Abstract:
A valve gate piston assembly, in an injection molding system, is positioned in a staggered arrangement such that one piston assembly overlaps that of another valve gate piston assembly thereby minimizing an overall centerline pitch between said piston assemblies. Such a superposed layout of piston assemblies enables larger diameter pistons to occupy a smaller footprint than would be realized should the same piston assemblies be positioned side by side. The use of relatively larger diameter pistons results in an increased force available, for the same input air pressure, to actuate the pistons and valve stems attached thereto, resulting in a higher valve stem closing force. A staggered piston assembly layout also allows independent air circuits to control each piston separately. A combination of multiple piston assemblies may be configured in such a staggered, overlapping array.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This patent application is a continuation in part patent application of prior U.S. patent application Ser. No. 11/848,823, filed Aug. 31, 2007 now abandoned. This patent application also claims the benefit and priority date of prior U.S. patent application Ser. No. 11/848,823, filed Aug. 31, 2007 now abandoned. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to, but is not limited to, injection molding systems, and more specifically to multiple valve gate piston assemblies. 
     BACKGROUND 
     It is commonly known, by a person having ordinary skill in the art, that there are, generally, two methods of gating in an injection molding system, and more particularly, in a hot runner system. The first type is a thermal gate or hot tip gate, which relies on a heated nozzle tip to be collocated inside a mold gate insert. When molten resin is injected, under pressure, through the hot runner system, terminating at the hot tip, the resin is forced into a mold cavity via a narrow aperture in the gate insert, namely the gate. When the mold cavity is packed full of resin, injection pressure is configured to maintain equilibrium between the pressures in the cavity as well as within the hot runner for a duration known as the hold and pack time. While the resin in the hot runner is maintained in a molten state during the entire molding process via various heaters installed therein, the mold gate insert and mold cavity are kept cooled to a temperature significantly lower than that of the hot tip in an effort to solidify the molded article as quickly as possible. In addition to the molded article solidifying in the mold cavity, a portion of the resin in the gate area where the nozzle tip resides also solidifies resulting in a slug. This slug remains behind, in the gate insert, when the mold opens and the molded article is ejected from its cavity, thereby precluding the flow of molten resin from the gate area. The slug is subsequently injected into the mold cavity with the next wave of pressurized, molten resin. The total time taken for the mold cavity to be filled, and the molded article to solidify and be ejected from the mold, is called the cycle time. The cycle time is greatly affected by the length of time it takes both the slug and the molded article to cool and solidify. The hot tip is juxtaposed within the cooled gate insert; thus, the slug must cool sufficiently such that ejection of the molded article does not pull a ‘string’ of still molten resin from the gate area. Further, the molded article must cool sufficiently such that the ejection process from the mold will not damage it. The longer time the molded article must remain in the mold for the above-mentioned reasons, the cycle time is increased resulting in less molded articles being produced for any given timeframe, resulting in unrealized revenue by the molder. Additionally, a small ‘vestige’ or remnant of resin still remains on the ejected molded article at the gate area which is usually undesirable and must be either covered up by a label or the like, or manually trimmed off, resulting in increased cost per molded part. Furthermore, if the cold slug is not totally remelted upon entry into the subsequent molded article, it can result in a cosmetic blemish or worse yet, a potential mechanical defect resulting in rejection of the molded article. 
     To overcome the many drawbacks of thermal gate design, as described above, purchasers turn to valve gating designs. A typical valve gate mechanism consists of a piston, reciprocating inside a cylinder which may be either pneumatically or hydraulically actuated. Alternatively, a mechanically or electrically actuated system may be employed. Attached to said piston is a valve stem which extends coaxially along the center of at least one portion of the resin melt channel in the hot runner, nozzle and/or nozzle tip, culminating in the mold gate area, in the closed position. To inject resin into the mold cavity, the piston, and hence the valve stem, is actuated to retract the valve stem from a position in the gate area so as not to restrict resin flow through the gate area. The molten resin is then injected under pressure to fill the mold cavity. Once the mold cavity is packed with resin, the injection pressure is stabilized and the piston is activated to translate the valve stem to its full forward position where the tip of the valve stem mates with, and closes off, the gate. The geometry of the tip of said valve stem is approximately sized to match that of the gate insert, such that the flow of resin is mechanically shut off completely and additionally such that the tip of the valve stem protrudes slightly into the mold cavity and hence the molded article itself. The purpose of this latter function is to eliminate any vestige or post left on the molded article, again which is undesirable from a molded article quality perspective. Since the valve stem acts to mechanically sever the resin in the gate area from the molded article, no time is required for cooling of a slug to eliminate resin stringing, but only that of the molded article itself for the purpose of a harmless ejection, and consequently, there is no slug to be forced into the mold cavity on the subsequent injection cycle. 
