Patent Publication Number: US-2005133946-A1

Title: Method and apparatus for molding a plate

Description:
RELATED APPLICATION  
      This application is a divisional under 37 C.F.R. 1.53(b) of U.S. application Ser. No. 09/655,134 filed Sep. 5, 2000, which application is incorporated herein by reference and made a part hereof. 
    
    
     TECHNICAL FIELD  
      The technical field relates generally to a method and apparatus for compression molding a plate from a thermosetting resin mixture that has a filler concentration comprising at least 50%, by weight, of a conductive, relatively incompressible material. More particularly, it pertains to a method and apparatus for making plates from thermosetting resins using a high percentage of a conductive, relatively incompressible filler material such as graphite.  
     BACKGROUND  
      Molded parts formed from molding materials comprising thermosetting resins having a high filler concentration of at least 50% graphite by weight are difficult to manufacture using existing techniques. Such molding materials are difficult to mold because they do not flow evenly across the surfaces of a cavity mold as the mold is closed and additionally because they often begin to cure before the material has flowed across the entire surface of the mold.  
      Plates having a high concentration of graphite fillers may be used as anode plates in lithium batteries and also may find use as fluid flow plates in fuel cell assemblies where both their electrically conductive and thermally conductive properties are needed. It is difficult and expensive to machine graphite plates with the extensive details necessary for them to function as storage battery electrode plates or as fluid flow plates in fuel cell devices. Similarly, because of the difficulties in molding graphite filled materials using existing molding processes, it has not been practical to mold materials having a high graphite content.  
      Thus, what is needed are systems, devices, structures, and methods to overcome the difficulties of forming complex molded structures of materials having a graphite content of at least 50 to 80% by weight.  
     SUMMARY  
      The above-mentioned problems with the manufacture of graphite filled resin plates for battery, fuel cell and other applications are addressed by the present invention and will be understood by reading and studying the following specification. Systems, devices, structures, and methods are described which accord these benefits.  
      In one embodiment, the invention is a process for compression molding of a fine grained mixture of a resin and at least 50% filler material into a highly detailed molded part. The process involves forming, at a first pressure, a preform of resin and filler mixture in a predetermined weight sufficient for molding the molded part, the cross sectional area of the preform being substantially less than the area of the part to be molded and positioning the preform in a mold cavity at about the geometrical center of the product outline. The process also involves molding the preform into the molded part by compressing the preform between mold segments defining the mold cavity in a molding machine which closes and applies pressure to the halves of the mold cavity to apply a second pressure to the molded part within a time interval sufficient to fluidize and flow the material to all parts of the mold cavity prior to the onset of curing of the resin in the mold.  
      An embodiment of the invention may be a machine for molding a plate from a thermosetting preform material using a thermosetting compression molding process. Such a machine may have a first platen and mold cavity portion and a second platen and mold cavity portion. In the machine the first and second mold cavity portions constructed and arranged for receiving in a mold cavity thereof a preform comprising a thermosetting resin and at least about 50% by weight of a graphite filler material. Such a machine may have a press for performing a molding operation by rapidly forcing the first and second platen and mold cavity portions together in an interfitting relationship, thereby compressing the preform material into a molded part with features therein defined by the first and second cavity portions.  
      In another embodiment the invention is a mold assembly for use in a machine for molding a plate from a thermosetting preform material using a thermosetting compression molding process driven by a high pressure press. The mold assembly has a first platen and mold cavity portion and a second platen and mold cavity portion. In the mold assembly the first and second mold cavity portions are constructed and arranged for receiving in a mold cavity thereof a preform comprising a thermosetting resin and at least about 50% by weight of a graphite filler material. The first and second mold cavity portions of the assembly are constructed and arranged for performing a molding operation when the press rapidly forces the first and second platen and mold cavity portions together in an interfitting relationship and compressing the preform material into a molded part with features therein defined by the first and second cavity portions.  
