building a vacuum /thermoforming machine

[drcrash]

Some useful references for beginning vacuum forming and vacuum former design:

Chad Veich has a nice intro to small, simple vacuum formers on rcuniverse.com:

http://www.rcuniverse.com/forum/Basi...m_81435/tm.htm

It's model plane-centric, but the issues are general.

The Studio Creations web site has a tutorial on stormtrooper costume-making that includes a nice, brief discussion of making and using a simple vacuum former, using your home oven. The vacuum former is little more than a board with a hole in it, some weatherstrip, and a couple of wooden frames for clamping the plastic. That's plenty good enough for a lot of stuff.

Ralis Kahn has an elegant design for a medium-sized over-and-under machine with its own oven on halloweenfear.com. Unfortunately, the little electric barbecue grill it uses as a heater is hard to come by and doesn't heat very evenly anyway. (But I've posted stuff about how to fix that on tk560.com.)

http://www.halloweenfear.com/vacuumformintro.html

For thin, forgiving plastics like styrene, it's not hard to make a good enough vacuum former for most purposes. If you want to form thicker and less forgiving plastic, you need a good oven, and maybe a high vacuum system. (Stronger than just a vacuum cleaner.)

The next step up is to build your own oven that heats very evenly. That's not difficult.

Thurston James's (excellent) book "The Prop Builder's Molding and Casting Handbook" has plans for a large flip-over vacuum former with its own oven. tk560.com has an article about actually building such a thing, with cost-saving substitutions. You can build a good 2 x 2 ft oven for about $100---or half that, because the ceramic insulators aren't really necessary.

The tk560 discussion board has a link to a nice calculator for helping adapt James's nichrome coil layout to different aspect ratios, sizes, or voltages.

If you're going to build your own oven, you should check out this thread on tk560.com's vacuum forming discussion board.

http://www.tk560.com/phpBB2/index.php

It's not hard to make an oven that heats evenly. Infrared is the way to go, unless you're up to the task of figuring out a good convection oven design. (Which could be even better, and not hard to make once you know how.) Cheap nichrome coil works fine.

One nice thing about resistive IR heating (nichrome coils, calrod bars, etc.) is that you can use the red glow from the coils to see pretty much where the IR is going. This allows you to identify and fix hot spots pretty well, if you turn out the lights and use a piece of ceiling light diffuser to view the (infra-)red distribution.

A reflective rectilinear box can do about as well as anything else, if you have uniform heating elements, evenly spaced, and the distance from the elements to the plastic is greater than the spacing of the elements. (That gives the IR from the elements a chance to spread out and overlap before hitting the plastic.) Cheap aluminum flashing works great, because aluminum is very reflective to infrared.

Beyond that, you want some extra heat around the edges to compensate for uneven convective losses from the other side (especially in an oven that only heats one side), and conductive losses to the clamping frame.

All of this stuff is (and a lot more) is discussed in various threads on tk560.com.

Hobbymolding.com also has a vacuum forming forum. (But it's less active.)

http://www.hobbymolding.com/hm_forum/phpBB2/index.php

You may also be interested in Doug Walsh's book "Do it Yourself Vacuum Forming for the Hobbyist." It's packed with useful information. (Most of that info is available somewhere on the web, but the book is not expensive and brings it all together.) You can find it on ebay or from his site [url]http://www.build-stuff.com[\url].

[drifter]

ok, i've had enough. i've been making car interiors out of fiberglass the last few months. but i've really had enough. it takes 3 hours to lay up the fiberglass properly for a singer door panel, plus the time you spend on trimming, bondo, etc. and the dust is just killing me, literaly
I AM GOING TO TRY THIS PROJECT
ok...so..the specs again. 1500/1500 milimeters (i'm from europe, romania so i'll give the sizes in metrical units)
i want to make it out of 9 500/500 mm modules, that you can lay up in different patterns(the basic 1500/1500, or say 1000/1500,2000/500,etc) that means, both the box and heater will be made modular.(heater above the table)
i have 3 tanks, i want to make vacuum in with a pump, and then use the vacuum from those tanks to draw out the air. ok, so, what do i need:
pump capacity, how much vacuum can be achived in the tanks, aproximatley (i'l post the size of the tanks the following days), heater capacity, (watts), per module (500/500 milimeters).
pleeease give me all the input you can. i'll post some sketcher theese days

