How to Prepare Your Model For 3D Printing Part 2: SLA

Missed part 1? Read here: How To Prepare Your Designs For 3D Printing – Part 1: FFF

Stereolithography (SLA) is a completely different technology to FFF. They both print layer by layer, but that’s about where the similarities end. FFF melts material and then extrudes it through a small nozzle onto a build plate, whereas SLA uses a laser to cure resin onto the build platform in the desired areas.

The specific printing process is different for each SLA printer, but the general concept is the same throughout photopolymer printing. Here we look at one particular type: inverted SLA.

Form 2 alongside resins on a table with two Form 2's in the background
Above: The Form 2 – an inverted SLA printer

How SLA Printing Works

Having a basic understanding of how SLA works can really help you to get successful and reliable prints.

A laser is reflected off finely calibrated mirrors, then through a transparent window and into a tank of resin. The build platform has been lowered into this tank leaving just one layer of resin between the platform and the bottom of the tank. The resin then undergoes a chemical reaction and solidifies in the places that the laser has hit.

After each layer is printed, the printer’s tank moves in order to peel the newly printed layer off the surface of the resin tank and then the build platform is raised to allow fresh resin to flow under the build platform.

This cure, peel, raise method is repeated layer by layer until the model is produced.

Part Orientation

When orientating your part, consideration has to be given to the movements of the printer in the peel process and the likelihood of the latest printed layer coming away from the rest of the model (delaminating).

You also need to ensure that the part has sufficient contact area for adhesion to the build platform and that support is kept off the important surfaces.

cube in slicing software
Above: a cube of dimensions 10x10x5mm. This model would be more likely to delaminate and so needs to be put at an angle.

Generally speaking, the larger and flatter the area is, the greater the chance of delamination. Models should be angled in such a way as to minimize the amount of surface area per layer. For most parts this is an angle of 30-45 degrees from flat.

a cube angled in slicing software
Above: The same 10x10x5mm cube but instead placed on an angle. This part has very little contact with the build platform and large unsupported areas.

Raft and Supports

There are two reasons we should use raft and supports:
1. As in the picture above, if a part is placed on an angle directly on the build platform, there would be very little contact area for the base and the print would likely fail.

2. The first couple of layers are over-compressed to ensure the part sticks to the build platform. This means the layers are slightly shorter than they should be. This is a problem if the part needs to be dimensionally accurate.

As in FFF, the raft is a complete base that goes under the model and allows for the model to be supported properly. As the raft is not a part of the model itself, the over-compression from the first layers is not important and can be ignored.

a cube with raft and supports in slicing software
Above: The 10x10x5mm cube angled with a raft and supports. This would print well and would prevent any delamination or dimensional accuracy issues.

Software will automatically generate the raft and supports for you where it computes they are most required. All supports can be removed or added individually afterwards if necessary. Software will also show up unsupported areas, so you can see where there may be issues before printing.

Suction Cups

If there are enclosed cavities in your model then these can create suction to the resin tank, very much like how suckers on tentacles operate. This can lead to delamination (as it is harder to unstick the part) and dimensional inaccuracy (as parts have stretched when they are pulled up).

If the suction cup is too strong, then the build platform may not even be able to raise at all and this will cause the printer to jam and the print to fail.

To avoid this, orientate any cup like cavities towards the build platform.

a trophy with supports and raft in slicing software
Above: A model of a trophy with a large suction cup. The software is warning the user of a potential suction cup (highlighted in yellow).
an upside down trophy with supports and raft in slicing software
Above: The same model, orientated to remove the suction cup. It prints upside down, so the bottom of the trophy would be pulled out of the resin tank last thus avoiding a vacuum forming.

Final Check

Once you think you have orientated and supported your part adequately, it is always useful to scan through the layers and check that there are no layers that are printing on nothing and no layers that are drastically larger than the one before. If there are, you may have to change your orientation or design.

a final check through each layer of a part using slicing software

It may seem complicated, but the software can do a lot for you, and alert you to any errors that are there.

Main points to remember

1. Minimize surface area by putting your parts on an angle
2. Keep dimensional accuracy by using a raft
3. Point enclosed cavities towards the build platform to avoid suction cups.
4. Check through the layers before you print.

If you want to learn more about printing with SLA, Formlabs have some very good instructional videos on their website.

Check out our range of photopolymer based 3D printers.