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Innovations in Additive Manufacturing
This sophisticated technology allows for extremely detailed final results
By Chris Brown, BSEE
Has the day of science fiction “replicators” arrived? Can we now order custom parts and have them magically appear before our eyes? We may not be racing across the galaxy in starships, but modern technology has already given us tasers instead of phasers, worldwide communication in the palm of our hand, single-serving hot coffee at the push of a button, and the ability to “print” almost anything that our hearts desire.
The first 3D printers were developed in the early to mid 1980s and used to create prototype parts for manufacturing companies. Previously, if an engineer needed to validate the form and fit of a newly designed part, it could be modelled out of clay or machined out of metal or plastic. Clay wasn’t particularly precise and machining was slow and expensive for short production runs. 3D printers provided reasonable accuracy with a relatively low part cost for short production runs. Thus, the Rapid Prototyping industry was born.
Subtractive Versus Additive Manufacturing
Most dental professionals are familiar with the concept of milling. Start with a disk, a block, or some other form of stock material and a 3D electronic file (STL) that defines the contours of the part to be manufactured, then use CAM software and a mill to precisely cut away unnecessary material until all that is left is the desired part. This is subtractive manufacturing, a process that generates a fair amount of waste, but can be relatively quick, precise, and efficient.
3D printing is a process that starts with an empty platform or tray. CAM software slices the STL file of the part into layers of a fixed thickness. The printer produces each layer, usually from the bottom up, creating the part (Figure 1). 3D printing is considered an additive manufacturing process. The method by which each layer is produced and the material involved varies based on the printer technology involved. Printing is generally not as fast or accurate as milling, but does allow the user to create complex parts, including those with undercuts, with ease and minimal waste.
Additive manufacturing equipment comes in all shapes and sizes, from table-top systems capable of printing single parts or small multi-part projects to large floor-standing systems able to print hundreds of parts at a time. The parts are built on a platform typically called the build tray, which generally travels in a downward direction after each layer is printed. The build envelope is the horizontal length and width of the build tray, plus the maximum height the printer is capable of working within. Build time is the time that it takes to print a complete project. Build time is always influenced by the number of layers in the project, and depending on the printing technology, may also be affected by the number of parts being printed.
Printing resolution is usually defined for both the horizontal and the vertical directions. The horizontal resolution determines a printer’s ability to accurately reproduce a shape or contour in the horizontal (x and y) direction and may be determined by a laser width or a material nozzle diameter. Vertical resolution is the smallest distance the build tray moves after each layer is printed, determining a printer’s ability to accurately reproduce a shape or contour in the vertical (z) direction.
For dental professionals, both horizontal and vertical resolutions are critical factors in build quality. The dark blue rectangles in the top of Figure 2 represent each printed layer. Note the areas indicated by the red arrows—these are where the printed layers do not closely match the contour of the desired part because of a large layer thickness. In the bottom of Figure 2, the light blue rectangles represent what happens when the layer resolution is improved by a factor of two. Think of the vertical steps being 50 microns instead of 100 microns. The areas indicated by the green arrows show a better contour fit than the same areas above. Further increasing the print resolution, or reducing layer thickness, continues to improve the contour matching. Be sure to keep in mind, though, that each layer takes a finite period of time to print. The more layers to print, the longer the print job will take. Typically printers are preset to operate at a single vertical layer resolution, however some will offer a thicker layer (higher speed) and thinner layer (high quality) settings.
Remember, 3D printing is a layer-by-layer additive process. Each successive layer is laid out upon the layer beneath it. This works well until there is an undercut contour. How can a printer produce a layer on top of an area that has no structure or base below it? The answer is “support material.” In some printers, it may be a special type of resin, plastic, or wax. In others, it may be unbound (sintered or melted) build material. Its sole purpose it to provide a base to support the next layer above the undercut region. The light blue material in Figure 6 shows how support material is used in undercut areas.
