What is Stereolithography (SLA) 3D Printing?

Stereolithography (SLA) is one of the common 3D printing technologies in rapid prototyping. This technology uses photosensitive resin as the raw material. This article explains the basic principle of Stereolithography 3D printing, describes how the technology works, the complete process flow step by step, and suitable materials.

Overview of Stereolithography (SLA)

Stereolithography (SLA) was one of the earliest rapid prototyping technologies developed. It is also one of the most deeply researched, most mature, and most widely used rapid prototyping technologies today.

Stereolithography uses photosensitive resin as the raw material. It focuses a laser (ultraviolet light) with a specific wavelength and intensity onto the surface of the photosensitive material. This makes the material solidify from point to line, and from line to surface in order, completing the drawing of one layer section. Then the build platform is raised or lowered by one layer thickness in the vertical direction, and the next layer is irradiated and solidified. This cycle of curing and moving is repeated, and the layers are stacked to complete the printing of a three-dimensional solid part.

Principle of Stereolithography

Stereolithography technology was first successfully developed by Charles Hull at the Massachusetts Institute of Technology in the United States in 1986. It received a patent in 1987. It is the earliest 3D printing technology to appear, and it is currently the most mature and most widely used 3D printing technology. It mainly uses photosensitive resin as the raw material and solidifies it layer by layer under computer control using an ultraviolet laser device. The SLA process can print prototypes with high surface quality and dimensional accuracy, and complex geometric shapes, in a simple, fast, and fully automatic way.

The printing effect of stereolithography is affected not only by the printing equipment, but also greatly by the performance of the photosensitive resin material. The printing material used must have suitable viscosity. After curing, it needs to have a certain strength, and the shrinkage and distortion during and after curing should be small. More importantly, to achieve high-speed and precise printing, the photosensitive resin must have suitable photosensitive properties. It needs to be fully cured under relatively low-energy irradiation, and the curing depth of the resin should also be appropriate.

The working principle of SLA is shown in the figure. Under computer control, the ultraviolet laser component scans and irradiates the surface of the liquid photosensitive resin point by point according to the data of the layered sections of the design model. This causes the thin layer of photosensitive resin in the irradiated area to undergo a polymerization reaction and solidify, thus forming a thin layer of cured printing. After completing the curing of one layer section, the build platform descends along the Z-axis by one layer thickness. Due to the flow characteristics of the liquid, the printing material will automatically form a new layer of liquid resin on the surface of the previously cured resin. Therefore, the irradiation component can directly perform the curing operation for the next layer. The newly cured layer will firmly bond to the previously cured part. The process of irradiation and descending is repeated in a cycle until the entire part is printed. However, after printing is completed, the prototype must still be taken out of the resin and undergo post-curing treatment. The final product is obtained through strong light, electroplating, painting, or coloring treatments.

It should be noted that because some photosensitive resin materials have very high viscosity, it is difficult for the liquid surface to level quickly in a short time after each layer is irradiated and cured. This will affect the accuracy of the printed model. Therefore, most SLA equipment is equipped with a scraper blade. After the build platform descends each time, the scraper performs a scraping operation. This allows the resin to be coated very evenly on the next layer. After light curing, higher accuracy can be achieved, and the surface of the final printed product becomes smoother and flatter.

The characteristics of SLA technology are high precision, good surface quality, and an almost 100% raw material utilization rate. It can be used to print parts with particularly complex shapes and very fine details. It is very suitable for rapid prototyping of small-sized parts. However, the disadvantage is that the price of both the equipment and the printing raw materials is relatively high.

At present, SLA technology is mainly used for making molds and models. It can also be used by adding other components to the raw material to replace wax patterns in investment casting. Although SLA technology has a fast printing speed and high precision, because the printing material must be based on photosensitive resin, and photosensitive resin inevitably produces shrinkage during the curing process, which causes stress or deformation, a major difficulty in promoting this technology at present is the urgent need for photosensitive materials with small shrinkage, fast curing, and high strength.

Process of Stereolithography

The process of stereolithography (SLA) technology can generally be divided into four stages: pre-processing, prototype making, cleaning, and curing treatment.

1. The pre-processing stage mainly involves data preparation work for the printing model. It specifically includes steps such as data conversion of the CAD design model, determining the placement orientation, adding supports, and slicing into layers.
2. The stereolithography process is the actual printing process of the SLA equipment. Before formal printing, the SLA equipment usually needs to be started in advance so that the temperature of the photosensitive resin raw material reaches the preset reasonable temperature. Starting the ultraviolet laser also requires some time.
3. Cleaning the model mainly involves wiping off excess liquid resin, removing and trimming the supports of the prototype, and sanding the step textures formed by layer-by-layer curing.
4. For various methods of stereolithography, post-curing treatment is generally required, such as overall post-curing treatment using an ultraviolet oven.

Characteristics of Stereolithography

The advantage of stereolithography technology is fast forming speed and high prototype precision. It is very suitable for making small-sized workpieces that require high precision and have complex structures.

However, stereolithography rapid prototyping technology also has two shortcomings. First, photosensitive resin raw materials have certain toxicity, so operators must take protective measures when using them. Second, the finished products of stereolithography perform very well in overall appearance, but the material strength is still not comparable to real manufactured products. This greatly limits the development of this technology and restricts its application mainly to prototype design verification. A series of processing steps is still needed later to turn it into industrial-grade products.

The equipment cost, maintenance cost, and material cost of SLA technology are much higher than those of Fused Deposition Modeling (FDM) and other technologies. Therefore, 3D printers based on stereolithography technology are currently mainly used in professional fields. Desktop-level applications are still in the starting stage.

Specifically, the advantages of SLA printing technology are as follows:

• SLA technology appeared early and has high technical maturity after many years of development.
• Printing speed is fast, the photosensitive reaction process is convenient, the product production cycle is short, and no cutting tools or molds are needed.
• Printing precision is high. It can print prototypes and molds with complex structures or shapes that are difficult to make with traditional technologies.
• The software functions are complete. It supports online operation and remote control, which is beneficial for production automation.

Compared with other printing technologies, the disadvantages of SLA technology are:

• SLA equipment is generally expensive, and the cost of use and maintenance is very high.
• It requires precise operation of toxic liquids and has strict requirements on the working environment.
• Due to material limitations, the available materials are mostly resins. This makes the strength, stiffness, and heat resistance of the printed products very limited, and they are not suitable for long-term storage.

Materials for SLA Printing

SLA is suitable for various resin materials. Materials can be selected according to the final use of the part, such as heat resistance, smooth surface finish, or wear resistance. The price of resins varies greatly. Standard materials cost about 70 RMB per liter, while special materials such as castable resin or dental resin cost about 500 RMB per liter.

Tolerance and Capacities

At Getzshape, our 3D printing services cover four main technologies, which are SLA, SLS, SLM, and FDM. Our SLA 3D printing tolerance and capacities are listed below.

Finishes for SLA Printed Components

• Sanding: A basic and important surface finishing step for printed parts. It uses sandpaper with different grits to remove the visible layer lines and make the surface smoother.
• Painting: First, the surface is sanded to make it smoother. Then primer is applied to help the paint stick better. After that, color paint is sprayed or brushed on. Painting can hide layer lines completely, give the part a nice color, and make the surface look smooth. It is widely used for display models, prototypes, and final products.
• Polishing: Making the surface of parts smooth and shiny. After basic sanding, special polishing compounds or tools are used to rub the surface. This process removes small scratches and makes the part look glossy.