Pause mid-print: embedding magnets, nuts and weights inside a part

Some things shouldn't be glued into a 3D print after the fact: a magnet holding a box lid will pop its glue after a few hundred openings, and a nut that's merely pressed into a hole spins when you tighten the screw. The tidier way is to have the printer wrap the object: print partway, stop, drop the object into its pocket, and keep printing on top. The object ends up sealed between layers of plastic, with no glue and nothing showing.

Every common slicer can do this. The hard part isn't the button — it's two small details: drawing the pocket to the right depth, and choosing the right layer to stop on, because slicers don't number layers the same way.

When to embed, and when to insert or glue

Drawing the pocket: three rules

1. 01The pocket is deeper than the object. The object's top must sit below the bottom of the layer that will cover it. If it stands proud by even part of a layer, the nozzle hits it on the first move after resuming. The easy way: take the object's height, add about one layer, and round up to a multiple of the layer height you'll print — at 0.2 mm layers, a 2.4 mm-thick M3 nut needs a pocket about 2.6 mm deep.
2. 02Clearance to match the fit. Magnets want a press fit — use the press-fit clearance in the handbook's fits section. Nuts and bearings only need to drop in. Print a test coupon with the pocket before the real part: FDM holes always come out smaller than drawn.
3. 03Flat pocket edges, a thick enough lid. The first layer over the pocket bridges empty space. The wider the pocket, the more that layer sags; a few solid layers above it flatten things out again. If the part is loaded in a way that pulls the object out, make the lid thicker — it's the only thing holding the object in.

For paired magnets — a lid and a box body — mark polarity before you print. Let two magnets snap together, mark the outer face of each with a pen, and place them into the two parts with the marked faces the right way round. Get the polarity wrong and the lid pushes away instead of pulling in, and the only fix is a reprint.

Choosing the pause layer: three slicers, three ways to count

The layer you want is the first layer that prints over the pocket's opening. In the preview, drag the layer slider up until you see the pocket covered for the first time — that's the target layer. The printer must stop after finishing the layer below it and before starting it. Each slicer expresses that point differently:

PrusaSlicer lets you attach a short message to show on the printer's screen when it pauses — say, "Drop the magnet in and resume" — which is well worth it when one print has several pauses. Bambu Studio uses M400 U1 as the default pause command in its own machine profiles: the printer waits until you press resume on the screen or in the app.

Seven steps, from drawing to part

1. 01

   Measure the object

   Use calipers to measure the real diameter and height — magnets and nuts are often a few hundredths of a millimetre off the size on the packet.

2. 02

   Model a closed pocket inside the part

   The pocket sits entirely inside, open to no face. Depth is the object's height plus about one layer, rounded up to a multiple of the layer height. Clearance to match the fit: press for magnets, loose for nuts and bearings.

3. 03

   Print a test coupon

   Cut out just the section with the pocket as a thin slice, print it, and try the object. Five minutes here saves the long print.

4. 04

   Slice and find the covering layer

   In the preview, drag the layer slider to the first layer that closes the pocket. Check the layer just below: the pocket must still be fully open and deeper than the object.

5. 05

   Insert the pause

   PrusaSlicer, Bambu Studio, OrcaSlicer: pick the covering layer itself. Cura: enter the layer just below it, counting from 0, or pause by height. Re-slice and confirm the pause marker is where you expect.

6. 06

   Have the objects ready before it stops

   Lay the objects out by the printer, mark magnet polarity, have super glue ready if needed. The slicer estimates the time to the pause — set a timer so you're there when it happens.

7. 07

   Drop in, check, resume

   Use tweezers to place the object, press it to the bottom of the pocket, and pick off any stray strings. Look from the side: nothing may stand above the layer surface. Only then press resume.

Common failures

• —The nozzle hits the object. The pocket isn't deep enough, or the pause came one layer early so the object stands proud. Recheck the pocket depth against a multiple of the layer height, and count layers the way your slicer counts them.
• —The magnet jumps to the toolhead. The pocket is too loose. Tighten the clearance or glue the bottom of the pocket before placing it.
• —A line around the part at the pause layer. While the printer waits, plastic can ooze from the nozzle and the part cools; the first layer after resuming often leaves a faint line. Shorter pauses show less. With warp-prone material like ABS, don't leave it waiting long — more in the troubleshooting guide.
• —The lid sags or breaks through. The pocket is too wide to bridge. Narrow it, or add a few solid layers above; bridging and overhang thresholds are in designing parts for FDM.
• —Forgetting a paused print. The printer will wait, but the part keeps cooling on the bed. Being there on time is part of the process.

Embedding an object mid-print is a design decision: the pocket, its depth and the pause layer are chosen on the drawing, not while the printer is running.

The same technique works for a colour change at one layer — PrusaSlicer has a dedicated colour-change command (M600) right next to the pause. Material choice for parts with embedded hardware is the same as for any functional part: see PLA vs PETG vs ABS.