3D printing: Additive manufacturing is becoming increasingly important in dentistry and dental technology especially for economic reasons. In addition to the processing of metal alloys and ceramics, the focus in dentistry is primarily on the additive processing of resins.
Veronika Greil, München
While in subtractive manufacturing the workpieces are milled or ground from a complete round blank using CAD / CAM-supported processes, in additive manufacturing the objects are built up layer by layer. The process for manufacturing the object consists of several steps:
- 3D model acquisition (intraoral scan or scan in the laboratory)
- Creation of an STL file (design of the object including support structures)
The first step is to generate the data for creating a 3D patient model. The digital impression can be taken directly on the patient using an intraoral scan or on a conventionally manufactured plaster model in the laboratory. Then the object and the support structures are individually designed and an STL file is created. STL files can be used for both, subtractive and additive manufacturing (Computer Aided Design CAD).
After determining the printing parameters such as the layer thickness or exposure time, as well as defining the manufacturing options, the next step in the manufacturing process is the actual manufacturing – the 3D print (Computer Aided Manufacturing CAM). The last step contains the post-processing, consisting of the removal of the support structures, manufacturer-specific cleaning, and post-polymerization.
Design of an individual impression tray in the design software (1) and after transferring it to the software of the 3D printer (2)
3D printing process of polymers
Three printing processes have established in the field of dental 3D printing:
- FFF (Fused Filament Fabrication)
- SLA (Stereolithography)
- DLP (Digital Light Processing)
When printing with the FFF method, wire-shaped thermoplastics are processed, which are melted in a chamber and then applied to the construction platform in the desired shape using a nozzle. The material then hardens again by cooling. A body is usually built up by repeatedly traversing a working plane line by line and then shifting the working plane upward in layers so that a three-dimensional object is created. With the FFF, there is no need for time-elaborating post-processing.
In contrast to this, liquid resin-based materials are used in both, the SLA and DLP processes, the matrix of which consists especially mono- and oligomers as well as photo initiators. While in stereolithography the resin is exposed point by point through a single spot and thus hardened, in digital light processing a whole layer is polymerized on the construction platform by means of a projector. Since a whole layer can be polymerized at once with the DLP process, a faster build-up rate is possible than with the SLA process. The layer thicknesses vary between 25 and 100 µm depending on the process, light source, and material.
Test specimens 3D printed using the DLP process
Pros and Contras
The 3D printing of polymers has been established in many areas of application and is proving to be the more attractive manufacturing process compared to subtractive manufacturing.
In contrast to the subtractive manufacturing, in which the restoration is milled out of a complete round blank, in 3D printing only the amount of material that is required for the object and the support structures is used. The ability to create cavities further reduces the amount of the required material. In addition to that, the additive manufacturing enables a large amount of object freedom and individual colour design. Therefore, individual complex components can be produced quickly and inexpensively with relatively little effort, without the need for molds, tools or similar.
Another advantage is that several restorations can be produced at the same time, which enables more economical and faster production. In addition, the devices required for additive manufacturing are generally of a simpler design and are significantly cheaper to purchase.
The limits of additive manufacturing are set by the often-limited precision and lower mechanical properties compared to subtractive machined blanks that are industrially manufactured under optimal conditions. Both the printing direction and the conversion rate and errors between the individual layers can have a negative effect on the stability and mechanical properties.
Advantages and disadvantages of 3D printing polymers
Applications in dentistry
The importance of additive manufacturing methods in dentistry is increasing continuously. The materials that are used depend on the respective area of application and the required material properties. In addition to conventional polymers, ceramic-infiltrated polymers for permanent, fixed restorations and polymers with added hydroxylapatite for patient-specific implants for reconstruction in the head and neck area are processed.
Requirements that are placed on dental 3D printing include high precision, dimensional accuracy after storage, adequate mechanical properties such as abrasion resistance and low brittleness, good surface quality and biocompatibility.
Established areas of application for 3D printing in dentistry are:
- Model production
- Auxiliary structures such as impression trays, splints, and implant drilling templates
- Temporary dentures
- Definitive dentures
- Reconstructions in the head and neck area
3D printing can easily be integrated in the already established digital workflow of computer-aided dentistry and time-elaborating work steps can be optimized and simplified. Now there is still little knowledge of 3D-printed restorations and their clinical properties, but the constant further development and establishment of new additive manufacturing processes reveal the great yet unused potential of digital dentistry.