Abstract:
A failsafe device, or pressure relief mechanism, for a shooting pot actuator in an injection molding machine. The shooting pot actuator has a multiple pusher rods mounted on one, or more, plates. Moving the plate holding the pusher rods depresses the shooting pot injection pistons and injects molten material into a number of mold cavities. To avoid damage to the machine from the pusher rods if an injection piston seizes, a failsafe device is used to mount the pusher rods to the plates. A shearing member is interposed, or sandwiched, between first and second apertures. Typically, the shearing member is a plate that, in normal operating conditions, blocks rearward movement of the pusher rod. However, when a predetermined shear force is applied to the shear plate, the shearing member shears and the pusher rod retracts within the channel, thereby alleviating the pressure. The failsafe device can be paired with a seizure detection system, using a laser beam, that detects piston and valve gate seizure, and provides appropriate notification or control signals.

Description:
FIELD OF THE INVENTION 
     The present invention relates to injection molding machines. More particularly, the present invention relates to the common control of multiple shooting pots in a injection molding machine, and a failsafe mechanism for preventing damage due to machine malfunction, such as injection piston or valve gate seizure. 
     BACKGROUND OF THE INVENTION 
     Employing control units, such as shooting pots, to introduce thermoplastic resins or other materials into a mold cavity in an injection molding machine is well known. Generally, a primary resin source feeds the material to a shooting pot reservoir which is, in turn, operated to feed a measured, or metered, quantity of the material into the mold cavity. U.S. Pat. No. 3,516,123, entitled “Injection Molding Machine”, to Lang; and U.S. Pat. No. 3,231,656, entitled “Apparatus and Method of Plastic Molding”, to Ninneman both disclose the use of shooting pots to provide accurately metered shots of resin to a mold cavity. Metering permits an accurate amount of material to be injected into a mold to ensure that a properly formed part is created and to prevent waste of material in the form of “flash”, etc. due to overfilled molds. Metering is generally achieved by controlling the distance by which an injection plunger in the shooting pot is retracted and advanced for each shot. 
     Other metering techniques are also well known. For example, U.S. Pat. No. 4,966,545, entitled “Staged Shooting Pot for Injection Molding, to Brown, shows how a single shooting pot can be operated to cause two sequential metered injections of the same resin into the same mold cavity. U.S. Pat. No. 4,460,324, to Van Appledom, entitled “Shot Cylinder Controller for Die Casting Machines and the Like”, shows how the injection speed of the piston of shooting pot can be controlled, thereby controlling the rate of injection of the resin into the mold cavity. 
     It is also well known to supply thermoplastic material to a multicavity mold through a hot runner system. The hot runner system can include a plurality of shooting pots, with at least one shooting pot associated with each mold cavity. 
     Hot runners systems can also be used for multimaterial injection, or coinjection, molding. Typically, two or more resins are injected, either simultaneously or sequentially, into each mold cavity to produce multi-layered molded structures. For example, a common application for multimaterial molding is the production of food quality containers from recycled plastic. Government standards require that any surfaces which contact the food be made of new, virgin, plastic. To take advantage of lower cost recycled plastics, manufacturers use confection techniques to encapsulate recycled material in a sheath of new plastic. U.S. Pat. No. 5,098,274 to Krishnakumar, entitled “Apparatus for Injection Molding of Multilayer Preforms”, and U.S. Pat. No. 4,717,324 to Schad, entitled “Coinjection of Hollow Articles and Preforms” both disclose injection molding machines for multimaterial applications. 
     Generally, individual control of the shooting pot strokes is provided in these prior art injection molding machines. Separate hydraulic actuation cylinders for each shooting pot injection plunger are mounted inside the machine&#39;s stationary platen. These hydraulic cylinders must be individually set for stroke to control the individual metering of the resins into the mold cavities. The setting of the cylinders can be a hazardous operation, which is performed manually and requires personnel to reach into the machine amongst the heated injection nozzles, close to hot surfaces and heated injection materials. Furthermore, the molding process has to be interrupted for this adjustment, which can cause significant loss of production time, especially in larger machines having up to ninety six injection plungers. 