      These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1 and 2  diagrammatically show a mold according to one embodiment of the present invention suitable for carrying out the method according to the invention;  
       FIG. 3A  is a cross section view of the cavity portions of the mold of  FIGS. 1 and 2  taken along the lines  3 A- 3 A of  FIG. 2 ;  
       FIG. 3B  is a detail view of a portion of the mold of  FIG. 3A ;  
       FIG. 4  is a cross section view of the cavity portion of the mold taken along the lines  4 - 4  of  FIG. 2 .  
       FIG. 5  is a sectional view of the mold of  FIGS. 1 and 2  taken together with the additional structure of one embodiment of the mold assembly;  
       FIG. 6  is a detail view of a portion of the mold assembly of  FIG. 5  showing the semi-positive land structure;  
       FIG. 7  is a detail view of a portion of  FIG. 5  showing the structure of the vacuum seals;  
       FIG. 8  is a sectional view of the mold assembly which shows the positioning of the heating cartridges and positioning of the vacuum ports;  
       FIG. 9  is another sectional view of the mold assembly which shows the positioning of the thermal sensors relative to the heater cartridges; and  
       FIG. 10  is a sectional elevation view of the mold assembly illustrating the positioning of the cylindrical support columns, the distribution of the ejection pins and the distribution of the support columns.  
    
    
     DETAILED DESCRIPTION  
      In the following detailed description of the invention, reference is made to the accompanying drawings forming a part hereof, and in which are shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention.  
       FIGS. 1 and 2  show, respectively, a male mold cavity portion  12  and a female mold cavity portion  14 . In operation mold cavities  12  and  14 , which are interfitting as described more fully below, are each mounted on a suitable platen and supporting structure in a suitable press in an abutting face to face relationship.  
       FIGS. 3A and 4  are sectional views of cavities  12  and  14  in a fully closed position as they would be in at the completion of a molding operation. The cross sectional view of  FIG. 3A  is taken along section lines  3 A indicated in  FIG. 2 . The sectional view in  FIG. 4  is taken along section lines  4 - 4  in  FIG. 2 .  
      As shown in  FIG. 1 , cavity portion  12  has a base portion  16  which provides support for the other mold cavity elements. Base portion  16  is manufactured from a tool steel such as H13 or any similar material. A peripheral edge  20  surrounds the actual mold area  22 . In  FIGS. 1 and 2  area  22  is illustrated in an embodiment for molding a complex plate which, in one embodiment, may be a battery plate or, in another embodiment, may be a fluid flow plate for a fuel cell. By battery plate, it will be appreciated, that we mean an electrode for use in a storage battery. By fluid flow plate it will be appreciated that we mean to describe fluid flow plates or collector plates which may serve one of a variety of functions in a fuel cell.  
      Various regions of the plates are shown in the mold area  22  of cavities  12  and  14 . For example, in one embodiment, a fluid flow plate surface molded by cavity portion  12  might serve as a cathode plate and a plate surface molded by cavity  14  might serve as an anode plate. In such a configuration flow channels for carrying a fuel medium such as hydrogen or any hydrocarbon gas and humidification water may be formed in the molded plate by mold features  24 . Such flow channels might also carry air/oxygen as oxidant, as well as humidification water and/or product water. The plates may have many different channel configurations which are not part of the present invention.  
      In addition to the flow channels that are molded into the plates by features  24 , peripheral apertures are initially molded into the plate by forming very thin diaphragms  26  of material in what is to become the peripheral apertures. After the molding operation is complete, the diaphragms  26  can be readily punched out and any remaining flashing removed to complete the forming of the peripheral apertures  28 . When fluid flow plates are combined in a fuel cell, a plurality of apertures  28  may combine to provide fluid manifolds for the fuel cell assembly.  
      In addition to the flow channels and peripheral apertures, mold cavities  12  and  14  also have further mold features  29  formed thereon for providing further apertures  28  on the fluid flow plates which may be used, for example, for alignment of the plates in a fuel cell assembly or for connecting the plates and directing fluid to and from the plates.  
