# What Does the SLA Resin Hollowing and Drainage Calculator Do?
This tool solves one of the most critical challenges in SLA, DLP, and LCD resin 3D printing: optimizing hollow models. Printing solid resin parts is expensive, heavy, and increases peel forces during the printing process. Hollowing the model reduces material usage, but introducing hollow cavities without proper drainage leads to trapped uncured resin and print failures due to vacuum forces. This calculator computes the ideal wall thickness, suggests the correct diameter and quantity of drain holes, and estimates the true resin savings by factoring in the liquid resin that inevitably remains trapped on the internal walls of the printed part. 1.5 mm Minimum recommended wall thickness for desktop SLA
2 holes Minimum number of vents required to break the vacuum
10-15% Resin volume reduction due to internal surface retention
30-70% Average total weight reduction achieved by hollowing
# How Wall Thickness Affects Resin Savings
Wall thickness is the primary variable that determines the amount of resin saved. A thicker wall increases structural strength but rapidly consumes more resin, reducing the efficiency of hollowing. Conversely, a wall that is too thin will be fragile, prone to warping during curing, and might fail to withstand the mechanical peel forces as the printer separates each layer from the FEP film. The goal is to find the sweet spot where the model retains its shape and utility while maximizing material savings.Pros and Cons of Hollowing Resin Prints
Advantages
- Massive reduction in material costs and overall print weight
- Lower surface area per layer reduces peel forces on FEP film
- Reduces thermal stress and warping during post-curing
Disadvantages
- Requires post-processing to wash and cure internal cavities
- Improperly placed holes can ruin the visual aesthetics of the model
- Risk of trapped liquid resin causing the part to crack later
# Understanding the Suction Cup Effect in Hollow Resin Prints
When a hollow model is printed, it forms an enclosed chamber as it dips into the vat. If this chamber lacks ventilation holes, each lifting cycle creates a strong partial vacuum (suction cup effect) between the cured layer and the release film. This force pulls on the print, leading to layer separation, layer lines, support failures, or even tearing the model completely off the build plate. Adding ventilation holes allows air to enter, neutralizing the pressure difference and ensuring smooth peel cycles.The Danger of Sealed Hollow Spaces
Never leave a hollow space completely sealed. Trapped liquid resin inside a printed part will slowly degrade the cured walls over time, eventually causing the model to crack, leak toxic resin, or completely disintegrate. Always include at least two holes to wash the interior and cure it with an internal UV light source.
# Drain Hole Placement Best Practices
- Place drain holes as close to the build plate as possible to allow the resin to escape early during printing.
- Always use at least two holes: one to let the liquid resin flow out and another to let air flow in.
- Orient the holes on non-visible surfaces, such as the bottom of bases or behind joints, to preserve aesthetics.
- Ensure that every isolated internal chamber or pocket has its own set of drainage holes.
# How the Calculator Adjusts for Geometric Complexity
Simple Geometry
Low detail models, spheres, or blocks. Traps minimal resin (approx. 5%) on flat interior surfaces.
Moderate Geometry
Character models or standard mechanical parts. Internal supports and curves retain moderate resin (approx. 10%).
High Complexity
Highly detailed miniatures, lattice structures, or complex hollow channels. Retains significant resin (approx. 15%+) due to capillary action.
# Understanding Resin Viscosity and Drain Hole Sizing
| Resin Class | Viscosity Level | Min Hole Diameter | Recommended Placement |
|---|---|---|---|
| Standard Resin | Low-Medium | 2.0 - 3.0 mm | Base or hidden flat surfaces |
| Tough / Flexible | Medium-High | 3.0 - 4.5 mm | Corners and joints to avoid peeling |
| Engineering / Castable | Very High | 4.5 - 6.0 mm | Direct line of sight for gravity drainage |
# When to Increase Wall Thickness Beyond 1.5 mm
Scale and Function dictate wall thickness
While 1.5 mm is a reliable minimum for small display figures, you should scale up the wall thickness for larger prints. For parts taller than 150mm, aim for 2.0mm to 2.5mm walls. For load-bearing mechanical components or parts printed with flexible/elastomeric resins, walls should be 3.0mm or thicker to prevent structural collapse under load.# Technical Glossary for Hollow SLA Printing
- SLA Hollowing
- The process of converting a solid 3D model into a hollow shell with a specific wall thickness to save resin and print time.
- Suction Cup Effect
- The vacuum force created when an enclosed hollow cavity is pulled away from the release film during printing.
- Peel Force
- The mechanical tension experienced by the model and supports as the cured layer is separated from the vat bottom.
- Resin Trapping
- The retention of liquid uncured resin inside internal corners, supports, and textures due to surface tension.
# Summary Checklist for Successful Hollow Prints
Preflight SLA Checklist
Verify hollowing thickness is appropriate for the model scale.
Confirm at least two drain holes are placed at the lowest printing points.
Check for isolated internal voids that lack ventilation.
Plan for internal washing with IPA and internal UV LED curing.