Porosity and die castings

A. As I understand your situation, the problem is not with the drawing specification/requirement, so much as it is a problem with the manufacturing process meeting it. IF your drawing clearly indicates that the part/component surface has a minimum requirement for surface roughness, then task 1 has been accomplished. IF the requirement is such that the surface roughness requirement can only be attained by machining (face milling, etc.), then perhaps task 2 needs to be addressed. My definition of task 2 is "the creation of sufficient detail on the drawing such that the part can be suitably manufactured by the die casting process".

The only reason to specify a porosity requirement is so that a casting process with poor control of porosity can be used. If this is necessary, then by all means create a requirement, but there is no inherent rule or specification that just because a part has been agreed upon to be manufactured by the die casting process that any potential part requirement (dimensional, strength, pressure tightness, etc.) that may be compromised by porosity is now exempt. Die casting companies frequently try to do this, but it is not true.

There are a number of ways to rectify this situation. The first is obviously place a requirement on the drawing for maximum porosity. Another is to use a vacuum die casting process, which substantially reduces or eliminates porosity. Gibbs Die Casting in the US is one casting vendor that uses this process. Resin impregnation of the casting may be another option to consider-- low viscosity resins infiltrate the casting porosity and seal it. You can obtain more information from the Loctite website. And lastly, your current die casting supplier should be investigating alternative gating methods, overflow positions, filling parameters (metal velocity/pressure profiles), etc. in order to reduce or eliminate the porosity in the affected areas.

I hope this was somewhat helpful. Feel free to reply if you want to discuss further.

A. I think what Kevin is implying is that regardless of the surface finish on a diecasting as cast, once you start machining it you cut through the 'skin' of the casting and are left with a very different kind of surface. I'm not sure, but I don't think your response was about that, Toby.

Are you saying, Kevin, that regardless of the machining method you use, you cannot get the porous innards of this casting to meet the surface roughness spec the drawing specifies?

A. Kevin,

Some applications require machined surfaces to be free of holes so as not not to corrupt seals.

Some companies have porosity on surface standards for items going into gas field applications.

I suggest that you give the customer a better casting by eliminating the porosity, by using high vacuum technique.

A. Kevin's issue(s) are very common and ALWAYS a headache. I've been in engineering & manufacturing for many years. The only way I have found to effectively remedy exposing porosity when machining is to include a specification for the casting and including a class and grade...and that spec needs to refer to ASTM E155 which are radiograph (X-ray) reference slides depicting the amount of porosity that can be tolerated. It's expensive because you can add the price of the NDT right to the price of the casting. Once you get a reliable process down, remove the requirement for the radiographing (only). Sorry I am at my residence so I do not have all of the specifications in front of me. Hot Isostatic Pressing is a nice way to reduce included porosity, but it's expensive as well. You need to ask yourself: "How valuable are these castings?". Another trick that is sometimes harder to do than it sounds is to design the product so that a little allowable porosity will not affect the function of the design. That can take considerable work. The best way, in any case, is to hold your foundry's feet to the fire, and also be willing to revise the design to accommodate the process.

A. Kevin,
I might be a little out of my element here, however I think that my 2 cents might help.

If I understand correctly, you are trying to find out which standard supersedes the other, porosity of cast surface vs surface quality of machined surface. If this is true, than I think your issue is more of a configuration management one than anything. In my experience, this problem is solved by separating the casting and machining drawings. The casting drawing includes your draft angles, ejector pin locations, and porosity spec. The machining drawing calls on the casted part as it's material requirement, in effect creating a one line BOM. The secondary operations, with applicable tolerances and surface quality specifications are included on this drawing.

I hope this helps.

A. Kevin,
I found this after a search as I have the same problem. we have many high volume parts that are machined castings with tight tolerance bores and they have high finish specs on them.
Now if the machinist sets up the machining operation and I give him high quality steel plate or bar he meets the finish all the time, but if I give him a casting he might not on some percentage. let us say we are trying to achieve a very low PPM rate. the castings in some cases have just enough fog that they do not meet the finish spec when the inspector checks.
Now all castings are porous they are just relative and to a degree. there are ASTM specs with grades or quality levels. and they involve cost.
So you can meet the intended casting quality level but not meet the machining finish spec on a percentage.
That is your problem and mine Kevin.
I do not know the solution except to change the design and allow for sleeves out of mill material that can repeat over and over to meet the spec.
so I think it is a design issue. either allow the casting flaws to the casting spec or redesign with sleeves.
Jess