[307startup]

The thing to remember with pressure is that it takes very little pressure to move something, the larger the object is, relatively speaking...the seal is very important, but I have used line water pressure from the city to make forms out of steel. By pinch-welding around the seam, pumping water into the form and releasing air pressure before continuing to pump water in, I was able to get some significant blows before needing to anneal the parts. With a 4'x8' sheet you are talking about 4608 in2...at 10 psi you are talking about 20 TONS of pressure on that sheet...1 psi should be more than adequate for forming ABS or PVC sheet.
However with vacuum you don't really need that powerful of a pump if you can get a decent seal as you lower the heated sheet onto the form...the problem is getting it to seal before appling the vacuum. If you have a raised lip that is as high as the highest point in your form, as you lower the sheet it will be sealed making it much easier to draw with a smaller vacuum source. OF course this means you are limited to the size of the part or making multiple parts with that 4'x8' sheet because you have the sheet drawing down the sides of the seal as well as the shape of the form.

[drcrash]

I AM GOING TO TRY THIS PROJECT
ok...so..the specs again. 1500/1500 milimeters (i'm from europe, romania so i'll give the sizes in metrical units)
i want to make it out of 9 500/500 mm modules, that you can lay up in different patterns(the basic 1500/1500, or say 1000/1500,2000/500,etc) that means, both the box and heater will be made modular.(heater above the table)

hmm... let me see if I understand this... You're saying that you want to build a heater and a platen that are each made out of 50cm (just under 20 inches) square modules, so that you can combine them into various sizes and aspect ratios to make, say, a 2 module x 3 module grid that's 100 cm by 150 cm (a bit shy of 40 x 60 inches), etc? Interesting.

What is the maximum size of thing you will be making, and what's it shaped like? In particular, how deep is the draw?

The volume of air you'll need to suck out (within a very few seconds) will depend on the volume of air under the plastic. If you'll be using negative (cavity) molds, that'll basically be the volume of the molds. If you'll be using positive (male) molds, you should think about the "tent" of plastic that you get when you stretch the plastic over the molds. Tall, peaky molds will make a bigger tent than short or gently convex molds, so there will be more air to suck out.

It's common for 2 foot x 2 foot (607 mm x 607 mm) formers to use about 3/4" inside diameter pipe (which is really about 20 mm inside). That lets a high-vacuum system suck about a cubic foot of air out from under the plastic in a second or so.

To scale that up, you want to increase the pipe's cross sectional area (and flow capacity) to match the increased volume of air under the plastic. If you increase all three dimensions of your molds by the same scale factor, you increase the volume of air under the plastic by the cube of that factor.

For example you double the platen and molds sizes in each of 3 dimensions, you increase the volume by a factor of 8. To increase the pipes' flow capacity by the same factor, you need to increase the diameter by the square root of 8, or a factor of about 2.83.

If your molds are a friendly shape, this may not be necessary, and you may only need to scale the pipe diameter up by the same factor as the platen, so that the cross-sectional area of the pipe increases with the area of the platen. (For example, if you were just using a big former to do a bunch of small parts at a whack, that would be true.)

i have 3 tanks, i want to make vacuum in with a pump, and then use the vacuum from those tanks to draw out the air. ok, so, what do i need:
pump capacity, how much vacuum can be achived in the tanks, aproximatley (i'l post the size of the tanks the following days), heater capacity, (watts), per module (500/500 milimeters).
pleeease give me all the input you can. i'll post some sketcher theese days

What sizes are your tanks?

