CAD, Slicing and the Path to a Print

Three stages, three jobs

Stage	Software	What you decide here
Model	CAD, sculpting or modeling software, or a downloaded file	Shape, dimensions, wall thickness, holes, fit between parts
Slice	A slicer for your printer (PrusaSlicer, OrcaSlicer, Cura, PreForm and others)	Orientation, layer height, walls, infill, supports, material profile
Print	The printer's firmware	Nothing new: it runs the instructions, with live adjustments such as speed or first-layer height

Most confusion comes from trying to fix a problem in the wrong stage. A hole that is the wrong size by design is a model problem. A hole that is the wrong size because of how plastic squeezes out of a nozzle is a slicer and calibration problem (see Resolution, Accuracy and Precision).

Stage 1: the model

You do not need CAD to start printing. Plenty of people print downloaded models for months. You need CAD, or another modeling program, when you want to make or change a design.

• Engineering CAD builds exact parts from sketches and dimensions. It is the right tool for brackets, enclosures, replacement parts and anything that must fit. It can export STEP for exact geometry, or a mesh such as STL or 3MF.
• Mesh and sculpting tools work directly with triangles. They suit figures, organic shapes and artwork, and export STL, OBJ or 3MF.

Whichever you use, keep the source file. A mesh export is a snapshot; changing a dimension later is easy in the original and painful in the exported triangles. The formats themselves are compared in STL, 3MF, OBJ, STEP and G-code.

When a slicer imports CAD geometry, it turns curved surfaces into triangles. Some slicers expose that conversion: Formlabs' PreForm lets you set a chordal tolerance (the largest gap allowed between the triangles and the true surface) and an angle tolerance for CAD imports. A looser tolerance allows a larger gap between the triangles and the true curve, so curved parts benefit most from a tight one.

Stage 2: the slicer

The slicer cuts the model into layers and works out every move the printer must make. Prusa's own walk-through for PrusaSlicer follows the order most slicers share:

1. Import the model. Drag it into the window or use the import command. PrusaSlicer accepts STL, STEP, 3MF, OBJ and AMF.
2. Orient and place it. Rotate the part, or use a place-on-face tool to pick which face sits on the bed. Orientation changes strength, supports and surface finish; see Print Orientation.
3. Pick the printer. The printer profile holds the bed size, nozzle size and machine limits. Set up during the slicer's first-run wizard, it is usually already selected.
4. Pick the material. Prusa notes that different materials need different temperatures, speeds and cooling, and that the slicer's material presets carry those values.
5. Pick a print profile. This sets layer height and the general quality versus time trade-off. See Choosing a Layer Height.
6. Decide supports, infill and brim. Supports hold up parts that would print in mid-air (Supports and Overhangs). Infill fills the inside. A brim adds extra first-layer outlines for adhesion.
7. Slice and preview. Step through the layers with the preview slider. Prusa's guidance is to check that the shape matches the model and that nothing prints in mid-air.
8. Export. Save the print file to a USB drive or card, or send it over the network if your printer supports that.

Leave the maker's profiles alone for your first prints. They are tuned starting points Starting point for that printer, and changing many values at once makes it impossible to tell which one helped or hurt.

Stage 3: the printer

The print file for FDM is G-code. Marlin's documentation describes G-code as "the programming language used to control 3D printers", and Marlin interprets commands sent from a host program, an SD card or a USB drive. Most of a G-code file is movement: Marlin's linear-move commands give a target position on each axis, an extruder position (how much filament to push), and a feed rate. Marlin notes that slicers tend to override the firmware's own feed rates.

Because the file holds machine moves for one setup, it cannot be meaningfully edited back into a model or reused on a different printer. If something needs to change, change the model or the slicer settings and slice again.

Where to fix what

Problem	Fix it in
Wrong overall size, wrong design dimension, part too thin	The model (or scale it in the slicer if the whole thing is uniformly off)
Needs supports, wrong orientation, too slow, layer lines too visible	The slicer
First layer too squished or not sticking	Printer calibration first; see First Layer Fundamentals
Stringing, gaps, blobs	Slicer material settings after calibration tests

Resin printing follows the same three stages with a resin slicer in the middle; the slicer writes the printer maker's own job file instead of G-code.