Abstract:
A die preparation and exchange method wherein dies to be inserted into an injection molding machine are electrically and fluidly heated prior to insertion so as to prepare the dies to make parts more quickly. The die exchange sequence wherein a first set of dies are removed from the machine and are replaced by a second set of dies has been changed from a generally serial sequence to a sequence wherein most steps are performed in parallel or simultaneously, greatly reducing the downtime associated with mold change.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to plastic injection molding machines and, more particularly, toward improved methods for exchanging molds for use in injection molding machines. 
   2. Description of Related Art 
   Injection molding machines have developed over the years to permit molding of a variety of parts, from small, simple plastic parts to rather large and complex plastic parts. For large parts, such as automobile bumpers, the injection molding machines and resulting injection molding dies are correspondingly large and heavy. 
   Due to the relatively short mold cycle time as compared to the rest of the factory, when using such large injection molding machines in a modern manufacturing environment, it is common to mold a particular part for a period of time with the first set of dies and then to exchange the first set of dies with a second set of dies to permit a different part to be molded. By switching the dies, the number of parts made with a single molding machine is increased. However, the die changing process has, in the past, been a time consuming operation requiring a significant amount of manual labor, and has resulted in a significant amount of down time for the machine. The manual labor involved in changing the dies is the result of the fact that each of the dies has a series of electrical, hydraulic, and cooling water connections that must be first unplugged from the first set of dies in order to permit the first set of dies to be removed from the machine. Thereafter, after the second set of dies is installed in the machine, these electrical, hydraulic, and cooling water connections must again be connected to the second set of dies. Since the number of connections may be between about 10-15, there is a significant amount of time involved, and is rather complicated for the operator. 
   Such large injection molding machines include an injector, a die assembly comprising a fixed die and a movable die, and die loading/unloading assembly including a movable mold transfer cart, and a part removal apparatus. The movable die is secured to a platan that is movable relative to the fixed die between a closed position and an open position. 
   The injector receives solid palletized or granular plastic material, heats and melts the plastic material, and pressurizes and injects the melted plastic into the die assembly. A screw type injector may be used in this regard. The die assembly receives the injected melted plastic and holds the plastic material until it solidifies, at which point the dies are moved apart to reveal the part. Ejectors are preferably built into the movable die, and serve to separate the molded part from the movable die. The part removal apparatus grasps the part, removes the part from the dies, and transports the part away from the dies. 
   The die loading/unloading assembly is used to replace or exchange dies in the machine, when necessary. In order to exchange a first set of dies in the machine with the second set of dies, which are stored on the mold transfer cart, the first set of dies are slid out of the machine and onto the transfer cart by the die loading/unloading assembly. Thereafter, the transfer cart is translated laterally to align the second set of dies with the opening in the machine, and then the second set of dies are slid into the machine by the die loading/unloading assembly. 
   More specifically, steps performed in a mold change sequence according to the prior art are illustrated in FIG.  1 . First, the hoses and cables are disconnected. This step takes an inordinately large period of time because each of the dies has a plurality of hoses and cables connected thereto, and each of the hoses and cables must be individually separately disconnected and moved out of the way. Thereafter, the molds are unclamped, and then the rear machine door is opened. The molds are carried out by the die loading/unloading assembly, which conventionally includes a drive roller and idle rollers built into the machine and the transfer cart. Thereafter, the rear door is closed, and then the transfer cart is moved to place the second set of molds into alignment with the rear door opening. Simultaneously, a data conversion is performed to adjust for the change from the first die set to the second die set. The platan is positioned to receive the second mold set, and then the rear door is opened. The platan positioning is performed after the data conversion, but partially simultaneously with the movement of the transfer cart. The rear door is opened after the platan is positioned, and then the die loading/unloading assembly carries the second set of dies into the machine. 