     As described previously, the piston and valve stem arrangement may be either pneumatically or hydraulically actuated, but it may be appreciated that while a hydraulic system is capable of generating very high pressures, it has the potential of leakage of hydraulic fluid and is more costly to install and maintain. Comparably, air is relatively inexpensive, operates at a lower and safer pressure, is cleaner and does not require as robust an installation a hydraulic system. For these reasons, pneumatic valve stem actuation systems are preferred, but in order to achieve higher valve stem closing forces, given a standard, readily available shop air pressure, generally between 80 and 120 pounds per square inch, a relatively large diameter piston must be used. A higher than normal valve stem force is required in instances when the injection pressure needed to completely fill the molded article is excessively high and so an equally high force is required to move the valve stem to the forward position to close the gate against the pressure of the resin in the molded article. Other examples where higher valve stem closing forces are necessary are when molding with high viscosity resins or with resins containing fillers such as carbon or glass fibers or the like. In these particular cases, if the valve stem does not have sufficient force to fully engage and close off the gate, the result will be a highly undesirable ‘crown’ or ‘post’ of resin protruding above the surface of the molded article. 
     The traditional solution to augmenting the valve stem closing force is to utilize a piston of a sufficiently large diameter which will achieve the necessary valve stem closing force for the same available air pressure. The conventional arrangement of valve gate piston cylinders in a hot runner is a side-by-side configuration, with the minimum center-to-center pitch between each piston cylinder being slightly greater than the diameter of one piston cylinder. However, should larger diameter pistons be necessary for higher valve stem closing forces, this arrangement necessitates increasing the pitch of the pistons respectively. The drawback of this natural progression of higher stem force requiring larger pistons resulting in larger pitch spacing between pistons is that it precludes the mold design from optimizing the number of molded articles able to be fitted in one hot runner mold, since the molded article may be significantly smaller than the pitch required between piston cylinders, which, in turn, impacts realized molded article production quantities. Another instance where an increased piston pitch can be detrimental in mold design is the case where multiple gates per molded article are necessary and a close proximity of gates is required to fit the geometry of the molded article. In both instances described above, a sufficiently small diameter piston may optimize the mold design spatially, but be inherently insufficient to produce enough stem closing force to overcome the nature of the resin or injection pressures. 
     One method and design of achieving multiple valve stems positioned in a close pitch configuration, yet activated with sufficient force, is to attach numerous valve stems to one large piston. While this may gain the advantage of air pressure acting on a large surface area of the piston and the potentially close proximity of valve stems, it does not offer the advantage of independent and/or sequential valve stem control. Sequential valve stem control is most advantageous when molding one article with multiple gates in order to optimize the resin filling and flow characteristics in said molded article, and can only be realized when each valve stem is actuated via a single piston fed by a dedicated air circuit. This arrangement allows for filling of a single, multi gated, molded article with molten resin using both the upper and the lower valve stems but sometimes with one valve stem slightly delayed in opening from the other valve stem to effectively manipulate the resin weld line. Valve stem timing is varied to move the weld line to a location of preference for reasons of strength or appearance of the molded article. 
     For the foregoing reasons, there is a need for an injection molding device and method of molding multiple, separate articles arranged in a close pitch array or having numerous, closely spaced gates per article, which may be sequentially filled, and have high quality gates. 
     SUMMARY 
     The present invention is directed to a device that comprises a plurality of valve gate pistons arranged in a staggered fashion in a cylinder block, the cylinder block designed to house at least the lower piston while having dedicated air passages sealed therein to enable air to activate the lower piston and a bore therethrough for passage of the adjacent valve stem. The pistons are configured in such a way that an upper piston overlaps a lower piston, the upper piston and lower piston each having a valve stem coupled thereto such that each piston and valve stem assembly reciprocate together. A cover plate attached to the top of the cylinder block encloses the lower piston bore such that air fed to the bore, both above and below the lower piston, will activate the lower piston, the cover plate also supporting the upper piston cylinder and upper piston and having an orifice therethrough for passage of the valve stem. A backing plate functions to enclose the upper piston cylinder such that air delivered via the backing plate will activate the upper piston as well as the lower piston via the cover plate. 
     In one aspect of the present invention, a plurality of backup pads are located between the cylinder blocks and the manifold, each backup pad surrounding a respective manifold bushing, such that the conduction of heat from the manifold to the cylinder block is minimized. 
     In another aspect of the present invention, one common backup pad is located between the cylinder block and the manifold and is mounted atop multiple manifold bushings such that the conduction of heat from the manifold to the cylinder block is minimized. 
     In yet another aspect of the present invention, the cylinder block is configured to have an integral cover plate such that a bore therein secures a lower piston cylinder whereby air delivered via the backing plate will activate the lower piston. 