     U.S. Pat. No. 4,632,653 to Plocher, entitled “Press with a Plurality of Injection Plungers” describes a common actuator for the injection plungers in a transfer molding machine. The injection plungers are actuated by a hydraulic drive acting on a single crosspiece. However, the shooting pot actuator disclosed in Plocher has several limitations and disadvantages which make it inapplicable to metered injection molding machines. Firstly, the shooting pots in a compression molding machine do not provide metered shots. Instead, each shooting pot is filled with an approximate amount of resin, and the injection pistons are actuated by the crosspiece to compress the resin into the mold cavity. Plocher discloses pressure compensating pistons and overflow channels to relieve the mold cavities in the case of overfilling, which results in non-uniform product and flashing. Also, there is no mechanism provided for adjusting the stroke of the injection pistons since precise control of the amount of resin injected into the mold is not critical in such a transfer molding process. Second, the crosspiece actuator in Plocher is located within the mold, which increases the cost of designing and manufacturing the mold. Also, such a design is impractical in machines with high clamp forces as the volume occupied by the crosspiece reduces the strength of the mold component in which it is located, thus increasing the likelihood of deformation of mold components when clamped. Further, the mold must be completely disassembled to obtain access for maintenance, adjustment, or replacement. 
     A device capable of actuating multiple shooting pots for metered multimaterial injection is disclosed in commonly assigned U.S. patent Ser. No. 09/050,095. The shooting pot described therein has a number of pusher rods attached to plates driven externally of the mold. The pusher rods extend through apertures in the stationary platen, and each abut against a respective injection piston to inject material into the mold. One problem, with the device as originally conceived, can occur when an injection piston seizes. If a single piston seizes, the actuator will attempt to overcome the resistance of the seized piston, potentially damaging the piston, its cylinder, or buckling its associated pusher rod. This can result in costly downtime and repair. Piston seizing is relatively common, and can occur for a number of reasons. Typically, piston seizures can be easily remedied if no permanent damage is done to the injection molding machine. 
     It is, therefore, desirable to provide a failsafe device for a shooting pot actuator that limits damage due to injection piston seizure. It is further desirable to provide a failsafe device that alerts an operator of an injection molding machine to a seizure condition in the machine. 
     SUMMARY OF THE INVENTION 
     In a first embodiment of the present invention, there is provided a failsafe device, or pressure relief mechanism, for a shooting pot actuator in an injection molding machine. The shooting pot actuator has a multiple pusher rods mounted on one, or more, plates. Moving the plate holding the pusher rods depresses the shooting pot injection pistons and injects molten material into a number of mold cavities. To avoid damage to the machine from the pusher rods if an injection piston seizes, a failsafe device is used to mount the pusher rods to the plates. A guide block, provided with a first aperture receives an end of the pusher rod. This first aperture is aligned with a second aperture in the plate to form a channel for receiving the pusher rod. In a presently preferred embodiment, a die plate, also provided with an aligned aperture secures the guide block to the plate. A shearing member is interposed, or sandwiched, between the first and second apertures. Typically, the shearing member is a plate that, in normal operating conditions, blocks rearward movement of the pusher rod. However, when a predetermined shear force is applied to the shear plate, the shearing member shears and the pusher rod retracts within the channel, thereby alleviating the pressure. For a forty-eight cavity, multimaterial injection molding machine the appropriate shear force is equivalent to approximately 45,000 psi plastic pressure in the shooting pot cylinder. 
     In a further aspect of the present invention the failsafe device is paired with a seizure detection system that detects piston and valve gate seizure, and provides appropriate notification or control signals. The detection system consists of a laser transmitter and a laser receiver aligned with a row of pusher rods. Sighting means are provided on the pusher rods, such as circumferential grooves coincident with the guide block, through which a beam transmitted by the transmitter passes to the receiver under normal operating conditions, i.e. when the pressure relief mechanism has not been activated. Appropriate circuitry is attached to the receiver to detect if the beam is interrupted, and to send notification signals to the machine operator, or control signals, such as an automatic shutdown signal. 
