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 details.
This article provides an overview of 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, where photopolymer resin is poured into a resin tank with a transparent bottom. The UV light precisely scans the 2D outline of the object, curing the photopolymer resin layer by layer.
As the UV light passes, a thin cured film will be created between the print platform and the bottom of the tank, adhering to the platform. The newly printed film is then peeled off the bottom layer (the peeling method depends on the machine's operation, perhaps by sliding or shaking the tank). After peeling, the print platform moves up a distance equal to the layer thickness, and the process repeats until the object is complete.
For successful SLA printing, reducing the pulling force during the peeling of printed layers is crucial. High stress is generated at the edges during object pulling, which easily leads to increased object failure rates and warping (where the object layer does not successfully attach to the print platform but instead sticks to the bottom of the tank).
The SLA printing process
Print Orientation
When orienting SLA parts, the biggest concern is the Z-axis cross-sectional area. The force involved in peeling from the resin tank is proportional to the 2D cross-sectional area of the printed object.
Because of this, parts are often printed at an angle on the print platform to reduce support, which is not the primary issue (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, support is 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 support is justifiable, reducing the 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 adding a large amount of support to the model.
In some cases, the design may require a lot of support, making SLA printing no longer cost-effective or detrimental to the part's appearance (once supports are removed), resulting in a visually unsatisfactory final product.
Limiting the number of horizontal parts, hollowing parts, and reducing the cross-sectional area are steps designers can take to optimize SLA designs.
Isotropicity
SLA prints are isotropic because the chemical bonds between layers during printing result in almost identical physical properties in the X, Y, and Z directions.
Regardless of whether the printed part is parallel or perpendicular to the print platform,
the final material properties of the part are not significantly affected.
Designing for SLA Printing
Print Features
The level of detail an SLA printer can produce depends on the size of the laser spot and the resin properties. General guidelines for SLA design are as follows:
| Feature | Description |
 | Supported Walls - Walls connected to the part on at least two sides are unlikely to warp. These should be designed with a minimum thickness of 0.4mm. |
 | Unsupported Walls - Walls connected to the rest of the part on fewer than two sides are highly susceptible to warping or detaching from the print. These walls must be at least 0.6mm thick, and designed with a filleted base (where the wall connects to the rest of the print) to reduce stress concentration at the joint. |
 | Overhangs - SLA printing has few issues with overhangs, unless the printed model lacks sufficient internal and external support structures. Printing without support often leads to warping, but if unsupported printing is required, any unsupported overhang must have a length of less than 1.0mm and an angle of at least 19° from the horizontal. |
 | Embossed Details (including text) - Any feature on the model that is slightly raised above the surrounding surface. Their height must be at least 0.1mm above the print surface to ensure clear detail. |
 | Engraved Details (including text) - Any feature recessed into the model. If these details are too small, they may blend with the rest of the model as it cures. 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 the model can be successfully printed, but designers must remember that wider bridges must be shorter than narrow 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 moving parts: 0.5mm. ● Clearances for assembly connections: 0.2mm. ● A 0.1mm clearance allows for a push or press fit. |
Resolution
SLA can achieve higher resolution than FDM because it uses laser light to cure the material. The XY-direction (or horizontal resolution) of SLA printing
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 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 with Z-axis layers of 150 to 400 microns.
Vacuum Breakdown
SLA machines print solid, high-density models, but if the functional part is a hollow model, it significantly reduces the amount of material required and
the printing time. It is recommended that hollow objects have a wall thickness of at least 2mm to reduce the risk of failure during the printing process.
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 breakdown."
Small failures (cracks/holes) will propagate throughout the part, and if not corrected, will lead to complete failure or part explosion. The drainage holes should have a diameter of at least
3.5mm, 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 | Applications |
| Standard Resin | Most commonly used for general printing and can provide high-detail surface finishes with resolutions smaller than 25 microns. These resins do not offer 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 and 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 toughness, elasticity, and high-temperature resistance. |
| Dental Resins | For general orthodontics, standard resins or castable resins are typically used. In the past year, Class 1 and Class 2 biocompatible resins have also become available for creating surgical guides using SLA technology. These resins are highly accurate and durable enough to be autoclaved before surgery. | Dental applications |
| Castable Resins | These resins are specifically 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 almost no ash or residue. | Jewelry, fine models, and investment casting applications |
A range of products printed with SLA resin (courtesy of Formlabs)
Post-Processing
A range of surface finishes can be achieved on SLA printed parts. The desired surface smoothness is often influenced by cost and application. For a detailed guide on the most common
SLA surface finishes, please refer to this article.
Limitations
Print Volume
The print volume of SLA printers is typically smaller than most FDM printers, except for industrial-grade machines.
The Formlabs Form 2 (a common desktop SLA printer) has a volume of 145mm × 145mm × 175mm,
while the Ultimaker 2+ (a common FDM desktop printer) has dimensions of 223mm × 223mm × 205mm.
When SLA print geometries exceed the printer's volume, they can be printed as smaller parts and then assembled.
The best method for bonding SLA printed parts together is to use 5-30 minute epoxy resin.
Cost vs FDM
Compared to filaments used in FDM printing, the volumetric cost of SLA resin is higher. Due to this, SLA printing is generally more expensive, but SLA's ability to print intricate
details represents a competitive option compared to many industrial 3D printing technologies. One kilogram of standard SLA resin typically costs around $150, while 1kg of ABS filament for FDM will cost approximately $25.
Material Properties
SLA parts are generally not suitable for producing load-bearing functional parts. The nature of SLA resin means that parts are brittle, not as stable over long periods as other 3D printing materials,
and undergo some changes.
Most SLA printed parts require post-curing in a UV curing machine. Post-curing allows the parts to achieve higher strength and become more stable.
Rules of Thumb
SLA is very suitable for small parts that require smooth surfaces (similar to injection molding) and high precision.
Support structures are very important for successful printing of precise SLA parts. If a good surface finish is required on a surface, the part should be oriented so that this surface does not
come into contact with support material (typically facing upwards).
SLA parts generally have poor mechanical properties and are best suited for non-functional prototypes, enclosures, and visual models.
Designing SLA features:
| Feature | Design Specification |
| Supported Walls | Minimum thickness of 0.4mm |
| Unsupported Walls | Minimum thickness of 0.6mm |
| Overhangs | Less than 1.0mm in length, and 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 press-fit connections |
| Holes | Minimum diameter of 0.5mm |
Original source:https://www.3dhubs.com/knowledge-base/how-design-parts-sla-3d-printing