     The present invention provides the ability to utilize pneumatic actuation of large pistons to yield sufficiently high valve stem forces to overcome high injection pressure and the resistance of filled or viscous resins when the molded article gate locations are spaced close together by more efficiently using available space by staggering the piston such that they overlap one another. Additionally, each piston may be sequentially activated through the use of individual pneumatic air circuits. 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a valve gate assembly showing a plurality of staggered pistons wherein valve stems are in forward and retracted positions. 
         FIG. 2  is a cross-sectional view of the valve gate assembly showing a plurality of staggered pistons having a one piece backup pad and a cover plate which is an integral portion of the cylinder block. 
         FIG. 3  is a plan view representation showing overlapping projected areas of the pistons. 
         FIG. 4  is an isolated view showing detail of the cylinder block, particularly the wall between the piston bore and valve stem bore. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now initially to  FIG. 1 , a valve gate piston assembly  90  of a hot runner injection molding apparatus, in accordance to the present invention, is illustrated. The components shown are not a complete representation of the entire hot runner, but rather only those necessary to convey the intention of the present invention. For clarity, the arbitrary directions of up and down, and assignments of upper and lower have been adopted, though it may be appreciated by those skilled in the art that, in use, the hot runner system may not necessarily be oriented in this fashion. 
     A manifold  100 , heated via inset heaters  105 , is used to convey resin in a molten state, by way of melt channels  110  contained therein. Manifold bushings  115  are installed through the manifold in a manner such that their axes are collinear with the gate orifice  290 . The proximity of the manifold bushings  115 , and hence the nozzle stack and gate orifice  290  location, is determined by the diameter of the piston cylinders  230 . A minimal centerline pitch ‘X’ is achieved by staggering placement of one piston cylinder  230  above another with sufficient clearance ‘Y’ to contain air and support the constant impact of the upper piston  160  and lower piston  165  against a cover plate  200 , as in  FIG. 1 , or cylinder bore  240 , as in  FIG. 2 . A distance between centerlines, ‘X’, of a first valve stem  125  and a second valve stem  127  is determined essentially by half the piston cylinder  230  diameter plus half the first valve stem  125  diameter, including sufficient wall thickness  232 , to locate valve stem bore  195  so as not to interfere with the airtight piston bore  155  or cylinder bore  240 . A projected area  300  of the upper piston  160  overlapping a projected area  310  of the lower piston, shown in  FIG. 3 , illustrates the reduced center to center distance, ‘X’. This technique of overlapping the piston cylinders  230  thus also allowing a minimum distance ‘X’ to be realized between gate orifices  290  is fundamental to locating multiple nozzles in close proximity to each other to inject molten resin in several locations on one molded article  295  or to a plurality of individual molded articles  295  which are closely collocated to optimize mold layout. This staggered arrangement also facilitates the use of larger piston cylinders  230  within the same footprint as if smaller diameter piston cylinders  230  were placed side by side, thereby offering a fourfold increase in the valve stems&#39;  125 ,  127  closing force. The distance ‘X’, previously described as a minimum distance between piston centerlines, may also be customized by a factor of ‘X’ depending on mold layout. This staggered piston design will apply for all pitches between the minimum, and two times ‘X’. At any pitch greater than ‘X’ times two, staggered pistons  160 ,  165  would be replaced with conventional side by side pistons. 
     Melt passages  120  in the manifold bushings  115  are in fluid communication with the melt channels  110  in the manifold  100  such that resin may be diverted at an angle within the manifold bushing  115  to direct flow ultimately to the gate orifice  290 . In addition to redirecting the flow of molten resin, another function of the manifold bushing  115  is to guide the valve stems  125 ,  127 . 
     In  FIG. 1 , a backup pad  130  surrounds a manifold bushing neck  135  for the purpose of standing off and insulating a cylinder block  140  from the manifold  100 . The backup pad  130  is made from a material which is preferably less thermally conductive than that of the manifold  100  in an effort to keep the cylinder block  140  relatively cooler than the manifold  100 . By minimizing the amount of heat transferred from the manifold  100  to the cylinder block  140 , the life of the piston seals  145  is prolonged and the wicking effect of resin weepage being drawn up the manifold bushing neck  135  is lessened. 
     In an alternative embodiment,  FIG. 2 , a backup pad  170  may support a plurality of manifold bushings  115 . In this configuration, the cylinder block  142 , resting on the integral backup pad  170 , may support a plurality of piston cylinders  230 . The integral cylinder block  142  also includes a cylinder bore  240  to house the piston cylinder  230  when it is installed in the lower position. 
     In  FIG. 1 , the cylinder block  140  is aligned to the manifold bushings  115  via bores  175 . Internally to the cylinder block  140 , in the center bottom of the piston bore  155 , a recess  180  is provided in the cylinder block  140  to allow for a coupler  185 , such as a clip, to attach to the manifold bushing neck  135  as a means for securing the cylinder block  140  thereto in the interest of ease of maintenance and assembly. 
