[jhudler]

I think I've changed my first E/G project to a vacuum chamber.

Something like 3.5w x 4.5l x 2.5h with a thick lexan cover.

Don't plan on pulling anything lower than 10-1 Torr (or 10-3) so this should suffice.

Comments?

Jack

[ckelloug]

Jack,

Kudos on the bubble counting. I'll do some griffith flaw size calculations and see whether the bubbles are likely to contribute to failures at the forseen loads. I think that some more additives may help the bubble problem although I think vacuum would probably be the best. For machine parts, assuming the bubbles do not dangerously lower strength or stiffness, they're probably okay.

As for the vacuum chamber, if your dimensions are in inches, it's probably okay. If those dimensions are in feet, gut instinct is that it could and would likely fail catastrophically and dangerously. Unreinforced E/G is an extremely bad idea for vacuum chamber walls because it fails in brittle fracture whereas metal is ductile. The failure mode will likely include shards. I would suggest aluminum or steel for a vacuum chamber and I would also suggest some time in a library with the ASME standards for vacuum chambers as catastrophic failures are bad. Also look into designing the chamber with leak before break criteria.

On the topic of Aluminum oxide, The plane strain fracture toughness of aluminum oxide is much higher than quartz: I speced it because as grains get bigger fracture toughness gets lower but large aluminum oxide grains will be a lot stronger than the associated quartz grains of the same size. Whether the effect is large enough to matter is unknown. Also, cast flat surfaces will be much more likely to succeed for the average home shopper as very few of us have a surface grinder at our disposal. I personally think that if you're going to have to grind the surface that it makes more sense to use iron.


Sandi,

I am not sure anybody on the thread has tried silica fume extensively. Fresh from the bag, it will behave as a thixotrope and greatly increase viscosity with some stiffness improvement hopefully. Run through a disperser or preferably an ultrasonic disperser , I believe it eventually break down into small enough particles to stiffen the material without a huge viscosity gain. A sufficient disperser is likely outside the means of the home shop as I estimate the cost at $10,000 US. Fresh form the bag, silica fume will likely make the mixture much harder to work with. It's probably needed at a rate of about 3% of the epoxy by weight.

I am personally in favor of testing silica fume and carbon black for their dispersion (nanometer sized particle) hardening effects but I won't be able to do so until I obtain my ultrasonic disperser. If you can post some results with silica fume out of the bag, it would certainly be interesting but I have some suspicions that without industrial strength equipment the success won't be as great as I predicted a long time ago on this thread.

Best of Luck,

Cameron

P.S. The concrete board for my shop walls has finally arrived. Yeah!

[jhudler]

P.S. The concrete board for my shop walls has finally arrived. Yeah!

Is that for tile, sound proofing and/or hanging heavy items... or do you need shrapnel protection

[jhudler]

As for the vacuum chamber, if your dimensions are in inches, it's probably okay. If those dimensions are in feet, gut instinct is that it could and would likely fail catastrophically and dangerously. Unreinforced E/G is an extremely bad idea for vacuum chamber walls because it fails in brittle fracture whereas metal is ductile.

Feet... works out to a little more than 8 tons of pressure on the outside on the largest area. Was thinking of getting creative and using stainless mesh and about 4 inch thick sides. Lot of bloody E/G!!!

I'm use to working around large vacuum chambers for coating mirrors, so I realize the danger involved. A cylinder would be the best, but it would be the most inconvenient.

[Eson]

Hey guys, what do you think of this?
http://www.ponoko.com/
ex. http://www.ponoko.com/showroom/ThomasSchickedanz/77
Download Starterkit and check pdf: http://www.ponoko.com/mingleandshare/design_tips

This would be perfect to make molds! Shouldnt cost more then a couple of 100 bucks.

[harryn]

Walter, I think your parts are too pretty to get dirty with real milling. Maybe you can gouge them up a bit before real use - sort of like getting that first scratch in a new car.

[walter]

Thanks Harryn! I did scratch my castings -->http://i233.photobucket.com/albums/e...g/6308e580.jpg
I went industrial on this one

As for the surface, I found an easy and cheap solution. You can throw your casting out of an airplane, when it hits the ground give it a little 60 grit sandpaper and paint it with epoxy/zeeospheres mixture. It'll be good as new.
There is no reason to sweat these things. You have to make sure your parts easily release from the mold, but not much beyond that. Thin coat of epoxy/zeeospheres will fix the rest.

[walter]

Jack, thank you for your work on the sample! How's the temperature reading (CTE)?

I apologize for the air bubbles and I'm sorry you had to go through the hole counting :-) The sample wasn't properly deaired and no deairing agent was used- I thought it was for temp reading- again I apologize.

All I have now is 15lb concrete vibrator which doesn't really work on 1lb samples. The actual castings will look much better. You've got all the mass and gravity and the vibrators are on for 1-2 hours, it should be a whole lot better. Still not perfect mind you..But better than samples.