If you use a one-stage system, with your tanks ganged together in parallel to act like one big tank, you need tank volume that's several times the amount of air you'll be pulling out from under the plastic. For example, if you have a cubic foot of air to evacuate, and two cubic feet of vacuum (tanks), sucking the air out will pollute the tanks so that they have 1/2 atmosphere of air in them---not a very strong vacuum. But if you have 10 cubic feet of vacuum, you won't pollute the tanks nearly as much---they'll only have about 1/10 an atmosphere of air in them, and pull almost as hard as if you had an infinitely large tank.

You may want to make a two-stage system, where you use one tank to basically pull the plastic down, then close it off and open another valve to the other tanks. That way, one tank will suck up most of the air without polluting the other tanks and weakening their vacuum. Once most of the air has been sucked into the first-stage tank, you can use the others to pull the remaining air out without polluting them much, and pull the plastic down really hard to get good detail.

Another version of that is to use vacuum cleaner pump/motor assemblies to do the initial pull-down, then use vacuum tanks to do the final hard pull. (That's what I generally do.)

A big advantage of that kind of two-stage arrangement is that you don't need nearly as much tank volume, or as much pump capacity to evacuate the tank(s) in a reasonable period of time. You can use cheap high-volume low-vacuum pumps to do the initial pull-down, and cheap low-volume high-vacuum pumps/tanks to do the final hard pull.

By the way, what kind(s) of tanks do you have? Some tanks built for several atmospheres of pressure will implode under less than one atmosphere of vacuum. (Ask me how I know.)

[tomoohoi]

I am making vacuum forming machine, but still encountering with difficulties.
the forming table size is huge 2400mm X 1000mm , the tank volume is appraxiomately 1 cubic meter , pump capacity 80000 pa (ie 23 in.ng).
it is pulling the plastic down but, stiil can't to get good detail ,
may be not enough capacity of vacuum? i liked two-stage system of tank , may i use vac shop for initial pulling? what will be the capacity?

hmm... let me see if I understand this... You're saying that you want to build a heater and a platen that are each made out of 50cm (just under 20 inches) square modules, so that you can combine them into various sizes and aspect ratios to make, say, a 2 module x 3 module grid that's 100 cm by 150 cm (a bit shy of 40 x 60 inches), etc? Interesting.

What is the maximum size of thing you will be making, and what's it shaped like? In particular, how deep is the draw?

The volume of air you'll need to suck out (within a very few seconds) will depend on the volume of air under the plastic. If you'll be using negative (cavity) molds, that'll basically be the volume of the molds. If you'll be using positive (male) molds, you should think about the "tent" of plastic that you get when you stretch the plastic over the molds. Tall, peaky molds will make a bigger tent than short or gently convex molds, so there will be more air to suck out.

It's common for 2 foot x 2 foot (607 mm x 607 mm) formers to use about 3/4" inside diameter pipe (which is really about 20 mm inside). That lets a high-vacuum system suck about a cubic foot of air out from under the plastic in a second or so.

To scale that up, you want to increase the pipe's cross sectional area (and flow capacity) to match the increased volume of air under the plastic. If you increase all three dimensions of your molds by the same scale factor, you increase the volume of air under the plastic by the cube of that factor.

For example you double the platen and molds sizes in each of 3 dimensions, you increase the volume by a factor of 8. To increase the pipes' flow capacity by the same factor, you need to increase the diameter by the square root of 8, or a factor of about 2.83.

If your molds are a friendly shape, this may not be necessary, and you may only need to scale the pipe diameter up by the same factor as the platen, so that the cross-sectional area of the pipe increases with the area of the platen. (For example, if you were just using a big former to do a bunch of small parts at a whack, that would be true.)



What sizes are your tanks?

If you use a one-stage system, with your tanks ganged together in parallel to act like one big tank, you need tank volume that's several times the amount of air you'll be pulling out from under the plastic. For example, if you have a cubic foot of air to evacuate, and two cubic feet of vacuum (tanks), sucking the air out will pollute the tanks so that they have 1/2 atmosphere of air in them---not a very strong vacuum. But if you have 10 cubic feet of vacuum, you won't pollute the tanks nearly as much---they'll only have about 1/10 an atmosphere of air in them, and pull almost as hard as if you had an infinitely large tank.

