Abstract:
A process for casting three-dimensional parts and sacrificial models from casting compounds forced by pressurized inert gas into transparent molds that are supported by transparent mold frames. The casting compound incorporates radiation activated photo initiators which cause polymerization and curing of the compound within the mold. An automated casting system is anticipated which is enabled by the use of very soft, stretchable and deformable material for the mold body and supporting the mold with radiation transparent mold frames. Oxygen and other gases in the atmosphere react with some casting compounds to create a sticky film on mold cavity walls which causes difficulty when removing a casting from a mold. This problem is eliminated by purging the mold cavity with an inert gas between casts.

Description:
BACKGROUND OF THE INVENTION 
     Lost wax casting, a 4000 year old process, is still used today in most types of casting operations that utilize sacrificial patterns. The jewelry casting industry is one of the more prevalent practitioners of the process. It has advanced over the years from using hand carved patterns to current production processes which use rubber molds for creating duplicate sacrificial wax patterns. In contemporary processes, molten wax is injected under pressure into the mold cavity and, because of the frailty of the wax, removed by hand when the wax has hardened. This is a labor intensive, costly process which negates the fact that wax is inexpensive. Wax is cheap and reliable but has limitations. Some plastics have been used for sacrificial patterns but because of the nature of injectable plastics, those patterns have been limited to simple parts. The rigid molds required by the plastic materials do not allow for the complex undercuts that are present in the majority of jewelry patterns. In other words, undercuts create a mechanical lock that does not allow the part to be released from the mold. 
     FIELD OF THE INVENTION 
     This invention relates to an automated method and enabling apparatus for casting complex three dimensional parts, such as sacrificial investment models, from fluid compositions which solidify as a result of polymerization after being poured into a mold. 
     DISCUSSION OF THE RELATED ART 
     An example of a direct casting technique using photocurable polymers as a casting medium is taught by V. Joyner in U.S. Pat. No. 6,375,887 which is incorporated herein by reference in its entirety. The patent teaches a process and apparatus for casting three-dimensional parts and sacrificial models from a class of casting compounds which are solidified by actinic radiation. The class of casting compounds include, 1) compounds which act as conduits for the actinic radiation during and/or after hardening and 2) compounds which are hardened by an actinic radiation induced chain reaction throughout the body of fluid casting compound. These compounds allow the part being cast to be cured within molds that are opaque. The apparatus consists of an actinic radiation source positioned to irradiate the casting compound containing a photoinitiator as it enters the mold and/or radiate into the mold cavity through the sprue hole, windows and/or venting holes to effect the solidifying process. 
     Another example of a direct casting technique using photocurable polymers is taught by S. Nakamura, et al. in U.S. Pat. No. 5,990,190. In this system, an actinic radiation transparent mold is surrounded by sources of curing radiation which penetrate the mold from six directions. The need to completely surround the mold with radiation greatly increases the complexity and cost of the process. Furthermore, the need to create the master mold from a radiation transparent material imposes severe limitations on the structural integrity of the mold and limits the geometry of the end product, i.e. the casting. 
     V. Kadziela, et al., U.S. Pat. No. 6,829,362 for “Soft Molding Compound” combines the teachings of the above two references. The photocurable polymer is irradiated through a transparent mold and through an opening into the mold cavity, see the paragraph commencing at column 8 lines 13. 
     Heretofore, systems such as those referenced above and all other known techniques employ rigid master molds that do not permit undercuts or complex three-dimensional features in sacrificial mold patterns, or in the case of flexible master molds, employ rigid, opaque mold frames. Additional problems are encountered when the casting material is an actinic initiated polymerizable compound. Such compounds leave a residue on mold cavity walls which reacts with oxygen and other gasses in the atmosphere to create a film on mold cavity walls that causes cast items to adhere to the cavity walls, making it difficult to remove castings when molds are used repeatedly, such as in the case of automated systems. 
