Patent Publication Number: US-2022234246-A1

Title: Pressure Casting of Submicron Ceramic Particles and Methods of Ejection

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/862,645, filed Apr. 30, 2020, now U.S. Pat. No. 11,305,457, which claims the benefit of and priority to U.S. Provisional Patent Application No. 62/842,580, filed May 3, 2019. The entirety of each of the foregoing applications is incorporated herein by reference. 
    
    
     BACKGROUND 
     Methods for slip casting ceramic bodies in molds are known. Traditional casting processes using non-porous molds require significant drying time to remove water from the cast body to provide sufficient shrinkage for removal. Molds often incorporate a taper to facilitate removal, limiting the shape of the cast ceramic body. 
     Ceramic bodies made by pressure casting methods using porous molds may be quickly removed by floating the cast from the mold surface. During the casting process, water that enters the porous structure of the mold is forced back through the porous structure and into the mold cavity. As water is forced between the cavity surface and the ceramic body, the cast part is ejected. Disadvantageously, water and ceramic slip forced through the porous mold wet the surface of the cast body, forming a tacky surface causing cracks or breakage. 
     Commonly owned U.S. Pat. No. 9,790,125, incorporated herein by reference in its entirety, discloses a vacuum-assisted slip casting process, and vacuum-assisted slip casting assembly comprising a porous mold surrounded by an enclosure. Optionally, it is disclosed that pressure may be applied above the enclosure to promote the process. It is further disclosed that a blank may be removed by opening the enclosure and removing the mold and the ceramic blank. 
     UK patent GB 2 372 958, discloses a pressure casting assembly in which a ceramic body cast in a tapered cavity and a method for releasing the cast part from the mold. Compressed air introduced into the mold cavity forces excess water from the cast piece prior to pushing liquid from an inner liner into the mold cavity causing the molded piece to float off the inner liner. 
     SUMMARY 
     Methods and apparatus are provided for pressure casting ceramic parts and rapid ejection of a cast ceramic body from a mold. A mold is described that has an impermeable cavity surface providing a ‘dry release’ of the cast object. Ceramic bodies produced herein may have smooth, non-tacky surfaces after ejecting from the mold. 
     In one embodiment, a method for making a ceramic block comprises: i. providing a pressure casting apparatus comprising a mold having a porous casting substrate and an impermeable cavity surface; ii dispensing a ceramic slurry into a mold cavity under pressure; iii. removing liquid from the slurry under pressure via the porous casting substrate; iv. consolidating ceramic particles forming a cast ceramic body within the cavity; v. removing the porous casting substrate from the apparatus to form a mold opening; and vi. ejecting the resulting ceramic body from the mold opening by application of pressure in a dry release process. 
     In one embodiment, the apparatus for pressure casting a ceramic body is comprised of two components, i. a mold housing comprising the mold cavity having impermeable side surfaces and a top surface through which ceramic slurry is delivered into the mold cavity, and ii. a removable porous body comprising a porous casting substrate on which the ceramic body is formed. 
     The casting porous substrate may comprise an average pore size sufficiently small to inhibit movement of ceramic particles into the pore volume of the casting substrate during the pressure casting process while allowing removal of the liquid component of the slurry. In one embodiment, a porous casting substrate has an average pore size of less than 1 μm. 
     Impermeable cavity surfaces resist penetration of the liquid component of a ceramic slurry under casting pressure. One or more openings into the mold cavity through the impermeable surface allows delivery of the ceramic slurry to the mold cavity under pressure. In a further embodiment, the apparatus comprises a dispensing lid as a separate component that engages with the mold cavity for dispensing slurry into the cavity. 
     In one embodiment, pressure for ejecting a cast ceramic body may be delivered through the openings in the impermeable surface of the mold. In one embodiment, pressure is applied to a top surface of the cast ceramic body through the inlet of the mold. For example, compressed air may be applied through a hose or nozzle aligned with the inlet used to deliver the ceramic slurry, or other opening in the impermeable mold. The porous casting substrate may be separated from the cast ceramic body, and the cast body is ejected from the cavity opening. 
