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vniebles
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Hi everyone, I was wondering if you guys could help me troubleshoot. I had some issues vacuum-bagging a 4' x 2' sandwich core panel. We used 18 oz. of epoxy resin/hardener for 27 oz of plain weave carbon fiber to follow a 40/60 ratio of resin to carbon fiber. We had a laminate sequence of 0/180/core/180/0. We had issues pulling a vacuum but we managed to achieve a vacuum level of 23 in Hg. During the layup, the carbon fiber seemed saturated with resin but I found today that the resin did not fully wet out the carbon fiber. We also had issues with striations on the surface and strands of breather sticking to the top of the layup. I believe our issues are a combination of not using enough resin (by not accounting for the resin uptake of the Nidacore) and not pulling a high enough vacuum. However, I would really appreciate any insight into this. I have attached some pictures of the panel for reference. Thanks!  
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Warren (Staff)
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If this is wet lay vacuum bagging then 23inhg is generally too much vacuum so could have made things worse by squeezing out more resin. Generally we tend to recommend around 6-7 inHg as a starting point only varying it to suit results. Also 60/40 is a fairly idealistic fibre resin ratio. Typically you would expect around 50/50 for wet lay up maybe a bit better with experience and tweaking the vacuum levels to suit results.
Certainly resin uptake of the core is worth considering here as well.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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ahender
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+xIf this is wet lay vacuum bagging then 23inhg is generally too much vacuum so could have made things worse by squeezing out more resin. Generally we tend to recommend around 6-7 inHg as a starting point only varying it to suit results. Also 60/40 is a fairly idealistic fibre resin ratio. Typically you would expect around 50/50 for wet lay up maybe a bit better with experience and tweaking the vacuum levels to suit results. Certainly resin uptake of the core is worth considering here as well. Interesting comment. Years ago I asked an Airtech support person some questions on vacuum bagging. He suggested pulling a full vacuum and after a short 'period of time' backing off to 10hg to let the fabric expand some. I honestly cannot remember what the length of time was. A little off topic. I have also seen many posts where people say the resin needs to fill 'a ll the voids'. I thought the only place the resin needed to be was as a coating around each fiber. I'm not discounting what you are saying, just have never seen 6-7 hg as a starting point stated before. Alan
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Warren (Staff)
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We've always recommended 20% vacuum which is around 6-7inHg.
Without testing, its hard to know the virtue of Airtechs method. But in theory it could help to get all the air out initially but it depends how long you leave it that way and how quickly the resin squeezes out. In our experience, high levels of vacuum usually means a fairly dry laminate with lots of pinholes and surface defects.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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vniebles
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+xIf this is wet lay vacuum bagging then 23inhg is generally too much vacuum so could have made things worse by squeezing out more resin. Generally we tend to recommend around 6-7 inHg as a starting point only varying it to suit results. Also 60/40 is a fairly idealistic fibre resin ratio. Typically you would expect around 50/50 for wet lay up maybe a bit better with experience and tweaking the vacuum levels to suit results. Certainly resin uptake of the core is worth considering here as well. Hi Warren, thanks for the reply! I definitely agree that 60/40 was unrealistic, I'll just try applying an equal weight of resin to carbon fiber next time, hopefully that should be sufficient for any resin uptake from nydacore. The common practice with all the design teams at my school is to pull a full vacuum for any composites work, so I also find it interesting that you recommend 20% vacuum only. I'll definitely try it out in our next layup too
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torsten Ker
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thats is an interesting info given by Warren.
Just for stiffness testing tI did a 1/2 sqm panel with 5x 600gsm biaxial and the ridgidity was not impressive and it was thinner then expected.
That is now explained because with me pulling full vacuum compressing the fibres.
Tthanks for that
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Warren (Staff)
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Compressing and consolidating the fibres are different but linked. In some cases a bit of compression is useful to avoid voids and excessive resin content. Also in some woven cloths it can be advantagous slightly as the straighter fibres after slight compression will perform mechanically better.