     OBJECTIVES OF THE INVENTION 
     It is a primary objective of the present invention to provide an automated system for creating sacrificial patterns through the use of soft, extremely flexible, transparent master molds that are purged with an inert gas between castings and encased in rigid, transparent mold frames. 
     An objective of the present invention is to provide a method and apparatus for casting a part, such as a sacrificial model for investment casting, from an actinic radiation curable material which is cast and cured in a mold supported by a rigid, transparent mold frame. 
     A prefered objective of the present invention is to provide a method and apparatus for casting a part, such as a sacrificial model for investment casting, from a radiation curable material which is cast and cured to a soft flexible state by radiation of a first frequency while in a mold supported by a rigid, transparent mold frame and cured to a final, ridged state by radiation of a second frequency after being removed from the mold. 
     A further objective of the present invention is to provide a method and apparatus for casting a part or a sacrificial model for investment casting from an actinic radiation curable material which is cast and cured in a mold that is purged with an inert gas before the liquid casting material is poured into the mold. 
     A further objective of the present invention is to provide a method and apparatus for casting a part or a sacrificial model, including undercuts and/or complex three-dimensional shapes, for investment casting from a radiation curable material which is cast and cured in a master mold that is flexible enough to allow the mold to be removed from the sacrificial part created therein by distorting and peeling the master mold from the sacrificial model. 
     Another objective is to provide a method for casting a radiation curable material incorporating a photoinitiator wherein the material is cured in the mold by an external source of radiation. 
     A further objective is to provide a method for casting a radiation curable material incorporating a plurality of photoinitiators wherein the material is cured to a first state while in the mold by an external source of radiation activating at least one photoinitiator and cured to a final state by activating at least one additional photoinitiator after the material is removed from the mold. 
     A still further objective of the invention is to provide a method for casting an actinic radiation curable material by irradiating the material as it is being pored or injected into the mold. 
     A further objective of the invention is to provide a method for casting an actinic radiation curable material by irradiating the material as it is being pored or injected into the mold, removing the material from the mold after it has cured to a first state and then irradiating the molded material by energy having a frequency different from the first source of irradiation to cause the molded material to reach a second state of cure. 
     Another objective of the invention is to photocure material within a mold by irradiating the material through a transparent mold frame. 
     Another objective of the invention is to photocure material within a mold by irradiating the material through a transparent mold frame and then photocure the material a second time after it is removed from the mold. 
     A further objective of the present invention is to provide a method and apparatus for casting a part or a sacrificial model, including undercuts and/or complex three-dimensional shapes, for investment casting from a radiation curable material which is cast and cured in a master mold comprised of more than two parts that create the mold cavity and is flexible enough to allow the mold to be removed from the sacrificial part created therein by distorting and peeling the master mold from the sacrificial model after the mold parts are removed from a stabilizing mold frame. 
     Other objects, features and advantages of this invention will be apparent from the drawings, specification and claims which follow. 
     SUMMARY OF THE INVENTION 
     The present invention teaches a process and apparatus for casting three-dimensional parts and sacrificial models from a class of casting compounds which incorporates a photo initiator and are solidified by actinic radiation. The class of casting compounds include, 1) compounds which act as conduits for the actinic radiation during and/or after hardening and 2) compounds which are hardened by an actinic radiation induced chain reaction throughout the body of fluid casting compound. These compounds allow the part being cast to be cured within transparent molds that are supported by transparent mold frames. The apparatus consists of an actinic radiation source positioned to irradiate the casting compound with energy of a first frequency while it is within a mold cavity. This radiation passes through the mold frame and mold wall to effect the solidifying process to a first state. A second radiation source is positioned to irradiate the partially cured casting with energy of a second frequency after it has been removed from the mold cavity. Separation of the molded part from the mold is enabled by purging the mold cavity with an inert gas prior to filling it with casting compound to eliminate sticky oxide residue on the mold walls and by the flexibility of the mold after removal of the stabilizing mold frame or the flexibility of the molded part before the second stage curing operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of the elements comprising the enabling apparatus of the present invention shown in their unassembled configuration. 
