Hollowing Resin Prints and Drainage Holes

Why hollow at all

Prusa gives three reasons. Hollowing saves a considerable amount of material, it needs fewer supports, and it lowers the area of each layer, which decreases the force needed to detach the print from the film after every layer (see Orientation and Supports). Large solid figures, busts and display pieces are the usual candidates.

Hollowing adds steps (holes, draining, washing the inside), and each one is a chance to trap resin. On a small part there is little resin to save, so solid is often simpler.

The two risks

• Trapped uncured resin. Prusa states it directly: without drainage holes, the uncured resin would end up trapped inside your finished print. That resin stays liquid. A part with liquid resin sealed inside is still a part that holds liquid resin, with all the handling and disposal rules that come with it.
• A suction cup. Formlabs lists hollow models among the shapes that trap air and resin against the film and cause suction. For enclosed hollows, Formlabs says to either fill the hollow or add drainage holes.

Wall thickness

In PrusaSlicer, the minimum shell thickness is 1 mm Maker spec. That is a software minimum, not a recommended value for every part. Other slicers set their own limits; check their documentation.

Prusa notes that low hollowing accuracy can make the wall thickness uneven, and recommends a higher accuracy setting for thin shells. Higher accuracy takes longer to compute.

PrusaSlicer also has a closing distance: it fills cavities narrower than that threshold. Prusa says this helps narrow pockets and sharp inner corners, where uncured resin would otherwise be hard to get out.

Drain holes

• At least two. Prusa suggests at least two drainage holes.
• Where they go. Put them where resin will collect and where they will hide: on the base, the underside, or a face that will be glued or painted. Think about which way the part will sit while it drains and while it washes, and put a hole at the low point of each pocket.
• Size and depth. In PrusaSlicer, hole diameter and depth are adjustable and holes are drilled perpendicular to the surface. Make the hole deep enough to pass through the wall; a hole shorter than the wall drains nothing.
• Holes without hollowing. PrusaSlicer can add holes to a model that was hollowed in other software. If the mesh is broken, Prusa notes the hole drilling may refuse to proceed; repair the model first.

Orientation for a hollow part

Orient the hollow so it does not form a closed cup facing the film while it prints; Formlabs' fix for a cup that cannot be avoided is a drainage hole. Combine this with the tilt and support rules in Orientation and Supports.

Draining and washing the inside

1. Before washing, with gloves on, tip the part over the vat or a resin-safe container so resin runs out of the holes.
2. Wash longer than you would a solid part. Formlabs' wash guidance says to extend the wash for hollow, concave and large parts and for parts with internal channels. Move solvent through the holes, for example with a wash bottle.
3. Let the inside dry completely before post-curing.
4. Tip the part over a paper towel and look for resin or solvent coming out of the holes. If any does, drain and wash again.

Full washing and curing steps are in Washing and Curing Resin Prints. Any resin that runs out is waste resin; see Resin Waste and Disposal. Formlabs' guidance is that liquid resin must not go down a drain or into household trash.

Decision table

Situation	Choice
Large display figure, holes can hide on the base	Hollow, at least two holes
Small part, little resin to save	Print solid
Hollow but no place for holes that can empty it	Print solid; trapped resin is worse than extra resin