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nosmos
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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
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Where to start? 4.5mm thick prepreg, you will need to debaulk/consolidate over few layers to compress the layers together and reduce voids. (put into the vac bag under full vacuum for a while (caveat - check your tech spec sheet for times), if you're careful you can reuse the materials for each debaulk/consolidation) Pop a thermometer into the over and actually check what temperatures you are getting, unless it's a fan oven it won't be anywhere similar around the oven space and even then the temp control most likely will be be optimal - I have heard of people on here using a kitchen oven but personally it's a big no no. With prepreg (and most elevated temp cure resins) you need to control the rise and dwell periods of the part carefully to get the best results. The few prepregs I've used need a slightly elevated debulk/consolidation, then once complete popped in the oven and raised to 40/50 odd deg for an hour or two to allow the resin to soften and flow then elevate the temperature to cure
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nosmos
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Thanks chris. debulking. Is this simply a case of laying up a ply then putting it under a vacuum for 15 mins? I am very new to this and can't see how to debulk anywhere on here.
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ChrisR
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Check your material data sheet for the specific times and number of layers etc but essentially: Layup surface ply (ply 1) - and really get it into every nook and cranny & make it conform to the shape wrap in NON perforated release film wrap in breather cloth bag & vac down (depending on your prepreg you may need to elevate the temperature) Hold under vacuum (& temp if required) for the time specified in your MDS, this could be 15mins, it could be 2hrs... Carefully unwrap everything & save as much as possible layup ply 2,3,4 etc (again see data sheet for recommended number of plys between consolidation) Make sure each ply is really pressed into the last and they conform to the mould shape - if there is a bridge now there will be a bridge after! <repeat as required> A tip that I use when I want to re-use the bag a few times on one part is to make the bag way oversize to allow me to cut a strip off the end (where the bag gum is) each time so I don't have to fart around trying to separate the gum or be two careful about it. I've found when working with prepreg (especially if I've not used it for a while), if your hands/fingers aren't tired at the end of it then you aren't pressing hard enough 
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sammymatik
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looks like a couple of issues... if you don't keep vacuum running you're not going to evacute the offgassing of the resin. You firstly need to seal the bag nicely, do a leak down test, and then keep the vacuum on during the cure cycle. Second, you need an oven controller. Without a good ramp up to temp, you won't give the resin enough time to flow properly and be able to evacuate any air or offgassing. Also you want to be sure to bag the part properly. You need to use the proper breather/bleeder to give the gas some place to go. I would also recommend using a perforate release film rather than unperforated. If you're having too much edge bleed, you can use an edge dam to prevent the resin from bleeding from the edges of the laminate. Basically for prepreg, you need good vacuum of at least 25in HG or so, a proper ramp profile as described by the resin system data sheets, and proper bag setup. Debulking is good and helpful but, probably less important if you do a good job during layup. If the part is thick then you should debulk the 1st ply and then every 4th ply. 10-15min debulk is fine. Just be sure to use a good squegee when laying up to try to remove as much air pockets as possible. You shouldn't hear popping when you run the tool over the fabric. If you debulk you shouldn't have issues. ' From the picture it looks like a oven temp issue and possible vacuum and bagging.
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ChrisR
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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
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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
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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 
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ChrisR
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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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