Concrete Mushroom Mold

v1.1 description:

v1.1 adds alignment tabs, makes the walls/floors thicker, and makes the cap mold also 2 parts. After trying it, these were all good moves to make.

Printing:

I printed this with the cheapest PLA I have. The molds were definitely reuseable with 2 walls and 20% infill. If you plan to make more than ~4 mushrooms, I would print it stronger than that.

You need 2 of each mold part. For the stem, print it standing upright, and paint the seams onto the outside of the mold. For the cap, print it on the flat edge of the outside, with the registration dots facing up.

Molding Concrete:

Apply some kind of mold release to the inside of the molds. I used bowling lane wax, and applied a heavy coating, and that worked well. Proper mold release is probably better.

Tape the two halves of each mold together with clear packing tape. Stretch it slightly as you go over the seams, and it will hold the molds firmly together. I did 2 wraps around the perimeter of the cap, and 2 wraps in 2 spots on the stem. I also recommend putting a single layer of tape across the seam on the bottom of the mold to limit how much water leaks out.

If you have a choice, a concrete mix with small aggregate will be better for getting bubbles out. Mix the concrete to a flowy consistency, pour it into the molds (slightly below the top is ideal), then tap the molds against the ground to shake out bubbles from the bottom.

Insert a piece of dowel, and use it to stir/shake slightly to release bubbles from the top of the mold. Center the dowel in the mold, leaving some exposed as a handle. Roughly every hour, rotate or wiggle the dowel to break it free from the concrete. You probably only need to do that 2-3 times before the concrete is mostly solid.

Once the concrete is fully dry, take off the packing tape. Try prying gently at each side of the mold. If it doesn't come off, use a rubber mallet, or a piece of wood to strike it lightly around the perimeter. Once one side is off, you can lightly tap the other side on the exposed inner face of the mold, and it should pop off easily.

Take out the casting dowels, cut a piece of dowel that's a little shorter than the two holes combined, and glue it in to one side, then the other. You can now lightly sand the mushroom to remove any visible layer lines from printing, and the seam line from the molds. Wear appropriate safety equipment when making fine dust particles.

Supplies:

• clear packing tape
• mold release
• concrete mix
• water
• a paddle mixer attachment and drill (optional; you can mix by hand)
• a small bucket
• 2 short pieces of dowel for molding, roughly 3" (75mm) long for the cap, and 4" (100mm) for the stem. I used 5/8" pine dowel, but anything from ½" (12mm) to 1" (25mm) should be fine.
• Another piece of dowel for final assembly (cut this to length after molding)
• Glue that will work on both concrete and wood
• A rubber mallet (optional)
• sanding paper (optional)

V1.0 description:

This is my first experiment in casting concrete with 3D printed forms. I learned a bunch, which I will attempt to relay here.

Printing:

I printed this with the cheapest PLA I have, because I was expecting to have to destroy the molds to unmold the concrete, and at this scale, the forces involved are small.

I designed the parts to be held together with packing tape, and with no registration features to minimize the plastic used. The stem is a 2 part mold, meant to be printed in the orientation it's modeled. The cap is a 3 part mold, meant to be printed with the flat outsides down. My slicer selected this side with the automatic alignment.

For both parts, you should “split” the STL to objects to move them appropriately in the slicer. It's best to paint the seams on the outside of the stem mold, so that they don't interfere with the fit, or molding.

Molding Concrete:

I first applied a bowling lane wax for a mold release, then taped the parts, making sure to stretch the tape a little bit for the second wrap around. The alignment wasn't perfect, but I did my best.

I picked a concrete mix with fiber reinforcement, because it advertised cracking resistance.

I mixed as little water as I thought I could get away with for pouring, and used the paddle mixer to strike the sides of the molds, hoping that would remove bubbles. I overfilled the molds, and pushed in a wooden dowel piece. After 1 hour, and again at 2 hours, I wiggled the wooden dowels to make sure I could remove it when the concrete was dry.

After the tape was removed, I couldn't pull the mold pieces off, and had to strike them with a rubber mallet. It took 2-3 blows per piece from 8 inches away swinging with “medium” force. Despite kinda thin walls, this didn't do a ton of damage to the plastic.

I used a silicone sealant alternative to glue the two pieces together with a dowel in between. Because I expect this to sit out in a garden, I don't anticipate large forces. Epoxy or many other things would make more optimal selections.

Problems: