Introduction

For many, the ability to print out nearly any object one could imagine from the comfort of their home is equal parts astonishing to inspiring. Now imagine if you could transform those relatively robust plastic creations into rigid long-lasting masterpieces. Well, I’m here to tell you that the process is surprisingly easy, and I’ll help guide you along every step of the way. Let’s get right into it, starting with the process overview, which details the key aspects of converting plastic 3D printed components into their concrete counterparts.

CAD Modeling: Turning the concept of your project into a three-dimensional object.

3D Printing: Converting the STL file into a tangible part, via additive manufacturing. 

Silicone Molding: Casting a negative silicone mold of the 3D printed parts.

Concrete Casting: Filling the negative silicone mold with concrete. 

Decorating: Arranging modular planters and adding decorative plants and materials.

How to Video

How to Video: https://youtu.be/40XSeIGZyiU

Concrete Planters

Project Introduction

When I started this project, I wanted to design a modular planter that was able to be quickly configured into different arrangements. The design would have to be relatively minimalistic to ensure ease of 3D printing, silicon molding, and concrete casting. In addition to these design constraints, I wanted certain components to be stackable, adding another level of modularity to the system. 

Design Considerations

When I started the design of this project I wanted to have the ability to quickly rearrange the planters and pavers to create different layouts. I achieved this modularity by sub-dividing the base shape which was an elongated hexagon into 3 unique shapes. The main shape is a pentagon, which has three different configurations, depending on the application of that component. For example, if you wanted to create a vertical stack up, you would start with a base planter and then add as many hollow planters as you would like. Because these shapes are derived from the base elongated hexagon, they can be added in a repeating pattern to create new and unique shapes.

Figure 1.0: Base Shape constructed using four hexagon pavers.

Figure 1.1: Two examples of the different configurations that are possible with the modular components.

Bill of Materials

• PLA Filament
• Mold Star 30 Silicone
• Quikcrete Mix
• Potting Soil
• Decorative Filler Rocks
• Decorative Moss
• Succulents
• Hot Glue Gun
• 3D Printer
• 330-Grit Sandpaper
• 220-Grit Sandpaper
• Plastic Sheet
• Mixing Bowl
• Mixing Stick
  

CAD Modeling

The first step is turning the concept of your project into a three-dimensional version, which will allow us to export as an STL. This STL can later be processed by the slicing software, and we will get into that in the next step. 

Slicing and 3D Printing

Before we can begin printing, we need to first convert our STL file into G Code. This is carried out by the software Snapmaker Luban, which “slices” the models into thin layers and creates tool paths in the form of G Code, allowing the printer to interpret the data and turn the compilation of individual layers into a printable part. For a more detailed explanation of slicing and G Code, check out this article: Slicing and G Code: The Bridge Between 3D Model and 3D Printer. 

Figure 1.2: Slicing the hollow planter component in software Snapmaker Luban.

Post Processing

Once the printing process is completed, we can begin post-processing the components. Due to the geometry, most of the parts don’t need support material during printing and therefore most of the surfaces on the print are smooth. The only exception to this was the hollow planter, which could be printed in an orientation that would allow for no support material. However, I wanted the parts all to be oriented the same to ensure the fillet on the top surfaces was consistent. I started by sanding the parts using 220 grit sandpaper, this removed any major surface blemishes and gave the parts a relatively smooth finish. From there I applied filler primer to all the parts and let them dry. The filler primer did an excellent job filling in any small voids or gaps between layer lines. From there I sanded the parts once again, this time using 330 grit sandpaper, resulting in an almost perfectly smooth surface finish. Although sometimes tedious post-processing these parts is a key step in this project to ensure a high-quality finished product. Since the silicon mold will capture any small blemishes in the part, which will in turn be reflected in the concrete casted parts. 

Figure 1.3: Sanding filler primed hexagon paver with 330 grit sandpaper.

Mold Setup

Since I used a trial bottle of Mold Star 30 silicone I had to optimize the shape of the mold shell. I started by arranging the components I planned to cast in CAD and then designed the shell around them. Making sure to leave enough space between components to ensure the silicon mold hard rigid enough walls to support the concrete during curing. The mold shell was also 3D printed although, I didn’t post-process it at all since the surface finish on the perimeter of the silicone mold wasn’t important. The next step was to hot glue the post-processed prints to a base, I used a plastic sheet from an old picture frame. Then I placed the mold shell around those components and hot glued it as well. It’s important to make sure there are no gaps between the mold shell and the base, otherwise, the silicone will slowly leak out since it’s not very viscous. I didn’t use any mold release spray for this project, but you could use some if you wanted to ensure easy removal of the 3D-printed parts once the silicone cured. In my case, I was able to remove the parts with minimal work even without any release spray. 

Figure 1.4: All components and mold shell layed out and hot glued to plastic base.

