How to make a bath bomb mold (3D-printed press mold)
The classic two-piece metal bath bomb mold only makes one size of sphere - and it dents, rusts and goes missing. A 3D-printed press mold fixes all of that: it's a matched two-part shell you squeeze your bath bomb mix into, in any shape and size you can model, and you can print a fresh one whenever you like. This guide walks through designing, printing and actually using one, and the bath bomb mold generator builds the two halves for you in seconds.
How a press mold works
You don't print the bath bomb - a bath bomb is pressed powder, not a printable material. You print a reusable two-part press mold: two halves that together form a hollow cavity in the shape of your bomb. You overfill both halves with damp mix, press them together so the mix compresses and bonds across the seam, then lift the halves away and turn out a finished bomb.
- Choose a shape & generate the mold. Pick a sphere, heart or custom STL; the generator builds two matched halves sized in true millimetres with a small overfill lip.
- Print the halves in PLA or PETG - no supports needed.
- Mix & pack your bath bomb batch, overfilling each half.
- Press, release & cure - squeeze the halves together, turn out the bomb, and let it dry.
Why print your own bath bomb mold
A printed mold gives you any shape and size, not just the one sphere a metal kit makes - hearts, eggs, stars, custom logos, mini bombs for testing. It's cheap (a few grams of filament per mold), you can print spares so a whole batch goes faster, and there's nothing to rust between uses. The mold halves are dimensionally consistent, so every bomb comes out the same size, which matters if you sell them by weight.
What you'll get
- Two matched mold halves that clamshell together into a hollow cavity in your chosen shape.
- A slight overfill lip around the parting line so you can mound the mix proud and compress it as the halves close.
- True-millimetre sizing - set the finished diameter and the generator sizes the cavity to match, with the press allowance built in.
What you'll need
- Filament: PLA is fine - the mix is dry and contact is brief. PETG is a small upgrade for a tougher, slicker cavity if you press lots of batches.
- Bath bomb mix: baking soda and citric acid (roughly 2:1) plus any add-ins - colour, fragrance, a little oil or butter.
- A spray bottle of witch hazel (or a light oil) to barely dampen the mix.
- A scale for repeatable batches and, optionally, cornflour as a light release dust.
Step by step
Choose a shape and generate the mold
Open the bath bomb mold generator, pick a built-in sphere/heart or drop in your own STL, and set the finished size. Preview the two halves on the virtual print bed and download the STL or 3MF. Aim a few millimetres over your target finished diameter - the mix compresses as you press, and the bomb tightens slightly as it dries.
| Setting | Suggested |
|---|---|
| Finished sphere diameter | 55–65 mm |
| Cavity oversize vs finished | +2–4 mm |
| Mold wall thickness | 2–3 mm |
Print the two halves
Slice both halves at 0.2 mm (or finer, 0.12 mm, for an extra-smooth surface) with 3 walls and 15% infill. No supports - the halves are designed to print cavity-up so the inner face needs no support. The cavity surface becomes the bath bomb surface, so keep the first layers clean and consider a finer layer height for detailed shapes.
Mix the bath bomb batch
Combine the dry ingredients first - baking soda and citric acid at roughly 2:1, plus colour, fragrance and any fine add-ins. Then spritz witch hazel sparingly, mixing constantly, until the powder just holds together when you squeeze a handful. It should clump, not feel wet. Too much liquid and the citric acid starts reacting early and the bomb won't release cleanly.
Overpack and press
Mound the mix proud above the rim of both halves - overfilling is what gives a dense, smooth bomb with no hollow centre. Press the mix in lightly with your fingers, then bring the two halves together and squeeze firmly so the mix compresses and the two domes bond across the seam. The overfill lip lets the excess compact rather than squish out.
Release and cure
Leave the halves closed for a minute so the mix sets. Tap the outside of one half gently all around to break the surface seal, lift it straight off, then turn the bomb out onto a soft surface (a sheet of bubble wrap or a foam cradle, never a hard counter). Let it cure several hours to overnight before handling or wrapping - fresh bombs are fragile until fully dry.
Print & release tips
- Finer layers, smoother bombs. The cavity walls transfer their texture to the cast, so 0.12–0.2 mm layers give the cleanest surface.
- Keep the mix dry. The single biggest cause of sticking and crumbling is over-spritzing. When in doubt, drier.
- Press harder than feels necessary. A firm squeeze is what bonds the halves and prevents a hollow seam.
- Use release sparingly. If a detailed shape sticks, dust the cavity with a little cornflour or wipe the thinnest film of skin-safe oil - too much leaves a greasy finish.
- Cure on something soft. Bombs are delicate before they dry; a foam or bubble-wrap cradle stops flat spots.
Troubleshooting
The bath bomb sticks in the mold
Almost always too much moisture. Spritz less so the mix just barely clumps, press firmly, and let it set in the closed mold a moment before releasing. A light cornflour dust or the thinnest wipe of oil in the cavity also helps it drop free.
The bomb crumbles or falls apart on release
Either the mix was too dry, or you didn't overpack and press hard enough. Add a spray or two of witch hazel until it holds a clump, mound the mix proud of the rim, and squeeze the halves together firmly so the mix compresses.
The two halves leave a visible seam line
A faint seam is normal - overfill more and press harder so the domes bond, then smooth the seam with a fingertip right after release. A little extra mix pressed into the line before it dries blends it in.
The surface is rough or pitted
Print the cavity at a finer layer height, overpack so the mix presses tight against every face, and keep the mix on the drier side so it doesn't pit. Lightly sanding the printed cavity once also smooths the cast face for every bomb after.
The bomb activates (fizzes) while still in the mold
The citric acid is meeting water too early - your mix or your hands are too damp, or the room is humid. Use witch hazel instead of water, work fast, and store the dry mix sealed. Press and release promptly rather than leaving damp mix sitting.
Bath bomb mold or silicone mold?
A press mold is right for bath bombs because the medium is dry pressed powder. If you're casting a liquid that sets - soap, resin, wax, plaster - you'll want a flexible silicone mold or a direct-pour rigid one instead. See the silicone mold guide for liquids that need fine detail and flexibility, and the mold release agent guide if you're fighting a stubborn release on any mold.