However with thick laminates, if compression has a significant difference on thickness then stiffness may suffer slightly in a similar principle to using a thinner core on a cored panel. I guess its a fine balance.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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Chris Rogers
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I agree with Warren that less vacuum is usually best - I use 15inHg as a starting point but often 8 is plenty, especially with thin laminates... you just bleed off less resin. The key is that you need to fully wet out the material before bagging - at least 50% resin by weight - and then use the bag - and release film and bleeder to pull off excess resin to get down to the 60/40 range. For thicker laminates and with experience you'll be able to use less resin in the initial wet-out. Unless you're using it to press material into corners - which you shouldn't do - less vacuum will only reduce compaction and bleed, but should still be fine. With core bonding it can be good to use very low vacuum to keep from bleeding out all the putty. Also, is there a way for resin or air to bleed out of the bottom skin? If the Nidacore is non-permeable then air can't escape. You'll get most of it with a careful wetout and good rolling with a bubble popper, but the core should allow air to escape, usually through small holes (in foam) but not sure about Nidacore... Alan, I am not sure I'd go along with the Airtech method you described - sounds like a good way to get lots of porosity. Usually backing off vacuum is asking for trouble... but there may be something to it that I don't understand.
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ahender
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+xI agree with Warren that less vacuum is usually best - I use 15inHg as a starting point but often 8 is plenty, especially with thin laminates... you just bleed off less resin. The key is that you need to fully wet out the material before bagging - at least 50% resin by weight - and then use the bag - and release film and bleeder to pull off excess resin to get down to the 60/40 range. For thicker laminates and with experience you'll be able to use less resin in the initial wet-out. Unless you're using it to press material into corners - which you shouldn't do - less vacuum will only reduce compaction and bleed, but should still be fine. With core bonding it can be good to use very low vacuum to keep from bleeding out all the putty. Also, is there a way for resin or air to bleed out of the bottom skin? If the Nidacore is non-permeable then air can't escape. You'll get most of it with a careful wetout and good rolling with a bubble popper, but the core should allow air to escape, usually through small holes (in foam) but not sure about Nidacore... Alan, I am not sure I'd go along with the Airtech method you described - sounds like a good way to get lots of porosity. Usually backing off vacuum is asking for trouble... but there may be something to it that I don't understand. Thank you Chris.
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Hanaldo
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+x+xIf this is wet lay vacuum bagging then 23inhg is generally too much vacuum so could have made things worse by squeezing out more resin. Generally we tend to recommend around 6-7 inHg as a starting point only varying it to suit results. Also 60/40 is a fairly idealistic fibre resin ratio. Typically you would expect around 50/50 for wet lay up maybe a bit better with experience and tweaking the vacuum levels to suit results. Certainly resin uptake of the core is worth considering here as well. Interesting comment. Years ago I asked an Airtech support person some questions on vacuum bagging. He suggested pulling a full vacuum and after a short 'period of time' backing off to 10hg to let the fabric expand some. I honestly cannot remember what the length of time was. A little off topic. I have also seen many posts where people say the resin needs to fill 'a ll the voids'. I thought the only place the resin needed to be was as a coating around each fiber. I'm not discounting what you are saying, just have never seen 6-7 hg as a starting point stated before. Alan Are you sure you have this the right way round? I remember seeing an Airtech research paper on the vacuum bagging process, and in that paper they were talking about taking advantage of the resin viscosity curve to get full compaction. They did it opposite to what you have said here though, they started off at 10hg and then as the resin viscosity increased as it started to gel they increased the vacuum until they were eventually at full vacuum just prior to the resin going to B-stage. The theory was to use the low initial vacuum to extract the air and squeeze a bit of excess resin out, and then by following the viscosity curve they could achieve full consolidation on the fibres without bleeding too much resin or getting excessive voids due to expansion at full vac. The paper claimed resin infusion-like results with void content of less then 1%, which is impressive for wet-lay. Will see if I can find that study again.
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