Your First 3D Print Is Impossible
Look closely at this shape. It is a triangle made of three straight bars, and every corner is a right angle, 90°. Three corners of 90° add up to 270°. But the corners of any flat triangle add up to exactly 180°.
This is the Penrose triangle. The Swedish artist Oscar Reutersvärd drew one in 1934, and the mathematician Roger Penrose and his father Lionel made it famous in 1958. Artists such as M. C. Escher built whole impossible buildings out of it.
Yet in Perth, Australia, a giant metal Penrose triangle stands in a park. Visitors photograph it from one special spot. And by the end of this guide, you will hold one in your hand, printed on your own 3D printer.
Make your guess. How can a real, solid object look like an impossible triangle?
- The bars are secretly bent, so the ends can meet
- Two bars never touch: they only line up from one spot
- It only works as a flat picture printed on card
Show answer
B. Two bars never touch: they only line up from one spot
The bars never close. One end floats in front of the other, and from exactly one viewpoint the near end hides the gap. Step to the side and the triangle falls apart. A camera shows the trick best, because a camera looks with one eye.
Before printing anything, see the trick for yourself. Where do you have to stand?
Find the impossible angle
Your goal: turn and tilt the bars until the gap reads 0 mm.
Only one view works: looking straight down the long diagonal of a cube, from one corner toward the opposite corner. From there, the top of the post sits exactly in front of the free end of the first bar, and your brain joins the two.
That shape is your first print. To make it, you need a few things. What exactly?
What you need
- A budget 3D printer, the kind that melts plastic string. The next section helps you pick one.
- A spool of PLA filament, the easiest plastic for beginners. Most budget printers take 1.75 mm filament: check yours.
- A computer that can install apps (Windows, Mac or Linux), for the slicer.
- A free Tinkercad account, to design the model in a web browser. Under-13s may need a parent or teacher to set one up.
- A sheet of plain printer paper, for levelling the bed.
- Dish soap or isopropyl alcohol, to clean the bed.
- An adult nearby for your first prints.
The triangle uses about 10 grams of plastic, so a 1 kg spool could make around 100 of them. First, the biggest choice: which printer?
Step 1: Pick a printer
A home 3D printer works like a robot glue gun. It pushes a plastic string, called filament, through a hot metal tip, called the nozzle. The melted plastic is drawn out in one thin, flat layer on a plate called the bed. Then the nozzle rises a fraction of a millimetre and draws the next layer on top.
A typical nozzle hole is 0.4 mm wide, about four sheets of paper thick. How thick is one layer on a typical print?
- About 2 mm
- About 0.2 mm
- About 0.02 mm
Show answer
B. About 0.2 mm
About 0.2 mm, the thickness of two sheets of printer paper. So the 60 mm tall triangle is a stack of 300 layers.
Budget printers come in a few kinds. Prices change often, so treat these as rough ranges at the time of writing (October 2026, in US dollars).
| Kind | How it works | Price |
|---|---|---|
| Bed-slinger | The bed slides back and forth under the nozzle. The most common beginner printer. | $150 to $350 |
| Mini bed-slinger | The same idea with a smaller bed, around 18 cm across. Plenty for the triangle. | $150 to $300 |
| Enclosed box | Faster, with a lid that keeps heat in for tougher plastics. Usually past a first budget. | $400 and up |
| Resin printer | Hardens liquid resin with light. Fine detail, but liquid resin irritates skin and needs gloves and fresh air. Not for a first printer. | $150 to $400 |
Popular beginner models at the time of writing include the Bambu Lab A1 mini, the Creality Ender-3 V3 SE and the Elegoo Neptune 4. New models appear every year, so read recent reviews before you buy. One feature is worth paying for: automatic bed levelling, where the printer measures its own bed before each print.
Most printers arrive mostly built: follow the maker's quick-start guide, load the filament and run the first-time setup. Now the printer is ready, but a printer only follows instructions. Where do the instructions start?
Step 2: Get or make the model
Every print starts as a 3D model: a file that describes the surface of a shape, usually saved as an STL or 3MF file. Websites such as Printables, Thingiverse and MakerWorld share millions of free models. Look for ones with photos of other people's prints: those are proven to work.