         FIG. 2  is a sectional view of the elements comprising the enabling apparatus of the present invention shown assembled into a mold assembly and receiving a charge of liquid polymerizable molding compound. 
         FIG. 3  is a sectional view of the mold assembly filled with molding compound and receiving polymerization initiating radiation. 
         FIG. 4  is a sectional view of the mold assembly shown during the initial stage of the demolding process with the mold frame removed from one mold half from which the polymerized part is partially extracted. 
         FIG. 5  is a sectional view of the mold assembly shown with the mold frame completely removed prior to the final demolding step. 
         FIG. 6  is a sectional view of the flexible mold halves freed from their mold frame and the molded sacrificial model demolded. 
         FIG. 7  is a sectional view of the elements comprising an alternate embodiment of the enabling apparatus of the present invention shown in their unassembled configuration. 
         FIG. 8  is a sectional view of the elements comprising the alternate embodiment of the enabling apparatus of the present invention shown assembled into a mold assembly and receiving a charge of liquid polymerizable molding compound. 
         FIG. 9A  is a schematic view of an exemplary automated production facility employing the mold assembly of the present invention and employing a single stage irradiation process. 
         FIG. 9B  is a schematic view of an exemplary automated production facility employing the mold assembly of the present invention and employing a two stage irradiation process wherein the second irradiation step occurs after a portion of the mold has been removed from the partially cured part. 
         FIG. 9C  is a schematic view of an exemplary automated production facility employing the mold assembly of the present invention and employing a two stage irradiation process wherein the second irradiation step occurs after the partially cured part has been completely removed from the mold. 
         FIG. 10  is a schematic representation of the mold purge operation illustrating the inert gas flow path. 
         FIG. 11  is a schematic representation of the mold evacuation process illustrating the inert gas flow path. 
         FIG. 12  is a schematic representation of the mold operation illustrating the inert gas flow path and resulting casting compound flow path. 
         FIG. 13  is a schematic representation of the mold purge, evacuate and fill system in the off position as a filled mold leaves station  1  of  FIGS. 9A , B or C. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the following description and claims, the terms “cure”, “cured”, “curing”, “solidified”, “solidifying”, “hardening” and “hardened” are used interchangeably to describe a transition of material from liquid to solid. The term “radiation” is used to identify the radiation which initiates the transition of material, polymerization, from a liquid to a solid which may be soft, flexible, hard, ridged or solid having enhanced properties, such as having qualities that enhance its use as a sacrificial model in a lost wax process. The frequency of polymerization initiating radiation is dependent on the photo initiator or initiators used in the casting compound. They may respond to radiation ranging from radio frequencies through microwave, infrared, the visible spectrum and ultraviolet. In the preferred embodiment, the polymerization initiating radiation is in the ultraviolet blacklight range, 352 nanometers, for one radiation step and in the ultraviolet bluelight range, 368 nanometers, for other radiation step, each of which performs as actinic radiation causing a liquid-to-solid phase change or flexible to ridged change in the casting medium. 
     The various embodiments of the invention are perfected through the use of a flowable casting medium,  13 , which may be poured or injected into a mold cavity and is hardened by radiation initiated polymerization. The casting medium is selected from a class of flowable compounds which include at least one photo initiator and become solidified, 1) when exposed to radiation and are transparent to the solidifying radiation whereby the casting functions as a radiation guide or light pipe during the curing process and/or 2) as a function of a chain reaction initiated by radiation. Actinic radiation of 352 and/or 368 nanometers is a preferred radiation for causing the transition of the casting medium from liquid to solid or from a first solid state to a state with enhanced properties for use as a sacrificial model in a lost wax process. Examples of typical photo initiators which may be used in the casting medium compound may be found in U.S. Pat. No. 6,025,114 issued to A. Popat et al. on Feb. 15 th , 2000 for “Liquid Photocurable Compositions”. In the preferred embedment, two or more photo initiators are incorporated into the molding compound and at least two are activated by radiation of different frequencies. The plural photo initiators cause the molding compound to transit to a first cure stage and then, controllably, to a second cure stage. By this process, the molded item may be cured to a flexible state to assist in demolding and later curded to a second state that facilitates use of the molded product. 
