Method and apparatus for molding a plate
A method and apparatus for molding a plate having highly detailed surface features from a thermosetting resin with a high concentration of a filler such as graphite. The method an apparatus provides for the formation of a preform of casting material and the placement of the material in a cavity mold which is precisely temperature controlled, has vacuum assist operating throughout the molding operation. The molding pressure is rapidly increased to flow the material through the molding cavity prior to the onset of curing and uniquely positioned ejector pins are provided for removal of the molded plate without damage to the detailed surface features.
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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 FIELDThe 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.
BACKGROUNDMolded 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.
SUMMARYThe 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
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.
As shown in
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
It can be seen in
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
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
Mold cavity portions 12 and 14 as shown in
In
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
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
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.
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.
In
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.
CONCLUSIONSystems, 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.
Claims
1. 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, comprising the steps of:
- 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;
- positioning the preform in a mold cavity at about the geometrical center of the product outline; and
- 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.
2. The process of claim 1 wherein the first pressure is at least 400 psi.
3. The process of claim 1 wherein the second pressure is at least 300 tons.
4. The process of claim 1 wherein the time interval is within about 0.5 to 3 seconds.
5. The process of claim 1 wherein the filler material is graphite.
6. The process of claim 5 wherein the graphite comprises at least about 50% of the mixture by weight.
7. The process of claim 1 wherein the forming of the preform is performed on a separate press
8. The process of claim 1 wherein the preform is maintained in a climate controlled environment between the forming and the molding steps.
9. The process of claim 1 wherein the molding process also includes applying a vacuum to the mold cavity as it is closed and maintaining the vacuum in the mold cavity throughout the molding process.
Type: Application
Filed: Feb 15, 2005
Publication Date: Jun 23, 2005
Applicant:
Inventors: Neal Benkofske (Elk River, MN), Pat Bell (Andover, MN), Mike Strantz (Ramsey, MN), William Hassell (Andover, MN), Charles VanHeel (Rogers, MN), Gerald Harms (Elk River, MN)
Application Number: 11/058,448