Casting Thor's Hammer
Project Background
In the summer of 2014, just before I began my college education, I went to an event in Texas at which the CEO of the company hosting the event was gifted a replica of Thor’s hammer, Mjolnir, by a fan who had made it himself. I was so impressed by the craftsmanship that had gone into it that I began thinking about the ways that I could go about making my own. It very quickly became the main project in my mind to create a mold positive of the hammer head from wax and use the lost wax casting method to cast the head from aluminum.
I had been watching the “King of Random” YouTube channel around that time, and they had just recently prepared a video series on constructing a soft-metal-melting foundry from Plaster of Paris and sand. They were able to melt large amounts of aluminum in a crucible made from a fire extinguisher, which I knew would be necessary to form my own ~28-pound hammer, so I built my own foundry over the next year or so.
The process of forming the wax head was extremely drawn out and even now is not 100% complete. I tested the cheapest idea first, forming a single silicone mold around a prop hammer and attempting to fill it with wax. When that failed to produce an aesthetically pleasing wax positive, I instead constructed the hammer from hand-made sections of candle wax which was the only wax that I had available to me at the time. By carefully heating and melting together the edges of the handmade components, I was able to create a highly detailed version of the hammer head modeled after the prop used in the Marvel movie franchise.
The next step in the plan, which has been on hold for years due to not having anywhere available in college to safely operate my foundry, is to move forward with the molding and casting, converting the wax positive into a negative (cavity) within a plaster investment and then pouring in molten aluminum. A small amount of drilling and boring will need to be performed on a mill to carve out the location where the handle will attach to the head. Following that I can work on the internal tapping using a standard-thread tap leaving only fine-detail filing, attaching the handle, and adorning it with woven leather straps and a brass end-cap that I will form through 3D printing and brass casting. I plan to display the hammer but never hit anything with it as aluminum is soft, and I mostly intend for it to be a fun 28-pound prop than a practical hammer.
I have had a lot of fun so far with learning to cast, modeling the hammer in Creo Parametric and animating it, and spending large amounts of time meticulously forming the wax positive and correcting errors in its surfaces. I look forward to finally firing up the foundry and melting the three thousand aluminum cans that I have stored, despite soda cans not being made from the best aluminum for casting, as well as the other scrap aluminum that I have collected.
Detailed Description
When I began modeling the Treasure Cube, I wished to improve as many of the official Treasure Cube’s shortcomings as possible to decrease the chance that my cube could be beaten by any method of opening it, through brute force or otherwise, besides solving it. Normal cubes are weak enough, with high stress concentrations occurring at internal corners that appear everywhere within the structure, but the design for the Treasure Cube does have a bit more structure to it than the conventional cube model. It supports each piece with a track rather than through the center faces being attached to a six-axis core, which leaves the entire center cavity completely open and available as a container in which to store items.
Modeling the hammer in Creo took a few hours to do with the help of images that I had taken of the surface patterns shaped into a cheap plastic prop of the hammer that I had purchased online. I traced out the detailed sections using splines and extruded them to give them dimension. The end-cap adornment required the use of modeling techniques that I had never used before, though, when I created it in 2017. I had to trace a front perspective view of the patterns onto a plane then project or “wrap” them onto a cylindrical surface to be radially extruded. I was able to apply that same method in my college senior design project, so it ended up being a useful tool to be able to navigate.
I had planned from the beginning to cast the aluminum hammer head directly around the handle to hold it permanently in place which would require the handle to be integrated into the wax positive and then the plaster investment. I later decided against this after becoming more familiar with thermal stresses and the threat that they pose to the integrity of my mold while the aluminum sits and solidifies. The expansion of the steel within the plaster could easily form a crack and ruin the entire mold. Furthermore, I deemed it a better idea to instead make the handle removable and attach to the hammer head via a threaded hole in its underside.
When not intrusive or excessive, I have found that it is usually better in the long run to design things with future maintenance in mind. A reasonable number of removable fasteners are the best route, when possible. If I decide to adjust anything or add any components to the handle, I will have a much easier time doing so without the handle being permanently fixed to the hammer head.
After the final detail touch-ups have been made, I can prepare the wax positive with extra wax sections called sprues which will leave behind tunnels in the plaster allowing air to flow out of the mold and metal into the mold during the pouring process. These sprues are to be attached at a 45-degree angle to the bottom face and a side face of the hammer head, ultimately parallel to each other, before the entire structure can be covered in a thin layer of liquidized investment powder. When the powder hardens into a thin plaster layer around the wax, producing an airtight shell, the entire wax head can be dipped into the liquid plaster (either after it has been de-gassed on a shake table or in a vacuum chamber) repeatedly until a plaster shell of sufficient thickness has formed. The shell captures the exact print of the wax and leaves a void in that exact shape when the wax is melted out.
The plaster investment is required to be fired at a high temperature to burn off the wax within it and set the plaster, baking it into a more solid structure completely devoid of water. If the plaster negative is formed without first removing as much air as possible from the liquid plaster, via vacuum chamber or shake table, the air bubbles have a high chance of expanding to the point of exploding the mold while bakes.
No other tasks remain once the mold is fired except for pouring molten aluminum into it which I plan to melt using the propane torch that I built and a standard 20-pound propane tank. The torch quickly heats the inside of the foundry when it’s closed to a few thousand degrees Fahrenheit. Pure aluminum melts at a fairly low 1220 degrees Fahrenheit, or 660 C, which is well within the temperature range that my propane torch can provide.
I submerged my plastic prop hammer head into water and measured the amount of water that was displaced to get an accurate volume measurement, and I found that nearly one gallon of water was displaced. My Creo model, whose dimensions are based on those of the prop, reports that the volume is 0.761 gallons, which is less than expected value based on my water test. Taking sprues into account and adding another 20-30% to give room for error, my plan is to melt slightly over a gallon of aluminum to be poured all at once into the mold.
To give the plaster mold more resistance to the pressures that it will be subjected to, I plan either to surround it with greensand (for which I have the components to make a batch but haven’t yet gotten around to actually mixing them) or slightly damp play sand. The mass and pressure of the surrounding sand should balance well with the liquid pressure inside of the mold from the aluminum being poured. The thicker the plaster section is, though, the less likely that liquid pressure will end up causing any problems.
I only get one chance with the wax positive that I made, so if the first casting attempt fails at any point, I have to start at the beginning and remake the wax hammer head. This is the other reason that I haven’t yet even formed the plaster negative, because I want to be sure that I have everything correct before going through with it in case I’m missing something or decide to take larger precautions. My crucible can hold slightly more than two gallons of metal, though I never expect to melt that much at one time due to the dangers that it presents, its large weight of around 50 pounds, and the fact that this is the largest casting project that I can currently dream up.
I expect that within the period of 2019 through 2020 I will have a good location to operate my foundry and the time to actually perform all of the steps involved in casting the hammer. The project has been drawn out for four years so far while I learned material science and important casting tips, techniques and facts. I look forward to getting it done both for the learning experience and the final product, then finally moving on to new casting project ideas.