Thread: Pressure Pot Mods

You might want to put the central handle back on as it allows one handed lifting and placement of the lid hehe. Well on the way though cheers ~ John

Thanks John
In fact I have the handle ready to refit as soon as the pressure test is completed. The indicators so far is that there are no leaks. The second hand compressor works a treat too. Hopefully I can do a cast after work tomorrow. Have some tubes wrapped and ready to go including a $50 note from the gold byers.

Does your compressor have a water trap ,you don't want any moisture any where near your castings and PR as it will give you problems ,drain the compressor tank on a regular basic will help as well It's all a learning experience and that's good hehe. Cheers ~ John

Yes, it has a drain cock and I drain it after each fill and close to prevent moisture re-entering when empty.

looking good

How long is your hose? I have a water stop on my compressor, but also at the pot, as the hose can collect a fair bit of moisture. My hose is 20m, so when using the pressure nozzle for cleaning, the first few blasts have spays of water

My air hose is only 3 metres long. The compressor is a 2 HP, 24 ltr tank, Arlec brand. I don't think that it has a water trap. I didn't get a manual with it as I bought it 2nd hand. I cast some blanks using the resin saver mold with no evidence of bubbles so far. The only issue I have now is that some resin is forced into the tube (I was warned that this might happen). I only pressure to 20-25 psi. When it sets I will drill it out, hopefully without spinning the tube inside the cast. I will wrap some masking tape around the lugs in the mold so as to make the tube a tighter fit to restrict the resin entering the tube. I think that the molds with the tapered end stoppers might be better as you can get a tight fit.
I will tackle the hurdles one at a time and I don't think that I have wasted my money with the pressure pot even if I can save some frustration by being bubble free. There is a minor air leak in the pot as it drops 2 -3 psi over night but didn't go below 20 psi.
Regards

If you are only losing 2-3psi, that's good enough, mine needs a new seal (but it is >30 years old). The water trap is a jar looking thing that would be on the outside of the compressor, if you don't have one, I'd recommend you put one on your pot.

The rubber stopper type molds can be used under pressure, I use mine at 45psi.

Cheers

Maybe a silly question, but why pressure?
Wouldn’t vacuum work better, it would draw out the air from the blanks, both wood and PR castings….at least it would appear to, and second hand A/C service vacuum pumps are cheap and easy to hook up, plus they have a gauge on the hose, and the moisture problem would be solved along the way.
Just wondering, as I see a lot of pressure pot threads here and on other forums.
.

Vacuum can tend to form more bubbles than it removes before the resin goes off. Pen blanks contain air and can take hours before all the air is extracted and resultant bubbles in the resin stops. Care is also needed with vacuum, resin boiling if too much vacuum, and expansion of the liquid resin.

Pressure expands the bubbles in the resin that then float to the top easier. Heating the resin slightly also helps with bubbles.

Hope this helps explain.

Cheers

Reason I asked is I used to cast model train parts, we used liquid styrene under vacuum, pressure tended to create voids in the molds, but these were solid steel dies, we would pt the die under vacuum and then introduce the liquid, which hardens quite quickly.
So the slower cure time for a resin would work better under pressure, right?
I understand that the vacuum can cause heat and bubbles, but I was curious.
Thanks for the explanation.
Ed

Originally Posted by edblysard

Reason I asked is I used to cast model train parts, we used liquid styrene under vacuum, pressure tended to create voids in the molds, but these were solid steel dies, we would pt the die under vacuum and then introduce the liquid, which hardens quite quickly...

Yep solid stuff is fine under vacuum, but wood is porous and always contains some % of air, even the really dense ones

Originally Posted by edblysard

..So the slower cure time for a resin would work better under pressure, right?...

Yes, but keep in mind under pressure, things take longer to set

Originally Posted by edblysard

I understand that the vacuum can cause heat and bubbles, but I was curious.

Vacuum doesn't cause heat, just under vacuum the boiling temperature reduces.

So the heat from any type of catalyst would occur quicker than expected, and the cure time would/might be too fast to get all of the voids filled and the wood impregnated completely before the resin set?
Makes sense to me now…the styrene sets within seconds of exposure to air, no catalyst needed, so the vacuum worked great with it…(you never really get all the air out with a vacume pump, you just reduce the atmosphere pressure)
Thanks again.