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Stereolithography
Stereolithography is one of the most important Additive Manufacturing technologies currently available. This involves the curing or solidification of a liquid photosensitive polymer through using irradiation light source, this causes a chemical reaction bonding large numbers of small molecules together. With the use of computer aided design or computer aided manufacturing (CAD/CAM) software, the UV laser will draw a pre-programmed design on to the surface of the photopolymer vat.

Photopolymers are sensitive to ultraviolet light, so the resin can be hardened and shapes a solitary layer of the ideal 3D piece. Then, at that point, the form stage drops down one layer, and a sweeper will re-cover the outer layer of the tank with resin. This cycle is repeated for each layer of the pattern until the 3D piece is finished. To get a better surface finish, the resin needs to be washed by a solvent.

History
Stereolithography process first appeared in the early 1970’s, it is the oldest AM process, Japanese researcher named Dr. Hideo Kodama invented this approach, using ultraviolet light to cure Photosensitive polymers. Later Charles W. Hull patented the technology in 1986 and described it as creating objects by successively “printing” thin layers of materials curable by ultraviolet light. This process was invented with the intent to allow engineers to manufacture prototypes of designs to reduce the time and be efficient.

In the 1984 Chuck Hulls patent was abandoned by the French General Electric Company due to other inventors filling their patent before him, but later in 1986 his patent was granted permission. Then Chuck Hull the co-founder of 3D systems produced the world first three-dimensional printer. The company that now sells 3D printers that use a variety of technologies. They range from entry-level kits to advanced commercial systems.

Stereolithography had success in the automotive industry where it achieved industry status, there are more demanding sectors where SLA could prove its significance in the future.

Basic principles of the technology
The process begins with an ultraviolet laser directed onto the surface of a liquid thermoset resin, the SL machine starts the 3D printing process by drawing the layers of the support structures, followed by the part itself. The build platform drops down once a layer is imaged on the resin surface, and a recoating bar moves across the platform to apply the next layer of resin. Layer by layer, the process is continued until the structure is complete. Newly constructed parts are removed from the machine and brought to a lab where any remaining resins are removed using solvents. The support structures are manually removed once the pieces are thoroughly clean. After that, pieces go through a UV-curing cycle to thoroughly firm the part's exterior surface. To avoid degrading, parts made in SL should be utilised with limited UV and humidity exposure.

Materials
There are 4 types of resin categories the following are: '''Different Types of materials used for stereolithography are: '''
 * Standard resins, which are used for general prototyping and modelling parts
 * Castable resins, for zero ash-content after burnout
 * Dental and medical resins, for the use of biocompatibility certifications
 * Engineering resins, for specific mechanical and thermal properties
 * Polycarbonate: A ceramic-filled material that has properties like strength, stiffness, and temperature resistance, but it can also be brittle.
 * ABS: Variations on ABS imitations include clear, low-viscosity resins that can be machined transparently. Opaque black plastic that blocks almost all visible light, even in thin areas. Transparent, colourless water-resistant plastic  for lenses and models for flow visualization. Micro-resolution resin that can manufacture parts with ultra-fine characteristics and strict tolerances.
 * Polypropylene/ABS blend: this is a Strong, white plastic which works well for joints like snap fits, assemblies, and other difficult applications.
 * Polypropylene: A flexible and durable resin that imitates polypropylene. This can withstand tough mechanical treatments and is great for fine details like sharp corners, thin walls, and small holes.
 * Polypropylene/ABS blend: this is a Strong, white plastic which works well for joints like snap fits, assemblies, and other difficult applications.
 * Polypropylene: A flexible and durable resin that imitates polypropylene. This can withstand tough mechanical treatments and is great for fine details like sharp corners, thin walls, and small holes.
 * Polypropylene: A flexible and durable resin that imitates polypropylene. This can withstand tough mechanical treatments and is great for fine details like sharp corners, thin walls, and small holes.

In polymerisation relatively small molecules, called monomers, combine chemically to produce a very large chainlike or network molecule, called a polymer.

These are the 5 phases of when Photopolymerisation occurs:


 * 1) Photoinitiator is mixed with a monomer
 * 2) Photonic excitation and free radical generation (from laser)
 * 3) Chain initiation
 * 4) Chain propagation
 * 5) Chain termination

Applications
Stereolithography is widely used for creating 3D models of several anatomical regions of patients. This helps doctors provide treatments while saving significant time and costs, Furthermore, it is customised to each individual person.

