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?
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
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?!
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
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.
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.
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
"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
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?
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
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.
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
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?
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.
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?
The Keeper (of too much crap!)
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
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
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
paint would seal it enough not to leak air (enough to vac-u-form
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