Ribbed concrete vase mold

A reusable mold for casting a ribbed vase or planter. The outer shell has a fluted wall that keeps it stiff under the weight of a full pour, and an inner core hangs down the middle to hollow the casting out, so what you demold is a vessel rather than a solid block.

It works with concrete, cement, plaster, Jesmonite and resin. What comes out can be a vase, a planter, a pen pot, a brush holder or a candle holder, depending on how you finish it.

The spoked top ring drops over the shell and holds the core centred, and the side clamps squeeze the seam shut while the cast cures. The seam is the one thing worth being fussy about: a clamp left loose is how you get a fin of concrete down the side of the vase.

The whole mold is 87 x 87 x 72 mm, so it fits on any print bed. It is genuinely reusable, because nothing in a concrete pour attacks PLA or PETG and the parts lift off the cast rather than being broken away, but only if you use release agent every single time.

What you need

• Concrete, cement, plaster, Jesmonite or resin, and something to mix it in
• Release agent on every cast: cooking spray, petroleum jelly or proper mold release
• Something to vibrate the mold with, even an orbital sander held against the outside with no paper on it
• A sanding sponge for the seam line, once the cast has fully cured

How to cast it

1. Seat the outer shell on the base plate, drop the inner core in, and fit the spoked ring over the top so the core sits centred.
2. Clamp the seam shut on both sides and check it is properly closed before you mix anything.
3. Coat every inside surface with release agent, the core included.
4. Mix a little wetter than you would for a slab, and pour into the gap between the shell and the core in two or three stages rather than one.
5. Tap or vibrate the mold after each stage so the trapped air rises. This is what decides whether the finished vase has pinholes in it.
6. Let it cure, then unclamp, lift the core straight up and out, and open the shell.