Is there anyone around that has experience with developing parts for sand casting? I am reverse-engineering something that was manufactured this way - I think - but I am not sure what the cast might’ve looked like.
I’d be really happy if someone might be able to take a look at the part and, if I get my act together, my model and offer insight into how it can be manufactured.
Thanks. 
No pictures? aww…
Anyways, this is as good a time as any to plug one of my favorite films
Here are pictures:
http://img261.imageshack.us/img261/9805/wicketgatecasting.jpg
http://img151.imageshack.us/img151/8462/p1010013z.jpg
It’d be nice to avoid destroying parts of the mold each time we make one of these, but they’ll be produced with low volume, so it’s not a deal breaker.
I haven’t done it…but as I understand it you need to make several molds to form the sand cores, then you need to put the sand cores together in the casting box, then pour the iron, let it cool and remove the sand.
The sand cores are not reused. The molds used to make the sand cores are reused.
Labor was inexpensive back then…
My limited understanding (electrical engineer talking about mechanical stuff alert!) is that sand casting can be done (is always done?) like investment casting, but in reverse. That guy has a lot of undercuts, which is why I’d believe they did it that way. That inner cavity around the axis looks like a nightmare for a mold that doesn’t get destroyed.
We actually did this in highschool on team 20/250. We carved a piece of foam to fit our driver’s hand, buried it and packed it in sand, and poured molten aluminum on top of it. The aluminum vaporized the foam, but left a cavity of the same shape. When it cooled, we pulled it out and sanded it. Perfection!
Are there any visible part lines?
you might try ceramic shell or some sort of investment casting instead of sand. If you can make a wax model of what you want or a rubber mold of the original you can fill with wax you may be able to do the burnout in a ceramics kiln or similar oven.
What kind of equipment do you have?
Erik is right that the undercuts that the center around the axis will be a real bear to cast. How big is this hub? The thinness of the spokes will also be an issue requiring pretty hot metal.
Of course if you are casting ferric stuff expect oxidation to be an issue. Any time you cast a very reactive metal (iron, copper etc) it is a good idea to cover the crucible with an inert gas like Argon.
YEARS ago I worked as a caster with precious metals and some brass/bronze. I can see how I would make this with the equipment I had then but all that was investment based and not too improvised.
Hope it gives some ideas
My boss seems to think that this part is either too big for investment casting or that the process is too cost prohibitive to be viable for the number of these we’ll need.
Ultimately, they’ll be produced for us by a vendor, so it’s not a matter of equipment, just a matter of design. I’m trying to faithfully recreate this part, for the most part, but I’m trying to be cognizant of any changes I make and how they’ll affect the casting process.
The undercut inside the sphere is the most challenging part, I think. We’re not entirely sure why it was manufactured that way in the first place. We considered that it might have been to keep the wall thickness as uniform as reasonably possible, but that sort of effort was not made anywhere else on the casting.
Edited to add: The part is, at its widest, about 20" in diameter and about 12" tall.
You are getting close to the max size for investment casting process. This appears to be a cast iron type part. As for how to model, typically all but the finished machined areas are cast to size. The area’s that will be machined need to have approximately .100 material added to the model to be machined after casting. For a sand casting without the fabrication of special cores (requiring more tooling and increasing the cost) the walls need to allow for the pulling out of the tooling very similar to an injection molding process. The tooling for sand casting last for long time so you can cast 1 part or hundreds. I hope this helps if you need more information either post here or pm my account.
There was a good video that I saw a long time ago about manufacturing skateboard trucks using sand casting. It was of the Independent Truck Company. I haven’t checked the internet if it is available electronically, but it showed a good example of small-scale sand casting.
There is a process that uses a process licensed by MIT for 3D Printing the cavity & Core. This process is similar to the Z-Corp 3D printer but uses sand and binder. They can make large parts with no tooling but they only get 1 part per printing of the Cope & Drag. There website is http://www.exone.com/eng/technology/x1-prometal/process_prometal.html.
My direct knowledge is limited, but here is what I believe to be accurate.
The center hub design appears to have been done that way to limit the stresses from thermally-induced dimension changes. If that were solid, the spokes would rip out of the outer shell when it cooled. The way it is done it can flex a little.
Investment casting made with an expanded polystyrene blank (in the gross shape you need) surrounded by sand could make this. Specifically called lost-foam casting, the idea is to pack sand around your styrofoam blank, fill it with hot metal (vaporizing the foam). The sand is reused, but the foam is lost. OK for low volumes and relatively coarse surface finishes. This is how things like high-end lighting fixtures are cast. The sand is re-used. Higher-end shops can make net-shape castings needing minimal machining with this process.
Sand casting uses a hard blank, the sand retains the shape as it’s pulled off the blank - you can’t make undercuts like this. You can use a foam insert for undercuts, but mixed castings like that are specialty stuff. This is how a large bell is cast. Again, the sand is re-used.
If you have selected a vendor, or find a potential vendor, ask THEM how they’d make it, especially ask if it is too large for lost-foam. It might not be.
Remember in the design to consider how the metal will get in, and how the air will get out. Usually you pour 20-30% more metal just to fill the sprues and vents. Air bubbles would be bad. Design these in from the start.
Madison,
Please send me the model to my e-mail address. I have personally designed over 40 sand castings for FANUC robots. Our volume is between 50 and 400 per year and sand casting is the most economical. Investment casting will be too much money. The biggest rule of thumb is to eliminate / reduce the number of cores required and also minimize the number of gates required. Gates are used to make sure the air entrapment is minimized.
Paul
If you can find it at a library, “Materials and Processes in Manufacturing” has a good introduction to casting processes. If sand cast, it sure looks like it will need some dry cores.