There are about 10 main rapid prototyping technologies, depending on how you summarize them. Which one should you choose for the medical device you design? Or maybe you developed something for cars, planes or a household appliance. Every industry and every product has its own requirements for prototypes. And you also have your own thoughts about this, don't you? Is it just to look good to show investors? Either you want to see if it works so you can skip the ending and settle for something cheaper, or maybe do some testing. They will break and break the thing, or heat it up, or submerge it in water.
For each of these products and applications, there is a technology best suited to creating the prototype. If I compared all the methods in an article now, the article would take an hour to read. And nobody has time for that these days. So let's stick with two techniques that cover around 30% of the prototyping market. SLA prototyping and SLS prototyping for plastics. So here is the comparison between SLA 3D printing and SLS 3D printing.
SLA printing development
SLA or Stereolithography is one of the oldest technologies in the world of 3D printing. It was discovered in Japan in the early 1980s. Dr. Kodama developed a device that used a tank of liquid polymer and a single UV laser.
The laser hardened the surface layer of the polymer and lightly dipped it into the tank. In this way, the hardened plastic served as the base for the next layer. Dr. However, Kodama did not have enough funding for his research, so he lost his patent.
In 1884, a group of French scientists were thinking about how to make a fractal piece (a very complex and intricate geometric piece) for their project and thought of a laser that could harden polymers. They presented a prototype, but did not pay due attention to commercialization and their idea was not taken seriously.
Also in 1884, American inventor Chuck Bell, the third person to develop such a method, filed a patent for the stereolithography process. He thought carefully and founded a company called 3D Systems to sell his idea. Today, 3D Systems is one of the largest companies producing 3D printers.
SLA printing technology
There are actually two methods of printing SLA. Let's call it forward and reverse SLA.
The direct method is the way SLA was originally invented. The printer has a tank of liquid polymer and a base plate that is a curing layer deeper than the polymer surface. A UV laser tracks the current cross section of the part (the program is written to cut the part into layers and generate the laser path for each layer) and you get the first cured layer. The plate then lowers a layer height and a special knife moves over the surface of the tank to ensure that the polymer completely fills the area above the plate. Then the next layer is cured.
The reverse method differs in terms of part orientation and movement. Here the base plate is transparent and inverted compared to the direct method. The laser cures the polymer across the plate and then the plate moves up instead of down. Therefore, you can compare this process to slowly removing an object from water. The only difference is that in the last method the object was originally in the water. With SLA printing, you create from the liquid in your tank.
Description and history of SLS
SLS is a similar 3D printing process. It was developed by Texas professor Carl Deckard. He was an engineer and tried to reduce the casting effort required for series production. He later sold his patent to 3D Systems. Later, 3D Systems was able to use SLS for metal powders, which also led to a revolution in the production of metal parts. Today this technology is the most used method for printing metal parts. During development it was discovered that the powder was very dangerous. It is very thin and easy to inhale. Therefore, unlike SLA, SLS technology is not very suitable for amateur desktop versions.
This process uses plastic powder as the main material. This powder is spread onto the base plate with a special knife. The laser then traces the cross section of the part, as with SLA. The base plate is then lowered by one layer and the powder is distributed again. Quite similar to SLA, don't you think?
materials
SLA This process uses polymers that become harder when exposed to UV light, called photopolymers. There are many different resin compositions, which is why they are differentiated mainly according to their use:
- Standard resins. They are the cheapest and have the worst mechanical properties, but their quality is very good and can even be compared to injection molding. Some of them are transparent, so you can make clear prototypes, for example for car lights.
- Technical resins. They can be used in real prototypes and are available in heat-resistant, rigid or flexible ABS format. They still maintain a good surface finish and their resistance is a little better.
- Medical resins. Among the photopolymers discovered for SLA printing, some are class I and II biocompatible, which means they can remain in contact with human tissue for a long time. This is a big advantage.
SLS. This technology was first used with nylon. Over the years, other materials have also been adapted for this technology.
- Nylon. This is by far the most common plastic on the SLS. It is much more durable, heat resistant and tougher than all SLA materials.
- Urethane. It is a simple flexible plastic suitable for casting simple polymer parts.
- Wax. The wax to be used is ideal for making cast models. Simply cover the model with the molding mixture and heat it until it melts.
post-processing
SLA. The main disadvantage of SLA printing is that it has many supports made from the same material. This makes it necessary to carry out cutting operations to remove the supports. It is also necessary to clean the cut area of all traces of the support. The entire process is quite time-consuming.
Furthermore, the cured part is not yet fully cured. Some areas between the layers respond only partially. Therefore, using some UV light chambers to cure the printed parts increases their strength. However, this is not absolutely necessary.
SLS. The part generally has a worse surface quality after SLS than after SLA. Therefore, SLS parts are mechanically ground and refined when good surface quality is required. This takes a lot of time.
Conclusion: what to use and when?
Overall, this process delivers high quality parts, but it is much more expensive than SLS (with direct SLA you cannot reuse the liquid plastic left over from the tank, it cannot be stored for a long time, so it is better to keep the printer loaded) and requires the brackets to be removed. Its advantage is the abundance of materials that can be used.
Use SLA for medical devices, clear parts with good surface finish, and some injection molded or cast models. Use it to show your product, the pieces are very good.
SLS offers much better strength and different properties than SLA. However, the surface quality is much worse (about 2-3 classes lower). The process is much cheaper than the SLA. You don't need any support structure (the previous layers of powder support the upper layers) and you can reuse the powder left over from previous use.
Use SLS if the part must withstand increased stress, corrosion, or vibration. Use SLS when surface finish is not an issue and the budget is low. Use it for yourself or for testing.
