December 20, 2023
Industry 4.0 has ushered in new beginnings by fostering the integration of advanced information technologies and intelligent production systems. Additive manufacturing, or 3D printing, plays a vital part in effecting the shift from traditional manufacturing to cutting-edge techniques and devices.
However, the murmurs of change were felt long back when Stereolithography (SLA) – the first 3D printing technology – burst onto the industrial scene during the 1980s to fulfil rapid prototyping needs. As the years went by, SLA became increasingly sophisticated. But the technology gained wider acceptance from the early years of 2000 when patents expired and desktop 3D printers became affordable for amateurs and hobbyists.
Also called the vat polymerisation process, it is a resin-printing technology. SLA uses a UV light source to selectively harden liquid photopolymers in a vat/tank to build prototypes and finished products that are accurate, isotropic and watertight. It can print objects both bottom-up and top-down, depending on how the core components – the light source, the build platform, and the resin tank – are arranged.
Types of printers
There are two kinds of printers, desktop and industrial, with each having its own set of advantages.
Desktops, meant for hobbyists, small businesses, and individual creators, are portable, user-friendly, require less resin, and designed to print smaller volumes with reasonably intricate and detailed features. They are affordable and accessible, but less than ideal for building functional parts.
Industrial printers are complex, faster (can build multiple parts in the build chamber), much costlier, more precise, and ideal for fabricating functional parts. They have larger build volumes, ensure much better quality, require more post-processing, and support a wide range of resin materials.
Process outline
When the CAD file is finalised, it must be converted into STL or OBJ data at the pre-printing stage. The 3D model is then sliced up into layers, creating commands for the printer, which will build one layer at a time. The thickness of these slices is called the layer height, which can range from the standard 50 microns to as low as 10 microns, depending on the capability of the machine. (1 micron=1/10000 of a millimetre).
There are two major types of SLA printing:
The top-down/right side up approach is used by industrial printers; and the bottom-up mechanism is found in desktop printers.
Top-down
The resin in the vat has to be absolutely still and stable for printing to commence. The laser is directed to the precise spots on the surface of the resin by a computer-controlled mirror system called the galvanometer. The light-sensitive substance is hardened/cured when it comes in contact with the UV beam. The laser first draws the support structures, followed by the actual part geometry. The build platform then descends the distance equal to the thickness of a single layer, and a resin-recoater sweeps across the vat to recoat it with fresh material for further exposure to the laser beam. This way, consecutive layers are built one on top of the other until the part is finished.
Bottom-up
The process involves building the part upside down, with the guided laser writing on the bottom-most layer of the resin through the transparent vat bottom. The build platform, already lowered to touch the bottom of the resin-filled vat, then rises by the height of one layer. The hardened polymer is peeled from the bottom of the vat but stays attached to the build platform. Then fresh liquid resin flows in from the edges of the partially built part and the laser leaves an impression on the second-from-bottom layer. This process is repeated till the object is printed.
Post-processing
When printing is complete, the object is separated from the platform. It’s time for sprucing up and enhancing its mechanical strength.
First, the support structures are removed gently. Then the part is thoroughly rinsed in alcohol (ethanol or isopropyl alcohol). After the bath, the part is to be dried properly, either naturally or using pressurised air, before post-curing. At this stage, there could be some gentle rounds of sanding to smooth out the rough patches on the surface.
Post-curing takes place in a UV oven. To improve aesthetics and functionality, optional post processes such as sanding, polishing, priming and painting could be undertaken. But this might affect tolerances and details.
Materials
SLA uses photosensitive thermoset polymers called resin. The liquid resin reacts to certain wavelengths of light, polymerising monomers and oligomers into rigid or flexible geometries. Depending on a host of factors, the wide variety of resins can be classified into the following categories:
Standard resins: Used for prototype fabrication and R&D, they offer good resolution and medium strength.
Engineering resins: These come in tough, durable, heat-resistant and flexible variants. For rigidity, they are reinforced with ceramic particles such as glass. Apart from rapid 3D prototyping, they have functional applications in various industries.
Medical and dental resins: Class I and Class II categories are used for custom surgical guides and dental-orthodontic applications, respectively. While the former offers a high degree of precision and smooth surface finishes, the latter is preferred for its biocompatibility and fracture-resistance.
Castable resins: Used for making, among other things, small jewellery with complex designs, they have zero ash content after burn out and offer a smooth finish.
Biomaterial resins: SLA is used in 3D printing of tissue to produce complex constructs that preserve cell viability and recapitulate the physiology of tissues. Biomaterial resins are formulated as aqueous solutions. They are either made up of biological polymers such as gelatin and dextran, or synthetic polymers such as polyethylene glycol.
Support structures: Supports are the cardinal element of an SLA print as they hold the model in place throughout the fabrication process. Both styles of printing require the use of support structures, as per design. Supports include rafts, scaffolding, and touchpoints. Not providing adequate support can lead to the model separating from the supports, misaligned layers during printing, warping of thin layers or features, deformation, visible lines and even breakage.
Orientation: Model orientation is crucial to print success. Orientation is the alignment of an object within the space of the build volume. It impacts stability, surface finish, printability, and precision. A majority of SLA parts are printed at a tilted angle (30 degrees-45 degrees).
Controlled environment: Keeping a controlled environment for 3D printing is important as heat and humidity can aggravate warping and drooping. Higher temperature also affects the viscosity of the resin.
Layer thickness: Layer height affects both printing time and printing quality. The thinner the layer height, the better the surface quality, culminating in smoother surface and clearer vertical details, but also longer build times. Thinner layers are also prone to warping.
Wall thickness: A supported wall, connected to other walls on two or more sides, should have a minimum thickness of .4 mm; otherwise, it might warp during the peel process. An unsupported wall, connected to other walls on fewer than two sides, should have a minimum thickness of 0.6 mm to prevent warping or detachment from the model.
Like all 3D printing technologies, SLA has its advantages and limitations.
It is preferred by designers and manufacturers for several reasons. Some of them are listed below.
Isotropy: Isotropic parts are equally strong in all directions (along the x, y and z axis), which make them ideal for applications like jigs and fixtures, end-use parts, and functional prototyping, where predictable performance is required.
Smooth surface finish and fine features: When it comes to smooth surface finish, SLA printers are the best, with performance comparable to CNC machining, injection moulding, and extrusion. The thinness of each layer and the laser beam make it possible to build prototypes and end-use products with intricate design.
Accuracy and part precision: SLA 3D printing delivers the tightest dimensional tolerance. It can be trusted to repeatedly create accurate and precise components, which explains their use in manufacturing and dentistry.
Speed: While the size, design (fine details) and precision finish of an object will impact printing time, SLA can produce a fully-functional part in just 24 hours. It also enables rapid prototyping with quick iterations, thus reducing product development time.
Costs: SLA is cost-effective as it can manufacture 3D items directly from the CAM/CAD file, without tooling.
Material choice: Compared to selective laser sintering, SLA is much more versatile. There are more than 50 resin options for materials.
Fragility: SLA printers are known for creating delicate and detailed parts. Long exposure to UV light, including sunlight, adversely impacts the physical properties and appearance of SLA parts, making them brittle and yellow in colour.
Wastage: SLA objects need support during the printing process, which take up build space and also substantially increase material usage. These supports are removed in the post-printing stage and can’t be recycled, leading to wastage.
Environment-unfriendly and health risks: Though many companies now offer biodegradable resins, a large variety of resins is considered toxic for the environment. Their chemical content can cause skin irritation, eye damage, respiratory problems in humans, or allergic reactions in humans.
Post-processing: Elaborate post-processing requirements can add to both time and costs.
Industrial SLA printers can cost a minimum of $60,000 and require trained technicians to operate. Professional-grade SLA desktop printers cost upwards of $3,000. Cost-optimisation is key for any business to survive and thrive in a competitive environment. Here are a few ways to streamline SLA printing costs.
Laser or Digital Light Processing technology (DLP) or LCD: All three are photopolymer-based 3D printing technology. Laser beams ensure superior quality but the laser printers are costly. DLP uses LED or digital micro devices and LCD uses LCD screens. Both are faster and more affordable than SLA but cannot match up to the latter’s surface resolution.
Hollowing out parts: Hollowing the model significantly reduces resin consumption and print time, which cuts down overall manufacturing costs.
Optimising part orientation: Getting the part orientation right can minimise supports and printing time. This, in turn, saves resin/powder, energy use, and labour.
Right choice of materials: It is important to match material properties with application of end-use products for best output.
Post-processing: Polishing, sanding, painting and other finishing touches to meet the desired specifications of the end-product add to the overall cost of the project.
Additionally, one needs to factor in maintenance and repair costs as well as labour charge as part of overall expenditure.
The global SLA 3D Printer market size was valued at US$ million in 2022. The SLA technology market is projected to witness a compound annual growth rate of 19.27% to grow to US$6.746 billion by 2028, from US$1.964 billion in 2021. The major players in the global stereolithography printing market are North America, Europe, China, Japan, Southeast Asia, and India.
SLA parts are used in many industries, including healthcare, architecture, automotive, jewellery, and aviation.
Healthcare: Due to its versatility, SLA is used in the development of tablets, microneedles, bio-medical scaffolds, dental prosthetics, and medical devices.
Architecture: SLA can create detailed scale models of buildings, landscapes, and urban plans, enabling visualisation and presentation of architectural designs
Automotive: Industry giants such as BMW, Lamborghini and Jaguar Land Rover have been using SLA for design, structural verification, replica prototypes, functional prototypes and casting.
Jewellery industry: Thanks to SLA technology, jewellers can craft delicate pieces with exquisite design.
Aviation: Many aircraft original equipment manufacturers are using SLA printed components in their models. In March 2021, Airbus extended its contract with Stratasys to develop SLA 3D-printed polymer components for airplane cabin interiors.
As the oldest additive technology, SLA is deemed a reliable technology for rapid prototyping and on-demand manufacturing. It is still evolving as researchers, engineers, innovators and entrepreneurs explore its possibilities to find solutions to complex real-life challenges.
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