Short answer
A print-in-place model prints its moving parts already interlocked, with a small air gap between every surface that should move. That gap - usually 0.3–0.5mm on a standard 0.4mm nozzle - is too wide for the layers to bond across, so the parts stay separate. Get the gap right, keep it clear of supports, and never scale the model, and the joint works the moment it leaves the bed.
How a part prints already assembled
An FDM printer builds a model one layer at a time, and it does not care whether two shapes in the same layer belong to the same part. A hinge pin and the knuckle around it are just two outlines, printed side by side. If they touch, the hot plastic welds them together. If a thin strip of air separates them all the way up, they stay two separate parts, trapped together - the pin cannot come out because the knuckle was printed closed around it.
That is the whole trick. Every print-in-place joint - a hinge, a chain link, the segments of a flexi snake - is designed so the moving pieces never share plastic, while their shapes keep them captive. The skill is in making the gap small enough for a snug, quality-feeling joint but large enough that it never bridges shut.
The clearance that makes it work
Plastic leaves the nozzle slightly wider than its programmed path and sags a little as it cools, so a gap drawn at 0.1mm in CAD prints as no gap at all. These are the typical results on a standard 0.4mm nozzle at 0.2mm layers:
| Gap | What happens | Use it for |
|---|---|---|
| 0.1–0.2mm | Usually fuses, or breaks loose stiff and gritty | Press-fits and snap-fits - not moving joints |
| 0.3mm | Snug joint; needs a short break-in | Small hinges, fidgets, tight-feeling mechanisms |
| 0.4mm · safe default | Moves freely straight off the bed | Most hinges, chains, flexi animals, boxes |
| 0.5mm+ | Loose, slightly rattly; very forgiving | PETG parts, large models, long chains |
Vertical gaps behave differently from horizontal ones. A gap beside a part can be any width the nozzle can resolve. A gap above a part - a knuckle sitting over a pin - is made of whole layers, so at 0.2mm layer height it can only be 0.2, 0.4, or 0.6mm. Good designs make those horizontal gaps at least two layers tall.
Five rules that stop joints fusing
Three kinds you will see most
Hinges
A pin printed inside a knuckle, with a gap all around. Boxes, wallets, and folding toys are built from them. The infinity cube is eight cubes joined by hinges that loosen into a smooth flip after a few minutes of play, the clamshell parts box uses a full-length lid hinge, and the pop-up card wallet combines several into a mechanism.
Chains and interlocking links
Each link is printed through its neighbour, never touching it. Because links can lie flat, chains are one of the most reliable print-in-place shapes - and a good first one to try. The chain-link fidget spinner is a compact example.
Flexi and articulated models
Animals, snakes, and figures made of short segments, each joined to the next by a small hinge or ball joint. They wiggle and pose straight off the bed - the snake fidget is the classic. These have the most joints of anything here, so they are the most sensitive to scaling.
Ordering a print-in-place model
Frequently asked questions
My print-in-place model arrived stiff. Is it broken?
Almost certainly not. A freshly printed joint has a few stray strands and slightly rough contact faces. Flex each joint gently through its full range a dozen times - most loosen into a smooth, free movement within a minute or two. If a joint will not move at all with firm finger pressure, stop and message us rather than forcing it.
Can I scale a print-in-place model up or down?
Not safely. Scaling shrinks or grows the clearance gaps along with everything else. Shrink a model to 70% and a 0.4mm gap becomes 0.28mm - often enough to fuse every joint solid. Scaling up keeps parts moving but makes them loose and rattly. If you need a different size, look for a version the designer published at that size, or ask us before you order.
Can print-in-place parts be printed in PETG?
Yes, and PETG joints are tougher - they flex instead of snapping. But PETG strings more than PLA, so it needs clearances at the generous end (0.4-0.5mm). Models tuned tightly for PLA may come out stiff in PETG. Our back scratcher is a print-in-place PETG part that works well; for most fidgets and toys, PLA is the safer choice.
Does layer height matter for print-in-place?
Yes. Horizontal gaps - the space between a pin and a part sitting above it - are built out of whole layers, so the slicer can only make them a multiple of the layer height. At 0.2mm a designed 0.3mm gap rounds toward 0.2 or 0.4. Most print-in-place models are designed for 0.2mm; we print them at the designer's intended setting unless you ask otherwise.
Do you check print-in-place files before printing?
When we slice your file we look at whether the joints will print, and we will tell you if a model looks likely to fuse - particularly if it has been scaled. We cannot guarantee a model we did not design, but popular, well-reviewed print-in-place models from libraries like MakerWorld and Printables print reliably.