The subject of this instructable is a Dodecahedral Concrete Planter cast on a 3D printed Mold. The aim for this guide is to be a reference for producing small Concrete Objects at home through the use of 3d printing as a means to manufacture the molds.
The idea is not only to manufacture a dodecahedral concrete planter, but to give a basic understanding of mold logic and 3D printed Mold finishing, as wuell as concrete chemistry/physics fundamentals on a level that allows for the design of effective molds and manufacturing of durable concrete objects with ease.
This instructable is on the Casting contest and on the Planter Contest, so, if you like it, please vote for it!
The Solidworks and STL files of this mold are available on this step
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Step 1: Mold Logic
Molds are a great way of replicating a shape consistently on a great variety of materials and, if done well, both the mold and the casts can be cheap, durable and strong. On this track, what better match than concrete and 3D printing, as concrete is very cheap and durable, and 3D printing allows for manufacturing of very complex shapes with almost no waste or manual labour.
All this being said, a mold can also be a great way to waste time and effort if designed improperly. Molds can be used to make very complex shapes, but we have to keep in mind at all times that the demolding process is limited by geometric factors, such as interlocking shapes, and physical factors, such as vacuum and friction.
Because of this, not all shapes are moldable and, when designing a mold, we have to ensure that, once the cast is set within the mold, there are no interlocking shapes between the cast and the mold, as well as very steep walls which would cause a lot of friction and break the cast upon demolding.
Therefore, as a general rule, we must ensure that all shapes have a positive angle with regards to the opening direction of the mold, of at least 3-5º.
Example: Dodecahedral Concrete Planter
A dodecahedron planter is a good example for understanding these principles of mold logic, as the angles are very pronounced and clear.
First off, it is evident that if the mold were a single part, the cast wouldn't be removable, as all the bottom faces would have a negative angle with regards to the demolding direction. therefore, this mould must be a 2 part mold. More complex shapes may require more parts.
Next, we can see that, even if we split the mold in half, there would be negative angles preventing it from opening, so the parting line must follow the outer perimeter.
With these 2 conditions, we can successfully mold a dodecahedron, but the next step is the cavity.
Evidently, the cavity cannot follow the shape of the exterior, as the mold would remain locked inside the cast, so it must be straight, but, even at 90º, the friction/vacuum between the cast and the mold would prevent it from demolding, eventhough there are no geometrical locks.
Because of this, the cavity must be an extrusion with an inward draft angle of at least 3º, more recommendably 5º.
After all this, I hope it is clear why the mold has the specific shape it has, and how to recognize which shapes are moldable and in which directions.
Step 2: Preparing the 3D Printed Mold for Casting
After printing both pieces, we must smooth the surface and prepare it for casting, so that the cast doesn't look like a 3d print.
The finishing process is very simple; first, sand the inner surfaces with a coarse sandpaper around 100 grit and apply a generous coat of spray Primer. Repeat this process 2 or 3 times with finer sandpaper untill you reach the desired smoothness.
To prepare for casting, apply a generous coat of vaseline to all the interior, ensuring there are no bulky remains, and close the mold. Next apply more vaseline along the outside seam to prevent leaks.
Step 3: Concrete Basics
Concrete is a very strong and durable composite material, composed mainly of 3 components:
1. Binder -> Cement
2. Coarse Aggregate -> Gravel
3. Fine Aggregate -> Sand
The binder is the element that sticks everything together, it is used in the smallest proportion, and is the one that actually cures.
The aggregates are inert, and, once glued together by the binder, are what forms rigid interlocking shapes that give the concrete it's strength.
The general proportion for concrete is 1Cement : 2Sand : 3Gravel : 0.5Water
To prepare concrete, the dry ingredients must be thoroughly mixed together first, and then the water slowly added in last.
Eventhough the concrete is prepared with water, it doesn't dry, but rather cure, in a process called hydration in which the cement chemically combines with the water. Because of this, it is preferrable that the concrete does not dry during the curing process and, in fact, it actually cures stronger underwater.
Cement, however, doesn't require much water to properly cure, and the desired consistency of a good concrete is a thick paste and NOT a runny liquid, as this would result in extremely brittle concrete.
Because of this, cement cannot simply be poured into a mould, as it wouldn't properly flow in. In order for it to flow, it must me vibrated. This will also allow any bubbles to come out.
For the case of small molds, I made a clamp with a DC motor and an offset weight, which allows me to easily vibrate and cast concrete into the molds. Such a tool is not mandatory, and it is possible to cast concrete with only manual aggitation, but it is extremely useful and practical.
Step 4: Casting the Planters
Now that you know all the basics of simple molds and concrete as a material, you are ready to cast your first planter.
For this I have prepared a timelapse of me casting a planter, so that you can easily see the process, as well as the proper consistency of the concrete.
For small concrete objects, the proportions I like to use are 1Cement : 2 fine aggregate : 2 coarse aggregate.
To determine how much concrete to make, you must first determine the volume of the mold. For this, fill it up with water and pour this into a plastic container.
You must prepare around 1.3 times the volume in dry powder, and mix it with around 0.4 times the volume in water.
For my cast I used Marble aggregates, and I also added some black concrete colorant, for a spatial look.
I hope this instructable encourages you to experiment with concrete casting and, if so, please share you results down below.
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