Idle Curiosity: castings with voids

I'm just wondering how they go about making a casting with a void in it. Like the inside of an axle housing, (the one-piece kind with a bubble to bolt in the differential) or the water passages in an engine block. My guess would be that they figure how much of the metal will solidify in a given amount of tine, wait that long, then drain the liquid in the center. Is that how they do it?

Reply to
B.B.
Loading thread data ...

In general, for metal castings, your guess would rate an "F"

I think there are some ceramic molding processes whhich work just the way you guessed however, but I've never heard of that technique used for metals.

The "void" you describe is filled with a "core", traditionally made from sand with a resin binder to give it enough strength to stay in one piece while the metal is poured.

Depending on the shape of the part and the core, there may be supports molded onto the core which extend through what should be solid wall surfaces in the casting. Typical of this are the engine block water passages you mentioned. The holes in the block left by the water passage core supports are usually closed off afterwards by pressed in metal plugs you may have heard referred to as "core plugs" or "freeze plugs" as if the coolant (if it doesn't have enough anti-freeze in it) freezes the expansion may push the plug out. Whether that can save the engine block from cracking in those circumstances has always been questionable to me.

The sand cores can be removed after the casting cools by vibrating or tumbling them, or even by washing them out.

There are quite a few other techniques used, such as centrifugal casting, where the mold is rotating as the metal is poured, but in general, what I just described is still the standard for cast iron parts, as well as for many other metals.

Hope this helped,

Jeff

Reply to
Jeff Wisnia

Hi B.B.

I'm not a long-time experienced foundry man, but I do have some answers because I have made a few mini-engine parts. In sand casting, you make "cores" out of sand that are replicas of the internal holes or passages that you want. These cores are typically made out of sand, formed in a mold called a core box, and held to together to keep it from crumbling using lots of different methods. The core is typically held in place by the sand patterns that are left in the cope and the drag (top and bottom of the sand mold). Some methods of gluing together sand to make the core, include a chemical called water-glass, mixed into the sand, which really hardens when gassed with CO2. Other cores are made out of binders that are 2-part chemicals that cure and harden and hold the sand in the shape you need. Or there's green sand, which is just moistened sand, or I have even heard of sand mixed with molasis and baked to harden it a bit.

After you pour the metal, the inner core of sand is chipped away and you have a hollow part.

In the lost wax casting, especially the solid mold kind (as opposed to the thin shell method) where you surround your wax pattern with something like Plaster of Paris, this soupy investment mixture seeps into all of the cavities including the hollowed out inside of the wax pattern (assuming that you've created a hole for the plaster to enter the inside of the hollow wax pattern).

Then you melt out the wax, fire the plaster to drive out the chemically bound water (so you don't create a steamy explosion when you pour in the molten metal) and you are left with a hollow plaster shell that will now accept the metal to be poured into it. The metal cools, and you chip away, or water blast away the outer and inner plaster shell. Removing an inner core of hard plaster can be tricky, especially if you have a long thin tube of plaster used to create a coolant passage or something like that.

Look up sand casting cores, binders, water-glass (sodium silicate I think) and then do a similar search on solid mold investment casting. Should help.

This is a very early intro to cores in castings. But I am pretty sure that foundries don't routinely pour out metal as it's hardening, thereby creating a hollow part. But it's not a bad idea, and I am NOT absolutely sure that's it's not done.

Have fun, Tom.

Reply to
Tom Kay

Jeff Wisnia wrote: In general, for metal castings, your guess would rate an "F" (clip) ^^^^^^^^^^^^^^ Jeff, let's give him at least a D+. I believe some small lead or pot metal castings can be done that way. Toy soldiers or maybe cheap chess pieces, which are open on the bottom, look to me like they are poured into a cold mold and then dumped.

Reply to
Leo Lichtman

Slush casting. 95% Zn, 5% Al is one alloy made specifically for the process. It's used mostly to make decorative items, and the molds typically are metal or high-temperature mold silicone. Lamp bases and candlestick holders are two examples often cited by people who make the alloys.

But these are very minor examples, and, from his engine-block example, the original poster wants to know the general processes. Inserted coring, as has been suggested, with cores usually made of sand with a crushable binder, so the metal shrinkage around the core doesn't cause hot-tears or cracks in the final casting.

Ed Huntress

Reply to
Ed Huntress

Right you are.

If you are still reading this thread B.B., here's a magnificent series of photos of some dedicated guys in Australia making their own aluminum Austin Healey engine blocks. It answers your question quite well.

I was damn impressed with the quality look of their wooden patterns. And I learned some things I'd never heard of before, like "filters" right in the mold to strain the molten metal and also painting portions of the cores to keep them from outgassing into the liquid metal. See:

formatting link
Jeff

Reply to
Jeff Wisnia
[...]

Hey, thanks! That's exactly what I wanted to find out. Now I just wonder what those filters are made from.

Reply to
B.B.

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.