How to Hollow a Model in Lychee

Hollow a model wrong and the print fails in spectacular ways: trapped resin pockets that explode under pressure, suction cups that tear the model off the plate, hidden cavities that pool half a bottle of expensive resin. Hollow a model right and a 75 mm dragon prints in half the resin and half the time of the same dragon solid. The difference is roughly five clicks in Lychee plus knowing where to put the drainage hole.

Why hollow at all

Resin cost per print scales directly with volume. A hollow model uses approximately 30 to 60 percent less resin than a solid model of the same external dimensions. That range is approximate and depends on wall thickness, model geometry, and how much air the cavity displaces. Print time also drops because each layer cures faster when the cross-section is smaller.

The savings are most significant on large models: dragons, terrain pieces, vehicles, character busts. At 75 mm and above, hollowing is worth the five minutes of setup. At infantry scale (28 mm to 35 mm), the wall thickness eats most of the volume saving and the effort is rarely justified.

The upstream article on printing bigger and hollow resin miniatures covers the conceptual side of hollowing. This article covers the exact Lychee workflow.

When not to hollow

Three situations where hollowing is the wrong choice.

Models smaller than roughly 50 mm in any dimension. At that size, a 2.0 mm wall leaves so little interior volume that the resin saving is small. Print solid and save yourself the extra steps.

Pre-supported files where the creator already hollowed the model. Many professional Patreon and MyMiniFactory files come as both solid and hollow variants. The hollow version has been designed by someone who knows where the drainage holes go and how thick the walls need to be for that specific model. Use it rather than hollowing the solid version yourself.

Water washable resin. This one is non-negotiable. Hollow models printed in water washable resin are prone to cracking as moisture enters through the drainage holes during washing and takes a very long time to evaporate from the enclosed interior. The wet interior resists curing and the pressure of trapped moisture eventually fractures the wall from the inside. If you are using water washable resin, either print solid or use a pre-hollowed file from a creator who has tested it specifically with that chemistry. The full explanation of why water washable resin behaves differently in hollow models is in Can You Really Wash Resin Prints in Water?.

The five-click workflow

The Lychee hollow workflow follows this sequence: Tools menu → Hollow → set wall thickness → preview the cavity → confirm. The sections below expand each step.

Step 1: Tools menu and the hollow dialog

In Lychee Free, the hollow tool lives in the Tools menu at the top of the screen. In some recent versions it has also appeared as a dedicated button in the left-side panel when a model is selected. Either route opens the same dialog.

Once open, the dialog renders the model’s exterior in solid form and displays the proposed interior cavity as a transparent or ghost volume inside. You can rotate the model normally while the dialog is open to inspect the cavity from any angle.

Step 2: Set wall thickness

Wall thickness is the single most important setting in the hollow dialog. It controls how much resin remains between the exterior surface and the hollow interior. Too thin and the model is fragile; too thick and most of the resin saving disappears.

The working range for miniatures and terrain is 1.5 mm to 2.5 mm.

1.5 mm is the practical minimum for fine miniatures. Below this, structural weakness becomes a real risk, particularly for wargame pieces that get handled repeatedly. Use 1.5 mm only when resin saving is the priority and the model will be display rather than gaming use.

2.0 mm is the workhorse setting. Strong enough for most miniatures, low enough resin cost to matter on large models. This is the number to start with on anything you are not sure about.

2.5 mm suits heavy terrain pieces and models with structural stress points: banner poles integrated into the body, thick weapon hafts, large scenic bases. At 2.5 mm the saving relative to solid is smaller, but the wall is solid enough to handle drops and gaming table life.

Step 3: Preview the cavity

Before confirming, inspect the proposed cavity using the ghost view. Three things to check.

First, the cavity must not break through any external surface. On thin edges of capes, wing membranes, or sword blades, the algorithm can try to place a cavity that reaches the exterior surface. If you see a region where the ghost volume touches or crosses the exterior mesh, increase the wall thickness until the breakthrough disappears.

Second, the cavity should be contiguous: one single interior volume, not several separate chambers. Multiple chambers can each trap resin independently and are much harder to drain. If Lychee’s algorithm produces multiple chambers on a complex model, a small increase in wall thickness usually merges them.

Third, check whether the algorithm has flagged any internal supports. Lychee’s hollow tool will sometimes note that the cavity requires internal supports on very large prints. For most miniature-scale work at 1.5 mm to 2.5 mm walls, internal supports are not needed.

