Patent Application
20110014589
This application claims priority to United Kingdom Patent Application No. 0805052.8 filed Mar. 19, 2008.
The invention relates to a method for making a dental blank of a ceramic material, as typically used for making dental restorations. The invention also relates to a press and a system for making dental blanks.
Dental restorations are often made of ceramic materials because ceramic materials generally provide relatively good physical, aesthetic and biological properties as commonly desired in the field of dentistry. Examples of such dental restorations are crowns, bridges, inlays, onlays, veneers, implants. One way to make dental restorations of ceramic materials includes the use of an automated process, in which a dental restoration precursor is machined from a ceramic blank. A type of blank as it is typically used in such process is made of pressed and pre-sintered ceramic particles. Relative to the use of a solid ceramic material, a so formed dental blank is porous and relatively soft so that it can be machined relatively easily. On the other hand such blank is sufficiently stable so that it can be handled in a machine.
Dental blanks for making dental restorations may be formed by pressing ceramic particles, like a ceramic powder, at relatively high pressure so that the ceramic particles block with each other and form a generally cohesive body of material. A subsequent pre-sintering step typically adds further mechanical stability to the body and thereby forms the dental blank which can then be used for producing a dental restoration precursor.
A dental restoration precursor obtained from such pressed and pre-sintered blank is typically further sintered, and subsequently polished or furnished with a veneer. Typically the precursor shrinks, generally proportionally, during sintering because the initially porous material reduces in porosity and increases in density. For this reason the restoration precursor may be initially larger, for example about 20% to 30% in each dimension, than the desired final shape after sintering, to account for shrinkage during the sintering step. It has been found desirable that the material structure of the blank is of a generally uniform density. This is because a non-uniform density or inhomogeneity of the blank material may cause the dental restoration precursor obtained from that blank to shrink non-uniformly in one or more dimensions during sintering. Thus, the precision of the final dental restoration may be adversely affected, resulting in wasted time and expense for a dentist.
To achieve a relatively uniform density of the blanks the ceramic particles are often pressed by isostatic pressing techniques. Isostatic pressing typically is based on the use of a hydraulic fluid, for example oil, water or an emulsion of both, to apply pressure to the ceramic particles generally uniformly from all sides, or isotropically. Typically isostatic pressing is used with cylindrical blanks because the cylindrical shape provides for relatively isotropic compaction, and therefore provides for a relatively homogeneous inner structure of such blanks However, there have been efforts to find alternative pressing techniques that are less expensive, and/or which can be reliably used for different blank shapes.
SE-7807661 describes a tool for isostatic pressing. The tool has an upper and a lower tool component which can be combined to form a mold. Each of the tool components have a membrane which join to form a closed space within the mold when the tool components are combined. A material to be pressed may be accommodated in the space between the membranes. Each of the tool components provide for a pressure medium to be supplied between the mould walls and the outside of the membranes, so that the pressure medium can be used to pressurize the membranes for pressing of the material accommodated in between.
It is still desired to manufacture blanks having any desired shape with a consistently high degree of homogeneity of the inner material. Also, it is a general desire to provide a relatively inexpensive manufacturing process. Especially for dental purposes there is also demand for a manufacturing process that provides for maintaining a high hygiene level during production.
The invention provides a method, a press and a system for making a dental blank. Preferably the dental blank is comprised of ceramic particles of a ceramic material based on zirconium or aluminum oxide. The dental blank is preferably usable in the preparation of dental restorations.
According a first aspect, the invention provides a method of making a dental blank. The method comprises the steps of:
(a) pressing ceramic particles to form a dental blank precursor; and
(b) pressing the dental blank precursor to form the dental blank,
wherein one of the steps (a) and (b) includes uniaxial pressing and the other one of the steps (a) and (b) includes isostatic pressing. Preferably the step (b) includes isostatic pressing. Step (a) accordingly preferably includes uniaxial pressing.
