[3D Printing News] How to Design 3D Printed Parts for SLA Printers

[3D Printing News] How to Design 3D Printed Parts for SLA Printers

A comprehensive guide to using SLA for 3D printing, covering the printing process, design specifications, material selection, and technical limitations.

Table of contents
introduce
Printing with SLA
Design SLA Printing
SLA material
Post-processing
limit
Rules of thumb

introduce

Solid-state lamination (SLA) is a 3D printing method that uses laser light to cure photosensitive polymer 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 using SLA to print parts, and discusses the most common SLA materials.

Printing with SLA

Printing process

A typical benchtop SLA machine includes a UV laser, which involves pouring UV-curable resin into a resin tank with a transparent bottom. The UV light can precisely scan the 2D contours of the object and cure the UV-curable resin layer by layer.

When UV light passes through, a cured film will be generated between the printing platform and the bottom of the water tank and adhere to the platform. The newly printed film is then peeled off from the bottom layer (the peeling method depends on the machine operation, such as sliding or shaking the water tank). After peeling, the printing platform moves to a distance of a high thickness and repeats the above process until the object is completed.

To ensure successful SLA printing, reducing the pull-out force of the printed layer during peeling is crucial. High stress is generated at the edges during object pull-out, which can easily lead to increased object failure rates and warping (caused by the object layer failing to adhere to the printing platform and instead sticking to the bottom of the tank).

SLA printing process

Printing direction

When positioning SLA parts, the biggest problem is the Z-axis cross-section. The force involved in printing the adhesive resin groove is proportional to the 2D cross-sectional area of ​​the printed object. Because of this, the part will be printed at a certain angle on the printing platform, and reducing support is not the most important issue (as shown in the figure below).
Minimizing the cross-sectional area along the z-axis is the best way to position SLA printed parts.


The model is oriented at a suboptimal level with a large z-axis cross-sectional area. In this direction, support is minimized, but the probability of printing failure is high.
Print volume = 33.39 ml, Print time = 2 hours 27 minutes


The model was repositioned at an angle to reduce the z-axis cross-sectional area. The significant increase in support is reasonable, reducing the likelihood of printing failures.
Print volume = 36.95ml, Print time = 4 hours 7 minutes

As a designer, it is important to understand why part orientation affects SLA print quality.
The necessity of positioning parts often leads to a reduction in the cross-sectional area along the z-axis, which in turn necessitates the addition of numerous supports to the model.
In some cases, the design may require a lot of supports, making printing in the SLA no longer cost-effective or detrimental to the appearance of the parts (once the supports are removed), resulting in a visually unsatisfactory final product.
Limiting the number of horizontal parts, hollowing out parts, and reducing cross-sectional area are steps designers can take to optimize SLA design.

isotropic

SLA printing is isotropic because the chemical bonds between the layers during printing result in almost identical physical properties in the x, y, and z directions.
Regardless of whether the printed parts are parallel or perpendicular to the printing plate.
The final material properties of the parts will not be significantly affected.

Design SLA Printing

Printing features

The level of detail that an SLA printer can produce depends on the size of the laser dot and the properties of the resin. General guidelines for SLA design are as follows:

feature	describe
	Supporting walls – The structure of the supporting wall is connected to the wall on at least two sides, so warping is virtually nonexistent. This should be designed with a minimum thickness of 0.4mm.
	Unsupported walls – walls that are connected to the rest of the print on two or more sides – are highly susceptible to warping or separation from the print. These walls must be at least 0.6 mm thick and designed with rounded corner bases (where the wall connects to the rest of the print) to reduce concentrated pressure at the joints.
	Overhanging SLA printing presents few problems unless the printed model lacks sufficient internal and external support structures. Printing without supports typically results in warping of the printed object, but if printing without supports is necessary, any unsupported protrusions must be less than 1.0 mm in length and at least 19° from the horizontal plane.
	Embossed details (including text) - Any features on the model are slightly raised above the surrounding surface. Their height must be at least 0.1mm above the printed surface to ensure clarity of detail.
	Sculpted details (including text) - any features printed on the model. These details may blend into the rest of the model if they are too small, so they must be at least 0.4mm wide and at least 0.4mm thick (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 (less than 21mm) than narrower bridges. Wider bridges have a larger z-axis contact area, which increases the likelihood of printing failures during peeling.
	Holes - Holes with a diameter of less than 0.5 mm on the x, y, and z axes may close (become closed) during the printing process.
	connect: ● The gap between moving parts is 0.5mm. ● The gap between the assembly and connection is 0.2mm. ●A 0.1mm gap allows for pushing or fitting.

Resolution

SLA achieves higher resolution than FDM because it uses laser light to cure the material. The XY-direction (or horizontal resolution) SLA printing resolution depends on the laser spot size, ranging from 30 to 140 micrometers. This is not an adjustable parameter for printing. The minimum feature size cannot be smaller than the laser spot size.
Z-axis resolution (or vertical resolution) ranges from 25 to 200 micrometers. 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-micrometer print and a 100-micrometer print. By comparison, desktop FDM machines typically print z-axis layers of 150 to 400 micrometers.

Breaking the vacuum

SLA machines print robust, high-density models, but printing hollow models significantly reduces the amount of material required and printing time. It is recommended that hollow parts have a wall thickness of at least 2mm to reduce the risk of malfunctions during printing.

If printing hollow parts, drainage holes must be added to prevent uncured resin from becoming trapped inside the final print. This uncured resin creates a pressure imbalance within the cavity, causing what is known as "vacuum breaking."
Small defects (cracks/holes) can spread throughout the part and, if left uncorrected, lead to complete failure or even the part exploding. Drainage holes should be at least 3.5 mm in diameter, and each hollow section should have at least one hole.

SLA material

The table below lists some of the more common SLA resins.

A range of products printed with SLA resin (provided by Formlabs)

Post-processing

A range of surface finishes can be achieved on SLA-printed parts. The desired surface smoothness is typically influenced by cost and application. For a detailed guide to the most common SLA surface smoothnesses, please refer to this article.

limit

Print volume

SLA printers typically have lower print volumes than most FDM printers, except for industrial-grade machines.
The Formlabs Form 2 (a standard desktop SLA printer) measures 145mm × 145mm × 175mm.
The dimensions of the Ultimaker 2+ (a standard FDM desktop printer) are 223mm × 223mm × 205mm.
When the geometry printed by SLA exceeds the size of the printer, it can be printed as a smaller part and then assembled.
The best way to bond SLA-printed parts together is to use epoxy resin for 5-30 minutes.

Cost vs FDM

SLA resin has a higher volumetric cost compared to filaments used in FDM printing. While SLA printing is generally more expensive, its ability to print intricate details makes it a competitive option compared to many industrial-grade 3D printing technologies. One kilogram of standard SLA resin typically costs around $150, while one kilogram of ABS filament used in FDM would cost approximately $25.

Material properties

SLA parts are generally unsuitable for producing heavy-duty functional components. The properties of SLA resin mean that the parts are brittle and, unlike other 3D printing materials, are not stable over long periods and will undergo some changes.
Most SLA-printed parts require UV post-curing. Post-curing allows the parts to achieve higher strength and become more stable.

Rules of thumb

SLA is ideal for small parts that require surface smoothness (similar to injection molding) and high precision.
Support structures are crucial for the successful and accurate printing of SLA parts. If a good surface finish is required, the part should be positioned so that the surface does not come into contact with the support material (usually facing upwards).
SLA parts typically have poor mechanical properties and are best suited for non-functional prototypes, housings, and visual models.

Features of SLA design:

Original source: https://www.3dhubs.com/knowledge-base/how-design-parts-sla-3d-printing