Abstract:
A method of manufacturing a molded article from a settable mixture includes the steps of in-taking a plurality of components into a plurality of separate material discharge containers sized relative to each other to provide the required mix ratio of the settable mixture. The components contained in the material discharge containers are discharged to a mold tool through a mixing head that receives each of the components separately and thoroughly mixes them into the settable mixture. The settable mixture flows from the mixing head to fill the mold tool. The mold tool rotates about an axis during the filling step to evacuate air from the mold tool. Pressure of the settable mixture is monitored during the filling step and rate of discharge of the settable component is varied in response to changes in pressure of the settable mixture.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a method of fabricating a molded article, and particularly to a method of delivering a settable mixture to a mold tool. 
   Many methods of fabricating molded articles include the mixing of at least two fluid materials to from a settable mixture that is discharged into a mold tool. The fluid materials typically include at least two reactive components that harden when mixed. Variations in mold tool temperature, settable mixture components and injection speeds can cause variations between molded articles fabricated using the same molding system. 
   A further challenge to current molding operations is the evacuation of air initially trapped within a mold. Typically an air passage is provided for the evacuation of air from a mold tool as the settable mixture is discharged into the mold tool. Air is expelled from the mold by the introduction of the settable mixture into the mold. For molded articles with simple shapes such an air exhaust passages is adequate. However, for molded shapes having extreme and dramatic shapes and contours air can become trapped within the mold. Further, the completed molded article may include cosmetically important surfaces that preclude the optimum placement of air exhaust passages. The entrapment of air within the mold tool causes voids to be present within the completed molded article, which provides an undesirable appearance and weakens the final molded product. 
   For these reasons it is desirable to develop a molding system that can accommodate variations in the molding process to provide a consistent, reliable and repeatable molding process. 
   SUMMARY OF THE INVENTION 
   An embodiment of this discloses a method of fabricating a molded article utilizing a low pressure molding process that varies a discharge rate of settable material in response to pressure in the mold tool to accurately control each molding sequence. 
   The system includes a two-part mold having a first part supported by a structure and a second movable part. The molded article is formed from a three-part mixture mixed by a mixing head. The fluid materials that compose the three-part mixture are stored in separate storage containers. The fluid materials are drawn from the storage containers by a material delivery system. The material delivery system includes a separate cylinder for each fluid material that forms the three-part mixture. Each cylinder is of a volume that is pre-sized to provide the correct ratio of fluid material required to form the settable mixture. 
   The material delivery system delivers the fluid material to the mold through the mixing head. The mixing head thoroughly mixes the fluid material and discharges the mixed components in the form of the settable mixture into the mold tool. Upon hardening of the settable mixture the molded article is removed and the mold is prepared for the fabrication of another molded article. 
   A sensor located within the mold monitors pressure of the settable mixture. The material delivery system varies the rate of discharge of fluid material to the mold tool through the mixing head in response to changes in pressure. As the pressure of the settable mixture increases, the rate of discharge is proportionally slowed to accurately control each discharge sequence from each material discharge container. The greater the pressure within the mold tool the slower the fluid material is discharged. 
   The method of fabricating a molded article of this invention provides a consistent, reliable and repeatable molding process for fabricating molded articles by varying the discharge rate of settable mixture into the mold tool to accommodate variations in the molding process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
       FIG. 1  is a schematic view of the molding system; 
       FIG. 1A  is a perspective view of the material discharge containers; 
       FIGS. 2A-D  are schematic views of movement of the mold tool; and 
       FIGS. 3A-C  are schematic views of the air escape passage within the mold tool during rotation of the mold. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , one disclosed embodiment is a method of fabricating a molded article with a molding system  10  including a mold tool  12  supported by a structure  14  such that the mold tool  12  maybe rotated about a horizontal axis  16 . The mold tool  12  includes a first mold part  18  and a second mold part  20  that are secured together to form a cavity  22  therebetween. Preferably, at least one of the mold parts  18 , 20  is movable away from the structure  14  to allow for demolding of the completed molded article  24 . 
