I’ve always been mesmerized by sand casting.
I’m not entirely sure where the fascination comes from but I’ve been known to binge watch episodes of How It’s Made and the segments that involve molten metal pouring into forms made of sand and emerging as solid objects as the sand crumbles away has always given me delight.
Some part of the interest undoubtedly has to do with reusability which is, I would argue, close kin to repeatability. Each component of this process is reusable: the pattern (the shape that’s used to make the mould, years ago they were made from wood), the casting sand, the metal itself, reusable all.
Admittedly, sand casting is a ‘creative interpretation’ of the repeatability assignment for this week but if you think about it, from a certain perspective, it’s sort of a universal jig.
Images of Raw Materials
Nice square lines scored on a piece of square 3/4” ply, completely botched by a poorly aligned miter saw. It was stupid of me not to check the alignment and cost me a fair bit of sanding to get things sorted.
Pocket screw jig action. Just because. Love it.
Given the scale at which I’m working there was no easy way a driver could have seated properly in the screw heads had the pockets been facing inward so I took a moment to think through the order of operations and be certain pockets facing out would work. Not an especially difficult mental calculation but this was my first go with pocket screws and I was still smarting from the miter saw fiasco.
Technically this block of wood, together with the Arduino, is called the pattern.
The wood block is deeper than the intended cast. You don’t want to have to pour right up to the lip of the mould and risk molten metal spilling over. With casts made from moulds with two halves, this function is provided by a conical shaped depression called the gate, which also helps regulate the flow of molten metal as it enters the mould.
I put a bevel on the block to make it easer to pull from the sand.
Talc powder is used as a release agent to keep the casting sand from sticking to the pattern.
Apparently the wood frame is referred to as a flask. Since there’s so little space in the flask, after my first cast I started depositing sand using a thin piece of card stock torn from a mailer.
At this point the sand has been packed – or rammed – to a height that covers the pattern.
Then the rest of the flask is filled, rammed, and scraped across the frame to create a flat surface so the mould sits flat when you flip it over.
The impression from my first mould was a mess. A lot of sand broke free from the mould when I removed the pattern.
A heavier dusting of talc and a lighter touch ramming the sand produced a better result.
Due to the No Open Flame policy at ITP, I had to find someplace to cast. I ended up pouring in the back alley behind my apartment building and returning to the apartment to re-pack the mould.
I got some strange looks from the super while waiting for the elevator after my second pour so I decided to relocate production to the privacy of my bathroom. The ready availability of a shower head directly above my work area seemed like a reasonable enough safety precaution.
Once you break the mould and remove the casting, the packed sand comes out in large chunks.
A steel ruler makes for a good thin edge to chop up the casting sand and get it ready to prepare another mould.
List of Tools Used
- T Square
- Miter Saw
- Sanding Wheel
- Pocket Screw Jig
- Cordless Drill
- Soft Bristle Painter’s Brush
- Screw Driver
- Rubber Mallet
- MAPP Gas Torch
- 6” Diameter Porcelain Crucible
- Pipe Wrench Pliers
- 12” Steel Ruler
Final Image(s) of the Work
Five multiples with the cleanest cast featured prominently in the foreground.
This is was first experience casting in sand.
I would have loved to have done this project in brass or bronze, but the temperatures required to liquify those metals are significantly higher than pewter, which is predominantly tin. Even brass and bronze alloys require in the vicinity of 1500 degrees or more before they flow, as opposed to the 500 degrees or so for pewter.
Clearly sand casting is a complicated process with many variables. The casts were inconsistent, to be sure but lack of knowledge and experience aside, no doubt some of the issues were to do with the shape of the object I was using as a positive. I’m sure lost wax casting would give much better results.
The casts appear to have gotten increasingly oxidized with each pour and I have no idea what that’s about. I know the clay that acts as binder burns away and eventually breaks down over time but that seems an unlikely explanation given I poured such a small number of casts. Come to think of it, once the pewter liquified I continued to apply heat to the crucible before the pour for a longer period of time so perhaps it’s something to do with that.
All in all I’d say I got one good casting and the rest were of varying of success. One nice thing about gravity casting with a relatively shallow mould is an imperfect mould results in an excess of material which can always be removed.
Or melt them down and go again.