STL Viewer BlogFrom the outside, the enclosure looks perfect. Then it prints with a wall so thin you can see light through it, or a cable channel that turns out to be blocked halfway, or a "solid" part that is actually hollow with a 0.6 mm skin. None of that is visible when you orbit around a shaded model. An STL cross section slices the model with a plane so you can see what is really inside, the same way a slicer will.
Short answer: Cut the model along X, Y or Z and look at the cut face to check wall thickness, hollow interiors, internal channels and hidden gaps. On a phone, the free x-ray view gives a quick look through the surfaces; STL Viewer's Premium Section tool makes a true cut with a solid face and shows its width, height and area.
Why look inside a model at all?
- Wall thickness on enclosures, vases, cups and helmets, where a thin spot means a weak or leaky print.
- Hollow models: figurines are often hollowed to save resin or filament, and you want to know the shell thickness and whether it has drain holes.
- Internal channels: cable routes, air ducts, water paths and screw bosses buried inside a part.
- Hidden defects: overlapping shells, internal faces left from a merge, or a gap that makes the mesh open.
- Overhangs inside: the ceiling of an internal cavity is an overhang the slicer must support or bridge.
Choosing the axis: X, Y or Z
A cut across an axis shows you the plane perpendicular to it. Turn on the XYZ axes indicator to keep your bearings.
| Cut across | You see | Best for |
|---|---|---|
| Z (horizontal) | One layer, as the printer will lay it down | Wall thickness all around, infill area, what each layer looks like |
| X | A side profile from left to right | Floor and roof thickness, vertical channels, overhang angles |
| Y | A side profile from front to back | Same as X, in the other direction; good for long parts |
A cut across Z is especially useful because it matches how the slicer thinks: each Z cut is essentially one layer before infill.
The free option: x-ray view
X-ray makes surfaces semi-transparent. Inner walls, cavities and embedded parts show through as darker overlapping layers. It's free and instant, and it answers yes/no questions well: is this figurine hollow? Is there a channel through this block? Are there loose internal pieces? It doesn't give numbers, and dense models can get visually busy. When you need a measurement, use a real cut.
The Section tool, step by step
Section is part of Premium. Here's how it works in STL Viewer:
- Open the model and tap Section in the tools.
- Pick the axis to cut across: X, Y or Z.
- Drag the section position slider to move the cutting plane through the model.
- The cut face is drawn solid, so walls stand out as filled bands and cavities as empty areas.
- Read the section's width, height and area for the current cut.
- Tap Show other side to see the other half.
If the plane misses the model, the app says it doesn't cut the model there. On a very big model shown as a simplified view, the cut notes that it's approximate. The section guide shows each step.
Worked example: a vase that leaks
A downloaded vase is 90 mm wide and 160 mm tall. You'll print it in PETG for real water.
- Cut across Z at 20 mm height. The section shows a ring: outer width 82.0 mm. The cut area is 7.4 cm². For a ring of outer diameter 82 mm, that area means a wall about 3 mm thick. Good.
- Move to 140 mm height, where the neck narrows. The ring now reads 48.0 mm wide with an area of 1.2 cm²: a wall of only about 0.8 mm. With a 0.4 mm nozzle that's two lines, likely to weep water.
- Cut across X through the middle to see the profile: the thinning starts around 120 mm.
Now you know before printing: either print with more walls in the slicer, or ask for a thicker version. You can estimate wall thickness from area like this: for a thin ring, thickness ≈ area / (π x diameter). 1.2 cm² = 120 mm²; 120 / (3.14 x 48) ≈ 0.8 mm.
When the cut reveals an open mesh
A closed model cut by a plane gives closed outlines. If the mesh has a hole at that height, the outline doesn't close, and the app tells you: "Open outline: the model isn't watertight here." That pinpoints where the hole is, which is useful when the info panel says the model isn't watertight but you can't see why. More in what watertight means.
Rule-of-thumb wall thicknesses
| Use | Suggested minimum wall (0.4 mm nozzle) |
|---|---|
| Decorative, no load | 0.8 mm (2 lines) |
| General parts | 1.2-1.6 mm (3-4 lines) |
| Watertight containers | 1.6 mm or more, plus enough walls in the slicer |
| Screw bosses, load-bearing tabs | 2 mm or more around the hole |
These are common starting points, not laws; material and print settings matter.
Putting it in your routine
Cross-sections are point 4 of the 8-point pre-print checklist. For hole spacing and exact distances, combine sections with Measure, described in scaling and measuring an STL. Scanned models in particular benefit from a look inside; see viewing 3D scans.
Note: A section shows the model's geometry, not the slicer's toolpaths. Your slicer's layer preview remains the final word on walls and infill.
You can try x-ray for free after you install STL Viewer from Google Play; Section comes with Premium, as a one-time purchase or a yearly subscription.
Feature details, including the free view modes, are on the STL Viewer website.
Frequently asked questions
How can I see inside an STL file?
Use a cross-section tool to cut the model with a plane, or an x-ray view that makes surfaces see-through. Slicers' layer previews also show the inside.
How do I check the wall thickness of a 3D model?
Cut it across Z at several heights and look at the ring of the cut face. For a thin ring, thickness is roughly the area divided by pi times the diameter.
What is a good minimum wall thickness for FDM printing?
With a 0.4 mm nozzle, 0.8 mm is a bare minimum; 1.2 to 1.6 mm is safer for general parts and containers.
Is the cross-section tool in STL Viewer free?
Section is a Premium tool. The x-ray view is free and lets you see internal cavities without cutting.