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Steve Broad
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Making a prepreg carbon front clip for my Lotus Elan. So far so good.  Mould with the reinforcement that caused the problem. I bagged the whole thing. This resulted in a differential pressure between the inner surface and the outer skin under the reinforcement which resulted in the mould sinking under the pressure!  
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oekmont
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Sorry to hear and see that. But this would have happened with any other bagging technique. It looks like your mould wasn't tempered high enough, or your mould resin just isn't suitable for the temperature.
Why is the central part of your mould black? did you use the same resin? What material did you use for the stringers? What is the layup of your mould, especially underneath the stringers?
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Hanaldo
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It's an interesting mould, but in my opinion that ribbing structure is totally unnecessary as well. That mould would be plenty stiff just from those parting barriers.
Anyway, I agree with oekmont - the pressure differential isn't the cause of the print. If it was, the mould surface would have blistered outwards, not collapsed inwards. And even this would have been extremely unlikely unless the mould is only one or two mm thick - a regular thickness mould of 5-6mm would quite easily withstand that pressure differential.
Unless of course, as oekmont mentioned, the mould couldn't handle the temperature. Whether that's due to too low a Tg, or just not having had a good enough post-cure prior to attempting this cure. I'd say this is almost certainly temperature related.
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Steve Broad
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Correct resin, gel and fibreglass. The central part is black as I ran out of fibreglass :-) Agreed, the ribs were not necessary, hindsight is a wonderful thing :-) Total of 8 layers with 6 under the reinforcement and two on top. Drawing below shows the construction and deformation cause, which was a combination of pressure differential and lack of post cure IMO. Without the reinforcement the lack of post cure wouldn't have been an issue. If I had post cured I might have gotten away with it. If I had just bagged the underside the pressure would have been constant on the other side and the deformation wouldn't have happened. Oh, well, hopefully I can rescue the bonnet. 
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Warren (Staff)
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Thanks for the pictures. I see what you mean by the ribbing. It is a shame as the rest of it looks really good. It is a shame there was the void in the ribs but as we discussed you probably don't need them or next time use something rigid in the ribs.
Hopefully the repairs will come out ok and you keep it down to a good weight too.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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oekmont
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As long as you didn't have bridging, it doesn't make a difference if you bagged the whole mould or just the inside. The air pushes against the outside mold with the mold with the same pressure, with or without the bag. As the pressure in your stringers should have been atmospheric pressure, I wonder why the bending didn't went outward, as the trapped air expanded with the temperature? The pressure inside the ribs should not be 0psi, but >atmospheric pressure?
Or did you drill venting holes from the outside? If so, the pressure inside the ribs would have been 0psi due to your bagging technique. The same would happen, if your upper two layers are not airtight.
The main mistake here was to use hollow profiles. You should always try to avoid air pockets in both moulds and parts, unless the air is securely trapped inside a foam or micro glass bubbles. Or at least honeycombs, but I saw heat problems with those, too.
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Steve Broad
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+xAs long as you didn't have bridging, it doesn't make a difference if you bagged the whole mould or just the inside. The air pushes against the outside mold with the mold with the same pressure, with or without the bag. As the pressure in your stringers should have been atmospheric pressure, I wonder why the bending didn't went outward, as the trapped air expanded with the temperature? The pressure inside the ribs should not be 0psi, but >atmospheric pressure? Or did you drill venting holes from the outside? If so, the pressure inside the ribs would have been 0psi due to your bagging technique. The same would happen, if your upper two layers are not airtight. The main mistake here was to use hollow profiles. You should always try to avoid air pockets in both moulds and parts, unless the air is securely trapped inside a foam or micro glass bubbles. Or at least honeycombs, but I saw heat problems with those, too. Yes, I know the air would be atmospheric, I was working on relative pressures. otherwise the 14 would be 0. With a vacuum within the ribs (I would be surprised if they were airtight, the combination of vacuum and heat would find a weakness) there is nothing to resist the pressure from the other side, which is why the mould deformed along the rib lines. The two faces of the bag were trying to meet but the ribs prevented the outside moving in so the path of least resistance was the part of the mould under the ribs. Differential pressure deformed the ribs as far as I can make out. Anyway, I won't make that mistake again. Yes, solid ribs would probably work but might still show through, albeit only cosmetically, so smooth surfaces are best, I think.
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Warren (Staff)
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How is the trimming and repair work coming on? Do you have a more or less final weight yet?
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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Steve Broad
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+xHow is the trimming and repair work coming on? Do you have a more or less final weight yet? Hi I have been adding foam core to splitter, again struggling with achieving a vacuum. However, that should be cured today so I will be able to turn the bonnet over and get sanding. I will advise on final weight. If the repair work is a failure, I will repair the bonnet section of the mould and make a new top and glue that on.
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Warren (Staff)
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Let us know how you get on. Hopefully you will be able to sort it.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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