WO2009120749A1

WO2009120749A1 - Cad/cam milling process for the fabrication of dental devices

Info

Publication number: WO2009120749A1
Application number: PCT/US2009/038193
Authority: WO
Inventors: Paul J. Cascone; Arun Prasad
Original assignee: The Argen Corporation
Priority date: 2008-03-28
Filing date: 2009-03-25
Publication date: 2009-10-01

Abstract

A method for fabricating a dental device using an improved milling technique is provided. The method uses a process in which the desired coping is milled from blanks made of a mix of metal powders and a polymer binder prior to densification. The step of densification incorporates debinding and sintering with or without infiltration. The method takes advantage of the advances in CAD/CAM technology to compensate for the shrinkage inherent when using mixes of metal and polymer powder to ensure proper dimensions in the final dental device.

Description

CAD/CAM MILLING PROCESS FOR THE FABRICATION OF DENTAL DEVICES

FIELD OF THE INVENTION

The current invention is directed generally to methods for fabricating dental devices, and more particularly to an improved CAD/CAM milling process for use in the dental industry.

BACKGROUND OF THE INVENTION

Presently, most dental devices are fabricated by hand using an investment casting process. However, the introduction of Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM] technology into the dental industry has made it possible to fabricate on a larger scale dental devices by machine milling of dental alloys. CAD/CAM is used to obtain digital images of a target object from which the object can then be milled. Specifically, the CAD/CAM camera takes a three dimensional image of the target area using infrared waves that are sent down to the area and back to the camera. An optical image is recorded with the 3D camera by projecting an invisible grid onto the field of view to record surface dimensions. As the grid is reflected off the surface, the camera interprets distortions in the grid as differences in height (Z values]. The camera's field of view must have a uniform reflective surface to accurately record the height values. A conventional CAD/CAM milling process generally involves:

1. scanning a die of a device using a scanner; 2. manipulating the scanned data on a computer and generating an electronic file;

3. sending the electronic file to an automated milling machine; and k. milling the device based upon the image in the electronic file. With respect to dental applications, computational analysis of the data captured by the optical image of a tooth allows for precise measurement of the heights of the structure, adjacent teeth and surrounding tissue. Representative examples of such CAD/CAM systems used in the dental field include the CEREC (Chairside Economical Restoration of Esthetic Ceramics) System manufactured by Siemens Dental Products Division and the PROCERA A11 -Ceram system of Nobel Biocare AD. In these systems, optical impressions of cavity preparations for restorative work are obtained and these impressions are then used for the manufacture of inlays, onlays, partial crowns, posterior crowns, anterior crowns, veneers, etc. Using CAD/CAM milling technology, it is possible to automate much of the fabrication process, speeding production and reducing overhead costs.

Despite these advantages, it has been difficult to find alloys that can be efficiently used with the milling process. Indeed, even though the dental marketplace is dominated by devices made of metallic alloys, currently the primary material used for milling dental copings is a ceramic--Zirconia. The reason for this is that current methods for processing alloys via milling are quite inefficient. For example, the metal utilization rate is less than 6% compared to investment casting where the metal utilization rate is over 95%. Also, the time for milling is very long. For example, it generally takes over two hours to mill one coping using a typical non-precious alloy, while milling the same crown in partially-sintered Zirconia takes on the order of 10 minutes. Moreover, softer precious alloys do not reduce the milling time significantly, and for these alloys the large quantity of waste presents a significant economic disadvantage.

SUMMARY OF THE INVENTION

The current invention provides a novel CAD/CAM milling process for fabricating dental devices using alloys of precious and/or non-precious metals.

In one embodiment, the current invention is directed to a method of fabricating a dental device, that includes combining a metal powder and a polymer binder to form a feedstock, fabricating a blank from the feedstock by metal injection molding, milling the blank into a preliminary dental device, and debinding and sintering the preliminary dental device into a final dental device. In such an embodiment, the preliminary dental device is enlarged sufficiently to compensate for the shrinkage caused by the debinding and sintering process.

In an alternative embodiment, the method includes combining a metal powder and a polymer binder to form a feedstock, fabricating a blank from the feedstock by metal injection molding, milling the blank into a preliminary dental device, debinding and partially sintering the preliminary dental device, and infiltrating the preliminary dental device with an additional material to form a final dental device.

