Minimum Useful Vacuum Level Appears to be 15 torr absolute pressure /29.3 inHg vacuum
Graybeard,
Awesome post about vacuum!!!!! 15 torr from your venturi is about what the pump I think Larry has is rated for. I believe either harryn or groger also reccomended venturi vacuum systems earlier but I incorrectly assumed the vacuum was a bit low.
The 5-10 torr numbers from Advanced Process Technology that I had been citing are from people who make equipment to vacuum degas critical stuff like MRI coils and potted high voltage components where it might be an issue if there were any imperfection. As Geof quite cleverly points out, thoroughly removing water vapor might be the reason they go so low. Geof's comment about hyperbole in manufacturer data is also valid. Water vapor is bad for epoxy aggregate adhesion in our case too.
From the consensus on the thread, it looks like 15 torr is realistically the minimum vacuum level for degassing epoxy. This is at the lower limits of the Advanced Process Technology Numbers and substantially higher than the millitorr numbers from Marks handbook.
<B>Does anyone disagree that a reasonable recommendation for vacuum on this thread is "Any vacuum source equivalent or better than 15 torr gage pressure and economically feasible". This is equivalent to 29.3 inches Hg of vacuum.</B>
More Hot air about degassing
Formulator epotek recommends 29 inches of vacuum but says that it should be applied and released as quickly as possible.
http://www.epotek.com/SSCDocs/techti...%20Tip%204.pdf
Formulator resins-online.com in the UK recommends brief degassing at 5 mbar= 3.75 torr gage pressure = 29.77 inches Hg Vacuum.
http://www.resins-online.com/resin-online-faq.php
This thread actually comes up in page 2 for epoxy degassing in Google!
Formulator PTM-W Recommends greater than 28 inches of vacuum.
http://www.ptm-w.com/dynamicdata/dat...procedures.pdf
PTM-W also has some high strength cast iron filled epoxies which look like they might be the kind of thing we like!!!
So it looks like 3 epoxy formulators (counting Crosslink from my last post) recommend more than 29 inches and one formulator just says it is more than 28.
Since Crosslink, Epotek and Resins-Online all recommend values very close to 29.75 inches of vacuum, I'm not so sure this was out of line as a target vacuum value, especially considering the fact that several of the vendors state that as short a degas time as possible is recommended. Less vacuum for a longer time may work but it doesn't seem reasonable to me to deliberately purchase a vacuum pump for this application which won't produce 29.75 inches Hg of vacuum.
It's still possible that an adequate mixture can be produced without vacuum using additives but I am convinced but the very strongest mixtures will still require vacuum.
It might not have any granite in it but it looks like we could just buy cast iron filled epoxy from PTM-W and be done early with higher strength and likely decent vibration damping. . . Sigh.
--Cameron