     A dry release process may be used to eject a cast ceramic part. In traditional processes, liquid retained in the pore volume of a porous mold is forced between the casting surface and the cast part to float the part off the casting surface. In contrast, in one embodiment where cavity side surfaces are impermeable, the cast part is ejected from the mold without re-introducing liquid from the casting process between the casting substrate and the cast body. 
     Thus, in a dry ejection process described herein, cast ceramic parts are ejected from the mold cavity without floating the part from the casting surface. Advantageously, cast ceramic bodies may be ejected immediately after casting without drying or shrinkage. For example, wet cast parts may be ejected that comprises up to 12 wt % liquid. 
     In a further embodiment, the apparatus comprises an ejection lid for ejecting the ceramic body. After casting a ceramic body, a slurry dispensing lid may be removed from the mold and replaced by an ejection lid. Pressure may be delivered through the ejection lid into the cavities of the mold. 
     A process is provided wherein the ceramic slurry is cast unidirectionally, inhibiting uneven build-up of the ceramic on side surfaces of the mold. In one embodiment, as casting occurs in the z-axis direction, build-up of ceramic material progresses from the casting substrate towards the mold top surface, and build-up on cavity side surfaces is inhibited. The thickness of the ceramic body increases uniformly in the x-y direction. 
     Through methods provided herein, zirconia ceramic powder having a median particle size less than 400 nm may be cast to form solid ceramic bodies with a smallest dimension (e.g. thickness) that is greater than 10 mm, or greater than 15 mm, or greater than 25 mm, or greater than 30 mm. Ceramic bodies having smooth surfaces, and a uniform build-up of the ceramic material through the x-y direction, lack cracks or breakage after drying. The resulting ceramic bodies may be suitable for use as ceramic mill blocks that can accommodate single or multi-unit restoration bodies, including, but not limited to crowns, veneers, bridges, and dentures. 
     In other embodiments, zirconia ceramic powder having a median particle size less than 400 nm may be directly cast to form solid ceramic bodies having thin walls (e.g., 5-10 mm, 0.5 mm to 5 mm, or 0.5 mm to 2 mm, or 0.5 mm to 1 mm), non-uniform shapes, or non-uniform sizes, such as wafers, tabs, rods, or dental restorations such as crowns, veneers, bridges, and dentures. 
     Pressure-casting processes described herein significantly reduce casting time of ceramic bodies compared to known vacuum casting processes. Dry ejection processes described herein significantly reduce the time for removing the cast ceramic body from the mold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are illustrations of one embodiment of a method for casting a ceramic body. 
         FIGS. 2A through 2  D are illustrations of an exemplary embodiment of an impermeable mold. 
         FIGS. 3A through 3C  are illustrations of two exemplary embodiments of an apparatus comprising an impermeable mold and lid. 
         FIGS. 4A through 4D  are illustrations of an exemplary embodiment of an exemplary lid. 
         FIG. 5  is an illustration of an exploded view of an exemplary embodiment of a mold set-up. 
         FIG. 6A  is an illustration of an exploded view of an exemplary for ejecting ceramic cast parts. 
         FIG. 6B  is an illustration of an exemplary method of ejecting a ceramic cast part from a mold. 
         FIGS. 7A, 7B, and 7C  are illustrations of an exemplary embodiment of a pressure casting apparatus and mold set-up. 
         FIG. 8 . is an illustrative representation of a traditional multi-directional casting process for casting a ceramic slurry in a porous mold. 
         FIGS. 9A and 9B . are representations of a ceramic body cast and removed by traditional pressure casting and removal techniques. 
         FIGS. 10A, 10B, and 10C  are illustrations of an exemplary embodiment of a ceramic casting system and method of ejecting a ceramic cast part from a mold. 
