A comprehensive guide to 3D printing with SLA, covering the printing process, design specifications, material selection, and technical limitations.
Table of Contents
Introduction
Printing with SLA
Designing for SLA Printing
SLA Materials
Post-processing
Limitations
Rules of Thumb
Introduction
Stereolithography (SLA) is a 3D printing method that uses laser light to cure photopolymer resin layer by layer. SLA is best suited for producing small, smooth parts with high precision and detail.
This article outlines the SLA printing process, demonstrates the limitations and advantages of printing parts with SLA, and discusses the most common SLA materials.
Printing with SLA
Printing Process
A typical desktop SLA machine contains a UV laser that precisely scans the 2D contour of an object and cures the photopolymer resin layer by layer in a resin vat with a transparent bottom.
As the UV light passes through, it creates a cured thin film between the build plate and the bottom of the vat, which adheres to the plate. The newly printed film is then peeled away from the bottom (the peeling method depends on the machine's operation, perhaps by sliding or rocking the vat). After peeling, the build plate moves up to a height equal to the layer thickness, and the process is repeated until the object is complete.
For successful SLA printing, it is crucial to reduce the peel force on the printed layer during separation. High stress is generated at the edges during peeling, which can increase the failure rate and cause warping (when the object layer fails to attach to the build plate and instead sticks to the bottom of the vat).
The SLA printing process
Print Orientation
When orienting SLA parts, the biggest concern is the Z-axis cross-sectional area. The forces involved in printing into the resin tank are proportional to the 2D cross-sectional area of the printed object.
Because of this, parts are printed at an angle on the build plate, and reducing supports is not the primary concern (as shown in the figure below). Minimizing the cross-sectional area along the Z-axis is the best method for orienting SLA printed parts.
Model oriented sub-optimally with a large Z-axis cross-sectional area. In this orientation, supports are minimized, but the likelihood of print failure is high.
Print volume = 33.39ml, print time = 2 hours 27 minutes
Model reoriented at an angle to reduce the Z-axis cross-sectional area. The significant increase in supports is justified by the reduced likelihood of print failure.
Print volume = 36.95ml, print time = 4 hours 7 minutes
As a designer, it's important to understand why part orientation affects SLA print quality.
The necessity of orienting parts to reduce the Z-axis cross-sectional area often leads to the need for a large amount of support material in the model.
In some cases, a design may require so much support that printing with SLA becomes cost-ineffective or detrimental to the part's appearance (once the supports are removed), resulting in a visually unsatisfactory final product.
Limiting the number of horizontal parts, hollowing out parts, and reducing the cross-sectional area are steps designers can take to optimize SLA designs.
Isotropy
SLA prints are isotropic because the layers chemically bond to each other during printing, resulting in nearly identical physical properties in the x, y, and z directions.
Whether a printed part is parallel or perpendicular to the build plate,
the final material properties of the part are not significantly affected.
Designing for SLA Printing
Print Features
The level of detail SLA printers can produce depends on the laser spot size and resin properties. General guidelines for SLA design are as follows:
| Feature | Description |
 | Supported Walls - Walls that are connected to the rest of the wall on at least two sides, thus having little to no warping. These should be designed with a minimum thickness of 0.4mm. |
 | Unsupported Walls - Walls that are connected to the rest of the part on fewer than two sides are very likely to warp or detach from the print. These walls must have a minimum thickness of 0.6mm and should be designed with filleted bases (where the wall connects to the rest of the print) to reduce stress concentration at the joint. |
 | Overhangs - These are rarely an issue with SLA prints unless the model is not sufficiently supported, both internally and externally. Printing without supports will usually lead to warping of the print, but if printing without supports is necessary, any unsupported overhangs must be kept shorter than 1.0mm and at an angle of at least 19° from the horizontal. |
 | Embossed Details (including text) - Any features on the model that slightly protrude from the surrounding surface. Their height must be at least 0.1mm above the print surface to ensure clear detail. |
 | Engraved Details (including text) - Any features engraved into the model. If these details are too small, they might merge with the rest of the model as it prints. Therefore, these details must be at least 0.4mm wide and at least 0.4mm deep (distance from the model surface to the recessed detail). |
 | Horizontal Bridges - Bridges between two points on a model can be successfully printed, but designers must remember that wider bridges must be shorter than narrower ones (less than 21mm). Wider bridges have a larger Z-axis contact area, which increases the likelihood of print failure during peeling. |
 | Holes - Holes with a diameter of less than 0.5mm on the x, y, and z axes may close up during the printing process. |
 | Connections: ● Clearances for active parts: 0.5mm. ● Clearances for assembly connections: 0.2mm. ● Clearances of 0.1mm for press-fit or snap-fit connections. |
Resolution
SLA can achieve higher resolutions than FDM because it uses laser light to cure the material. The XY-direction (or horizontal resolution) of SLA prints
depends on the laser spot size, which can range from 30 to 140 microns. This is not an adjustable parameter for printing. The minimum feature size cannot be smaller than the laser spot size.
