How to orient hollow models and cavities on a Formlabs SLA?

How to orient hollow models and cavities on a Formlabs SLA?

In the previous articles we talked about the basic process architecture and methodology of Formlabs SLA, and part orientation and support structure generation for the same. In this article, we will cater to hollow models and cavities and in the process understand the nitty-gritties of part orientation and the impending implications if they aren’t oriented properly.

Let’s take an example of a bottle. A bottle has an open end (mouth) and a closed end (base). If you were to fabricate this bottle (shown below) in Formlabs SLA, how would you orient it?

Basic understanding says that one would orient the bottle upright to minimize support structures. But one should note that in Formlabs SLA, the actual part, during printing, will be oriented upside down with respect to the part orientation shown in the Preform software. Thus, a bottle oriented upright in Preform will actually be fabricated upside down, as shown in the image below.

Wrong Part Orientation. The yellowish-orange color shows cups.

In the given orientation, what this will do is, it will result in the mouth of the bottle facing the resin tank, which will result in the formation of “Cups”. Cups are hollow volumes or cavities in a model. When the mouth of the bottle faces the resin tank, it results in air and resin entrapment. This results in suction forces building up in the part which can separate the individual part layers and result in print failure. The highlighted part (yellowish-orange) shows the probable zones for print failure. For such an orientation, the entire bottle is prone to failure.

To give an analogy, have you ever tried pushing an empty mug upside down into a bucket full of water, and then tried pulling it out? What would you experience? You’ll observe that you require more force to pull it just out of the water due to the suction forces acting on it. Because when you submerge an empty mug upside down, air gets trapped inside the hollow volume. This scenario is similar to 3D Printing a part in Formlabs SLA wherein, in this case, the mug is your part geometry (bottle) and the water is your resin material.

Part experiencing suction forces. Courtesy: Formlabs

To counteract this phenomena, it is highly recommended that hollow parts and cavities should be oriented with their openings facing the build platform. Thus, for the given part geometry, a better and preferred orientation for the part would be to orient the opening of the bottle facing the build platform, as shown in the image below. We can thus minimize/mitigate the suction forces acting on the part as there is no air and resin entrapment.

Correct part orientation

Note: Use the Orientation tool to rotate cups so that the opening of the cavity either faces the build platform or is placed at an angle. Regenerate supports after orienting the model. Orienting cups at an angle or to face the build platform prevents vacuum from forming between the part and the bottom of the resin tank during printing.

A rule of thumb:

For any cavity spanning more than a few layers, it is recommended that the opening of the cavity shouldn’t face the resin tank, otherwise it’ll result in the formation of Cups.

Another important thing to note is that, for the correct part orientation shown above, the bottle will have a lot of support structures inside its hollow volume. And if the opening is small, as it is in any standard bottle design, it will be difficult to remove these supports from the internal sections. In such a scenario, we advise that the bottle be split in half and the individual parts be manufactured separately. The two halves can later be assembled together. The advantage of manufacturing the bottle in two parts is that, one can easily remove the supports, and it becomes easy to post-process the part.

Cross-sectional view of the bottle showing the internal support structures
For models that contain internal hollows or enclosed volumes (for example, a hollow sphere), use CAD software to either fill in the hollow or add drainage holes to minimize suction effects during printing and to avoid trapping resin inside the part.