     The cylinder block  140  is configured to have a piston bore  155  for reciprocating travel of the lower piston  165  which is open above said piston for installation and maintenance access. A first passage  190  is drilled in the cylinder block  140  to provide air to the underside of the lower piston  165  to cause it to move upward. A valve stem bore  195  is through the cylinder block  140  to allow valve stem  125  to pass therethrough. Said bore  195  is sealed at the mating surface between the cylinder block  140  and the cover plate  200  to prevent air leakage up the valve stem bore  195  using sealing member  205 , such as an o-ring. 
     Each piston  160 ,  165  has a valve stem  125 ,  127 , installed therein, the valve stems  125 ,  127  being removably coupled to the pistons  160 ,  165  via a fastener  210 , such as a set screw. Therefore, when the lower piston  165  strokes up and down in the piston bore  155 , the corresponding first valve stem  125  also travels the same distance. The first valve stem  125  will be longer than the second valve stem  127 , by nature of the overlapping piston cylinder arrangement, and is dependent upon such factors as piston  160 ,  165  geometry and the clearance ‘Y’ between the piston cylinders  230 . 
     The uppermost stroke of the lower piston  165  is determined by the lower piston top  215  striking the cover plate  200  in  FIG. 1 , or the underside of cylinder bore  240 , in  FIG. 2 , just as the uppermost stroke of the upper piston  160  is determined by the upper piston top  217  striking the underside of cylinder bore  240  in the backing plate  235 . The cover plate  200  in  FIG. 1  is mounted to the cylinder block  140  with aligning components, such as dowels, and fasteners, such as screws, and serves to provide a back stop for the lower piston top  215 . 
     A second passage  220  above the lower piston  165  allows for air to enter the piston bore  155  forcing the lower piston  165  downward. Conversely, a third passage  225  through the cover plate  200  is in fluid communication with first passage  190  in the cylinder block  140 , allowing air to cause the lower piston to move upward. Additionally, the cover plate  200  supports and locates the piston cylinder  230  which houses the upper piston  160 . 
     A backing plate  235  at least partially houses the piston cylinder  230  inside a cylinder bore  240  by compressing it against the cover plate  200  to contain air therein to activate the upper piston  160 . The backing plate  235  feeds air to both the upper piston  160  and the lower piston  165  for their, reciprocating actuation and accomplishes this via dedicated passages drilled in the plate which terminate at particular locations in and around the piston bores. To cause the lower piston  165  to move the second valve stem  127  and a stem tip  245  to the gate closed position  250 , pressurized air is introduced through the lower piston forward passage  255 . To cause the lower piston  165  to move the second valve stem  127  and the stem tip  245  to the gate open position  260 , pressurized air is introduced through the lower piston retract passage  265 . To cause the upper piston  160  to move the first valve stem  125  and the stem tip  245  to the gate closed position  250 , pressurized air is introduced through the upper piston forward passage  270 . And to cause the upper piston  160  to move the valve stem  125  and the stem tip  245  to the gate open position  260 , pressurized air is introduced through the upper piston retract passage  275 , via a plurality of feeder holes  277  to the underside of upper piston  160 . It may be noted that lower piston forward passage  255  is in fluid communication with second passage  220  and that lower piston retract passage  265  is in fluid communication with third passage  225 . To ensure all air passage connections and bores are hermetically sealed, each is surrounded by a groove  280  for receiving a seal  285 , such as an o-ring. 
     In operation, when the valve stems  125 ,  127  are cycled back and forth via pneumatic actuation of the pistons  160 ,  165 ; it causes the stem tip  245  to move in and out of the gate orifice  290 . For example, the piston  165  in the down, fully forward or closed position  250  results in the stem tip  245  residing in the gate orifice  290  sealing off flow of pressurized, molten resin to the molded article  295  via an intimate fit between the stem tip  245  and the gate orifice  290 . Similarly, a piston  165  in the up, fully retracted or open position  260  pulls the stem tip  245  out of the gate orifice  290  with sufficient clearance such that molten, pressurized resin can flow without obstruction through the gate orifice  290  into the molded article  295 . 
     Description of the embodiments of the present inventions provides examples of the present invention, and these examples do not limit the scope of the present invention. It is to be expressly understood that the scope of the present invention is limited by the claims. The concepts described above may be adapted for specific conditions and/or functions, and may be further extended to a variety of other applications that are within the scope of the present invention. 
     Having thus described the embodiments of the present invention, it will be apparent that modifications and enhancements are possible without departing from the concepts as described. Therefore, what is to be protected by way of letters patent are limited by the scope of the following claims: