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knightshade
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Joined: 19 Jun 2005
Posts: 123
Location: Rochester NY

PostPosted: Wed Oct 12, 2005 12:12 am    Post subject: Reply with quote

crashmann -

I noticed that you mentioned that you were making your final molds out of Hydrostone plaster. I haven't worked with it before and was curious how much that differed from Hydrocal White.

I know that Hydrocal puts out quite a bit of heat while curing. It didn't seem like enough to melt plastic - but i've never tried it. My experience to date has been using it to make negatives from molds made with oil based clays. I assume that hydro-stone also heats up - but hasn't been hot enough to warping hips?

I've read up on the "technical" specs - and it looks like hydro-stone is harder once dry - and has a shorter work time. The numbers don't really give me a good reference to understand how much more durable a mold made out of Hydrostone might be - but harder seems to be a good quality Smile

Hydrocal White:
http://sculpt.com/catalog_98/CastingMaterials/GYPSUMS/gypsum4.htm
Code:

1 Hr. Compressive Strength     1,000 psi     6.9 MN/m2
Dry Compressive Strength    5,000 psi    34.5 MN/m2


Hydro-stone:
http://sculpt.com/catalog_98/CastingMaterials/GYPSUMS/gypsum7.htm
Code:

1 Hr. Compressive Strength     4,000 psi     27.6 MN/m2
Dry Compressive Strength    10,000 psi    69.0 MN/m



Any recommendations on vendors?
I did a quick search tonight... best price that I noticed was just under $40 for a 100 lbs. bag (plus shipping).
Would some type of local vendor be likely to carry it?
Hardware? Pottery? Other?
(Time to break out a phone book I suppose...)
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crashmann
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Joined: 27 Sep 2005
Posts: 501

PostPosted: Sun Oct 16, 2005 10:46 pm    Post subject: Reply with quote

Hi Knightshade,

Actually, I was looking for a vendor that sold UltraCal 30 in Maryland (UltraCal 30 is supposed to capture fine details). So I went to http://www.gypsumsolutions.com, clicked on Distributors, then clicked on MD. The only place that sold any kind of USG plaster was a construction supply company - they only carried HydroStone. I read the technical specs, and I liked the idea of it being a harder material. I tried plaster of Paris from Home Depot, and it left a chalky mess on whatever it touched, and was very brittle.

The Hydrostone does heat up as it's curing. We made a chest piece about 8" deep, and I'd say it warmed up to ~150 degrees.

In the video, you can see steam coming off the elbow buck! I haven't noticed any warping from the heat (using .060 HIPS). Really, we have more trouble with making sure the mold stays level, and not warping from the weight of the plaster. It's not enough to affect the mold, but you need to be prepared as you're pouring it out.

I bought some UltraCal 30 from New York at The Compleat Sculptor, but the shipping basically doubled the price of the plaster! Definitely try to find a local dealer.

Another important recommendation - use a scale when mixing the plaster. I bought a 50 pound digital scale from here:
http://www.americanweigh.com

You've read the technical specs, be sure to weigh the plaster, then do the math to calculate the amount of water required. With HydroStone, it's 3 parts plaster to 1 part water. As I'm sifting the plaster into the water, I always end up with a dry top - there's just not enough water! But then I use a paint mixer chucked in a drill, and after a couple of minutes there's no lumps Smile


Be sure to avoid mixing bubbles into the plaster by keeping the impeller below the surface in your 5 gallon bucket.

Good luck!

Charlie
TI-386


Last edited by crashmann on Tue Mar 28, 2006 9:46 pm; edited 3 times in total
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knightshade
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Joined: 19 Jun 2005
Posts: 123
Location: Rochester NY

PostPosted: Mon Oct 17, 2005 6:49 am    Post subject: Reply with quote

Sounds like some great info.

I've never seen a bit like that for mixing paint. Looks like I have a new gizmo to look for. I can see that it would be much better than the fork that I'm using to mix stuff Smile

I had found a vendor about 70 miles away (Buffalo) at that same site.
As I have an event there in two weeks - I was hoping to make a detour and pic up some hydrostone.
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peterb



Joined: 29 Nov 2005
Posts: 4
Location: South Africa

PostPosted: Mon Dec 05, 2005 1:14 am    Post subject: Reply with quote

To summarize, I think the formulas go something like this

You choose the total Watt output of your oven, lets say you want 2300 Watts. First thing you do is calculate how many Amps you're going to need:

W = V * A

A = W / V = 2300 / 115 = 20 amps

Now to calculate the total resistance needed to draw 20 amps from 115 volts

Rt = V / A

Rt = 115 / 20 = 5.75 ohms

That's the total resistance needed, however there are 4 elements each in parallel, so the calculate the resistance of each element, we use the formula

1/Rt = 1/R1 + 1/R2 + 1/R3 +1/R4


but since each of the 4 elements are supposed to be the same, this formula can be re-written as