     
    
    
     DETAILED DESCRIPTION 
     A method and an apparatus are provided for pressure casting a ceramic body. Further, a method and apparatus are provided for the rapid ejection of the cast ceramic body from a mold. A mold set-up comprising an impermeable cavity surface and a porous casting substrate facilitates a ‘dry release’ of the cast object after pressure casting. Ceramic bodies produced herein may have smooth, non-tacky surfaces after ejecting from the mold, and reduced cracking and breakage after drying. The resulting ceramic bodies may be suitable for use as ceramic mill blocks that accommodate single or multi-unit restoration bodies, including, but not limited to crowns, veneers, bridges, dentures, and the like. 
     With reference to  FIGS. 1A and 1B , a pressure casting set-up for unidirectional casting is illustrated. A mold set-up  100  is shown that comprises a mold cavity  101  in which a cast body is formed. A porous casting substrate  102  on which ceramic particles are deposited and a mold  103  define the mold cavity  101 . An inlet  105  is provided through a mold top surface  106  which a ceramic slurry is delivered under pressure, and liquid from the slurry passes through the casting substrate  102  into the porous body  107 . An opening of the mold cavity  101  is in communication with the porous casting substrate  102 , and the cast ceramic body  108  is ejected through the opening after casting. The surface of the mold cavity perpendicular is impermeable to liquid and solid components of the slurry during the casting process. Optionally, the top surface of the mold cavity is also impermeable to the slurry components. 
     The mold  103  comprises a material suitable for resisting penetration of the liquid or solid components of the ceramic slurry during a pressure casting process while maintaining dimensional stability of the mold cavity. Suitable materials include, but are not limited to polytetrafluoroethylene, alumina, acetyl plastic, and the like. In one embodiment, the mold comprises a rigid, monolithic structure wherein a solid block of material, such as plastic is milled to form the cavities. In another embodiment, the mold comprises a porous or permeable structure having an impermeable coating on cavity-facing side surfaces. 
     An impermeable mold  200  is exemplified in the illustrations of  FIGS. 2A through 2D . A mold  200  may comprise a single mold cavity  201 , or more than one mold cavities  201  for simultaneously casting multiple ceramic bodies. Cavity-facing side surfaces  202  of the mold are impermeable to both the liquid and solid components of the ceramic slurry under pressure casting conditions, throughout the casting process. As seen in  FIG. 2A , one or more inlet ports  203  extend through the top  204  of the mold to deliver ceramic slurry into the cavities  201 . A lower surface  205  of the mold shown in  FIGS. 2C and 2D , is in intimate contact with the porous casting substrate (illustrated in  FIG. 3B, 306 ), when the two components are assembled. 
     The porous casting substrate comprises a median pore diameter that is sufficiently small to inhibit movement of ceramic particles into the pore volume of the casting substrate. Ceramic particles form a layer on the porous casting substrate and continue to build in thickness, as liquid from the ceramic slurry is removed by passing through ceramic particles into the porous casting substrate. The porous casting substrate  102  may have a submicron median pore diameter. In some embodiments, the median pore diameter is less than 3 μm, such as less than 1 um, or between 0.1 μm and 0.6 μm. In some embodiments, the ratio of median pore diameter of the casting substrate to median particle size of the ceramic component may be from 10:1 to 1:1, such as from 5:1 to 1:5. 
     The porous casting substrate may comprise one or more porous materials including, but not limited to plaster, such as gypsum, or a porous plastic such as hydroxypropyl cellulose, or copolymers of acrylic acid and methacrylic acid, and the like. The casting substrate should have sufficient rigidity to maintain the cavity shape during the casting process. In other embodiments, the porous casting substrate may comprise a filter paper, cloth or membrane backed by a supporting structure or material, such as a metal filter. 
     In a further embodiment, as illustrated in  FIGS. 3A through 3C , an apparatus  300  comprises a mold  301  and a slurry dispensing lid  302 . The slurry dispensing lid engages with an upper surface  309  of the mold  301 . Slurry introduced to an upper surface  307  of the lid  302  is dispensed under pressure into mold cavities  304  through passages  308  that align with inlet ports  203  in the mold  301  when the lid and mold are assembled. As exemplified in the exploded view of  FIG. 3B , the dispending lid  302 , mold  301  and porous casting substrate  306  form an apparatus. A bottom surface  303  of the mold and openings of mold cavities  304  are in intimate contact with the porous substrate  306  when assembled. 
