Translation from the original German by Niq Oltman
When 3D printing in resin, you will need support structures, and it’s hard to get them just right. Your slicer software can add them automatically, but this rarely works perfectly. Adding them manually requires quite a bit of knowledge. This guide will help you set up supports correctly and get to know the important parameters. We’ll cover:
- Why you (almost always) need supports in resin prints, and where to place them
- How to assess auto-added supports, and how to improve them manually
- Differences between resin and FDM printing when setting up your prints
Even with the common FDM printing method using molten plastic, for many 3D prints to come out correctly, it’s essential to add the right support structures. And if you’re printing in liquid resin, you need to take even more care. Relying blindly on auto-added supports, or leaving them out entirely, is likely to cause problems.
Why are supports so important? In resin printing, a light source (laser, LED, or projector) is used to cure the liquid resin — to cause a chemical reaction that makes it solid. Like all 3D printing methods, this takes place layer by layer, but compared to FDM printers it’s inverted. The current layer being cured is at the bottom of the resin container, which is transparent. When the layer is complete, the object being printed is moved slightly upward, peeling off the bottom of the container, to make room for the next layer. Without adequate support structures to connect delicate parts of the workpiece to the (upside-down) printing platform, the freshly cured layer may get stuck to the bottom of the container and become separated from the workpiece. This will almost certainly ruin the print. Worse, it can damage the delicate film that forms the container bottom in many printers, causing critical leaks.
Choosing a Slicer
SLA printing typically uses tree-type supports, unlike the lattice-type supports often used in FDM printing. A piece of software called a slicer is used to prepare your 3D print job, including adding the necessary supports.
The features of some popular slicers for resin printing are compared in the table below. There are many more differences between these slicers. Their approaches to generating support structures vary greatly, as do the available options for tuning this process. Note also that none of the slicers shown is directly capable of pre-processing a 3D model for printing in resin.
In this guide, I’ve chosen PrusaSlicer as an example for demonstrating the important parameters. You can use this free software not only for the SL1 from Prusa Research, but for any other resin or FDM 3D printer.
For PrusaSlicer and all the others, you’ll initially need to enter the specifications of your actual printer. Slicers do come with pre-set profiles for some printers, but these usually include only the manufacturer’s own products. For PrusaSlicer, this is the SL1; for Z-Suite, it includes Zortrax’s resin printers.
Almost all resin prints require support structures if you want them to come out correctly. For instance, look at the Benchy boat, a popular benchmark model for testing 3D printers. You’ll find that it doesn’t have any big overhangs; that’s one reason it became popular for testing FDM printers. It’s useful for testing resin printers as well — you usually don’t need to worry about supports inside the model.
However, some support structures outside are still needed. PrusaSlicer generates them automatically if you slice the model using its standard settings (Figure A). When you’re printing in resin, you’ll almost always want to have a gap between the build plate and the object — 3mm to 10mm in height — to make room for a raft or attachment layer support (in PrusaSlicer it’s called a pad.). That’s because the initial layers of a 3D print in resin will be cured for much longer than the rest (by a factor of 3, even up to 10) to ensure that they’re affixed very strongly to the build plate. Should your workpiece peel off the plate during printing, your print is inevitably ruined — just like FDM, only upside down.
As an added benefit, you’re unlikely to damage your finished print when you take it off the build plate, which is usually done with a scalpel, utility knife, or metal scraper. The tool will only touch the supports, not the model itself. In PrusaSlicer the parameter for this gap be found under Print Settings –> Support Material –> Pad.
If you’re printing an overhang with a slope of around 45° or more (depending on your material and your printer), you should definitely include supports. In the Benchy model, we can spot a nice example, which PrusaSlicer detected automatically (Figure B): The lip of the anchor hawse hole on the boat’s sloped side wall forms a small overhang at an angle of roughly 90° — practically horizontal.
With FDM, this little overhang would be negligible, as the material is always suspended from the fresh filament being pushed out of the print head. But with resin printing, this protrusion may be too much.
There’s much less of a problem where horizontal surfaces straddle a cavity with no supports in between, like a bridge. For example, the Benchy cabin roof can be printed without adding supports because it’s not very long.
When models are sliced, some layers may contain parts that aren’t connected to anything else in the layer, or to the building platform (Figure C). Without supports, these islands are doomed to fail; they’ll remain stuck to the bottom of the container. You can detect islands by inspecting each layer in the slicer, looking for parts that hover freely in the air (Figure D). However, you can typically avoid this scenario entirely by adding automatic supports.
Another critical situation is where large areas are connected to the rest of the object in just a few places. When a part like this peels off the resin tank, the adhesive forces are particularly strong, so you should add additional supports to make sure these areas are bonded well to the rest of the model.
Loading and Orienting Models
Once you’re done configuring your slicer to work with your printer, loading a 3D model works just like you’re used to with FDM printers. The next step is important, though (and many users don’t do it): for resin prints, unlike FDM, you should rotate a flat-bottomed model so that its underside is slightly tilted — not parallel to the printing platform. If you leave the model as-is after importing (Figure E) and don’t change the orientation, the forces caused by the initial flat layer (Figure F) detaching from the bottom of the resin tank may deform or even rip apart your model — despite having proper supports.
Rotated slightly, a model’s large base surface is less of an issue, as there is less surface area per layer that needs to peel off after being printed (Figure G). In addition, you’ll need far fewer supports. The layers in the final printed model won’t be horizontal, but as resin printing typically creates very thin layers, you’re unlikely to notice this.
For most cases, I recommend rotation by 10° to 30° (rotating around a single axis is sufficient). To do this in PrusaSlicer, press the R key or click on the Rotate icon at the left edge of the screen (Figure H). PrusaSlicer can also apply rotation automatically (right-click on the object, and select Optimize Orientation). However, this tends to rotate the model in unexpected ways, which may greatly increase the time for printing as well.
In PrusaSlicer, it’s super easy to get automatic support structures. There’s a Supports setting in the right-hand panel to choose if you want supports only between the build plate and the model (“Support on build plate only”), or between different points in the model as well (“Everywhere”). The first option is often sufficient for simple models such as the Benchy boat.
Click on Slice Now, and the software will generate the appropriate structures. Until you slice, the model will be shown resting on the build plate; after slicing, you’ll see that the raft layers mentioned earlier have now been applied, too.
The auto-generated supports usually aren’t bad, so it’s a good idea to create automatic supports first, and then add or remove supports manually as needed. You should do this wherever your previous prints had problems, or where you find some of the typical issues we mentioned above: flat or long overhangs, large surfaces, or loose parts (islands). Figure I gives an overview of some things that have turned out to work well (Good), and some that didn’t (Bad). Supports are shown in grey and the workpiece in green. Some objects are only partly shown to make things clearer — try to visualize them extending past the orange-colored cut surface.
You can click on the model to add or remove supports, and you can change global settings for the supports in Print Settings –> Supports. Figure J shows you the components that make up a support structure. Every support structure has a base. It needs to be large enough to remain stable under the weight of the remaining structure (2–5mm in diameter and 0.5–2mm in height, depending on your material).
The support pillar connects the base to the head. You can set the pillar’s diameter. Advanced slicers can also link or join several pillars automatically to create a more stable structure; in PrusaSlicer you can set a maximum “bridge length” and “pillar linking distance,” for example.
The part that connects the support and the model is known as the head. To ensure a secure connection, it will typically penetrate 0.1 to 0.7mm into the object. The head diameter at the top (normally between 0.2 and 1mm) controls the amount of force that the support structure can take. Be aware that large structures will tend to leave imperfections on the printed object. Confusingly, the length of the head is labeled as the head width. You shouldn’t make it too long — 2mm to 5mm is known to work well.
In general, you can depend on PrusaSlicer’s standard settings for these support parameters; then if there’s a problem, you can work your way toward the right values.
Slicing and Printing
The slicing process usually runs quite fast on PCs and laptops (depending on the software used, the machine’s processing power, and the size of the 3D model). If you run NanoDLP on a Raspberry Pi or a Pi Zero directly in the printer, slicing may well take up to several minutes — but after this, the software will also control the printing process directly.
Depending on your combination of slicer and printer, the final step may vary significantly. Using PrusaSlicer with Prusa’s SL1, you just need to transfer the exported file to the 3D printer. With Z-Suite and Zotrax printers, it’s roughly the same. With other printers, there’s a bit more to do.
PrusaSlicer creates files ending in .sl1, but these are really just ZIP archives. You can simply change the extension to .zip and unpack the archive. Doing so will reveal a collection of images that will be projected sequentially by the display below the resin tank, serving as masks for curing. Where the image is white, light will illuminate and cure the resin; black areas mean no light (Figure K).
Many common resin 3D printers will accept such a sequence of images (including a configuration file) as their input. Some printers — old or very cheap ones in particular — don’t include a “real” computer. They require an image signal (usually via HDMI) as well as G-code commands for additional control. For these, I recommend using a Raspberry Pi with NanoDLP. As NanoDLP offers only rudimentary features for supports, it’s best to feed it with STL files that already include all your support structures. You can create the supports in PrusaSlicer and export the result in STL format (Figure L).
Fine-Tuning Your 3D models
When printing in resin, it can be worthwhile to hollow out your objects to save on your material. In contrast to FDM, resin printing always creates a solid mass — it can’t do lesser-density infill. Many slicers (PrusaSlicer excepted) lack support for hollowing, nor do they help you with fixing defects on surfaces or in volumes. But other free software exists to do this, like Meshmixer.
Meshmixer imports 3D models from STL, PLY, AMF, OBJ, or 3MF formats. To modify the shape of your model, check out the various modeling tools under Sculpt. Should you find defects (such as holes) in your surface lattice, there’s an Auto Repair under Analysis –> Inspector. Another useful feature can detect overhangs for you (Analysis –> Overhangs) — you’ll definitely need to add support structures to the areas marked as affected. Under the Edit menu, there are more tools to be found: for instance, to split your 3D model into parts, or to hollow it out and generate holes for liquid resin to drain out, all in one go. Finally, you’ll export the file. For resin printing, the best option is to export directly into STL Binary format.
Beyond this, you’ll need to gain some experience of your own. Some workpieces and resins require more custom setups. For example, flexible resins tend to bend out of shape quickly, often buckling under their own weight. My recommendation here is to add more supports, but you should get advice from the manufacturers of these special materials as well.
If you’re a beginner, I suggest that you start by using the available automatic features, and working with simple (and well-tested) models from the community. Once you’ve gained some more experience, you can try changing some of the parameters for controlling the structures (but avoid changing too many parameters at once). When you’re getting frustrated because your prints keep failing, it’s a good idea to manually include additional support structures right from the start instead of relying on auto-added supports blindly — regardless of what slicer you’re using. At worst, you’ll end up removing more supports from your printed object than what’s strictly necessary, but there’s less chance of your print failing — and you having to clean the resin container in the aftermath.
Nonetheless, the available slicers have come quite a long way, and chances are they’ll be capable of auto-generating perfect supports at some point in the future.
Key to Good Prints: Your Printing Environment
Proper supports are critical for resin printing, but it’s also important to have the right environment for printing. For one, your resin should not be too old — when stored correctly (as per the instructions on the bottle), most resins should last you for about a year. Before filling your printer, give the resin bottle a thorough shake so the included pigments can distribute evenly. After shaking, let the bottle rest for a while to allow air bubbles to escape.
Keeping track of temperature is essential, too, and often under-appreciated. In my experience, printing in a cold environment such as your basement workshop or an air-conditioned server room tends to produce many failed prints. It’s much better to print where it’s warm — preferably around 77°F (25°C) or slightly above. Better still is to have integrated heating elements that warm up the air inside the 3D printer, or warm the resin directly. Depending on your material, the air should reach a temperature of 95°–122°F (35°–50°C); for the resin, 85°–105°F (30°–40°C). Until you’ve got sufficient experience, be careful fooling with heat settings. While a print is in process, the resin and ambient air will usually keep a stable temperature, as the illuminating display will create sufficient heat all by itself.