Frustrating VARTM leaks

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#1

Hey everyone, I am hoping that some of the more experienced infusion people on here can help me with a problem I am encountering.

I will describe our problem and then our-set (as much as I can legally explain)

Our problem is as follows: We are in a manufacturing environment and over the past 2-3 months we have been experiencing a pesky problem. We are using the VARTM process to infuse parts. After the bags are ready the operators leak check the parts for 1 hour with a required 0 leakage (the spec says 1"hg in 5min but since the problem has come up we are trying to get in much longer leak checks). The operators then infuse the part and close off the resin feed line when the infusion is complete. This is where the problem is coming in, while the part is waiting for cure it will randomly spring a leak. Sometimes after 5min, sometimes 1 hour or 2 hours, who knows! If the operators notice the leak right away then can (most times) use infusion bagging tape to stop the leak. If it goes unnoticed then our part is ruined.

I have tried many things to help solve this problem. I was convinced that the WL7400 bags we were using were becoming too brittle towards the lower end of our in-spec humidity requirements. So we switched bags but the same results are persisting.

Our infusion set-up is as follows: 2 part resin system degassed for 15minutes

pee ply extends 2" beyond part and 4" where the infusion pack will sit

greeflow mesh cut 1" inside of layup

infusion pad (fitted with a t-fitting and hose to resin feed)

spiral tubing wrapped in peel ply (to make sure the spirals do not cause bag leaks) which extend the lenght of the part and 1/3 down the sides of the parts as well.

Thanks in advance for any help that I may receive

#2

I think you posted this on engtips. Have you tried leak checking your just the tool to see if it has a crack. Sounds like it might be leaking.

#3

Yes, we have 13 tools, all have been leak checked and they are not leaking. This is why I am confused.

#4

How long do you leave them under vacuum before leak checking? How about your gauges? are they accurate? Are sure the air is coming from a leak in the bag? Perhaps you are infusing too fast?

#5

We leak check for AT LEAST one hour before infusion. As for the gauges, all of them have recently been calibrated by our quality calibration guys. I was reading about infusing too fast but there is no sign of anything wrong during infusion, whether it would be crazy race tracking, or air bubbles being left behind, etc. We have not changed anything in our process which would alter resin flow. Where the air is coming from is beyond me at this point. That is why i am hoping to get some knowledge from people with far more experience than myself. when this first started happening I also suspected the bushing in the indexing pins might be leaking , but the tools have all (just last week) gone through their preventative maintenance inspection which included leak checking the tools. the tools saw no leak drop in 2 hours.

#6

Have you changed resin batches/drums? it might be going bad/bad batch that is releasing too many VOCs. You say if the techs notice the leak, they can fix it…where do they fix it? The perimeter sealant? The bag randomly? Distro sharp spot? Infusion/vent tube placement?
We just went through having 10 panels being made, some turned out fine, some had air randomly show up in the distro media…not in the part though. I think it’s just weird gravitational forces changing in the earth’s crust.

#7

your not letting the resin drip out of you resin inlet after clamping off are you? as soon as the resin is gone from atmospere side air can easily get air in through the pinched line. using a valve and kepping the end in the resin after turning off can help prevent this or just have the end of the tube facing up to prevent drainage.

#8

There is too little info.

You say the operators detect the leak (most of the times) and are able to fix it with some tape. What is leaking then?

#9

To answer more of your questions:

It been happening with both our previous and this resin batch. As for where the leak usually occurs it is usually in the bag but varies as to where it is. The operators never actually find out exactly where because they rush to throw down as much bagging tape in the area that they notice the leak. I guess my main problem is that I can’t understand how, after infusion is done, the leak suddenly springs up seemingly out of thin air. I have actually seen a part that we left leak check for a whole night until we infused the next morning only to spring a leak after infusion. I have tried getting different kinds of bagging material but the parts still leak with the same frequency. I was reading something that TET wrote stating that a 1"Hg is a small leak that is in many specs (including ours) and we always meet and greatly exceed this value but the problem persist. I am convinced that our problem is occuring during infusion itself as any small leak anywhere would be detected with more than a 12 hour leak check on the parts pre-infusion.

#10

sounds like they notice a hole in the bag during infusion and patch it with sealant, what I dont understand is why if they are doing up to a 1 hour vac test that they are not picking that up, you should see a massive drop. Perhaps the method that is choosen to isolate the part from the vacuum isnt cutting vac completely off giving the illusion of a “perfect” bag. are you pinching the vac source off or using a valve? You might want to give these a look over.