The method preferably provides at least to some extent for independent controlling of the general shape, and the material homogeneity of the dental blank. For example, one pressing step may be adapted and/or controlled to achieve a relatively precise general shape of the blank. In a subsequent pressing step may be adapted and/or controlled to increase the material homogeneity of the blank, preferably without substantially changing the shape of the blank. This may be advantageous because relative to a single continuous process the invention preferably allows splitting of the blank manufacturing process into separate steps, each allowing the application of appropriate parameters (for example pressures, pressing speeds etc.), and the use of appropriate tooling. The invention, for example, may provide for pre-shaping blanks in a relatively easy, robust and inexpensive process. The so formed blank precursors may be subsequently increased in their material homogeneity rather effectively in a process that would probably be less effective in shaping. The invention may therefore provide for minimizing the overall process time required for making dental blanks. Further the invention may provide for a relatively homogeneous inner material structure of the dental blanks independently from their shape. The invention may also provide a cost effective method of making blanks, and further may provide for dental blanks to be manufactured at relatively high quality.
Preferably uniaxial and isostatic pressing are only used separate from each other. For example, isostatic pressing may only be performed when uniaxial pressing is inactive. Further uniaxial pressing may only be performed when isostatic pressing is inactive. In particular the step (a) may comprise a step of releasing the dental precursor from pressure. Therefore the material of the dental blank precursor may relax between steps (a) and (b). For example, the material density of the dental blank precursor may decrease during relaxation. This may provide for a better material homogeneity of the dental blank in the step (b).
Preferably uniaxial pressing comprises pressing of the ceramic particles or the dental blank precursor from generally mainly opposite sides, for example two opposite sides, preferably by moving at least two pressing dies towards one another. Preferably such two pressing dies during pressing also move relative to a die plate which laterally restrains the ceramic particles or the dental blank precursor.
During step (b) “pressing the dental blank precursor to form the dental blank” the dental blank precursor is preferably in touch with a pressing member. The pressing member is preferably arranged between the dental blank precursor and a hydraulic fluid. Further the pressing member preferably transmits pressure received from the hydraulic fluid to the dental blank precursor. The pressing member is preferably adapted to generally conform to the shape of at least a portion of the dental blank precursor. This also includes that the pressing member is adapted to generally conform to a shape change of the portion of the dental blank precursor. Such change of the shape may, for example, result from a compression of the dental blank precursor during the step (b). Therefore the pressing member is preferably also adapted to generally conform to the shape of a portion of the dental blank. The pressing member is preferably flexible or deformable. In particular the pressing member may be a membrane, preferably a flexible membrane.
Preferably the general shape of the dental blank is mainly provided by the step (a) “pressing ceramic particles to form a dental blank precursor”. In contrast, the dental blank preferably does not obtain its general shape as a result of an initial shape of the pressing member used in step (b). In one embodiment step (a) provides the dental blank precursor with a shape that substantially corresponds to a proportionally enlarged shape of the dental blank. Further, step (b) may substantially proportionally (in three dimensions) reduce the dental blank precursor provided in step (a) in shape. For example, the dental blank precursor may be shaped generally cuboid in step (a). Then the cuboid shape may generally be maintained in step (b), but the length, width and height of the cuboid may be reduced proportionally. Other shapes, however, are possible like cylindrical shapes, or shapes having an elliptical or other suitable profile. Generally any such profile (including a rectangular and circular profile) may extend generally straight or curved to form the overall shape of the dental blank precursor and/or blank.
In one embodiment step (a) increases the material density of the bulk of ceramic particles used to form the dental blank precursor. Preferably step (a) increases the material density of a bulk of ceramic particles by a factor of between about 1.7 and 3, preferably by a factor of between about 2.2 and 2.3. The material density of the dental blank precursor of the ceramic particles is preferably between about 2.5 g/cm3 and 3.0 g/cm3, preferably about 2.9 g/cm3. In another embodiment step (a) increases the material density of a bulk of glass ceramic particles by a factor of between about 1.5 and 6.7, preferably by a factor of between about 2.9 and 3.1. The material density of the dental blank precursor of the glass ceramic particles is preferably between about 1.2 g/cm3 and 2.0 g/cm3, preferably about 1.5 g/cm3.
