What 3D Printing Is

The short version

A 3D printer makes a physical object from a digital file by building it up in thin, flat layers, one on top of the next. Each layer is a slice of the model. Stack enough slices and you have the whole part. Formlabs defines it as creating physical objects by building them up layer by layer from digital designs.

The file starts as a 3D model, drawn in CAD software or downloaded. A program called a slicer cuts that model into layers and writes the instructions the printer follows. See CAD, Slicing and the Path to a Print for that chain, and STL, 3MF, OBJ, STEP and G-code for the files involved.

Additive, subtractive and formative

There are three broad ways to make a shape out of a material:

Approach	How it works	Examples
Additive	Adds material layer by layer, only where the part needs it.	FDM, resin (SLA, DLP, MSLA), powder (SLS)
Subtractive	Starts from a solid block and removes material by cutting, drilling or grinding.	CNC milling, CNC routing, turning
Formative	Forces material into a mold.	Injection molding

A CNC router carving a sign and a 3D printer building a figure are opposite processes: one removes wood until the shape is left, the other adds plastic until the shape exists. That difference drives most of what each one is good at.

FDM in one paragraph

FDM (fused deposition modeling, also called FFF) feeds a plastic filament, such as PLA or ABS, into a heated nozzle and lays the melted plastic down as thin lines that trace each layer. Motors move the nozzle across the layer in X and Y, then step up in Z (or lower the bed) for the next one. Formlabs notes that FDM printers often trade part quality for simplicity and affordability, and that FDM parts tend to show visible layer lines and can be inaccurate around complex features, and that the layers do not bond to each other as strongly as the lines within a layer. See Parts of an FDM Printer.

Resin in one paragraph

Resin printing (SLA, DLP and the LCD-masked MSLA machines most hobbyists own) uses a light source to cure liquid photopolymer resin into solid plastic, one thin layer at a time, usually building the part upside down from a platform that lifts out of a vat. Resin gives much finer detail and smoother surfaces than FDM, and Formlabs describes resin parts as highly isotropic, meaning about equally strong in every direction. The cost is a wet workflow: parts are washed to remove uncured resin and then post-cured, and uncured resin is a skin and eye hazard. Formlabs' Grey Resin SDS, for example, lists serious eye irritation and allergic skin reactions and calls for gloves, eye protection and adequate ventilation; follow the SDS for the resin you use. See Parts of a Resin Printer and Resin Protective Equipment.

For a full comparison and how to choose between them, read FDM versus Resin Printing.

What 3D printing is good at

• Complex shapes. Formlabs says 3D printing excels at complex geometries and rapid iteration. Organic curves and interior shapes that a cutter could not reach are built the same way as a plain box.
• One-offs and small batches. There is no mold and no fixture to make. Changing the design means changing the file, so a custom part costs the same effort as a standard one.
• Fast prototypes. A part can go from a CAD model to your hand in hours, so you can test a fit, change the design and print again the same day.
• Small, intricate parts. Formlabs notes additive methods are typically better suited to small parts and highly complex or intricate designs.

What it is bad at

• Surface finish. Machining excels at smooth surfaces, tight tolerances and strong solid materials, per Formlabs. An FDM part shows its layers; sanding and filling take real time.
• Strength across the layers. On FDM, the bond between layers is the weak link. Prusa compares FFF prints to wood: stronger in one direction than another. Which way you stand a part on the bed changes how strong it is. See The Three Axes and Layer-by-Layer Printing and Print Orientation.
• Printing in mid-air. Each layer has to sit on the one below it. Steep overhangs and bridges need help from supports, which leave marks. See Supports and Overhangs.
• Speed at volume. A print takes the same time whether it is the first copy or the hundredth. Formlabs notes that subtractive processes become more competitive as batch sizes grow.
• Size. The part has to fit the printer's build volume, or be split into pieces and joined.

What a print actually takes

1. A model. Download one or draw your own. It has to be a closed, solid shape for the slicer to fill it.
2. Slicing. Choose the printer and material profile, set layer height, walls, infill and supports, and export the print file. Layer height is the first big choice; see Choosing a Layer Height.
3. Printing. The printer works through the layers, often for hours. Watch the first layer go down.
4. Finishing. Remove supports. On resin, wash and post-cure. Sand, prime or paint if the part needs it.

If you have an FDM printer in front of you, the next step is Your First FDM Print.