The Z-direction resolution (or vertical resolution) ranges from 25 to 200 microns. Choosing the vertical resolution is a trade-off between speed and quality. For parts with few curves or details, there is almost no visual difference between a 25-micron print and a 100-micron print. For comparison, desktop FDM machines usually print Z-axis layers from 150 to 400 microns.
Venting
SLA machines print solid, high-density models, but if the functional parts of the print are hollow, the amount of material required and the printing time will be significantly reduced. It is recommended that hollow objects have a wall thickness of at least 2mm to reduce the risk of failure during printing.
If printing hollow parts, drainage holes must be added to prevent uncured resin from being trapped inside the final printed object. This uncured resin creates a pressure imbalance within the cavity and causes what is known as "vacuum breaking."
Small failures (cracks/holes) will spread throughout the part and, if not corrected, will lead to complete failure or even an explosion of the part. Drainage holes should be at least 3.5 mm in diameter, and each hollow section should have at least one hole.
SLA Materials
The table below lists some of the more common SLA resins.
| Resin Type | Description | Application |
| Standard Resins | Most commonly used for general printing and offer high-resolution surface finishes with details smaller than 25 microns. These resins do not provide special material properties and are generally more brittle than standard FDM materials. | Ideal for non-functional, high-detail prototypes or models. |
| Engineering Resins | SLA resin manufacturers have recently entered the engineering field by simulating common engineering plastics, offering elastic, high-temperature resistant resins similar to ABS or polypropylene. These resins provide superior engineering performance without sacrificing print quality, but at a higher cost. | Applicable for tough, elastic, and high-temperature resistant parts. |
| Dental Resins | For general orthodontic purposes, general-purpose resins or castable resins are typically used. Class 1 and Class 2 biocompatible resins released in the past year can now also be used with SLA technology to create surgical guides. These resins are highly precise and durable enough to be autoclaved before surgery. | Dental applications |
| Castable Resins | These resins are specially designed for detailed and intricate functional prints and are formulated for direct investment casting. This resin can produce very small details, with a minimum feature size of 0.2mm. When properly cured, the resin burns out with virtually no ash or residue. | Jewelry, fine models, and investment casting applications |
A range of products printed with SLA resins (provided by Formlabs)
Post-processing
A range of surface finishes can be achieved on SLA printed parts. The desired surface smoothness is usually influenced by cost and application. For a detailed guide on the most common SLA surface finishes, please refer to this article.
Limitations
Build Volume
SLA printers typically have smaller build volumes than most FDM printers, except for industrial-grade 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) measures 223mm × 223mm × 205mm.
When an SLA print geometry exceeds the printer's volume, it can be printed as smaller parts and then assembled.
The best method for bonding SLA printed parts together is using a 5-30 minute epoxy.
Cost vs. FDM
SLA resins have a higher volumetric cost compared to the filaments used in FDM printing. Due to this, SLA printing is generally more expensive, but SLA's ability to print intricate details makes it a competitive option compared to many industrial 3D printing technologies. One kilogram of standard SLA resin typically costs around $150, whereas 1kg of ABS filament for FDM would cost approximately $25.
Material Properties
SLA parts are generally not suitable for producing load-bearing functional parts. The nature of SLA resins means that parts are often brittle, not as stable over time as other 3D printing materials, and undergo some changes.
Most SLA printed parts require a UV post-curing machine. Post-curing can give parts higher strength and make them more stable.
Rules of Thumb
SLA is ideal for small parts that require a smooth surface finish (similar to injection molding) and high precision.
Support structures are crucial for successful and accurate SLA printing. If a good surface finish is desired on a particular surface, the part should be oriented so that this surface does not come into contact with support material (usually facing upwards).
SLA parts generally have poor mechanical properties and are best suited for non-functional prototypes, enclosures, and visual models.
Designing features for SLA:
| Feature | Design Specification |
| Supported Walls | Minimum thickness of 0.4mm |
| Unsupported Walls | Minimum thickness of 0.6mm |
| Overhangs | Less than 1.0mm in length, at an angle of at least 19° from the horizontal. |
| Embossed Details | Minimum height of 0.1mm |
| Engraved Details | Minimum width of 0.4mm, minimum depth of 0.4mm |
| Connections | 0.2mm for assembly connections and 0.1mm for fit connections |
| Holes | Minimum diameter of 0.5mm |
Original source:https://www.3dhubs.com/knowledge-base/how-design-parts-sla-3d-printing