Pressure Vacuum Casting Impregnation Rig

I have a number of fluid and compressed gas parts in the present and future and needed to make an impregnation rig. In past lives impregnation was just SOP for anything that needed to be water or air tight, especially for sand castings. I didn’t really want to take this on but lot charges from the for-hire companies were prohibitive at $400. There are a number of different resins but one of the most common is Methachrylate monomer. Loctite’s brand for such is Resinol. There are a heat and or anaerobic curing versions. 90c is the heat (only 200F or so for a few minutes) cure version, and the anaerobic version just cures with time at room temperature. I selected 90c because after activation it has long shelf life whereas the anaerobic curing type must have air continuously bubbled through it after activation to keep it from curing. Besides having very good chemical resistance it also acts as a modest cutting lubricant and can help machining but it doesn’t dimensionally change the part so can be done post machining as well.


There are several variations on the process. In the first you just place the castings in the resin, pull vacuum (<=10 Torr is recommended) and then re-pressurize. The second evacuates and then immerses the castings, then re-pressurizes to atm pressure. The last is the same as the second but re-pressurizes to several atms requiring a pressure capable chamber. In speaking with the Loctite folks, they indicated that vacuum is the most important and doing the second process at sub 10 Torr and re-pressurizing to 1 atm achieves ~90% of the elevated pressure PV process……so that sealed it for me .…But, you need a chamber that can allow the castings to be actively moved into the resin while at high vacuum. That build is what this thread is about.


The impregnation rig is just a vat that can be evacuated and then re-pressurized. I needed a bigger vacuum chamber so I bought this unit with compressor. It’s an 8.5 gallon stainless vat, clear polycarbonate lid with silicone seal, compressor and valving. So I wanted to make a “drop in” immersion device and for that I needed a pressure tight insert, which……you got it…..I decided to cast in lost foam.

That is documented here.

http://forums.thehomefoundry.org/index.php?threads/casting-impregnation-rig-insert.68/


The insert was a design on the fly effort. I went lost foam because I only needed one piece and I thought I could make a good near net shape part which turned out to be the case though not without incident.


So here it is with the insert casting in place. I need to build the internal basket and sealed lifting mechanism. It can be set up with a basket or bracket to attach the castings. It will just lower castings in to the resin with a crank mechanism which has a cup seal on one end and blind bushed hole on the other. That’s it so far. More later.



Best,
Kelly

 

Even with degassing, for thin walled parts it's pretty much inevitable for a percentage to exhibit some degree of this type of leakage. It's often more practical to impregnate than to try to control microporosity in complex and thin walled parts. The cylinder head that was used as my casting stock actually became scrap because of the issue and it's a high volume Chevy LS head. See here:

http://www.alloyavenue.com/vb/showt...pregnation-Rig&p=206228&viewfull=1#post206228

Already confirmed thus the impregnation rig build.

Sorry no pictures though. The castings looked great both cosmetically and polished under magnification. If I get a chance I'll see if the frothing affect in the water tank can be photographed.....before and after......but soapy water on the external surface with the inside pressurized is a better bet.

Best,
Kelly

 

You know... I didnt pressure test my thin wall castings until after I had shot peened them... Makes me wonder now...

 

Yah, that's why I had asked you about that and why I thought I might get away without. If your part is a nice uniform wall, you had good clean metal that was degassed and consistent from batch to batch, mold sand very consistent, pour rate....etc, etc, and all other variables the same.....you're probably ok.

But if you made any appreciable number of them and different lots, ....well?

If you shot peened them, you work the surface which tends to seal them up......problem is, it tends to be temporary. Not to worry...all flat heads leak somewhere

Best,
Kelly

 

I gather I have a few people scratching their heads on this and I guess I kind of jumped over some important pieces of information. Yes castings tend to be porous and leak especially thin walled castings. Casting impregnation has been around for about 70 years. Anyone care to guess what one of the most common materials was back then?........Sodium Silicate. Yes water glass. In fact, SS is the active ingredient in the typical radiator sealing additives and they are basically a casting impregnation process in progress.


The water neck casting I made looked great. It leaked. After 10-15 minutes it would puddle coolant in the logo.



Now I could have just said pour in a can of Bar-Seal radiator sealant and call it a day but I wouldn’t want to do that to a pristine cooling system because it tends to get into other places where you don’t want it and these go on relatively high value collector cars.


I should have known better but thought I’d get away with it…..wrong! The small amount of wax in that area during the casting process probably wasn’t helpful. In any case, it was far more sensible to just seal up the castings than try to control the micro-porosity along with everything else. So I turned to vacuum impregnation. If I only wanted to do this one part, I would attach a radiator hose to one opening, plug the rest, and fill the radiator hose with sealant (Methachrylate) with the part in an orientation such that the internal passages were dry, pump the air out, then turn it upside down so the internal portion of the part fills up with sealant, pressurize, then remove the Methachrylate and cure with temperature >90c. The above would probably work just fine with radiator sealant as well.


