Preparing Models for Printing#

Overview#

A model that is perfect for rendering or drawings can still be unprintable. Preparation is the discipline of checking the model against the physical limits of the process before you spend hours of machine time. The four topics that matter most are units, orientation, wall thickness and tolerances, followed by a general manifold and geometry check. This page treats them in order and finishes with a pre-flight checklist.

The numbers below are typical for desktop FDM with a 0.4 mm nozzle unless stated. Resin, powder-bed and metal processes have their own minimums, covered on their process pages. Always confirm with a test print on your own machine, because nozzle size, material and calibration move every number by 10-50 percent.

Where to find it#

Step by step#

1. Units#

1. Open Browser > Document Settings and check Units. For almost all printing, choose millimeters.
2. If a design was started in inches, you can change the document unit; existing geometry keeps its true size and only the display changes. This is different from scaling.
3. When you export, check the unit option in the export dialog if it offers one (Save As Mesh and 3D Print both offer a Unit Type; STL itself is unitless). If the slicer shows a part at 1/25.4 or 25.4 times the expected size, the unit assumption at one end is wrong.
4. Prefer 3MF where possible. It stores units explicitly, so the slicer does not have to guess.
5. Never "fix" a unit error with the Scale command inside the design unless you really intend to change the part. Fix the export setting instead.

2. Orientation#

Orientation decides strength, surface finish, support volume and print time, so decide it on purpose.

1. Identify the largest flat face that can go on the bed. Flat bottom faces print well and stick well.
2. Identify the load direction. FDM parts are weakest between layers (Z direction). A hook loaded in tension should have its layers running along the load, not stacked across it.
3. Identify critical cylindrical features. Holes and pins printed with their axis vertical come out round; printed horizontally they become slightly oval or flattened on the top (roof) and need support.
4. Identify overhangs greater than about 45 degrees from vertical (FDM). Rotate to reduce them, or add chamfers to the design to make them self-supporting.
5. Identify cosmetic faces. Faces touching supports or the bed look worse. Put the less visible side down.
6. Decide the orientation in the slicer or in the 3D Print preview rather than rotating the Fusion body, unless you want the orientation to be part of the design record. If you do rotate the body, do it with Move/Copy and a timeline-recorded rotation, so you can roll back.

3. Wall thickness#

1. Measure walls with Inspect > Measure or with Section Analysis. Look at the thinnest wall, not the average.
2. For FDM with a 0.4 mm nozzle, aim for walls that are a multiple of the extrusion width. Typical extrusion width is 0.4-0.5 mm, so 0.8, 1.2, 1.6 mm are clean. A 1.0 mm wall might produce a single bead plus a gap-fill artifact.
3. Minimum recommended structural wall: 1.2 mm (three perimeters) for FDM; 0.8 mm is the practical floor for light-duty cosmetic walls.
4. Ribs and fins: thickness about half to equal to the wall they reinforce, height up to about three times the thickness. Add a fillet at the root.
5. If you use Shell (Modify > Shell) to hollow a part, remember that a hollow part must usually still contain drain holes for resin and powder processes. See the SLA and SLS pages.

4. Tolerances#

1. Decide which dimensions are fit-critical (shafts, holes, snap-fits, mating faces) and which are not.
2. Create user parameters for clearances, for example clr_slip = 0.2 mm, clr_press = 0.05 mm, clr_loose = 0.4 mm.
3. Use those parameters in Extrude distances, hole diameters and offsets, so that you can change them in one place after a test print.
4. Print a tolerance coupon (a small set of holes and pegs at different clearances) before the real part. See the print-in-place and tolerance page.

5. Geometry health check#

1. Each printed object should be a closed, manifold solid. Surface bodies that do not close must be stitched or thickened first (see the surface-modeling section).
2. Avoid zero-thickness faces, coincident bodies and overlapping solids. Combine overlapping bodies with Modify > Combine (Join) so the slicer sees one volume.
3. Remove tiny features below the process resolution (details smaller than 0.4 mm are ignored or distorted on FDM).
4. Check for very sharp points or knife edges; they print as stubs.

Options reference#

Tips and pitfalls#

Tip: Chamfer the bottom edges of parts with a 45-degree chamfer of about 0.5 mm. It hides first-layer squish ("elephant foot") and makes the part easier to remove from the bed.

