What am I do in wrong?

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What am I do in wrong?

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nosmos	nosmos posted 10 Years Ago HOT Topic Details
Group: Forum Members Posts: 3, Visits: 41	Hello everyone, thanks in for taking the time to read this, as I really need the help. I have tried twice now to make this part and both times I get the same problem. I am using the ec pre preg starter kit, and by all accounts it has had nothing but good reviews. There for I am sure I am doing something wrong. I am forming a profile 4.5mm thick on a bent ally sheet. I am getting a lot of resin flash, and lots of what I presume is what you refer to on here as pin holes. Although they are bigger than pin holes. I cured the first attempt in my kitchen oven at 100 degree c, and the second attempt at 85, just in case the oven was actually hotter than it said. Both cured ok except this problem. I am having trouble sealing the bag and have no way at the moment to keep the vacuum pump attached. If I am loosing the vacuum slowly, could this cause the problems I'm having? any help gratefully received.
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ChrisR	ChrisR posted 10 Years Ago HOT Post Details
Group: Forum Members Posts: 363, Visits: 3K	You shouldn't use a perforated release film with prepreg, you are not trying to remove resin from the layup, any air will find it's way out to the breather fabric during when you stick it under vacuum. Epoxy doesn't outgas during the cure (polyester and vinylester yes they do), that's why you can use it in closed mould and once you've pulled the full vacuum and done a drop test on it you "should" be able to disconnect the vac pump for the entire cure (some leave it on for insurance though) but if you do and you have a slight leak somewhere then you are going to screw your pump. What you can do to help air escape during debaulk and final cure is to put a few strands of glass running across the part and out the sides of the stack (but not outside the vag bag/gum tape) this will allow dry flow path for the air to get out. (you can use a perforated film if you are trying to bleed resin out, but in the case above that's the last thing you'd want to be doing)
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sammymatik	sammymatik posted 10 Years Ago HOT Post Details
Group: Forum Members Posts: 3, Visits: 9	epoxy can outgas... I guess it would depend on the resin itself. Mostly it's the removal of entrapped air, which is outgassing. The airplane manufatures that do OOA will use perf instead on non perf, at least in the small civil craft manufacturing that I've seen. The perf also gives you a vertical breathe rather than edge. You can also use edge damn breather, we do this with non perf. The edge damn breather actually increases tensile pull tests by abou 2k psi, which is significant in structural parts. The amount you will bleed with perf is relatively minor. If you're using a 42% ratio pre preg, this should put you right at 60/40 fiber ratio. With the perf you're just ensuring that you get a good resin flow and path for any air or volatiles to escape. You can also use a variety of perf film to control bleed, really it's not an issue. If your part is that critical then do testing to determine which perf will give you the exact resin content your application requires. And you should always have vacuum running during cure. If you turn off the vacuum then you will lose any possibility of the gases escaping because there is no resin flow. This is also why you want a ramp in temperature. You need to give the resin time in the liquid stage for the gases to move. If the vacuum is off, then you just get compaction from the air pressure, but not the suction. Also how can you be sure that your part is maintaing vacuum during cure? Maybe you messed up the bag when you put it in the oven? Maybe the heat has caused the sealant tape to create a leak? Running the pump with a leak will not hurt the pump at all... If your leak is that bad that the pump is laboring during cure, then you probably need to rebag your part. and for the bag or debulk, you always put breather across the entire part, on top of the release film. If you don't use breather, then you can trap areas of air that will have no vacuum and thus no compaction. There are self breathing bags that we use for debulk... quick draw bag. But for final bag you need breather/bleeder. this is all stuff that I've learned from manufacturing experience in both small and large craft production. I would definitely trust these methods as I've built many primary structural parts and have also done the testing to verify much of these processes. I often find that with composites people have ideas they think are correct or just common sense but, when asked for data or empirical testing, they don't have any. It seem like black magic often time because testing is laborious, difficult, expensive, and time consuming. I always tell people to do tests. Many many tests until you can refine your process to meet your needs. As with any project, there are many many variables and what result you want depends on these sometimes tiny variables. Like the edge damn breather... I didn't think it would make much difference, but even a small detail as this actually can produce fairly varied results. Sometimes it doens't matter so much. A bottle opener probably won't need that extra bit of strength but a wing spar definitely will.
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Matthieu Libeert	Matthieu Libeert posted 10 Years Ago HOT Post Details
Group: Forum Members Posts: 955, Visits: 3.4K	I follow a bit of Sammymatik and ChrisR on here... Here is what I think and works well for me. - Pump always on - NON Perforated film, no peelply, just prepreg and non perf film - Dam to prevent bleeding of resin - NEVER put a part in the oven that still has a leak, if so, rebag till perfect vacuum with droptest of 4 hours. otherwise you will pull air trough your prepreg - 4,5 mm is a thick part to start testing with, start with some thinner lay up maybe - Kitchen ovens are not made for prepreg Testing is the key to succes Matthieu Libeert Founder MAT2 Composites X Sports website: www.mat2composites.com
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ChrisR	ChrisR posted 10 Years Ago HOT Post Details
Group: Forum Members Posts: 363, Visits: 3K	Sammymatik, you are right in everything you say from your experience (and from the sounds of it, rather extensive) I've had no difference in results (in my experience) by not running the pump all the time therefore saving the leckly bill and although I do primarily make structural parts I'm not really after the extra few % of strength as deflection control is my primary concern. but with no offence intended to the original poster, he is inexperienced with the material, running before you can walk and all that. With all things, you need to learn the basics, typical way of doing things and the rules, once you understand this in depth you can break the rules and go off piste. You wouldn't learn about quantum mechanics without knowing first learning basic physics first and working up. There is one thing I do disagree with but that depends on the pump i suppose, if you use the EC pump then you shouldn't run it with any sort of a leak as it will spew oil vapour into the air, contaminate any surface (or fabric) it falls on, overheat, use up the oil & burn out. (and Sammymatik, just so you know you're not just talking to a keyboard numpty, I first started working with composites (repairing boats) 20+ yrs ago, I'm a Structural Engineer did my MSc on structural connections in composites, designed a couple concept stage composite bridges, used composite sections in a few structures and have built my own workshop/small factory for producing the parts I design & watersports equipment to name but a few things!)
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