Basic Problems With 3D Printing Slicing Software
Most failed prints trace back to the slicer, not the machine. This guide covers the basic problems 3D printing slicing software throws at engineers: broken meshes, wall gaps, over-extrusion and feature loss. It is written for design and manufacturing engineers who need to decide whether a part should be printed at all or machined from metal.
What the slicer actually does
A surface model goes in, machine commands come out. Every error downstream starts here.
Where slicing goes wrong before the first layer
A slicer reads an STL, 3MF or OBJ file and converts the surface geometry into toolpaths, layer by layer. It does not see a solid part. It sees triangles and a set of rules about how thick each wall should be. That gap between the CAD model and the triangle mesh is where most basic problems in 3D printing begin.
Engineers usually blame the printer when a part comes out weak or undersized. In our experience the root cause sits earlier: a mesh with flipped normals, a wall thinner than two extrusion widths, or a hole smaller than the nozzle can resolve. None of these show up as obvious errors in the preview.
The fix is not always a slicer setting. Sometimes the part should not be printed at all. A bracket that needs ±0.005 mm on a bore, or a thread that has to hold torque, belongs on a CNC machine. Knowing which side of that line a part falls on saves weeks of iteration.
- 1Check the mesh firstNon-manifold edges and flipped normals cause gaps the slicer cannot fill.
- 2Minimum wallTwo extrusion widths is the practical floor for a load-bearing wall.
- 3Feature sizeHoles and slots below roughly 0.8 mm often close up after printing.
- 4Tolerance budgetFDM holds about ±0.3 mm; metal CNC holds ±0.005 mm.
Mesh errors that stall the slice
An STL stores geometry as a shell of triangles. If two triangles share an edge incorrectly, or a vertex sits in the wrong place, the file is described as non-manifold. Slicers handle this differently. Some repair silently. Others refuse the file or produce a preview full of holes.
Open edges are the most common defect. They appear when a CAD export drops a face or when a model is mirrored without stitching. A slicer fills the gap with an approximation, which usually shows as a thin web or a missing wall on the finished part. Repair tools inside the slicer can close small gaps, but a badly broken shell is better fixed in CAD.
Inverted normals are quieter. The surface looks closed, yet the slicer treats inside as outside and reverses the fill direction. The result is a part with the correct outline and no internal structure. Most slicers flag this only in the layer view, so scroll through the layers before you commit to a long print.
- 1Non-manifold edgesMore than two triangles meet on one edge; the shell is not watertight.
- 2Open edgesA face is missing; fill material leaks through the gap.
- 3Inverted normalsInside and outside are swapped; walls print with no infill.
- 4Self-intersectionTwo surfaces pass through each other; the slicer picks one at random.
Slicing symptoms and the usual cause
Match the symptom in the preview or the finished part to the likely root cause before changing settings.
| Symptom | Likely cause | First check |
|---|---|---|
| Missing wall in preview | Open edge or hole in mesh | Run a mesh repair, inspect layer view |
| Solid block, no infill | Inverted normals | Check face orientation in CAD |
| Weak thin walls | Wall under two extrusion widths | Thicken wall in CAD to 1.2 mm or more |
| Holes print undersized | Nozzle cannot resolve small diameter | Drill or ream after printing, or machine the part |
| Stringing between features | Travel moves ooze molten plastic | Increase retraction, lower nozzle temperature |
| Warped base on long part | Thermal shrinkage over large footprint | Add brim, enclose printer, split the part |
| Bore out of tolerance | FDM cannot hold tight bore size | Move to CNC machining for the bore |
Wall thickness, infill and the trade-offs behind them
Wall thickness controls strength more than infill does. A part with three 0.4 mm perimeters and 15 percent infill is stiffer than the same part with one perimeter and 50 percent infill. The reason is simple: the outer skin carries bending load, while infill mostly holds the skin apart. If a printed bracket flexes, add walls before you add infill.
