How to design parts for SLA 3D printing

Stereolithography (SLA) is a 3D printing process that uses a laser to cure photopolymer resin layer by layer. SLA is best suited for producing small, smooth parts with fine details requiring a high level of accuracy.

This article outlines the SLA printing process, presents the advantages and limitations of designing components to be printed with SLA and discusses the most common SLA materials.

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What is the SLA printing process?

The SLA printing process is highly accurate but can be time-consuming. To achieve the best results, a designer should keep part orientation and isotropy in mind.

SLA printing

A typical desktop SLA machine contains a UV laser to cure a specific layer of a component from a tank of photosensitive resin. The bottom of the tank is transparent and the UV laser is precisely controlled to trace a 2D contour of the object. The laser cures the resin, forming a solid layer.

Each pass of the laser creates a very thin slice of the object. This thin slice is stuck to the layer below it (or the build plate) and the tank bottom. The newly printed layer is then separated from the bottom. Depending on the machine, this can involve peeling, sliding or shaking the tank. The build plate then moves down a layer in thickness and the process is repeated again and again until the part is completed.

For a successful SLA print, it is critical to reduce the forces on the newly printed layers during the separation stage. The separation stage creates areas of high stress along what is, in some cases, a razor-thin edge that can cause high failure rate and warping if the part sticks to the tank bottom rather than the build plate.

Print orientation

When orienting a part for SLA, the biggest concern should be the z-axis cross-sectional area. The forces that determine if a print sticks to the tank are directly proportional to the print’s 2D cross-sectional area. This is why the part is printed to the plate at an angle, and support reduction is not a primary concern (as shown in the images below). When you are orienting parts for SLA prints, try to minimize the cross-sectional area along the z-axis.

For designers, it is important to understand how and why part orientation impacts the quality of an SLA print. When you need to orient components so that the z-axis cross-sectional area is reduced, it can result in the model requiring a significant amount of added support. In some instances, a design may need so much support that printing in SLA no longer proves cost-effective. Or the necessary support can be so detrimental to the appearance of the part that, once support has been removed, the result is visually unsatisfactory. To optimize a design for SLA, limit the number of horizontal members, hollow out components and reduce the cross-sectional area.

Isotropy

SLA prints are isotropic because the layers chemically bond to one another as they print, resulting in nearly identical physical properties in the x, y and z directions. Whether the part is printed parallel or perpendicular to the build plate, its final material properties are not noticeably impacted.

How do you design for SLA printing?

SLA is best suited for producing small, smooth 3D printed parts with fine details to a high level of accuracy.

Print features

The detail level an SLA printer can produce depends on laser spot size and resin properties. General guidelines for designing for SLA are as follows:

Resolution

SLA can achieve much higher resolutions than FDM because it uses a laser to solidify the material. SLA printing resolution in the xy-direction (or horizontal resolution) depends on the laser spot size, and can range anywhere from 30 to 140 microns. This is not an adjustable printing parameter, as the minimum feature size cannot be smaller than the laser spot size.

Resolution in the z-direction (or vertical resolution) varies from 25 to 200 microns. Choosing vertical resolution involves a trade-off between speed and quality. For a part that has few curves or fine details, a print at 25 microns versus a print at 100 microns shows little visual difference. By comparison, a desktop FDM machine typically prints z-axis layers at 150 to 400 microns.

Hollowing and cupping

SLA machines print a solid, dense model. However, if the print is not intended to be a functional part, hollowing the model significantly reduces the amount of necessary material and print time. To reduce the risk of failure during printing, keep the walls of the hollowed print at least 2mm thick.

When 3D printing a hollow part, add drainage holes to prevent uncured resin from getting trapped inside the final print. Uncured resin creates pressure imbalances within the hollow chamber, causing what is known as cupping. Small failures, such as cracks or holes, spread throughout the part, causing complete failure or part explosion if uncorrected. Drain holes should be at least 3.5mm in diameter. At least one hole must be included per hollow section.

What materials do you use for SLA 3D printing?

This table describes common SLA resins.

Designing for SLA post-processing

A range of surface finishes can be achieved on SLA-printed parts. The desired surface finish is often governed by cost and application. Read this detailed guide on the most common SLA surface finishes.

What are the limitations of SLA 3D printing?

For all its capabilities, SLA 3D printing also has constraints. These include the printers’ smaller build volumes, higher costs, and the brittle nature of materials used. 

Print volume

SLA printers generally have a much smaller build volume than most FDM printers, with the exception of commercial machines. The Formlabs Form 2 (a common desktop SLA printer) has a build volume of 145mm × 145mm × 175mm while the Ultimaker 2+ (a common FDM desktop printer) offers 223mm × 223mm × 205mm. When SLA print geometries exceed the printer capacity, they can be printed in smaller sections and then assembled. The best method for bonding SLA-printed components together is a five- to 30-minute epoxy.

SLA versus FDM costs

The cost by volume of SLA resin is higher than the filament used for FDM printing. SLA prints are therefore usually more expensive. However, SLA can be used to print intricate details, which makes it a competitive option compared to many of the more industrial 3D printing technologies.

Cost comparison: A liter of standard SLA resin typically costs around USD$150 while a 1-kilogram roll of ABS filament for FDM costs around USD$25.

Material properties

SLA 3D printed parts are generally not suitable for producing functional parts that can withstand subjected to loads. The nature of SLA resins means that parts are brittle, less stable than other 3D-printed materials over long periods and inclined to experience some creep. Most SLA-printed parts require curing in a UV chamber post-print. Post-curing enables parts to reach their highest possible strength and become more stable.

What are the best design practices for SLA?

• SLA is ideal for small parts that require a smooth surface finish—as compared to injection molding—and a high level of accuracy.

• Support structures are critical for successfully printing accurate SLA parts. If a good finish is required on a surface, the part should be oriented so that the surface does not contact support material, thus typically upward facing.

• SLA parts typically have poor mechanical properties and are best suited for non-functional prototyping, enclosures and visual models.

What are the design specifications for SLA 3D printing?

Want to learn more about 3D printing? Read our full guide: What is 3D printing?