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SLA shares many characteristics with Direct Light Processing (DLP ), another photopolymerization technology. For simplicity, both technologies can be considered equal.

Here's how the process works:

1) 2) 3)

1) A platform is placed in the tank with liquid photopolymer, at the same height from the resin surface.

2) Then the UV laser selectively cures the required areas of the photopolymer resin according to a predetermined algorithm.

The laser beam is focused on a given path using a set of mirrors called galvos. Then the entire cross-sectional area of ​​the model is illuminated. Therefore, the resulting part is completely solid.

3) When one layer is finished, the platform moves to a safe distance and the mixing foot inside the tub mixes the resin.

This process is repeated until the part is printed. After printing, the part is not fully cured and requires further post-processing under the UV lamp . At the end of UV illumination, the part acquires very high mechanical and thermal properties.

The liquid resin solidifies through a process called photopolymerization: during solidification, the monomer carbon chains that make up the liquid resin are activated by an ultraviolet laser and become solid, creating strong, inextricable bonds with each other.

The photopolymerization process is irreversible, and there is no way to convert the resulting parts back into a liquid state. When heated, they will burn, not melt. This is because the materials that are produced by the SLA technology are made from thermoset polymers, as opposed to the thermoplastics that FDM uses.

On SLA systems, most print settings are set by the manufacturer and cannot be changed. The only inputs are the layer height and the part orientation ( last, locates the supports ).

The typical layer height in the SLA print ranges from 25 to 100 micron .

The lower the layer height, the more accurately the complex geometry of the model will be printed, but at the same time the printing time and the likelihood of failure will increase. The layer height of 100 microns is suitable for most common geometries and is the golden mean.

Another important parameter for the operator is the size of the platform. It depends on the type of SLA printer. There are two main types: orientation top to bottom and orientation from bottom to top .

In the first case, the laser is above the tank, and the part is face up. The platform sits at the very top of the resin vat and moves down after each layer is sintered.

In the " bottom up " pattern on the SLA printer , the light source is placed under the resin tank (see picture above) and the part is built upside down.

The tank has a transparent bottom with a silicone coating that allows the beam of light to pass through but prevents the cured resin from sticking. After each layer, the cured resin separates from the bottom of the tank as the platform moves up. This is called the sintering step.

The " bottom to top " orientation is mostly used in desktop printers such as Formlabs. The " top - down " orientation is used in the industrial SLA printer .

Printers SLA " bottom up " are easier to manufacture and operate, but the size of the possible print will be smaller, since the forces applied to the part during the sintering stage can cause printing to fail.

Top-down printers, on the other hand, can print very large parts without much loss in accuracy. The wide possibilities of such systems naturally cost more.

The following are the main characteristics and differences between the two orientations:

Popular brands:

FORMLABS

Printable size: Up to 145 x 145 x 175 mm

Typical layer height and print accuracy: 25 to 100 µm and ± 0. 5% (lower limit: ± 0.010 to 0.250 mm) respectively

Popular brands:

PRISMLAB

Print area size: Up to 1500 x 750 x 500 mm

Typical layer height and print accuracy: 25 to 150 µm and ± 0.15% (lower limit ± 0.010 to 0.030 mm) respectively

Supports are always required at Print SLA . Structural structures are printed from the same material as the part and must be manually removed after printing.

Part orientation determines the location and amount of supports. It is recommended that the part be oriented so that surfaces that require maximum quality do not come into contact with supports.

In different types of SLA printers, support is used in different ways:

For top - down printers , support requirements are the same as FDM . They are essential for accurate printing of overhangs and bridges ( the critical overhang angle is typically 30 degrees ).

The part can be oriented in any position and is usually printed flat to minimize the number of supports and the total number of layers.

In printers like " from bottom to top " everything is more complicated. Overhangs and bridges also need to be supported, but minimizing the cross-sectional area of ​​each layer is the most important criterion.

Forces applied to the part during the sintering step can cause it to come off the platform. These forces are proportional to the cross-sectional area of ​​each layer.

For this reason, the parts must be oriented at an angle, and minimizing supports here is not a primary concern.

On the left - a detail oriented on the SLA printer "from top to bottom" (support minimization).

On the right is a part oriented on the SLA printer "from the bottom up" (minimizing the cross-sectional area).

Removing supports for an SLA printed part

One of the biggest problems with the accuracy of parts made with SLA is twisting. This problem is similar to the deformation in FDM when materials shrink.

During curing, the resin shrinks slightly when exposed to the printer's light source. When shrinkage is significant, large internal stresses develop between the new layer and the previously cured material, causing the part to twist.

SLA printed parts have isotropic mechanical properties. This is due to the fact that one pass UV beam is not enough to completely cure the liquid resin.

Further passes help the previously hardened layers to fuse together. In fact, in the SLA of printing, curing continues even after the printing process is complete.

To achieve the best mechanical properties, parts printed using this technology should be post-cured by placing them in a chamber under intense ultraviolet radiation ( and sometimes at elevated temperatures ).

This greatly increases the hardness and heat resistance of SLA , but does not make it stronger. Rather the opposite.

For example.

Test specimens printed with standard clear resin on a SLA desktop printer have, after curing, almost 2 times the tensile strength of ( 65 MPa compared to 38 MPa).

Can operate under load at higher temperatures ( 58 degrees Celsius, compared to 42 degrees ), but their elongation at break is half as much ( 6.2% compared to 12% ).

If you leave the part in the sun, then nothing good will come of it.

Prolonged exposure to ultraviolet radiation has a detrimental effect on physical properties and appearance. The part may curl, become very brittle, and change color.

For this reason, before using the part, it is recommended to apply a spray of transparent acrylic paint resistant to UV .

Materials for SLA printing are available in the form of liquid resin. The price per liter of resin varies greatly - ranges from $50 for standard material to $400 for specialty materials such as casting or dental resin.

Industrial systems offer a wider range of materials than desktop systems SLA printers that give the designer more control over the mechanical properties of the printed part.

SLA materials ( thermosets ) are more brittle than materials made using FDM or SLS ( thermoplastics ) and for this reason SLA parts are not typically used for functional prototypes that will be subjected to significant stress. However, new advances in materials development may change this in the near future.

The following table lists the advantages and disadvantages of the most commonly used resins:

Parts printed with SLA technology can be processed to a high quality using various methods such as sanding and polishing, staining and mineral oil treatment. Widely developed articles about post-processing can be found on the Internet.

Transparent resin housing cover for electronics in various finishes. From left to right: removal of the main support, wet sanding, UV irradiation, acrylic and polishing

Pros:

• SLA 3D printers can produce parts with very high dimensional accuracy and complex geometries.

• The parts will have a very smooth surface, making them ideal for visual prototypes, for example.

• Special materials are available such as clear, flexible and cast resins.

• Parts printed using SLA technology tend to be fragile and not suitable for functional prototypes.

• The mechanical properties and appearance of these parts deteriorate over time. They are adversely affected by exposure to sunlight.

• Supports and post-processing when printing are always required.