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I have a 3D printer in which I can print the attached object for use as a planter. I am assuming that the easiest molding process would be to use a silicone mold. As a first timer for creating a mold, my plan is to

  • print the cylindrical container to surround the object
  • glue the base of the object to the base of the box
  • pack some clay on the inside so that the resulting wall thickness is sufficient for concrete
  • coat the inside of container and object with a release agent.
  • pour the silicone into the container and let it set.
  • break away the container
  • use knife and cut a zig-zag pattern on the side of the mold
  • pull out the object
  • close the zigzag pattern and tape it up ready for concrete pouring.

What grade of silicone should I use to minimise difficulty pulling out the object from the mold. A silicone with low viscosity? Have I missed some crucial steps?

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2 Answers 2

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Alternative to a silicone mold

You could just as well print the pot twice in different sizes and use those prints as a mold. You'll probably need to print both sizes at the same time because you cannot fit a slightly smaller pot through the opening of the large pot. You can print them with thin walls because they only have to withstand some liquid concrete, but no wear and tear.

Take a square wood or a board and glue the upper edge of the smaller pot to it. Then center the bigger pot over it and glue the edge. You now have a mold you can fill with concrete.

Once the concrete is completely cured, you'll have to break away the outer 3D print. You can chose to keep the inner print as a water barrier or break it away as well for a pure concrete look.

Complex outside

This flower pot has a very complex surface. That's what makes it so attractive for 3D printing and the reason why such objects were very rare before 3D printing: it cannot be mass-manufactured in the conventional way.

The rough shape is a cylinder with some very prominent protrusions. If you cast the outside of the pot in a single mold, it will be very hard getting the pot back out after the mold cured. If you also cast the inner space as a single mold, it will be almost impossible to get the concrete cast out of there without either destroying the mold or the cast.

You need to cut the outside mold vertically into at least 2 parts, but 3 are probably better. Due to the irregular surface, it will be hard to create clean lines. It's better to seal the opening of the pot, lay the entire mold container onto one side and pour molding silicone up to 1/3 of the volume. Put some big indentations into the silicone (with something like wooden dowels) to help you align the mold parts.

Once the first part is completely cured, remove the dowels (but not the printed pot!) and cover the whole mold with a very generous amount of mold release. Rotate the mold by 90° and pour a second part of the mold like above (including indentations for alignment). Once that's cured, stand the container upright, add more mold release and pour the third part.

Complex inside

You'll encounter the same problem in the cavity of the pot: fill it with silicone and you'll have a really hard time getting it out in one piece again.

I propose finding (or printing) any cylindrical object that is almost as big as the cavity at its smallest part. When you fill the cavity with molding silicone, suspend the cylinder in a way that it fills as much of the cavity as possible without actually touching the walls. Once the mold is completely cured, you can first remove the cylinder, which should give the silicone enough room to be removed as well.

Keep in mind that the cylinder is part of the mold and needs to be inserted when you pour the concrete.

Mold container

Concrete is thick and heavy. If you don't want your pour to distend or leak, you need either very thick silicone or a structure that gives the silicone more stability. You should not "break away" the molding container at all. Instead, pull the cured mold out, separate the parts and remove the original printed pot.

When you want to pour the concrete, reassemble the mold parts and insert them into the container to give them stability.

Wall thickness

Why would you "pack some clay on the inside so that the resulting wall thickness is sufficient for concrete" when you could 3D print the pot with a sufficient wall thickness? At the very least, I would greatly increase the thickness of the bottom of the pot.

If you want to save on printing material, a better solution might be filling the entire inside of the pot with clay except a smooth cylinder (very similar to the one I suggested for making the inside mold). That makes the molding process much easier, but it might create weaknesses in the wall.

The finished concrete pot would have the structured outer wall but a simple, smooth cylindrical inside.

Molding silicone

You'll need a 2 part molding silicone for this project and it should be pourable. Be careful when mixing both parts to avoid introducing air bubbles. Pour the liquid silicone in a thin stream into the mold (that removes bigger air bubbles) and don't pour directly over the object. Instead, start pouring at a side wall or the bottom of the mold container and let the silicone rise up around the object to avoid trapping air bubbles.

Concrete

You'll want a rather liquid concrete mixture to make sure that all the protrusions are completely filled. Shake, tap or vibrate the filled mold to help air bubbles escape.

