Designing for a 3D printer

A shape can look perfect on your screen and still come out of the printer as a sagging mess. The screen does not care about gravity, and your design software will happily let you draw a part that floats in mid air or hangs off into nothing. The printer cares very much. It builds your object one thin layer at a time, from the bottom up, and each new layer has to land on something solid underneath it.

So before you draw the lemon squeezer, you need a handful of rules that live in the gap between "looks fine" and "prints well." None of them are hard. Each one comes from the same simple fact about how the machine works, and once you understand the why, you will start to see your designs the way the printer sees them. This chapter teaches those rules. In Chapter 13 you will apply every one of them to the squeezer itself.

Think in layers, from the bottom up

Here is the one idea that explains almost everything that follows. An FDM printer (the kind we are using, which melts plastic filament and lays it down) cannot print into empty air. It deposits a line of soft, hot plastic, and that line needs the previous layer beneath it to rest on while it cools and hardens. Build a tall stack of these lines and you get a wall. But ask the machine to put plastic where there is nothing below, and the plastic droops or falls.

Keep that picture in your head: the print head can only add a little to what is already there. Every rule below is just a consequence of it.

Overhangs and the 45 degree rule

An overhang is any surface that leans outward as it rises, so that each new layer hangs out a bit past the one below. A little overhang is fine. The new line of plastic only needs most of its width supported by the line under it, and the cooled plastic is sticky enough to hold the rest. But lean out too far and each layer is mostly hanging over open space, and the print starts to droop, curl, and look rough.

The rule of thumb is about 45 degrees from vertical. Measure the angle between the wall and straight up. Steeper than 45 degrees (closer to vertical) prints cleanly. Shallower than 45 degrees (closer to horizontal) starts to sag. Here is the picture:

   GOOD (about 45 deg)          BAD (steep overhang, ~70 deg)

        |  /                          |        ____
        | /  each layer               |   ____/    <- droops,
        |/   rests on the             |__/            curls down
        |    one below                |
   _____|_____ bed              _______|_______ bed

On the left, every layer is offset only slightly, so it has plenty of support. On the right, each layer juts way out past the last one, and the unsupported plastic sags.

You have three ways to deal with a bad overhang:

1. Reorient the part. Often a surface that is a nightmare in one orientation is easy in another. Turning the whole model over can move an overhang into thin air to a position where it rests on the bed instead.
2. Redesign it. Replace a flat overhanging ledge with a chamfer (an angled cut) so the surface stays within 45 degrees. This is the cleanest fix because it removes the problem entirely.
3. Add supports. Supports are temporary plastic scaffolding the slicer prints under the overhang, then you snap them off afterward. They work, but they waste plastic, leave rough marks where they touched, and take time to remove. We met them in Chapter 5. Treat them as a last resort. A good beginner habit is to design the supports out rather than lean on them.

For the lemon squeezer, this is the single biggest reason we print the bowl sitting flat with its open side facing up. The bowl walls flare outward only gently, so they stay inside the safe angle, and the inside surface (the one that touches juice) never needs a support touching it.

Bridging: spanning a short gap

There is one happy exception to "no printing in air." If the plastic only has to cross a short gap between two solid points, it can stretch across like a rope bridge. This is called bridging. The print head lays a line from one edge to the other, and as long as the gap is not too wide (a few centimeters at most, less is safer), the line stays taut enough to harden flat.

Bridging is what lets you print a flat ceiling over a hole or a horizontal opening without any support underneath. It will never be quite as smooth as a normal surface, but it works. If the squeezer ever needs a flat roof over an opening, a short bridge beats a pile of support material.

Wall thickness: think in nozzle widths

Your printer pushes plastic through a nozzle, and the standard nozzle is 0.4 mm wide. Everything the printer draws is made of lines (sometimes called perimeters or walls) that are roughly that width. This has a direct consequence for how thick you should make walls.

