Vacuum Pump Rating

I'm trying to vacuum form a sheet of 0.75mm plastic about two feet square. My first idea as to why initial results are poor is the rating
of the pump. Its a recon unit with absolutely no markings on it. Does anyone know how I can test it to get an idea of how much vacuum it will pull, eg in Hg or bars? Is that how theyre rated? One test:It sucks enough air from a 3 litre plastic bottle to nearly crush the bottle in seconds. My vacuum table is about 2 feet square and needs a space of 2-3 litres emptying. The inlet to the pump is only 4-5 millimetres diameter, does that matter? Many thanks Breathless
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Get a 40 inch glass tube and stick it vertically in an open jar of mercury. Put the hose on the upper end and start sucking - in principle. You could also do the same with a 40 ft hose filled with water and hung off a cliff or building. Anyways, that's how they're rated. I don't know of a conveinent way of calibrating them, except maybe with a vacuum gage.

Well, at least you understand the concept of flow rate fitting into the whole thing. Your plastic isn't all that thick (about .030). Have you tried using a big shop vacuum? That ought to cover 80% of hobby jobs. If that doesn't work either, I'd look into:
Are you leaking? Seal the system as best as possible - use plastic tape and silicone RTV sealant.
Do you start the pump AFTER you've put the part on the form? Start the pump BEFORE and keep it running until the plastic is cold..
Is the plastic hot enough? It should literally be sagging in the frame. Make sure it is soft everywhere.
Is the plastic clamped into the frame securely?
Is the mold geometry just too hard to form?
Do you have enough holes in the platen to allow the air to be evacuated from under the plastic quickly?
Micro-Mark sells a yellow and black book on vac-forming that I found very useful.
HTH, KL
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Thanks Kurt
I omitted to mention the detail that I have a gauge attached to the airline! When the plastic bottle squashes it goes up to about 21 inches Hg. I assume therefore that 9 inches are leaking from pipes/airfilter/pipe&bottle seal etc. Of course I also assume that 21 inches is fine for vacforming .75mm plastic? Thanks for all the tips on heating but my problem lies before this. My vacuum table registers hardly any inches Hg with all the surface holes sealed with tape (leaky tape?). Hence I built a simple wooden box 5 inches cubed, popped my pipe in (sealed) and still only 4-5 inches of vacuum. Clearly my airtight wooden constructs are flawed! Edges are all screwed and sealed with silicon caulk, but the caulk is silicon bathroom sealant, is this good enough? Re holes in the top of the two feet sqr platform, I can find no definitive rule as to using many small holes (I have 400 all 3mm diam) or a few large ones. What about one huge hole the size of the mould? Re shop vac, having invested in this pump which is quickly evacuating 2.5 litres to 21Hg in seconds Im reluctant to abandon it and shopvacs dont come cheap here (UK) Re book if you want to see the platen Im reading Prop Builders Molding Casting Book: 24 inch sqr table, qtr inch air gap, 400 Oles in the metal top. Does plywood leak?! Many thanks,
Breathless
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The book says to use plywood, so that ought to be airtight enough if the joints are sealed properly. But a few coats of polyurethane wouldn't hurt. Plywood often has 'tunnels' running through it, so it'd be a good idea to seal all the edges of each board with putty or calking before assembly.
Did you apply the caulking during or after nailing the platform together? You'd get a better seal if you apply it like glue between all surfaces. Also when attaching the pipe flange to the platform, and when attaching the sheet metal (at least around the edges).
Did you include a large vacuum tank and valves, as described in the book? How large a tank? Everything should be connected like this:
pump -- check valve -- tank -- main valve -- platform
The check valve is a one-way valve to keep air from entering the tank from that side when the pump is turned off. You turn on the pump and close the main valve to build up a strong vacuum inside the tank, then turn off the pump. You push the heated plastic down on the mold & platform, and immediately open the valve. The vacuum in the tank should draw the air from the platform very quickly.
Did you use pipes or hoses? The book says to use 3/4" pipe between the platform and the tank. If that connection is too narrow, the tank won't pull air from the platform fast enough. You also want it to be as short as possible.
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Remember that air will not move through 400 holes totaling 10 sq inches nearly as well as one hole of 10 sq inches.
I would be hesitant to "dump the pump" myself. I thought you might have a shop vac handy that you could use as a check of the system.
KL
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Kurt Laughlin wrote:

