How to Hollow a 3D Model for Resin Printing

A solid resin print of anything bigger than a chess piece wastes resin, prints slower and can tear itself off the build plate. Hollowing fixes all three, as long as the walls are thick enough and the resin has somewhere to drain.

Why hollow a resin print?

Hollowing a 3D model for resin printing saves most of the resin, shortens the print and removes the suction forces that break large solid parts. Formlabs' own worked example cut a part from 228.90 mL of resin to 51.88 mL at a 2 mm wall, a 77% saving. The shell also needs fewer supports, so there is less cleanup afterwards.

Resin. A solid model's volume grows with the cube of its size, so a 10 cm bust uses eight times the resin of a 5 cm one. Standard resins ran roughly $25 to $80 a liter across the price guides we checked as of October 2026, with engineering and dental resins well above that. A shell replaces that cube with a surface, which is why the savings are so large on anything chunky.

Peel forces. Resin printers cure each layer against the bottom of the vat, then lift the platform to peel it off the film, and the force of that peel scales with the cured area. A hollow model's cross-section is a thin ring instead of a whole slice: less area, less force, fewer prints ripped off their supports.

Cupping. This is the one that surprises people. Formlabs describes it plainly: a hollow or convex section "acts as a suction cup and traps air while printing." As the platform lifts, the trapped pocket expands, the pressure inside drops, and if the wall is too thin it buckles inward. Formlabs calls that failure a blowout. A hollow model without drain holes is a textbook cup. So hollowing without drain holes can make a print less reliable than leaving it solid.

How thick should the walls be?

For most hobby resin prints, a wall of 1.5 to 3 mm is the working range, and 2 mm is the usual starting point. Formlabs' hollowing guide uses a 2 mm wall in its example; third-party guides for consumer printers cluster around the same figure. Go thicker for large pieces, thin flat panels and brittle resins, and thinner only for small parts that will not be handled.

Printer makers publish minimum printable wall thicknesses that are far below this. Formlabs lists 0.2 mm for the Form 3 and Form 4 generations and 0.4 mm for the Form 2, as of October 2026. Those are the thinnest walls the printer can resolve, not what you should hollow to. A 0.4 mm shell the size of a fist will crack when you pull the supports, warp during washing as it soaks up isopropyl alcohol, and may blow out during the print itself.

A few rules of thumb that hold across printers:

Whatever you pick, the wall must be a real wall everywhere. A fingertip, a sword, an ear is already thinner than two walls, so a correct hollowing tool leaves those parts solid rather than carving a sliver of air into them. If yours does not, the model gets fragile exactly where it is already fragile.

Where do drain holes go, and how big?

Put at least one drain hole at the lowest point of the model as it will sit on the build platform, and make it 3 mm or larger for a hobby printer. Formlabs recommends a second hole on another surface so air can enter while resin leaves, and gives printer-specific minimums of 2.5 mm (Form 3) and 3.5 mm (Form 2), as of October 2026.

Placement matters more than size. Formlabs' support staff traced one user's repeated blowouts to drain holes placed on the sides of the model instead of near the build plate. The cavity fills with resin as the model rises out of the vat, and that resin has to get out downward, through a hole that is submerged in the vat for as much of the print as possible. A hole at the top does nothing until the print is finished.

Two holes beat one. With a single hole, resin has to drain against air trying to get in through the same opening, which is slow and leaves a pocket at the top. A second hole, ideally also low on the model but on a different face, lets the cavity breathe. On a bust, the common choice is one hole under each shoulder or one under the base and one at the back of the neck.

Size is a trade-off between draining and hiding. Bigger holes drain and rinse faster. Smaller holes are easier to plug and hide after painting. Three to four millimeters is the usual compromise; vehicle builders go to 6 or 8 mm because there is nowhere visible to hide a hole anyway.

Slicer hollowing vs hollowing in the sculpting tool

You can hollow at two points in the pipeline: in the slicer after export, or in the sculpting tool before export. Both work. They fail in different ways.