   The molding system  10  further includes a material delivery system  26 , and fluid material storage containers  28 A,  28 B, and  28 C. Fluid materials to form the settable mixture are stored in the storage containers  28 A-C and include delivery hoses  30  A-C attached to material discharge containers  32  A-C included in the material delivery system  26 . Preferably the fluid materials that compose the settable mixture include a catalyst, a foaming agent, and a matrix polymer. It is within the contemplation of this invention that other fluid materials may be used to form the settable mixture. 
   Fluid pumps  34  A-C disposed along the delivery hoses  30  A-C aid in moving fluid from the storage containers  28  A-C to corresponding material discharge containers  32  A-C. Each of the material discharge containers  32  A-C includes valve assemblies  36  A-C to control the intake and discharge of fluid material. Preferably a controller  38  actuates the valve assemblies to  36  A-C to allow filling of the material discharge containers  32  A-C. A worker skilled in the art would understand that the pump and valve assemblies may be of any type known by one skilled in the art. 
   Referring to  FIG. 1A , the material discharge containers  32 A-C are sized relative to one another such that one shot of fluid material contained from each container  32  A-C provides a predetermined ratio required to form the settable mixture. Each material discharge container  32  A-C is sized to contain a predetermined volume of fluid material based upon the desired ratio that must be mixed to create the settable mixture. Further, multiple material discharge containers containing the same fluid material component can be used to provide the proper ration for the settable mixture. In this way the fluid material component that makes up the largest portion of the settable mixture will fill multiple containers instead of one container much larger relative to the other material discharge containers. 
   Each material discharge container includes a piston  40  attached to a piston rod  33  that is in turn attached to a drive plate  45 . Note that each material discharge container includes a piston rod attached to the piston  40 , however in  FIG. 1A  some of the piston rods  33  are not shown to improve clarity. A drive  42  actuates the drive plate  45  thereby moving the pistons  40  within the material discharge containers  32  A-C. Preferably the drive  42  hydraulic cylinder disposed centrally within the material delivery system  26  and includes a drive rod  43  attached to the drive plate  45 . Actuating the drive  42  to move the drive plate  45  actuates the pistons  40  to draw into or expel fluid material from the material discharge containers  32  A-C. A controller  38  actuates the drive  42  to provide a required discharge rate to the mold tool  12  depending on application specific molding requirements. The material discharge system is described in further detail in the co-pending application Ser. No. 09/662,302 titled “Rapid Discharge Multiple Material Delivery System” that is hereby incorporated by reference. Fluid material from the material discharge containers  32 A-C flows through discharge hoses  40  A-C to a mixing head  44 . 
   Referring back to  FIG. 1 , the mixing head  44  thoroughly mixes the various separate fluid materials to form the settable mixture. The mixing head  44  is best described in pending application Ser. No. 09/662,662 titled “Mix Head Assembly for a Molding Material Delivery System”. The settable mixture flows from the mixing head  44  through a single hose  48  to a mold valve  50  of the mold tool  12 . 
   The controller  38  controls movement of the drive  42  and actuation of the valve assemblies  36  A-C. A sensor  52  for monitoring pressure of the settable mixture is disposed within the mold tool  12 . A second sensor  54  can also be positioned to monitor pressure in the hose  48  between the mixing head  44  and the mold tool  12 . Each sensor  52 ,  54  communicates fluid pressure levels to the controller  38 . The sensors  52 ,  54  and the controller  38  are of conventional construction and would be recognized as such by a worker skilled in the art. 
   In operation, the molding process is initiated by intaking fluid material from the storage containers  28  A-C to the material discharge containers  32 A-C. Fluid is drawn into each of the material discharge containers  32 A-C by driving the piston  40  upward with the drive  42 . Fluid flow between the storage containers  28  A-C and the material discharge containers  32  A-C is aided by the fluid pumps  34 A-C. The controller  38  actuates the valve assemblies  36  A-C to fill each of the material discharge containers  32  A-C. The delivery system  26  is now ready to discharge the components to the mixing head  44 . 
   Referring to  FIGS. 1 , and  2  A-C, the second mold part  18  is first moved into a position adjacent the first mold part  20 . The first and second mold parts  18 , 20  are then secured together and lifted to a first position I (FIG.  2 B). The mix head  44  is connected to the mold valve  50  of the mold tool  12  by way of the hose  48 . The controller  38  opens the mold valve  50  and actuates the valve assemblies  36  A-C to allow flow of the fluid material to the mold tool  12 . 
