Abstract:
The invention includes a system and method for making injection molded dental appliance apparatuses having an injector module removably attached to a dental flask, a press and a controlled compression apparatus, wherein the press and compression apparatus introduce pressure into the dental flask and the injector module, which pressure may or may not be maintained during curing of the appliance. The invention also includes a system for dispensing dental appliance chemicals having at least one chemical dispensing apparatus and a mixing container; whereby a separate dispensing apparatus is used for each chemical and the chemicals are dispensed and mixed in the same container.

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
     This Application claims the benefit of pending U.S. Provisional Application No. 60/038,630, filed on Feb. 19, 1997. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a molding system and method for making injection molded dental appliances such as dentures, retainers, prostheses and similar appliances. The invention also relates to a means for dispensing materials which may limit operator exposure to harmful chemicals. 
     SCOPE OF THE PRIOR ART 
     Conventional molded dental appliances and methods of making them are widely practiced by dental laboratories. Most molded dental appliances are constructed in a dental flask containing a mold, also called an investment, of the appliance to be formed. The investment is made of plaster or other investment material and is molded around a wax model of the desired appliance, a wax-up, which is embedded in the dental flask. The wax-up may include items such as retainer wires or artificial teeth. Once the investment has hardened, the wax is removed from the dental flask, creating a cavity in the investment of the shape of the desired appliance. The uncured appliance material is then introduced by hand or by conventional injection methods. In conventional methods of packing, approximately three to four tons of force is required in order to squeeze out excess appliance material. After packing, the material is cured in the flask by conventional means such as by conduction heating, thereby forming a dental appliance of the desired shape. 
     The use of conventional molding systems and methods poses several problems for the practitioner. Often, too much material is added to the investment cavity, resulting in overpacking of the flask. Also, the high pressures can cause distortion of metal frameworks. In addition, when applied to partial dentures, tooth movement frequently occurs, requiring time-consuming reconstruction. Additionally, the material may undergo shrinkage during curing. Overpacking, excess pressure and shrinkage can all result in inaccurately dimensioned dental appliances. In addition, opening and subsequent addition or subtraction of material to the dental flask can cause a variety of significant problems, including but not limited to nonuniform or incomplete curing, the entrapment of air bubbles within the appliance, dislodging of embedded items such as teeth or retaining wires, premature appliance fractures (due to nonuniform preparation of materials), and inaccurately dimensioned appliances. 
     In addition to the problems disclosed above, conventional methods of mixing the materials may expose the operator to potentially harmful fumes and chemicals. For instance, many dental appliances are created using resins, such as acrylic, urethane or epoxy resins. A number of these resins are currently believed to pose a risk of cancer to exposed persons, and all may be harmful if absorbed through the skin or lungs or ingested, or if fumes emitted from the resins are inhaled. For example, in a conventional acrylic resin preparation of the type generally used in dentistry, the resin material is prepared on a benchtop by combining a fine, loose powder polymer with a liquid monomer and mixing by hand with a small stick or spatula until uniform. The polymer powder readily disperses into the air, and may be inhaled by the operator or absorbed through the skin. The liquid monomer is volatile, and may be inhaled in its gaseous state. In addition, the possibility of spillage of the liquid monomer is a constant threat to the operator. Moreover, upon combination of the powder and the liquid, a chemical reaction causes the immediate release of fumes, which may also be harmful to the operator if inhaled. In another example, epoxy resins are generally formed from two or more thick liquid components which are hand mixed on a benchtop, the process of which also releases potentially harmful vapors. In addition, as with virtually all commercially available uncured resins, mixed but uncured acrylic or epoxy material may be harmful and irritating if contacted with skin. 
     Moreover, conventional molding systems further cause an economic disadvantage to operators in that the flasks and attendant equipment used for each type of material are different. For instance, traditional flasks and injector modules for forming heat-curable appliances are different than traditional flasks and injector modules used for forming microwave-curable appliances. Thus, operators must purchase entirely separate equipment if they are to have the desired freedom to use both types of materials. 
