Gluing and splitting big parts

Every printer has a wall around it — a build volume you can't print past. Sooner or later you'll design something that doesn't fit inside it, and the answer isn't a bigger printer, it's a smarter model: cut the part into pieces that each fit, print them, and join them back into one. Done carelessly, that's two halves and a smear of glue that shows a seam and snaps under load. Done well — with alignment features designed in and the split placed with intent — it's a part that's stronger, cleaner and barely betrays that it was ever in pieces. Splitting is a modelling task first and a gluing task second.

Glue is chemistry, so it depends on the plastic

Adhesives don't all work the same way, and which one bonds your part depends entirely on what it's made of. Cyanoacrylate (CA / superglue) is the fast general-purpose choice — it grabs almost anything in seconds, but the joint is thin and brittle and hates a gap. Solvent cement (plastic cement) is the strong one for ABS and ASA: it doesn't glue the parts, it welds them, softening both faces so the plastic flows together and fuses into a single mass — a solvent weld is nearly as strong as the part itself. Epoxy is the gap-filler and the diplomat: it's structural, it bridges loose fits, and it bonds dissimilar materials that nothing else will hold together. PLA takes CA and epoxy happily. The hard cases are the low-surface-energy plastics that resist paint — polypropylene and polyethylene — along with nylon, which is hard to bond for a different reason: it's semicrystalline and chemically resistant. All of them need special primers or purpose-made adhesives, and sometimes won't reliably glue at all.

The CAD move: split with alignment built in

Here's where the model does the heavy lifting. A butt joint — two flat faces pressed together — has nothing to locate the halves, so they slide out of register while the glue sets and the seam ends up stepped and weak. Instead, design the join into both halves. Add locating features: pins on one side and matching holes on the other, a dovetail or a tongue-and-groove that keys the pieces together, registration bosses that only seat one way. These do two jobs at once — they line the halves up automatically, and they add gluing surface area so the bond is far stronger than two flat faces could ever be.

Two details make or break it. First, leave a glue-gap allowance: model the pins a hair smaller than their holes and the mating faces with a few tenths of clearance, so glue has somewhere to sit and the halves actually close instead of bottoming out on a tight fit — the same clearance reasoning as Tolerances and fits. Second, place the split where it hides and where it's strong. Put the seam on a flat or naturally hidden face so it disappears after a little cleanup, and orient the cut so the joint runs across the layer direction rather than along it — a glued or welded seam can be stronger than the weak inter-layer bond it replaces, effectively repairing the Z-axis weakness the print would otherwise have.

Gluing is one way to join. When you want a join you can take apart — bolted, threaded, captive-nutted — that's a different toolkit, covered in Threads, fasteners and hardware, and the upstream orientation and seam decisions live in From model to print.

That closes the loop on what happens after the bed. One last thread ties all of it together: every allowance you've dialled in here — the sacrificial skin, the coating clearance, the glue gap, the shrink compensation — only stays true if the machine that prints it stays true. Keep the printer tuned and maintained, its nozzle clean and its axes square, and the numbers you worked so hard to get right keep landing where you put them.