     In another aspect of the present invention, there is provided a multimaterial injection molding machine incorporating the failsafe device and detection system. The machine includes a mold cavity, and at least two shooting pots that provide material to the mold cavity. Each shooting pot has an injection piston for expressing material into the mold cavity. A shooting pot actuator is attached to the machine. It has a first plate and a second plate, and each plate carries pusher rods that abut respective injection pistons. The first and second plates are sequentially driven to advance their respective pusher rods against the injection pistons. A pressure relief mechanism, or failsafe mechanism, is used to mount each each pusher rod to its respective plate. A guide block, provided with a first aperture receives an end of the pusher rod. This first aperture is aligned with a second aperture in the plate to form a channel for receiving the pusher rod. In a presently preferred embodiment, a die plate, also provided with an aligned aperture secures the guide block to the plate. A shearing member is interposed, or sandwiched, between the first and second apertures. Typically, the shearing member is a plate that, in normal operating conditions, blocks rearward movement of the pusher rod. However, when a predetermined shear force is applied to the shear plate, the shearing member shears and the pusher rod retracts within the channel, thereby alleviating the pressure. For a forty-eight cavity, multimaterial injection molding machine the appropriate shear force is equivalent to approximately 45,000 psi plastic pressure in the shooting pot cylinder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
     FIG. 1 is a schematic illustration of a prior art multimaterial hot runner system for a four cavity mold; 
     FIG. 2 shows a cross section of a prior art multimaterial hot runner system in the vicinity of one nozzle assembly; 
     FIG. 3 shows a cross section of a portion of a prior art multimaterial injection molding machine, including a common shooting pot actuation assembly with all pushers in the retracted position; 
     FIG. 4 shows a rear view of the machine of FIG. 3 in the direction of the line D; 
     FIG. 5 shows a cross section of the machine of FIG. 3 along the line A—A; 
     FIG. 6 shows a cross section of the machine of FIG. 3 along the line B—B; and 
     FIG. 7 shows a cross section of the machine of FIG. 3 along the line C—C. 
     FIG. 8 shows the machine of FIG. 3 with the first set of pushers advanced; 
     FIG. 9 shows the machine of FIG. 3 with both the first and second sets of pushers advanced; 
     FIG. 10 shows a cross section of a portion of a multimaterial injection molding machine according to the present invention; 
     FIG. 11 shows a side view of a first embodiment of a failsafe mechanism for the shooting pot actuation assembly of FIG. 10; 
     FIG. 12 shows a cross section of the failsafe mechanism of FIG. 11; 
     FIG. 13 shows a side view of a second embodiment of a failsafe mechanism for the shooting pot actuation assembly of FIG. 10; 
     FIG. 14 shows a cross section of the failsafe mechanism of FIG. 13; 
     FIG. 15 shows a cross section of the machine of FIG. 10, along the line E—E; and 
     FIG. 16 shows a cross section of the machine of FIG. 10, along the line F—F. 
    
    
     DETAILED DESCRIPTION 
     For purposes of illustration, the present invention will be described with reference to a dual hot runner injection molding machine as shown in the drawings. As will be apparent to those skilled in the art, the present invention can be generally employed in any injection molding machines having multiple shooting pots for which common control is desired. 
     A prior art shooting pot actuator is shown in FIGS. 1 and 2, wherein FIG. 1 shows a shows a schematic and FIG. 2 shows a cross section of a portion of a hot runner system for an injection molding machine which accommodates two thermoplastic resins, or other material to be molded, indicated generally at reference numeral  20 . One resin is provided from a source identified as Extruder A, the other resin is provided from a source identified as Extruder B. While the illustrated embodiment shows two resin sources A and B, it is entirely within the scope of the invention to utilize one, two or more sources. The portion of the hot runner system  20  leading from Extruder A is shown in solid lines, and the portion of the system leading from Extruder B is shown in dashed lines. 
     As shown in FIG. 1, the materials supplied by Extruders A and B are fed to mold cavities  22 ,  24 ,  26  and  28  through corresponding individual confection nozzles  32 ,  34 ,  36  and  38 . Extruder A supplies a heated manifold M a  which, in turn, communicates with each nozzle  32 ,  34 ,  36  and  38  via hot runners or channels  42 ,  44 ,  46  and  48 , respectively. Rotary valves  52 ,  54 ,  56  and  58  operate to control charging of shooting pots, or injection cylinders,  62 ,  64 ,  66  and  68 . 
     Correspondingly, heated manifold M b  leads from Extruder B to each nozzle  32 ,  34 ,  36  and  38  via hot runners  72 ,  74 ,  76  and  78 . Rotary valves  82 ,  84 ,  86  and  88  control charging of shooting pots  92 ,  94 ,  96  and  98 . 