Our triangle is simple enough to make yourself, in Tinkercad. Tinkercad is a free CAD app (computer-aided design: software for drawing exact shapes) that runs in a web browser. You build things by stacking simple shapes, like digital LEGO.
- Start a new design and drag a Box onto the workplane.
- Click the box, then click a size number and type 60 × 10 × 10 mm. That is bar 1.
- Copy bar 1, paste it, and set the copy to 10 × 60 × 10 mm. Slide bar 2 so the two bars form an L, flush at the outside corner.
- Add a third box, 10 × 10 × 60 mm: the post. Stand the post on the far end of bar 2, flush with both outer sides.
- Select all three boxes and click Group. Then click Export and choose .STL.
| Box | Size (mm) | Corner at (x, y) |
|---|---|---|
| Bar 1 | 60 × 10 × 10 | 0, 0 |
| Bar 2 | 10 × 60 × 10 | 50, 0 |
| Post | 10 × 10 × 60 | 50, 50 |
You now have an STL file of the triangle. But a printer can't read an STL. What turns a shape into instructions?
Step 3: Slice it
A slicer is a free app that cuts your model into flat layers and plans the nozzle's path through every one: where to move, how fast and how much plastic to push. It saves the plan as a file the printer reads, usually G-code. Popular free slicers include Cura, PrusaSlicer, OrcaSlicer and Bambu Studio. Your printer's maker usually recommends one with settings ready for your printer.
Your goal: press Play and watch the triangle print. Then pick the vase, switch between 0.1 mm and 0.3 mm layers, and compare the side close-up.
Each layer gets two outlines, called walls, and a sparse criss-cross inside, called infill. A solid part would waste plastic and time; with infill, the inside is mostly air but the part stays strong. The first and last few layers are packed solid, so the bottom and top come out smooth.
Slicers have hundreds of settings, but for a first print only five matter:
| Setting | What it does | Ours |
|---|---|---|
| Filament | Picks the plastic, which sets the temperatures. | PLA |
| Layer height | How thick each layer is. Thinner looks smoother but takes longer. | 0.2 mm |
| Infill | How full the inside is. | 15 to 20% |
| Supports | Scaffolding under parts that would hang in mid-air. | Off |
| Bed adhesion | Extra plastic that helps the first layer grip: a skirt (a loop around the part) or a brim (a flat ring stuck to the part's edges). | Skirt |
The slicer also asks one more question: which way up should the part sit? Every layer has to be printed on top of something, either the bed or the layer below. So what happens to a wall that leans outward?
The 45° rule
A wall leans outward as it goes up. How far can it lean before the layers start to droop?
- Not at all: walls must go straight up
- About 45° from straight up
- Almost flat, about 80°
Show answer
B. About 45° from straight up
Most printers manage about 45°. Each new line of plastic can hang about halfway past the line below it and still be held up. Lean further and most of each line sits on air, so it sags. That is the 45° rule, and a face that leans past it is called an overhang.
When an overhang can't be avoided, the slicer can build supports: thin scaffolding you snap off after printing. Supports cost plastic and time and leave rough marks, so first try turning the part.
Your goal: turn each part until no face is blue. There are four parts, and the last one is a puzzle.
Our triangle is lucky. With the L flat on the bed and the post standing up, every face is flat on the bed, straight up, or flat on top. No overhangs, so no supports.
Suppose you laid the triangle down so the post rested flat on the bed. Bar 1 would then stick straight out sideways from the top of bar 2, 50 mm up. What would happen without supports?
- It prints fine, because PLA is stiff
- The first layers of bar 1 land on thin air and droop into a tangle
- The printer refuses to start
Show answer
B. The first layers of bar 1 land on thin air and droop into a tangle
Bar 1's first layers would have nothing underneath them, so the plastic would sag and curl. The slicer would need supports all the way up from the bed. Standing the post up avoids all of that.
The file is sliced and the orientation is right. The print can still fail in its first five minutes. Why?
Step 4: Nail the first layer
Most failed prints fail on the very first layer. The first layer is the foundation: if it doesn't grip the bed, nothing above it will. Two things decide it: the gap between nozzle and bed, and how clean the bed is.
The gap has to be the same everywhere on the bed, which is called bed levelling. Printers with automatic levelling measure the bed with a sensor. On a printer without one, you level by hand with a sheet of paper.
