Nov 19, 2018

SLA Printing – 5 Tips and Tricks for Great Resin Prints

Unlike thermoplastic filament printers, which typically use a 0.4-mm nozzle, a resin 3D SLA printer may have a laser spot size of 140 microns. The result? The smoothest and most precise 3D prints you've ever seen, if you know what you're doing...

SLA Printing – Tips & Tricks Background

Benchy printed on Formlabs Form 2 SLA Printer Source: https://www.flickr.com/photos/3dbenchy/24614369271

Stereolithography, or SLA, is the oldest and still the most precise form of additive manufacturing.

Traditional SLA printers consist of a build platform in a vat of liquid photopolymer resin. A UV laser, focused on the surface of the resin precisely draws each layer, curing the liquid into a solid. Between layers, the build platform is lowered, and a blade spreads new resin atop the previous layer so that the next can be drawn.

One disadvantage of such printers is that the volume of resin in the vat must always be a little more than the maximum build volume. Thus, a large quantity of resin is required even for smaller prints.

The vast majority of today’s liquid 3D printers are inverted SLA printers. They address the problem of large vats by printing “upside down”. The laser is directed up through the bottom of the resin tank, which has a transparent bottom, and the build plate rises out of the resin tank one layer at a time. This makes it possible to print with only a small volume of resin in the tank.

This article provides some tips and tricks specific to resin 3D printing, in particular to inverted or top-down printers.

For more general information on SLA, head on over to our article on the technology: Stereolithography (SLA) – 3D Printing Simply Explained.

SLA Printing – Tips & Tricks 1. Design for SLA Printing

A rotated model with supports. Source: PranjalSingh IITDelhi / Wikimedia Commons

This article from 3D Hubs recommends the following guidelines when designing parts for SLA printing:

  • Supported walls (walls connected to other structures on both sides) should be at least 0.4 mm thick.
  • Unsupported walls should be at least 6 mm thick and should be filleted at the base.
  • If printing without supports, any unsupported overhangs must be kept less than 1 mm in length and at least 19° from level.
  • Embossed details (including text) should be at least 0.1 mm in height above the surrounding surface.
  • Engraved details (including text) should be at least 0.4 mm wide and at least 0.4 mm deep.
  • When creating horizontal bridges, keep wide bridges shorter than narrow bridges.
  • Holes should have a diameter of at least 0.5 mm.
  • Allow 0.5 mm of clearance between moving parts.
  • Allow 0.2 mm of clearance for assembly connections.
  • Allow 0.1 mm of clearance for a snug fit.

SLA Printing – Tips & Tricks 2. Prepare Your Printer

Your build plate must always be level (relative to the bottom of the tank). Source: Flickr

Just as with FDM printing, it’s vital that you level your build plate. Since SLA printing typically has a much higher resolution than FDM, even very small discrepancies can cause your print to separate from the build plate.

Furthermore, ensure that your resin tank and resin are clean, not expired, and within the proper temperature range.

SLA Printing – Tips & Tricks 3. Prepare Your Model

Generating holes using Meshmixer. Source: herbweiner.com

Slicing your model for resin 3D printing is different from slicing for FDM. The two are similar in that you generally want to avoid 100% infill in order to save material and time. However, the honeycomb infill typically used for FDM is not appropriate for resin 3D printing.

The problem is that excess liquid resin generally becomes trapped in hollow areas. Therefore, to avoid weakening the print, this resin must be drained after printing. Unless your slicing software can handle the following tasks, you will need to manually perform them prior to slicing your model:

  • Repair your model if it is non-manifold, i.e. invalid. (Models are non-manifold if they, for example, have areas with no thickness, internal faces, or disconnected edges or vertices.) Unfortunately, there are many non-manifold models on Thingiverse and similar sites. The Medieval Castle on Thingiverse is one example. Slicers such as Cura usually handle such models without issue, but others do not. In such cases, models must first be repaired using Netfabb, MakePrintable, or similar services.
  • If the model contains large solid portions, hollow it out using Autodesk Meshmixer (free for Windows and Mac), leaving weep holes at the bottom for the excess resin to drain. A good standard is to add two holes, each with a radius of 1.5 mm. Make sure to scale the model to the desired size prior to hollowing it out. Otherwise, the walls may be thicker or thinner than you intended.
  • The abovementioned article from 3D Hubs also recommends orienting the part to minimize large Z-axis cross-sectional areas. This, in turn, minimizes the peeling forces on the part each time the platform moves upward to begin a new layer. Note that rotating the model to achieve this orientation may require supports that would not have been required if the part were oriented parallel to the build plate.

SLA Printing – Tips & Tricks 4. Professionally Remove and Clean Your Print

A magnetic stirrer. Source: Amazon

When removing and cleaning your model, use nitrile gloves to protect your hands from resin. A set of #17 and #18 X-Acto knives may work better than a spatula to remove the print from the build platform, but work slowly to avoid chipping the model. You can allow excess resin (including resin inside of hollow models) to drain back into the resin tank. Scooping the excess resin off the build platform works well with a squeegee.

Rinse the model in isopropyl alcohol. If the model is hollow and has drain holes, allow the alcohol to partially fill the model, cover the drain holes, and then gently shake the model to rinse the inside. The longer you leave the model in the alcohol, the duller it will become, so it is best to keep the rinse duration very brief — never more than 10 minutes. You can hold the model in the alcohol and use a toothbrush to clean off any excess resin, then allow the model to drain.

If you’re looking for a more advanced rinsing solution, you might consider a magnetic stirrer. This tool uses a rotating magnetic field to cause magnetic capsules immersed in the isopropyl alcohol to stir the alcohol while rinsing the part. This stirrer works well with the Tupperware Modular Mate Square 4 (23 cup capacity, 9 inches tall) with a 3D printed PLA insert to keep the stir bar in the center of the container.

This rinsing solution emulates the Formlabs Form Wash, except that it lacks a timer and doesn’t automatically lift the part out of the alcohol.

If you need help to determine when your Isopropyl Alcohol solution is depleted and needs to be replaced, you can measure the resin concentration using a specific gravity hydrometer. (This will work for isopropyl alcohol solutions up to 93%.) It’s convenient to perform this measurement in a graduated cylinder. Here’s a useful spreadsheet showing the specific gravity of various concentrations of resin in isopropyl alcohol.

If the model has supports, remove them after rinsing the model. You can use a pair of small diagonal cutting pliers.

SLA Printing – Tips & Tricks 5. Properly Cure Your Print

UV curing station (for SLA printers). Source: romanjurt / Thingiverse

For the best quality, the print should be cured after drying, ideally using 405-nm UV light. Commercial solutions such as the Formlabs Form Cure and the XYZprinting UV Curing Chamber exist, but you can build your own curing chamber for far less money. Here are a few examples:

License: The text of "SLA Printing – 5 Tips and Tricks for Great Resin Prints" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.

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