Introduction: How to 3D Print Molds for Small Batch Concrete Casting

Recently I took my first step into the exciting world of 3d printing. The result? I learned how to utilize basic CAD modeling and a 3d printer to create quick and precise custom molds for small batch concrete casting.

Step 1: Overview

About a month ago I visited the Annex (Instructables' old shop location in SF) to chat with shop guru Steve about what people had been making with the Objet Connex 500 3d printers. Steve showed me gadgets made with multiple materials and moving parts, crystal clear resins, a small model ship that had broken from insufficient wall thickness, and a super duper detailed character design created by somebody in the movie business. Steve explained how the resins, packaged in what looked like giant sized "ink cartridges" could be combined to create unique material characteristics (rigid, flexible, translucent, clear). He also walked me through the cleaning process, a necessary step where support material from the initial print must be removed with a combination of hand tools and pressure washing. I was surprised to find that both the support material and even the final cured resins were fairly toxic. After picking up a few 3d printed Instructables robots I was advised to wash my hands thoroughly. I also noticed that the general feel of the cured resins was somehow 'off' from the consumer plastics I was used to touching on a regular basis. 

Fast forward one month later to Instructables' new diggs at Pier 9 nearing completion and the last two weeks of my Artist in Residency quickly closing in. I had the green light from NoahW to get creative with the newly relocated armada of Objet Connex 3d printers. I was excited to finally use these crazy machines, but still wasn't sure how I would approach the material issues that I had learned about during my visit with Steve. 

I love making things that are seen and touched on a regular basis; useful things inspired by everyday life. With the toxicity and visceral quality of 3d printed materials hindering my thought process, I spoke to Randofo (friendly and helpful featured author at Instructables) about utilizing the good characteristics of 3d printing while avoiding the bad ones. He suggested I create molds. What a great idea! Making molds would allow me utilize the precision of CAD software and 3d printing while fabricating the final product(s) in a secondary material. I immediately recalled my recent desire to make small scale concrete castings...

*Photo credit: audreyobscura

Step 2: What You'll Need

In contrast to most of my instructables, which feature instructions on how to make one particular design, I chose to make this project inherently process based. My intent here is to inspire people to create their own designs using the increasingly accessible resources for CAD modeling and 3d printing.

That being said, here's what you'll need:

• Free CAD Software Application​ like Autodesk 123d Design or Autodesk Inventor Fusion
• Objet Connex 3d Printer w/ Vero and Tengo Resins
• Casting materials:

a. Duracal gypsum mix
b. 2 Medium sized buckets
c. 2 Putty knives or trowels
d. A mixing stick OR a hand drill mixer contraption
e. Mold release
f. Dust mask
g. Water

Step 3: Ideation

While thinking about the material characteristics of concrete (clean lines, minimalist aesthetic, and weight) I found inspiration in an existing product by Blue Lounge called Sumo. Sumo is a clever little product that keeps charger cables from falling off your desk or table when they are not being used. While the injection molded Sumo uses micro suction pads underneath to literally stick to a desk, I decided I would make a cord weight utilizing the characteristics of concrete to solve the same problem. A simple mockup using sticky notes helped me to envision the scale I would need to achieve a good weight in concrete. I also needed to figure out how big the slot underneath would need to be in order to allow a charging cord to slide back and forth without any trouble; hence the calipers in the photograph. I landed on a 3" x 3" x 1" form with a cord slot that tapered from .5" to .15".

Step 4: Using CAD to Model Your Object

If you've never used a CAD modeling application before, I encourage you to give 123D design a try. The pared down interface is about as warm and inviting as a CAD program can get. It also happens to be a great stepping stone to Inventor Fusion, which is what you'll need for more complex modeling capabilities.

Though I explained earlier that this instructable is not meant to inform the creation of any one specific design, I felt that including imagery of my cord weight build process would be useful to those who were new to CAD modeling. An important note to keep in mind here is that there are always multiple routes you can take to model an object. Knowing which method to use comes down to a combination of preference and your understanding of the toolsets. I tend to envision objects through an orthographic lens, so I prefer to shape my models through the use of two-dimensional sketches.

*Thanks to fellow Artist in Residence M.C. Langer for allowing me to hijack his work computer to make this Instructable!

Step 5: Creating a Mold From Your Object

You can use your modeled object to create a mold in just a few simple steps. In 123D Design this process is a matter of 1) Creating a cube slightly larger than the size of your object, 2) Positioning the cube overtop your object, and 3) Subtracting your object from inside of the cube using the "combine" tool. This process literally leaves a negative impression of your original model within the cube. 

Step 6: Printing a Flexible Mold

After you've exported your mold as a .stl file, you'll need to setup the print. This step is pretty simple with an Objet printer. Just "insert" your file onto the virtual printer tray in Objet Studio and select your material. I achieved a semi flexible material suitable for concrete casting with a combination of Vero and Tango resins. Print time will vary depending on the size of your models. I happened to be working on this project at the same time GorillazMiko was making his 3D Printed Skateboard Wheels. I think our print time ended up being somewhere around 8-10hrs together. 

Step 7: Casting

Once you've cleaned off the printer tray and removed the support material from your prints, you will be ready for casting. I tried a variety of concrete mixes including standard concrete, a patching compound, and a gypsum mix recommended by fungus amungus called Duracal. Duracal yielded the best results by far, with the added benefit of having a super quick curing time (about 1-2 hrs compared to standard 24 hrs concrete curing time). I won't go into too much detail with mixing because there are already some great instructables (below) on how to do just that. Just make sure you remember to coat your molds with mold release before pouring in your mix!!!

A great resource if you're new to concrete:

Step 8: Finishing

I added cork to the collection of desktop objects I made to prevent scratching and improve grip. While I left the concrete itself unfinished, a smoother or more polished look can also be achieved by applying concrete sealer and wax. So there you have it. My last project as an Artist in Residence at Instructables. I hope this project inspires you to rethink 3d printing and how it might be utilized to make beautiful, useful things. :)

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