stained 356 castings from foundry

[BrianE22]

Well, either from the foundry or the machine shop. It starts out as a very light discolorization on the casting like something was spilled on it. After going through powder coating and baking of the powder coat the stain turns dark brown and bleeds through the powder coat paint.

The machine shop says that's the way they got them. The foundry says there's no way the parts were stained while they had them.

The parts are made from a permanent mold. The foundry heat treats them and "wheelabrates" them. They go direct from the foundry to the machine shop so I don't get a chance to see them until after they are machined. It just started happening on the last shipment of 300 parts. Now it's happening again on the latest group of 300 parts.

There's no problem with the sand castings produced from this foundry (just this permanent molded part) or from any of the other parts machined at this machine shop. So my question is, have you ever seen stained parts come from the foundry?

 

[swall]

Is the stain on only the cast surfaces or is it also on the machined surfaces? Are these castings heat treated prior to machining?

 

[BrianE22]

Some of the machined surfaces show a bit (lighter colored) of it -almosts seems as if the stain soaked into the metal deep enough to still show up after some machining. But the stain is definately worse on the unmachined surfaces.

Yes, the castings are heat treated by the foundry prior to machining.

I tried to upload a picture but there were errors. I'll try again later on.

 

[BrianE22]

Here's a picture of a machined and painted casting. The view is of the unpainted interior of the gearbox. You can see a slight discolorization on the face of the machined hub just up and slightly to the left of the middle of the housing.

 

[swall]

I suspect something in the machining coolant is attacking the aluminum. That is the only explanation for staining both on machined and unmachined surfaces.

 

[BrianE22]

I was putting my money on the machine shop causing the problems. However, on the latest group of castings delivered to the machine shop the machinist was careful to look them over before putting them into the machine. He's sending me the stained ones. So, supposedly, these never got into a machine.

The machinist machines several of our other 356 castings (sand castings) and there's no stain problems with them. He says he uses the same coolant and machines on each of the parts.

 

[CastMetal]

Could be excess mold release (spray or a wax) (unlikely due to appearance on machined surfaces other than that looks similar), very concentrated coolant (likely), or some third unknown source. Whatever it may be the powder coating preparation seems to have been inadequate and should bear some scrutiny, if you can see an oil residue on an area to be painted you probably should make sure its cleaned well.......

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

 

[cloa]

There is a very similar post in Machine and Machining Subforum.

http://www.eng-tips.com/viewthread.cfm?qid=310436

 

[TomDOT]

Brian, have you tried a wipe with various solvents to see what (if anything) removes the stain? The stain pattern looks like some kind of spilled liquid.