- Heat the bed (many guides level warm, because heat changes the gap a little). Keep your fingers away from the hot nozzle.
- Use the printer's menu to move the nozzle to its lowest point over one corner.
- Slide the paper between the nozzle and the bed, and turn that corner's knob until the paper drags a little.
- Repeat for every corner, then go round a second time, since each corner nudges the others.
Next, clean the bed. Fingerprints are oil, and plastic can't grip oil. Wash the plate with dish soap and warm water, or wipe it with isopropyl alcohol if the printer's maker says that is safe for your plate. Then hold the plate by its edges.
Now start the print and watch the first layer go down. With practice, you can read a first layer like a detective.
Your goal: diagnose six first layers. Look at both drawings, then pick the cause.
If the first layer looks wrong, stop the print, fix the cause, and start again. Five minutes of fixing beats two hours of spaghetti. And what exactly is spaghetti?
When prints go wrong
| Problem | What you see | Try this |
|---|---|---|
| Won't stick | Lines drag around or peel up on the first layer. | Redo the paper test, clean the bed, and print the first layer slower. |
| Stringing | Thin hairs between parts, like a spider web. | Lower the nozzle temperature by 5 °C at a time, and keep filament dry. |
| Spaghetti | A tangle of loose plastic loops. | Stop the print. The part came loose or had nothing under it: fix the first layer or add supports. |
| Warping | Corners curl up off the bed. | Add a brim, clean the bed, block drafts, and check the bed temperature. |
| Clicking, thin lines | The motor clicks and little plastic comes out. | Check the spool isn't tangled, raise the nozzle slightly, or clear a clog using the maker's guide. |
Halfway through a print, the part breaks loose and the nozzle starts drawing loops in mid-air. What should you do first?
- Let it finish: it might fix itself
- Stop the print from the printer's screen
- Reach in and push the part back into place
Show answer
B. Stop the print from the printer's screen
Stop the print. The printer can't see, so it will keep drawing in the air until the file ends. Never reach in near the hot nozzle. Let it cool, peel off the mess, and find the cause, which is usually the first layer.
With a clean first layer and no overhangs, your triangle should finish in about an hour on an older budget printer, and faster on a newer one. Then comes the best part. Can you make it look impossible?
Step 5: Photograph the impossible
When the print finishes, let the bed cool for a few minutes: many prints pop off a cool bed on their own. If the plate is a bendy steel sheet, take it off and flex it. If you use a scraper, always push away from your hand.
Now the trick. Put the triangle on a table with the post at the corner nearest you. Step back about a metre and look down at it at a slant, about 35° below level, with one eye closed (or through a phone camera, zoomed in a little).
Move your head until the top of the post sits right in front of the free end of bar 1. The triangle snaps shut.
Why one eye? Your two eyes see from two spots a few centimetres apart, and your brain uses the difference to judge depth. One eye, or one camera, has a single viewpoint, so the gap can hide. Stepping back helps too: up close, the nearer end looks bigger than the far end, and the edges won't line up.
So what will you print next?
What to print next
Each project below adds one new skill. Search for them on Printables, Thingiverse or MakerWorld, or design your own in Tinkercad.
- A name tag or keychain. Add Tinkercad's text shape to a flat plate. New skill: designing with text.
- A calibration cube. A 20 mm cube to measure with a ruler. New skill: checking your printer's accuracy.
- An overhang test. A staircase of faces from 20° to 70°. New skill: finding your own printer's real limit, beyond the 45° rule.
- A print-in-place hinge. Two parts printed already joined, with a small gap between them (often 0.3 to 0.5 mm, depending on the printer). New skill: clearance. Too small a gap and the hinge fuses solid.
- A chain or a flexible animal. Dozens of print-in-place joints that come off the bed already moving. New skill: printing across short gaps without supports, called bridging.
- Your own illusion. The Penrose staircase works the same way: two ends that only line up from one viewpoint. New skill: designing from a single camera angle.
Stuck on any of these? Ask Phy, for example:
- My first layer peels up at the corners. What should I check first?
- How do I design a print-in-place hinge in Tinkercad, and how big should the gap be?
- Why does closing one eye make the Penrose triangle work?
Pick one project, slice it, and watch the first layer go down.