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SLA resin printing

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Frank3k:
This is a long post. The TL;DR: SLA printing quality is as good or better than Shapeways, it's stinky and messy. Pictures at the bottom.

I recently bought an AnyCubic Photon, a Stereolithography (SLA) 3D printer that works by shining UV light through a high resolution mask to cure a liquid resin. If you've seen or used those expensive UV-cure glues, it's roughly the same type of material. Once cured, it's a hard and brittle plastic like most model resin or Shapeways high resolution parts.

I also have an FDM (Fused Deposition Modeling) printer that I've use to print models. FDM printers commonly use PLA (polylactic acid) or ABS filament, which is heated until it melts and is then extruded layer by layer through a narrow nozzle to build up a three dimensional object. The print time is determined by the time spent by the print head moving in XYZ and by the complexity, resolution and orientation of the model.

Each type of printer has its uses, advantages and disadvantages. FDM printers can print fairly large objects in a wide range of materials and speeds. Once printed, the object is ready to go, unless optional post processing steps are needed.
FDM printer resolution is limited by several factors, but typically do not print with vertical (Z axis) resolutions smaller than 0.1mm (100 microns or about 0.004"). While this is more than enough resolution for most objects, it's not ideal for small scale models, and the horizontal artifacts created by the print layers can detract from the appearance of the model.

There are ways around this - with PLA, you can prime and sand the model until the lines go away or you can buy a special type of PLA that can be bathed in alcohol to remove the print lines. With ABS, you can prime and sand the model or use Acetone or similar solvents to smooth the surface and remove or reduce the artifacts. Both of these methods have the potential to damage or obliterate small details on the model.

An SLA printer like the AnyCubic Photon or the Sparkmaker are limited in the size of models they can print (the printer's print volume). The Z axis resolution can be 25 - 100 microns, up to 4 times better than an FDM printer (it can be pushed to print 10 micron layers, so 10x better than FDM). The XY resolution is around 40 - 50 microns.

Unlike an FDM printer the print time for an SLA printer is determined by the Z axis resolution and the time required to expose and harden each layer of resin. Printing five copies of the same object (as long as they fit in the build volume) takes just as long as printing one copy.

Both types of printers work by slicing a model into many layers. On an FDM printer, the slices determine the path of the print head. In an SLA printer, it defines the mask displayed on the LCD. Clear areas are exposed to UV and harden, dark areas stay liquid.

Most consumer SLA printers use an LCD screen to display the slices. This screen is covered by a resin vat. The bottom of the vat has a thin, clear film of FEP (Fluorinated ethylene propylene) which presses against the LCD screen. FEP is used because few materials stick to it.

Resin is added to the vat and the print is started. A build plate (aluminum or steel) moves towards the FEP film until it is a layer thickness (50 or 25 microns) away from the film, trapping a thin layer of resin between the two. The LCD screen and UV backlight are turned on, exposing the resin to a cross section of the object. After a few seconds exposure (it varies by resin) the resin has hardened and stuck to the build plate (and hopefully not the FEP). The light is turned off and the build plate moved up to the next layer. This allows more resin to flow in the new space. The process is repeated until the object has been printed.

Once the printing has finished, the object will be hanging upside down from the build plate, dripping uncured resin. The print has to be removed from the build plate and the uncured resin removed by washing it in a container of Isopropyl or Denatured (Methylated in some places) Alcohol. After this wash, the alcohol is removed by rinsing the object in water. The resin isn't fully cured at this stage, so the print will be somewhat soft and delicate. Placing the object in sunlight for a few minutes or under a 405nm UV light will fully cure the object and it will be ready to use.

In an FDM printer, the filaments are solid until melted. PLA and especially ABS will have a strong smell while printing, but this is easily mitigated with an enclosure. When the printing is done, the smell is gone and there's little waste material left, unless it's support plastic.

In an SLA printer, the liquid resin is sticky and can be very smelly. Gloves, eye protection and a mask are not just recommended, but required. During printing, the resin smell will be strong, since the AnyCubic (and SparkMaker) do not have good filtering. There are aftermarket solutions that add an activated carbon filter that eliminates the odor, but that's an extra part that has to be added.

Once the printing is done, the print has to be removed from the build plate, which will be gooey with uncured resin. This will consume many paper towels. I have a small tray that I use to remove the print from the build plate and keep the resin contained. Washing the print takes alcohol and more paper towels and curing the print will require a UV lamp (I use an LED lamp used to cure fingernail polish). The build plate then has to be cleaned of resin. If you don't plan on printing in the next couple of days, the resin vat has to be emptied and the unused resin filtered (in case there are small bits of cured resin floating in it) and stored in a different container, as to not contaminate the original resin bottle. This resin can be reused, though. The emptied vat has to be cleaned thoroughly (which requires more paper towels and care not to scratch or puncture the FEP film). It's only then that the printing has been completed.

There are currently quite a few sub $500 SLA printers on the market; with careful shopping, the AnyCubic Photon can be bought for well under $400 (that's what I did). The Sparkmaker printer is $219 on Amazon. It has roughly the same resolution as the Photon, but a smaller build area. While the AnyCubic Photon comes with 250 ml of resin (about $25 worth), tools, etc. The Sparkmaker doesn't include resin at that price. Other printers include the Wanhao D7 and the Monoprice Mini Deluxe 3D Printer (a rebadged Wanhao D7).

My first print was the default Anycubic test cube, which prints without any supports. The transparent green resin is hard to photograph:



My second print was of the 1/35 Lehaitre tracked motorcycle that I printed late last year on my FDM printer. This model was designed to print at the absolute limit of my FDM printer and it failed to print many of the fine details on the model. The SLA printer could do much finer details, but I went ahead and printed the same file. Again, the clear plastic is hard to photograph:



Against a dark background, (which is a great novel), it doesn't improve the image by much:




Here is the original FDM print, with the inspector and the SLA print:



I had to break out the microscope to see the printing artifacts, which are very shallow:



Note that the elliptical knobs for the vent covers are perfectly reproduced, down to the protruding bolt  at one end:



The support structures are needed to keep the part tilted, so uncured resin doesn't pool in the model. The supports will probably get reused as scaffolding on some other model. Note that at 50 microns, the surface finish looks better than Shapeway's best material... there are reasons for this, partly because Shapeways goes for speed and low print quality to maintain their volume.

 I'll paint this model this weekend; I doubt that the printing artifacts will be very noticeable. 

GTX_Admin:
Great post! :smiley:

Jeffry Fontaine:
Thank you for the excellent review on this 3D printing system.  The finish on your model definitely makes the efforts by Shapeways pale in comparison. 

Brian da Basher:
That's one of the best write-ups I've seen on 3D printing yet and I greatly appreciate how you went into detail about the differences between the types of printers.

When you share this stuff with us it makes me feel lucky we've got such knowledgeable BtS members!

Many thanks, Frank!

Brian da Basher

Frank3k:
Thanks guys. Next time I print something on the AnyCubic, I'll add something I've had printed at Shapeways, for a direct comparison.

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