An Introduction to SLA 3D Printing

March 12, 2021

Stereolithography, known as SLA, was the first 3D printing technology – having been patented in 1986. SLA continues to be accessible and affordable to those with 3D printing needs. SLA is helping those in manufacturing cut on printing times and costs without compromising quality.

How Does SLA Work?

SLA creates objects by using an ultraviolet laser beam to selectively cure a polymer resin layer by layer. The materials, which are photosensitive thermoset polymers; come in a liquid form. SLA belongs to the group of additive manufacturing technologies, also known as vat photopolymerization. SLA is generally the best 3D printing technology that is typically used for surfaces needing a smooth finish.

SLA is extremely accurate. Its parts have the highest resolution and finest feature detail. SLA is very popular due to its versatility. Due to recent improvements in print speed and materials, it can be used for virtually anything, competing with engineering thermoplastics in mechanical properties and injection molding in finish and detail. 

Another huge advantage of SLA is printing parts within 24 hours. Where many production technologies have multi-day print cycles, being able to start a machine and a few hours later have printed parts is a huge advantage of this technology when quick turn parts is a necessity.

SLA resins include:

  • A medical grade, high flexibility polypropylene simulant

  • A high impact resistance and rigidity ABS simulant

  • White, black and clear acrylic

  • Ultra high temperature (460F HDT) and tensile strength 75 MPa Peek simulant

  • Ultra high elongation and flexible elastomers

  • Burnout “wax” resins for investment casting metals like bronze, aluminum and steel

  • Many more new materials available and in development

An aspect of SLA’s flexibility is being able to swap between resins with ease. A material change over from one build to another can take as little as 15 minutes. Running multiple materials within a single day is possible with an experienced service provider.

The Steps to Use SLA

Let’s go through the steps for using SLA:

  • First, the build platform is positioned in a tank of liquid photopolymer. This is to be positioned one layer height away from the surface of the liquid. 

  • A UV laser then creates a layer by curing and solidifying the photopolymer resin. The way this works is that the laser is focused in the predetermined path using mirrors. These are called galvos. The produced part is fully solid due to the whole section being scanned. Parts usually are started with selectively creating support material, or scaffolding which mounts the part to the plate and positions it in the tank for accurate curing.

  • The platform then moves at a safe distance once the layer is finished. After this, the sweeper blades then re-coats the surface. This is repeated until the part is completed.

  • Once the printing is done, you will see the part in a green state that is not fully cured. Excess resin is cleaned off the part so ambient UV light does not cure it onto the part altering the geometry. The part will then often require further post UV and thermal post curing to achieve maximum mechanical and thermal properties. Sometimes parts can achieve maximum properties just by waiting a few days for full cure.

  • Once parts are cured enough for handling the supports must be removed and the witness areas must be sanded smooth. Often a final touch with baby oil or polish is added to for sheen.

Support Structure is a Must

Support structure is something that is absolutely needed for SLA. Support structures use the same materials as the part, but are manually removed after printing. This part, true to its name, helps in determining the location and amount of the support that is needed. The support structure should be oriented so visually critical surfaces do not touch the structure. 

Dealing with the risk of Warpage

A common problem that you will see with SLA is curling or warping. This happens due to resign shrinking slightly during the solidification and curling stage. This is due to the exposure from UV.  This is similar to thermal warping common with SLS and MJF. The best way to avoid this is to design parts with adequate structure to hold it’s shape as it cures. Typical good plastic part design for injection molding is good design for all 3D printing technologies to avoid these issues. Properly designed ribs and filleted/radiused edges alleviate this problem. 

Relative Cost of SLA

The major drawback with SLA is cost, at least compared to some higher volume production technologies like Jet Fusion (MJF) and Selective Laser Sintering (SLS.) SLA can be very cost effective for high quality short runs of injection mold quality parts, but it may not be the best 3D printed option for hundreds to thousands of parts. Although print speed and machine costs have come down SLA still has a comparatively expensive material cost and a high amount of finishing labor required compared to other technologies like MJF or SLS. The resin costs can often cost hundreds of dollars per KG and the support material leaves witness marks that needs to be cleaned up and sanded smooth by hand – labor costs that can be quite expensive. The relatively small bed also limits production speeds compared to MJF and SLS for large production runs. That said, when compared to the costs of machining or injection molding, when specialized materials, ultra high finish quality or extreme details are needed, SLA is an extremely cost effective way to achieve these ends.

UV Decay of SLA Parts

Since SLA parts are UV cured photopolymers, they don’t stop reacting to light once fully cured. Over time UV exposure will actually cause the parts to start to break down. This break down can weaken mechanical properties and cause parts to yellow. SLA parts are generally not suitable for environments with extreme daily sun exposure over a period of months or years. A good UV clear coat or paint can limit or prevent this breakdown entirely.

Experts in SLA

Using SLA should always start with a conversation with an expert. This is where RapidMade steps in. With experience in SLA, traditional 3D printing, thermoplastics, and more, you can get a free quote to find out how RapidMade can help with your next 3D print project.

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