Resin degassing problems 2

[Neil_99]

Hi
Im working on a carbon fiber project. I have a 4.5cfm 1/2 HP vacuum pump which worked fine until recently. I know what proper degassing is like. After 60/90 seconds bubbles form, rise, and collapse on themselves leaving only residual bubbles. Takes about 3 mins.
What i have now, is that under vacuum the tiniest bubbles form, like foam on a pint of beer. They dont get bigger, they do rise, but there is no collapse, this beer head like foam just stays there it doesnt change, it doesnt do anything and leaves me with unusable resin.
Ive have checked/replaced oil, taken the pump apart (it was just as bad after as it was before so i dont think thats caused any problem) checked all the seals, cleaned the small filter, there is literally nothing left to check.
When it WAS working fine, the pressure guage went beyond the max 30inHg probably going +1, now it goes up to 30 and its just not working the way it was when it went over.
Its driving me mad. Its my second pump, my funds are limited i just want to get the damned thing working again.
Any ideas?
Cheers!

 

[Compositepro]

You may have an air leak somewhere in your plumbing or resin tank, or a restriction in your vacuum line. It takes only a tiny leak to raise the pressure one inch of mercury when using a small pump. This is assuming that your problem actually is the level of vacuum.

A stable residual foam can also be caused by not having enough volatiles in the resin itself. A volatile is a liquid at ambient pressure that has a vapor pressure, which will cause boiling at low pressure. Boiling volatiles will almost instantly expand and pop any air bubbles. You can add a few drops of acetone to the surface of the resin before pulling vacuum to get rid of foam.

 

[Neil_99]

Many thanks CP. So just drop a few drops of acetone on the resin, don't mix it in?
I ordered a new pressure guage but i fear im clutching at straws here. Can these guages ever develop leaks?
Cheers!!

 

[Compositepro]

Leaks are most common at connections that are disconnected regularly, valve stems, and tank lid seals.

Acetone can only expand bubbles if it can get into the bubble. Mixing into resin is effective but makes subsequent removal more difficult. You can pull vacuum to get bubbles to rise to the surface and then add a few drops and pull vacuum again, so you do not boil off the acetone before the bubbles rise to the surface.

You can also draw resin off of the bottom of the container so that bubbles on the surface are of no importance.

 

[Neil_99]

Thanks CP. Whn i stick my finger in the valve it still doesnt pull full vacuum (according to the guage) so some leak in the chamber lid setup can be eliminated.
Should get my new guage tomorrow. If both guages show the same slightly less than full vacuum, then it has to be the pump right?
Cheers!

 

[Compositepro]

Only you can say, as we cannot observe or measure anything. If you have taken the pump apart it may very well not be working correctly now, as they are very tightly toleranced machines and not easy to work on. The thickness of the gasket used in some areas is critical to functioning correctly. Most dial gauges are very crude instruments for measuring vaccuum. A mercury or oil U-tube manometer is far more sensitive.

 

[Neil_99]

Thanks CP. Forgive me, im quite new to this work, what and where is the gasket on a vacuum pump?
Also something i noticed attempting to degass yesterday (its still not right). When the pump is at max pull (which is possibly not full vacuum, thats my guess) there is little to no vapour coming out the chimney. I know from bagging that when there is a leak a lot of vapour is produced. Does this suggest i AM pulling full vacuum? If i am, then im really confused why i cant degass properly...?
Thanks!

 

[Compositepro]

The rotor and vanes in the most common type of vacuum pump are sandwiched between two side plates sealed with a paper gasket. This controls the side clearance of the rotor. If this is too wide by even 0.001", there will be internal leakage (exhaust-side back to suction-side), which limits the ultimate vacuum, and will not cause any increase in exhaust flow.

To measure exhaust flow you can bubble the exhaust flow into a jar of water. But, be careful that you do not allow water to be sucked back into the pump (like when you stop the pump).

Vacuum pumps are rated in liters/minute. When the inlet pressure is 1/100 of an atmosphere,the mass flow rate will be 1/100 of the initial rate,if everything is working perfectly. The type of oil used and it's purity is also important.

 

[jari001]

Just my 2 cents:

If you can, you should run a chamber evacuation test of your vacuum pump on an empty vacuum chamber to see if you repeatedly get the baseline performance of the unit. If you have already done that, what did you observe during this?

Getting a new, calibrated gauge is another good diagnostic move. Hopefully you will have it in hand soon and it can help you eliminate some variables.

 

[Neil_99]

Thanks guys.
This is really depressing, i mean its not like there arent enough other things to go wrong making carbon fiber parts!
I got decent pressure guage, cost me €40 so i think its a reliable bit of kit. It shows the pump is pulling full vacuum 30InHg/-1 bar. And still i get this crappy micro bubble beer froth foam. No large bubbles form. Just froth. Doesnt collapse down. I have a both a fast and slow hardner, ive tried those, same thing, tried a totally different resin from a different manufacturer, same thing.
If its not the pump, and its not the resin, what the hell else can it be??
Thanks!!

 

[Compositepro]

What happens to the froth when you release the vacuum? That does not have to be infused into the part (you draw resin from under the froth). If it is drawn into the part it will not necessarily cause any voids. Parts are not cured under vacuum. If the resin is cured under a high vacuum it will almost certainly have voids.

I assume you are having void issues in your parts. What are they? Flow front control during infusion, along with other factors is critical to getting good infusions. You may be chasing a red herring with this froth.

 

[3DDave]

What resin is it? Have you contacted the resin supplier? You say different resins had the same problem - are they the same age?