     Thus, there is a need for an improved dental injection molding system which minimizes inaccurately dimensioned appliances, which protects operators from exposure to potentially dangerous fumes and chemicals and which allows operators the freedom to use at least some of the same equipment with more than one type of material. 
     As will become apparent from a detailed reading of the description, the system of the invention offers significant advantages over the prior art systems described above, most significantly including a system for pressurized and continuous injection in a closed system even during curing, which greatly minimizes the risk of inaccurately formed or non-uniform appliances. In addition, the injection system of the invention can accommodate conduction heat and microwave curing, both under pressure and without pressure. Lastly, use of the dispensing system of the invention will minimize operator exposure to harmful fumes and chemicals, regardless of the chemistry used to make the material. Other objects and advantages of the present invention will become obvious to the reader and it is intended that these objects and advantages are within the scope of the present invention. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a pressurized dental injection molding system whereby material may be continuously pressurized in a mold up to the point of cure of the material. 
     It is a further object of the invention to provide a system for making injection molded dental appliances that can compensate for shrinkage or overfilling or underfilling. 
     It is also an object of the invention to provide a system for making injection molded dental appliances which at least partly vents the injector module and flask during injection. 
     It is yet another object of the present invention to provide an injection module for making injection molded dental appliances that may be used with both conduction heating and microwave curable material. 
     It is still another object of the present invention to provide a dispensing system that minimizes operator exposure to chemicals and vapors. 
     The invention includes a system and method for making injection molded dental appliance apparatuses having an injector module removably attached to a dental flask, a press and a controlled compression means, wherein the press and compression means introduce pressure into the dental flask and the injector module, which pressure may or may not be maintained during curing of the appliance. The invention also includes a system for dispensing dental appliance chemicals having at least one chemical dispensing apparatus and a bag; whereby a separate dispensing apparatus is used for each chemical and the chemicals are dispensed directly into the bag and mixed in the bag. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic view of a dental injection molding system apparatus made in accordance with the principles of the invention, featuring a press, an assembled flask, an injector module and a compression device. 
     FIG. 2 is an exploded view of a flask and an injector module made according to the invention. 
     FIG. 3 is a schematic diagram of an assembled flask and sprue former of the invention. 
     FIG. 4 depicts the material dispensing system of the invention. 
     FIG. 5 shows a separated flask of the invention containing a formed mold for an appliance made in accordance with the principles of the invention. 
     FIG. 6 shows insertion of appliance material into an injector module of the invention. 
     FIG. 7 illustrates the continuous injection apparatus of the invention whereby the appliance is microwave cured under continuous pressure. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a preferred embodiment of an injection molding system made in accordance with the principles of the present invention. The system includes a press  2 , a compression device  8 , a flask  4 , and an injector module  6 . 
     The press  2  includes a vertically moveable platform  10 , a compression plate  12 , and opposing first  14  and second  16  vertical platform posts running from the underside of compression plate  12  through platform  10  to compression device  8 . The compression plate  12  is bolted to the platform posts  14  and  16  respectively by first and second bolting means  18  and  20 . A press piston  19  connected to the compression device allows the platform  10  to be raised or lowered along first  18  and second  20  platform posts. 
     The compression device of the preferred embodiment illustrated in FIG. 1 includes a controller means for controlling various factors, such as pressure, speed and time. The compression device further includes a plurality of switches and visual and audible indicators, including a pressure indicator  22 , a cycle completion indicator  24 , stop and start buttons ( 28  and  30  respectively) and a selector switch  32 . The compression device  8  further contains a compression means that is housed within the compression device. The compression means may be any means of raising and lowering platform  10  with press piston  19 , which means are known in the art and which include an electronic controller for controlling factors such as pressure and speed of injection. 
