3d printing

Designing for SLA/DLP Printing

Print features

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

Feature	Description
	Supported walls - Walls that are connected to other structures on at least two sides, so they have very little chance of warping. These should be designed at a minimum of 0.4 mm thick.
	Unsupported walls - Walls that are connected to the rest of the print on less than two sides, and are at a very high chance for warping or detaching from the print. These walls must be at least 0.6 mm thick and should be designed with filleted bases (where the wall connects to the rest of the print) to reduce stress concentrations along the joint.
	Overhangs - Pose very little issue with SLA printing, unless the model is being printed without adequate internal and external support structures. Printing without supports often leads to warping of the print, but if printing without supports is necessary, any unsupported overhangs must be kept less than 1.0 mm in length and at least 19° from level.
	Embossed details (including text) - Any features on the model that are raised slightly above the surfaces around them. These must be at least 0.1 mm in height above the surface of the print to ensure the details will be visible.
	Engraved details (including text) - Any features which are imprinted or recessed into the model. These details are at risk of fusing with the rest of the model while printing if they are too small, so these details must be at least 0.4 mm wide and at least 0.4 mm thick (distance from surface of the model to recessed detail).
	Horizontal bridges - Bridges between two points on a model can be successfully printed, but the designer must keep in mind that wider bridges must be kept shorter (less than 21 mm) than thin bridges. Wider bridges have a greater z-axis area of contact increasing the chance of print failure during peeling.
	Holes - Holes with a diameter less than 0.5mm in the x, y, and z axes may close off during printing.
	Connections: 0.5mm clearance between moving parts. 0.2mm clearance for assembly connections. 0.1mm clearance will give a push or snug fit.

Resolution

SLA is able to 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) is dependent upon the laser spot size and can range anywhere from 30 to 140 microns. This is not an adjustable parameter of printing. 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 is a trade-off between speed and quality. For a part that has few curves or fine details, there will be little visual difference between a print at 25 microns versus a print at 100 microns. By comparison, a desktop FDM machine will typically print z-axis layers at 150 to 400 microns.

Hollowing and cupping

SLA machines print a solid, dense model but if the print is not intended to be a functional part hollowing the model significantly reduces the amount of material needed as well as print time. It is recommended that the walls of the hollowed print be at least 2 mm thick to reduce the risk of failure during printing.

If printing a hollow part, drainage holes must be added to prevent uncured resin from getting trapped inside the final print. This uncured resin creates pressure imbalances within the hollow chamber, and will cause what is known as “cupping”. Small failures (cracks/holes) propagate throughout the part and will cause complete failure, or part explosion, if not corrected. Drain holes should be at least 3.5 mm in diameter, and at least one hole must be included per hollow section.

Rules of thumb
SLA is ideal for small parts that require a smooth surface finish (comparable to injection molding) and a high level of accuracy.
Support structures are critical to successfully printing accurate SLA parts. If a good finish is required on a surface the part should be orientated so that that surface is not in contact with support material (typically upwards facing).
SLA parts typically have poor mechanical properties and are best suited for non-functional prototyping, enclosures and visual models.