Step 4: Confirm

Click the green check or the confirm button in the hollow dialog. Lychee applies the hollow operation to the model mesh and the exterior appearance does not change. The model now has a wall and an interior cavity.

Step 5: Add drainage holes

This step is critical and is the most common omission that causes hollow prints to fail.

Without drainage holes, two things go wrong during printing. First, liquid resin fills the cavity during the print as the build plate dips into the vat. Second, as the build plate lifts on each peel cycle, the sealed cavity creates a vacuum that pulls against the plate. On large prints, this suction force is enough to tear the model off the plate mid-print or to crack the walls.

After printing, a hollow model without drainage holes traps uncured liquid resin permanently inside. That liquid resin will crack the model from within as it slowly expands and contracts with temperature changes.

Lychee has a dedicated drill or hole tool accessible from the same hollow dialog or from the Tools menu after hollowing. Use it to add drainage holes.

Minimum two drainage holes. One at the lowest point of the model in its print orientation, and one at a high point to allow air to vent as resin drains.

Orientation is everything for drainage hole placement. The lowest point during printing is determined by how the model sits on the build plate, not by the model’s anatomical logic. A dragon tilted at 45 degrees for printing has its lowest point at the tip of the tail or the foot, not at the belly. Place the drainage hole at whichever point is physically lowest during the print run.

Diameter: 3 to 4 mm for miniature work. Smaller holes clog with partially cured resin and do not drain reliably. Larger holes are visible and weaken the wall near the opening. 3 mm is the minimum; 4 mm is better on anything above 75 mm.

Hide the holes where possible. The underside of a base, the interior of a recessed area, the soles of the feet. The model drains just as well and the holes are invisible from any painted display angle.

The water-washable trap, again

It is worth repeating because it catches beginners who read quickly. If you are using water washable resin, do not hollow the model. Water enters through the drainage holes during washing. It evaporates slowly from an enclosed cavity. The wet interior resists UV curing. Over days or weeks the moisture builds up enough internal pressure to crack the wall. Solid prints in water washable resin are fine; hollow prints in water washable resin have a substantially higher failure rate. Use alcohol-washable resin for hollow work.

The drainage hole rules in summary

The rules are simple but the consequences of getting them wrong are expensive.

• At least two holes per model: one at the lowest print point, one at a high vent point.
• 3 to 4 mm diameter for most miniature work.
• Placement is relative to print orientation, not to anatomical logic.
• Hide holes on undersides and recessed areas where possible.
• After printing: hold the model at the drainage hole and let pooled liquid resin drip back into the vat before moving to the wash station.

Re-supporting after hollowing

If you added supports before hollowing, Lychee generally keeps them in place because they attach to the external surface, which has not changed. Check the support tree after hollowing to confirm all supports are still attached to valid geometry.

New internal supports are not needed for most miniature-scale hollow work at the wall thicknesses covered here. The cavity is small relative to the model and the wall thickness carries the structural load. If Lychee flags unsupported sections after hollowing, they are almost always on the exterior surface and a small number of additional manual supports resolve them.

The support settings guide for resin miniatures covers tip diameter and placement logic in detail if the post-hollow support check reveals gaps.

What to do after printing a hollow model

Before moving the model from the build plate, hold it at the drainage hole and let pooled liquid resin drain back into the vat. Give it thirty to sixty seconds. On large prints this step recovers a meaningful amount of resin.

At the wash station, ensure the wash medium reaches the interior through the drainage holes. On alcohol-washable resin, a squirt bottle aimed at the drainage hole and a brief soak is enough. Shake out any remaining liquid, dry, and cure as normal. The interior does not need to be fully transparent to light for curing: the wall thickness at 1.5 mm to 2.5 mm is thin enough that UV reaches the interior surface through the walls in most UV curing stations.

Where this fits in the Lychee workflow

Hollowing happens after importing and orienting the model but before adding supports. The sequence is: import → orient → hollow → add drainage holes → support → arrange → slice.

If you are new to Lychee and working through the full slicer workflow, the Lychee Slicer guide for beginners covers the complete five-step sequence. If orientation is the part you are not confident about before hollowing, the resin print orientation guide for miniatures covers the decision logic for tilting and placing models on the plate.

Hollow printing is one of those skills that pays for itself the first time you run a large model. A 75 mm dragon that would have taken two full bottles of resin solid takes one at 2.0 mm walls with proper drainage holes. The five minutes of Lychee setup is never the bottleneck.