Hollow models and drain holes — avoiding trapped resin in large prints

Hollow models and drain holes are essential considerations for anyone working with large resin prints. Properly designing hollow prints and strategically placing drain holes can prevent cured resin from becoming trapped inside your model, saving you time, material, and headaches during post-processing.

Key takeaways

• Hollowing large resin prints reduces material use, print time, and curing complications but requires careful planning.
• Drain holes enable trapped uncured resin to escape, preventing discoloration and excessive weight.
• The size, placement, and number of drain holes depend on the model’s design and resin flow dynamics.
• Design software tools and slicer settings often include hollowing and drainage features to simplify this process.
• Post-print techniques like vacuum degassing and careful flushing further ensure trapped resin removal.

Why hollow models matter in large resin printing

Large resin prints typically mean significant volumes of liquid resin inside the model. Solid prints that size can become very heavy and consume large quantities of resin, which increases cost and print duration. More importantly, uncured resin trapped inside can compromise the print’s quality and longevity.

By hollowing your model during preparation, you create an internal cavity that drastically reduces resin use and weight. The hollow areas are enclosed by thin walls, typically 1.5mm to 3mm thick depending on your printer’s resolution and resin strength. This approach can cut material volume by 50% or more.

However, hollowing introduces the problem of trapped uncured resin inside the voids after printing. Without an outlet, this resin cures semi-transparent or cloudy over time, discoloring the model and preventing full post-curing inside the cavity. This is where drain holes become critical.

How drain holes work: principles and benefits

Drain holes are small openings placed in hollow models designed to allow liquid resin to escape after printing and before post-curing. They serve several functions:

• Resin drainage: Allowing uncured resin to flow out during and after the print process.
• Improved curing: Exposing the interior cavity to UV light or heat during post-curing.
• Preventing bubbles: Letting trapped air escape, which can cause voids or weak spots.
• Weight reduction: Letting excess resin drain out minimizes the weight added by residual fluid.

Typically, drain holes are between 2mm and 5mm in diameter, balancing easy resin flow with model aesthetics and structural integrity. Placement and quantity depend on the model’s geometry, orientation, and size.

Best practices for hollowing and adding drain holes

Hollowing your model

Most 3D sculpting or CAD software includes tools to hollow models by offsetting inner surfaces. Some resin slicers (like ChiTuBox or Lychee) also have integrated hollowing features.

• Target wall thickness: Usually 1.5–3mm depending on resin strength and printer layer height.
• Avoid sharp internal corners which trap resin; round angles inside cavities.
• Consider the entire build orientation. Orient your model to facilitate drainage and resin flow.

Drain hole placement

• Position drain holes at the lowest points of the hollow cavity relative to build orientation so resin can flow out easily.
• Use multiple drain holes if the model has multiple internal cavities or is very large.
• Include at least one vent hole near the top of the cavity to allow air to enter as resin drains.
• Keep holes small to maintain aesthetics but large enough to prevent clogging with cured resin or support debris.

Hole design considerations

• Chamfer or slightly flare the edges of drain holes to prevent sharp stress points.
• Design holes to be easily accessible for post-print cleaning tools like syringes or tweezers.
• Consider using removable plugs or resin-friendly caps to close holes after drainage if sanding and finishing demand.

Tools and software features to streamline hollowing and drainage

Modern software can simplify these tasks dramatically:

Software	Hollowing Features	Drain Hole Tools	Notes
ChiTuBox	Auto hollow with wall thickness control	Manual hole placement + hole size adjustment	Popular resin slicer, free version available
Lychee Slicer	Auto hollow, automatic hole creation option	Automatic and manual drain hole options	User-friendly UI for novices
Meshmixer	Manual hollowing via offset shells	Hole creation via brushes and primitives	More hands-on but very flexible
Blender	Manual hollow modeling with modifiers	Hole modeling with boolean operations	Steeper learning curve but highly customizable
PreForm (Formlabs)	Hollow function with hollow shell settings	Manual hole design, no auto drain hole	Specific to Formlabs printers

It’s best to experiment with your chosen software’s features and test prints from your usual brand of resin to optimize wall thickness and hole size/placement.

Post-print drainage and cleaning procedures

After printing, follow careful steps to remove remaining trapped resin:

1. Drain the resin: Invert your model over the resin vat or a container to drain out as much resin as possible through the drain holes.
2. Flush the cavities: Use a syringe or compressed air to push isopropyl alcohol through the holes to flush uncured resin out.
3. Vacuum or pressure chamber treatment: Vacuum chambers can help pull liquid resin from cavities; a pressure chamber can speed curing but requires drainage before.
4. Inspect under bright light: Shine a flashlight through the holes to ensure no liquid resin remains trapped.
5. Post-cure with holes uncovered: Allow UV light or heat to cure interior surfaces through the holes.
6. Seal drain holes: After curing, plug or fill holes with resin or putty for aesthetic finishing if desired.

Checklist: Hollow printing with drain holes

Step	Importance	Tip
Model hollowed with 2–3mm walls	Prevents excessive resin use	Avoid too thin to prevent breakage
Drain holes placed at lowest cavity points	Essential for resin drainage	Use multiple holes if complex shape
Vent holes included	Allows air in to facilitate flow	A small hole near highest cavity point
Holes sized 2–5mm	Balances drainage vs aesthetic	Test hole size on sample pieces
Post-print drainage with syringe/air	Removes trapped resin	Flushing with IPA recommended
UV cure with holes exposed	Ensures full resin cross-linking	Avoid sealing holes before curing
Final holes filled or capped	Improves visual finish	Use resin or filler putty

FAQ

How thick should walls be when hollowing resin prints?

For most standard resin printers (25–50µm layer height), walls of 1.5mm to 3mm thickness are ideal. Thinner than 1.5mm risks fragility and warping, while thicker walls reduce material savings.

Can I print hollow resin models without drain holes?

While possible, you’ll very likely trap uncured resin inside large cavities causing cloudiness and resin curing failures. Drain holes significantly improve print quality and ease post-processing.

Where should I place drain holes on a complex model?

Place drain holes at points where liquid resin naturally pools – typically the lowest points relative to build orientation. Add vent holes on top regions to allow air in during drainage.

What size should drain holes be?

Drain holes between 2mm and 5mm diameter optimize resin flow while staying discreet. Smaller than 2mm risk clogging, larger than 5mm may be visually obvious or cause strength issues.

Can I seal drain holes after curing?

Yes, after fully draining and post-curing, drain holes can be filled with resin or putty and sanded to create a smooth finish. Some people use small removable plugs or caps during curing for convenience.

---

With properly hollowed models and well-placed drain holes, large resin prints become more economical and reliable. Incorporate these careful design and post-processing steps to avoid trapped resin issues, improve your prints’ structural integrity, and reduce overall print time.