Draft for Lost Wax Investment Casting? 1

[AXNRXN]

Hey All. Not sure what category this should go in, so I'd start here.

I'm working on an investment cast stainless marine product. I'm not that familiar with investment casting. It seems to me that no draft is required on the investment cast itself due to the ceramic mold being broken or vibrated off after the casting process...any undercuts would just crumble off. However, must of the wax patterns used to make the casting are formed in an injection molding process. So, in order to pop the wax pattern out of the mold, a standard draft angle would be required.

Do I have this correct? Does the wax pattern need a draft angle? Or, does the wax shrink enough in the mold that it would pop out anyway?

Thanks for any clarification!

 

[MikeHalloran]

You need to talk to the person who will make the molds for the preforms.
Sometimes they can make the mold such that no draft is required.
More often than that, they will put the parting line somewhere other than you would, so if you guess and put the draft in your model or drawing, it's probably going the wrong way anyway.


Mike Halloran
Pembroke Pines, FL, USA

 

[drawoh]

AXNRXN,

Talk to your fabricator.

Most investment casting patterns are made in aluminium dies. When I specified one, I was told that the draft would be so slight, I would not see it. I was told to not call one up.

If your foundry disagrees with this, they are right.

--
JHG

 

[3DDave]

You haven't accounted for the amount of shrink the wax will undergo as it cools and the lack of friction between the wax and the pre-form mold. I'd think draft is only required on internal surfaces to prevent trapping the wax. Apparently ceramic pre-forms can be inserted into the wax molds to handle parts that are difficult otherwise, but there will be a cost for those.

Also note that a part can be assembled from a number of wax parts. If the parts are individually moldable, then there is no problem, aside from ordinary metal flow in the final casting process. Even most of those could be handled with vacuum/pressure molding.

As others have mentioned - your supplier will tell you what he can do.

 

[drawoh]

3DDave,

There is investment casting wax, and then there is wax that can be dissolved with acid. This allows you to make hollow investment castings. You can investment cast just about anything if you are not stuck with something stupid, like a budget.

--
JHG

 

[3DDave]

How does one prevent the metal from filling the void in the hollow wax part? Any links? (Asking for a friend.)

Also - an artist I spoke with who did bronze castings told of taking a house-fly and trying to lost-fly cast it. He said it worked pretty well except the membranes of the wings were too thin and so he only got the larger veins. Apparently the legs came out OK.

 

[EdStainless]

Find a local caster that you believe that you can work with.
Show him the finished part that you really want, and tell him where you can make some compromises.
And then let him tell you what is possible, and what is simply expensive.
I cast some small pump impellers suing ceramic inserts. The results were great and in this case worth the cost.
For prototyping there are also people making waxes using AM (additive manufacturing) and making both ceramic cores and entire shells by AM. These are costly, but they will get you real parts without hard tooling.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[KENAT]

As others suggest, I wouldn't go too far without finding the foundry you plan on working with to finalize the design and go to production.

You can follow all the design guides they put on their websites or in DFMA training... and then when you go to order the part discover that due to various 'secret sauce' or 'art of casting' issues changes to your design are still required. Some analysis tools may help here, but you may need reasonable volume of designs to justify the cost.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[arunmrao]

Investment casting parts can be designed without fear of undercuts or mould drafts. The wax gets melted and flows out.
With advent of 3D sand printing, even sand moulds can be made without fear of parting line, mould draft or undercuts. Technology is changing the way we think.

I had the opportunity to visit Aristo Cast facility recently. They have excellent 3D printing facility for making investment casting and promise to deliver in 48 hours!!

Hope it helps.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[racookpe1978]

But you DO have to make sure the wax can boil out, melt out (flow) or get pushed out through the vent pipes when the metal goes in. I've only done a little casting s=design, but found even the relatively simple pattern that I needed required some "tweaking" for vents and pour openings by the casting shop. Expect a few trials and errors to start out.

 

[btrueblood]

Ra, I think you are talking about lost wax sand casting? Residual wax is usually cooked off (burnt out) in firing an investment casting ceramic shell, but Ra's comments still apply - the foundry will know about venting and gating to get the best results for you. As with any casting, you should think about where you can live with some porosity.

The website for SigniCast has a "knowledge center" with a lot of good info. on a really modern IC shop

http://www.signicast.com/knowledge-center

. I'd recommend reading their "design guide" for a pretty good intro to the kinds of things they can do, and what IC is capable of in general (caveat: my current employer is too small in volume to ever use them, and their capabilities are in some cases quite a bit above the average IC shop, so don't go against the advice above to find a vendor and plan to walk through your part with them).

 

[drawoh]

3DDave,

The process...

[ol 1]
[li]Use your first die to cast the inside pattern in acid dissolvable wax[/li]
[li]Drop the inside pattern into your second die, and cast the investment casting pattern.[/li]
[li]Use acid to dissolve the inside pattern.[/li]
[li]Drop your investment casting pattern into the sluury to make your casting mold.[/li]
[li]Make your casting.[/li]
[/ol]

Note how you pay for two casting dies, two casting patterns, and the acid dissolve process. Don't design undercuts!

--
JHG

 

[3DDave]

drawoh,

Thanks, when I was thinking hollow I was thinking roto-mold hollow, with a continuous shell around a void. There are a ton of ways to make a deep recess.

 

[tbuelna]

How does one prevent the metal from filling the void in the hollow wax part? Any links? (Asking for a friend.)

Hollow (negative draft) areas in an investment casting are often made using soluble wax cores. First a core is molded from soluble wax, and then is placed in the pattern mold and over-molded with pattern wax. The soluble wax core is then washed out of the wax pattern using some type of solvent. The soluble wax core requires some features to locate/support it in the pattern mold, similar to cores used in other processes like sand casting. The inner surface of the hollow area is coated with shell material just like the exterior surfaces are.

One thing to consider is removing the shell material from the hollow areas in the casting. Common methods for removing shell material are shot blast and water jet, which may not be effective in enclosed spaces with limited line-of-sight access.

https://www.investmentcasting.org/paper_detail.asp?PaperID=922

Another method for creating very small diameter hollow passages in investment castings uses pipe cores made from molybdenum wire. The molybdenum wire has a melting point higher than most casting alloys, and after casting the molybdenum core material is dissolved using acid.