1/Rt = 4 * (1/R)

or
4 * (1/R) = 1/Rt

so

4 * (1/R) = 1/5.75 = 0.1739 

1/R = 0.0434
R = 23 ohms

so each element must have a resistance of 23 ohms. To calculate the end-to-end resistance, we add each of the 4 element's resistance

Re = 4 * R = 92 ohms


Now, according to www.infraredheaters.com, the 22AWG wire is in 10 coiled foot lengths with a total resistance of 360 ohms, and an outside diameter of 0.30"

1 coiled foot = 360 / 10 = 36 ohms

1 coiled inch = 36 / 12 = 3 ohms

To get 92 ohms we will need

92 / 3 = 30.66 coiled inches = 2.55 coiled feet


Which works out to be about 1/4 of the 10 coiled foot length for sale at www.infraredheaters.com
Now, for intrest sake, 22 AWG = 0.025 inch in diameter, so to calculate the number of coils in 1 coiled inch

Coils = 1 / 0.025 = 40


To calculate the length of each coil, you can take the outside diameter of the coil (0.30" from the website), subtract half the wire's diameter, and then use the formula to calculate the circumference

CenterDiameter = 0.30 - (0.025 / 2) = 0.2875

Circumference = PI * Diameter = 3.1415 * 0.2875 = 0.903

So 1 coil is 0.903 inches, therefore

1 coiled inch = 40 * 0.903 = 36.128 straight inches


so your 2.55 coiled feet is approximatly 1105.51 straight inches
you can buy 10 foot lengths from www.infraredheaters.com, so the total straight length of 10 coiled feet

10 * 12 * 36.128 = 4335.397 straight inches


if the total resistance of 10 coiled feet is 360 ohms, 1 straight inch is

360 / 4335.397 = 0.083 ohms per striaght inch


therefore our 2.55 coiled feet has a total resistance of

1105.51 * 0.083 = 91.79 


which comes damn close to the 92 total ohms needed, and each segment would have to be 7.65 coiled inches long
You can use those formulas to calculate a 5 or more segment oven.


Ok, to calculate the length needed to output 2300 wats at 230 volts, it would be

2300 / 230 = 10 amps

Rt = 230 / 10 = 23 ohms
4 * (1/R) = 1/Rt = 0.0434
R = 92 ohms
Re = 92 * 4 = 368 ohms
CoiledLength = 368 / 3 = 122.66 coiled inches = 10.22 coiled feet

So for people in 230 volt countries, you'll need 4 times the length of wire, but you can run it off half the amps.
For metric people, a direct conversion from www.infraredheaters.com works out to a wire of 6.35 mm diameter with a resistance of 3.2 ohms per meter.
However, the closest wire I can find is 6mm diameter, with a resistance of 3.8 ohms per meter, which means that I would need 96.84 straight meters to get 368 ohms

Please note that these calculations are in an "ideal" world

If there are any errors, please notify me, and I'll make changes
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jegner
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Joined: 30 May 2003
Posts: 2144
Location: Texas, USA

PostPosted: Mon Dec 05, 2005 8:26 am    Post subject: Reply with quote

WOW! Some great math there and that sounds about right for the length f material that John H. and I used for our ovens.

Thanks for sharing!
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crashmann
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Joined: 27 Sep 2005
Posts: 501

PostPosted: Mon Dec 05, 2005 9:05 pm    Post subject: Reply with quote

As Darth Vader would say... "Impressive. Most impressive." Razz

It looks like you went all out with the formulas just like I did, and even further! Your numbers came out very close to mine, so you are right on track. Especially the calculation for number of ohms per coiled inch. I just used a ruler, measured out 1 inch, stretched it so the coils weren't touching, and took a reading. It came out to ~3 ohms.

Once you've got your oven wired up and plugged in, please let me know how it runs based on the calculations. Initially I was disappointed with the output from my oven, and found I needed to reduce the resistance (shorten the nichrome wire) in order to get decent heat output. I chalked it up to AC power calculations being a little lower than DC power.

In the end, I just stretched the wire a little further until the 20 amp breaker popped, then backed off.

Oh, and thanks for calculating the total straight length of the nichrome. 4,335" is incredibly long! That's 361 feet! Longer than an (American) football field, and with plenty left over!

Good luck!

Charlie
TI-386
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thebluecanary
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Joined: 02 Feb 2006
Posts: 123
Location: St. Louis, MO

PostPosted: Tue Feb 07, 2006 8:32 am    Post subject: Reply with quote

Oh this will be very helpful. I was getting ready to hunt down all the old formulas and things to do what you've already done for me. =) But you mentioned breaking it over 2 seperate breakers. Would you just basicly wire up two small seperate 'ovens' inside the one oven? Like one set of outter coils, and one set of inner coils? I was thinking of doing this (I have two outlets that actually run off 2 differnet 20 amp breakers right next to each other.) The guy who wired my house before me was on crack, but he did wire up one hell of a nice workroom.
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crashmann
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Joined: 27 Sep 2005
Posts: 501

PostPosted: Thu Feb 09, 2006 10:44 pm    Post subject: Reply with quote

You are exactly right, there will be two separate coils inside the oven. That's a great way to spread the load across two separate breakers, and get plenty of heat.

