Since the early 1980s 3D printing has developed from an interesting experiment to high speed dental manufacturing technology. Dental Review reports...
In the UK of 1981, Margaret Thatcher had just been polled as Britain’s most unpopular Prime Minister and the country was embroiled in the Falklands war. Meanwhile in Japan, Hideo Kodama – working with thermal research specialist Nagoya Municipal Industrial Research Institute – had published the first account of a functional rapid prototyping system using an acrylic-based material called photopolymer. A solid, printed model was built up in layers, each of which corresponded to a cross-sectional slice in the model. Two years later, in 1983, Charles Hull developed his stereolithography machine. He was granted a patent in 1986.
Stereolithography enabled designers to create tangible 3D models from virtual, CAD designs. A vat of liquid photopolymer is precisely hit using a digitally guided UV laser beam. The light-exposed liquid is instantly transformed into solid plastic. After a few minutes the solidified liquid had been moulded into the required 3D model. It was expensive, it wasn’t very precise in engineering terms, and the margins warped during drying, but from that time on creative types could theoretically produce rapid prototypes and test designs without a huge upfront investment. In 1989 Scott Crump, a co-founder of Stratasys Inc. filed a patent for a process called fused deposition modelling (FDM) — the proprietary technology still held by the company today. The FDM patent was issued to Stratasys in 1992. The fledgling technology had taken to the air, but it still had a way to go before it became good enough for dentistry.
Modern 3D printing
Since its inception 3D printing has been recognised as an exciting technology full of incredible potential. More recently biocompatible frameworks to support failing organs have been printed, as have kidneys. Medical devices have been printed with the patient’s own cells to obviate rejection. Complex parts for aerospace engineering and intricate, precision components for formula one racing cars and racing bikes have been manufactured. 3D printing entered the dental arena by offering rapid and precise anatomical model making, implant guides, denture components, orthodontic appliances and much more. Some of the leading names in dental technology have invested a lot of time and money in developing the process. The holy grail is seen to be an affordable, desktop printer for the dental lab or surgery. Over the course of this series we will look at the leaps 3D printing has made during the last decade and investigate the latest machines.
What is 3D printing?
Modern 3D printing faithfully reproduces complex CAD detail with virtually no loss of surface resolution and none of the wastage associated with other manufacturing processes. It is extremely good at producing highly detailed and complex surfaces. In the case of metal dental frameworks, for example, dental specific, CE marked Co-Cr alloys can be used, with the final parts manufactured within an ISO 13485 quality system. Put simply, a layer of metal powder, 20 microns thick, is consolidated with a super fine laser beam and the dental framework is built up layer-by-layer.
Alternately PolyJet printing using biocompatible materials can now be used for the rapid printing of models and orthodontic devices; and materials to create acrylic-based denture components are also available. Further developments in denture manufacture are expected soon, but are currently shrouded under a non-disclosure mantle.
Working under the general umbrella of “Additive Manufacturing”, the starting point for any dental 3D printing process is a digital model created using one of a variety of 3D scans, which is then manipulated by dedicated CAD software programmes. The model is then digitally sliced into layers, thereby converting the design into an algorithmic file readable by the 3D printer. The material processed by the 3D printer is then layered according to the required design and the chosen printing process. There are many different types of 3D printing technologies, which process different materials in different ways to create the final object. Functional plastics, metals, ceramics, and sand are all routinely used for industrial prototyping and production applications. However, plastic is currently the only widely used material — usually ABS (Acrylonitrile-Butadiene-Styrene) or PLA (polylactic acid). However, there are a growing number of alternatives, including Nylon, and a growing number of entry level machines have been adapted to work with foodstuffs, including sugar and chocolate. What kind of printing is there and what is it used for in dentistry? We look at that next.
Types of 3D Printing
As stated above, Stratasys produced the world’s first FDM machine. This technology uses plastic and an extruder to deposit layers on a print bed. Modern Stratasys FDM-based 3D printers produce parts in production-grade thermoplastic materials for aerospace, automotive, medical, dentistry, and many other industries. Modern FDM is often used to build complex geometries and functional parts, including prototypes, low-volume production pieces, manufacturing aids, jigs and fixtures.
PolyJet is a 3D printing process that jets and cures thin layers of liquid photopolymer using UV technology. It is capable of printing in 16-micron layers and can be cured at several different levels of hardness (or durometers). It can also print different colours during the build process for greater reality. PolyJet dental materials have been specifically designed for digital dentistry and orthodontic applications, including stone models and aligners. These rigid, opaque materials have a natural-looking peach colour and combine extremely accurate detail with high dimensional stability.
Still going strong after all these years, stereolithography (also known as SL or SLA) is commonly used to produce concept models, master patterns, large prototypes, and investment casting patterns. SLA has been used for decades to create anatomical models, including the complex maxillofacial sinus in full detail, and has aided precision implant placements.
Laser sintering (also known as Selective Laser Sintering, SLS or LS) uses a CO2 laser to heat and fuse durable thermoplastic powder to build tough and versatile parts with a high fracture strain. LS produced components and prototypes are strong and lightweight, plus heat and chemically resistant.
Direct Metal Laser Sintering
Direct metal laser sintering (DMLS) fuses powdered metal and alloy materials with a high-wattage laser to produce robust, metal parts, including patient specific bridges, crowns, and partial dentures – printed using bio-compatible cobalt chrome powder. For many technicians DMLS is the main technology of choice for dental applications. 3D Systems, EOS Concept Laser, Renishaw, SLM Solutions & Realizer all sell metal 3D printers developed specifically for the dental industry.