You may want to make a two-stage system, where you use one tank to basically pull the plastic down, then close it off and open another valve to the other tanks. That way, one tank will suck up most of the air without polluting the other tanks and weakening their vacuum. Once most of the air has been sucked into the first-stage tank, you can use the others to pull the remaining air out without polluting them much, and pull the plastic down really hard to get good detail.

Another version of that is to use vacuum cleaner pump/motor assemblies to do the initial pull-down, then use vacuum tanks to do the final hard pull. (That's what I generally do.)

A big advantage of that kind of two-stage arrangement is that you don't need nearly as much tank volume, or as much pump capacity to evacuate the tank(s) in a reasonable period of time. You can use cheap high-volume low-vacuum pumps to do the initial pull-down, and cheap low-volume high-vacuum pumps/tanks to do the final hard pull.

By the way, what kind(s) of tanks do you have? Some tanks built for several atmospheres of pressure will implode under less than one atmosphere of vacuum. (Ask me how I know.)

[drcrash]

I am making vacuum forming machine, but still encountering with difficulties.
the forming table size is huge 2400mm X 1000mm , the tank volume is appraxiomately 1 cubic meter , pump capacity 80000 pa (ie 23 in.ng).
it is pulling the plastic down but, stiil can't to get good detail ,
may be not enough capacity of vacuum? i liked two-stage system of tank , may i use vac shop for initial pulling? what will be the capacity?

It's hard to say what's going on without a lot more information, and preferably pictures. (Of your bucks, your platen seal, your plumbing, and some test pulls.)

How much air is trapped under the plastic when you pull the plastic down over the bucks? How good is your seal? How hard is your plastic to form?

Those things will determine how much airflow you need for the basic pull-down and to keep up with leaks until the plastic has cooled below its thermoforming range.

You need a tank big enough to suck up most of the initial air under the plastic without unduly weakening the vacuum in the tank. Or if you go two-stage with high-volume low-vacuum pump such as a shop vac, you need enough CFM's to get the air out, and enough inches of mercury (typically very few) to suck the plastic down most of the way. (If the latter is a problem, maybe because your plastic is very thick, you could go with a two-tank two-stage system.)

How big are your pipes? Are your valves that big and full-ported?

Are you getting the plastic hot enough?

Do you have a vacuum gauge? What happens during a pull?

Do you have an infrared thermometer? (That's worth 20 bucks from Harbor Freight for such a big project.)

[tomoohoi]

Thanks a lot for reply,

I am planning to make plastic moulds for concrete floor tiles using vacuum forming .

1. the buck is 300mm X 300mm X 20 mm wooden square board , and 600x600x 20 mm and other sizes,

2. the sealing , is so big problem , but i think i did it, a little bit vacuum go out from sealing,

3. i am using air comprosser instead of vacuum pump, air compressor sucks air from tank and creating vacuum with 0.75 kgf/sq.cm (ie 73549 pa ) in tank. i have vacuum gauge , it points up to 1 kgf/sq.cm , when second pulling down finished its points stopped between 0.4-0.5 kgf/sq.cm , but mould is still in not good details


4. and air compressor is used also evacuate air between plastic and buck , ie used as initial pull down. ( i've read your post about 2 stages pull down , i think this is very good idea)

5. the plastic sheet is PVC ,thickness 0.5 mm , i tested it in many tempreture 200C , 250C , and 350 C etc... i have thermometer.

in low temperature , the poor details mould appears , in high temperature too big sagging and too much webbing, mould details is better than used low temperature , but still not good

So, what i shall do? may be i need vacuum pump with bigger capacity?

[tomoohoi]

forgot to say other things,

the tank size is with diameter 500 mm and heigth 1300mm cylinder.
pipe is 25 mm diameter

[drcrash]

How big are the plastic-holding frames and platen you're actually using, and how do you create a seal around the edge? Does the plastic pull down around the platen and form around the edge of the platen to make a seal directly, or do you have a gasket, or does it form around the upper edge of a box around a female (concave) mold... or what?

