Why Hollow Resin Prints Crack, Dent and Leak

You hollowed the model because 400 grams of resin for one bust is absurd, and the slicer made it look like a free decision: click, set a wall thickness, save two thirds of the material. Then the part came off the plate with a wall sucked inward, or split along a seam a week later, or started weeping something sticky out of a hole you drilled to stop exactly that. Hollow resin prints fail in a small number of specific ways, and all of them come from the same change — you turned a solid object into a sealed vessel with liquid inside it, and almost every step after that treats the two differently.

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Read Which Hollow Failure You Have

Six symptoms, six different moments in the process. Find yours before you change a setting, because the fixes point in opposite directions.

The first two are printing problems and are solved by geometry and speed. The last four are post-processing problems and are solved by getting liquid out of the part before you do anything else to it.

Hollowing Trades One Force for Another

An MSLA printer builds every layer against a film and then peels the part away from it. The force that peel takes scales with the cross-sectional area being lifted, which is the whole engineering reason to hollow: a solid 60mm-wide base presents its entire footprint to the film on every layer, and the supports have to carry that.

Hollowing removes most of that area. What it adds is a volume of liquid resin inside a shell, and a shell that behaves like a suction cup if the liquid and the air cannot move. In an unvented cavity, the plate lifts, the pressure inside drops, and the thinnest wall in the assembly deflects to make up the difference. A fraction of a millimetre per layer across four hundred layers is a dent you can see from across the room.

So hollowing is not a free reduction in force. It is a reduction in force on the condition that the cavity breathes. Where this overlaps with the general failure modes — sheared layers, lost supports, cold resin — our walkthrough of why resin prints fail covers the solid-part versions of the same physics.

Two Holes, Not One, and Where They Go

One drain hole is the single most common mistake in hollow printing, and it is worse than none at all because it looks like the problem has been addressed.

A cavity with one hole airlocks. Resin cannot flow out unless air can flow in, and through a 3mm hole those two things cannot pass each other reliably. The cavity stays full, the part still suctions, and you have a sealed vessel that happens to have a hole in it.

What works:

• Two holes minimum, 3–4mm across. Below about 2.5mm, surface tension in a viscous resin wins and almost nothing drains.
• One at the lowest point in the printed orientation — the end hanging nearest the vat — so resin leaves under gravity while the print is still running.
• One at the highest point of the same cavity, so air replaces it. Opposite ends, not adjacent.
• Check the holes in the sliced preview, not in the model view. A hole placed on the model's geometric bottom ends up near the top once the part is tilted 20–30° for printing, which is the orientation you actually want for everything else.
• Scale up with the part. Fist-sized and above, 4–5mm and a third hole; a hole that drains a 50mm cavity does not drain a 200mm one in the same time.

Hide them where paint or a base will cover them, not where they are structurally convenient. A hole in the middle of a thin wall is a crack initiation site, which is the next section's problem.

The Wall Thickness That Survives Curing

A resin printer will print a 0.6mm wall perfectly. It will not survive, and the failure is not during printing — it is cure shrinkage.

Photopolymer shrinks as it finishes polymerising. On a solid part that stress is distributed through the bulk. On a hollow shell it concentrates in whatever is thinnest, which is why hollow parts craze, bow and split along walls rather than at joints. Workable floors, assuming a display piece handled gently:

Material is the other half of this. Standard display resins are formulated for detail and surface finish, and they are brittle — low elongation at break, which is exactly the property a thin wall needs. If your part is large, thin-walled and going to be handled, print it in an ABS-like or tough resin and accept slightly softer detail in exchange for a wall that bends instead of splitting. Cracks that appear after the cure rather than during it are covered from the other direction in why resin prints warp and crack after curing.

One more constraint people hit late: exposure and wall thickness interact. A wall tuned at one exposure is a measurably different wall at another, which is the practical argument for standardising on one resin you have calibrated — Chitu Systems sells its Conjure standard resin in 10kg for people printing at that volume, and one calibration across ten kilos beats twenty calibrations across twenty bottles.

Washing a Part That Has an Inside

A hollow part arrives at the wash station full. Not damp — full, or at least coated internally with liquid resin that gravity has not finished moving. Everything that makes washing work on a solid part fails here: you cannot see the surface, you cannot brush it, and the part floats.

The sequence that works:

1. Drain it on the plate first, holes down, for a few minutes. Most of the volume leaves before the part ever reaches alcohol, and that alcohol stays clean for longer.
2. Flush the cavity directly. A syringe or a squeeze bottle of 99% isopropyl through the lower hole, out through the upper one, until what comes out is clear rather than milky.
3. Then wash normally, in two stages — dirty alcohol first, clean alcohol second. Hold the part under with the holes facing up so the cavity fills rather than bobbing; a floating part washes on one side only.
4. Keep it short. Total alcohol contact under about ten minutes. A long soak swells the polymer and softens fine detail, and a thin hollow wall has less material to lose.
5. Drain and dry completely, holes down, before curing. Alcohol left inside cures into a tacky film and softens the wall around it, which is the hollow version of a print that stays sticky after curing.