Stereolithography (SLA) is a method of 3D printing that employs an ultraviolet (UV) laser to cure or harden photopolymer resin. The liquid resin is held within a reservoir (Figure 3) and the laser traces the pattern of each part, instantly hardening the material. Undercut areas are supported by the uncured resin.
Digital Light Processing (DLP) is very similar to SLA printing. However, rather than using a laser to trace each part individually, a UV light is projected for all parts simultaneously across the entire reservoir, dramatically speeding up printing time.
Jetted Photopolymer Printing is accomplished with two types of resin. One is called a model or build resin, and the other is a support resin. Both resins are liquids stored in cartridges within the printer. The resins are sprayed out of micro print heads, similar to the types used in desktop inkjet printers. Once the resin hits the build platform, a UV light cures the material instantly (Figure 4).
Inkjet 3D Printing is similar to Jetted Photopolymer, but the build and support materials are temperature sensitive rather than UV-light sensitive. The build and support materials are solids at room temperature and are heated to liquid forms for printing. Once the materials are sprayed out of the print heads, they rapidly cool and harden (Figure 5). Inkjet printed materials are generally resins or waxes.
Fused-Deposition Modeling (FDM) is another method of printing that involves build and support materials that are solids at room temperature. They are generally spool-fed rather than stored in a reservoir. The materials are melted at the spray nozzle (Figure 6) before delivery to the build platform. FDM printers are capable of printing metals or plastics, depending on the type and operating temperatures of the print nozzles.
Selective Laser Sintering (SLS) or Selective Laser Melting (SLM) is a printing process that like SLA, uses a reservoir of build material and a laser to harden the material (Figure 7). It differs in that the build material is a metal, plastic, glass, or ceramic powder, rather than a liquid. The laser traces each part and either melts or sinters the powder, binding it together. As the build platform is lowered, more powder must be added to the reservoir. It is often required that sister reservoirs are moved into place, layer-by-layer. Support in undercut areas comes from the non-cured/sintered build material.
Nearly every form of additive manufacturing requires some type of post-processing procedure before the part can be used. Generally, removal of support material is the first step. It may be as simple as lifting the parts out of a bed of unused build material as in SLA or SLS/SLM printing. Jetted resin, Inkjet, and FDM support material may be removed by soaking parts in a liquid that dissolves or softens it. A second curing or cleaning step may also be required, particularly for resin-based printers. The important thing to note is that additional processing is almost always necessary, particularly for printing in dental applications.
Today in dentistry, SLA, DLP, and Jetted Photopolymer resin printers are regularly used for printing models. These technologies generally have the speed necessary to be productive in our industry, and they have the resolution and accuracy to provide sufficient detail in dental models. The materials may not be what the industry is used to working with, but they are improving, as is the ease and simplification of post-processing. Wax patterns for casting metals or pressing ceramics are being printed with DLP and Inkjet 3D printers. SLS/SLM printers are being used for printing metal copings, frameworks, and in some cases, full contour metal restorations. Surgical guides for implant placement are being created with Jetted Photopolymer and SLS/SLM printers. FDM printers are being used for denture try-in models.
One can only imagine how far 3D printing will go. Medical researchers are printing human body parts. A European company is printing temporary restorative materials (not yet approved for use in the US). Although 3D printing technology is mature and proven, it continues to evolve. The availability of FDA-approved materials for inside the mouth and our own creativity will spur the development of new products and services. Printing bite splints, mouth guards, complete dentures, and even final restorations are certainly not too far in the future. Thankfully, more and more of today’s dental technicians are learning to use CAD software. They will be the ones to lead us not to the final frontier, but to next frontier in the ongoing evolution of dental technology.
Chris Brown, BSEE is the business manager of Apex Dental Milling in Ann Arbor, MI.
For more information on additive manufacturing be sure to take the author’s free, on-demand webinar, 3D Printing: A Model for Dental Laboratories at dentalaegis.com/go/idt673.