I assume that you are forming over male (convex) molds, and that the webbing you're seeing is around the edges... is that correct? Is your mold shape a simple shallow box? Is there a space under it for air to flow from around the edges and under the mold, to a vacuum hole?

Do you know if there's good airflow from all over the mold into your vacuum pipe?

If you're actually using a 2400 mm x 1000 mm (94.5 inches x 39.4 inches) platen, a 25 mm (one inch) pipe is rather small. I'm not sure if that would be a problem for what you're doing, with very shallow molds. I suspect that it should be okay.

25 millimeters (about one inch) is usually about the right size for a 2 ft x 4 ft (1218 mm x 609 mm) platen. (About three quarters of a square meter, as opposed to your 2.4 square meters.) But if there's not much air under your plastic because your molds are so shallow, that is likely not the problem.

There may be an airflow bottleneck somewhere else, though... not enough vent holes, paths from them to the pipe that are too small, valves that are too small.

Do you know if the openings inside your valves are about the same diameter as the pipe? Some valves restrict flow even when they're fully open, but some don't. If yours do, that could be slowing down your initial pull-down, so that the plastic cools too much before it's formed.

Do you get good detail in some places on your molds, and not others, or is the quality of the forming pretty uniform?

If it's uneven, you may not have enough airflow from some parts of the mold, or you may not be heating the plastic evenly.

Does your thermometer tell you the actual temperature of the plastic, or just the temperature of the air in the oven? An infrared thermometer is good for telling how hot you've actually gotten the plastic, and can be cheaper than a big sheet or two of test plastic. It can also tell you which parts of the plastic are not getting as hot as others. (You point it at a small area and it tells you the surface temperature there.)

How much are you letting the plastic sag before forming it? (Unfortunately, I'm not experienced with big sheets of plastic, so I don't really know how far your size of plastic should sag.)

By the way, if you are using a huge platen and much smaller molds, you should probably make some smaller frames and a gasket so that you can use smaller sheets of plastic for tests. That is easy and cheap to do. I can hunt up some links if you're interested. If using plastic just big enough for one floor tile mold works better, that may tell you something useful.

Paul

[tomoohoi]

i attached picture of scheme of my vacuum processing , sorry it not so nice depicted by hand but hope you can understand it.
as for the sealing, it was difficult to keep sealing when use gasket between frames , so i did sealed platen , the plastic pull down around the platen and form around the edge of the platen to make a seal directly. The frames just for holding plastic sheets. ( not for sealing)

the frame which holds plastic sheet is adjustable , ie , plastic size is variable, if smaller plastic down to platen other remained area of platen has covered and sealed with tape.

Yes , mould is male , and it has spaces under it to air flow around it.
i've checked the diameters of pipe and valve ,and diameter of opening valve is little bit smaller than pipe's.

yes, sometimes i get good detail in some places on molds, specially in rear side .the quality of the forming not uniform . but sometimes.

thermometer tell me just the temperature of the air in the oven.

the manometer tells maximum - 0.75 kgf/sq.cm ( ie -73550 pa )

I've tried many times in many way to get nice forming , I've got tired , may be last hope is to change vacuum pump with high pressure?
or does any way to increase vacuum capacity ? i have 2 air compressors , is there any way to increase pressure by combining 2 air compressor ?

[drcrash]

So, what i shall do? may be i need vacuum pump with bigger capacity?

I think your pump is fine, if you're pulling a 1-cubic-meter tank down to 20+ inches of mercury. (You're getting around .72 atmospheres of vacuum, right?---so at most you could get around 50 percent more forming pressure if you had an almost perfect vacuum.)

As long as you're taking most of the air out of a big tank, there isn't a whole lot of room for improvement in terms of how much forming pressure you get.

The air compressor likely doesn't pull enough air per unit time to work as the first stage of a two-stage system, though. For the first stage, you need high volume, not high vacuum. (That's why I use a good vacuum cleaner for the first stage of my little machine. It doesn't pull very hard, but when the resistance is low, it pulls a lot of air in a hurry.) For the first stage, it's more important to get most of the air out quickly, rather than to pull it down very hard.