In still another embodiment, the shape and dimensions of the preliminary dental device are determined by a CAD/CAM technique.

In yet another embodiment, the preliminary dental device is enlarged by up to about 20%. In such an embodiment the enlargement of the preliminary dental device may be controlled by either or both the ratio of the metal to the binder and/or the particle size distribution of the feedstock. In still yet another embodiment, the final dental device is at least 99% dense. In such an embodiment the density of the final dental device may be controlled by either or both the ratio of the metal to the binder and/or the particle size distribution of the feedstock.

In still yet another embodiment, the method includes collecting any residual feedstock remaining after milling and prior to debinding and sintering, and reprocessing the remaining feedstock for use in a new blank.

DETAILED DESCRIPTION OF THE INVENTION

The current invention addresses the deficiencies of the conventional CAD/CAM milling procedure used in the manufacture of dental devices by combining the CAD/CAM procedure with elements of metal injection molding techniques. Specifically, the invention utilizes metal injection molding as a first step preliminary to a CAD/CAM milling process for the fabrication of dental alloys. This inventive combination eliminates both the wastage of material and the long milling times typical of conventional milling techniques.

Metal injection molding (M IM) is the most commonly used technology to accurately fabricate small parts from a variety of alloys. Exemplary conventional M IM processes are described in U.S. Patent Nos. 5,91 1 , 102; 6,027,686 and 6,444,167, the disclosures of which are incorporated herein by reference. In summary, the conventional M IM process follows the following steps:

1 . A metal powder is blended with a polymer binder into a mixture called a feedstock. 2. The feedstock is injected under high pressure into a prefabricated mold.

Typically the mold contains multiple cavities and the mold cavities are oversized by about 20%. The percent of the oversize relates to the ratio of the binder to the metal powder used in step 1 .

3. After molding, the parts are heated in a furnace, first to remove the polymer binder, and then to sinter the metal powder. In this step, the parts shrink about 20% to over 99% dense.

The current invention utilizes the first few steps of the basic M IM process to fabricate an alloy blank formed of a combination of an alloy powder and a polymer binder. The blank is then milled to shape a desired dental device, and the device is heat treated to remove the polymer binder and to sinter the alloy powder. Specifically, the fabrication process in accordance with the current invention follows the following process steps:

1 . Mix metal powders with a polymer binder and inject blanks for milling, for example, cylinders, in accordance with a conventional metal injection molding process as discussed in Steps 1 and 2 above;

2. Mill the desired dental coping from the blank; and 3. Debind and sinter the milled part in accordance with Step 3 of the conventional metal injection molding process above.

With regard to the final step of the current method, it should be understood that the shrinkage typically associated with the debinding and sintering steps of the MIM process can be compensated for using a CAD/CAM technique. Specifically, the milled copings from the MIM blanks can be made larger using CAD/CAM software to compensate for such shrinkage. A more detailed description of CAD/CAM as used in association with the manufacture of dental devices can be found in U.S. Patent No. 7,01 1 ,522, the disclosure of which is incorporated herein by reference. As is well- known in the field and will be understood by those skilled in the art, the enlargement percentage would not only be a function of metal:binder ratio, but also of particle-size distribution, the alloy composition and the sintering temperature. These parameters can also be optimized to provide density close to the density of the equivalent cast metals. Alternatively, to reduce the amount of shrinkage experienced during the manufacturing process, it is possible to use a modified MIM process that incorporates an infiltrate. In such an embodiment, the preliminary article is debinded, partially sintered and then infiltrated with a second material to form the final article. Because during this modified process only partial sintering is performed in the densification stage, the amount of shrinkage is small or none. However, there are voids left in the partially sintered skeleton. To fill these voids the partially sintered skeleton is filled with another lower melting temperature material to create the dense device. Thus, the size of the preliminary device does not need to be enlarged as much to compensate for shrinkage. To use this alternative process, the melting range of the powder used to make the MIM blanks must be higher than that of the infiltrating material(s).

