Specs for a Vacuum Chamber and Pressure Chamber

cavx

cavx

Master Member
EDIT: Can't seem to edit the title which should read "What Pressure Is Required For Pressure Casting And What Vacuum Is Needed To Degas Resins and Rubber?

Hi Guys,

I am wanting to buy a Vacuum Pump that can be used to degas as well as also be used (via a tank) to really make my Vac Form Table perform.

Right now I am using a vacuum cleaner for the Vac Form Table and whist it works on small items, it does not provide enough suction for larger jobs. As I understand this, I need a tank that can be de-pressurized by a Vacuum Pump. The pump is shut off once full vacuum is achieved and the suction to the table is controlled by a ball valve. If that makes sense.

Q: How large would the tank need to be?

I am thinking that this tank could also be used as a Vacuum Chamber.

Q2: Can I do this or am I best to have a separate chamber?

I also would like to get a pressure chamber for casting. And specs for this as well would be very much appreciated.

Q3: What pressure is actually needed for casting?
 
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This is kind of what I had in mind for the Vacuum/Pressure chamber.

I am not exactly sure where to mount the hardware and will probably have this laid on its side so long as it can done up quickly for casting.

I will require an ball valve on the side (the inlet can be used to connect to the Vac Form Table), an outlet to the Vacuum Pump, a Vacuum gauge and then a an input for the compressor and a pressure gauge. Is there anything I've missed?

attachment.php


Whilst the Vacuum holds the lid on, I will need attachment points for the pressure system. Is 4 enough? The tube is 250mm PVC. It has a 6mm wall thickness and is the same type of pipe I used when I built a Vacuum Chamber in 2006. I never owned a pressure pot before, so am really guessing this.

I've drawing the lid (perspex) and base (PVC?) at 25mm thick. From memory, the base on my last Vac Chamber was only about 10mm or 12mm and whilst it did seem to bow slightly in during operation, was plastic welded to the tube so never split or broke.

I am just an not sure if this thing needs to be able to hold 100PSI as I have read that just 20PSI will crush bubbles in casting.

Experts please chime in.
 
Thanks for replying. I need to look further on line before purchasing a pump.

I got a quote for that chamber at $100.00 and that is with 25mm thick lids and base, so pretty happy with that.

I found a company that sells Vacuum Pumps in both dry and oil. For the price difference I think the oil is the way to go, but the price is considerably higher than I was hoping to pay. I also learned that I can use old LPG cylinders as vacuum storage and the units that are outside their usable dates for holding gas can be bought cheap.
 
I would not recommend trying to build your own pressure chamber since paint pots are readily available and easy to convert for casting resin. For thin resin, 32-35 psi is usually adequate, and most all the paint pots can do 50psi easy so there is plenty of safety. Unless you really, really know what you are doing, a homemade pressure pot is really just a bomb.

For a vacuum pump, the oil type will pull a deeper vacuum than a dry type. While not as critical for the vac-forming application, you will want the oil-type pump for degassing silicone rubber. You need to pull at least 28"Hg for that, and the closer you can get to 30"Hg the better. Dual-stage rotary pumps are the best, but I have a cheaper single-stage that works just fine down to 29"Hg.

Hope this helps :)
 
Thank you Fireball Joe for taking the time to respond. I understand how dangerous a pressure pot can be and why I am asking how much pressure should I be using and about clips to hold this thing together.

attachment.php


This is how I am thinking this needs to be done with the rods being no less than 3/4" threaded rods (might even go to 1") and large wing nuts and washers to hold them down. The clamps would be welded and made from 1/4" steel. I don't think there is anything considered 'over kill' with something like this.

Anyway, things have moved faster than I though they would and this arrived today.

attachment.php


And this is why I am keen to get my tank and vacuum pump ASAP. I think I might as well get the oil pump, just not happy about the price, so will continue shopping around.

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The heat was good, but there was just not enough suction to pull this down. It is made from 3mm ABS. I am also in the process or a complete redesign of the Platen, Vac Box and frames that hold my plastic.
 
You are not asking for design suggestions, but TAP Plastics has a simple design that I was going to copy "some day".

Vacuum_Pump.jpg


Very similar to your idea, but a steel pipe section welded to a steel plate.
 
So maybe I need to finish the Vac Chamber (as this is how I built my last one in 2006) and look at steel for the pressure pot. Two chambers means more floor space, but hey, I would rather lose some floor space than my life or limbs should this plastic thing pop.

It took time, but a rep from Aluminite got back to me about the pressure and it is 80PSI, so allot.

Also after reading Volpin Props' blog, I have began a complete redesign on the Vac Form table and started on a new heating housing today. I do have a concern about the heating elements though. What insulates (apart from the ceramic holders) the coil wire?
 
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cavx,

I'm still trying to decide which way to go for my vacuum chamber but I do have this Pressure Pot from Harbor Freight that I use for casting my own blanks for the pens I turn. The tank is approx. 10" in diameter by 14-15" deep and has a rounded bottom. This means you'll probably need to make some type of rack or platform in order to keep your forms level.

I have a small compressor that I keep down in the basement just for charging this tank. I usually have 6-8 blank forms (3/4" PVC tubes) in a custom-made rack at a time and charge the tank to 60-70 PSI. Using Alumilite, I generally leave the blanks under pressure for 25-30 minutes, and have yet to have issues with air bubbles in my castings.

Hope this helps,

R/Derby
 
Here is mine-- Just a lil ole thing-- 20" in diameter and 24" in height-- need to upgrade from my 6cfm to a 9 cfm vacuum pump tho--
 
The issue with pressure chambers is two-fold. 1-- being able to seal the chamber in time that the application of pressure will actually do some good. 2-- safety.

I solved these issues by having my chamber custom-built by a very knowledgeable welder. My chamber is a box, made of a large section of 'square' steel tubing. The wall thickness is at least .375 inches. To the back of this box is welded a steel plate, the same thickness. The front has a 'frame' made of .5 x 1" bar stock. The door is .375" steel, cut to fit freely inside the box. There are two brackets welded to the top, inside the box, for the door to rest on in the open position. The door has a rubber gasket applied to it, so when it is pulled down (much like a garage door) it seats against the frame and pressure is applied. The pressure pushes the door against the inside of the frame and thus remains closed until the pressure is relieved. With this arrangement, I can stick a mold into the chamber, close it and apply pressure within 5 seconds or so. This chamber was tested to the limit of my compressor, 120 psi, and did not explode. The door warped a tiny bit, but I've been using this for about 8 years and it is still going strong.

I realize this kind of chamber, being custom built, is not in everyone's future. I hope that this description can be used for someone to have their own built, to their own size. The guy who made mine was in the business of installing high pressure stainless steel dairy equipment, so he was about the best welder to do this kind of thing. Who ever you may find to do this work, he'd better be well acquainted with high pressure use of his work!!

Scott
 
TopSnipe said:
cavx,

I'm still trying to decide which way to go for my vacuum chamber but I do have this Pressure Pot from Harbor Freight that I use for casting my own blanks for the pens I turn. The tank is approx. 10" in diameter by 14-15" deep and has a rounded bottom. This means you'll probably need to make some type of rack or platform in order to keep your forms level.
Click to expand...

A vacuum chamber can be quite simply a tube and two flat plates with rubber seals. The suction holds the lids/base on. It would easy to convert a pressure pot into a vacuum chamber, but not the other way around.

I have a small compressor that I keep down in the basement just for charging this tank. I usually have 6-8 blank forms (3/4" PVC tubes) in a custom-made rack at a time and charge the tank to 60-70 PSI. Using Alumilite, I generally leave the blanks under pressure for 25-30 minutes, and have yet to have issues with air bubbles in my castings.

Hope this helps,

R/Derby
Click to expand...

It does very much, thank you. So the resin is set in that short time?
Does anyone use one of those Vac Pumps that runs off a compressor?

Eaglewood said:
Here is mine-- Just a lil ole thing-- 20" in diameter and 24" in height-- need to upgrade from my 6cfm to a 9 cfm vacuum pump tho--
Click to expand...

