Additive Manufacturing

How Stereolithography (SLA) works

What is Stereolithography?

Stereolithography (SLA) is an additive manufacturing (AM) process that creates three-dimensional objects from liquid resins. When Charles Hull patented the process in 1984, he coined the term “stereolithography.” SLA is now one of the most popular rapid-prototyping technologies in the family of AM processes.

Most AM systems require that build material is liquefied, deposited in an ultrathin layer and solidified. With SLA, changes in material state are reversed — a liquid becomes a solid. To accomplish this, SLA depends on the process of photopolymerization. Light from an ultraviolet (UV) laser causes cross-linking — the bonding of one polymer chain to another, which hardens the polymer.

How SLA Works

First, it is necessary to prepare a CAD file that defines the parameters of a three-dimensional object. Next, CAD data is used to prepare an STL file which “slices” the digital representation of the object into multiple cross-sections.

This information directs the stereolithography apparatus to create the desired 3-D object. The apparatus consists of a tank filled with liquid photopolymer, a perforated print bed, an ultraviolet laser and an STL file to direct the movement of both the laser and the print bed. The use of a tank filled with a liquid photopolymer led to the adoption of the term “vat photopolymerization” to describe the process.

As the UV laser selectively strikes thin cross-sections of liquid resin, they are cured or hardened in milliseconds. The laser is positioned either above or below the vat of resin. Shrinkage is managed because any heat generated dissipates in the bath of liquid resin. The print bed is lowered incrementally to progressively print successive layers from the bottom up. Adjacent layers instantly bond during the curing process.

When the process is complete, the operator lowers the vat of liquid resin to expose the object. Isopropyl alcohol is often used to remove residual liquid resin from the object. It is possible to perform additional curing in an ultraviolet oven to further harden the object.

Model geometries determine whether support structures are required during printing. Since the SLA process utilizes only one type of resin at a time, support structures are made from the same material used to create the desired object. Therefore, supports cannot be dissolved away; they must be removed mechanically. Specialized software calculates the number and placement of supports. In post-processing, supports are broken away or removed with pliers. To facilitate removal, the rib-like supports have small tips that minimize contact with the object.

Points of contact between the object and the supports are often wet-sanded, which also prepares the object for priming and painting when desired. A clear UV acrylic may be applied to hard plastic parts.

SLA Materials and Applications

One big advantage of stereolithography is the wide variety of liquid photopolymers compatible with the process. The different resins used in stereolithography yield plastics varying in pliability, hardness and toughness.

Durable resins produce wear-resistant ball joints, snap-fits and impact-resistant cases. Tougher resins that emulate the properties of acrylonitrile butadiene styrene (ABS) are used for more rugged prototypes.

Castable resins produce finely detailed objects, allowing jewelers to proceed directly from digital files to a 3-D object ready for investment casting. The SLA process offers qualities that jewelers value, including smooth surfaces and highly accurate prong placement.

High temperature resins endure hot fluid and air flows, making them a good choice for heat-resistant fixtures and mold prototypes. Flexible resins simulate the performance of soft rubber, making it an appropriate selection for the production of grips, handles and parts designed to cushion impact. Flexible resins are also used to add soft-touch ergonomic functionality to certain products.

Since the SLA process can be used to create high-resolution objects with smooth surfaces, it is implemented to fabricate metal-plated prototypes that can replace more cumbersome sheet metal prototypes. It is also used to print models for displays and demonstrations. SLA systems produce investment casting patterns and vacuum casting masters. Some resins result in rigid, functional prototypes for manufacturing, while others are perfect for the creation of visual prototypes used in market research and photography shoots.

Stereolithography is increasingly used to fabricate biocompatible objects for dental applications. Biocompatible transparent resins are ideal for printing long-lasting orthodontic appliances like retainers. Other resins are used to print dentures and models of crowns and bridges.

SLA medical modeling systems produce medical prototypes and anatomical models. For example, researchers at the Walter Reed National Military Medical Center studied the viability of SLA for rapid prototyping related to head and neck reconstruction. The study, “Accuracy of Rapid Prototype Models for Head and Neck Reconstruction,” related to the use of SLA models in the development of highly specialized prostheses. Another study at Walter Reed looked at the use of anatomical models fabricated through stereolithography.

Other potential applications for the SLA process continue to emerge. For example, it is possible to suspend ceramic microparticles in a photopolymer resin to build ceramic objects. In post-processing, firing in a programmable kiln cures and hardens the ceramic object. When desired, glazing completes the fabrication process.

SLA Advantages

The stereolithography process can produce highly detailed, durable objects with smooth surfaces. Intricate structures and smooth surfaces are possible in part because of the 20-50 micron resolutions realizable with typical SLA systems.

In general, the process is ideal for the production of low-volume parts and prototypes with complex geometries. Accurate, repeatable production of end-use parts is yet another advantage. The use of UV-resistant durable resins can extend the lifespan of SLA parts subject to sunlight or other UV light sources.

Stereolithography differs from most AM processes in that it can be used to produce transparent objects, a real advantage in rapid prototyping of optical items and headlight covers. Transparency is also a plus in the production of dental appliances, anatomical models and architectural models. In post-processing, fine sanding, polishing and clear coating optimize transparency.

Rapid prototyping is a key advantage of stereolithography. In fact, SLA is one of the most popular AM processes for quick and efficient prototyping. Traditionally, the prototyping process is dependent on repetitive design modifications following frequent retesting.

In this kind of design environment, the inherent efficiency of the SLA process significantly reduces lead times. With lead times measured in hours or days, stereolithography is highly conducive to prototype development. Although fused deposition modeling (FDM) is popular for cost-effective prototyping, SLA is the preferred prototyping method when fine detail and smooth surfaces are desired.

Since the SLA process inherently minimizes resin shrinkage, parts and prototypes exhibit good dimensional accuracy. Careful control of light intensity, exposure time and layer thickness further manages shrinkage. It is also possible to use mathematical models to account for anticipated shrinkage.

Today's SLA systems do more than produce finely detailed objects. As the technology has evolved, machines have increased in size. It is now possible to print 3-D parts up to three meters long, for example, engine block models.

Although stereolithography is among the most mature of the seven major AM processes, it remains one of the most popular for modeling, rapid prototyping of high-resolution objects and the production of customized plastic parts.