Here are some old samples, deairing agent on the left. Still a ton of air! Both samples were barely vibrated.
_

[jhudler]

I apologize for the air bubbles and I'm sorry you had to go through the hole counting :-) The sample wasn't properly deaired and no deairing agent was used- I thought it was for temp reading- again I apologize.

Oh wow no need to apologize. I should have asked if deairing agents were used. I just assumed you had (BTW my BYK-A 525 sample arrived today, wow are they fast).

It was my thought to count them up and then ask Cameron if he could analyze the volume of air-entrapment to get a better idea if this would have a appreciable effect on strength.

I'm setting up the laser system as we speak (err type), still haven't settled on a temperature chamber yet.

BTW, The sample on the left definitely show a substantial reduction in air.

Jack

PS. Actually I used Photoshop to help me count holes, twas easy!

[walter]

I'm 2 weeks behind but pushing on. I have only one day per week- very frustrating..










There will be no reinforcements of any kind and I'm going to use large foam cores in the column and base, it has to stay under 150lbs fedex limit. I'm going to donate it to the E/G community for testing purposes.

I guess you could say that it's being made to fail... True.

But if it doesn't fail ----> better watch out, metal fabricators.


(I'm looking straight at you Macgyver!)

[the4thseal]

looking great!!!!!!!!!!!!!!!!!!!!!!!!

[greybeard]

.....................
PS. Actually I used Photoshop to help me count holes, twas easy!

Jack, I'd be v.grateful if you could give me an outline(pun intended) of how you did that.
I've wondered if I could use a scanner plus software to give me a particle count with sand. If I could add in an idea I've had on object size via software, I could do the aggregate analysis using photo editing/vector progs without any hardware.
Regards
John

[greybeard]

Cameron - the photocopies arrived this am. Many thanks for giving me some bedtime reading
Regards
John

[walter]

Thanks The4thseal!


Eson,

re: Mold material- anything you can get your hands on will work. MDF, wood, plywood- you name it.
$40 per 4' x 8' sheet would be a good price. Even the el cheapo plywood, see the big mold here (bottom of post):
http://www.cnczone.com/forums/showpo...&postcount=541


Thanks everyone for posting your comments and ideas. Keep 'em coming!
_

[dilbert]

Air entrapment:

Size of bubbles in one square inch:

>.03 2
>.025 5
>.016 16
>.008 108
<.008 ~140

Note: the top part had a much larger set of bubbles, which was to be expected.

Cameron: Can you extrapolate the volume gas sphere's per cu/in?


Jack

From your numbers it looks like you are producing about 7% porosity in 1 inch cube. ie .07 inches^3
-regards
dilbert

[lgalla]

Jack,at 3.5WX4.5LX2.5H in feet,I will totally agree with Cameron and add a Kevlar vest may be in order.We have not yet determined if a vacuum will be able to de-air the mix as it is very thick and heavy like pizza dough.
If vacuum works on such a heavy mix,a vacuum bag is the way to go.Cover the E/G in the mold with a bleeder sheet which is a sheet of Mylar with perforations .
Place a bleeder or fabric or cheap carpet ontop and place the casting in a bag.
Equal pressure all round=no shrapnel.Again this is not proven for such heavy mixes.I imagine vibrating while vacuuming would be in order
Larry.

[jhudler]

Larry,
My only problem with vacuum bagging is the pressure placed on the mold.
Soon as I can I'll run and FEA in Autodesk Inventor to see if what I have planned can hold it.

Couple of things have me stopped; one I don't have complete data on the E/G mix (tensile,elasticity, etc...), and two; simulating a vacuum in Inventor. I'm sure it can do it, just don't know how to yet.

Jack

[lgalla]

Jack,sorry may have a hard time with a good explanation.If the whole mold and E/G are in the bag,this results in pressure 360 degrees or equal all round which poses no stress on the mold.If any part of the mold was a hollow this would present a problem as the vacuum would crush the mold.Hope this helps or another member can elaborate on the subject.

[jhudler]

Larry,
That's what I meant, I don't see anything I need to cast that doesn't have some type of void or cantilever that would be affected by bagging.

Jack

[lgalla]

Jack, sucks doesn't it,Possibly mixing under vacuum and post vibration is the answer.I have mixed under vacuum in a 10 gallon pressure pot or paint tank which can withstand the negative pressure.My mixes are a far cry from the E/G 10% by weight or 20% by volume.My mixes are 50-50 by weight which is poured into the mold then vibrated for compaction and results in a void free class A surface.These references are to "densified granite"or solid surfacing material,such as counter tops in MacDonalds.
Since this is new technology it may take time to fine tune.Walter is in the ballpark.A home run is eminent soon,possibly a triple play.
Larry