Hollow resin prints: drain holes, suction cups and orientation

Why hollowing changes how an SLA part behaves

A solid part uses resin through its full thickness and takes longer to print. According to Formlabs documentation, printing a model as a hollow object conserves material and decreases print time, and it can also be a functional requirement, for example when a part needs to stay light. That is why hollowing becomes an early habit once you move to the resin process for larger objects.

A cavity can change the forces during SLA printing. Formlabs explains that an upward-facing cavity in PreForm traps air between the part and the tank; the resulting suction can make the part stick and detach. The guide links this scenario to possible print failures, but a print symptom alone does not establish that cause.

Formlabs distinguishes two purposes for drainage holes: reducing suction cups in SLA and preventing material from remaining trapped in SLA or SLS models. This recommendation does not mean that one hole or one placement resolves every case; check the geometry and applicable design specifications before finalizing a model.

The PreForm Hollow tool and what it does not decide for you

According to the Formlabs page on hollowing, the Hollow tool is available starting with PreForm 3.41.0. It opens from the toolbar to the left of the 3D scene and offers three settings: Wall Thickness, Feature Size and Accuracy. Formlabs states that model regions thinner than twice the selected Wall Thickness will not be hollowed, which explains why some details stay solid.

The documentation also flags topology: if a model is a union of multiple smaller pieces rather than one defined body, PreForm hollows each piece separately, which may produce unexpected results. Check the section view and tool output; merge geometry in CAD only if that matches the model you intend.

Reducing Feature Size smooths internal surfaces, but PreForm may take longer and generate a larger FORM file. Increasing Accuracy makes internal features match outer features more closely, at the cost of a longer calculation. Hollow All, Hollow Selected and the Reset buttons let you hollow everything, a selection, or restore the original models. What the tool does not do is add drain holes for you: Formlabs advises creating them in a 3D design package.

Reading the cross-section of an imaginary decorative shell

Take an original, purely hypothetical shape: a decorative dome with a flat base, something like a shelf cloche, with no mechanical function. No file, print or measurement accompanies this example. It only exists so you can reason about a section. Cut vertically, the part shows an outer skin, an inner skin created by hollowing, and a wall of constant thickness between them. The top of the dome, thinner than twice the chosen wall, stays solid under the rule Formlabs describes.

Picture the shell in PreForm with its opening facing up: Formlabs describes this configuration as one that can trap air and create a suction cup. If the flat base closes the cavity in the chosen orientation, consider how the interior could be drained; this thought experiment does not establish that a particular pose or opening will work for a real model.

The section exposes the real question: you need a continuous path between the cavity and the outside, placed where resin will naturally run in the final orientation. That reading, not wall thickness alone, drives everything that follows.

Mapping drain paths before you place the holes

For the imaginary shell, trace on a section view the path between the cavity and the outside in the chosen orientation. For internal or enclosed hollows, Formlabs recommends filling the volume or adding holes in a design package to minimize suction. Compare these options against the model and check the applicable print specifications before choosing an opening location.

If you keep the cavity, compare possible opening locations in a section view and check that they connect to the intended geometry. The source gives no drain-hole diameter; it directs readers to the printer's design specifications for minimum supported wall thickness. Do not infer an opening size from that wall-thickness rule.

Keep in mind what the holes do not guarantee. They limit suction and trapped resin during printing, but they do not prove the interior will be washed, dried and cured correctly. Resin post-processing remains its own step, and a cavity that has stopped dripping is not necessarily clean.

Orienting the part: reduce suction risk and weigh tradeoffs

The Formlabs orientation page sums up four general rules for its SLA printers, while noting that not every recommendation applies to every model or print job. Print larger sections of the part closer to the build platform, and directly on it when possible, to constrain the part and prevent warping. Minimize the XY cross-sectional area of each layer by orienting the part vertically along its longest dimension, and minimize the change in area between layers. Orient features so they branch off a common base rather than merge. And a stiff, rigid material can be self-supporting when the angle to the horizontal is greater than 45°.

For a hollow shell, include the PreForm cup check when considering orientation. Formlabs says an upward-facing cavity can trap air; its tool lets you view flagged areas. The hollowing page also recommends drain holes to minimize suction. Do not turn this advice into a mandatory angle or pose for every part.

Print supports come next, not first. Formlabs recommends orienting complex, aesthetically critical or mating surfaces away from the supports as much as possible, and notes that faces printed parallel to the platform may show slight dimpling from the release texture of the Light Processing Unit.

Warping, visible surfaces and checks before you start the job

Formlabs documentation describes several warping artifacts seen on Form 4 and Form 4L generation printers: shrinkage from sudden changes in cross-sectional area, curling of edges, curling of large surfaces, and bat-winging on faces tilted at a shallow angle. For the shrinkage and edge-curling cases, the cited strategy is reorienting the model at 45° to the horizontal. These are documented specific cases, not a single angle that fits every part.

Formlabs describes direct platform printing as a way to limit some warping, but notes that initial layers may be oversized in XY. A critical fit may therefore require model-specific compensation. For the hypothetical dome, compare that pose with a supported one and examine the drain path in each; a flat base does not establish a universal hole location.

Before starting, check topology and the hollowing result, areas that remain solid, the path from each cavity to the outside, flagged cups, and the tradeoff between supports and visible surfaces. Multiple bodies are not prohibited: inspect what the tool produces. Choose the resin and its wash/post-cure process before finalizing the file, since their constraints affect design. Formlabs also notes that heavy parts may sag during post-curing without enough support, especially with softer materials or longer cycles.

Sources and limits

• Hollowing models in PreForm | Formlabs
• Model orientation best practices for SLA printing | Formlabs

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.