Layer height trades time against surface finish. A 0.1 mm layer gives a smoother side wall and roughly double the print time of 0.2 mm. FDM surfaces land around Ra 3–10 μm as printed, which is far from the Ra 0.8–1.6 μm we hold on machined faces. If a part needs a sealing face or a sliding fit, printing alone will not get there.
Supports are the other hidden cost. Overhangs beyond about 45 degrees need support material, and that material leaves witness marks on the surface. You can design around this by adding chamfers, reorienting the part, or splitting it into two printed pieces that bolt together. Each option changes the part number, so decide early.
- 1Perimeters before infillThree walls beat 50 percent infill for bending stiffness.
- 245-degree ruleOverhangs past this angle need support and leave marks.
- 3Layer height0.1 mm doubles print time for a smoother side wall.
When printing is the wrong answer
Printing shines for fit checks, jigs, enclosures and low-load covers. It struggles when a feature has to hold a tolerance, seal a fluid, or survive repeated load. A printed bore rarely lands inside ±0.1 mm. Bores that must accept a bearing, a dowel pin or a threaded insert belong on a mill or a lathe.
Material behavior decides a lot of this. Printed ABS, PC and PA parts are anisotropic: they are weaker along the layer direction. A bracket loaded across the layers can delaminate well below the bulk strength of the same plastic. Machined 6061-T6 or 17-4PH has no layer direction and holds its properties in all axes.
The practical split we use at GreatLight: print the prototype for form and fit, then machine the production part from aluminium, stainless or titanium. A 5-axis machine holds ±0.005 mm and a fine surface of Ra 0.2–0.8 μm, so the transition does not force a redesign. We quote and return a DFM analysis within 12 hours.
- 1Print forFit checks, jigs, enclosures, low-load covers, fast iteration.
- 2Machine forBores, threads, sealing faces, load paths, tight tolerances.
- 3Materials on our side6061-T6, 17-4PH, Ti-6Al-4V, Inconel, PEEK and more.
Common questions from engineers
Why does my sliced preview show holes that are not in the CAD file?
The mesh is not watertight. Open edges or non-manifold geometry force the slicer to guess, and it fills the gap with an approximation that often shows as a hole or a thin web.
Run a mesh repair in the slicer for small gaps. For a badly broken shell, go back to CAD, stitch the surface, and export again as 3MF or STL.
What is the smallest hole a slicer can print reliably?
As a rule, keep hole diameter at least 1.5 times the nozzle diameter. With a 0.4 mm nozzle, that means holes below about 0.8 mm will close up or print undersized.
If a hole has to be accurate, print it undersized and ream it, or move the part to CNC machining where a bore can hold ±0.005 mm.
Why is my part weak even though I set 50 percent infill?
Infill is not the main load path. The outer perimeters carry bending and tension. One thin wall will fail before dense infill helps.
Increase perimeter count to three or more, and thicken the wall in CAD to at least 1.2 mm. Printed plastics are also weaker across layers, so orient the part so the main load runs along the layers, not through them.
Can I hold ±0.005 mm on a printed part?
No. FDM holds roughly ±0.3 mm on a good day, and resin printing is better on small features but still not in that range. ±0.005 mm is a machining tolerance.
If a drawing specifies ±0.005 mm or ±0.0002 in, the feature should be cut on a CNC mill or lathe, not printed.
Should I split a large part to avoid warping?
Splitting helps when the footprint is large and the material shrinks. Long flat parts curl at the corners as they cool.
You can also add a brim, enclose the printer, or change to a lower-shrink material. If the part has a sealing face or a tight bore, machine it from aluminium instead of splitting it.
Do I need to redo the CAD model to switch from printing to CNC?
Usually not. The same solid model can be printed or machined. What changes is the tolerance callout and the surface finish spec.
Send us the STEP file with your critical dimensions marked. We return a DFM analysis and a quote within 12 hours, and production can start within 24 hours.
Send us the part that printing cannot hold
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