      In  FIGS. 3A and 4   a  fluid flow plate  30  is shown between the halves  12  and  14  of the mold  10 , which is illustrated in the closed position. It can be seen in these sectional views that a projecting flange or edge portion  20  of mold cavity portion  12  engages an inner peripheral groove  32  in the surface of cavity mold portion  14 . Details of that interfiftting relationship are shown more clearly in  FIG. 6  which is a detail of the region  34  of  FIG. 3A . That interfitting relationship results in the provision of a fully positive shear edge which completely surrounds the mold cavity region.  
      It can be seen in  FIG. 6  that the upper portion  36  of wall  38  of the inner groove region  34  has a draft angle α which facilitates the mating of the two mold cavity portions  12  and  14  by providing a guide for peripheral flange  20  as the mold is forced into the closed position by the press. The draft angle α in one embodiment is about approximately 5 degrees.  
      The lower portion  40  of wall  38  is within about approximately 1 degree of perpendicular to parting line  42  of the mold so that there two mold portions  12  and  14  are maintained in their aligned relationship in the final stages of the molding operation. In one embodiment, the height of the substantially vertical portion  40  of wall  38  is about approximately 0.200 inches before the transition to the upper portion  36  of wall  38  which is angled at and angle of a.  
      As shown in  FIG. 6 , the peripheral flange or edge  20  of mold cavity portion  12  does not contact mold cavity portion  14  along the parting line  42  when the cavity portions are fully closed. This is done so that a gap remains which is sufficient to receive the small amount of excess casting material which is forced out of the mold cavity as it is closed. That excess material may form a burr. Because the burr is so thin relative to the molded item  30 , which in one embodiment may have a thickness at the edge of approximately about 0.15 inches, it can be readily removed. In one embodiment the separation of the parting lines of the two mold cavity portions is about approximately 0.007 inches. The semi-positive parting line feature described above has been found to be necessary in molding high filler content materials which are relatively incompressible in the mold as compared to the high resin content materials that have heretofore been considered suitable for thermoset compression molding operations.  
      The same semi-positive parting line feature which is used around the perimeter of the molded parts is used to mold the diaphragms  26  as illustrated in the detail view  44  of  FIG. 3A  which forms  FIG. 3B . In the detail of  FIG. 3B  the molded item  30  is shown with a thinned web region  46  formed by a split parting line between the two mold portions  14  and  16 . The thinned web region  46  in one embodiment surrounds each of the diaphragms  26  and serves to facilitate the removal of the diaphragm prior to assembly of the plates  30  into a fuel cell assembly. Region  46  in one embodiment may have a thickness of about approximately 0.007 inches and a width of about approximately 0.3 inches. The remainder of diaphragm  26  in one embodiment has a thickness of about approximately at least 0.025 inches.  
      Mold cavity portions  12  and  14  as shown in  FIGS. 1 and 2  in one embodiment may include detail molding inserts  50  and  52  which allow a single basic mold to be modified for producing several variants of a molded structure. In the sectional views of  FIGS. 3A and 4 , the installation of mold inserts  50  and  52  are also shown.  
       FIG. 5  is an elevation view of the cavity mold portions  12  and  14  and also shows aspects of platens which support them on the press. The region  34 , which is shown in detail in  FIG. 6 , illustrates the positive shear edge joining of the mold cavities as well as the split parting line semipositive land features. A further detail  58 , shown in  FIG. 7 , illustrates aspects of the implementation of the vacuum assist feature of one embodiment of the molding system.  
      In  FIG. 7  the sealing aspects of the structure which allow for the provision of a vacuum assist in the molding process are shown. A sealing ring  60  is mounted on a platen  62  on which mold portion  14  is mounted. It circumferentially surrounds mold cavity portion  14 , as may be in  FIG. 10 , a sectional view of the mold assembly.  
