I need the added strength of Kevlar instead of fiberglass and can’t use carbon fiber because my application needs to be electrically non-conductive. I’ve read that Kevlar is difficult to wet and therefore not recommended for vacuum infusion. On the other hand, I’ve read elsewhere that difficult wetting can be managed with higher vacuum pressure. Is vacuum infusion/RTM viable for Kevlar?
There are no problems with Kevlar in infusion. Just make sure you let the vacuum work some time that all the humidity can be sucked out the Kevlar. Kevlar is hygroscopic.
Thank you very much. This was very helpful!
Indeed. Results with aramides are usually better with infusion than with hand laminating.
Moisture indeed is a problem. Aramides can take up water up to 5% of its weight.
Is there a typical amount of time to vacuum humidity out before injecting resin? Or is there a way to know when the water is mostly gone?
I think a good rule of thumb is 30 minutes if you’re at RT. Make sure the vacuum draw is still open during this step as vapor boiling off into the bag will reduce vacuum.
If you have connected a digital gauge, you see the gauge “hanging” at 20 mbar or so. (vapour pressure of water at RT)
Once the water is gone, the pressure goes down again.
Caveats:
If you have a leak, you can arrive at 20 mbar, and then the pressure drop stalls. (a flow meter helps)
If you use balsa, the time for water to boil off can be considerable.
If you do not have a digital gauge, you see nothing. Analogue gauges are basicly useless. (I use them to see if vacuum is “on” or “off”)
I do have a digital differential pressure gauge for HVAC. I’m confused what is meant by “boiling off”. Is heat applied?
Look at this chart; you can see the curve at which pressure/temperature the moisture vapor point. So for example, if it is 68F and your pressure level in your part will not go below 22mbars, this means that moisture in the laminate still has not boiled off. Once it drops below that point then you know the moisture has been removed.
Forgot to add the chart…
What kind of part are you making if you don’t mind me asking? Cutting Kevlar can be fun…
Or boring. I just use a box cutter.
course…I only use prepreg (Thermoset and thermoplastic)
Excellent! thank you.