Mixing Silicone

There were a few reasons I chose Mold Star 30 silicone for this project. The primary reason was that this specific silicone rubber doesn’t require a vacuum for degassing and has a low viscosity, which allows it to easily flow into small areas. To prep, the platinum silicones are mixed 1A:1B by volume, meaning no scale is necessary. I chose to pour each bottle into a common container and mixed them until a homogenous solution was achieved. Once the silicone was ready to be poured I slowly began filling the Mold shell, making sure to fill in all the little channels between the components and the shell. The last step in the silicone molding process was to agitate the silicone. This is a very important step, which removes any trapped oxygen bubbles within the silicone. If bubbles aren’t properly evacuated from the silicone, it will likely impact the surface finish on your final concrete parts once you cast the silicone mold. This could have been avoided had I used a vacuum chamber to degas the silicone, although I didn’t have one at my disposal. In the end, the features that were generated in the concrete parts due to residual trapped air bubbles ended up being a surface feature I came to appreciate and enjoy in the parts.   

Figure 1.5: Filling the mold shell with Mold Star 30 silicone. 

Component Removal

I started the component removal process by pealing away the flexible plastic base followed by pressing the silicone through the mold shell, leaving just the silicone mold with the entrapped 3D printed parts. From there it was as simple as pressing the 3D printed parts out since most parts were glued directly to the plastic base. Except for one part, which required me to add relief cuts to the silicon mold. These cuts would prove to be extremely useful once it came time to remove the concrete cast parts.

Figure 1.6: Silicone mold, once all 3D printed parts have been successfully removed.

Mixing Concrete

There is an expansive list of potential concrete mixes one could use for a casting operation like this. I opted for the Quikcrete sand/topping mix since the powder was relatively fine and I was able to easily source it from my local HomeDepot. The first step in mixing the concrete was to pour an appropriate amount of concrete powder into a mixing bowl, then added water. The general rule of thumb is to get the concrete to a consistency similar to that of oatmeal. You want the concrete to be wet enough to flow into the silicone mold nicely, but not too wet. When concrete is mixed with too much water it dries weaker, overly porous, and is more prone to cracking. 

Figure 1.7: Concrete mixing process, just before adding water and stirring the mixture.

Concrete Casting

When casting the silicone mold with concrete, it’s important to pour slowly. This helps to limit the number of air bubbles that are trapped within the mold. To aid in the evacuation of trapped air, I once again agitated the silicone mold by vibrating it. This brought the vast majority of air bubbles to the surface, although there were still some trapped lingering bubbles. These air pockets ended up giving a unique surface finish on the final parts and became something I liked.    

Figure 1.8: Carefully filling the silicone mold with the concrete mixture.

Post Processing

Once the concrete had dried completely I removed the parts from the silicone mold. The next step was to sand down the small amount of “flash” on the bottom surface of the parts. This is caused by the silicone mold not being filled all the way with concrete. When this happens the concrete experiences capillary action, which causes the concrete to stick and rise on the perimeters of the mold. Additionally, I sanded a few regions on the hollow planter parts to ensure they would have a slip fit with other components when stacked.   

Figure 1.9: Sanding concrete hollow planter with 220 grit sandpaper block.

Decorating

With our post-processed components completed, it's time for the fun part. I started by laying out an arrangement that I liked. From there I decided which areas would have plants and soil, and the areas which would only have rocks and moss. The stack-ups of the hollow planters were filled with soil, to help provide nutrients for the succulents I planted within them. As a finishing touch, I added some small rocks around the succulents and filled a few other areas with rocks and moss. I opted for a succulent and moss-themed design, but there are many other styles one could choose. For example, a desert-themed planter with cactus and sand.  

Figure 2.0: Adding small river rocks to cover the soil surrounding the succulents.

Finished Product

Alternate Applications

The process of turning 3D printed parts into a silicone mold and then using it to cast in another material could be applied to a wide range of projects. Although the complexity of these 3D printed parts is somewhat limited, there are design choices that can be made to overcome these limitations. Additionally, the material you choose to cast with can vary as well, for example, you could replicate this project but use chocolate instead of concrete. The possibilities are nearly endless. If you’re interested in trying out this manufacturing process but aren’t sure where to start, here are some project ideas to get you started:  

• Concrete Vases
• Desk Organizers
• Concrete Coasters
• Candle Holders
• Phone Stands
• Business Card Holder
  

Conclusion

At first glance, it probably seemed like a daunting task to produce a consumer-quality concrete component from a few 3D printed parts and some silicone. But once the manufacturing process is broken down into its fundamental steps, it becomes a much easier project to comprehend. Hopefully, as you’ve progressed through this article you’ve developed a better understanding of how to turn 3D printed parts into concrete masterpieces and found some inspiration for your own DIY projects along the way.

Now go out there and Make Something Wonderful！