Choosing the Right 3D Printer: FDM vs. SLA

SLA (Stereolithography Apparatus) falls under the Vat Photopolymerization category of 3D printing technology. This was the world’s first ever patented 3D printing technology and was invented by Charles Hull, an American engineer, in the mid-1980s.

SLA uses a liquid resin material to print objects. This technology uses a powerful laser to form the 3D object. In this technology, a laser source sits at the bottom of the vat filled with liquid thermosetting resin. The laser is selectively flashed onto the liquid photosensitive resin and the exposure to the laser cures (hardens) the material. This process is followed until every single point on the layer is exposed and hardened. Once completed, the process moves towards the next layer and then to the next. This process continues until the entire model is cured and the resulting 3D print is ready.

COMPARISON FOR CHOOSING THE RIGHT 3D PRINTER

FDM: The surface finish achieved through FDM printers is coarse and the layer lines are clearly visible due to the minimum layer height. The line thickness as seen from above is typically around 400 microns (the bead resulting from the diameter of the nozzle). The line thickness as seen from the side can be adjusted typically from 50 - 400 microns - this is called resolution or layer height.

SLA: Because SLA printers utilize a laser for “drawing” each line of the part, the resulting lines can be much smaller and more precise resulting in a smooth surface finish. The approximate width of the laser and the resulting cured lines is 20 microns.

ACCURACY 

FDM: FDM 3D printing achieves good dimensional accuracy for larger features. For smaller features, the dimensional accuracy is hampered and sometimes is not consistent. This can also be hampered depending on the 3D printer type (DIY, desktop, professional, or industrial FDM printer). Choosing the right 3D printer becomes a task because the accuracy also depends on other factors like the calibration and slicer settings. Materials also play a role in the accuracy through their shrinkage properties. One example of a precise 3D printer – the MakerBot METHOD  – utilizes a unique heated build chamber allowing for a high level of dimensional accuracy.

SLA: As an SLA printer is able to achieve a very fine resolution, the dimensional accuracy is unmatched by most of the other 3D printing technologies. If the primary objective is surface finish and accuracy on fine features (such as in jewelry or dentistry), SLA is a great choice.

PART STRENGTH 

FDM: FDM printers tend to have an advantage when it comes to producing durable, usable, long-lasting parts due to their ability to print in known polymers and composites. Printing in nylon carbon fiber, for example, will allow for a lightweight part that is incredibly strong.

SLA: Typically, SLA printers are known more for their creation of delicate and detailed parts rather than for strong parts. This is due to a combination of factors. For one, cured resins tend to be fairly brittle. Another reason is that unlike FDM prints, SLA prints cannot be printed solid as such parts will result in cracks, warping, and often print failure. There have been some new stronger resins in development, but they are far from perfect and have often not been proven in the field as have many of the known polymers and composites on the FDM side.

EASE OF USE 

FDM: FDM printers are fairly straightforward – insert plastic filament in one end, and extruder out the other. This simplicity and relative cleanliness has made FDM printers a choice for those who want to use them in an open office or educational environment. The parts come out dry and clean, and often there is just the singular post-processing of removing supports by breaking them off. The remaining material is easily stored in hard plastic filament form for the next print.

SLA: While SLA printed parts can be nice looking, there is a little extra work that goes into the process. The nature of printing parts in a liquid resin can be too much of a hassle for some due to the resulting mess – parts come out sticky, and resin can find its way into the larger workspace. Also the resin can be highly toxic - requiring the use of goggles, gloves, and other protective equipment. 

Post-processing involves multiple steps – first washing the print to remove excess resin, followed by clipping off supports which requires clippers (be careful not to remove features that are actually the print!). Lastly, a further curing process using a UV lamp is recommended to finish the part. The remaining material of liquid resin must be kept in the tray away from light and has a short shelf life once in the tray.

This chemical-laden process means that SLA printers are most often found in closed labs.

3D PRINTING COSTS 

FDM: FDM is the most affordable technology available in the entire space. It is not only inexpensive in terms of the actual machine but also in its operations as the materials are comparatively less expensive than those for other technologies. As a result, products manufactured through FDM 3D printing tend to be less expensive  than for comparative products.

SLA: The SLA printer is a costly machine. It involves the use of expensive parts like laser source and scanning mirrors. The materials are also expensive. Additionally, almost all the models require some amount of support structures and so 3D printed output also gets expensive. 

SIZE 

FDM: FDM printers are available in a wide range of build volumes. From smaller desktop versions to large-format industrial versions, the build volumes can be manufactured as per the need of the user. The largest FDM printer packs a volume of approximately 2,475 liters.

SLA: SLA printers have a limitation on their build volumes. They are generally not very big. The largest SLA printer has a volume of approximately 618 liters.

SUMMARY

FDM printers and SLA printers have proven popular across many industries and their use continues to grow. While some may choose one technology over the other, many see the benefit of utilizing both alongside each other and benefiting from each technology’s strength. For example, a designer at a company might choose to use SLA to get a detailed look at a final part, while an engineer might use FDM to produce a part in a production material, such as ABS, for prototype testing. 

If you have to choose one or the other – ask yourself if you need a more robust part printed in more common materials, or if you need a highly detailed part that looks nice. If you need a stronger, usable part, an FDM printer might be the way to go, but if you just need a nice looking, detailed part, the SLA printer might be your choice. Either way, 3D printing is a great way to quickly take a digital design and make it a physical reality.

Previously Featured on Makerbot's website.