   Once the second set of dies are inside the machine, the rear door is closed. Once the door is closed, the dies are clamped into place, a mold thickness step is performed, and the hoses and cables are reconnected. Mold thickness refers to calibration of the machine to the size of the dies to establish zeroes or set point positions. As when the hoses/cables are disconnected, reconnecting the hoses and cables takes and inordinately large period of time. When reconnecting the hoses and cables, care must be taken to make sure that the correct connections are made. Thus, locating the hoses/cables, and correctly reconnecting them to the mold takes a great amount of time. 
   As will be appreciated, the vast majority of the prior art mold change steps are performed sequentially or serially. Therefore, apart from speeding up any of the steps, there is little that can be done to improve the conventional serial operation of the prior art method and, more specifically, little that can be done to reduce the length of the prior art mold change method. 
   With reference to  FIG. 2 , a prior art machine operating method surrounding the mold change sequence of  FIG. 1  is disclosed. As will be seen, the machine operating method also employs a generally sequential operation. Following production of the last part using the first set of dies, the R/B home is performed wherein the part removal apparatus is moved to a pre-programmed position by use a pendant control mechanism. Thereafter, the screw unit of the injector assembly is retracted, and then rust protection is applied to the first set of dies. Following application of the rust protection, the mold is closed and the cores are drained. Thereafter, the first set of dies is exchanged with the second set of dies, as discussed hereinbefore with reference to FIG.  1 . Once the second set of dies are installed in the machine, the safety drop bar is set whereby the safety bar is moved into a position by a servo-driven screw to place a mechanical stop in place to prevent the movable platan from advancing forward anytime the machine front door is in the open position. Thereafter, an operational check, which is a check performed by the operator to visually confirm hydraulic units are functioning properly, is performed, and the screw unit is advanced. Since the R/B attachment change, wherein the part grasping head or chuck has been replaced with a part grasping head or chuck adapted to receive the new part, has been previously performed, the dies are monitored until plastization or melting of the plastic pellets occurs and then parts can begin to be made. As noted before with regard to the mold changing sequence, since the steps in the overall machine operating method associated with the machine operation surrounding the mold change are sequential or serial in nature, it is difficult or impossible to significantly reduce the time required. This has been an often-criticized problem with large injection molding machines that the industry has tolerated for years. 
   Since the second set of dies are cold when they are inserted into the machine, the second set of dies must be heated up to a molding temperature before good parts can be produced. Typically, the second dies are heated by electrical heating means built into the mold. When the plastic within the mold becomes liquid, which may take up to about 30 minutes, molding may begin. However, due to required normalization of the dies, it is commonly necessary to make a series of parts, sometimes up to a dozen parts, before good parts are made. Therefore, in addition to a great loss of time, the conventional mold-changing technique results in significant amount of waste. 
   As will be appreciated by those skilled in the art, machine down time resulting from die changes reduces the production of the machine in a given time period, and needs to be minimized to increase the productivity and efficiency of the manufacturing operation. 
   Therefore, there exists a need in the art for a method of reducing the downtime associated with mold change. Moreover, there exists a need in the art for speeding connection and disconnection of hydraulic and electrical connections during a mold change. 
   SUMMARY OF THE INVENTION 
   The present invention is directed toward a method for reducing a downtime of an injection molding machine during exchange of dies. The present invention is further directed toward a method and apparatus for speeding connection and disconnection of hydraulic and electrical connections associated with dies used in an injection molding machine. 