     While the schematic of FIG. 1 shows a hot runner system  20  leading from two sources, Extruders A and B, transporting conditioned thermoplastic resins to a four cavity mold, it is entirely within the scope of the present invention to service forty-eight, or more, mold cavities originating from one, two or more sources. 
     As shown in FIG. 2, a central manifold block  102  is maintained at an appropriate temperature range by heating elements  104 . For example, if the resin is polyethylene terephthalate (PET), the central manifold block can be maintained at a temperature ranging from approximately 500° to 550° F. Channels  106  and  108  receive plasticized resin from Extruder A. Rotary valve  112 , in circuit with channel  108  and operated by link mechanism  114 , controls the charging of reservoir  116  of shooting pot, or injection cylinder,  118  each of which is equipped with an injection piston,  122 . Rotary valve  112  is formed with a transverse throughbore  124  and is shown in FIG. 2 in the closed position. The reservoir  116  communicates with channel  126  which, in turn, leads to the nozzle assembly  32 . Nozzle assembly  32  functions to inject the resin into a mold cavity (not shown). 
     Similarly, for the path leading from Extruder B, a manifold block  130 , which can be a separate segment from manifold  102  or a part thereof, is maintained at an appropriate temperature range by heating elements  132 . For example, if the resin is ethylene vinyl alcohol copolymer (EVOH), the central manifold block can be maintained at a temperature ranging from approximately 400° to 440° F. by heaters  132 . Channels  134  receives plasticized resin from Extruder B. Rotary valve  144 , in circuit with channel  134  and operated by link mechanism  133 , controls the charging of reservoir  136  of shooting pot, or injection cylinder,  138  each of which is equipped with an injection piston  142 . Rotary valve  144  is formed with a transverse throughbore  146  and is shown in FIG. 2 in the closed position. The reservoir  136  communicates with channel  140  which, in turn, leads to the nozzle assembly  32 . 
     Nozzle assembly  32  includes a central spigot  146  in thermal contact with manifold block  102 . Spigot  146  is formed with a through channel  148  through which the resin can flow to a nozzle gate  152 . As shown, a valve stem  166  moved by an a piston  168  controls the opening and closing of gate  152 . Other gating systems, as are well known to those of skill in the art can be used to control the injection of resin through nozzle assembly  32 . 
     Spigot  146  is supported in a housing  158  which is spaced from spigot  146  substantially through its length by an insulating air gap  162  to maintain the resin from Extruder B at its optimum processing temperature as it progresses to gate  152  through a channel  160 . 
     Generally, to inject the two resins from Extruders A and B into each mold cavity, the set of injection pistons  122  for the resin supplied by Extruder A is first advanced to displace a metered amount of the first resin into the mold cavity, partially filling it. This is followed by advancing injection piston  142  to displace a metered amount of the second resin supplied by Extruder B, again only partially filling the mold cavity. Finally, a second feeding of the first resin directly through channel  126 , bypassing shooting pot  116 , fills the mold cavity and packs out the molded articles. As is well understood, the particular sequence chosen for producing the molded articles will depend on the desired final structure, and can include simultaneous, as well as sequential, injection into the mold cavity. 
     FIGS. 3-9 show side and rear views of an injection molding machine incorporating an embodiment of the present invention. In FIG. 3, a mold  180 , including hot runner system  20 , is mounted between a clamp unit  184 . Clamp unit  184  generally comprises a stationary platen  190  and a movable platen  192 . Mounted to the exterior of the stationary platen  190  is a common shooting pot actuation assembly  196 . While in the illustrated embodiment, and the following description of the present invention, the shooting pot actuation assembly  196  is mounted to the stationary platen  190 , it is fully within the contemplation of the inventors that assembly  196  can be mounted to whichever platen is adjacent hot runner  20 . 
     Shooting pot actuation assembly  196  generally comprises a frame  198 , a shooting pot actuator  200 , and drive means  202 . Frame  198  has four columns  204 ,  206 ,  208  and  210  secured to stationary platen  190  in a generally rectilinear pattern, as best seen in FIG. 4, by bolts  212 . A drive support  214 , spaced from the rear of the stationary platen  190  by the exposed length of columns  204 ,  206 ,  208  and  210  is mounted to the ends of the columns and secured by bolts  216 . To drive support  214  are attached first and second drives  218  and  220 , the operation of which will be further described below. Drives  218  and  220  can be hydraulic rams, linear electric motors, or any other suitable drive. 