       FIG. 1  illustrates the mold assembly  10  of the present invention in its unassembled state immediately before assembly. It is comprised of a plurality of inter-fitting sections which form a mold cavity,  21  of  FIG. 2 , when assembled. The inter-fitting sections are formed from one of a class of materials which have the property of being radiation transparent, yielding to the touch, stretchable, pliable and shape retentive. In the preferred embodiment depicted in  FIGS. 1 through 6 , the mold has a bifurcated mold body  20  fabricated from a soft, flexible, radiation transparent material which is easily distorted to allow removal of complex three dimensional molded parts which may include undercuts. Each half of the mold,  11  and  12 , includes a cavity,  17  and  18  respectively, which form the mold cavity  21  when the mold halves  11  and  12  are mated. The mold halves,  11  and  12 , include a plurality of pins  13  which secure the mold to an assembly conveyor. Because of the soft flexible nature of the material from which the mold  20  is formed, it cannot maintain its integrity or the shape of the mold cavity during filling. Therefore, a mold frame is required to support the mold halves. The frame is comprised of transparent halves  15  and  16  which are dimensioned to enclose the mold halves  11  and  12  and stabilize the body of flexible mold material to allow injection of a liquid molding compound and prevent distortion of the mold  20  during the filling and polymerization processes. Means such as anchoring pins  19  function to secure the mold frame to an assembly conveyor. 
       FIG. 2  illustrates the mold assembly  10  configured with the mold halves  11  and  12  pressed together by the mold frame halves  15  and  16  to create the mold  20  incorporating the mold cavity  21 . A casting medium composition  22  may be poured in or injected under pressure at this point because the soft, flexible mold  20  is stabilized by the mold frame. 
       FIG. 3  depicts the cavity of the mold  20  filled with the liquid casting composition  21  which incorporates a photo initiator that causes polymerization which causes solidification when the radiation from source  23  reaches the material after passing through the radiation transparent mold frame and mold. In the preferred embodiment the radiation source  23  generates ultraviolet actinic rays which pass through the mold frame,  15  and  16 , and mold  20  to initiate polymerization and convert the liquid compound  21  into a solid casting  24 . Actinic radiation is used in the preferred embodiment but the invention contemplates the use of a broad range of radiation, such as, but not limited to, radio frequency, high frequency, microwave, x-ray, infrared, visible light, blacklight ultraviolet and blacklight blue ultraviolet. The radiation used to initiate polymerization is a function of the initiator used in the casting compound and the transparency of the mold and mold frame. 
     As the casting compound  21  in the irradiated mold cavity hardens, it becomes the cast part  24  and mold frame  16  is removed as depicted in  FIG. 4 . This allows mold half  12  to become destabilized to the point where it will stretch and its cavity  18  deforms as the casting  24  is separated by moving the mold half  12  away from the frame stabilized mold half  11 . 
     After the mold half  12  is removed from the casting, mold frame  15  is removed from mold half  11  as illustrated in  FIG. 5 . This allows the casting  24  to be extracted from the mold cavity  17  because when the frame  15  is removed, the mold half  11  is free to stretch and deform and thus release the casting. When the casting  24  is free of both mold halves  11  and  12  as in  FIG. 6 , it may be conveyed to additional processing stations which may include additional radiation initiated polymerization, deburring, polishing and packaging. 