Make your walls a clean multiple of the nozzle width and the slicer can fill them with whole lines: 0.8 mm is two lines, 1.2 mm is three, 1.6 mm is four, 2.0 mm is five, and so on. Pick an awkward thickness like 0.5 mm or 1.0 mm and the slicer is stuck. The wall is too thin for a tidy number of lines and too thick to ignore, so it may leave a gap down the middle or skip the wall entirely. Walls thinner than one nozzle width (under 0.4 mm) often will not print at all.

For the squeezer bowl we will use a wall of about 2.4 mm, which is six lines. That gives the bowl enough stiffness to take the press of a hand and a lemon without flexing or cracking.

Don't be confused. Wall thickness and tolerance/clearance are two different things. Wall thickness is how thick the solid shell of a single part is (the 2.4 mm of plastic that makes up the bowl). Clearance is the deliberate empty gap you leave between two separate parts so they can fit together. One is solid plastic; the other is air. We will get to clearance below.

Bed contact and footprint

The first layer is glued to the build plate, and everything balances on it. A part with a wide, flat base is stable while it prints and afterward. A tall, skinny part standing on a tiny base is asking for trouble: the printer's moving head can knock it loose, or it can simply tip over partway up.

If a part must be tall and narrow, give it a wider base, or add a brim in the slicer. A brim is a flat skirt of extra plastic, one layer thick, printed around the base to increase the contact area with the bed. You peel it off after printing. The squeezer's parts are wide and low, so this is not a big worry for us, but keep it in mind for spouts, handles, or any thin sticking-out feature.

Orientation and strength

This rule surprises people. An FDM print is not equally strong in all directions. Because it is built from stacked layers that are fused together, it is weakest between the layers. Push or pull in a way that tries to peel the layers apart and the part will split along a layer line, like pages tearing out of a book. Push along the layers and it is much stronger.

So when you design and orient a part, ask: which way does the force go when someone actually uses it? Then arrange the layers so that force is not trying to separate them.

The lemon squeezer gets pushed down and twisted as you grind a lemon half against the reamer. That is exactly the kind of load that could peel layers apart if you orient things carelessly. Printing the bowl flat (open side up, base on the bed) lays the layers in horizontal sheets across the bottom, so the downward press squeezes the layers together rather than pulling them apart. The central reamer cone rises out of that same base, and we keep its load running down its strong axis into the bed. We will work through the exact orientation in Chapter 13.

Orientation and surface quality

Orientation does not just affect strength. It changes how each surface looks and feels. As a rule, up-facing and outward surfaces print smoothly, while down-facing surfaces that needed support come out rough, pitted with little marks where the support touched.

This matters a lot for the squeezer, because the inside of the bowl and the reamer cone are the surfaces that touch your juice. You want those smooth: smooth is easier to clean, and a rough, pitted surface has tiny crevices that trap pulp and bacteria. This ties directly into food safety, which gets its own treatment in Chapter 14. For now the design takeaway is simple: orient the part so the juice-contact surfaces face up and never sit on supports.

Tolerances and clearances for parts that fit together

If your design is more than one piece (say a separate strainer that drops into the bowl, or a lid), the pieces have to fit. And here is the catch: prints come out slightly oversized and slightly rough. The plastic spreads a hair as it is laid down, and the surfaces are textured, not glass-smooth. If you design two mating parts at the exact same size, they will jam.

The fix is to leave a small deliberate gap, called clearance or tolerance, between surfaces that must meet. A common range is 0.2 to 0.4 mm of gap. How much you leave decides the kind of fit:

• Press fit (tight): a small gap, around 0.1 to 0.2 mm. The parts go together firmly and stay put by friction. Good for things you assemble once.
• Loose fit (sliding): a larger gap, around 0.3 to 0.5 mm. The parts slide together and come apart easily. Good for a lid you remove often or a strainer you lift out to rinse.