Rapid prototyping with 3D printing lets engineers turn their ideas into working concepts, this can further help them develop mass productions or alter them if the product is imperfect. This reduces the overall manufacturing costs and defects, increases quality, speeds up assembly, and maximises labour effectiveness.

3D printers are multifunctional devices for immersive learning and advanced research. While supporting the STEAM curriculum in science, engineering, arts, and design, it can promote creativity and introduce your expertise to students. Jewellery professionals use CAD and 3D printing to quickly prototype designs and create large batches of ready-to-cast parts. Digital tools allow them to create consistent, sharp, and detailed works without the hassle and variability of wax carving.

Hearing specialists and earmold labs use digital workflows and 3D printing to make higher quality custom ear products more consistent for applications such as hearing aids behind the ears, hearing protection, custom earplugs, and earplugs are manufactured in large quantities.

Digital dentistry reduces the risks and uncertainties caused by human factors and provides greater consistency, accuracy, and accuracy at all stages of the workflow to improve patient care. 3D printers can produce a wide variety of high-quality custom products and devices at low cost, with excellent fit and reproducible results.

High resolution physical models are widely used in sculpture, character modelling, and prop creation. 3D-printed parts are also used in stop-motion movies, video games and bespoke costumes with detail in the appearance. Aerospace components are also manufactured using this technique, this is due to having a complex geometry. Engineers designing and manufacturing for commercial and military aerospace platforms need flight worthy components made from high performance materials.

Advantages
There are many advantages of using SLA to manufacture parts therefore engineers, designers, and manufacturers choose SLA because of its fine features, smooth surface finish, extreme portion accuracy and precision and mechanical properties like isotropy, water-tightness, and fabric versatility.

Stereolithography is an excellent choice for rapid prototyping and project designs that require the production of extremely accurate and finely detailed components. Complex internal and external contours and shapes can be created with this process, for example a blood test sample plate, as it has fine grooves that need to be precisely made.

There are a lot of types of resins that can be used depending on what properties are required for the components, this is good because more variety of parts can be produced.

The surface finish and accuracy are very good compared to the other Additive Manufacturing techniques like SLS and inkjet printing, this makes it more suitable for numerous components. Parts used to make sand casting tools; they are also used for flow test visualisation transparency for fluid flow, this helps analyses data for research.

This process does not require any attendant while it is running as it runs automatically and stops when the process is finish making it more efficient. It also runs at a fast pace making it more efficient than the other AM processes, functional parts can be manufactured with a day. The time it takes to build a part depends on its complexity and the size. It can take a few hours to build small simple component and more than a day to build a large complex component.

3D printing is still a new manufacturing technique which means there are still improvements being made to make it cheaper and more efficient to use. Further advances will help this technique be used in more industries where it has not yet been introduced to or has limitations within.

Disadvantages
There are several disadvantages that this process has, Tolerance and surface finish are not mutually exclusive. When a part is oriented one way, it may result in increased accuracy but a less appealing surface finish. Orienting the part in a different direction, on the other hand, may result in a smoother finish with less exact tolerances.

Stereolithography is considered an expensive process as it is very complex and needs time and energy to run, as the machines can also cost from around £3,000 to £750,000 and with this the need of CAD/CAM which adds extra costs.

This is an untidy process and requires a finishing off process to help clean off any excess resin, this means delays in creating the components and having another step in the process to complete it. Newly made parts need to be washed, further cured, and then dried.

A support structure must be modelled and designed in CAD for each part, this would slow down the process if there are several components that need to be made.

Large, thin, and flat structures are difficult to produce accurately, as they are manufactured layer by layer. There is also a limit of materials or resins that can be chosen for this process.

Post process is used to clean off any access resin left on the surface, which is a waste of material, this needs to be reduced to a small amount as particular resins are expensive to buy or produce. This is also and environmental issue. Post processing methods used include water jetting, sanding, a chemical soak and rinse, air or heat drying, assembly and others.

Stress generation on solidification can lead to distortion the process is not controlled properly during the machine set up phase and the choice of resin used.