     Shooting pot actuator  200  is mounted on columns  204 ,  206 ,  208  and  210  for sliding movement between drive support  214  and the rear of stationary platen  190 . In the illustrated embodiment, actuator  200  has two parallel and separately movable plates  222  and  224 . A first group of pushers  226  is secured to first plate  222 . Pushers  226  are arranged to correspond to the position of each of the injection pistons  142  in their respective set in mold  180 . Similarly, a second group of pushers  228  are secured to second plate  224 , and are arranged to correspond to the position of injection pistons  122  in their respective set. Pushers  226  and  228  can be screwed into plates  222  and  224 , or can be secured with “bayonet” mounts, or in any other appropriate manner. Ideally, the mounting method ensures that each mounted pusher  226 ,  228  extends from its respective plate  222 ,  224  to a substantially identical extent. 
     Pushers  226  and  228  extend through bores  230  and  232 , respectively, in stationary platen  190  and abut injection pistons  142  and  122 . The arrangement of pushers  226  and  228  depends upon the placement of shooting pots  138  and  118 , and their respective injection pistons  142  and  122 , in the hot runner system  20 . FIG. 7 shows an arrangement suitable for a forty-eight mold cavity confection molding machine for making preforms. To accommodate a number of different shooting pot arrangements, pushers  226  and  228  can be detached and rearranged as desired on plates  222  and  224 , or, separate plate-pusher assemblies can be provided for different molds  180 . It is contemplated that standardized injection piston spacings can be employed to permit molds to be interchangeable, as described below in more detail. 
     Plate  222  can be driven reciprocally along columns  204 ,  206 ,  208  and  210  by corresponding drive  218 . As best seen in FIGS. 5 and 6, drive  218  comprises two hydraulic cylinder pistons  236 . Plate  224  is similarly driven by drive  220  which comprises two hydraulic cylinder pistons  234 . Since plate  222  is disposed in front of plate  224 , piston bores  238  are provided in plate  224  to accommodate the passage of pistons  236  and to permit free movement of plate  222  with respect to plate  224 . Similarly, bores  239  are provided in plate  222  to permit the free passage of pushers  228  therethrough. Depending on the configuration of pistons  236 , bores  238  and  239  can be replaced by cutouts, or omitted altogether if the pushers would not interfere. 
     The position and linear velocity of plates  222  and  224  can be sensed by linear position sensor means  240 . Sensor  240  can be a magnetic, opto-electronic or other suitable sensor, such as those manufactured by Temposonic Inc. Sensor  240  is fixed to frame  198 , or otherwise fixed relative to plates  222  and  224 . The sensor  240  can be attached to a suitable control system (not shown) for conventional electronic and/or programmable control of the actuator  200 , as is well known to those of skill in the art. 
     Referring to FIGS. 3,  8  and  9 , the operation of the actuator  200  will be described with respect a multimaterial injection sequence. Prior to the below described injection sequence, the clamp unit  184  is activated to clamp together the mold  180 , in a manner well understood by those of skill in the art. The injection sequence begins with pushers  226  and  228 , and plates  222  and  224 , in a retracted position, as shown in FIG.  3 . In the retracted position, the free ends of the pushers  226  and  228 , which abut the injection pistons  142  and  122  in the hot runner system  20 , limit the rearward movement of the injection pistons  142  and  122 , and, hence the volume of material that can be received in shooting pot reservoirs  136  and  116 . Adjusting the retracted positions of plates  222  and  224 , by adjusting the rearward stroke of their respective cylinder pistons  234  and  236 , thereby effectively meters the amount of material can be accepted by each shooting pot  136  and  116  from Extruders B and A. 
     Once the shooting pots  136  and  116  are filled with the desired amount of material in the manner described above, plate  224  and its pushers  228  are advanced to actuate the set of injection pistons  122 , thereby injecting the metered shot of material from each reservoir  116  into its respective mold cavity. Pushers  228  are advanced by a forward stroke of cylinder pistons  236  acting upon plate  224  in the direction of the arrow F, as shown in FIG.  8 . Bores  238  and  239  permit plate  222  to move forvard without affecting the position of plate  222 . The position and speed of plate  224  during the forward stroke is sensed by sensor  240 . Sensor  240  relays the information to the control system which, in turn, controls the speed and distance travelled by the pushers  228 . 
