apparatus with the pistons in the starting position of the first phase of the injection sequence. 
     FIG. 3 is a side sectional view of the present invention showing the apparatus at the beginning of the second phase of the injection sequence. 
     FIG. 4 is a side sectional view of the present invention showing the apparatus at the end of the injection sequence. 
     FIG. 5 is a side sectional view of the chill block of the present invention. 
     FIG. 6 is a side sectional view of a portion of a modified version of the present invention in which the cross-sectional area of one of the ganged pistons is different from the cross-sectional area of the other of the ganged pistons. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a side sectional view of a well-known prior art injection-molding apparatus  100  comprising a mold portion  102  and an injection piston  104 . Mold portion  102  is comprised of a left mold portion  101  and a right mold portion  103 . Injection piston  104  is mounted for reciprocation within an injection cylinder  106 , and a mold cavity  108  is formed by the walls of mold portion  102  when left mold portion  101  and right mold portion  103  are brought together. A well-known hydraulic cylinder-and-piston forcing means  110 , operably coupled to injection piston  104  as by a mechanical linkage  112 , causes injection piston  104  to reciprocate within injection cylinder  106  in a manner well-known to those skilled in the art. Such devices as injection molding apparatus  100  are well-known for making a molded part to a desired specification by forcing a specific amount of molten material from injection cylinder  106  by injection piston  104  into a mold cavity  108  of the specific and desired size and shape of the molded part. A well-known vacuum pump, not shown but in communication with the interior of mold cavity  108 , is often used in conjunction with the prior art device to evacuate gasses from mold cavity  108 . 
     Referring to FIGS. 2-5, the die-casting vacuum apparatus  20  of the present invention is seen to comprise an injection piston  22  mounted for reciprocation within an injection cylinder  24 , and a vacuum piston  26  mounted for reciprocation within a vacuum cylinder  28 . A mold cavity  30 , of a desired size and shape, is provided for forming a molded part therewithin of said desired size and shape, and a vacuum line  32  is provided that causes mold cavity  30  to be in communication with vacuum cylinder  24 . Injection piston  22  is in communication with mold cavity  30  and, as injection piston  22  reciprocates into injection cylinder  24 , injection piston  22  forces molten material  34  within injection cylinder  24  into mold cavity  30 . Prior to initiation of the molding process for each part, molten material  34 , preferably molten metal, is poured into apparatus  20  through an opening  36  in the top of injection cylinder  24 . Typically, and preferably, about 30% more molten material  34  than needed for the finished molded part is poured into opening  36  of injection cylinder  24  to compensate for overflow, etc., during the molding process. Vacuum piston  26  is mounted for reciprocation within vacuum cylinder  28  so that, as vacuum piston  28  reciprocates out of vacuum cylinder  28 , air is evacuated from within mold cavity  30  through vacuum line  32 . Vacuum line  32  preferably includes well-known filter means  38  interposed between mold cavity  30  and vacuum piston  26  for trapping and catching any molten material  34  that escapes from mold cavity  30  into vacuum line  32 . 
     Injection piston  22  is caused to reciprocate into and out of injection cylinder  24  by forcing means  40  such as, for example, a well-known hydraulic cylinder-and-piston that includes a driven piston  42  that reciprocates within a hydraulic cylinder  44  in a manner well-known to those skilled in the art using a well-known source of hydraulic pressure  46  supplied to and from hydraulic cylinder  44  through well-known bidirectional valves  48 ,  50  interposed within supply lines  52 ,  54 , and with valves  48  and  50  being respectively controlled using control signals  56 ,  58  supplied by control means  60 , hereinafter discussed. 
     Apparatus  20  further includes ganging means  62  for coupling injection piston  22  to vacuum piston  26 , thereby creating simultaneous coordinated movement of the two pistons. Ganging means  62  is preferably a mechanical linkage or coupling such as, for example, an arm  64  directly coupling injection piston  22  to vacuum piston  26 . As the injection piston  22  reciprocates into the injection cylinder  24 , the vacuum piston  26  concurrently caused to reciprocate out of vacuum cylinder  28  at the same rate of speed. 
     It should be understood that, because vacuum piston  26  is directly coupled to the injection piston  22 , the vacuum created within vacuum line  32 , and thus the rate of gas evacuation from the mold cavity  30 , by vacuum piston  26  is directly proportional to the rate of injection of molten material into the mold cavity by injection piston  22 , and it will be understood that the rate of injection of molten material into the mold cavity is necessarily proportional to the stroke velocity of injection piston  22 . If injection piston  22  and vacuum piston  26  have the same transverse cross-sectional area, and if ganging means  62  is a direct one-to-one mechanical linkage as shown in the drawings, then the constant of proportionality will simply be the ratio of the two transverse cross-sectional areas of the two pistons. FIG. 6 shows an alternate embodiment of the invention in which the transverse cross-sectional area of vacuum piston  26 ′ is substantially the same as the transverse cross-sectional area of injection piston  22 ′, in which case the rate of gas evacuation from the mold cavity will be the same as the injection rate of molten material into the mold cavity. In contrast, and to show the different ways in which the invention may be configured, FIGS. 2-4 show an embodiment of the invention in which the transverse cross-sectional area of the vacuum piston  26  is larger than the transverse cross-sectional area of injection piston  22 , thereby causing the rate of gas evacuation from the mold cavity to be greater than the injection rate of molten material into the mold cavity by the ratio of the transverse cross-sectional areas of the two pistons. Similarly, if ganging means  62  were not a one-to-one coupling, but instead employed gears or levers or the like to cause the stroke velocity of the vacuum piston to be a multiplier constant (greater or lesser than 1.0) times the stroke velocity of the injection piston, then the rate of gas evacuation from the mold cavity would be understood to be this multiplier constant times the injection rate of molten material times the ratio of the transverse cross-sectional areas of the two pistons. 
     Apparatus  20  preferably includes regulating means  66  for selectively regulating the vacuum within vacuum line  32  to mold cavity  30 . Regulating means  66  preferably includes a well-known vacuum break valve  68  having a mechanically-operated adjustable diaphragm that is actuated by an electrical solenoid  70 , with solenoid  70  being controlled by electrical signal  72  from control means  60 , hereinafter described, and with vacuum break valve  68  being interposed between vacuum line  32  and the atmosphere. An example of an acceptable vacuum break valve  68  is the model D-52 vacuum breaker sold by IMI Cash Valve Inc., located in Cullman, Ala. When vacuum break valve  68  is not actuated, it does not modify the vacuum within vacuum line  32  because the diaphragm within vacuum break valve  68  is closed. When vacuum break valve  68  is actuated by solenoid  70  under the direction of control means  60 , the diaphragm within vacuum break valve  68  opens to the atmosphere, causing the vacuum within vacuum line  32  to become a preselected constant vacuum that may be adjusted by adjusting the degree to which the diaphragm within vacuum break valve  68  opens to the atmosphere, thereby pre-selecting the amount of atmospheric pressure passed when the vacuum break valve is actuated. 
     The injection sequence is divided into two phases, a first portion and a second portion. During the first portion of the injection sequence, injection piston  22  moves at a first velocity, vacuum break valve  68  is not actuated (i.e., is closed), and the vacuum in the vacuum line  32  is thus proportional to the first velocity, as heretofore described. During the second portion of the injection sequence, injection piston  22  moves at a second velocity, which is preferably much faster than the first velocity, vacuum break valve  68  is actuated (opened) by electrical signal  72 , and the vacuum in the vacuum line  32  is caused to be a preselected constant vacuum that may be adjusted by adjusting the diaphragm of vacuum break valve  68 . 
     Apparatus  20  includes control means  60  for selectively switching regulating means  66  from a first mode, when injection piston  22  is in its first portion of movement, to a second mode when injection piston  22  is in its second portion of movement. When regulating means  66  is in the first mode, the vacuum break valve  68  is in a closed position thus preventing atmospheric pressure from entering the vacuum line  32  during the first portion of the injection sequence. When regulating means is in the second mode, the vacuum break valve  68  is open, thus allowing some atmospheric pressure to enter the vacuum line  32  and thereby creating a preselected constant vacuum in the vacuum line  32  during the second portion of the injection sequence. The preselected constant vacuum has been found to preferably be 15 to 25 inches (381 to 635 mm) of mercury, depending on the size of the part being molded. Control means  60  receives an electrical triggering input signal  74  from a detector  76 , such as a well-known proximity switch, located adjacent the rod  50  of injection piston  22 . Control means  60  receives electrical triggering signal  74  from the detector  76  when injection piston  22  has reached the end of the first portion of the injection sequence, as shown in FIG.  3 . This signal causes the second portion of the injection sequence to be initiated, and injection piston  22  is caused to quickly accelerate to the second velocity by fully-opening bi-directional valves  48 ,  50 . The operator of the apparatus  20 , when setting up the apparatus for molding a new part, adjusts the position of the detector  76  so that the electrical triggering input signal  74  is generated for control means  60  at the point where the molten material  34  just reaches the bottom of the mold cavity  30 , just prior to beginning to fill that portion of the void of the mold cavity that will become the molded part, so that the injection piston moves at the second velocity during the time when molten material is filling that portion of the mold cavity  30  that will become the molded part. The velocity curve of the injection sequence is preferably a step function, having a first constant velocity during the first portion of the injection sequence and an almost-immediate increase to the second velocity during the second portion of the injection sequence. At the same time that control means  60  receives the triggering signal  74  from detector  76  that causes initiation of the second phase of the injection sequence, control means  60  emits electrical signal  72  to regulating means  66  that causes regulating means  66  to switch from its first mode to the second mode. The opening of the vacuum break valve  68  of regulating means  66  allows some atmospheric pressure to pass through the diaphragm of regulating means  66  as described hereinabove, thereby creating a preselected constant controlled vacuum in the vacuum line (rather than the vacuum of the first mode, in which the vacuum is proportional to the first velocity of the injection piston). This preselected constant controlled vacuum is maintained during the second phase of the sequence while the molten material  34  is rapidly injected into the mold cavity  30 . The entire second portion of the injection sequence lasts approximately 0.5 to 1.0 seconds and the vacuum break valve  68  must be controlled with a high degree of precision for accurately timed opening and closing during repeated operation over many cycles. When the second portion of the injection sequence is complete, as shown in FIG. 4, the vacuum break valve  68  is again placed in its closed position, and injection piston  22  and vacuum piston  26  begin their return strokes to their original starting positions as shown in FIG.  2 . Any air trapped in vacuum cylinder  28  at the end of the forward stroke is released on the return stroke through a one-way check valve  78  located in the vacuum cylinder  28 . As it cools, the molten material  34  solidifies in the mold cavity  30 , thereby forming the desired molded part. 
     Apparatus  20  may be used in combination with a mold  80  comprised of a first mold portion  82 , a second mold portion  84 , and a mold cavity  30  that is formed when the first mold portion  82  and second mold portion  84  are brought together. The mold  80  also has a well-known chill block  86  interposed between mold cavity  30  and vacuum line  32  where overflow of excess molten material  34  is forced during the second portion of the injection sequence. As best seen in FIG. 5, the chill block  86  is formed by a sequence of mating channel portions  88  in the first mold portion  82  and second mold portion  84 . When the first mold portion  82  and the second mold portion  84  are brought together, the channel portions  88  of the chill block  86  form a serpentine path through which the vented gasses and vaporized molten material  34  pass. The area of each channel portion  88  decreases as it approaches the vacuum line  32 . The purpose of the chill block  86  is to cool the excess molten material  34  before it reaches the vacuum line  32 . However, any molten material  34  that escapes into the vacuum line  32  will be trapped by filter means  38  before reaching the interior of the vacuum break valve  68 . 
     The apparatus  20  has a minimum of moving parts, and only the moving injection piston is in contact with the molten material. This factor eliminates so-called “down time” during production and, as a result, the present invention increases production efficiency as compared with the prior art. It is well-known that prior art die-casting apparatus require a large amount of energy for operation, approximately 50% of which is used to produce the desired molded part or component. In comparison to the amount of energy already used by prior art die-casting apparatus  100 , the additional energy required for operation of vacuum cylinder  28  of the present invention is negligible. Thus, the present invention requires relatively no additional energy consumption. 
     Although the present invention has been described and illustrated with respect to a preferred embodiment and a preferred use therefor, it is not to be so limited since modifications and changes can be made therein which are within the full intended scope of the invention.