Stereolithography (SLA)

Prusa SL1 Resin Printer and CW1 Curing & Waching Machine
Resin Types: Prusa Tough (Orange, Black)

Introduction

Stereolithography (SLA) belongs to a family of additive manufacturing technologies known as vat photopolymerization, commonly known as resin 3D printing. These machines are all built around the same principle, using a light source—a laser or projector—to cure liquid resin into hardened plastic. The main physical differentiation lies in the arrangement of the core components, such as the light source, the build platform, and the resin tank. SLA 3D printers use light-reactive thermoset materials called “resin.” When SLA resins are exposed to certain wavelengths of light, short molecular chains join together, polymerizing monomers and oligomers into solidified rigid or flexible geometries. SLA parts have the highest resolution and accuracy, the sharpest details, and the smoothest surface finishes of all 3D printing technologies, but the main benefit of the stereolithography lies in its versatility. Material manufacturers have created innovative SLA resin formulations with a wide range of optical, mechanical, and thermal properties to match those of standard, engineering, and industrial thermoplastics.

Diagram of the key parts of a typical desktop SLA printer

Principle

When the process starts, an ultraviolet laser “draws” the first layer of the print into the photosensitive resin. Wherever the laser hits, the liquid solidifies. The laser is directed to the appropriate coordinates by a computer-controlled mirror. Most desktop SLA printers work upside-down. That is, the laser is pointed up to the build platform, which starts low and is incrementally raised. After the first layer, the platform is raised according to the layer thickness (typically about 0.1 mm) and the additional resin is allowed to flow below the already-printed portion. The laser then solidifies the next cross-section, and the process is repeated until the whole part is complete. The resin that is not touched by the laser remains in the vat and can be reused. After finishing the material polymerization, the platform rises out of the tank and the excess resin is drained. At the end of the process, the model is removed from the platform, washed of excess resin, and then placed in a UV oven for final curing. Post-print curing enables objects to reach the highest possible strength and become more stable.

Why Choose SLA 3D Printing?

Isotropy

Because 3D printing creates parts one layer at a time, completed prints may have variations in strength based on orientation of the part relative to the printing process, with different properties in X, Y, and Z axes. Extrusion-based 3D printing processes like fused deposition modeling (FDM) are known for being anisotropic due to layer-to-layer differences created by the print process. This anisotropy limits the usefulness of FDM for certain applications, or requires more adjustments on the part geometry side to compensate for it. In contrast, SLA resin 3D printers create highly isotropic parts. Achieving part isotropy is based on a number of factors that can be tightly controlled by integrating material chemistry with the print process. During printing, resin components form covalent bonds, but layer to layer, the part remains in a semi-reacted “green state.” While in the green state, the resin retains polymerizable groups that can form bonds across layers, imparting isotropy and watertightness to the part upon final cure. On the molecular level, there is no difference between X, Y, or Z planes. This results in parts with predictable mechanical performance critical for applications like jigs and fixtures, end-use parts, and functional prototyping.

Watertightness

SLA printed objects are continuous, whether producing geometries with solid features or internal channels. This watertightness is important for engineering and manufacturing applications where air or fluid flow must be controlled and predictable. Engineers and designers use the watertightness of SLA printers to solve air and fluid flow challenges for automotive uses, biomedical research, and to validate part designs for consumer products like kitchen appliances.

Accuracy and Precision

Industries from dental to manufacturing depend on SLA 3D printing to repeatedly create accurate, precise components. For a print process to produce accurate and precise parts, multiple factors must be tightly controlled. The combination of the heated resin tank and the closed build environment provides almost identical conditions for each print. Better accuracy is also a function of lower printing temperature compared to thermoplastic-based technologies that melt the raw material. Because stereolithography uses light instead of heat, the printing process takes place at close to room temperature, and printed parts don't suffer from thermal expansion and contraction artifacts.

Fine Features and Smooth Surface Finish

SLA printers are considered the gold standard for smooth surface finish, with appearances comparable to traditional manufacturing methods like machining, injection molding, and extrusion. This surface quality is ideal for applications that require a flawless finish and also helps reduce post-processing time, since parts can easily be sanded, polished, and painted. In comparison, FDM and SLS printers typically print Z-axis layers at 100 to 300 microns. However, a part printed at 100 microns on an FDM or SLS printer looks different from a part printed at 100 microns on an SLA printer. SLA prints have a smoother surface finish right out of the printer, because the outermost perimeter walls are straight, and the newly printed layer interacts with the previous layer, smoothing out the staircase effect. FDM prints tend to have clearly visible layers, whereas SLS has a grainy surface from the sintered powder. The smallest possible detail is also much finer on SLA, given 85 micron laser spot size on a high end desktop model, in comparison with 350 microns on industrial SLS printers, and 250–800 micron nozzles on FDM machines.

Material Versatility

SLA resins have the benefit of a wide range of formulation configurations: materials can be soft or hard, heavily filled with secondary materials like glass and ceramic, or imbued with mechanical properties like high heat deflection temperature or impact resistance. Material range from industry-specific, like dentures, to those that closely match final materials for prototyping, formulated to withstand extensive testing and perform under stress. In some cases, its this combination of versatility and functionality that leads to companies to initially bring resin 3D printing in-house. After finding one application solved by a specific functional material, it’s usually not long before more possibilities are uncovered, and the printer becomes a tool for leveraging the diverse capabilities of various materials.