     FIGS. 1-3 illustrate a preferred embodiment of a microwavable flask  4  and injector module  6  of the invention, which flask  4  includes a bottom section  34 , a center spacer section  36  and a top section  38 , each section having a plurality of bolt holes  44  through which a plurality of bolts  42  are inserted upon assembly of the flask sections. 
     The bottom section  34  also includes a knockout plate  50 , which fits into opening  51  and assists in separation of the sections upon completion of the method of the invention. Impressed into bottom section  34  is a bottom cavity  46  in which a cast and wax-up is inserted. The cast and wax-up and methods of their making are well known to practitioners of the art, and all of those methods are suitable for use with the dental flask and injector module of the invention. 
     The center spacer section  36  includes a spacer plate having an outer edge  48  and an inner edge  49 , wherein the outer edge is shaped to be in registry with the outer edge of bottom  34  and top  38  sections, and the inner edge is shaped to be in registry with the edges of the bottom cavity  46  of the bottom section. 
     The top section  38  includes two vent openings  52  and  56 , and one sprue former opening  54 , into which a bushing  58  is inserted, although the number of vents, sprue former openings and bushings may vary in other embodiments of the invention. When the flask sections are assembled in registry with one another as illustrated in FIGS. 2 b  and  3 , each set of bolt holes  44  (one set in each of the three sections  34 ,  36  and  38 ) are in registry with one another. Although the embodiment illustrated here shows three bolt holes, other embodiments with more or less than three bolt holes in dental flask  4  are contemplated to be within the scope of this invention. The bushing  58  includes an opening  60 , which fits into sprue former opening  54 . In another embodiment, the top section includes a top plate for use with pressureless curing The top plate includes a slide valve (not shown) that is closed when curing without the injector module is desired by the operator. The dental flask illustrated in FIG. 2 is constructed from material which is microwave conducting and which can withstand injection pressures of at least 3000 lbs. Other embodiments of the flask of the invention include a flask for use with conduction heating which can withstand injection pressures of at least 3000 lbs. and which is compatible with the injection methods of the invention. 
     The injector module  6  (used to inject the curable material into the appliance mold within the flask) is then assembled and attached to the reassembled flask  4 . In the preferred embodiment of a microwavable flask  4  and injector module  6  as illustrated in FIG. 2 a,  the injector module  6  includes a plate  68 , upon which is mounted a tubular injection cylinder  70  having a cavity  71  and with a plurality of bolt holes  44  in registry with those previously described. The injection cylinder  70  and plate  68  further have an injection orifice (not shown) which is constructed to be in alignment with the sprue opening in the dental flask. Disposed about the outside of the injection cylinder is a threaded pressure retaining band  72 . The injection cylinder cavity  71  is designed to accommodate a solid cylindrical piston  74 , having a piston cap  76  with a patterned exterior edge ridge  78 , and top  80  and bottom  82  surfaces. This piston  74  is fitted to be removably inserted into the injection cylinder cavity  71  up to the bottom surface  82  of the piston cap  76 . The injection cylinder  70  may further includes a tubular pressure cap  84  with a substantially flat cover  86 , which includes a spring mechanism  85  and pressure retaining ring  90  which is threaded on the interior surface  92  to be compatible with the pressure retaining band  72  on the injection cylinder  70 . The pressure cap  84  and the pressure retaining ring are used for curing under pressure, as illustrated in FIG.  2 . At least one view hole  88  disposed in the side wall of the pressure cap  84 . Disposed about the exterior surface of the pressure cap  84  is a pressure retaining ring  90 . In another embodiment of the injector module used for pressureless curing, no pressure cap or pressure retaining ring are used. In the embodiments described herein, the injector module is constructed of a metal which does not conduct microwaves. The universal metal injector is a novel feature of the invention which may be used to inject material into both microwave flasks and conduction heating flasks, and may be used for pressured or pressureless curing. If used with pressure cap  84 , the injector module applies continuous pressure and venting of the material during injection and either microwave or conduction heat cure, and holds the material in the injector in an uncured state while exposed to microwaves. 
