8. On Investment Casting, Part 1

(This one turned out very very long. Turns out carefully describing a workflow for a general audience takes a lot of words! It will thus appear in two parts.)

Investment casting is a manufacturing method for shaping metal into a desired form with a high degree of precision. In brief, the method involves starting with a replica of some desired form made out of a sculptable and relatively flammable material like wax or PLA, putting the form in a metal tube and surrounding it with a highly heat-resistant but brittle material that captures form well like plaster or water glass, and putting the metal tube in a furnace to completely destroy the flammable material so as to leave behind negative space. Then, one pours molten metal into a space left in the top of the tube, waits for the metal to cool somewhat and solidify, and finally plunges the whole tube into water to finish the cooling process and destroy the hard outer layer to recover the cast metal. Lost-wax casting is a particularly longstanding form of investment casting where the flammable and moldable material is wax, especially beeswax, and the heat-resistant outer layer can be some combination of fine silica sand - dry or wet, lime - the calcium compounds, not the fruit, and gypsum - also known as plaster of Paris.

I have used investment casting in the past to make highly detailed coins from 3d models, pins from sprigs of plant matter, and on one occasion, a large seal stamp from a combination of my own 3d-printed design and hand-molded wax. Despite my relative lack of expertise, the finished products capture extreme amounts of detail and to the untrained or inattentive eye are often indistinguishable from professionally-cast products. I will describe my workflow in detail as I undertook it during 2017~2019 at Pumping Station One, an excellent makerspace in the North Side of Chicago, with deep thanks to my good friend SN for having encouraged my joining the space, to AA who ran the Small Metals area at the time and whose expertise and aid were extensive and extremely valuable, and to DLS, who frequently assisted in pours and gave excellent advice.

First, come up with an idea for a finished product. Without an idea, there will be no metal object. Here are some ideas, some of which I saw to completion, others which I heard of others trying, and some which I never got around to: a game piece, a piece of jewelry like a ring or a pin or a pendant, a coin, a seal stamp, cosplay items like buckles or Star Wars money, insects and chunks of hive, a chopstick rest, or a sauce dish. Factors that might make an idea especially suitable might include a desire for repeatability and multiple copies - especially if they are small and multiple of them can be cast at a time, a desire to carefully capture precise details of an object, and feasibility of wax form construction.

At this stage you should also pick a metal or alloy that you want the final object to be made from. The most popular choices are copper and its alloys, bronze and brass. This is primarily due to their cheapness, relatively low melting point, and easy sourcing. Fine silver is another major choice, though sterling silver is more popular for durability and cost and can be made by adding 2.5% copper - or, practically speaking, bronze scraps or especially 1947~1982 pennies - by mass to 97.5% silver, a ratio of 3:37. Aluminum, easily obtained from drink cans, is yet another standard choice, though the problems with oxidation during the pour are much worse with aluminum than with other metals. For more committed choices, gold and its assorted alloys formed by addition of silver or copper are suitable choices, especially for jewelry like wedding rings; for a less expensive option I used heavily, the addition of ~1% germanium by mass to a sterling silver alloy results in an alloy called argentium silver, whose germanium content forms a passivating layer slowing tarnishing and improving the whiteness of the silver. On the other hand, iron and its alloys melt at too high a temperature to be suitable, and likewise for platinum and palladium; zinc and lead are tempting choices due to their low cost and melting point, but both are highly toxic in vapor form and should be avoided.

The size of the object is even more important: you will be constrained by the size of the metal tubes available to you, called flasks, and by the cost of the metal, if your choice of metal is a relatively expensive one. The practicality is by far the more important consideration, though if you’re casting in silver or especially gold, cost will quickly become an issue - though silver is much less expensive than you might initially expect.

Next comes making the flammable positive form. It’s not necessary that the positive be literally flammable, just that it melt or burn out cleanly under the high heat of a furnace. It should also hold fine detail well. Some suitable materials include various waxes, wood and other organic matter (e.g. sprigs of rosemary or dead insects), polylactic acid (PLA), and specialty wax-based resins for use in stereolithography (SLA) printers. The last two cases indicate the use of a 3d printer of some kind, be it SLA or FDM (fused deposition modeling), while the former cases generally indicate shaping by hand or taking directly from the environment. That said, you may want to make or order a silicone mold if you have access to a wax injector and intend to make many copies of the same object. If you want, you can even combine multiple construction methods: for example, you can dip vegetation in wax to fix its form, or use molding wax to sculpt a grip for an otherwise 3d-printed seal stamp.

After that comes flask assembly, including surrounding - investing - the positive form with the heat-resistant brittle material. You will want additional molding wax on hand, both hard wax in stick form of varying diameters and softer molding wax that can be softened by the heat from hands; to work the material, I recommend the kind of soldering iron-like “wax welder” I had access to. Use a large-diameter wax stick as the main sprue line, connecting the depression in the rubber bottom of the flask to the positive form above it. Mound up soft wax around it to hold it in place and to form what will be a funnel after the burnout, and use a combination of thinner rods and softened wax to join the sprues to your positive form. Depending on the form, you may want to sprue it in multiple places, and to orient the positive form to be somewhat tilted from the vertical, to comfortably fit the flask, allow easy dislodging of any bubbles that might form in the plaster, and especially to promote smooth flow of molten metal. Spray the whole assembly of wax and positive thoroughly with debubbling solution, which can be made from a mix of half water, half rubbing alcohol, and a few drops of dish soap. Carefully seat the flask on the rubber base; the positive should be horizontally centered in the flask, no less than ¼“ from the flask wall and no less than ½” from the flask top, which will become the bottom; the extra space is to avoid having molten metal break through the plaster and spill out.

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