In another embodiment step (b) increases the material density of the dental blank relative to the dental blank precursor. Preferably step (b) increases the material density of the dental blank precursor of a ceramic material by a factor of between about 1.02 and 1.4, preferably by a factor of between about 1.08 and 1.1. The material density of the dental blank of a ceramic material is preferably between about 2.9 g/cm3 and 3.4 g/cm3, preferably about 3.15 g/cm3. In another embodiment step (b) increases the material density of the dental blank precursor of a glass-ceramic material by a factor of between about 1.02 and 1.9, preferably by a factor of between about 1.1 and 1.3. The material density of the dental blank of a glass-ceramic material is preferably between about 1.6 g/cm3 and 2.2 g/cm3, preferably about 1.7 g/cm3.
Other factors and densities are possible as appropriate for other materials used with the invention.
In another embodiment step (b) may be performed with a plurality of dental blank precursors generally simultaneously or in parallel. The step (b) may be performed generally simultaneously in the same press and/or in two or more presses. This may allow minimizing the cycle time required for pressing, for example. In one embodiment of the invention the method may comprise a step (c) of placing a plurality of dental blank precursors in different predetermined positions in a press. Dental blank precursors may, for example, be placed arranged in a generally two-dimensional pattern, for example side by side along two rows, on the pressing member.
In another embodiment of the invention at least two steps of the method of the invention are performed automated in a sequence. A preferred sequence of steps is step (a), and step (b), in the order as listed. Another sequence of steps may be step (a), step (c), and step (b), in the order as listed. The sequence of steps may also be repeated. A method of the invention may thus comprise:
Such a method may be advantageous for concatenating a method step comprising uniaxial pressing with an a method step comprising isostatic pressing. For example, this may allow for concatenating an uniaxial press inline with an isostatic press in case one of the isostatic or uniaxial presses has a longer cycle time relative to the other.
In another embodiment the ceramic material has a relatively low content of binders, or is generally free of binders. Preferably the amount of binders is below 5% by weight, for example between about 2% to 4%. A ceramic material as it may be used with the invention may comprise between 90 and 99% by weight zirconium oxide, and preferably 91 to 97.25% by weight zirconium oxide. The ceramic material may further comprise 0-1% by weight aluminium oxide. The ceramic material may also be based on aluminium oxide, meaning the ceramic material may comprise 90 to 99% by weight aluminium oxide and 0 to 1% by weight zirconium oxide. Further, the ceramic material may comprise 0-10% by weight of at least one of hafnium oxide, yttrium oxide and oxides from gallium, germanium, and indium, as well as 0.0005 to 1.5% by weight of colouring additives, selected from the group consisting of the oxides Fe2O3, Er2O3 and/or MnO2. The ceramic material is preferably selected to be compatible for use in human bodies.
In another aspect the invention relates to an isostatic press for making a dental blank. The isostatic press comprises:
Preferably the pressing members in the area in which they encase the dental blank are generally not supported by solid parts of the mold, in particular in a stage in which the pressing members are separated.
In one embodiment of the invention the first and second pressing members are flexible. Preferably one or both of the first and second pressing members have a thickness of between about 0.05 mm and 5 mm, preferably between about 0.05 mm and 0.10 mm.
In another embodiment the size and shape of the chamber generally corresponds to a cuboid having dimensions of between about between 10 mm×10 mm×10 mm and 30 mm×200 mm×200 mm, preferably between about 25 mm×25 mm×40 mm and 40 mm×25 mm×70 mm. The general size of the chamber may be provided mainly by a correspondingly sized receptacle in one of the first and the second pressing members, or by a correspondingly sized receptacle formed between both pressing members.
For example, the first pressing member may have a receptacle which is covered by the second pressing member when the first and second pressing members are combined. In this case, the first pressing member may have a receptacle of a generally cuboid shape, and the receptacle in size and shape may generally correspond to the size and shape of the chamber. Further, the corresponding second pressing member may be generally flat in at least an area covering the receptacle. Therefore a dental blank or blank precursor placed in the receptacle may be typically generally flush with the opening of the receptacle.
In another embodiment the first or the second pressing member may have a plurality of receptacles and the corresponding other pressing member may be generally flat in at least the areas covering the receptacles.
Further, the first and the second pressing members may have a first and second plurality of receptacles, respectively, with the first and second receptacles in combination forming chambers for encasing dental blank precursors. Therefore dental blanks or blank precursors received in the receptacles may project over the openings of the receptacles. This may provide the advantage of facilitating gripping of the blanks or blank precursors for insertion in or removal from the pressing members. This may be particularly advantageous if the press is used in an automated process.