In the slicer. CHITUBOX Basic, which is free, has a Hollow tool with Hollow out and Peel modes, a Wall Thickness setting, a Precision slider from 0 to 1 that controls the triangle density of the inner surface, and optional infill. Its Dig Hole tool cuts circular, hexagonal or square holes of a set diameter and depth and can keep the plug for you to print as a cork, as of October 2026. Lychee Slicer also hollows; a third-party comparison published in August 2026 places its voxel hollowing and automatic hollow blockers in the paid Plus tier, so check Lychee's current feature list before counting on it in the free version.

Slicer hollowing is convenient because it happens where you are already placing supports. Its weakness is that it works on whatever mesh the export gave it: intersecting layers, an inside-out face or a thin region come out duplicated on the inside, where you cannot see them.

In the sculpting tool. Hollowing before export means the mesh you ship to the slicer is already a single closed shell, and you can check its wall before supports enter the picture. Autodesk Meshmixer had a well-liked Hollow command, but Autodesk no longer updates it and points users to Fusion, as of October 2026. Blender's Solidify modifier produces a shell on simple shapes but, in the words of a developer who wrote a hollowing add-on to replace it, "generates inadequate geometry for any non trivial model." Sculpting apps with a voxel remesher are better placed here, because a voxel offset of a surface is a clean shell by construction.

The right answer for most people is both: hollow in the tool that can produce a watertight shell, then let the slicer confirm it and add supports.

How to hollow a sculpt in Re:Form

Re:Form is our tool, so read this section knowing that. It is a browser sculpting app on WebGPU, currently v0.8.8 Alpha, and as of that release it has a Hollow command built for exactly this job. The steps below are the whole workflow from finished sculpt to slicer.

1. Set the real size. Resin printers think in millimeters, and so does the Hollow tool. The Hollow window has a Print height field at the top: type the height you will print the model at, in millimeters, and the wall, the holes and every size in the window are worked out at that height. Exports in every format come out at it too, so the slicer reads the true size. Until you type one, the window assumes 100 mm tall and says so. Get this right first, because a 2 mm wall on a model the tool thinks is 2 mm tall is no wall at all. (Print height sets the same thing as the Unit mm slider under View ▸ Grid, said the easier way round.)

2. Open Tools ▸ Hollow… The window describes what it is about to do: make the active layer a shell with a wall of the thickness you set and drain holes at the bottom, through the Remesh. Step 1's Print height is its first row, and the plan underneath shows the model's size in millimeters so you can confirm it took. The window can be dragged by its title if it is in the way of the model.

3. Set the wall. The Wall slider runs from 0.4 to 20 mm in 0.1 mm steps and starts at 2 mm, the figure this post has been recommending throughout. The window states the consequence in plain terms: parts thinner than two walls stay solid. A 4 mm sword blade with a 2 mm wall is left as it is, not turned into a fragile tube.

4. Choose the drain holes. The Drain holes row offers None, 1 or 2. It starts at one. The holes are placed at the bottom of the model, its lowest point as it stands, which is where the resin needs to leave. With two, they sit apart on the bottom; if the bottom is too small for two of the size you chose, the window says so and makes one. Hole size runs from 1 to 12 mm and starts at 3 mm. Choose None and the window warns you that resin or support material will stay trapped inside and most printers want at least one hole.

5. Read the plan. Before anything changes, the window lists what will happen: the model's dimensions, the wall and hole settings, and the remesh resolution it will use. The tool picks a resolution fine enough for about three voxels across the wall, so the shell is not stepped or pinched, and warns when the wall is too thin for the remesh at this size, when the wall would be as thick as the model, and when a fine remesh will take a while.

6. Apply. The shell is built through the Remesh, the same path Boolean uses, so the result is one clean watertight piece: an outer surface, an inner surface offset inward by the wall, and one or two holes joining them. It is one Undo. If the wall looks wrong, press Ctrl+Z, change the slider and apply again.

