Lost Wax Investment Castings 6

[JSBCintl]

What is the difference between silicon and water glass in an investment casting. The water glass method seems to be much cheaper and poorer surface finish. Secondly, what is the the Investment cast process that yields the best surface finish and closest to "near net shape" . The parts are approximately 5 in. in Dia. and valves for a pump and therefore need to be very smooth. See drawing attached.

 

[EdStainless]

With a part that size distortion is also a big factor. Your backing stucco layers will be important.
There are a number of different slurry wash systems. How the metal reacts with the shell is important also.

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Plymouth Tube

 

[arunmrao]

Selection of molding process will depend on the quantity of pieces to be produced. Your part appears ideally suited for investment casting. Water glass process to my mind is the sodium silicate/CO2 system. This is cheap system,but the surface finish is poor in comparison to investment cast part. For investment cast parts,there are 2 constraints,the die cost,other than the process cost and most foundries will need at least 10000 pieces per batch.

Getting a near net shape by investment casting is possible and a discussion with die maker and the foundry will help.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[MikeHalloran]

You need to discuss the print in detail with a founder ASAP.

I'd expect considerable difficulty filling the investment cavity without producing lots of flaws, but I'm not a founder.

Similar parts almost always extend the guide ribs all the way to the valve plate proper, without necking things down to a shank like the .900 diameter. I suspect there's a reason.


Mike Halloran
Pembroke Pines, FL, USA

 

[ornerynorsk]

It's also not just the type of ceramic used for the shell, but the number of layers. A good investment casting firm will develop the shell method without you having to be overly concerned of it. They need to be fully aware of your requirements on the front end.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[CastMetal]

Mike,

Cast parts in general, but more important in gravity fed than pressurized(diecast, centrifugal etc.). Try to avoid areas like the .900" central shaft because it restricts the flow of material during cooling and will result in large amounts of porosity in the area beyond the restriction.

Comprehension is not understanding. Understanding is not wisdom. And it is wisdom that gives us the ability to apply what we know, to our real world situations

 

[blacksmith37]

As noted : let the foundry figure out how to gate it; it may take a couple trials on the number and location of gates. I would suggest not telling the foundry what binder you want (we used ethyl silicate), or particle size of the various dip-coats. You may also want to read about investment casting vs shell molding/casting.

 

[JSBCintl]

Blacksmith37, do you believe that a shell molded casting will have better surface finish, and lower cost that investment cast?

 

[EdStainless]

My experience is that investment gave better surfaces.
With shell if you go to finer sand to improve finish you get less venting and may have more gas defects. It is a delicate balance.
With investment they can pick a particle size for the first coats to give surface and then move to coarse to give enough strength.
They can also change the mold temp at pour to control fill and solidification.
Investment will cost more but has the chance to deliver a vastly superior part.

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Plymouth Tube

 

[JoeyV87]

In answer to your original question, the best ceramic shelling material currently used commercially is a full alumina system. It costs an arm and a leg, though, and most places will settle quite happily for a zircon-based prime coat followed by as few layers as they can get away with of aluminosilicate stucco in an aluminosilicate slurry. There are other systems out there (my place uses 100% silica) but they tend to be for more specialised uses.

For surface finish the most important factor is the prime coat. Thicker slurries give better finish, as do finer stuccos. As previously pointed out, however, both these factors can lead to gas defects.

With this part in particular, it may be a lengthy development process as the part has a chunky section in the middle and no easy way of gating to it through a thick section.