      Sealing ring  60  is, in turn, received in a corresponding circumferential slot  64  which is formed in platen  66 . In order to provide an adequate seal for the evacuation of the mold cavity, sealing ring  60 , in one embodiment, is provided with sealing o-rings  68 ,  70  and  72  which are seated in slots  74  in the outside vertical face  76  of sealing ring  60 . In one embodiment, a further sealing o-ring  78  is provided to seal sealing ring  60  against platen  62 , to which it is mounted, in one embodiment by a bolt or other suitable fastener  80 .  
      A vacuum is drawn in the mold cavity after the mold portions  14  and  16  are moved into a closing position so that the o-rings  68 ,  70  and  72  of sealing ring  60  engage the surface of circumferential slot  76 . Manifolds  82  and  84 , as shown in the sectional view of  FIG. 8 , provide communication from the parting line of the closing mold through platens  66  and  62  respectively to vacuum ports  86  and  88  as shown in  FIG. 9 , an elevation view of the mold assembly. Ports  86  and  88  are in turn connected to a vacuum pump which is not shown. The entire molding operation is conducted under a vacuum which removes outgassing byproducts of the curing of the thermosetting resin, avoids the creation of high porosity regions in the molded product and by assisting the molten graphite resin mixture in spreading uniformly throughout the entirety of the mold cavity.  
      In order to accomplish the accurate molding of the molded items such as molded plates including battery plates or fluid flow plates for use in fuel cell assemblies, it is necessary to maintain the mold temperatures at a predetermined temperature throughout the molding process. This is accomplished in one embodiment by providing plates  62  and  66  with a plurality of 1800 watt cartridge heaters  90 . Suitable cartridge heaters are available from D-M-E Company, 29111 Stephenson Highway, Madison Mich. 48071. In order to hold very tight temperature ranges during molding, a plurality of temperature sensors  92  are provided and embedded in platens  62  and  66  as shown in  FIG. 8 . In one embodiment temperature sensors  92  are thermocouple sensors whose signals are provided to a temperature control apparatus, not shown, which varies the power delivered to cartridge heaters  90 .  
      In one embodiment, a plurality of temperature sensors  92  are each arranged on a centerline drawn between pairs of adjacent cartridge heaters  90 . A temperature control circuit, not shown, uses the output of a sensor  92  to control the operation of the two nearest cartridge heaters  90 , with all of the temperature sensors being monitored to control the cartridge heaters to hold the temperature uniformly across platens  62  and  66  and mold cavity portions  12  and  14  to hold the temperature to a set point selected, in one embodiment, to lie between 270 and 320 degrees Fahrenheit and to hold that selected temperature within a range of between 5 and 10 degrees Fahrenheit across the mold surfaces on both halves of the mold. The combination of multiple cartridge heaters controlled by multiple closely spaced sensors and the thermal mass of the mold cavities  12  and  14  and platens  62  and  66  provides the thermal control and stability that are required for the molding of fluid flow plates for fuel cell assemblies.  
       FIGS. 8, 9  and  10  illustrate generally details relating to the ejector pin aspects of one embodiment of the mold assembly. At the top and bottom of the mold assembly are clamp plates  96  and  98 . Adjacent to the four corners of the upper and lower portions of the assembly are actuators  94  which drive a lower ejector plate  102  and an upper ejector plate  104 . Each of the ejector plates  102  and  104  carries a plurality of vertically upstanding ejector pins  100  which have the distal ends thereof disposed for engaging the molded part or fluid flow plate  30  at the ejection points  106  indicated on mold cavity portions  14  and  16  as shown in  FIGS. 1 and 2 .  
      In order to minimize the potential for damage to plate  30  by its ejection from the mold cavity portions  14  and  16 , the majority of the ejection points  106  are positioned within the removable diaphragm areas  26  or in the distal corners of fluid flow plate  30 , removed as much as possible from the intricate structure of the fluid flow paths in the active area  108  of fluid flow plate  30 . By providing multiple ejection points within regions  26 , sufficient release force can be brought to bear on part  30  to release it as the mold portions  14  and  16  are opened, even though the diaphragm portions are joined to the rest of the fluid flow plate  30  by extremely thin webs  46  which are intended to be readily broken to allow their removal from flow plate  30  prior to assembly of the fuel cell assembly. Providing ejector pins  100  which bear on both faces of molded plate  30  as the mold cavity opened assures a smooth, rapid release of plate  30  from the mold assembly.  