Your problem may be that the pump can pull hard enough, but takes too long to get most of the air out. If your plastic cools before most of the air gets out, it will not get pulled down hard until it's too late.

It sounds like the pump and tank you've got would work fine for a one-stage system. If you want two stages, you either need a higher volume pump, low vacuum pump, or a second tank. (A tank can pull hard, and can also pull a lot of air in that first second if you open a big enough pipe into it.)

Have you tried opening the valve to the tank immediately when the plastic gets to the platen, rather than waiting for the pump to pull most of the air out?

I'm not sure I understand your two-stage plumbing. I'm guessing that two of the valves are to control whether you pull air from different parts of the table, for forming different-sized plastic, and that you don't change their positions during a pull. Is that correct?

(You need to make sure that you're only pulling air from holes under the plastic. If your plumbing pulls air from uncovered holes, it will mostly pull air from outside, and never pull the plastic down hard.)

[pimpbike]

From what I can see from the drawing, there is a whole bunch of redundant plumbing going on. I can't see why you'd have three valves the way you've shown under the platen. Unless you have the platen in separate chambered areas but then that wouldn't make sense either. I think because you don't have a check valve on that one pipe that you've labled for initial pull down in front of the compressor you may be even loosing vacuum through the pump itself especially if it' not running the pump while you try to form. I have never tried forming PVC but I have easily formed ABS and HIPS.
I think your system should be set up like the following attached Drawing.

[One of Many]

First and foremost, compressor intended as pressure piston pumps are not really all that efficient in pulling a vacuum as they are for pressure. Even more so if they are reed ported. I am sure you already know that and use what you have, but you are fighting an uphill battle as I view your process.
A Gast rotary vane pump would suit the purpose much better. The shop vac is fine for pulling air through its fan, but invariably restricted, flow unloads and the vacuum drops drastically.

The initial pull is the most critical. That should come direct from the tank stored vacuum. The pump will be far to slow volume wise to get the material moving before it chills off against your mould. The rule of thumb we used was the tank capacity had to be 4-6 times the largest mould capacity for that particular machine. Some excess to cover leakage is handy. A drop in vacuum at the part was normally around 20-30% max from 29hg. If you cannot maintain at least 20hg by the end of the initial pull on the largest mould, the part would be too cold to expect a tight pull during the vac building back to 29hg. There is not much surface area left for the vacuum to act on once the material is against the mould. If it does not get pulled while at forming temp, all bets are off.....it will never get formed tight. The first 15 seconds out of the oven is the working envelope if all other variables are taken into account. A cold mould will reduce that to nil.

The temp consistency over the entire sheet should be under your control. If not, learn how to screen off hot spots and be very aware of air currents in the immediate vicinity. Open doors or shop fans can have an effect there. An enclosed oven helps, but an automated cycle would reduce the need to open the door for constant inspection of the material sag on the first few parts.

DC

[pimpbike]

I do 2'x4' forming on my home made machine. My edwards high vac Rotary vane pump will pull 27 hg on my 120 gallon tank. The largest forming I do only drops to 26hg. I heard of the 2 stage systems but I didn't like all the extra fuss so I went overkill on the tank. The parts I make don't need super definition so the top heat is sufficent. I know with a bottom heater especially on the 1/8 abs would help. I have tried my mold in a dual top/bottom heater machine and it gives better results.

This is the largest product I form is from 2'x4' sheets of abs over a male mold It takes literally 2 secs and the forming is done.

[tomoohoi]

Thanks a lot guys ,

I'll check things which you advised ,

and please advise me to choosing vacuum pump , attached is a catalogue, which one is best ? they can create 30 in.hg vacuum in tank?

and please explain me what is the ultimate vacuum/pressure of vacuum pump ? what it means ? 0.3 pa of ultimate pressure is ok? how about and 5 pa of ultimate pressure ?

[One of Many]

You can think of vacuum as something less than the ambient 15psi of pressure acting on your skin. Without that 15psi of positive pressure we would not survive. An ultimate vacuum is the same as 0psi or 29-30" Hg. Many pumps cannot achieve ultimate vacuum or they struggle getting near it. In other words, there is no such thing as a negative PSI, only a relative reference below our ambient atmosphere. There are many numbering systems to gage that by, Hg, pa and atmospheres are only a few. The web should have conversion charts available.