From what I can gather he's using a lab type pump and expecting it to move a serious amount of air very quickly, ain't gonna happen. A shopvac is probably the best choice for rapid air movement in a vauform setup. My Gast will pull 29" Hg with ease, it just takes time to evacuate the chamber (standard polycarb vacuum dessicator, probably 1.5 gallon total volume) and it certainly isn't fast enough for a large vacuform setup. The shopvac move more air in a given time but with a limited pull (inches of water vs. inches of mercury) but the pull should be more than enough for hot plastic sheet.
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Many thanks everyone for your well informed comments:
Caulking applied to platform sides after fixing but the ply layers I fixed with screws and glue. Wood glue literally soaks into the grain so I assumed, maybe rashly, that the glue seal would be adequate.
Yes maybe my pump is a "lab type"? The only inlet is a hole about 7mm diameter
"Hoses"! Here's the good bit. The lab pump inlet fooled me into thinking I could use, wait for it, car windscreen washer hoses and their connectors. Since this is how I evacuated a 2.5 litre plastic bottle in seconds I thought I was on the right track, Oh Well. Despite my doubts that a big platform hole could work if it had to push the air through a small hole I am bowing to your greater experiences. I've fitted metal and plastic gas plumbing pipes, connectors, pipe reducers, thread tape today. Already the pull is greater just by covering the two inch hole with my hand.
The flange has always bugged me so rather than wait ages for someone to weld a pipe to a flange for me I've mated the two with the epoxy resin I made my moulds with. Ages to cure, watch this space, and please don't tell me epoxy resin leaks!!
Tanks and valves: I've worked on the principle that one could suck air from the platform prior to the hot plastic descending, hence avoiding the need for a tank and valve. The draw I need is less than an inch high.
If one big hole is better why do people build platforms with many small holes? Have I drilled 400 holes in the ply and the aluminium cover for nothing?
Breathless
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My 1989-vintage Ron Charles Vacuum Forming machine has a 9x12 inch platen that is simply perforated steel plate, there must be well over a thousand holes in it. For suction, it uses, believe it or not, the motor, impeller and volute unit from a Kirby Vacuum Cleaner, which moves a helluva lot of air, very quickly.
For successful vac-forming, it's necessary to remove the air between the hot plastic and the platen & master fast, fast enough so that it sucks down the hot plastic almost instantly, before it has a chance to cool. You mention .75mm styrene, that is the same (for all intents and purposes) as .030" thick, and that thickness of styrene is eminently formable by vacuum, provided it is hot enough (it should be as soft as a sheet of soft rubber!).
Laboratory vacuum pumps are fine for many things, but vac-forming really isn't one of them, as lab pumps are just too slow. They get their advantage by removing air slowly, but completely, to at least 29" of mercury, which I understand to be a mechanically perfect vacuum for most applications.
Take the advice you have already gotten, use a shop-vacuum to extract the air, those are more than powerful enough for vacuum forming, and certainly quick enough, due to the volume of air they are capable of moving per minute--lab pumps are not, from all the lab pumps I've seen.
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Andrsn1 wrote:

Maybe I'll convert the Kirby if the wife decides she wants a lighter vacuum cleaner someday.

Close, IIRC it's more like 29.5" Hg but once beyond that theory takes over and you pumps rated in millitorr. Those are really only good for some types of chemical reactions.

Oh you can buy some fast enough, got 10 grand minimum?
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OK maybe I'll have to scour the UK for a large vac. I beleive I may need about 4 horsepower which in eurospeak is 3 kilowatts. Does that sound OK?
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Four horse sounds adequate. I've managed to collect a few shop vacs whilst "treelawn shopping." Most were lines that were discontinued and apparently the previous owners didn't want to be bothered finding the bags/parts for them. How much does one go for in your market? Cheers,
The Keeper (of too much crap!)
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Theseeker wrote:

Caulk it too.

Sounds right for a lab type.

You can use hoses for vacuum but they're not cheap, something on the order of $100 for 20' of vacuum rated hose.

As long as you got it sealed wel it should be fine.

Ho much volume are you trying to evacuate? Height has nothing to do with it, it's volume of air being moved that counts.

Many small holes distribute the airflow more evenly and allow many different sizes of master on the platform and still allow for good airflow.
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I'm trying to move about 2.5 litres. If this lab pump evacuates 2.5L from a plastic bottle in seconds I assume a platen/table should do the same, assuming I can solve the leaks! BTW dont bother trying to use resin to seal the pipe and flange. Unless you handle it delicately youll end up with a hairline crack like Ive just done.
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Go to a pet store and get some aquarium sealer. RTV remains pliable for life. hth
The Keeper (of too much crap!)
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snipped-for-privacy@aol.com wrote in message

I'm not a vac-u-form expert but I have done some...
I keep looking at this thread and it keeps bothering me. You don't need a high vacuum to do the job. You need high volume.
The forming needs to be done in about a second (or less). The air movement cools the plastic sheet very quickly (and it hardens).
The quicker, the better.
Also, keep the volume of your air chamber (under the plastic) as small as possible (without making it small enough to obstruct the airflow).
If you have a small chamber, there will be less air to evacuate.
IMHO, small powerful Shop Vac (or a household vacuum cleaner) with a 1.5" hose (or larger) would be better than a Lab Pump with a small hose.
I also suspect that painting the wooden walls with several layers of glossy paint would seal it enough not to leak air (enough to vac-u-form properly).
Peteski
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Thanks everyone, bit of a delay replying since I'm trying an old pressure cooker as a vacuum tank. Obviously the remarks above about evacuating as fast as poss make sense. My pressure cooker has taken me to the lofty heights of just over half an atmosphere, does that seem enough? I've yet to aquire a check valve so I can't try it on plastic. Does anyone know a better nearly-ready-made DIY tank than the pressure cooker? Evacuated
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In the commercial painting industry there are "paint pots" that are pressurized to deliver paint to a sprayer. That's the way to go. hth
The Keeper (of too much crap!)
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A vacuum dessicator of course.
snipped-for-privacy@aol.com wrote:

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I heard Austin Powers wrote a book about these things...
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