Additive Manufacturing: It's a Positive Thing

Actually, additive manufacturing is a relatively new concept that encompasses 3D printing and related technologies that automatically produce a product from a digital design file. Start with an empty platform or tray and add material, layer by layer, to create something from nothing. It’s not magic; it’s a wonderful combination of technology, materials, and creativity coming together to provide amazing flexibility in short-run manufacturing.

Five basic technologies are used in today’s additive manufacturing systems:

• Stereolithography (SLA)/digital light projection (DLP)

• Selective laser melting (SLM)/selective laser sintering (SLS)

• Jetted photopolymer

• Inkjet

• Fused deposition modeling (FDM)

Each of these technologies builds layers (Figure 1), and starts with an empty build tray, or platform. SLA, DLP, SLS, and SLM involve the platform being submerged or buried in the build material. SLA and DLP use an ultraviolet (UV) laser or projected light in specific areas on the build tray to harden, or cure, the UV-sensitive material. Selective laser sintering and SLM incorporate a laser to sinter or melt the metal or plastic build material connecting each new layer to the one beneath it. Jetted photopolymer printers spray a UV-sensitive resin from extremely precise miniature print heads. The resin is then immediately cured with a UV lamp. Inkjet 3D printing systems also have print heads but use temperature-sensitive resins that harden once the build material cools. Fused deposition modeling systems use temperature-sensitive resins, but extrude the spool-fed build material onto the platform.

All additive manufacturing technologies require the use of support material. When build material is placed above an undercut, it must somehow be supported. In SLS and SLM systems, the dense unsintered or unmelted build material can act as support. In some cases, little feet or pegs are also printed to help support the layers above the undercut. For SLA and DLP systems, the uncured resin in the reservoir provides the necessary support. Jetted photopolymer, inkjet 3D, and FDM systems usually have a secondary support material that is applied in the same manner as the build material.

Regardless of the additive manufacturing system utilized, a secondary process is always necessary before the parts are ready for use. It may be as simple as blowing away unsintered powder, or it may require cutting, cleaning, soaking, or curing.

Additive manufacturing systems place material only where it’s needed. In subtractive manufacturing, such as milling, unnecessary material is removed. Additive manufacturing technologies generate far less waste than subtractive technologies. As production volumes increase, additive manufacturing systems typically become more efficient. The 3D printing build platforms are generally much larger than the raw material stock used in milling machines. For example, a typical build platform for a dental product 3D printer is 150 mm x 150 mm or larger. The raw stock material blank for most milling machines is a 98-mm-diameter disk or smaller. Not only do additive technology systems give more room to parts in a build project, but the technology can achieve much tighter nesting density. The diameter of the largest tool in a milling machine determines the minimum distance between parts because it must pass between them. The 3D printers have no such restriction.

Because additive manufacturing systems build by layering, the height of a part impacts the time it takes to print. Most of the 3D printers in our industry have layer thicknesses that range from 16 microns to 50 microns. A part with a 10-mm height could have as many as 625 layers or as few as 200 depending on printer technology. Some printers even allow for different layer height settings from one print job to another.

Among the first uses of additive manufacturing in the dental industry were wax patterns used for casting or pressing. Inkjet-style or DLP printers could print as many as a 100 patterns in a single print job. Eventually, software became available for designing removal partial denture (RPD) frameworks, and soon 3D printers were being used to print frameworks for casting. When dedicated chairside digital impression systems were introduced, at least one system manufacturer adopted an SLA crown and bridge model solution. Costs of printers were initially higher than they are today, and only high-volume users could see profits when using the technology.

Today smaller, lower-cost printers are available for wax patterns. The size is equivalent to a shoe box, and production capacity is limited. For some laboratories, this can be a good fit. However, in many cases, laboratories are abandoning their own casting departments and sending their coping or framework designs to production centers using SLM or SLS printers for non-precious and semi-precious substructures. A new generation of print and investment materials has created a more reliable workflow for printed and cast RPD frameworks. At least one company has successfully mastered direct SLM and SLS printing of partial frameworks.

Jetted-resin printers are in production centers and laboratories as the business model now exists to print crown-and-bridge-quality models from a new generation of digital impression systems or even scans of traditional impressions. The same printers are also being used to produce models for orthodontic appliances, aligners, and retainers.

Every day, both resin-based and SLM or SLS printers are being employed to create surgical guides for dental implant placement. Temporary restorations produced from dental-office 3D printers are being placed in Europe now.

Looking into the near future, it’s very likely we will see broadened use of additive manufacturing for full dentures. At least one company has developed a laboratory-involved workflow to produce denture try-ins. It’s only a matter of time before we are able to print nearly the entire denture. A recent 501(k) approval was granted for an SLM/SLS-printed sleep apnea device.

The future for additive manufacturing will be determined by three factors: Can a material be printed? What are the physical properties of the material? Can the material be approved for use in the mouth?

Although a number of metals, nylons, and resins are being printed within the dental industry, many materials, particularly on the restorative side, still can’t be printed. Remember, the current additive manufacturing technologies are sintering, melting, jetting, and extruding a material. While pressable glass ceramics can obviously be melted, the operating temperatures, pressures, and material viscosity may not be favorable for printing, at least not yet. We sinter milled zirconia, but it has first been pressed to achieve a very high material density. Zirconia in powder form on a build tray can never have that density. It will be interesting to see if materials are developed or modified that make them printable with current technologies, or if a new method can be developed to print currently available and proven materials.

The next challenge is the properties of the material. How strong is the material? Is it flexible? How long will it last? Is it esthetic, or does it even need to be? Companies have been experimenting with printing dental ceramics, but they haven’t been able to maintain the density, strength, and esthetics of currently milled or pressed materials. Interestingly, some of the higher-end printers are capable of simultaneously printing multiple materials. That means someday we should be able to print appliances that have dual material properties.

Finally, just because a material can be printed and has the desirable material properties, it doesn’t necessarily mean it’s safe for use in the mouth. Most of the materials being printed for use in the mouth today are limited to short durations. Future material testing and development will eventually produce materials that will satisfy FDA requirements for long-term oral use.

The industrial world is printing cars (Local Motors), jet engine parts (GE), and rocket engines (SpaceX). Even a wrench was recently printed on the International Space Station. Not many people are surprised that rocket scientists are using additive manufacturing. They may be surprised at how much additive manufacturing is being used in the dental industry and how much potential the technology has for us.

Chris Brown, BSEE, is the Manager of Aclivi Consulting in Pinckney, MI.