       FIG. 3C  illustrates a further embodiment of a mold  313 , in which a single ceramic body may be formed in the shape of a large disk. A top surface of the mold may attach to a lid  302  and slurry may be delivered from the top of the dispensing lid  312  under pressure through an opening  311  in the top of the mold  313 . A mold cavity in which the ceramic body is formed, comprises an a mold wall  314  and a cavity opening  315  that are in direct communication with a porous casting substrate when assembled. After casting, the ceramic body is ejected through the mold opening  315  after separating the porous casting substrate from the mold. 
     In one embodiment, a dispensing lid  400  as illustrated in  FIGS. 4A through 4D , comprises an upper surface  401  having openings  402  that connect to passages  403  through the thickness of the dispensing lid. A passage  403  may overlay an inlet port  203  of the mold cavity. In an alternative embodiment, the opening  402  may form a passage  403  through a plug  405  that engages with the inlet  203  on the mold. In one embodiment, the lower surface  404  of the slurry dispensing lid  400  comprises a raised surface that forms an edge  406  providing a gap  310  between the mold  301  and lid  302  when assembled. The edge  406  may facilitate separating the lid from the mold after casting. In a further embodiment, the upper surface  401  of the dispensing lid may be substantially flat, or may comprises a rim  408 , for example, for holding slurry or securing a slurry pot. 
     In a further embodiment, the mold cavity may be open on both the bottom and the top surfaces. The dispensing lid may serve as the top surface of the mold cavity when the lid and mold are assembled. 
     In  FIG. 5 , an exemplary embodiment of a mold-set up  500  is illustrated in exploded view. A mold  501  is positioned between a porous casting body  502  and a lid  503 . Mold cavities  504  have a lower opening that is in communication with an upper surface  505  of the porous substrate, and inlet ports  508  on the top surface that are in communication with passages of the dispensing lid  503 . Slurry is dispensed into the lid upper surface  506  through passages  507  that align with inlet ports  508  to fill the cavity  509 . In one embodiment, the passages  507  fit within cavity inlet ports  508 . In another embodiment, the passages  507  encircle the cavity inlet port. 
     After pressure casting, the slurry dispensing lid  503  may be removed. Ceramic from the casting process may build up within the passages and form a column of ceramic material within and between the inlet port and the passages. Ceramic columns may be broken by removing the lid from the mold. In one embodiment, both the slurry dispensing lid  503  and the porous casting substrate  505  are separated from the mold after casting. A force may be applied directly through the inlet ports to the ceramic body within the cavity to eject the ceramic body through the cavity opening  504 . 
     In another embodiment, the slurry dispensing lid is replaced with a release apparatus  600  to commence an ejection process. In the embodiment illustrated in  FIG. 6A , passages  601  extend through the thickness of the release apparatus  600  and align with mold cavities  602  in the mold  603 . A force may be applied through the passages  601  into openings  604  on the top surface  603  of the mold, and into the cavities  602 . For example, a gas, such as compressed air, may be delivered through passages  601  into the cavities, and/or onto the ceramic bodies  605 , as illustrated in  FIG. 6B , to eject the cast ceramic bodies  605  from an opening in the bottom of the cavity. 
     In some embodiments, a pressure of 0.5 psi or greater, such as approximately 5 psi or greater, such as from 7 psi to 15 psi, or from 20 psi to 80 psi, or from 20 psi to 30 psi, may be introduced into the cavity to eject the ceramic body from the mold. Pressure may be applied, for example, by mechanical piston, or compressed gas, such as compressed air. In one embodiment, compressed air may be delivered, for example via one or more hoses, fittings, nozzles and the like, that align with passages in the release apparatus. Simultaneously or sequentially ejection of multiple ceramic cast bodies from the mold may be suitable for use in continuing manufacturing processes. 
