Direct Digital Manufacturing

Direct Digital Manufacturing (DDM) is an additive layered manufacturing process which generates the final functional part with desired material directly from a CAD model. This fabrication process is highly suitable for making customized parts at a small quantity. Like rapid prototyping processes, DDM

Fig. 8.32 The 4-implant dental bar: (a) SLA pattern; (b) metal bar cast from Ni-Cr alloy (Delli, 2006)

is advantageous over traditional processes on fabricating parts with internal features and complex geometries.

Selective Laser Melting (SLM) is a process of fabricating 3D metal parts using a technology similar to Selective Laser Sintering (SLS) but with higher laser power. SLM generates metal parts in layers by selectively melting the metal powder with a focused laser beam (Vanderbroucke et al., 2006). Metallic powder is fully fused in slices to produce a part of full density. Standard metals like stainless steel and hot worked steel are commonly used materials in this process. Other materials like cobalt-chromium alloys and titanium alloys can also be used to fabricate 3D parts with this process. A 3-implant dental bar fabricated by SLM from a titanium alloy is shown in Fig. 8.35(a).

Another DDM process for producing fully functional metal parts directly from CAD models is 3D metal printing, which prints the part layer-by-layer from metal powder. After a layer of metal powder is spread on the platform, the binder material is selectively dispensed over the layer of metal powder using inkjet printing technology. The zone filled with binder holds together while the other area is left with loose metal powder. After building one layer, the platform is lowered by one layer thickness and a new layer of powder is spread. The process is repeated until the complete green metal part has been built. The unbound powder is removed and the green metal part is sintered

Fig. 8.35 (a) The 3-implant dental bar fabricated by SLM with Ti-6Al-4V; (b) physical part of same geometry fabricated by 3D printing with stainless steel (Delli, 2006)

Fig. 8.35 (a) The 3-implant dental bar fabricated by SLM with Ti-6Al-4V; (b) physical part of same geometry fabricated by 3D printing with stainless steel (Delli, 2006)

to obtain the final metal part. A stainless steel dental bar fabricated by this process is shown in Fig. 8.35(b).

8.7 Conclusion

Automating the design and fabrication ofdental restorations involves acquiring the patient's oral digital data, surface reconstruction from the digital data, use of the surface information to design dental restorations, and fabrication of physical parts from the digital designs. Acquiring the patient's oral data can be done with a contact or non-contact digitizer. A contact digitizer is relatively inexpensive and fairly accurate. However, it is slower and can be used only on a stone model. A non-contact digitizer can acquire data faster and a miniature optical device can be placed inside the patient's mouth for direct data acquisition. The acquired digital data in either case needs to be converted into a surface representation such as a triangular mesh in STL format for purposes of visualization rendering and further data processing for digital design and fabrication. This requires surface reconstruction from the digital data when the data represents a set of points on the object surface.

When the digital data is from CT or MRI images, the data can be converted into a voxel model, with its surface evaluated and rendered using a marching cube algorithm. After the acquired digital data has been converted into a surface or solid representation, the digital design process can begin. This may involve, e.g., using a B-spline function to represent the surface of a tooth and to edit it by adjusting feature points and curves. Design of a dental bar for a restoration involving implants can also be automated by using the measured data to determine the positions and orientation of the healing abutments, with which to construct the cylinders and connecting bar of the dental bar model. Finally, the design model representing an inlay/onlay, crown, bridge, or implant dental bar can be transferred into a physical dental part of the same geometry through a digital fabrication process such as CNC machining, rapid prototyping plus investment casting, or direct digital manufacturing.

Dental automation available from commercial CAD/CAM systems today has made the creation of some restorations, such as veneers, inlays, onlays, or crowns, possible in a single visit to a dentist. However, in-office technology cannot be used for more complex restorations such as bridges to replace missing teeth. In these situations, stone models must be sent to the dental lab where a laboratory automation system could be used to generate a high-strength zirconia framework that is manually veneered with porcelain to complete the restoration. Interestingly, twenty years after the introduction of in-office CAD/CAM technology, there is still only one in-office system while more than ten laboratory-based CAD/CAM systems are available today. This probably reflects the fact that although one-appointment, in-office milled restorations are convenient, the more important current issue may be the dental technician manpower shortage, which is addressed with laboratory automation systems.

In spite of the progress with dental laboratory technology, present systems for automating the design and fabrication of dental restorations involving implants have very limited capabilities and are largely under development. Automating this design and fabrication process will greatly reduce the cost and time of treatment and also improve the quality and consistency of implant-based restoration design and fabrication. Development of advanced CAD/CAM systems for such restorations is a critical area of future research in dental automation.


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