An ultrasonic cleaner is genuinely useful here, because it moves solvent through a volume you have no physical access to. Run short cycles and keep the temperature down — warm alcohol accelerates the swelling you are trying to avoid. Do all of it in nitrile gloves, and cure the waste solid before binning it; the full handling routine is in resin safety and post-processing basics.

What Trapped Resin Does Weeks Later

If liquid resin stays inside a cured shell, it does not go away and it does not cure. UV cannot get through an opaque wall, so the inside of your part stays a reservoir of uncured photopolymer indefinitely.

Three things follow, in roughly this order. It weeps — out of the drain holes, out of micro-porosity at the layer lines, usually after the part is painted and on a shelf, lifting the paint from underneath. It stays a skin sensitiser, because uncured resin on the outside of a finished model is chemically the same liquid it was in the vat. And where a translucent wall lets a little light through, the resin inside cures slowly and unevenly, shrinking against the shell and producing a crack with no visible external cause.

The fix is always removal rather than containment. Flush it out and dry it. Sealing the holes with filler traps the problem and guarantees you meet it again later.

The Cure Station Is Where Sealed Cavities Fail

Curing is exothermic, and a cure lamp adds its own heat on top. A solid part sheds that into the room. A hollow part contains a volume of air — and possibly some residual alcohol — that expands as it warms, inside a shell whose walls are 2mm of softening polymer.

That produces the bulged wall, the split at a hole, and occasionally a part that visibly deforms while you watch. Three habits remove it:

• Cure in short runs with the part turned, rather than one long exposure from one side. A station with a rotating plate and a timer makes this the default; our comparison of wash and cure stations covers chamber size, which matters once your parts are the size that needed hollowing.
• Never cure a hollow part submerged in water. Water curing is a good technique for solid parts — it excludes oxygen and holds the temperature down — but a cavity fills with water you then cannot get out.
• Make sure the holes are open before it goes in. They are the pressure relief. A hole plugged with a blob of cured resin from the drain step is not a hole.

When Not to Hollow At All

Hollowing is a material-cost optimisation for display models. It is the wrong instinct for:

• Miniatures under about 50mm. The resin saved is pennies, and a 1mm wall on a 32mm figure is thinner than the detail standing on it.
• Functional parts. Hollowing removes exactly the material that resists bending, and a uniform slicer offset is not a rib structure. If cost is driving it, the part may want a different process entirely.
• Parts that must hold a dimension. A shell distorts under cure shrinkage more than a solid block does, and the error is not uniform — see why resin prints come out the wrong size.
• Anything that will be sealed, glued shut or embedded. You lose access to the cavity forever, and the cavity is where the uncured resin is.

A useful middle option that gets ignored: hollow to a generous wall and leave the slicer's lattice infill off. Closed lattice cells are dozens of tiny sealed cavities, each one a small version of every problem above, and none of them drainable.

Hollowing in the Slicer Versus in CAD

Slicer hollowing applies a uniform inward offset to the mesh. It is fast, it is fine on simple forms, and it has two characteristic failure modes on detailed models. Where the model is thinner than twice the wall thickness — a blade edge, a horn, a cape edge — the offset self-intersects and the slicer either punches through or leaves a sliver of unsupported geometry. And on a complex model the result is often several separate cavities rather than one, each needing its own pair of holes, which nothing warns you about.

If you have the source model, hollowing in CAD lets you vary wall thickness where the part needs it, put ribs across long spans and place the holes deliberately. If you only have a mesh, hollow in the slicer but then check the result in cross-section view before you commit to a six-hour print, looking specifically for breakthroughs and isolated voids.

Either way, hollow before you generate supports. Hollowing afterwards invalidates the support placement, and the usual outcome is a support landing on a wall that is now 1.5mm of unbacked shell. That wall will dimple, and the vat film takes the rest of the punishment — the slack-and-clouded-film problem in why resin vat film fails arrives much faster on a machine printing big hollow parts badly.

When the Part Belongs Somewhere Else

There is a size past which this stops being a settings problem. A 300mm display piece on a desktop machine is a sequence of compromises — hollowed hard to fit the plate, split to fit the build volume, bonded afterwards along a seam that then has to be hidden. Any one of those is routine; all three at once is a weekend.

A shop running large-format MSLA or industrial SLA prints the piece in one go, at a wall thickness chosen for the part rather than for the plate, and handles the washing and curing of something that size as a process rather than an evening. If you are hollowing mainly to get a part to fit, that is the signal — and if you are buying a machine instead, read the plate size and lift behaviour per model in our resin printer buyer's guide or across ELEGOO's resin machines, both of which list build volume per model so the comparison is possible at all.

For the one-off large piece, browse providers by process and location and get a quote against the file you already have. It is usually cheaper than the third attempt.

Start with the cheap diagnosis tonight, though: open your last failed hollow print, look at where the holes ended up in the printed orientation rather than in the model, and see whether the cavity could ever have drained at all.

Hero photograph by Azeez Maree, via Unsplash.