Thing is, the above would only work for this part and I have a number of parts I want to impregnate and seal up so a dipping rig is a universal solution….cry once. Also, I wouldn’t impregnate the thermostat sealing surface with this approach and it may weep over time.


Here’s what I originally envisioned for the rig. It’s stupid-simple.




Thing is, I didn’t want to drill a hole in the bottom of my new vacuum chamber because 90% of the time I’ll be using it to autoclave resins. I also would like to use the stainless vat to boil water for the cure cycle for impregnated parts.


I also thought of maybe substituting two 5-gallon buckets with snap lids for the vats shown in the diagram but I didn’t think the seals would hold high vacuum and if a plastic lid ever failed at high vacuum, I’d have 4 gallons of Methachrylate all over my shop……and I might mention it is considered hazardous material until it is cured at 90c.


So I decided to build the rig originally described in this post and I’ll be able to do batches of any part that can fit in the rig. The key to this approach will be whether I can get the rig sealed up tight enough with the additional seals. BTW, that vacuum rig is amazing. I pumped it down to a few Torr and then shut the isolation valves and that thing sat in that condition all day while I worked in my shop and didn’t budge from the original pressure reading, at least as near as I could measure. -Hope that helps.


Best,
Kelly

 

Thought I’d resurrect this thread since I vacuum impregnated a batch of castings today. I took a different route than how this thread started out. Instead of using the casting insert earlier in the thread I modified the lid with a automotive bulkhead fitting that I fit with an O-ring to seal on the sliding rod. This allows me to suspend castings above the impregnation sealant and then reposition the castings, immersing them into the resin, all while at high vacuum. Then you raise the rig back to atmospheric pressure and that dives the resin into the pore of the casting. Then rinse in water and cure in boiling water for 5-10 minutes.

Here’s the impregnation rig.



Here’s the rig, rinse bucket, and cure tank of boiling water.



Here’s a video of the process.



There is some interesting color variation. The thermostat housings have not been impregnated but apparently were cast from different alloys. The water necks have been impregnated and cured and took on a light golden tan hue…….I wasn’t expecting that.



Here’s the whole lot after impregnation.



I need to pressure test them but expect them to be sealed up good and tight.

Best,
Kelly

 

Believe it.

Yes, and around nearly as long as commercial aluminum casting. The early sealants were sodium silicate.

Yes sectioned them and they looked great under low magnification......leaked, well sweated would be a more accurate description but enough to puddle. You'd need to read the whole thread. It's H2 porosity. Some of this metal was remelted a number of times times and was not degassed. Some believe H2 porosity can be eliminated with furnace atmosphere....not true. Even with degassing, I can tell you it's still difficult to get a decent yield in sand casting thin walled aluminum parts if they need to be hermetic.

Best,
Kelly

 

Though H2 can be introduced by incomplete combustion and to this extent fuel tune can reduce hydrogen dissolving in molten aluminum, the primary source of H2 dissolving into the melt is water vapor in ambient air and it's along for the ride and being introduced with the combustion air. If you live in the desert, better, if you live in the tropics, worse. I think there were a couple other recent threads where it was discussed in more detail.

http://forums.thehomefoundry.org/index.php?threads/furnace-measurements.178/page-2#post-2848

http://forums.thehomefoundry.org/index.php?threads/degassing-lance.204/page-2#post-3902

Best,
Kelly

 

Formation of oxides is one area where furnace atmosphere does matter and proper burner tune can reduce oxidation. However, a layer of dross can actually slow and inhibit the rate at which H2 can dissolve into the melt because H2 is not soluble in nor easily diffuse through Al2O3. -Best not to skim until you're ready to pour. That said, I don't notice an appreciable difference in the amount of dross in my electric than I do in others fuel fired furnaces. Now, the quality of the melting stock does make a noticeable difference.

H2 porosity is due to its solubility in molten aluminum and then coming out of solution when the aluminum freezes causing the porosity. As far as gases not much else is soluble in molten aluminum including the gaseous byproducts of decomposed and vaporized polystyrene. Turbulence can form oxides but that is true of about any casting process done in air and isn't limited to lost foam......but that is why I use the hollow foil sprue and try to get to pour to stabilize in the cup/sprue/gate as soon as possible in my lost foam pours.

Best,
Kelly

 

<~ 1/4".

 

I've had success with thinner walls on some parts and features, but for gravity fed sand casting it's sort of a practical limit IMO. Interaction with and off-gassing of the mold medium binder, control of pour temp, rate of heat loss to the mold, metal fluidity and quality....it all means lower yield/higher scrap and part variation.....and yes, more susceptibility to H2 porosity, especially for hermetic sealing parts. A lot of the aluminum automotive parts I cut up are ~1/8" wall but they're permanent mold parts and that's a different ball game with respect to the above. It can be taken farther with investment, shell, and vacuum or pressure assisted processes.

In most cases it's easy enough by making design allowances to produce parts that are mechanically sound enough, but when it comes to being hermetic, impregnating is more practical, and thus a very commonly used process for insuring the sealing barrier in commercial castings.

Best,
Kelly