   In accordance with the method of the present invention, the machine includes a first set of dies installed within the machine and a second set of dies stored on a mold transfer cart of the machine. The method includes the steps of preheating the second set of dies, disconnecting electrical cables and hoses from the first and second sets of dies, unclamping the first set of dies, and simultaneously opening a rear door of the injection molding machine. The method further includes the steps of, following unclamping of the first set of dies, moving the first set of dies onto a mold transfer cart and, immediately following placement of the first set of dies on the mold transfer cart, moving the cart to place the second set of dies in position to be installed in the injection molding machine. While moving the second set of dies in position for installation in the injection molding machine, the die platans are positioned to receive the second set of dies and, at least partially simultaneously with the platan positioning step, the second set of molds are moved into the injection molding machine. Thereafter, the second set of dies is clamped within the injection molding machine and the hoses and cables are reconnected to the second set of dies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and further features of the invention will be apparent with reference to the following description and drawings, wherein: 
       FIG. 1  is a flow chart illustrating steps of a mold change method according to the prior art; 
       FIG. 2  is a flow chart illustrating method steps of machine operation including a mold change method according to the prior art; 
       FIG. 3  schematically illustrates an injection molding machine according to the present invention; 
       FIG. 4   a  is a schematic perspective view of a connection assembly secured to a first movable die according to the present invention; 
       FIG. 4   b  is a schematic perspective view of a connection assembly secured to a second fixed die during a preheat step; 
       FIG. 5  is a flow chart illustrating steps according to the mold change method of the present invention; and, 
       FIG. 6  is a flow chart illustrating a machine operation method including a mold change method according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIG. 3 , an injection molding machine  100  according to the present invention is schematically illustrated. The injection molding machine includes an injector  102 , a first die assembly  104  comprising a first fixed die  104   a  and a first movable die  104   b , a die loading/unloading assembly  106  including a translatable mold transfer cart  106   a , a part removal apparatus  108 , and a second die assembly  110  comprising a second fixed die  110   a  and a second movable die  110   b.    
   The first fixed die  104   a  is secured to a fixed die platan  112   a , and the first movable die  104   b  is secured to a movable die platan  112   b . The movable die platan  112   b  is moved relative to the fixed die  104   a  between a closed position and an open position. When the movable die platan  112   b  and associated movable die  104   b  are in the closed position, the first set of dies  104  is in position to mold a part. When the movable die platan  112   b  moves the movable die  104   b  to an open position, the molded part may be removed from the first die assembly  104 . 
   The injector  102  receives solid palletized or granular plastic material, heats and melts the plastic material, and pressurizes and injects the melted plastic into the die assembly  104 . A screw type injector may be used as the injector. The first die assembly  104  receives the injected melted plastic and holds the plastic material until it solidifies, at which point the dies  104   a ,  104   b  are moved apart to reveal the part, as discussed previously. Ejectors are preferably built into the movable die  104   b , and serve to separate the molded part from the movable die  104   b . The fixed die  104   a ,  110   a  includes electrical resistance heaters (not shown) to keep the plastic liquid as it moves into the die assembly  104 , and each of the fixed and movable dies  104   a ,  104   b ;  110   a ,  110   b  has a cooling water circuit through which cold water is circulated to assist in cooling and solidifying the molded plastic part. 
   The part removal apparatus  108  grasps the part, removes the part from the die assembly  104 , and transports the part away from the dies. In this case, the part removal apparatus  108  is an overhead-type trolley carrying a part-grasping head  108   a . The part grasping head  108   a  is specially adapted to hold the particular part being molded. Therefore, when the first set of dies  104  is exchanged for the second set of dies  110 , the part grasping head  108   a  must also be exchanged. In this regard it is noted that the prior art method employed a manual step to transfer the robot to an attachment exchange (chuck change) position prior to performing any operational checks. However, the present invention makes this function automatic when the injection carriage is retracting. The inverse is likewise true in that following chuck change the robot is returned to operating position by activation of one push button on the operators control panel. Therefore, significantly less user interaction is now required to change the chuck or grasping head than was required with the prior art method. 
   The die loading/unloading assembly  106  is used to replace or exchange dies in the machine  100 , when desired. In order to exchange the first set of dies  104  in the machine with a second set of dies  110 , which are stored on the mold transfer cart  106   a , the first set of dies  104  are slid out of the machine and onto the transfer cart  106   a  by the die loading/unloading assembly. Thereafter, the transfer cart  106   a  is translated laterally (to the right in  FIG. 3 ) to align the second set of dies  110  with the opening in the machine, and then the second set of dies  110  are slid into the machine by the die loading/unloading assembly. 
