Concrete Card Holder

with Magnet & Steel Ball


A business card holder designed by Chloe Madison (@clomads) as she was exploring concrete as a medium. It began as a 3D model that was 3D printed, molded, and then cast in concrete. It features a steel ball that interfaces with an embedded magnet to keep cards in place.
It was sold via Chloe Madison's Etsy store and then under the Voidbox brand on Amazon through FBA. We no longer have the facilities to manufacture these, but hope to someday in the future.

Source Overview

  1. 1.
    Parametric design source in Fusion 360 format - for making modifications
  2. 2.
    STL file of final design mesh - for 3D printing
  3. 3.
    Choosing filament and pre-processing for molding.
  4. 4.
    Molding process
  5. 5.
    Pourinig concrete
  6. 6.
    Finishing process
  7. 7.

Fusion 360 Parametric 3D Model

The Fusion 360 file is the original source design from Chloe Madison (@clomads). Parametric design allows you to see how it was made and make prefered changes along the way. If you just want to 3D print the model, read the next section.
Also included in this file is a box for mold making and a sphere for reference. A reference to the magnet in the production model is not made in this file, but should be assumed to be at the back of the cylindrical cavity.

STL Mesh for 3D Printing

TODO: Upload STL and post to Thingiverse & Printables
You can get various versions from the a360 link above for now. Don't print the ball unless you really want to, it's really just for reference.
Download this file if you just want to download and make the thing as it was designed. This requires a 3D printer and knowledge of slicing programs.
You're more than welcome to stop at a 3D printed version. We have a couple ourselves, but if you want to know more about how to make it out of concrete, read on.

Choosing filament and sanding prints

Before printing the 3D model you should consider that you'll likely have to sand it for the molding process. This is mainly because 3D Printing, especially FDM (most common), is an imperfect process and those imperfections should be removed before making a mold from your 3D Printed part. In our experience this means printing in ABS. While a fine layer height of 0.1mm is a nice to have, you can get away with 0.2mm no problem. The most important thing to consider is wall thickness as you may end up breaking through to infill when sanding if you don't have enough layers.
There is a lot of information on the internet about finishing parts. Choose the method that you feel most comfortable with or have access to. I personally like using super glue to fill in large gaps.
While sanding the 3D print is highly reccomended, it may not be necessary if you're making only one or two. Your goal is to make the mold as smooth as possible so sand the print and then fill any gaps with super glues and sand some more. You'll likely also end up sanding the final concrete part too, so it's up to you where you want to spend your time.

Molding process

We reccomend Mold-Star 15 Slow Platinum Silicone mold from Smooth-On, but everyone has their preferences. Make sure you have a container that can hold the liquid around your 3D print while it cures. Mix Part A & Part B thouroughly and pour carefully over your part. Make sure to reference the documents of your mold material to make sure you're doing the thing right. When the mold is cured (usually no longer that 24 hours) take your container apart and carefully remove the 3D print from the mold. You may have to use a hobby knife to clean up some areas.
I highly suggest googling or youtubeing mold making to help with your process on this step.

Pouring Concrete

Choosing your cement mix appropriately is important if you want to obtain a specific look. We've played around with various mixes and have landed on a not-scientifically-measured mix of Portland Cement, Glass Fiber Fill, and cement plasticiser. This mix gives us a smooth finish after sanding and the fiber fill creates strength and can many times be seen in the final finish as a 'whispy' design within the concrete. The Plasticiser reduces water consumption by the cement, therefore increasing strength, though this is meant to be a functional art piece and I'm going for style over durability. An interesting occurance of this mix is sometimes the appearance of spiderweb like micro-fractures across the entire surface of the object. This doesn't appear to be a detriment to the strength of the object as a whole.
Additional options for concrete mix include adding sand or stone aggregates which will significantly increase strength. Though, they do give the final product a much more rough surface that can still be sanded, but will kill all your sandpaper and have sex with its mother.
In all honesty though, you will reduce your ability to efficiently use sandpaper on concrete items with sand in their mix and will likely have to move to some sort of grinding wheel if you've added gravel... you will literally be cutting through rock. I am about to start experimenting with diamond grinding pads which are easily available on Amazon.
Let us know what your favorite concrete mix is!

Finishing Concrete

This section assumes you've decided to use our mix above of Cement, Glass Fiber, and Plasticiser.
We sand the outside surfaces with a random orbit sander at 60 grit to cut the 'skin', 220 to smooth it out, and 400 polishes it. The inside flat surfaces go through a similar grit cadence, but with an oscilating tool and triangular diamond pads. The cylindrical cavity for the ball is the hardest to sand. We're still looking for a good mechanical means to do this and have recently settled on a 1-inch dowel or pipe with sand paper around it and going at it by hand.
Feel free to direct any questions about technique, etc. to the issues tab on GitHub.