       FIGS. 8, 9 , and  10  also illustrate other aspects of the mold assembly  10 .  FIG. 10  is a top sectional view of mold assembly  10  which illustrates the distribution of support columns  110  which provide mechanical strengthening of mold cavity portions  14  and  16  within mold assembly  10 . At the center of mold the mold cavity where the active area  108  of the flow plate  30  is molded, there are six support columns  110  which provide needed support to the portion of the cavities molding the active area. The necessary room for providing the concentration of support columns  110  in the region of the mold assembly  10  where the active area  108  is molded is due in part to the fact that that area does not include ejector pins  100  which are positioned at other regions throughout the cavity area with support columns  110 . Relatively thicker support columns  112  are provided in the outer portion of the mold cavity.  
      In  FIG. 8  it can be seen that platens  62  and  64  have side locks  112  which provide for positive and aligned closing of mold assembly  10 .  
      In order to carry out a molding operation using the mold assembly  10  according to the present invention it is first necessary to take the fine graphite and resin mixture and prepare it into a mold preform prior to moving the material to the mold cavity. According to one embodiment of the invention the material is placed in a separate press to form a generally cylindrical preform having a diameter of about approximately 3 inches. In one embodiment the mixture is approximately about 75% graphite and the remainder is comprised substantially of thermosetting resins. In one embodiment the preform is compacted at a pressure of at least about 400 psi to form the preform which is, in one embodiment, about 3 inches thick. In one embodiment plate  30  is about approximately 10 inches by 12 inches and about ¼ inch thick at its thickest point. The preform is placed on one of the cavity mold portions  12  or  14  at a point as close as possible to the geometrical center of the cavity. The mold cavity portions are preheated to a predetermined temperature between 270 and 320 degrees Fahrenheit.  
      The press is then operated to close the mold portions  12  and  14  rapidly together as a vacuum is drawn on the vacuum manifolds  82  and  84 . The closing time for the mold must be bring the pressure up to the full 300 tons of molding pressure in a period between about 0.5 and 3.0 seconds while the temperature of the mold is maintained. If the mold is not closed quickly enough, the resin graphite mixture may start to cure before the material has flowed to all areas of the mold cavity. If the mold is closed too quickly, the material is not sufficiently fluid to freely flow to all areas of the mold, even with vacuum assistance. In order to perform the molding process, the closing time of the mold assembly must be shortened substantially below the times that are practiced in conventional compression molding applications. In one embodiment where the molding is done using a 300 ton press, the addition of accumulators to allow for a more rapid build up of compression force may be necessary.  
      In another embodiment of the invention, the preform may not be subjected to a compression process. In that embodiment, the molding material of graphite and resin is run through a plasticizer heating process to produce a suitable preform for introduction into the molding cavity and molding according to the same process.  
      It is an important part of the process to consolidate the powdered molding material into a preform having an area which covers only a fraction of the area of the mold cavity. Our experience in molding with preforms which covered substantially the entire area of the surface of the mold cavity resulted in a high rejection rate because of poor material flow in the mold and a failure to reliably achieve uniform porosity product. Using the limited area preform in the center of the mold has surprisingly led to the molding of more uniform product despite the fact that the material is required to move further within the cavity during the mold closing process step.  
     CONCLUSION  
      Systems, devices, structures, and methods have been described to address situations the molding of resins with high graphite filler content to form fluid flow palates for use in fuel cell assemblies. This application is intended to cover any adaptations or variations of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention includes any other applications in which the above structures and fabrication methods are used. Accordingly, the scope of the invention should only be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.