A peg-in-hole example: if the peg is 10 mm across and you want it to slide, make the hole about 10.4 mm. If a strainer ring must drop into the bowl and lift back out, leave it loose. If two halves should click together and stay, go tighter. Printers vary, so when a fit really matters, print a small test piece first rather than committing to a whole part.

Holes, and why they come out small

Round holes deserve special attention. They tend to print slightly undersized, partly because the plastic shrinks a touch as it cools and partly because the printer's path rounds off on the inside of a curve. So if you need a hole to end up a certain size, draw it a little oversized to compensate.

There is a second problem with holes that run horizontally (with the round axis sideways, parallel to the bed). The top of such a hole is an overhang, the worst kind, a curve leaning further and further out, so it sags inward and the hole comes out squashed at the top. The trick is to design a teardrop shape instead of a circle: a circle with a little pointed roof on top. The angled roof stays within the safe overhang angle, so it prints clean, and the hole stays usable.

   round hole (sags)        teardrop hole (prints clean)

        ____                       /\
       /    \                     /  \
      |      |                   |    |
       \____/                     \__/

Fillets and chamfers

A fillet is a rounded edge; a chamfer is an angled, flat-cut edge. Both are worth adding for three reasons. They make a part stronger, because sharp inside corners concentrate stress and are where cracks like to start, while a rounded corner spreads the load. They are kinder to the hand, with no sharp edges to dig in, which matters on a kitchen tool you grip. And a chamfer on the bottom edge fights a common first-layer flaw called elephant's foot, where the bottom layer gets squished out wider than the rest, leaving a bulging lip. Angling that bottom edge gives the squish somewhere to go, so the base stays clean.

Minimum detail size

Finally, respect the size of the nozzle. Anything close to or smaller than one nozzle width (0.4 mm) is at the edge of what the printer can render. Tiny raised text, thin pins, fine grooves: below roughly a nozzle width they blur, merge, or vanish. If you want embossed text on the squeezer base, keep the letters chunky and the lines well above 0.4 mm wide. The ridges on the reamer cone need to be coarse enough to print and to bite into a lemon, not delicate decoration.

Tying it back to the brief

Every rule here points back to what the squeezer has to be (the brief in Chapter 11): a sturdy, smooth, easy-to-clean kitchen tool that survives being pressed and twisted, holds together if it is multi-part, and presents a clean surface to the juice. Strength comes from orientation and wall thickness. Smoothness comes from orientation and avoiding supports. A good fit comes from clearances. None of it is guesswork once you know why the machine behaves the way it does.

Takeaways

• The printer builds bottom up, one layer on the last, so every design rule comes from "what is underneath this plastic?"
• Keep overhangs steeper than about 45 degrees from vertical. Fix bad ones by reorienting or chamfering before you reach for supports.
• Short gaps can bridge unsupported; use that for flat ceilings over holes.
• Make walls a multiple of the 0.4 mm nozzle width (the squeezer bowl is about 2.4 mm, six lines). Avoid sub-nozzle-width walls.
• Give parts a wide, flat footprint, or add a brim, so they do not tip or pop off.
• FDM parts are weakest between layers. Orient the squeezer so pressing and twisting squeeze layers together, not peel them apart.
• Up-facing surfaces print smooth; down-facing supported surfaces print rough. Keep juice-contact surfaces facing up (see Chapter 14).
• Leave 0.2 to 0.4 mm clearance between mating parts. Tight gap for a press fit, larger gap for a loose fit.
• Holes print undersized; horizontal holes sag at the top, so draw them oversized or as teardrops.
• Add fillets and chamfers for strength, comfort, and to fight elephant's foot.
• Keep details well above one nozzle width or they will not print.

👉 You now know the rules of the game. Next we will learn a precise way to express a design in writing, where you can set a wall to exactly 2.4 mm and a clearance to exactly 0.3 mm and have the computer build the shape for you, in Designing in code with OpenSCAD.