7. Check it. Tools ▸ Print check… reports whether the layer is closed, how many pieces it is, its size and volume in millimeters, its triangle count, and any open borders, flipped faces or walls thinner than the Min wall you set, which starts at 1 mm. A hollow model's inner wall is counted as a cavity, not a fault, so a correctly hollowed shell passes. The volume figure is also your resin estimate: compare it before and after hollowing and you have your saving in milliliters.

8. Export. STL is the slicer-native choice and is on the Pro plan; OBJ is on every plan and every resin slicer opens it. The format guide covers the difference, but for a single hollow shell it does not matter much. Both come out at the Print height you set.

The Hollow and Print check tools work on the Free plan, which is the full toolset up to 6 million triangles with OBJ export, no card and no watermark on the model. Pro is $9.99 a month or $99 a year and adds STL, GLB and FBX, the roughly 80-million-triangle budget and a commercial license. You sign in with a free account before sculpting, and it is desktop browsers only: Chrome, Edge, Firefox or Safari 26, nothing to install. Re:Form is alpha software, so keep the save-and-check habit, and if a hollowed export misbehaves in your slicer, write to [email protected] with the file.

Should you hollow every model?

No. Hollowing pays off on anything with a cross-section wider than about 20 mm, and it is often pointless or harmful below that. A single 32 mm tabletop miniature saves perhaps a milliliter of resin and gains a drain hole you have to fill. Functional parts, pegs, threads and anything that will be drilled or screwed should stay solid.

Two cases where leaving it solid is right even on a big piece: when the model will be sanded or drilled, because breaking through into a cavity is the end of that part, and when the inside cannot be rinsed, because uncured resin left in a cavity keeps curing, warps the shell and can leak through a paint job months later. If you cannot drain and rinse it, do not hollow it, or add bigger holes until you can.

Cleaning and curing a hollow print

A hollow print is not done when it comes off the plate. The cavity is full of uncured resin, and if it stays there it will leak, warp the shell or crack it when the trapped resin eventually cures and shrinks.

Drain the model over the vat for a few minutes, holes down, before you wash it. Then wash the inside, not only the outside: swirl isopropyl alcohol through the holes, or use a syringe or squeeze bottle to flush the cavity, and drain it again. Shake out as much alcohol as you can, because alcohol trapped inside softens the wall.

Cure with the holes open, turning the model over partway so UV reaches the inside through the holes. Only then plug them, with a printed cork if your slicer kept one, a drop of resin cured by a UV torch, or putty under the paint. A sealed shell with liquid resin inside is a time bomb; one that was drained, rinsed and cured is a finished model.

Hollowing tools compared

If you sculpt in Re:Form, hollow there. If you sculpt elsewhere and export a clean watertight mesh, CHITUBOX Basic's Hollow and Dig Hole are free and do the job. The one approach to avoid is hollowing a mesh you have not checked, because every tool copies the mesh's problems onto the inner wall.

For the rest of the print pipeline, the sculpting-for-3D-printing guide covers detail density and export, the mesh repair post covers watertightness and thin walls, and the tabletop miniatures workflow covers scale and supports for small figures. To try the Hollow tool on something of your own, open Re:Form in a desktop browser.

FAQ

Does hollowing weaken a resin print?

A hollowed print with a 2 to 3 mm wall is strong enough for display pieces and most handled models; the parts that break are the thin ones, and a correct hollowing tool leaves anything thinner than two walls solid. Hollowing does weaken parts that will be drilled, sanded through or loaded in bending, so leave those solid.

Can I hollow a model after it has been sliced or supported?

Not without starting over. Hollowing changes the mesh, so supports placed before it no longer match, and a sliced file has no mesh left to edit. Hollow first, add drain holes, orient the model with the holes toward the plate, then support and slice. If you hollow in the slicer, do it before the support step for the same reason.

How many drain holes does a hollow print need?

At least one at the lowest point of the model as it sits on the build platform, and preferably two so air can enter while resin leaves. Formlabs recommends one as close to the platform as possible and at least one more elsewhere. Make them 3 mm or larger on hobby printers, and bigger still on large pieces with nowhere visible to hide a hole.