Microseal for porous Al castings 2

[JohnNabors]

Has anyone here tried Microseal sealant for porosities in cast aluminum? I'm ordering some for a test drive, just curious if any of you have used it, what sort of results you obtained, and if so what tips you might offer in the application of it. The porosities I'm trying to seal are sponge-like formations with pores about .005" to .015" that are allowing slow lubricating oil leakage in bearing housings for rotory lobe dewatering pumps. The link to the Microseal website is below.

Thanks -John

http://www.microleak.com

 

[BobM3]

We've used it for years now. It plugs some of the holes but not all.

 

[unclesyd]

Haven't used Microseal but have used similar composition materials applied by vacuum impregnation.
Our use was both for pressure containment and prevent adsorption of oil onto a surface.
The base material Bakelite is a very chemical resistant material that can take a little temperature.

 

[kenvlach]

Microseal may be adequate for your application, but it cannot get into the smallest porosity. Also, the solvent-borne phenolic product cannot be as good as that produced by vaccum impregnation of pure resin. It also leaves a surface film which may need removal in your application. But, give it a try & report back.

The vacuum impregnation resin monomers are rinsed from the surface before heat curing, so it is possible to anodize or chemfilm the impregnated aluminum castings. Info on one resin:

http://www.nealuminum.com/loctite.htm

 

[yates]

Would HIP (Hot isostatic pressing) be too expensive ?. This certainly gets rid of porosity.

 

[tripleZ]

Yes, but the HIP process has the potential to kill your dimensional consistency, especially if the levels of porosity differ in specific areas of the part. With .015" pores, you've got some sizable pits. You may try contacting Casting Impregnators of WI (depending on where you're located). I know some folks who use them continually.

 

[arunmrao]

Why should there be sponge like defect in your casting? Is the foundry not able to control this defect.

On a regular basis if you are looking into salvaging some of the leaked castings, vacuum impregnation techniques are successful at low temperatures.

Hipping is a good tool,provided costs permit such an operation.

 

[BillPSU]

Master brake cylinders are manufactured from aluminum using permanent mold processes and the melting practices are followed religiously purging the melt with nitogen. The master cylinders are still vacuum impregnated as a final step to eliminate/reduce leakage.

 

[swall]

BillPSU--your case for 100% impregnation sounds like a case of poor value analysis.My company made tens of millions of automotive master cylinders machined from permanent mold 319-T6 alloy. We did not impregnate and never had a single one leak from porosity.It is also important to note that most of the impregnation resins are not resistant to automotive brake fluid. The phenolic Microseal is the obvious exception.

 

[arunmrao]

swall,

I have raised the same issue. Especially for a bearing housing a simple static component if it has sponginess the foundry should accept responsibility and replace a defect free one.


I have visited a few aluminium die casting units and no where was vacuum impregnation offered as a solution.

Early in my career I had made steel valve body castings and I was new to the trade. The castings leaked in hydro test. Someone advised me to use this technique of vacuum impregnation. I was elated. They cleared the test bench but in the field they failed. The fluid temperature was high and all the resin impregnated melted.

 

[swall]

Arunmrao--I agree with you fully. The foundry should find the root cause of the porosity and fix it. Now, having said that, I will admit that in a manufacturing environment,one often finds that a lot of money has been put into the castings (machining time) by the time the leakage is discovered. Then it becomes a matter of either wait for new castings or try to salvage what you have to meet shipments. JohnNabors--just out of curiosity, what alloy are these castings and what is the casting process?

 

[JohnNabors]

Swall-

The material is 356 alloy as cast, the process is green sand with a wooden pattern.

 

[arunmrao]

Yes you are perhaps talking of hand mold castings. Check the green sand properties and the mold quality. Alternately for a few castings change over to CO2/sodium silicate process. The sponginess in the castings might just vanish.

It is perhaps the moisture in the sand or the volatile matter that are creating the problem

 

[horsetrainer]

I'm new to the forum and appreciate all of the experience and knowledge.

It is difficult some times achieving defect free Al casting when using sand, perm. mold, or HPDC, however it is doable. A356 lends itself well to not only these processes but also squeeze and my favorite, semi solid casting. I agree with BillPSU, swall, and arunmrao that a well controlled process should achieve the desired absence of defects. Removal of hydrogen and oxides from the melt is key at the beginning, then making sure there are no other factors such as excessive die lubes or in the is case volatile binders in the sand etc. I believe the best way to plug holes is not to have any to start with.

Mike

 

[Nadimuthu]

Swall,
356 castings in green sand can be cast to have shrinkage defect free castings(sponginess ) as mentioned by arunmrao.
In moulding have sufficeint vents and ensure a sand to metal ratio of at least 4:1.
In Melting use min 1.5 to 2% Sodium modification.
You can see detailed notes on my thread "356 sand castings."
Have a very clean melt.
Also use ceramic foam filters 20 ppi to ensure clean metal into the castings.
Preferred gating ratio when using filters just after downsprue to runner is 1:1.1:1.2.
Gate your thick sections through open ,hot risers and use antipiping compound for topping the riser .

THese are preventive actions.
Microseal is a corrective action and any porosity which is more than 0.4mm cannot be sealed by any selant.
Also porosities which are continuous and open up to a surface on machining will not be served by any sealing action before machining.
Hope these provide you with good inputs for a lasting solution.