Laura Galloway, director of marketing, Boston Micro Fabrication (BMF)
Chuck Hull invented the first stereolithography (SLA) printer in the early 1980s and this led to the co-founding of 3D Systems, one of the leading companies in the 3D printing industry today. Since then, there have been several variants of SLA, all fundamentally based on the vat photopolymerisation process, namely using light to cure a photosensitive material. There are other polymer 3D printing methods, such as fused deposition modelling (FDM), material jetting (MJ) and selective laser sintering (SLS). They all work on the same basic 3D printing premise of taking digital data and building a part layer by layer, and they all have their unique advantages and disadvantages.
Vat photopolymerisation as a category has the core advantage of allowing a higher resolution and surface finish to be achieved. It is why it is often the approach for dental, hearing aid, jewellery and other high-quality product applications. Historically, a significant disadvantage of vat photopolymerisation has been the limited range of materials and ability to mimic engineering grade plastics. This has begun to change in recent years as new companies have entered the market, introducing the materials needed to bring the aforementioned advantages of this method to more end-use applications.
The ability to achieve high resolution, accuracy and precision is dependent on multiple factors, including:
• the resolution of the optics;
• the precision of the mechanical systems in the machine;
• the control of the exposure and the resultant curing;
• the interaction between the part and required support structures; and
• the overall size of the part and the ability to control tolerances across the build.
This article looks at how laser-based SLA, digital light processing (DLP) and projection microstereolithography (PμSL) compare.
Stereolithography
SLA uses an ultraviolet (UV) laser to draw, or selectively cure, each layer of the 3D model in a vat of liquid polymer (resin). The laser beam is directed across the printing bed using mirrors driven by galvanometers, curing resin point-to-point as it moves. Once the first layer is complete, the printing bed moves down if it is a bottom-up system or up if it is a top-down system, and the laser starts to cure the next layer. The minimum laser spot size helps to determine the part resolution that can be obtained. The resolution and repeatability of the process can be affected by the quality and robustness of this galvanometer (galvo) system. X, Y resolution of the printer translates to several key part characteristics, namely surface finish, edge details and crispness, minimum feature size, and part tolerance (accuracy and precision).
For SLA, support structures are required to (1) hold the part to the base (this is especially true if building bottom up) and (2) to support overhanging structures.
SLA systems currently on the market can typically achieve an X, Y resolution of 50 µm, a minimum feature size of 150 µm and an overall tolerance of +/-100 µm.
Digital light processing
DLP differs from SLA in that it uses a projector to flash each 3D model layer image onto the resin, thus curing entire layers at once. The resolution, accuracy and precision are determined by the resolution of the projector (the current standard is 1,080 pixels), the optical character of the projection lens and the tightness of the mechanical X, Y, Z movement, respectively. Many DLP systems do not have X, Y movement, as the image covers the entire printing bed. Therefore, only the Z stage movement is required. To increase the printing bed size, the magnification of the projection lens increases, but comes at a cost of decreased resolution; this is because the digital micromirror device (DMD), a semiconductor chip covered in an array of microscopically small mirrors, is currently limited in size.
DLP systems also require support structures for overhanging geometry, and in the case of systems that use a bottom-up approach, supports are needed to secure the part to the build platform. Typically, the more supports required, the more the support-side surface finish is compromised.
DLP systems currently on the market can typically achieve an X, Y resolution of 25–50 µm, a minimum feature size of 50–100 µm and an overall tolerance of +/- 75 µm.
Projection microstereolithography
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![Image 3a PuSL-diagram[1].png](https://www.cmmmagazine.com/downloads/4032/download/Image%203a%20PuSL-diagram%5B1%5D.png?cb=69c91ae58ccbb16cfea184f75c029860&w={width}&h={height} "Image 3a PuSL-diagram[1].png")
Projection microstereolithography (PµSL).
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Projection microstereolithography (PµSL).
Boston Micro Fabrication developed PµSL as a variant to the DLP process, the objective being to increase the resolution but maintain the ability to print larger or multiple parts at high tolerance. This has been accomplished by adding a few key elements. First, a high precision lens is placed between the projector and the resin batch, thus focusing the resolution to desired levels.
Second, the printing bed moves in the X, Y plane, enabling the printing of one large part as well as many small parts. The PµSL method employs projection zones and these create multiple areas for printing a part or parts in, depending on size. Either one larger part can be printed by stitching the projection zones together or many smaller parts can be printed through nesting (packing). In the case of the latter, if necessary, movement in the X, Y plane allows for imaging of a layer multiple times at a maintained resolution.
Finally, the PµSL process is top down, meaning that not only are less support structures required but the damage to small features is reduced and a transparent membrane can be used to eliminate bubbles. No supports are needed to secure the bottom surface to the print surface and limited supports are required for overhangs due to the interaction of often lightweight parts and buoyancy of the resin.
PµSL systems can achieve an X, Y resolution down to 2 µm, a minimum feature size of 10 µm and a dimensional tolerance as high as +/-10 µm.
Summary
Vat photopolymerisation techniques afford the advantages of high-quality parts and an increasing range of materials. DLP systems provide the speed required to make them a viable choice for end-part production applications. PµSL has now added the resolution, accuracy and precision required for high-quality parts such as electrical connectors, medical devices and MEMS. As the market continues to evolve, hardware, software and material developments will continue to open up new adoption opportunities for 3D printing in end-part production.
Boston Micro Fabrication