     In some embodiments, vacuum may be used instead of, or in addition to, the positive pressure applications described above. Vacuum may be applied and delivered, for example, via one or more hoses, fittings, nozzles and the like that align with passages at the exit of the mold cavity in order to facilitate dry ejection of the ceramic cast bodies from the mold. 
     Where the mold is comprised of an impermeable material that does not retain liquid and/or solid components of a slurry during a casting process, the ceramic body is ejected in a dry release process. In this embodiment, the ceramic body is ejected without releasing water from the mold and into the cavity. Thus, in one embodiment, a dry release process is used for ejecting the cast part from the mold cavity wherein the ejected body is not floated off the mold cavity-facing surface with liquid. In other embodiments, the dry ejection process may be used for slip casting methods such as vacuum casting. 
     Ceramic bodies may be ejected from the mold without drying to remove residual liquid component from the cast ceramic body. In some embodiments, ceramic bodies ejected from the mold comprise more than 8 wt % residual liquid, such as between 8 wt % and 15 wt % liquid, or between 10 wt % and 12 wt %, residual liquid from the slurry. In some embodiments, the ceramic parts are ejected without shrinkage, and have substantially the same dimension as the inner diameter of the mold, after ejection and prior to drying. In some additional embodiments, the ceramic parts are ejected prior to drying while having less than 2.0% shrinkage, such as less than 1.0% shrinkage, such as less than 0.5% shrinkage, such as less than 0.1% shrinkage, such as less than 0.05% shrinkage relative to the inner dimension (e.g., diameter) of the mold. 
     A release agent may be applied to one or more surfaces of the mold cavity. Release agents include but are not limited to lubricants such as petroleum jelly, oleic acid, and the like, for example, that are at least partially insoluble under casting conditions. 
     Casting and ejection methods described herein may be used with ceramic slurries that include, but are not limited to, alumina, zirconia, boron carbide, silicon carbide, spinel, and barium titanate. Ceramic slurry may comprise ceramic materials suitable for use in manufacturing dental restorations, such as crowns, veneers, bridges and dentures. In some embodiments, ceramic material suitable for use in dental applications may comprise zirconia, alumina, or combinations thereof. Zirconia ceramic material may comprise stabilized, partially stabilized or fully stabilized zirconia ceramic material. 
     In some embodiments, zirconia powders may include yttria-stabilized zirconia that has been stabilized with approximately 0.1 mol % to approximately 8 mol % yttria, such as approximately 2 mol % to 7 mol % yttria, or approximately 2 mol % to approximately 4 mol % yttria, or approximately 4 mol % to approximately 6 mol % yttria. Specific examples of yttria-stabilized zirconia powders include yttria-stabilized zirconia commercially available from Tosoh USA, such as Tosoh TZ-3YS (containing 3 mol % yttria, or 3Y), Tosoh PX485 (containing 4 mol % yttria, or 4Y), and Tosoh PX430 (containing 5-6 mol % yttria, such as 5.5Y. Commercially available zirconia powder may have a measured particle size D(50) of about 600 nm or more, which constitute agglomerations of particles of crystallites having an actual particle size of about 20 nm to 40 nm. As used herein, the term “measured particle size” refers to measurements obtained by a Brookhaven Instruments Corp. X-ray disk centrifuge analyzer. The comminution processes described herein may reduce the measured particle size of the zirconia powder contained in the slurry from the D(50)&lt;600 nm, to a range of D(50)=100 nm to 400 nm, such as D(50)=200 nm to 300 nm. 
     In additional embodiments, zirconia powders may have a measured particle size D(50) of 100 nm or less. The comminution processes described herein may be used to reduce the measured particle size of the zirconia powder contained in the slurry from the D(50)&lt;100 nm, to a range of D(50)=20 nm to 90 nm, such as D(50)=30 nm to 70 nm. These zirconia powders may include yttria-stabilized zirconia that has been stabilized with approximately 0.1 mol % to approximately 8 mol % yttria, such as approximately 2 mol % to 7 mol % yttria, or approximately 2 mol % to approximately 4 mol % yttria, or approximately 4 mol % to approximately 6 mol % yttria. Specific examples of yttria-stabilized zirconia powders include yttria-stabilized zirconia commercially available from Inframat Corporation, USA, such as 4039ON-9501, 4039ON-9502 (containing 3 mol % yttria, or 3Y), 4039ON-8601 (containing 8 mol % yttria, or 8Y) or mixtures thereof. 
     In additional embodiments, the nano powders are mixed with submicron powders, such as in a comminution process described herein to get a mixed powder with a D(50)&lt;200 nm, to a range of D(50)=20 nm to 180 nm, such as D(50)=40 nm to 100 nm. 
     The yttria-stabilized zirconia powders may include alumina at a concentration of 0 wt % to 0.25 wt %, such as 0.1 wt %, relative to the zirconia powder. Optional additives include coloring agents and esthetic additives, such as metal oxides and metal salts, or other metal-containing compounds used to obtain dentally acceptable shades in final sintered restorations. In some embodiments further processing aids such as binders and dispersants may added to the slurry. 
     Dispersants suitable for use in casting the green body promote dispersion and stability of the slurry, and controlling the viscosity of the slip. Dispersion and deflocculation occur through electrostatic, electrosteric, or steric stabilization. Examples of suitable dispersants include nitric acid, hydrochloric acid, citric acid, diammonium citrate, triammonium citrate, polycitrate, polyethyleneimine, polyacrylic acid, polymethacrylic acid, polymethacrylate, polyethylene glycols, polyvinyl alcohol, polyvinyl pyrillidone, carbonic acid, and various polymers and salts thereof. These materials may be purchased commercially or prepared with well-known techniques. Specific examples of commercially available dispersants include Darvan® 821-A ammonium polyacrylate dispersing agent commercially available from Vanderbilt Minerals, LLC; Dolapix™ CE 64 organic dispersing agent and Dolapix™ PC 75 synthetic polyelectrolyte dispersing agent commercially available from Zschimmer &amp; Schwarz GmbH; and Duramax™ D 3005 ceramic dispersant commercially available from Rohm &amp; Haas Company. 
     The liquid component may comprise water, organic solvent, inorganic solvent, and combinations thereof. A ceramic slurry comprising a liquid component and a ceramic component, may comprise a ceramic loading of between 20 wt % and 90 wt %, such as between 40 wt % and 80 wt %, based on the total weight of the ceramic slurry. Zirconia powder and dispersant are added to the liquid component, such as water, to obtain a slurry. The slurry may be subjected to a comminution process by which the zirconia powder particles are mixed, deagglomerated, and/or reduced in size. Comminution is performed using one or more milling processes, such as attritor milling, horizontal bead milling, ultrasonic milling, or other milling or comminution process, such as high shear mixing, ultra high shear mixing capable of reducing the zirconia powder particle sizes described herein. 
     Ceramic slurry dispensed into the mold may be cast at a pressure up to approximately 1000 psi, such as from 20 psi to 600 psi, or 40 psi to 600 psi, or 40 psi to 300 psi, or greater than or equal to approximately 50 psi, such as 50 psi to 600 psi, or 50 psi to 200 psi. Devices for casting the slurry under pressure, include, but are not limited to, commercially available pressure casting machines for casting ceramics, or a pressure pot coupled to an air condenser. In a further embodiment, vacuum may be applied to the porous mold to aid removal of the liquid component. 
     In one embodiment, a method for making a ceramic block is provided that comprises i. providing a pressure casting apparatus that comprises a mold having an impermeable cavity surface and a cavity opening that is in direct contact with a porous casting substrate; ii delivering a ceramic slurry into the mold cavity and casting the slurry under pressure greater than 20 psi; iii. removing liquid from the slurry under pressure via the porous casting substrate; iv. consolidating ceramic particles forming a ceramic green body within the cavity; v. removing the porous casting substrate from the apparatus to expose a mold opening; and vi. ejecting the resulting ceramic green body from the mold opening by a dry release process using a pressure greater than 20 psi. 