Tip: Replace horizontal round holes with teardrop or diamond shapes so they no longer need support. Draw a teardrop in the sketch with the point up, and extrude-cut it.

Warning: Holes print smaller than designed, and external dimensions print slightly larger on many machines. If you compensate, do it through a named parameter rather than editing every sketch dimension by hand.

Warning: Do not trust a thin wall just because Fusion draws it. If Section Analysis shows a wall thinner than your extrusion width, the slicer will either skip it or fill it with a weak single line.

Tip: Use Modify > Fillet generously on internal corners. Printed parts crack at sharp internal corners because stress concentrates at the layer boundary.

Warning: Do not scale an entire body to compensate for shrinkage unless the material is known to shrink uniformly (ABS and nylon do shrink; PLA barely). Scaling changes holes and pins in the wrong direction relative to each other. Apply compensation to the critical dimensions only.

Practice exercise#

Design a small project box lid that has to slide onto a 40 x 30 mm box with walls of 2 mm. Create parameters wall = 2 mm, clr_slip = 0.25 mm, and box_w = 40 mm. Build the lid so its internal width is box_w + 2 * clr_slip. Add a 0.5 mm chamfer to the bottom edge. Run Section Analysis through the lid and confirm the thinnest wall is at least 1.2 mm. Then orient the lid with the opening facing up in your slicer, print it, and measure the fit. Expected result: the lid slides on with light friction. If it is too tight or loose, change only clr_slip and reprint.

Related#

• 3D Print Workflow Overview
• Print-in-Place and Tolerance Design
• Exporting STL and 3MF
• Shell, draft and scale (03-solid-modeling)
• Units and document settings (01-getting-started)

Deeper notes: why these numbers exist#

It helps to understand where the rules of thumb come from, because then you can adjust them when your printer, nozzle or material differs.

Extrusion width and walls. An FDM printer lays down a bead of molten plastic whose width is roughly the nozzle diameter, often slightly wider (about 110-125 percent). A wall is built from whole beads. If the wall thickness is not close to a multiple of the bead width, the slicer must either squeeze in a narrower bead (gap fill), leave a void, or round the wall up or down. Rounded walls are why a nominal 1.0 mm wall can come out as 0.8 mm or 1.2 mm depending on slicer settings. If you change to a 0.6 mm nozzle, your clean wall steps become 0.6, 1.2, 1.8 mm; parameterizing the wall as wall = 3 * nozzle_w is a neat way to keep the model honest.

Why holes shrink. Molten plastic that forms the inner perimeter of a hole is pulled inward by cooling and is also pushed inward by the bead's round cross-section and by slicer path compensation. Small holes lose a larger fraction of their diameter. Use a drill or reamer on critical holes, or model them 0.1-0.3 mm oversize and measure.

Why layer orientation matters for strength. Each layer bonds to the one below by partial melting. That bond is weaker than the continuous filament along the layer. Typical tensile strength in the Z direction can be 30-70 percent of in-plane strength. Therefore, avoid features whose main load path crosses layers, such as thin vertical pegs bearing side loads. Make them wider, add fillets, or orient them to lie flat.

Thermal effects. Large flat footprints in high-shrink materials curl at the corners. Round the corners of the footprint (a 3-5 mm radius on vertical corners) and avoid very long thin footprints. Chamfers and fillets on vertical edges are cheap in Fusion and reduce peeling forces.

Quick pre-flight checklist#

Run through this list before every non-trivial print:

1. Units are millimeters, and the exported size matches Inspect > Measure.
2. Each body is a closed solid; surface bodies have been stitched and thickened.
3. Overlapping bodies have been combined, or intentionally kept as separate objects.
4. Thinnest wall is at least the target for your process and nozzle.
5. Critical clearances are driven by named parameters.
6. Bottom edges have a small chamfer, and vertical corners have a radius where practical.
7. Overhangs over about 45 degrees are chamfered, re-oriented or planned for support.
8. The part fits inside the build volume in the chosen orientation, with margin for brim and skirt.
9. Fasteners have a plan: heat-set inserts need a hole slightly smaller than the insert, and screws need clearance or thread-forming holes depending on the design.
10. The file format and refinement are suited to the part (3MF, medium or high refinement).