Vacuum Forming

[Kazzer]

A few years ago, I was given a vacuum/pressure gauge by Larry Kuntz, and today I rigged it to a little compressor I've been using to pressure a 10 gallon tank I use for spray painting with a little detail gun.

The idea was to put the tank on the input side of the compressor, to pull a vacuum on it. The vacuum gauge showed 20, which I presume meant -20psi.

My question is, will minus 20psi from a ten gallon tank be enough to pull better than a vacuum cleaner?

[He Who Shall Not Be Named]

If it's a differential pressure gage then it is zeroed with a scale either side of zero -- one side positive pressure, the other negative pressure. The negative pressure can't go below 14.7 psi. So, your gage is likely calibrated in inches of Mercury.

Takes 30-inches of mercury -- the weight of a column of the stuff fighting a vacuum atop it with sea-level air pressure on the bottom of the Mercury, where a balance between the weight of the Mercury and force of atmospheric pressure is achieved.

So, on your gage, pulling 30-inches of mercury is a perfect (you lab-rat's, back off!) vacuum. 20-inches of mercury is pretty good, but nothing to write home about -- but good enough.

You need the evacuated volume tank to present that partial vacuum to the work over time -- the volume tank will keep a partial vacuum between the sheet and plenum, with air-pressure pushing down from atop, for the duration of the state-change as the work cools. The set-up always leaks and the volume tank covers that reality.

David

[Kazzer]

I thought the gauge was rigged like that, so my little compressor will do a fair job, probably better than the shop vac?

As for the forming table/platen, I made one out of wood and pegboard, but as I didn't make the frame big enough to go around the box, I'm starting over. I think this will cure all my sealing problems as the plastic sheet will now lay on the platen all around the outside. DUH!

If I make the volume/space under the pegboard as small as possible, then that should make the 10 gal vacuum tank more efficient, shouldn't it? Lets say we have a 10 gal tank = 2310 cu inches, and I make the platen 15x 12x 2 = 360 cu inches, that's going to provide some good 'suck' when I pull the plug on the tank, isn't it?

[He Who Shall Not Be Named]

I thought the gauge was rigged like that, so my little compressor will do a fair job, probably better than the shop vac?

As for the forming table/platen, I made one out of wood and pegboard, but as I didn't make the frame big enough to go around the box, I'm starting over. I think this will cure all my sealing problems as the plastic sheet will now lay on the platen all around the outside. DUH!

If I make the volume/space under the pegboard as small as possible, then that should make the 10 gal vacuum tank more efficient, shouldn't it? Lets say we have a 10 gal tank = 2310 cu inches, and I make the platen 15x 12x 2 = 360 cu inches, that's going to provide some good 'suck' when I pull the plug on the tank, isn't it?

Yes, less air to evacuate.

The big multiplier is the surface area of the work -- the more area air-pressure has to work on, the more force presented onto the heated plastic, the better it conforms to the shape of the pucks and face of the PLENUM!

David

[Subculture]

The tank should provide a better vacuum than a vacuum cleaner. The latter give high flow, but only a few inches of mercury at best.

What you could try with your tank system is rigging the compressor in line with the tank via a non return valve, and keep it switched on whilst the plastic cools down, that will help keep the pressure on the moulding.

[trout]

If I make the volume/space under the pegboard as small as possible, then that should make the 10 gal vacuum tank more efficient, shouldn't it? Lets say we have a 10 gal tank = 2310 cu inches, and I make the platen 15x 12x 2 = 360 cu inches, that's going to provide some good 'suck' when I pull the plug on the tank, isn't it?

The maximum suction is the same for large are or small space - that is dependent on the vacuum, it just gets there quicker with the smaller volume, meaning less time for plastic to cool down and more time to form around mold while warm. So, simple answer is yes.

[Kazzer]

OK! Enough BS, back to business.

While I am excitedly waiting for the vacu-former to arrive, I still felt it was worth pursuing the manufacture of a smaller unit.

I've poked around in hardware strores, looking for 'stuff' to make the heater with and no light bulbs went on. And then - there it was - in my basement - staring me in the face - 2x4 steel studs! I'd had some remodeling done to 'this ol' house, and there were a few studs left over, quietly minding their own business, leaning in a corner.



I cut 5 pieces about 15" long and pop riveted 3 together to make the floor. They have formed the 'legs' of the unit, and keep that hot surface well away from the table.






I purchased some special heater elements and installed them into the supplied spring clips, then wired them to a metal switch box.

Sharp edges of the studs are covered in aluminum tape.



Yup! It works! and it's darned hot.




Next, I'm going to make the sheet holder frame. I think this is getting easier.

[Kazzer]

Well, this is a comedy of errors! (Isn't every prototype?)


After a bit of trial and error, I realised that to get a really good seal on the platen, I needed to make the frame holding the sheet fit completely around the platen, so the soft plastic would form its own seal around the edge. Getting the height right so the frame would flip over and surround it was fun. It looks like the platen needs to be about 1" higher than the heater box, and the frame about 1/2" wider.

My other BIG mistake was to make the platen out of wood. I should have simply made the front and back of the heater box longer, and incorporated the platen into it. That would have eliminated the 2x4 holding the frame hinge.



Next problem! I kept burning the underside of the plastic sheet. It was getting very hot in the middle before the sides had melted sufficiently. Those two heater bars were simply too close to the sagging plastic (sort of dangerous too! Now where did I put that ol' fire extinguisher?)
I had temps of 310 in the middle and 265 around the edges.



One of my employees (he's into R/C Cars) told me a pal of his was covering the nichrome wires etc with sand to even out the temps. So, I tried it!

Yup! That works, but the whole thing now takes a while to heat up and cool down.



Here's the first one! It's not bad!


Certainly good enough for making Skipjack weights!



On my next attempt, I went for more heat and waited until I had 300 around the edges, which resulted in 350 in the center. I also raised up the part on some washers to try to achieve a sharper edge around the bottom.




The result was that the SHop-vac pulled the plastic right through the platen holes. I got a couple of folds in it too Dang-blast-it!

If I can conjure up the enthusiasm, I'll make MkII now. I have a better handle on it. Got to get that plastic further away from the heater elements.

[He Who Shall Not Be Named]

You're sneaking up on it, Mike!

David