     One preferred method of using the injection system to make molded dental appliances includes the steps of assembling the dental flask, creating an investment, or mold, for the desired appliance, attaching the injector module, preparing the curable material, injecting the curable material into the flask using the compression device of the invention, curing the curable material under pressure or without pressure, disassembling the flask and finally releasing the finished appliance. 
     The dental flask of the preferred embodiment illustrated in FIGS. 1-3 and  5 - 7  is assembled by first inserting knockout plate  50  into the bottom section  34  of dental flask  4 . Investment material is then prepared according to known methods and placed into the cavity  46  of the bottom section  34 . A cast and wax-up which have been formed in accordance with methods well known in the art are positioned in the assembled bottom cavity  46  of the bottom section  34 . The center section  36  is then positioned upon the bottom section  34  such that the bolt holes  44  in each of the two sections are in registry. Additional investment material is then added to the flask on top of the cast and wax-up. 
     Once the flask is filled with investment material, the flask is reassembled. The reassambled flask may include either a top section for use with continuous pressurized injection during curing, or it may be reassembled with a top section for pressureless curing, as described above. The top section  38  is then positioned such that the bolt holes  44  of the top section are in registry with the bolt holes  44  of the bottom and center sections ( 34  and  36  respectively). Bolts  42  which extend from the top section into the bottom section of the flask are then inserted into the bolt holes and hand tightened. Vent openings  52  and  56  allow excess investment to be released from the assembled flask upon tightening of the bolts  42 , which vent openings  52  and  56  subsequently become sealed by solidification of the investment material. Prior to the hardening of the investment, the sprue former  62  is then positioned in the sprue former opening  54  such that the sprue former contacts the wax-up, thus forming the passage  63  in the investment material through which curable material is injected (as illustrated in FIG.  5 ). Once the investment is hardened, the sprue former  62  is removed. The cast and wax -up in the invested flask is then heated for a time sufficient to soften the wax. The top section  38  then separated from the bottom  34  and center  36  sections and the wax is removed, leaving a cavity in the investment as shown in FIG.  5 . This cavity forms the mold for the appliance which is subsequently filled with uncured material as described below. The flask sections are then reassembled but not bolted. 
     In order to attach injector module  6  to reassembled flask  4 , the plate  68  with injection cylinder  70  is aligned with dental flask  4  so that bolt holes  44  in plate  68  are in registry with bolt holes  44  in dental flask  4 . The bolts  42  are then inserted into bolt holes  44  and hand tightened. The bolts are then tightened using press  2  and compression device  8  of the invention. This step involves placing the dental flask  4  with attached plate  68  and injection cylinder  70  on platform  10  so that the cavity  71  of the injection cylinder  70  is facing the lower surface of the compression plate  12 . The various controls on the compression device  8  are then used to raise the platform  10  and compress the flask so that the bolts  42  may be tightened. Once platform  10  stops rising, bolts  42  are tightened, and platform  10  is lowered. 
     Once the dental flask  4  and injection cylinder  6  are assembled and bolted together as described above, the material may be prepared using the dispensing system of the invention. The dispensing system illustrated in FIG. 4 depicts preparation of an acrylic resin material, and includes a powder dispenser  98  with lever  100 , a cup  102 , a funnel  104 , a funnel stand  106 , a stopper  108 , a liquid dispenser  110  with a lever  116 , and a container  96 , such as the bag illustrated in the preferred embodiment shown in FIGS. 4 d - 4   f.  The dispensing system may also include an umbrella  112 , as shown in FIG. 6, which may be used to seal the container  96 . The container  96  may be any container which will not degrade when filled with the material, which is capable of being closed and which is thin enough to allow the operator to assess through touch the consistence of the material. 