Good luck!

Charlie
TI-386
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thebluecanary
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Joined: 02 Feb 2006
Posts: 123
Location: St. Louis, MO

PostPosted: Fri Feb 10, 2006 8:30 am    Post subject: Reply with quote

crashmann wrote:
You are exactly right, there will be two separate coils inside the oven. That's a great way to spread the load across two separate breakers, and get plenty of heat.

Good luck!

Charlie
TI-386


Looking over, I think breaking it at the 'C' point would be best. That gives about 2 coils of the same lenght, one for inner one for outer. I'll have to give it a try and see what I get. If it doesn't work right, well, lets just hope it does. =)

edit: so 11.5 ohms for the total of each segment to keep the 20 amps.
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crashmann
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PostPosted: Fri Feb 10, 2006 10:05 pm    Post subject: Reply with quote

As my math teacher would always say... "Let's see your work" Smile

Current = Voltage / Resistance

Current = 120 vac / 11.5 ohms

Current = 10.4 amps
--------------------------------------
Power = Voltage * Current

Power = 120 vac * 10.4 amps

Power = 1248 watts (per coil)

Total Power = 2 * 1248 watts = 2496 watts

Not too shabby. My oven (according to the formulas) 20.5 amps * 120 vac = 2460 watts, so you would have a little more power output. And no fear of tripping a breaker Smile

Don't forget, you've got to plug in the shop vac somewhere, and that motor pulls a good amount of juice too! Hopefully turning on the vac doesn't cause the breaker to trip (or you could run the vac on a third circuit).

Charlie
TI-386

(Did I mention it's a good idea to keep a fire extinguisher handy? Razz)
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Stomper
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Joined: 01 Oct 2005
Posts: 473
Location: Woodburn, Oregon USA

PostPosted: Sat Feb 11, 2006 7:59 pm    Post subject: Reply with quote

If you are curious about the BTU output, there are 3.41 BTU's per watt,
so you are putting out a total of:


2496 x 3.41 = 8,511.36 BTU's of heat.
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thebluecanary
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Joined: 02 Feb 2006
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Location: St. Louis, MO

PostPosted: Mon Feb 13, 2006 8:35 am    Post subject: Reply with quote

crashmann wrote:
As my math teacher would always say... "Let's see your work" Smile


Yeah yeah, I always got marked off in school for not showing my work. But I got the right answer, why do I need to show my work? =P

I plan on running the oven from either two 20 amp breakers, and the vac from one 15 amp. But one of the 20 amps in on the other side of the room, lets say 10 - 15 feet away. I might try running the Vac from this breaker, and one of the ovens from the 15 amp ones. As long as the math is right. =)

crashmann wrote:

(Did I mention it's a good idea to keep a fire extinguisher handy? Razz)


Oh I am so going to get one for electrical fires. Lowes should have what I need. I'm going to wait till its a little warmer out, just because.
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jegner
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Joined: 30 May 2003
Posts: 2144
Location: Texas, USA

PostPosted: Mon Feb 13, 2006 8:52 am    Post subject: Reply with quote

Safety is important! Wink Understanding what the oven does is important too. When I started working on my oven, it was hard to find anyone that knew much about what I was trying to do. [One reason I started this site and this board]

Just be sure to use 10 gauge wire when wiring the oven, all the way back to the switch. Then use a good heavy duty wire [I used left over 10 guage with ground wire] to the plug.

Test in short intervals. Then shut everything down, and make sure all is ok.

I never had an issue with a 20amp garage circuit, unless I tried to run the shop vac on the same circuit.

Jim
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thebluecanary
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Location: St. Louis, MO

PostPosted: Mon Feb 13, 2006 9:11 am    Post subject: Reply with quote

jegner wrote:
I never had an issue with a 20amp garage circuit, unless I tried to run the shop vac on the same circuit.


My lights are on the same 20amp circuit, so splitting it just seemed to be a good idea.
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crashmann
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Joined: 27 Sep 2005
Posts: 501

PostPosted: Mon Feb 13, 2006 11:33 pm    Post subject: Reply with quote

jegner wrote:
Safety is important!

...

Test in short intervals. Then shut everything down, and make sure all is ok.


Oh, you can't imagine how nervous I was when I flipped the switch! (Thus the reason why I used a 2 x 4 to turn it on and off Razz) I also only ran for a few minutes, shut down, started to breathe again, then ran a little longer.

As long as you're splitting the oven and vac load across separate breakers you should be fine.

I prefer to run my oven outside so the obnoxious fumes don't build up in my house. But when the temperature drops, or it's very windy, I run the oven just inside the basement door with fans running. The fumes may not be toxic, but some of the folks I've seen at GE Polymershapes are a few beers short of a 6 pack! Good ventilation is very important!

When you flip the switch, you've got to holler out the mandatory "Fire in the hole!"

Charlie
TI-386
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