Die casting questions: choice of Al-Zi-Mg alloys 1

[eks32]

Hi,

I'm designing a part with a size and complexity comparable to a
standard retractable utility knive. These knives seem to be diecast
from whitemetal (some alloy of Al,Zi,Mg) but my spec on surface
hardness is tougher than the samples I've tested.

This app is cost sensitive, and my reading of past articles suggests
that 1) stainless can't be diecast and 2) other methods are a good bit
more expensive; I guess that's why I don't see any SS utility knives.
Still, I'd like to understand the tradeoffs between a soft stainless
steel (or at least rust resistant steel) and some whitemetal alloy.

Qs:
1) Steel can't be diecast the way whitemetal is, right ?

2) What's the cheapest unit cost process that can compete with diecasting
for steel?

3) Are there any references for a really tough whitemetal alloy
(diecastable)? Weight is not an issue, cost is.

Thank you,

 

[kenvlach]

Read about die casting alloys, equipment & processes at this link. Note that aluminum, with a melting point roughly 1000[sup]o[/sup]C below SS, is considered a high temperature die casting metal:

http://www.diecasting.org/faq/

One method of forming small, complex, high quality SS parts is “Metal Injection Molding. It is very similar to plastic injection molding but with metal powder blended with a binder...” The binder is then baked out, and the part is sintered at high temperature. The process is cost-effective for complex parts, where it may eliminate joining, machining & surface finishing operations. Perhaps reduce the cost by using less (thinner) of the higher strength SS vs. a typical die cast alloy. See
Metal Injection Molding Design Guidelines

http://www.alberox.com/MIM_design_guide.pdf

 

[metman]

I am not at all sure of the cost effectiveness but you might want to compare SS stamping as a poosibilty. A punch and die set made from Tool Steel should not be too costly with current technology of CadCam/CNC. The Tool Steel unit cost is expenxive but mass of Tool Steel per part is low.

 

[arunmrao]

I concur with metman about producing SS stampings. Quite often I am stumped by Purchase Managers asking if I can produce a steel casting by die casting route. ( Common refrain is I have seen it in Germany).

It is nearly impossible for me to explain the limitations of die casting process and the intricacies involved.

 

[eks32]

Hello,

I apologize for not being too clear in my initial description.

The required internal complexity (keys, lips & webbing) is equivalent
to those utility knives. Since, the outside surface can't be stamped
in, this part would require at least two stampings,so I think the best
candidates are

1) a hard Al/Zi alloy, possibly nickel plated surface
2) finding a cheap way to do investment casting in steel.

Thanks for the replies and references,

 

[swall]

Do you have a cost target, or the projected selling price for the knife? That's what will ultimately guide your materials and fabrication selection process. You're probably familiar with the "Superknife" type of retractable blade utility knives. I bought one about 4 years ago because they were cool; it was $30. Now the clones are $10.

 

[kenvlach]

For cost reasons, choose a die cast alloy in common usage. Go to Dynacast

http://www.dynacast.com

[free registration required], then browse the alloy info. The 'Materials Comparison' tables & on-line 'Materials Selector' guide may be helpful.

Steels can be die cast in expensive refractory metal (e.g., Mo, W) dies, but I don't know of much use commercially. The preheat temperatures needed are higher than can be reached by the usual method of flowing hot flowing through passages in the mold.

Intricate SS parts are often made by investment casting (aka. lost wax, lost foam). Slower & more expensive, though. Shell moulding is similar; less expensive at greater quantities. For a simplified overview of the various moulding & casting processes, see

http://www.foundryonline.com/foundvpr.htm

Ken

 

[eks32]

thanks for the additional references.

I'll review and let people know how it turns out...