Preparing Files for 3D Printing: STL, STEP, and Design Guidelines

The quality of a 3D print is fundamentally limited by the quality of the file it is printed from. Mesh errors, insufficient wall thickness, and incorrect tolerances produce prints that fail during printing or fail to function after. A few checks before sending files save significant time.

File Format

STL is the universal format for 3D printing and is accepted by all slicers. It represents geometry as a mesh of triangles. The mesh resolution (number of triangles) determines how well curves are approximated — export at high resolution for parts where surface smoothness matters.

STEP (STP) is a CAD exchange format that preserves exact geometry rather than a mesh approximation. Some slicers (Bambu Studio, PrusaSlicer) can import STEP files directly and generate their own mesh internally at whatever resolution is needed. STEP is preferable when available.

OBJ is accepted by most slicers. Functionally similar to STL for 3D printing purposes.

Non-Manifold Geometry

A manifold (or watertight) mesh has no holes, no zero-thickness surfaces, and no self-intersections. Non-manifold errors confuse slicers — they may not know which side of a surface is inside the part and which is outside, leading to missing sections, incorrect infill, or complete slice failure.

Check for non-manifold geometry in your CAD tool before exporting. Meshmixer, Netfabb (free online version), and PrusaSlicer all include mesh repair tools that identify and fix common errors.

Wall Thickness Guidelines

Walls below minimum will either be skipped entirely by the slicer (less than 1 perimeter width) or produce structurally weak features. Design with the nozzle diameter in mind — walls that are exact multiples of the nozzle diameter (0.4, 0.8, 1.2, 1.6mm) produce cleaner, stronger perimeters.

Tolerances for Mating Parts

FDM printing produces parts slightly smaller than modelled on flat surfaces and slightly smaller on holes due to thermal contraction and the way the nozzle deposits material inside a circular feature.

Clearance for press fit (interference): 0mm (or slight negative) on shaft/hole diameter

Clearance for sliding fit: 0.2mm on shaft/hole diameter (0.1mm per side)

Clearance for free sliding: 0.3–0.4mm on shaft/hole diameter

Thread fits: Standard thread fits generally work for M6 and above. Below M5, design clearance slightly larger.

Designing for Support Reduction

Supports add print time, use material, and always leave surface marks where they contact the part. Design to minimise them where possible:

• Overhangs below 45° typically print well without support
• Self-supporting angles can be extended to 50–55° on most printers with good cooling
• Holes perpendicular to print orientation benefit from a slight teardrop profile at the top
• Chamfer inside corners at the top of holes and pockets to reduce bridging distance
• Orientate the part so the most critical surfaces face up (better finish) and the least visible surfaces face down where supports would be needed

Sending Files for Printing

When sending files to P3DA (or any service bureau), include:

• The model file (STL or STEP)
• Intended material
• Any tolerance-critical dimensions and their required accuracy
• Quantity needed
• Any post-processing requirements (sanding, painting, hardware installation)