In short, the method in accordance with the current invention combines metal injection molding processes with a CAD/CAM milling operation to create a novel fabrication methodology that addresses the major problems of milling alloys using conventional techniques without requiring the use of exotic materials. In particular, a typical milling operation would entail casting a block of solid metal and then machining that block to create a component. In contrast, the block (made of polymer and metal powders) milled by the process of the current application has a consistency similar to a machinable wax, making it much easier to mill. As a result, the process in accordance with the current invention has the following advantages:

• It substantially reduces the time of milling, since the machine mills a metal powder/binder combination, not dense, hard metal; • It eliminates almost all wastage of the alloy, since the residual powder/binder material can be reused for feedstock; and

• It can be used with any of the typical alloy compositions used with standard MIM or CAD/CAM operations.

DOCTRINE OF EQUIVALENTS Those skilled in the art will appreciate that the foregoing examples and descriptions of various preferred embodiments of the present invention are merely illustrative of the invention as a whole, and that variations in the steps and various components of the present invention may be made within the spirit and scope of the invention. For example, it will be clear to one skilled in the art that additional processing steps or alternative configurations would not affect the improved properties of the method of the current invention nor render the method unsuitable for its intended purpose. Accordingly, the present invention is not limited to the specific embodiments described herein but, rather, is defined by the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method of fabricating a dental device, the method comprising: combining a metal powder and a polymer binder to form a feedstock; fabricating a blank from the feedstock by metal injection molding; milling the blank into a preliminary dental device; and debinding and sintering the preliminary dental device into a final dental device; wherein the preliminary dental device is enlarged sufficiently to compensate for the shrinkage caused by the debinding and sintering process.

2. The method of claim 1 , wherein the shape and dimensions of the preliminary dental device are determined by a CAD/CAM technique.

3. The method of claim 1 , wherein the preliminary dental device is enlarged by up to about 20%.

h. The method of claim 1 , wherein the enlargement of the preliminary dental device is controlled by the ratio of the metal to the binder.

5. The method of claim 1 , wherein the enlargement of the preliminary dental device is controlled by the particle size distribution of the feedstock.

6. The method of claim 1 , wherein the final dental device is at least 99% dense.

7. The method of claim 1 , wherein the density of the final dental device is controlled by the ratio of the metal to the binder.

8. The method of claim 1 , wherein the density of the final dental device is controlled by the particle size distribution of the feedstock.

9. The method of claim 1 , further comprising: collecting any residual feedstock produced after milling and prior to debinding and sintering; and reprocessing said feedstock for use in a new blank.

10. A method of fabricating a dental device, the method comprising: combining a metal powder and a polymer binder to form a feedstock; fabricating a blank from the feedstock by metal injection molding; determining the shape and dimensions of the dental device using a

CAD/CAM technique; milling the blank into a preliminary dental device; and debinding and sintering the preliminary dental device into a final dental device; wherein the preliminary dental device is enlarged sufficiently to compensate for the shrinkage caused by the debinding and sintering process.

11. The method of claim 10, wherein the preliminary dental device is enlarged by up to about 20%.

12. The method of claim 10, wherein the enlargement of the preliminary dental device is controlled by the ratio of the metal to the binder.

13. The method of claim 10, wherein the enlargement of the preliminary dental device is controlled by the particle size distribution of the feedstock.

14. The method of claim 10, wherein the final dental device is at least 99% dense.

15. The method of claim 10, wherein the density of the final dental device is controlled by the ratio of the metal to the binder.

16. The method of claim 10, wherein the density of the final dental device is controlled by the particle size distribution of the feedstock.

17. The method of claim 10, further comprising: collecting any residual feedstock produced after milling and prior to debinding and sintering; and reprocessing said feedstock for use in a new blank.

18. A method of fabricating a dental device, the method comprising: combining a metal powder and a polymer binder to form a feedstock; fabricating a blank from the feedstock by metal injection molding; milling the blank into a preliminary dental device; debinding and partially sintering the preliminary dental device; and infiltrating the partially sintered preliminary dental device with a metal material having a melting point lower than that of the feedstock to form a final dental device, wherein the preliminary dental device is enlarged sufficiently to compensate for the shrinkage caused by the debinding and partial sintering process.