Ceramic Mold vs Good Finish

[ckrantz]

All,
I plan on venturing into casting a little more.. I've done some hobby casting of stupid stuff out of aluminum(bored on a weekend). Bad results, but i was using sand.

Now here is what Id like to do. I want to start casting rear differental covers out of aluinum. I was hoping to put together a really nice mold. At this point cost will be worth quality in the end. So what type of mold will I need to make for a great finish. Is it really possible to cast aluminum and not have a rough finish? I've seen casted aluminum and there was no machining necessary.. maybe some glass bead blasting for a smooth finish, I'm not sure how it was done. I just saw the finished part and was amazed.

Does anyone know if using a 2 part ceramic compound give me much cleaner results then if I was using casting sand? I'm starting out as hobby now, but I'd like to actually make some money off of it later once I perfect it(or at least retail quality). Also is it possible to glass bead or tumble casted aluminum to a nice smooth finish if the casting process wasn't perfect?

EDIT* I'd like to add that I would like this to be a perminate mold, not a Lost Foam or Wax setup.

I didn't realize there is a mold making section.. can a moderator move this post to the proper section?

[Brian Kidd]

Well I'm pretty sure you can get a good surface with a permanent mold but you might spend a bit $ on tooling. Investment casting will give you magnitudes better surface finish over sand casting. With investing your first dip needs to be made of real fine particulates then your first stucco will also have to be real fine. After that you can start using larger and larger grain size. The company I work for is one of the largest investment casting companies in the world, we make things anywhere from the size of a finger nail to 70+ inch diameter aircraft engine parts. You can actually make wax injection tooling by pouring epoxy up against a master to create your tool cavities. We do this quite often for temporary tooling.


B.Kidd

[ckrantz]

I will be using a finished piece to make the mold with, not a waxed or plastered piece. It is made from aluminum.. I planned on pouring the ceramic aorund it.. Does ceramic usually give better results than sand? I don't know what type of finish ceramic gives.

From what I've been reading, with ceramic I would only need to spray it with a mold release then pour my aluminum. And the ceramic can withstand up to 2000 degrees f. So I can heat up my mold to about 1400 then pour the aluminum.. hopefully it will keep from the freezing action the aluminum can do.. I'm hoping this sounds right?? maybe?

[Brian Kidd]

Well you won't be able to invest your pattern then if you are going to use a finished piece because you won't be able to pull the shell off the master without ruining the shell. That's why I suggested going with a temporary style tool where you would dam up your master where a parting line can be and pour epoxy against it for one side of the mold. After you pull that side off you would then damn it up again for the opposite side of the mold and pour against it again. That way you will then have a mold you can either inject or possibly even pour wax into for a pattern that you can melt out of the shell. Otherwise it sounds like you will only be able to go with sand casting. I know that there are many types of sands availiable and that the surface can be better than your run of the mill sand foundry so it still might yield what you are looking for. I suppose it also depends on what kind of quantity you are looking for because your molding rate would probably go up in sand casting if you only have 1 master that you can mold against 1 at a time.

Hope this helps.

B.Kidd

[ckrantz]

ahh I didn't relize that the ceramic may bond itself to the aluminum. Let me give you some kind of an idea of what the part I'm trying to cast looks like. Think of this as a Bowl and the mouth of the bowl is flat. So what I was thinking about doing was making a box out of steel or aluminum. I was going to place the mouth on the bowl on the top of the box and then then put the top back on the box.. lightly glaze the piece with release. Then pour ceramic on the inside of the box.. this will make the mold for the outside of the bowl. Then drill and tap holes on the top and run bolts on the inside.. then drill a hole in the middle and pour the ceramic on the inside of the bowl. The bolts will be used to fasten the ceramic to the top of the box.. like ribar does with concrete.

I've attatched 3 pictures. One is my "idea" for a mold box.. and the others are basically what the part looks like. Tell me if you think I'm headed for failure or possibly success.

If need be I "can" make a plaster mold first and then pour wax.. then from there I'll have a wax master. Is there a certain type of wax I can buy that is better to use for master making?

Thank you again for answering my questions. I'd like to work a day in your shoes to see what your company does and the methods behind it.

[Brian Kidd]

Sorry I've taken so long to answer. Actually with a simple parting line on the part your idea looks like it may work pretty good. It would be fairly cheap to try to see if it pans out before moving to more labor intensive methods. I got to admit I am not a casting expert, my current capacity here at work is creating CAD models to build plastic patterns on our Stereolithography machine for prototyping before we commit to a tooling design. It would really be interesting to see how well it comes out.


B.Kidd