What 3D printing actually is

Before you buy anything, spend a few minutes on the idea itself. Once the core picture clicks, everything else in this book (choosing a machine, picking plastic, slicing, designing a lemon squeezer) becomes a set of details hanging off one simple concept. So let us start with that concept and build up slowly. No background is assumed. Every term gets explained the first time it appears.

The core idea: adding, not removing

Most ways of making a physical object fall into one of three families.

• Subtractive manufacturing starts with a solid block and removes material until the shape is left behind. Carving wood with a chisel, drilling a hole, milling metal on a machine: you begin with too much and cut away the rest. The leftover material becomes waste (sawdust, metal shavings).
• Molding (also called formative) pours or presses a soft or liquid material into a mold and lets it harden into the mold's shape. Injection molding makes most of the plastic objects around you, from bottle caps to LEGO bricks. It is fast and cheap per item, but you first need an expensive metal mold, so it only pays off across thousands of copies.
• Additive manufacturing, which is what 3D printing is, builds the object up from nothing by adding material a little at a time until the shape exists. No block to carve, no mold to pay for.

That last family is the one that makes a 3D printer special. Because you add material only where the object needs it, you can make shapes that would be awkward or impossible to carve or mold, and you can make exactly one of something without any setup cost beyond the plastic itself. For a hobbyist this is forgiving in a very practical way. If your design is wrong, you change a number on your computer and print again. You are not throwing away a carved block or a machined mold, just a bit of plastic and some time.

Don't be confused. "3D printing" and "additive manufacturing" mean the same thing. "Additive manufacturing" is the term used in factories and engineering papers. "3D printing" is the everyday name for it. When someone says one or the other, they are pointing at the same idea: building an object by adding material in layers.

FDM: the kind of printer this book uses

There are several ways to add material in layers. This book focuses on one of them, because it is cheap, reliable, and good at exactly the kind of object we want to make. It is called FDM, short for Fused Deposition Modeling. You will also see it called FFF, for Fused Filament Fabrication. The two names mean the same process. (FDM was a trademark, so other makers coined FFF to describe the identical thing without using the trademarked term.)

Here is what physically happens, step by step.

1. The raw material is a long, thin plastic thread wound onto a spool, like fishing line but thicker (commonly 1.75 mm across). This thread is called filament.
2. A motor grabs the filament and pushes it forward, feeding it into a metal part that is heated to roughly 200 degrees Celsius. That heated part is the hotend, and the tiny hole the plastic comes out of is the nozzle (typically 0.4 mm wide).
3. At that temperature the plastic softens into a thick paste, about the consistency of warm glue. It is squeezed out through the nozzle in a fine strand.
4. The nozzle moves across a flat surface called the build plate or bed, laying down that strand to draw a thin, flat 2D shape, like piping a single outline of icing onto a cake.
5. When that flat layer is finished, the printer moves the nozzle up (or the bed down) by one small step, and draws the next flat shape directly on top of the first. The fresh hot plastic fuses to the slightly cooler plastic just below it.
6. Repeat. The object grows upward from the bottom, one layer stacked on the last, until the full shape exists.

That is the whole trick. A 3D object is just a tall stack of 2D layers, and the printer makes it by drawing one layer, stepping up, and drawing the next.

Moving in three directions: X, Y, and Z

To draw layers and stack them, the machine needs to move in three directions. These are named with letters borrowed from math.

• X is left and right.
• Y is forward and back.
• Z is up and down.

X and Y together cover the flat plane of the bed. Any single layer is drawn by moving in X and Y. Z is height. Every time a layer finishes, the printer makes one small Z move to begin the next layer higher up.

The size of that Z step is the layer height, and it is one of the most important numbers you will meet. A common value is 0.2 mm, meaning each layer is two tenths of a millimeter thick. Thinner layers (say 0.12 mm) give a smoother surface because the steps between layers are smaller, but the print takes longer because there are more layers to draw. Thicker layers (say 0.28 mm) print faster but show coarser ridges on the surface. You can feel these layer lines by running a fingernail up the side of almost any printed part. They are the signature of FDM.

A simple shape, built layer by layer from the bottom up:

  layer 6   ####          <- nozzle drew this last, on top
  layer 5   #  #
  layer 4   #  #
  layer 3   #  #
  layer 2   #  #
  layer 1   ######        <- nozzle drew this first, on the bed
            ============   the build plate (bed)

Each row is one pass of the nozzle. Stack enough rows and you
have a hollow cup. The Z axis is "which row," X and Y are the
shape of each row.

From an idea to a solid object: the pipeline

A printer by itself does not know what you want. There is a short chain of steps that turns a thought into a physical thing, and it is the same chain every time.

  an idea
     |
     v
  a 3D MODEL        (you design it, or download one)
     |
     v
  a SLICER          (software that chops the model into layers
     |               and writes the machine's instructions)
     v
  G-CODE            (a plain list of moves: go here, push this
     |               much plastic, heat to this temperature)
     v
  the PRINTER       (follows the G-code line by line)
     |
     v
  a solid object

Three of those words are worth pinning down now, because the rest of the book returns to them constantly.

• A 3D model is a digital description of a shape, a file on your computer. You can design your own in a program called CAD (Computer-Aided Design), which you will start doing in Chapter 8, or you can download a model someone else made.
• A slicer is a separate program that reads your model and decides exactly how the printer should build it: how thick the layers are, how fast to move, how hot to run the nozzle. It is named "slicer" because its main job is to slice the model into the horizontal layers we just described. You will learn to drive one in Chapter 5.
• G-code is the output of the slicer: a long text file of simple commands the printer obeys in order. You rarely read it by hand, but it helps to know that "send a print to the printer" really means "hand it a G-code file."

Don't be confused. The model and the print are not the same thing. The model is the digital file, weightless and editable, living on your computer. The print is the physical plastic object the machine produces from it. You can print one model a hundred times, or change the model and never print it. When a tutorial says "open the model," it means the file. When it says "remove the print from the bed," it means the real object you can hold.

FDM is not the only kind

So you have the full picture, here are the other main 3D printing technologies. You will not use them in this book, but it helps to know FDM is one option among several.

• Resin printing (the common types are called SLA and MSLA) works completely differently. Instead of melting a plastic thread, it uses a shallow tank of liquid plastic called resin and hardens it with light. A screen or laser shines a pattern, the light cures (solidifies) a thin layer of the liquid, the build plate lifts a fraction of a millimeter, and the next layer is cured below it. Resin printers capture much finer detail than FDM, which makes them popular for miniatures and jewelry. The cost is that the work is messy, the uncured liquid resin is toxic and needs gloves and ventilation, and a part made this way is not suitable for anything that touches food. That last point alone rules it out for a lemon squeezer.
• Powder methods like SLS (Selective Laser Sintering) spread a thin layer of fine plastic powder and fuse grains together with a laser, layer by layer. These machines are mostly industrial and expensive, mentioned here only so the word is not a mystery if you meet it.

This book uses FDM because it fits a beginner and fits our goal. FDM printers and filament are inexpensive, the process is forgiving of mistakes, the machine is not dangerous to stand next to, and the parts it makes are strong, solid, and functional. A lemon squeezer is exactly that kind of part: a tool that needs to be sturdy and handle some force, not a delicate display piece. Chapter 3 covers which plastics to feed it.

What FDM is good and bad at

Honesty here saves you frustration later. FDM has a clear personality.

Two of those weaknesses matter for our project. "Overhangs" are parts of a shape that stick out over empty space, like the underside of a horizontal arm with nothing beneath it. Because each layer needs something under it to land on, steep overhangs need temporary support material, a topic for Chapter 9. And food contact is a real consideration: an FDM print can be made reasonably safe for occasional kitchen use, but it takes deliberate choices about plastic, cleaning, and the tiny grooves between layers where residue can hide. That is important enough to get its own chapter, Chapter 14.

Takeaways

• 3D printing is additive: it builds an object by adding material in layers, instead of carving it away (subtractive) or shaping it in a mold (formative).
• FDM (also called FFF) melts a plastic thread called filament in a hot nozzle and draws layer on layer on a bed, building the object from the bottom up.
• The printer moves in X and Y (the flat plane) and Z (height); layer height (often 0.2 mm) trades surface smoothness against print time.
• The pipeline is always: idea, model, slicer, G-code, printer, object. The model is the digital file; the print is the physical result.
• Resin printing gives finer detail but is messy, toxic, and not food-safe; FDM is cheaper, forgiving, and good for sturdy functional parts like our squeezer.

👉 Now that you know what the machine does, the next question is which machine to stand in front of, so Chapter 2 walks through choosing a printer (or skipping the purchase entirely).