I just got myself a small Vacuum Pump which is just 3CMF. It will be interesting to see how long it takes to evacuate the air from the chamber or surge tank (for the Vac Table). Whilst I do want to degas, the main reason for the purchase at this stage is for the Vac Form. I guess I can always make a smaller Vac Chamber if need be.

CaptCBoard said:
The issue with pressure chambers is two-fold. 1-- being able to seal the chamber in time that the application of pressure will actually do some good. 2-- safety.

I solved these issues by having my chamber custom-built by a very knowledgeable welder. My chamber is a box, made of a large section of 'square' steel tubing. The wall thickness is at least .375 inches. To the back of this box is welded a steel plate, the same thickness. The front has a 'frame' made of .5 x 1" bar stock. The door is .375" steel, cut to fit freely inside the box. There are two brackets welded to the top, inside the box, for the door to rest on in the open position. The door has a rubber gasket applied to it, so when it is pulled down (much like a garage door) it seats against the frame and pressure is applied. The pressure pushes the door against the inside of the frame and thus remains closed until the pressure is relieved. With this arrangement, I can stick a mold into the chamber, close it and apply pressure within 5 seconds or so. This chamber was tested to the limit of my compressor, 120 psi, and did not explode. The door warped a tiny bit, but I've been using this for about 8 years and it is still going strong.

I realize this kind of chamber, being custom built, is not in everyone's future. I hope that this description can be used for someone to have their own built, to their own size. The guy who made mine was in the business of installing high pressure stainless steel dairy equipment, so he was about the best welder to do this kind of thing. Who ever you may find to do this work, he'd better be well acquainted with high pressure use of his work!!

Scott
Click to expand...

WOW nearly 10mm wall thickness. Thank you for that. I spoke to an engineer yesterday who basically said he can custom build one for me that will not explode if I really want to go down that path but it won't be cheap. He seems to think that if I start with a Vacuum Chamber and degas my rubber and elastomer first, I will have a better result. If I still need a pressure pot after that, come back and talk to him.
 
tonykch said:
I am a huge fan of my vacuum chamber I built from Harbor Freight parts.

Paint Pressure Tank - 2-1/2 Gallon
Electric Vacuum Pump - 2.5 CFM

That little 2.5 CFM pump will suck most of the air out of my 2 1/2 gallon chamber within 10 seconds.
Click to expand...

Thanks for sharing. I seem to think the last Vac Pump I owned back in 2006 was about the same. It could pull -27HG on the gauge in about 16 seconds. My chamber was about the same size as the one I am making this time.
 
I have and use the Tap Plastics unit.This is my pressure pot,one is on sale at Amazon.it is made by All American Canning #921.Seal the pop off valve hole with silicone,add a quick release fitting and your good to go.Been casting with these for over 12 years now.
 
falcondesigns said:
I have and use the Tap Plastics unit.This is my pressure pot,one is on sale at Amazon.it is made by All American Canning #921.Seal the pop off valve hole with silicone,add a quick release fitting and your good to go.Been casting with these for over 12 years now.
Click to expand...
Amazon sells everything these days. I think the idea of a "pop off valve" is good. How much pressure does it require to blow out?
 
cavx said:
Amazon sells everything these days. I think the idea of a "pop off valve" is good. How much pressure does it require to blow out?
Click to expand...

These are used for food canning,for resin work the hole must be sealed or it would pop at about 20psi.The dial says 30,but I've put 50,more than enough to burst any bubbles in the mix.
 
Doing a quick search now on Amazon and found this

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Product Specifications
Part Number CAT-365
Power Sourceair-powered [what does that mean?]
Item Package Quantity1
Maximum Pressure80 PSI [what we need :)]

Product Features

5 Gallon Pressure Pot
Removable Caster Wheels
Easy to use agiator
Internal dimensions are 12-1/2" wide x 11" deep w/ lid applied.
HVLP Spray Gun, Paint Hose and Air Hose

$475, looks like it is ready go.

I doubt I could have one made at that size for that price.
 
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