     Next, as shown in FIG. 9, plate  222  and its pushers  226  are advanced to actuate injection pistons  142 , thereby injecting the metered shot of material from each reservoir  136  into its respective mold cavity. Pushers  226  are advanced by a forward stroke of cylinder pistons  234  acting upon plate  222  in the direction of the arrow G. The position and speed of plate  222  are sensed by sensor  240  to control the speed and distance travelled by the pushers  226 , as described above. An injection of material from Extruder A is then fed directly to the nozzle  32  to pack the mold, and the gate  152  is closed. 
     The coinjection molding operation then proceeds as in conventional machines. The material injected into the mold cavities is permitted to cool, the clamp unit  184  is released, and the finished product is ejected from the mold. 
     As will be apparent to those skilled in the art, such a shooting pot actuator is not limited to two plates, but can be extended to three or more plates-pushers and corresponding sets of shooting pots, as desired. Nor is the actuator limited to sequential injection of the multiple resins. Combinations of sequential and/or simultaneous movement of the push rods are possible to cause like injections of the respective resins. 
     The actuator assembly  196  can also be incorporated into a transfer molding system, as described in co-pending U.S. Provisional Application Ser. No. 60/078,587, filed Mar. 19, 1998. As described therein, the injection pistons are pulled backwards from their forward stroke position at the same rate as the shooting pots are being filled to reduce the acetaldehyde content of the finished articles. In this case, to incorporate the actuator assembly  196 , the pusher rods  226 ,  228  are fixed to the injection pistons to permit the controlled retraction of the injection pistons, and a control system monitors and controls the rate at which the pistons are pulled backwards. 
     The present invention incorporates a failsafe mechanism into the prior art shooting pot actuator  200 , as shown in FIG.  10 . In certain situations, one or more injection pistons  122 ,  142  can seize. Since the actuator  200  operates on multiple pistons, the seizure of a single piston  122 ,  142  can have catastrophic effects. The actuator will attempt to overcome the resistance of the seized piston(s) and will likely cause damage, such as buckling pusher rods  226 ,  228 , and/or damaging the piston and its associated cylinder. This is equally true of valve gate seizures. To avoid such a situation, plates  222 ,  224  are provided with a number of failsafe devices  250 ,  252 , typically one per pusher rod attached to a plate. 
     Referring to FIGS. 11-14, failsafe devices  250 ,  252  are shown in greater detail. FIGS. 11 and 12 show failsafe devices  250  deployed on plate  224  for attachment to pusher rods  228 . FIGS. 13 and 14 show failsafe devices  250  deployed on plate  222  for attachment to pusher rods  226 . 
     Referring first to FIGS. 11 and 12, which show a side view and a cross section, respectively, of failsafe devices  250  attached to plate  224 , each failsafe device  250  consists of a guide block  252 , a shearing member, such as shear plate  254 , and die plate  256 . Guide block  252  and die plate  256  are provided with apertures  258 ,  260 , respectively. Apertures  258  and  260  are aligned with each other, and with an aperture  262  formed in plate  224 . The aligned apertures  258 ,  260  and  262  form a channel  264  that has a diameter slightly larger than the outer diameter of pusher rod  228 , such that pusher rod  228  is held in a sliding fit within channel  264 . Shear plate  254  is placed between guide block  252  and die plate  256  such that it blocks channel  264 . Guide block  252 , shear plate  254  and die plate  256  can be made of any suitable material, such as machined steel or aluminum, as will be apparent to those of skill in the art. 
     In operation, failsafe device  250  protects the injection molding machine from damage caused by seizure of injection pistons or valve gates. Generally, when such an over-pressure situation occurs, a pusher rod can be subject to increasing longitudinal pressure as plate  224  is advanced. Shear plate  254  is designed to shear, or punch through, at a predetermined pressure to permit its associated pusher rod  228  to retract within channel  264  to relieve the excessive pressure applied thereto. For example, in a forty-eight cavity injection molding machine, it has been found that damage to the mold components and pusher rods occurs when the plastic pressure in the shooting pot exceeds approximately 60,000 psi. Therefore, incorporating a safety factor, shear plate  254  is designed to shear at approximately 45,000 psi of plastic pressure. As used herein, “plastic pressure” is defined as the pressure in the shooting pot, or a force of equivalent resistance, and “shear pressure” is defined as the plastic pressure, or a force of equivalent resistance, at which a shearing member is designed to shear. For the shear plates  254  associated with plate  224 , this translates into a force of approximately 5400 lbs calculated by dividing the shear pressure by the shooting pot area, which equates to 221 psi of hydraulic pressure applied to plate  224 . For the shear plates  272  associated with plate  222 , this translates into a force of approximately 31500 lbs calculated by dividing the shear pressure by the shooting pot area, which equates to 224 psi of hydraulic pressure applied to plate  222 . In both cases, this allows a safety factor of approximately 4 between the operating force and the shear force. The design of shear plates  254 ,  272  is a matter of standard engineering design, and can be modified, as desired to incorporate greater or lesser safety factors and shearing forces, depending on the desired application. 
     Referring to FIGS. 13 and 14, showing a side view and a cross section, respectively, of failsafe devices  252  for attaching pusher rods  226  to plate  222 . Failsafe devices  252  are similar in construction to failsafe devices  250 . They also consist of a guide block  270 , a shearing member, such as shear plate  272 , and a die plate  274 . However, because the pusher rods  226  are subject to greater pressures than pusher rods  228 , they must have an increased diameter to avoid buckling at the predetermined shear pressure of 45,000 psi. To maintain the same shear properties, rod  226  has its end  276  machined down to the appropriate diameter to allow shear plate  272  to shear to prevent damage to mold components. Consequently, apertures  278  and  280 , formed in die plate  274  and plate  222 , respectively, are of substantially the same diameter as the machined end  276 , while an aperture  282 , formed in guide block  270  has a diameter coinciding with the thicker portion of pusher rod  226 . The aligned apertures  278 ,  280 , and  282  form a channel  284 . 
     In operation, failsafe device  252  operates as described above. If a pressure in excess of the calculated shear pressure is applied to any pusher rod  226 , end  276  of the pusher rod  226  will shear, or punch out, the shear plate  272 . Pusher rod  226  will then retract within channel  284 , thereby relieving the over-pressure and preventing damage to the mold components and pusher rod. 
     While the failsafe devices  250 ,  252  have been described as having a shear plate that shears in an over-pressure situation, the shearing member can be a shear pin or other analogous component, as will occur to those of skill in the art. 
     A further feature of the present invention is a system for detecting over-pressure situations, such as piston or valve gate seizure. Referring to FIGS. 15 and 16, the detection system generally consists of a series of paired transmitters  290  and receivers  292  placed at opposite edges of plates  222 ,  224 . Circumferential grooves  294 , or other sighting means, are provided on each pusher rod  226 ,  228  (as shown in FIG.  11 - 14 ). Transmitters  290  and receivers  292  are paired and aligned with the upper surface of the grooves  294  which extend beyond guide blocks  252 ,  270  on each plate. Each transmitter/receiver  290 / 292  pair services a horizontal row of pusher rods. In a presently preferred embodiment, transmitters  290  and receivers  292  are laser transmitters and receivers. 
     In operation, when the pressure applied to a pusher rod exceeds the predetermined shear force, its associated shear plate shears and the pusher rod retracts within channel  264 ,  284 . This causes groove  294  on that pusher rod to become misaligned with the rest of the grooves  294  in its row. This breaks the laser beam travelling between the transmitter  290  and receiver  292 . Appropriate circuitry and processing hardware and software, as are well known to those of skill in the art, are attached to the transmitter/receiver pair to detect such a broken beam, and to provide appropriate automatic shutdown of the machine, or alarm and warning signals to the machine operators who can then shut the machine down, and effect appropriate repairs. 
     The failsafe device and seizure detection system of the present invention provide certain advantages over the prior art. Chief among these is the reduction in damage to mold components and pusher rods that can occur when an excess of force is applied by a pusher rod. This results in savings in repair costs, and reductions in machine downtime. The detection system also permits quick detection of a seized piston or valve gate such that the injection molding machine can be shut down and repaired when a problem arises. 
     The above-described embodiments of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art, without departing from the scope of the invention which is defined solely by the claims appended hereto.