     The powder dispenser lever  100  is used to dispense a measured amount of polymer powder into the cup  102 . (FIGS. 4 a  and  b ). The powder is then poured into funnel  104 , a spout  118  of which is blocked by the stopper  108 , and which funnel  104  is held upright by use of the funnel stand  106  (FIG. 4 c ). The container  96  is then disposed around a nozzle  114  of the liquid dispenser  110 , and an appropriate amount of liquid monomer is then dispensed into container  96  by movement of lever  116  (FIG. 4 d ). The container  96  containing the liquid monomer is placed below funnel spout  118  and stopper  108  is removed, causing powder to flow into container  96  (FIG. 4 e ). The top portion of the filled container  96  is then closed and the contents gently mixed by hand (FIG. 4 f ). The use of the material dispensing apparatus and the method described above minimizes aerosolized powder and exposure to potentially harmful vapors, and reduces the risk of operator exposure to potentially harmful chemicals and vapors. This particular embodiment illustrates preparation of an acrylic resin compound, but it is understood that any powder/liquid or liquid/liquid combination may be used with the dispensing apparatus of the invention, provided the material resulting from the mixed components is sufficiently viscous for use with the mold and injector module of the invention. 
     Once the container  96  containing the material is prepared, the filled container  96  is placed into the injection cylinder cavity  71  such that the empty top of container  96  extends out of the top of the injection cylinder  70 . If an umbrella  112  is used, the top of the container  96  may then be threaded through an opening  116  in the umbrella  112  and the umbrella  112  pushed down until it reaches the material, as shown in FIG. 6, with the empty top of the container  96  extending through the opening  116  of umbrella  112 . The top of the container is then folded and the filled container  96  is inserted into the cavity  71  of the injection cylinder  70 . The injector piston  74  is then inserted into the injection cylinder cavity  71 , thus compressing the filled container  96 . 
     If the continuous pressure embodiment of the injector module is used, the pressure cap  84  is placed over the piston  74 , with the piston cap ridge  78  visible through the pressure cap view hole  88 . The pressure retaining ring  90  is then disposed around the piston cap  76 , with the interior surface  92  of the ring  90  removably resting upon the lower rim  120  of the pressure cap. 
     In yet another embodiment of the system, the dispensing system may not be used and the material may be loaded directly into the injector module without use of a container. 
     The assembled dental flask  4  and filled injector module  6  are then placed on platform  10  of press  2  so that in the pressureless embodiment, the top of piston  74  is facing the lower surface of the compression plate  12 , and in the pressured embodiment, the top of pressure cap  86  is facing the lower surface of the compression plate  12 . The compression device is then activated to cause injection of the material into the investment through the injection orifice in the injector module, by pressure applied to the injector module  6  and dental flask  4 . In the continuous pressure embodiment of the invention, once the injection is substantially completed, the pressure retaining ring  90  is tightened about the pressure retaining band  72 , thus causing spring mechanism  85  to exert pressure on the piston  74 , which creates pressure in the dental flask  4 . Upon completion of injection, the flask and injector module are removed from the press. 
     The pressurized dental flask  4  with attached injector module  6  is then cured, for instance by microwave as shown in FIG.  7 . Until the point of cure (hardening) of the material, the pressure in the dental flask  4  allows for continued injection of uncured material at a constant pressure as necessary to fill void spaces within the investment without causing overfilling of the investment. In addition, the continuous pressure exerted on the appliance minimizes distortion of the appliance due to shrinkage during polymerization, or cure, of the material. 
     In the pressureless injection system of the invention, the injection of material into the flask is as described above, but after injection, the slide valve on the top section of the flask is closed to seal the material within the flask, and the injector module  6  is removed. The flask is then cured as described above. 
     After cure, the dental flask  4  is cooled. In the pressurized embodiment, the pressure retaining ring  90 , pressure cap  86 , piston  74  and bag  96  are removed from the injection cylinder  70  prior to cooling. The cured appliance is removed and finished in the conventional manner. 
     It should be understood that various changes and modifications to the preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be within the scope of the claims.