   The machine further includes a die pre-heating assembly, including a preheat controller  120  and a pre-heat connection assembly  122 ′. The pre-heat controller  120  is provided to supply heating fluid via inlet and outlet hoses  125 ′,  127 ′ to the second set of dies  110  and to provide electrical power to the resistance heaters within the fixed die  110   a  of the second set of dies  110  via the pre-heat connection assembly  122 ′. Generally, and as noted hereinbefore, the movable and fixed dies  110   b ,  110   a  include or define cooling water circuits therethrough by means of which cooling water, during use of the dies to form parts, may be directed through the dies  110   a ,  110   b  to help keep portions of the dies at a reduced temperature during use and thereby assist in solidification of the plastic part. During the preheating stage, the present invention takes advantage of these cooling water circuits; however, instead of cooling water, the present invention circulates heated water through the cooling water circuits to preheat the dies  110   a ,  110   b . Accordingly, the second set of dies  110  is preheated by the electrical resistance heaters in the fixed die  110   a  and by the heated water circulated through the cooling water circuit of the fixed and movable dies  110   a ,  110   b . Therefore, each of the second fixed and movable dies  110   a ,  110   b  is brought up to an elevated temperature closer to the molding temperature before they are inserted in the machine, normalizing the dies and, therefore, permitting almost immediate molding of parts with the second set of dies  110 . 
   A connection assembly  122  is schematically illustrated in  FIG. 4  secured to the first movable die  104   b , it being understood that a generally identical connection assembly is secured to the first fixed die  104   a . It is further noted that, with the exception of the hydraulic lines/connections  124   b , the connection assembly  122  is substantially identical to the preheat connection assembly  122 ′ described hereinbefore. As shown, the connection assembly  122  includes a backing plate to which a plurality of electrical cables  124   a  and hydraulic hoses  124   b  are secured. Similar hoses and cables, prior to the present invention, were separately and individually connected to the mold. However, with the present invention, the hoses  124   b  and cables  124   a  may be secured to the die or removed from the die with one connecting or disconnecting operation by means of the connection assembly  122 , respectively. As will be appreciated by those skilled in the art, and with reference to  FIGS. 3 and 4   a , a connection assembly is secured to each of the first movable and fixed dies  104   b ,  104   a . The connection assembly provides electrical connectors for various electrical components disposed within the molds (i.e., resistance heaters, position sensors, limit switches, etc.) and hoses for communication of fluid to the mold for various purposes (i.e., hydraulic lines to permit movement of movable units within the die  104 ). 
   For the second set of dies  110 , a connection assembly  122 ′ is connected to the fixed die  110   a  to connect the resistance heaters therein to the pre-heating controller  120 . Insofar as the preheating step does not require hydraulic actuation or movement of internal die parts, only electrical cables  124   a′  are shown as being connected by the preheat connection assembly  122 ′ to the die  110   a  ( FIG. 4   b ). As noted hereinbefore, heated water provided by the preheat controller  120  is directed, via an inlet  125 ′ and an outlet  127 ′ through the fixed and movable dies  110   a ,  110   b  to bring them up to, or close to, a molding temperature and serves to normalize the dies. Due to the large heat sink represented by the dies when molding a large part, this pre-heating step may take a rather long time (i.e., two hours). 
   The die changing method of the present invention is illustrated in FIG.  5 . As will be appreciated from the following discussion, as compared to the prior art method illustrated in  FIG. 1 , the inventive method greatly speeds the die exchange sequence and thereby reduces downtime. It is noted that, prior to the die exchange sequence, the second set of dies  110  are preheated using the preheating assembly, described hereinbefore. First, the preheat connection assembly  122 ′ and the heated water inlet hoses  125 ′,  127 ′ are disconnected from the second set of dies  110 . The hydraulic hoses  124   b  and electrical cables  124   a  are disconnected from the first set of dies  104  by operation of the connection assemblies  122 . The free preheat connection assembly  122 ′, which was previously connected to the second fixed die  110   a , is now stored on a stand  123  that is disposed on the transfer cart  106   a  at a location convenient for all potential die storage locations. The heated water hoses  125 ′,  127 ′ may also be secured to and stored on the stand  123 . The stand  123  is positioned and adapted to securely retain the preheat connection assembly  122 ′ in a convenient location for subsequent access. 