The style of pump you posted is normally used for Refrigeration repairs and vacuum fixtures. They will evacuate a tank, but my experience has found them to be high maintenance and low volume at 20"Hg.

The link below is a Grainger source we use, but may be available in your area from a Gast distributor as well. At least there are model numbers for you to look for. There are other brands that function and spec' out similar in regards to oiless rotary vane pumps.

Grainger page 1043 of Cat. 398


If you look at the table at the page bottom where it states "Free Air CFM @ Vacuum (In. Hg.)". This shows how many cubic feet per minute the pump can pull into the tank. Most are rated as "free air at 0 Hg". While that does help get the tank to full vacuum rapidly, I'd rather see what its cfm rating is at 20Hg to restore any loss in the tank after a forming cycle.

This Gast 1.5hp Model would be a nice choice. I have seen these pumps with 30+ years of service on them and very little troubles.

DC

[tomoohoi]

Here is attached catalogue from another pump manufacturer , i've choosen X-25 model , others are too big ,and expensive,
and also picture of another manufacturer's pump

please see this catalogue and picture and advise me

You can think of vacuum as something less than the ambient 15psi of pressure acting on your skin. Without that 15psi of positive pressure we would not survive. An ultimate vacuum is the same as 0psi or 29-30" Hg. Many pumps cannot achieve ultimate vacuum or they struggle getting near it. In other words, there is no such thing as a negative PSI, only a relative reference below our ambient atmosphere. There are many numbering systems to gage that by, Hg, pa and atmospheres are only a few. The web should have conversion charts available.

The style of pump you posted is normally used for Refrigeration repairs and vacuum fixtures. They will evacuate a tank, but my experience has found them to be high maintenance and low volume at 20"Hg.

The link below is a Grainger source we use, but may be available in your area from a Gast distributor as well. At least there are model numbers for you to look for. There are other brands that function and spec' out similar in regards to oiless rotary vane pumps.

Grainger page 1043 of Cat. 398


If you look at the table at the page bottom where it states "Free Air CFM @ Vacuum (In. Hg.)". This shows how many cubic feet per minute the pump can pull into the tank. Most are rated as "free air at 0 Hg". While that does help get the tank to full vacuum rapidly, I'd rather see what its cfm rating is at 20Hg to restore any loss in the tank after a forming cycle.

This Gast 1.5hp Model would be a nice choice. I have seen these pumps with 30+ years of service on them and very little troubles.

DC

[One of Many]

A 2 CFM pump will not resolve your initial problem and may add more delays. The style of pump I posted is quite a bit larger, but will handle the application. The second one you posted is again an oil filled pump. Oiless pumps as I posted are 1/3 the cost of those large oil filled pumps. We just purchased 2 similar models for a vacuum chuck fixture at near $3000us each.

I'd recommend you look for a used oiless rotary vane pump and a seperate motor. Surplus and scrap dealers may have something just laying around for cheap. Spending money on the inadequate lower CFM pump would be wasteful if you expect time efficient cycles using a large tank.

Best of luck,

DC

[cod]

I have a robinair 15600 VAC pump hooked to a 50 gal tank. It works great . Cost me around 180$, rebuilt. HVAC folks have told me these are good, dependable pumps and seldom need repair.

It pumps at 6cfm and takes a few minutes to get to 25"

[tomoohoi]

I don't understand , why vacuum pump (with 12 CFM , 0.3 pa ultimate vacuum ) create not strong vacuum in tank, (only 20" ???)

as i understand , if ultimate vacuum is 0.3 pa , 30 in.hg - 0.3 pa = 98067 pa - 0.3 pa , ie, allmost 30 in.hg strong vacuum it tank , is it that ?

0.3 pa ultimate vacuum = 30" - 0.3 pa pressure vacuum in tank .
ie , vacuum gauge which connected to tank will show 30" ???