Starting infusion for the first time. I’ve heard many different answers to the question of how long and how much of a vacuum to hold once the infusion is finished. Anywhere from full vacuum to gel, reduced vacuum to gel and turn off the vacuum once the infusion is done. Even the nice guy at the MTI website has given two different answers. What is correct? I’ll be using the MTI tubing.
If you’re using MTI tubing, I would think you could just clamp off the supply line and shut off the pump once your infusion is completed, since the MTI tubing effectively seals off the vacuum supply automatically as it comes into contact with Resin.
Their isn’t just one way to infuse a part. The key is find the technique that works for you and you get that from research which you are doing by coming here and also experiance/experiments. The resin system can play a part in your decision of how much vacuum to have, some resin systems give a spec. With the resins and materials I use I leave full vacuum on until I demold. If you look in the MTI hose thread you will see some 12"x12" test panels I made with the MTI hose and those were kept at full vac until demold. The only time I’ve want the vacuum lowered or vented was with prepreg and autoclave cure. Also I only play with epoxy…
I’ve been wrong before, but I believe maintaining a vacuum post-infusion using MTI hose is pointless because the only thing being kept “at vacuum” is the inside of the hose. Maybe the vendor will chime in…
I’ve been leaving vacuum on using MTI hose although the manufacture claims you don’t need to. In my infusions I have still been drawing a vacuum even after the resin reaches the hose. Maybe it’s because the resin doesn’t completely encompass the hose at first after reaching it.
Although I have yet to achieve a perfect infusion out of my first four.
HI all
Just a quick question, why do you leave the vacuum pump switched on ? i close the infusion clamp and give it 30 seconds and clamp the vac side.
Chris
How long do you check to see if you have any vacuum drop over your part? I find even a good bag will lose some vacuum over time. So although I dont infuse (at the moment) When I vacuum bag parts I keep the pump on for at least 4 hours just to ensure it has good pressure over the part and the resin has cured enough to be beyond the gel stage. I found parts just seem to not be as good without keeping a vacuum over them.
Hi Richard
I leave the bag for 30 mins and check the gauge, it its been able to hold vacuum for that amount of time all should be ok. We use enough electric in the house as it is with the kids leaving lights on plus X box etc 
Chris
True… It is just when it heats up the vacuum can drop and when I have parts in the oven it is usually topped up again after an hour or so… You can build a vacuum reservoir using an old calor gas bottle and making up a couple of fittings to pull a vacuum down and then feed though to pull the bag down. Just a simple manifold. You can either leave that connected or after an hour so connect that into the bag.
Hi Richard
Yes just to save any confusion my components are room temp cured so it stays a constant temp.
Chris
I don’t infuse but when bagging I use a programmable vacuum controller. It turns the pump on whenever pressure increases above the set threshold.
I also mean exothermic heat. Some parts do get quite hot depending on the resin you use. When I wet lay I do keep the pump on for about 4 hours. When pre-preg parts are cooked I prefer to keep a vacuum on at all times but Vic who’s oven I use will use the reservoir and just top it off every hour.
I am looking into making a pressure switch to turn the pump off and on when it loses vacuum. I would put the switch onto the reservior chamber.
I have used a switch like that, and I do not like them AT ALL.
Although they have the right principle: switching on the pump when vacuum is lost behind a set level, and turned off again at a desired level, the range between the 2 vacuum pressures is not adjustable, and WAY to large, and not even constant (anywhere from 100 mbar to 200 mbar, or 3 to 6" Hg)
They are cheap though…
A better option is this one, which works good from 100-1000 mbar:
vacuumschakelaar
Or this one (Danfoss RT 121):
I only have used the MTI hose and I leave my vacuum on full until I demould. I don’t care if the thing seals or not, stuff happens so I just leave it on to be safe.
The MTI hose is not pemeable for air after it is wetted by the resin, but that does not mean that no air can travel into the part! It means that the resin blocks / slows down significant evacuating any air where the MTI hose is wetted out. At the areas where it is dry it still allows air to travel through the membrane.
If your vacuum is 100% tight you can clamp both lines because no air can get in your part and there should be no air in the part.
I prefer to let the pump running until the part is cured. If you use the MTI hose you don’t have to care for lowering the vacuum after infusion.
If you do not work with the MTI hose you should lower the vacuum to minimize the voids that could be created by the vacuum that is stil working to the laminate.
Thanks for the info. Based on what I’ve read, I’m going to keep the vacuum on until cure. I’m doing some test panels, however the final part is pretty large and I’m not sure if I will be able to ensure a 100% leak free setup. I can see how at least some areas of the MTI hose will remain open.