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  • Wow. Thanks for the detailed suggestions! Certainly a lot to think about for this project. I just thought that the good old fashioned concrete would be more appealing than plastic for this kind of shape. Feb 22 at 21:58
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I won't duplicate all the good information in Elmy's answer; this will be supplementary ideas.

  1. If concrete is the material you want to cast, here are some considerations.

    • Just to clarify terminology, "concrete" implies a cement-based material that contains stones, which won't be compatible with a thin wall or fine surface detail. If you want a cement-based material, you'll need something containing cement and very fine sand, that can be mixed to a pourable consistency and cure strong. There are a few pre-mixed materials that do well for this kind of application. Rapid Set's CementAll is a popular one, generally available at big hardware chains. ShapeCrete and SculptCrete would also be good. A similar material that would also work is polymer-modified plaster, which is a mix of plaster and acrylic resin, sold as Jesmonite and various other brands.

    • You can go reasonably thin with ceramic, but not cement. Cement is brittle and has low tensile strength (easily pulls apart or breaks under a load that stretches it). If you want to use cement, plan on a reasonable wall thickness.

      Another implication relates to the mold. Small, thin surface detail will easily break off during demolding. So the mold needs to be designed to avoid undercuts that will catch in the mold. Avoid a "sock" mold (a thin, elastic mold that you peel off), and incorporate mold seams in undercut locations so the mold can be removed without pulling on the cement.

  2. Mold considerations

    • Elmy's answer suggests ways to facilitate removing the mold from the inside -- making the inside cylindrical or making a thin, flexible mold of the inside that is held in place by a removable cylindrical core. If you go the second route, there's an additional helpful approach. Much of the problem interior detail will be out of sight. Modify the 3D model so the inside basically follows the shape of the outside, especially where visible at the top, but doesn't have fine surface detail or undercuts that will snag.
    • As a practical matter, this design will probably require at least the outside to be a flexible mold, like silicone (it would take an insane number of rigid sections to avoid undercuts). Silicone is expensive. You can reduce the amount required by 3D printing an external shell. Make the silicone layer just thick enough to capture the surface detail and provide some material for good alignment of sections. Contain that in a 3D printed shell that follows the gross shape. Make that shell in sections that clamp together.
  3. Possible silicone alternative

    There is a very inexpensive, reusable molding material you can make at home that can replace silicone for casting a variety of materials. The primary ingredients are gelatin and glycerine. An online search for "gelatin glycerine mold" will yield a lot of variations on the recipe. A couple of good ones for a start are Reusable Molding Material: Homemade... - Observations and Gelatin And Glycerine Mold-Making Recipe, Cheap And Reusable • Ultimate Paper Mache. It generally behaves like a soft silicone mold. The resulting mold isn't as durable as silicone in terms of reusing it many times, but it can be melted down and reused to make a new mold.

    I didn't research how well this material works with cement. It's soft, so it could distort under a lot of weight. It's also possible that the high alkalinity of the cement could affect its properties. It would probably work best with a fast-setting cement mix so the water doesn't have too much time to affect the material and potentially degrade surface detail.

    This material is very flexible, so it might make it easier to demold the result. It would also offer the possibility of casting fragile detail and undercuts, then demolding by melting any residue that doesn't easily peel out. The drawback of melting it to demold is that cement is very porous, so some of the melted material would probably soak into the cement and affect the surface appearance (which may or may not be good). If any cement debris sticks to the mold, it will settle out when you remelt the material.

  4. Plan on experimenting

    Using cement for this kind of object is really pushing the boundaries. Assume that it will take a lot of trial and error to get everything right.

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  • many thanks for this. So I presume undercuts are just regions where if one pulls on the mold it also pulls on this undercut feature? Some examples of a mold seam would be great! Feb 25 at 8:20
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    @user9106985, yes, basically an undercut is anywhere the object is bigger than the mold opening you need to remove it through. But it's especially a problem if what catches on the mold has an angle that the mold can't easily slide off from. re: seam, imagine you wanted to make a mold of a hook. You'd embed the hook flat, half way into one side of the mold and then embed the other side in another layer of mold. When you separate the mold at that seam, there's nothing locking the hook in. Do that with undercuts so the mold separates on either side of the undercut.
    – fixer1234
    Feb 25 at 22:18

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