   The free connection assemblies  122  that were associated with the first die assembly  104  are now retained in a desirable position by a suspension assembly including support wires and counterweights, as is known in the art, and then the first set of dies is unclamped from the machine and, simultaneously with the unclamping step, the rear door is opened. Once the dies are unclamped, which takes slightly longer than the time required to open the rear door, the first set of dies  104  is carried out of the machine and onto the die transfer cart by the die loading/unloading assembly. Once the first set of dies  104  is on the cart  106   a , the die transfer cart  106   a  is moved laterally (to the right in  FIG. 3 ) so as to place the second set of dies  110  in position for introduction in the machine, it being noted that prior art step of closing of the rear door is skipped as unnecessary. While the die transfer cart  106   a  is moving, a data conversion and platan positioning is performed. After the die transfer cart  106   a  has moved the second set of dies  110  into a loading position, but before the platan has reached its final position, the carrying-in of the second set of dies  110  into the machine is begun. In this regard it is noted that the prior art method step of opening the rear door is not necessary since the rear door is already open. After the second set of dies  110  is installed in the machine, the rear door is closed, and the dies  110  are clamped in position. While the dies are being clamped, the mold thickness or calibration step is performed. Thereafter, the hydraulic hoses  124   b  and electrical cables  124   a  are connected to each of the movable and fixed dies of the second set of dies  110  via the connection assemblies  122  previously secured to the first set of dies  104 , the cold water circulating hoses are connected to the inlet  125  and outlet  127 , and the mold exchange process is complete. 
   It is noted that the time required to perform the mold exchange method of the present invention is significantly shorter than that of the prior art described hereinbefore with regard to FIG.  1 . This reduction in time is attributable to the time savings from use of the connection assembly, the parallel operation of a series of the method steps, described hereinbefore, as well as mechanical changes to the operation of machine components that speed-up the mold changing. For example, although the same motor and mechanical system is used in the die loading/unloading assembly as in the prior art system, the motor frequency has been increased to the maximum rated frequency of the motor and, therefore, the motor has been sped-up relative to its operation under in prior art. These time savings are compounding and, based upon testing conducted by the inventors, have resulted in an average time reduction from 681 seconds (11 minutes, 21 seconds) under the prior art to 236 seconds (3 minutes, 56 seconds) with the inventive method. This amounts to a 445 second (7 minutes, 25 seconds) savings each time the molds are changed. 
   A machine operation method including a mold change method according to the present invention is illustrated in FIG.  6 . Following a production of the last part using the first set of dies, the R/B home step is performed. Simultaneously, the screw unit of the injector assembly is retracted. The rust protection step taught in the prior art is avoided, and is instead performed after the first set of dies is removed from the machine. After the R/B home step, the mold is closed and the core is drained, and then the first set of dies are exchanged with the second set of dies, as described hereinbefore with reference to FIG.  5 . Once the second set of dies are installed in the machine, a series of essentially simultaneous steps are performed. These include setting the safety drop bar, performing an operational check, and advancing the screw unit. With the R/B attachment change being previously performed, the dies are monitored until plastization occurs, and then parts can begin to be made. 
   As evidenced by the foregoing, the overall downtime of the machine from the making of the last part with the first set of dies to the making of parts with the second set of dies is reduced from greater than 25 minutes (1514 seconds) with the prior art method shown in  FIG. 2 , to less than 10 minutes (543 seconds) with the present invention. Accordingly, more than 15 minutes in machine downtime is eliminated each time the dies are changed.