Sand Cast Engagement Rings

February 2018   Bianchini-Love

Project Documents

This is a special project that we are excited to present. During our last semester as undergraduates at MIT, we set out to create our own engagement rings. While the final rings are made of platinum, a pair of bronze and stainless steel rings were also made as part of the experimental process. In addition to the rings, we also created a wooden box to protect and house the rings when not on a finger. Built in the Course 3 foundry at MIT, the final product is a unique set of rings that have a special meaning to both of us.

Process overview

In the video below, you'll get an overview of our process in the form of just photos and video snippets.  We will go into more detail in the sections following.

We are both fans of old things that look like how they were made.  Since Bibit was using the foundry at MIT to work on a larger sand casting project in the fall of 2017, she got us insider access to the MIT Course 3 foundry in the basement of the Infinite Corridor.  Our tastes plus this unique resource led us to make sand cast rings.

Our material choice was platinum due to its hypoallergenic and robust properties, though not for its availability and associated cost.  Because it would have been infeasible for us to get enough platinum to fill a large sand casting sprue and runner system, we developed a way to achieve a sand cast look through investment casting a form that was replicated from a sand cast ring.  More on this later.

The process we created in order to achieve the sand cast look in platinum rings.  Note:  there is a non-fuzzy version of this uploaded as "process_diagram.pdf" that you can view instead for more of the detail -- see the file listed under "Project Documents".

Test rings

Part 1 is the test rings.  This section will highlight each of these steps.

To determine what size we needed and what shape we liked, we first made CAD models of different ring profiles and sizes.  We then 3D printed several of these to test their look and fit to our actual fingers.  We were able to 3D print these for free using Ultimaker 2 3D printers in MIT MakerLodge.

CAD model of a test ring sizer.

We incorporated a size label on each to make things easier.  Note that the ring size is a letter, indicating that it is a UK size.  This is because we found we could get a free ring sizer from the British company Wedding Rings Direct.  We used this sizer to get an idea of what sizes we would actually print out and test.

Bibit wearing one of our test ring size pieces.

This was a very iterative process -- look at how many different sizes we tested!

Sand casting

Part 2 is sand casting.

We began our pursuit for "sand cast" platinum rings by sand casting bronze rings.  These bronze rings would later be replicated in platinum to create our final rings.

Making a custom sand casting flask

The typical flasks that are used for sand casting in the MIT Foundry are much too large compared to the size of a ring.  Thus, we made our own sand casting flask out of aluminum tubing, aluminum L extrusion, and some standard fasteners.  We decided aluminum was fine for our purposes because we were expecting to cast only up to a few times using them.  If we wanted to use these flasks for higher orders of magnitude, we would have used a metal with a higher melting temperature.

The original idea (left) and actual construction (right) of our small sand casting flask.

The original design featured bolts that would apply clamping pressure between the cope and drag halves.  After constructing the flask in this manner, we noticed that this method did not adequately locate the two halves consistently.  Thus we gave up some clamping pressure in order to mount the two halves in a way that would guarantee better alignment.  This compromise was okay because the rings are small enough to not require high clamping pressure.  If we had experienced significant flash on our cast parts, we would have reconsidered this strategy (however, it was not a problem).

3D printing the sand casting pattern

To first begin the sand casting process, we needed to create patterns.  After selecting the size that fit our fingers best, we made a CAD model of a pattern that incorporated both Henry and Bibit's rings as well as gates and runners.  We additionally added thickness towards the finger hole so that the as-cast rough sand finish could be polished during post-processing.  We left the outside wall the same because we wanted the rough finish on the outside.

The CAD model of the pattern (left) and the 3D printed patterns (right).

We ended up using this right angle between the rings because it provided a more direct flow of bronze into each ring.  We first used a different runner geometry and did not achieve proper filling of our rings.  This redesign (as well as a pouring temperature increase) would allow us to get a successful pour.

Pouring

We packed two of our dual-ring patterns so that we would have two attempts per ring in each pour.  Our first pour attempt featured the patterns with straight runners (not pictured above).  As mentioned, it was unsuccessful in that the ring cavities did not fill entirely.

Our first unsuccessful pouring attempt.  Only 1 out of the 4 ring cavities filled completely, and 2 of them filled not at all.

We then knew we had to make some changes to get a successful pour.  With the redesign of the gates and runners as well as pouring at a higher temperature, we were able to get 4 successful rings out of our second attempt.

Henry cuts out a funnel for the sprue.

The bronze ingots were melted in an induction furnace (left).  The second pour showed much lower viscosity than the first pour (right).

Henry opened up the flask after the bronze had time to cool (approximately 15 minutes since the parts were so small) (left) to discover that all 4 ring cavities had successfully filled (right)!

Post-finishing

Post-finishing for the bronze rings included sawing off the runners, filing the gates, sanding the insides of the rings until they fit each of our fingers, and polishing the insides.  We did all of these steps in MIT MakerWorkshop.

First we filed the roughest portions out of the insides of the rings.  We did this first so we could hold the ring for easy access using the sprue and runners.