In another embodiment the first and second pressing members encase a dental blank precursor. In this case preferably all sides of a dental blank precursor are surrounded by and in contact with the at least one of the first and second pressing members.
In another embodiment of the invention the press may be adapted to perform at least step (b) of the method of the invention.
Another aspect of the invention is related to a system for making dental blanks. The system comprises:
The system may further comprise a pick and place system for moving dental blank precursors between the isostatic press and the uniaxial press. Preferably, the pick and place system is adapted for moving dental blank precursors from the uniaxial press towards the isostatic press. The pick and place system thereby may also be indirectly coupled with one or both of the uniaxial and isostatic press. For example, the dental blank precursors may be stored in an output buffer associated or connected with the uniaxial press, and fed from the output buffer to the pick and place system. The pick and place system may then load the dental blank precursors into an input buffer associated or connected with the isostatic press. A system combining a uniaxial and a isostatic press may provide for an automated manufacturing of dental blanks. As an advantage the throughput of the manufacturing process may be maximized. Further, the automatic handling may provide for maximizing the hygiene during manufacturing of dental blanks because manual handling steps may be minimized. Such system may also allow for optimizing the cooperation of the two different pressing techniques according to the invention.
Still another aspect of the invention is related to a kit, comprising at least a part of an isostatic press, and instructions for connecting the isostatic press and an uniaxial press. This may provide for easy adaptation of an isostatic press with an uniaxial press, for example, to facilitate implementation of the method of the invention in a manufacturing plant for making dental blanks
The invention is described in the following by way of example only with reference to the accompanying figures, in which:
The terms “upper”, “lower”, “top” and “bottom” as they may be used to designate locations or parts in this specification are used for ease of explanation only. The so designated parts or locations may in other examples be arranged differently, for example at any angle or orientation, as appropriate.
In contrast to
For pressing the dental blank precursor 16 the fluid may preferably be pressurized to pressures of between about 10 MPa and 1000 MPa, in more particular between about 50 MPa and 700 MPa, preferably between about 100 MPa and 400 MPa.
As shown, the dental blank precursor 16 is almost entirely surrounded by the hydraulic fluid with only the relatively thin and flexible pressing members arranged between. Therefore the pressing forces arriving at the dental blank precursor are relatively isotropic. This may result in a relatively homogeneous inner material structure of the blank. In one embodiment of the invention the pressing members are considerably thinner than 30 mm, preferably between about 0.05 mm and 5 mm, and in particular preferably between about 0.05 mm and 0.10 mm. Preferably the flexibility of the pressing members is mainly provided by a relatively low thickness of the pressing members in relevant areas rather than by a soft material. Materials as they may be used for a pressing member are, for example polyurethane, polyethylene, polypropylene, but also rubbers, silicones, latex, thermoplastic elastomers, for example. Different materials may also be combined, for example layered. The pressing member may also comprise a coating providing for relatively low surface energy, for example a polytetrafluoroethylene coating. This may help to separate the blank form the pressing member.
The pressing members may comprise a reinforcement layer, for example a wire mesh. Such reinforcement layer is preferably provided in areas of the pressing member that are adapted to encase the blanks or blank precursors.
The pressing members 21, 22 in the example of
An alternative configuration of the pressing members is shown in
The lower part 34 comprises a flexible pressing member 32 having a plurality of receptacles 35. Some of the receptacles 35 are illustrated as containing a dental blank precursor 36. The upper part 33 of the press 30 also comprises a flexible pressing member 31 which in this case is generally flat. The upper and lower pressing members 31, 32 when combined, as shown in
As shown in
The dental blank precursors 36 may be loaded in the lower pressing member 32 automatically, for example by a pick and place system. The dental blank precursors 36 may be delivered from a previous manufacturing step to a pick up location from which they are picked up and placed in empty receptacles of the pressing member 32. After pressing of the dental blank precursors, the so formed dental blanks may be removed by the same or another pick and place system, and the receptacles may be filled again.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0805052.8 | Mar 2008 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US09/37106 | 3/13/2009 | WO | 00 | 9/16/2010 |
