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Got vac pump...what's next? (progress update-09 Mar)
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Stomper
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Joined: 01 Oct 2005
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Location: Woodburn, Oregon USA

PostPosted: Fri Jan 18, 2008 9:14 pm    Post subject: Reply with quote

cod wrote:
huh, I calculate 108 U.S. gallons- but that can't be right, can it ?


could you pour 108 gals of milk in your tank? Wink


her's my math:

cylinder volume = r*r*h*Pi

19.75 is roughly inner radius, assuming .25" tank walls

= 26,945.5175 cubic inches

divide by 1728 for cubic feet= 15.593470775463

convert to US gals = 116.64



bahhh. I get different results every time!

good luck.


I get nearly the same results as you do... (based on the outside diameter), then "eyeballing" the difference you got for the inside diameter you are right there AFAIK. Wink
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drcrash
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PostPosted: Sat Jan 19, 2008 9:59 am    Post subject: Reply with quote

Are the given dimensions right? Is that tank almost three and a half feet in diameter? Are the dimensions reversed, maybe?
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ANH trooper
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Joined: 20 Oct 2005
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PostPosted: Sat Jan 19, 2008 4:54 pm    Post subject: Reply with quote

Sorry my mistake Embarassed

It's 40" circumference and 22" tall. The width is 12" so I suppose the radius is 6".

Yeah....I'm thick Laughing
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drcrash
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PostPosted: Sat Jan 19, 2008 8:11 pm    Post subject: Reply with quote

ANH trooper wrote:
Sorry my mistake Embarassed

It's 40" circumference and 22" tall. The width is 12" so I suppose the radius is 6".


Somebody check my math here...

A 40 inch circumference would give a diameter of 12.73; I'm guessing that's a better measurement than the 12 inches?

Assuming something in between, and allowing for a bit of thickness to give the inside diameter, call it 6.25.

The cross-sectional area of the circular way is pi times 6.25 squared, or about 3.142 x 39, or about 122.54 square inches. Multiply that by 22 to get about 2696 cubic inches.

2696 cubic inches divided by 1728 (cubic inches per cubic foot) is about 1.56 cubic foot, which is 11.67 U.S. gallons.

Minus a little for the roundedness of the end domes (?), I'd say that's about an 11 gallon tank.
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ANH trooper
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PostPosted: Sun Jan 20, 2008 7:29 am    Post subject: Reply with quote

Cool thanks Dr Very Happy

What do you reckon about the size of the tanks? Will one be enough for a 24" x 24" platen or would you use two?
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ANH trooper
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PostPosted: Sun Jan 20, 2008 10:02 am    Post subject: Reply with quote

Take a look at this diagram and let me know what you think.


Valves 2 and 3 are closed while a vacuum is being pulled and then valve 1 is closed to hold the vacuum in the tanks. Valve 3 is then opened once the heated plastic hits the platen to suck down the plastic. Valve 3 is then shut and valve 2 opened to allow the vacuum pump to keep pulling until the plastic has cooled.

Valves 2 and 3 will be up near the platen and valve 1 will be down below out of the way and will not be used in the forming process, only to hold the vacuum in the tanks.

I am not competely sure if the tanks have to be shut off while the vacuum pumps keeps the pressure applied to the plastic, but I would imagine it would so there is less air to be removed.

I just want to know what you guys think before I plumb this lot up.
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Stomper
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PostPosted: Sun Jan 20, 2008 10:48 am    Post subject: Reply with quote

IMO you have too many valves to fiddle around with.

1.
Delete the separate circuit that valve-1 is on

2.
Put valve-2 on the outlet of the tee going to the platen. (between the platen and the tee)

3.
Place the vac-gauge where valve-2 currently is.

You now will have a two-handed valve assembly that you have a better chance of operating "simultaneously" instead of in "stages", which could pose problems when you have hot plastic that is solidifying quickly. When I get around to building my machine, I'm going to use a 3-way valve for 2-stage operation.
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drcrash
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PostPosted: Sun Jan 20, 2008 11:40 am    Post subject: Reply with quote

Check out the second diagram in the first posting in this thread over on CNCzone:

http://cnczone.com/forums/showthread.php?t=38493

(That's linked from my www.vacuumformerplans.com site, BTW.)

If your vacuum pump can keep ahead of leakage around the edge of the platen, one medium-small tank will work fine. (I think an 11 gallon tank should be fine for a 2 x 2 foot machine.) You only use the tank for the initial pulldown, then close it off and use the vacuum pump alone for the long hard pull.

The initial pulldown will pollute your tank, but when you close it off, the pump will pull a higher vacuum, limited by the ratio of the leakage rate to the pumping rate. When you're finished, you close the valve to the platen and open the valve to the tank to re-evacuate the tank.
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ANH trooper
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PostPosted: Sun Jan 20, 2008 2:37 pm    Post subject: Reply with quote

Thanks for the input!

I've taken your advice and altered the design. How's this?
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Stomper
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PostPosted: Sun Jan 20, 2008 4:25 pm    Post subject: Reply with quote

Looks perfect, and it is the same (schematically) as the one Dr. Crash referred to in his post. Wink
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ahillworks
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PostPosted: Mon Jan 21, 2008 5:51 pm    Post subject: Reply with quote

Now that we see this I wounder if my design is ok. I see there are two valves. I have just one. Whats the major difference. I have exta pipe i can always change what I have to make things better if needed. I just want some one to explain why having the two would make a differ.
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Stomper
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PostPosted: Mon Jan 21, 2008 7:16 pm    Post subject: Reply with quote

The pictures above illustrate a two-stage vacuum system.

When you open the valve to your tanks to make a pull, you are pulling air into them and this decreases your "vacuum power" as the tanks refill, so what you do is shut that valve to your tanks after the plastic has formed to your molds from the vacuum in the tanks, while leaving the other open to the platen so that the vacuum pump will pull against the seal of the plastic & the platen. This is a better arrangement because the vacuum level will increase faster as the volume of air is much smaller than the tanks... providing you don't have massive leaks between your platen surface & the plastic sheet. You can have much tighter pulls against your molds, and get away with using a smaller vacuum reservoir (tanks).
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ahillworks
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PostPosted: Tue Jan 22, 2008 1:39 pm    Post subject: Reply with quote

Got it will change this when i get home I already have all the piping and a ball valve. Knew I had extra for a reason =). See I would wasted money on PETG this week and wasted more time with less results. I knew I waited for a reason.
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ANH trooper
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PostPosted: Wed Jan 23, 2008 2:26 pm    Post subject: Reply with quote

Here's a couple pics of my machine so far.




I added the bottom shelf to store my plastic. The lifting mechanism is working, but still need to fine tune it and figure out the best way of keeping it in the upright position.

I need to get the vacuum tank modified to accept my plumbing. I thought it would be too easy to use the regulator on the bottle Laughing

My coil wire has arrived and I am using Supalux heat resistant fire board for the backing. I beleive this is the same stuff TJ uses. I haven't even marked out the spiral yet, but I do have all the cotter pins as I am going to make it like Dr.Crashs' design.
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ahillworks
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PostPosted: Wed Jan 23, 2008 4:25 pm    Post subject: Reply with quote

I love it but how did you do the pipping? I am trying to figur it out. I had to re do my pipes the other day.
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