     In the alternate embodiment depicted in  FIGS. 7 and 8 , the mold is comprised of four sections,  61  through  64 , which minimize under cuts in the part to be cast in the assembled mold body  20 . The sections are fabricated from a soft, flexible, radiation transparent material which is easily distorted to allow removal of complex three dimensional molded parts which may include undercuts. Each section of the mold includes a cavity,  71  through  74 , which form the mold cavity  21  when the mold sections  61 ,  62 ,  63  and  64  are mated. The assembled mold includes a plurality of pins  13  which secure the mold to an assembly conveyor. Because of the soft flexible nature of the material from which the mold  20  is formed, as in the preferred embodiment, it cannot maintain its integrity or the shape of the mold cavity during filling. Therefore, a mold frame is required to support the mold halves. The frame may be the same as used in the preferred embodiment, i.e., comprised of transparent halves  15  and  16  which are dimensioned to enclose the mold sections,  61  through  64 , and stabilize the body of flexible mold material to allow injection of a liquid molding compound and prevent distortion of the mold  20  during the filling and polymerization processes. Means such as anchoring pins  19  function to secure the mold frame to an assembly conveyor. 
       FIG. 8  illustrates the mold assembly  10  configured with the mold sections  61 ,  62 ,  63  and  64  pressed together by the mold frame halves  15  and  16  to create the mold  20  and form the mold cavity  21 . A casting medium composition  22  may be poured in or injected under pressure at this point because the soft, flexible mold  20  is stabilized by the mold frame to a point where the mold cavity is ridged but gasses trapped in the mold cavity may escape but the casting compound  22  may not. Under certain circumstances it is desirable to construct the mold frame from a plurality of sections,  65  through  68 , which are dimensioned so that the mold sections,  61  through  64 , are pressed together so tightly that gasses or casting compound cannot escape. This minimizes extrusion ridges at the joints of the mold to simplify clean up of the cast part after it is removed from the mold. In such cases. one or more small holes,  75 , may be punched in one or more of the mold sections  61  through  64  to allow gasses to escape during the cavity filling operation. The holes may be very small, such as might be created by a 22 gauge needle. 
     The procedures illustrated by  FIGS. 1 through 8  may be accomplished manually or by a variety of automated means. One such automated means is illustrated by  FIG. 9A  as an exemplary automated application enabled by the present invention. 
     In  FIG. 9A , the mold assembly  10  is filled with a liquid casting compound containing a photo initiator as it passes through station  1 . The mold halves  11  and  12  and mold frames  15  and  16  are secured to the conveying means  30  by a plurality of securing devices  13  and  19 . The retaining devices  19  used to hold the mold frames to the conveyer are releasable but the retaining means  13  securing the mold halves to the conveyer are not. After the mold cavity  21  is filled, either by pressure injection via a system such as illustrated in  FIGS. 10 through 11  or simple pouring  40 , the mold assembly continues along the conveyer to station  2  where it is subjected to polymerization initiating radiation from one or more sources  23 . The radiation passes thought the mold frame members  15  and  16  and mold  20  to initiate polymerization of the liquid compound within the mold cavity  21 . 
     The mold assembly  10  proceeds to station  3  where it arrives after the casting compound  21  has polymerized into a hardened casting  24 . At this station, mold frame  16  is removed, leaving the mold half  12  unsupported but secured to the conveyer. At station  4  the conveyer splits in two, dividing into separate paths, a primary path  31  along which mold half  12  travels and a secondary path  32  which mold half  11  follows. As the paths diverge, mold half  12 , which is secured to its conveyer  31 , is peeled away from the casting  24  as conveyer  31  turns away from conveyer  32 . The angular velocity of both conveyers is maintained constant, i.e., the relative speed of the conveyers changes so the mold halves are always on the same radial. 
     At station  5 , frame  15  is removed from mold half  11  so that mold half will be free to stretch and deform as the casting  24  is removed and dropped on conveyer  33  which transports the casting to a finishing and packaging means  34 . Because the mold halves are maintained on a common radial, they mate up as the conveyers  31  and  32  converge at station  6  where the mold frames are joined to form the complete mold assembly  10  to begin a repeat of the casting and unmolding process. 