   The molding process continues with the controller  38  actuating the drive  42  such that the pistons  40  discharge the fluid material from the material discharge containers  32  A-C to the mixing head  44 . The fluid materials flow from each material discharge container  32  A-C through the discharge hoses  46  A-C to the mixing head  44 . The mixing head  44  combines the fluid materials into the settable mixture. A single hose  48  carries the settable material from the mixing head  44  to the mold valve  50  to begin filling the mold tool  12  with the settable mixture. 
   Referring now also to  FIGS. 3  A-C as well as  FIGS. 2A-C , during the filling step, the mold tool  12  is rotated about the horizontal axis  16  from the first position I, shown in  FIGS. 2B and 3A , through an intermediate position, shown in  FIG. 2B , to a second position II, shown in  FIGS. 2C and 3C , to aid the evacuation of air from the cavity  22 . Air trapped within the mold tool  12  exits through air escape passages  56  during rotation of the mold. As the cavity  22  becomes further filled with settable material, air can become trapped within contours of the cavity  22 . Rotation of the mold tool  12  flushes out air trapped in such contours such that upon complete fill of the cavity  22 , substantially all air is evacuated from the mold tool  12 . Evacuation of the air from the mold tool  12  prevents air bubbles from being trapped within the finished molded article  24 . The amount of rotation is determined by the shape and contours of the cavity  22  and the molded article  24 . Further, the shape of the molded article  24  will determine how quickly and to what degree the mold tool  12  is to be rotated. The speed and degree of rotation would be understood by one skilled in the art to be application sensitive and thereby any rate and magnitude or rotation of the mold tool would be understood to be within the scope of this invention. 
   Referring to  FIG. 1 , during the filling of the mold tool  12 , the sensors  52 ,  54  within the mold tool  12  monitor pressure levels of the settable mixture. The controller  38  receives information about pressure of the settable mixture within the cavity  22  of the mold tool  12  from the sensor  52 . The discharge of the settable mixture is varied in response to changes in pressure within the mold tool  12 . The controller  38  slows the race of discharge of the settable mixture proportionate to the increase in pressure. The greater the pressure within the mold tool  12 , the slower the controller  38  operates the drive  42 . The controller  38  controls the rate at which the drive  42  moves the pistons  40  within the material discharge containers  32 A-C are driven downward to expel the fluid material for the settable mixture. Pressure increases in the mold tool  12  due to thickening of the settable mixture and because as the mold tool  12  fills the restriction to the flow of the settable mixture increases. The rate of discharge or injection speed of the settable mixture into the mold tool may vary during each discharge sequence in order to accurately control the discharge of the settable mixture into the mold tool  12 . The changes in conditions such as mold temperature and settable material temperature are accommodated by varying the discharge rate in response to pressure within the mold tool  12 . 
   The controller  38  shuts the mold valve  50  to interrupt flow of the settable mixture upon reaching a predetermined pressure level. The sensors  52 , 54  monitor the pressure level and will shut down the process if the pressure level deviates from a predetermined range. In this way, pressure spikes upward or downward indicating a problem during the molding process initiate a process shut down. 
   The method further includes the step of flushing the mixing head  44  of any settable mixture after completion of the filling step. The flushing step expels any settable mixture remaining in the mixing head  44  or the hose  48  to prevent hardened settable mixture from blocking the mixing head  44  or hose  48 . 
   The hose  48  is disconnected from the mold valve  50  and the mold tool  12  is rotated to the first position about the horizontal axis  16  and the settable mixture is allowed to cure. Referring to  FIG. 2D , the mold parts  18 , 20  are separated to allow the removal of the molded article  24 . During the separation of the mold parts  18 , 20  air is applied to both sides to free the molded article. One part of the mold  12  is moved clear of the structure  14  to allow demolding of the molded article  24 . Each of the mold parts  18 , 20  is then prepared for another molding sequence. 
   The foregoing description is exemplary and not just a material specification. The invention has been described in an illustrative manner, and should be understood that the terminology used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications are within the scope of this invention. It is understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason, the following claims should be studied to determine the true scope and content of this invention.