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Traditional subtractive methods required outrageous sums of money: The basics of additive manufacturing

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Advances in additive manufacturing engineering are now more approachable and affordable than ever. (Image: Olga Ilina/Shutterstock)

Mon. 5. December 2022

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Traditionally, the more accurate methods of forming metal into the desired shapes have used subtractive manufacturing, which is the process of removing material to create a part. For years, engineers have used subtractive machines such as lathes and mills to create the custom parts that they have designed. However, subtractive manufacturing is limited by the reach of the tools and by the machine’s axis of movement. This is where the advent of additive manufacturing makes all the difference.

Rather than removing material to expose a part, additive manufacturing lays down material to build up the part. This is an engineer’s dream because it removes many design barriers that had previously prevented the building of parts that are more advanced. An easy way to conceptualise it is to think of it like building a house: it is far more practical to build the house layer by layer with bricks than it is to carve the house out of a giant block of stone.

Additive manufacturing has grown in popularity in recent years, and many unique methods have been developed to allow for parts to be made of a broad spectrum of materials. The most common type of additive manufacturing that may be familiar to readers is fused deposition modelling (FDM). This process involves extruding a plastic filament through a heated nozzle similar to a hot glue gun. An FDM printer moves in three axes as it lays down the plastic layer by layer until eventually the desired part has been formed.

There are a number of other additive manufacturing methods, including resin-based printing. With resin printing, a UV light-sensitive resin is exposed to UV light, causing it to harden, to form parts with great precision. The UV exposure source can be controlled through a variety of methods, such as liquid crystal display (LCD). This process uses a transparent LCD screen similar to that used for a computer monitor. The LCD is programmed to essentially display an image of the layers required to build up the parts, and in doing so, the LCD darkens specific pixels, creating a mask that only exposes the resin according to where the layer requires.

Once that resin has been polymerised, the printing bed the layer is sitting on is shifted, allowing for the next layer to be deposited and interact with the UV light. LCD printers have a drawback, as the UV exposure to the LCD can damage it over time, requiring the screen to be replaced.

Other resin systems operate somewhat similarly, such as stereolithography (SLA), which scans a UV laser over the resin to build the layers, and digital light processing (DLP), which uses an array of micro-mirrors which function like individual pixels in a screen by reflecting UV light on to and away from the image transfer optics. With the DLP process, a whole layer can be exposed at a time, leading to a faster build time.

“This is an engineer’s dream because it removes many design barriers that had previously prevented the building of parts that are more advanced.”

These processes have found their way into many fields, including dentistry, for fabricating retainers, dentures and aligners, for example. However, resin printing has a downside: it is weak. Typically, building things like expanders and bone plates requires a strong biocompatible material such as titanium. Invented in 1995, the process of direct metal laser sintering (DMLS) is employed in many professional fields as a state-of-the-art manufacturing technique for producing robust parts with complex geometry.

While the DMLS process is similar to the SLA technique, it utilises a multi thousand-watt fibre laser that scans over a level bed of fine titanium powder rather than a UV laser scanning over resin. With DMLS, the particles of titanium melt and bind together to form a single metal structure. This process has proved valuable in orthodontics, as the additive aspect allows printer operators to build models with topography optimisation, a computer design process that generates complex organic models that are lightweight but strong. Before DMLS, having parts manufactured in titanium with traditional subtractive methods required outrageous sums of money. However, thanks to the advancements in machine technology, custom titanium parts are now available to a greater audience.

Editorial note:

This article was published in CAD/CAM—international magazine of dental laboratoriesissue 2/2022.

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