     Cast ceramic bodies made by the methods described herein may have a smallest dimension greater than 10 mm, or greater than 15 mm, or greater than 20 mm. In one embodiment, a ceramic green body in the shape of a solid mill block or disk having a thickness greater than 10 mm is formed from a binderless slurry of yttria-stabilized zirconia with an average particle size of less than or equal 300 nm. The resulting body has sufficient green strength to withstand the ejection processes described herein, and may further withstand handling and/or molding or shaping before sintering. 
     In other embodiments, zirconia ceramic powder having a median particle size less than or equal to 300 nm are directly cast to form solid ceramic bodies having thin walls (e.g., 0.5 mm to 5 mm, or 0.5 mm to 2 mm, or 0.5 mm to 1 mm), non-uniform shapes, non-uniform sizes, or irregular cross-sectional dimensions such as near net shape mill blanks, wafers, tabs, rods, or dental restorations such as crowns, veneers, bridges, and dentures. 
     Pressure-casting processes described herein significantly reduce casting time compared to known vacuum casting processes. Dry ejection processes described herein significantly reduce the time for removing the cast ceramic body from the mold; by eliminating a drying step, a ceramic body comprising more than 8 wt % of liquid may be ejected within seconds of completing casting. Thus, the methods described herein for producing strong green bodies comprising yttria-stabilized zirconia having small particle size are suitable for use in automated manufacturing processes. 
     EXAMPLES 
     Example 1 
     A zirconia ceramic body suitable for use in milling single or multi-unit dental restorations was formed by a pressure casting process and ejected by a dry release process. 
     A pressure-casting apparatus  700  substantially according to  FIGS. 7A through 7C , was used to cast a zirconia ceramic slurry. As seen in  FIG. 7A , the apparatus comprised a mold  701  having a single cavity with dimensions of 98 mm diameter×30 mm depth. The mold cavity was made from acetyl plastic, and the mold dispensing lid  702  was comprised of a separate component made from acetyl plastic. The plastic material of the mold cavity was impermeable to the slurry during the casting process. As seen in  FIG. 7B , a porous polymer casting body  703  having a porous casting substrate (with a median pore size of 1 μm), was placed on top of the mold cavity, prior to inverting the set-up for the casting process. 
     As illustrated in  FIG. 7C , the mold set-up was inverted, so that the mold lid  702  was above the mold  701  which rested on the porous casting substrate of the plaster body  703 . A ceramic slurry was prepared comprising water, dispersant, and an yttria-stabilized zirconia dental ceramic having a median particle size 280 nm (D50). The binderless slurry had a ceramic loading 79 wt % and was placed in a slurry pot  704  which was on top of the mold lid  702  after inverting the set-up. The ceramic slurry was dispensed from the slurry pot and through an inlet  105  in the mold lid  702  filling the cavity  101  at a casting pressure of approximately 174 psi. 
     As illustrated in the casting process of  FIG. 1 , casting proceeded unidirectionally (according to the direction of the arrows) by delivery of the slurry towards the porous casting substrate  102  from the top of the impermeable mold  103 , and the thickness of the ceramic increased uniformly throughout the x-y direction of the cast body. 
     A ceramic green body (98 mm diameter×25 mm thickness) was cast in 2.5 hours with a pressure of approximately 174 psi. Upon completion of the casting process, the porous casting substrate was removed from the apparatus to expose the cavity opening through which the ceramic body was ejected. The ceramic body was ejected by a dry release process with the application of air pressure  104  of 40 psi into the cavity supplied by an airline inserted through the inlet  105  at the top surface of the plastic mold. No water was pushed into the cavity from the mold during the ejection process. After ejection, the ceramic body had a dimension substantially the same as the inner diameter of the mold, and comprised about 10 wt % of liquid from the casting process. 
