Investment form disintegrating

[zargnut]

Hi,

I am a novice to casting. I made a nice 16gal propane fired smelter and am working with small volumes of aluminum.  I have been making silicone molds and trying to use lost wax to cast some aluminum.  The smelter, metal prep, and silicon molding and wax creation has been fine. I first tried a plaster of paris shell, which disintegrated. so I got some Ransom and Randolph Ultra-vest Maxx Investment and poured a solid form, suspending my wax in it. 

I first used a convection oven to melt out most of the wax with gravity into a can. Then I fired the form in the smelter, and within about 5-10 minutes it essentially crumbled? I can speculate on several points of failure, but was hoping folks could point me to a "duh" reason, to cut down on failed attempts.

1) I didn't let the form really dry before heating it

2) I am not using a burn out oven, don't have one... firing it in the smelter

3) I mixed the investment about 8:1 with sand  (investment:sand) to make up some volume 

I was hoping to do the investment / sand / investment / sand layering technique, the the investment powder I got seems to cure chemically within a really short time period.. so won't permit keeping a bucket of liquified investment, for repeat dipping. Did I get the wrong stuff? Can I still use this stuff up making solid forms around my waxes?

thanks for any newbie advice you can give.

dan

 

I  inve

[ThomasPowers]

First of all smelting is taking ore and refining it into metal. Melting or foundry work is taking metal and melting it and casting things from it. Please do not confuse the terms!

Your 3 suggestions sound like possible failure modes. Your investment should have come with explicit instructions, I would follow those closely and not try to experiment till you get good at it; (I do most of my casting with petrobond.)  Have you visited backyardmetalcasting.com for forums dedicated to casting?

[Frosty]

Take a class and follow the product directions, humans have been doing this stuff for a couple thousand years, no reason to figure it out. What are you using for PPE?

Frosty The Lucky.

[zargnut]

Hi,

Thanks for the correction.. I will look for a local class, would be fun. 

The investment came as powder in a bag in a box... no instructions.  I did just find an online instruction sheet which does note a gradual burnout 2 hr at 150 C, increase over 5 hr to 1350 C. This part would be hard to do without a formal burnout oven.

RE: PPE. I am using leather welding jacket and bib , 3M 7500 series respirator with P100 filters, Safety Glasses behind a rated faceshield. Outdoor only work. Heavy insulated furnace gloves.  I like to think that I am novice, but not stupid...

 

[Frosty]

I just realized I forgot to welcome you to the Forum, glad to have you.  If you'll post your general location in the header you might be surprised how many of the IFI gang live within visiting distance.

A 1350C burnout isn't for casting al, that's bronze territory. It's also in the temperature range where you have to have the right kind of sand or it'll vitrify, melt or powder. It might be the sand degrading your molds.

Try holding your burnout temp to say 600-650C range after the 150C burnout cycle. Try it on a test piece of your investment rather than a mold. There's no good reason to experiment on molds, there's too much work involved to learn the properties of your investment.

I haven't done any casting in I don't know how long and that was mostly al in green sand. We investment cast bronze a couple times as assignments in metal shop class but had nothing to do with formulating, mixing or anything but applying the investment. The method we used was to dip the form let it dry, dip a second time and afterwards sprinkle sand on the wet investment let it dry and repeat. By dry I mean it wasn't dripping or wet wet but still pliable.

After saying that I have to warn you those memories are on the order of 45 years ago.

Frosty The Lucky.

[zargnut]

Hi thanks for the welcome. I live in Baltimore, MD. I added to my profile. 

Sounds like a good plan.  I also got some green sand, but thought this would be "easier"... think I misjudged the complexity of prepping lost wax... This will be a great opportunity to learn....  I have a thermocouple on order, will see how well I can regulate my furnace... But worth a shot.

thanks again for your advice.

d

 

[Frosty]

We eyeballed al and found the best casting temp was when it was just liquid, hotter is NOT better. Sometimes it looked like lumpy mercury coming out of the crucible and those were the golden pours. However do NOT judge your alloy by my 45+ year old memories. Make small test molds and experiment till you find what works best.

You also have to choose your al carefully if you're using scrap. Al cans and structural shapes are extrusion alloys and not so hot for casting. Try to source thin castings to use, in high school we tried hard to find old motorcycle engine heads and transmission cases as the al was alloyed to fill thin molds.

Frosty The Lucky.

 

[zargnut]

 

Helpful. I will monitor the temp once I can get a thermocouple into the furnace.

I luck out on scrap... I do more recreational fabrication and welding than any forgery work... My metal shop has a great scrap shelf and frequently has aluminum block cutoffs that are clean, uniform alloy, and small enough to fit into a crucible. 

I just got a small burnout oven, 6x6x4.5" that should work for these small castings.. I figure my kids can use it as a low temp 2000F clay kiln.

I will report back once I get my act together.  thx

 

[BIGGUNDOCTOR]

When I worked at a foundry that made ingots for the dental industry we did build ups on the molds with various sized silica sands. We would wet the wax in a slurry that was constantly being stirred, then dip in a very fine sand that had an air blast from below to fluff it up. It was then hung on a conveyor chain that took it through a dryer. On each pass through it would get re-wetted, and dipped in coarser silica sand to build up the mold. When it was done it was around 1/4" -5/16" thick. To remove the initial amount of wax we put them upside down in a steam autoclave. The pressure in the autoclave kept the molds from breaking as the wax would normally expand when heated. Our burnouts were done at 1,600°F for the nickel based alloys, and the gold alternatives. For the chrome cobalts they were burned out at 2,100° F. We were normally pouring around 1,450°F, but the chrome cobalt was a white hot 3,250°F . That was back in the mid 90's when I worked there.

How big of a pour are you doing?

[zargnut]

really small stuff...  The biggest crucible I have is #4 Graphite, maybe 15cm high x 10cm wide.

I have definitely seen the method you are describing. I will need to find an alternative investment slurry, one that does not cure chemically, as this one won't remain liquid  for repeat dipping. My stuff is small enough that I can use this up making solid forms... I can then look for an alternative investment ceramic slurry.  I do have some petrobond, and suspect I will have to use that for any larger pours I wish to do. 

d