Returning to a thread from November, at last I have some vacform
success using an old propane tank between the lab pump and the
platen/table. My teeny pump evacuates the 44 litres tank to 18 inches
mercury in about two minutes and my plastic of 16 inches by 10 inches
is showing pretty good detail.
One more tweak to follow all the good advice I got so far: I know I
can improve the seal between the frame the plastic is clamped in and
the table. From what Ive read so far totally airtight is not necessary
esp when the tank swooshes the air in so quick. Does anyone use rubber
oven seal? I assume thats heatproof enough. The frame is going to
contact the seal as it comes off the oven. My book and web sources
seem so vague in saying "airtight seal" but not How! Metal to metal or
wood to metal is hardly airproof.............
Silicone gasket material in a tube will work just fine. Apply a thin layer
of Vasaline to the surface you don't want the silicone to stick to and a
bead of silicone to the other surface. Press the two togeather and wait for
the silicone to set up.You can get the gasket material at any auto parts
store. HTH Pete
Ive used that on cars, will need a bigger tube though for my frame I
The bathroom type of silicon seal is the one Ive used liberally on my
contraption, but will that be heatproof I wonder?
You are sort of headed in the right direction. I manage the
engineering department at a commerical "vacuformer" (we use a more
generic term "thermoforming") and can give you a little advice.
You've got MORE than enough vacuum, in terms of static pressure. Where
you are lacking is the RATE at which you can remove the air between
the plastic and the mold. Yes, a good seal will help, and it is easy
to do**, but it will only it be a marginal improvement. Most homemade
vacuform machines employ vacuum cleaners, not vacuum pumps. Only a
modest vacuum is needed. But, you need to create it between the
plastic and the mold nearly instantaneously. Your surge tank is an
excellent idea, (we use surge tanks on our machines as well) but I
suspect that your vacuum lines are pitifully small (1/4"?). This
creates a lot of resistance to flow that slows the process down. The
amount of air moved by your pump during the critical second or two
that the plastic is forming is negligible. Focus of the surge tank.
It's doing all the work. I'd recommend moving it as close as possible
to the mold. Increase that vacuum line to 1 to 1.5" diameter (PVC
pipe). If you are using a portable propane tank with a tiny valve
opening, look at removing the valve so you can get some real flow.
Consider a bigger tank.
And just for grins, you might try the vacuum cleaner...
** Don't try to seal to the frame. Too many leaks. The inside
dimensions of your frame should be slightly bigger (1/8 to 1/4" per
side depending on the thickness of your plastic sheet) than the mold
box or baseplate. Their edges in contact with the hot plastic create a
seal very nicely. You can use a thin layer of hardware store 100%
silicone if you wish. It won't hurt. We don't bother.
Greg Reynolds, IPMS
On Thu, 20 Jan 2005 21:21:42 +0000, firstname.lastname@example.org wrote:
I >suspect that your vacuum lines are pitifully small (1/4"?).
Im using 15mm copper pipe but yes the pump inlet is about qtr inch.
Tank evacuates now to 22 inches in 2-3 minutes.
The pipe from the tank to the mold is about two feet cannot rearrange
that til I am working in more spacious surroundings.
When I got the tank I had two 22mm pipe connectors welded in the side,
the gas valve on the top us closed off. I do not rely on the pump to
draw down the plastic, I gave up that route! I only use the release
valve between the tank and the mold.
Is your silicon from a car shop or a bathrrom shop? Does it matter re
heat? Ive stacks of cheap bathroom sealant
As long as the label reads 100% silicone, you're good. There are a lot
of cheap sealants with acrylic in them (some are sold as "paintable").
They won't last nearly as long. (Neither on your vac-u-form machine
nor your house!)
Thanks everyone. I got a bit sidetracked in thinking the seal between
plastic and table was vital. In fact the soft plastic makes its own
seal and the results are great. I may widen the piping between tank
and table to an inch sometime...
This thread is nice and I have a couple related questions:
1) When looking for a vacuum pump I learned that thermoforming requires
25-30 inches of mercury in vacuum -- but what about the time that it
takes to reach that? One book mentioned a auto intake vacuum but I did
find these on the web: http://www.blowerwheel.com/vacuum-pumps-gast.htm
They have the "Hg but not enough speed? My plastic will be quite
2) How does having cavities in your mold effect your vacuum
requirements, if at all? I am going to try molding a part with a
series of 1/2" deep cavities.
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