     The resulting green body had a smooth surface, lacking cracks or breakage, and uniform build-up of the ceramic material through the x-y direction of the ceramic body. 
     Example 2 
     A nano zirconia ceramic body suitable for use in milling single dental restorations was formed by a pressure casting process and ejected by a dry release process. 
     A pressure-casting apparatus substantially according to  FIGS. 10B and 10C , was used to cast a zirconia ceramic slurry. As seen in  FIG. 10B , the apparatus comprised a mold  1003  having seven cavities each with ˜60 sq. mm cross section and a 16 mm depth. The mold cavity was made from acetyl plastic, and the mold dispensing lid  1000  was comprised of a separate component made from acetyl plastic. The plastic material of the mold cavity was impermeable to the slurry during the casting process. The mold cavity was located on top of a porous polymer casting body  1006  having a porous casting substrate (with a median pore size of 1 μm). 
     As illustrated in  FIG. 10C , the mold lid  1000  was above the mold  1003  which rested on the porous polymer casting body  1006 . A ceramic slurry was prepared comprising water, dispersant, and an yttria-stabilized zirconia dental ceramic having a median particle size 65 nm (D50). The binderless slurry had a ceramic loading 50 wt % and was placed in a slurry pot  1007  which was on top of the mold lid  1000  after inverting the set-up. The ceramic slurry was dispensed from the slurry pot and through the inlet  1001  in the mold lid  1000  filling the cavity  1002  at a casting pressure of approximately 174 psi. 
     The casting proceeded unidirectionally by delivery of the slurry towards the porous polymer and the thickness of the ceramic increased uniformly throughout the x-y direction of the cast body. 
     A ceramic green body (60 sq.mm cross section by 9 mm thickness) was cast in 5 hours with a pressure of approximately 174 psi. As shown in  FIG. 10A , upon completion of the casting process, the porous backing was removed from the apparatus to expose the cavity opening through which the ceramic body  1005  was ejected. The ceramic body was ejected by a dry release process with the application of air pressure of 80 psi into the cavity supplied by an airline inserted through the inlet  1004  at the top surface of the plastic mold. No water was pushed into the cavity from the mold during the ejection process. After ejection, the ceramic body had a dimension substantially the same as the inner diameter of the mold, and comprised about 15 wt % of liquid from the casting process. 
     The resulting green body  1005  had a smooth surface, lacking cracks or breakage, and uniform build-up of the ceramic material through the x-y direction of the ceramic body. 
     (Comparative) Example 3 
     A traditional pressure casting apparatus was used to cast a ceramic slurry. The apparatus comprised a porous two-part mold, having a top part  801  and a bottom part  802  that formed a mold cavity  800  with dimensions of 98 mm diameter×30 mm depth when assembled. The median pore size of the porous mold was less than 1 μm. 
     A ceramic slurry substantially according to Example 1, was delivered into the cavity  800  through an opening  803  at a casting pressure of 174 psi. Casting occurred multi-directionally, as ceramic particles deposited on multiple surfaces and corners (e.g., side, top and bottom) of the mold cavity surface, as illustrated in by the arrows in  FIG. 8 . All cavity surfaces were porous, and contained water and slurry within the porous mold from the casting process. Water was also removed from the porous mold through a space  804  in the porous mold. 
     A ceramic green body (98 mm diameter×25 mm thickness) cast in 40 minutes. The mold top  801  and mold bottom  802  were separated to expose an opening in the top of the mold. Air pressure was delivered to the space  804  in the mold through an air fitting  805  forcing water and slip back through the porous mold structure, into the cavity between the ceramic green body and the mold cavity surface to float the ceramic part off the cavity surface through the top of the mold. 
     As illustrated in  FIGS. 9A and 9B , the resulting green body  900  had a tacky surface, which resulted in cracks on the surface. Ceramic build-up was greatest on top, bottom and side surfaces  901  with less ceramic build-up in the center  902  of the body. Uneven distribution of ceramic particles throughout the body resulted in a weaker center causing breakage during drying (e.g., at ambient temperature, or oven at 30° C.).