       FIG. 9B  illustrates an exemplary preferred embodiment wherein a radiation source,  63 , generating a frequency different from radiation source  23 , initiates a second polymerization process which causes the solid casting to transition from a first stage to a second stage having properties more desirable in a sacrificial model. For instance, the properties of the casting may transition from a soft or flexible solid to a hard or ridged solid or from a solid with poor burn-out qualities to a solid with properties more desirable for a sacrificial model to be used in a lost wax casting process. As illustrated in  FIG. 9B , radiation from source  63  is preferably applied directly to the casting  24  after an inter-fitting section  12  of the mold  20  is removed. 
       FIG. 9C  illustrates another exemplary preferred embodiment wherein a radiation source,  63 , generating a frequency different from radiation source  23 , initiates a second polymerization process which causes the solid casting to transition from a first stage to a second stage having properties more desirable in a sacrificial model. For instance, the properties of the casting may transition from a soft or flexible solid to a hard or ridged solid or from a solid with poor burn-out qualities to a solid with properties more desirable for a sacrificial model to be used in a lost wax casting process. As illustrated in  FIG. 9C , radiation from source  63  is preferably applied directly to the casting  24  after it has been removed from the mold  20 . 
       FIG. 10  schematically illustrates the principles of the preferred mold filling process. A gas pressure regulator  41  controls the outlet pressure of an inert gas source  42  to prevent the gas from blowing out through the matting faces of the mold sections or vent holes  75 . The gas is coupled to a gas control switching valve  43  inlet port via conduit  44 . This valve controls gas flow mutually exclusively between gas conduits  45  and  54 . The valve is set to the purge position which allows the gas to flow from conduit  44  via the gas control valve  43  into conduit  45  which is connected to the fill switching valve  46  via a one way, nonreturn valve  47  and then into the mold via connector  48 . The fill switching valve includes inlet and outlet ports which are mutually exclusively connected to the valves switching outlet, the mold fill port, which is connected to the mold cavity&#39;s sprue via the mold connector  48 . The inert gas purges the air from the mold cavity  21  to eliminate the adverse effects caused by the casting compound reacting with oxygen and other air source contaminants. The primary adverse effects eliminated are those that cause the cast part to stick to the mold cavity walls and thus inhibit removing the casting from the mold. The preferred inert gas is argon because it is heavier than air and sinks to the bottom of the mold cavity to ensure complete purging of air. 
     The fill valve  46  is then moved to the evacuate position as shown in  FIG. 11 . In this position, pump  49  partially evacuates the mold cavity via conduit  51  and returns the inert gas to its storage container  42  via conduit  52 . The partial vacuum thus created in the mold cavity assists the casting compound fill operation and eliminates the possibility of trapped gas pockets that would degrade the cast part. The reduced pressure within the mold cavity  21  during the evacuation process causes seams at the mating faces of the mold sections and vent holes to seal. 
     As the mold cavity  21  is evacuated, the gas control valve  43  is moved to the fill position and the gas pressure regulator  41  increases the pressure applied to conduit  44  to a value which will force casting compound into the mold cavity when the fill control valve  46  is placed in the fill/purge position as illustrated in  FIG. 12 . With the fill control valve in the fill position, the pressurized inert gas applied to the casting composition reservoir  53  via gas control valve  43  and conduit  54  forces casting compound  22  into conduit  55  via one way valve  56 . The fill control valve  46  is then placed in the fill/purge position as illustrated by  FIG. 12  and the pressurized casting compound flows into the mold cavity  21  until pressure sensor  57  detects the back pressure created when the mold  10  is filled. This causes the gas and fill valves to be turned to the off position as shown in  FIG. 13  and the fill connecter  48  to be removed from the mold  10 . If the conveyer was stopped during the fill operation, it is restarted and moves until the next mold is positioned at fill station  2 . In the preferred embodiment, the inert gas used is argon because it is heavier than air and readily sinks to the bottom of the mold. 
     While preferred embodiments of this invention have been illustrated and described, variations and modifications may be apparent to those skilled in the art. Therefore, we do not wish to be limited thereto and ask that the scope and breadth of this invention be determined from the claims which follow rather than the above description.