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Steve Broad
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+xIf your experience with infusion is limited Steve, I'd concur with Oekmont that pre-preg offers, in theory, a more straightforward process in many ways for a first-time user.
Key with it is to ensure that the fabric is in intimate contact with the mould, to avoid any bridging that results in a rough, pin-holed or broken surface. Before starting, I'd make up lots of cutting templates to take car of the detailed contours on the front. If done tidily, everything will look great even with loads of joins so time put into planning the simplest ways to achieve coverage without need to pull, push and twist the fabric too much will really pay off.
In fact, the trickiest part, unless you're used to making up big envelope bags, will be getting a perfectly sealed bag once everything's laid in. I'd also enlist another pair of hands to help handle any big sections of fabric as it's a pig to deal with in big sections as it just sticks all over the place as you're trying to lay it in.
If you've settled upon this route, then put your effort into designing the mould so that, where possible, you'll be releasing it directly away from the part rather than relying on things coming out at an angle or needing to be flexed. This will, hopefully, guide you on where best to create the parting lines.
I have made many items using wet lay vacuum bagging and a fair number of small parts with prepreg but no infusion, although I bought all the stuff many months ago. But, despite the huge extra cost of prepreg, it is probably the way to go. Just need to decide on how to split the mould.
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MarkMK
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If your experience with infusion is limited Steve, I'd concur with Oekmont that pre-preg offers, in theory, a more straightforward process in many ways for a first-time user. Key with it is to ensure that the fabric is in intimate contact with the mould, to avoid any bridging that results in a rough, pin-holed or broken surface. Before starting, I'd make up lots of cutting templates to take car of the detailed contours on the front. If done tidily, everything will look great even with loads of joins so time put into planning the simplest ways to achieve coverage without need to pull, push and twist the fabric too much will really pay off. In fact, the trickiest part, unless you're used to making up big envelope bags, will be getting a perfectly sealed bag once everything's laid in. I'd also enlist another pair of hands to help handle any big sections of fabric as it's a pig to deal with in big sections as it just sticks all over the place as you're trying to lay it in. If you've settled upon this route, then put your effort into designing the mould so that, where possible, you'll be releasing it directly away from the part rather than relying on things coming out at an angle or needing to be flexed. This will, hopefully, guide you on where best to create the parting lines.
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Steve Broad
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+xThe problems with difficult infusions are: It is much harder to get a frayless finish in places where you need to cut the cloth. With painting, this isn't a problem for you. The flow media is much harder to get into complex shapes, than everything used in a prepreg process. on complex 3d shapes the resin front easily flows around some higher or lower shapes, trapping some voids, resulting in pinholes or (In worst case) unimpregnated areas. Split mold infusion is a different topic on its own. If you are careful during the layup, get the bag tight into the corners, and follow the heating instructions, prepreg parts always come out nicely. I never had a failed part. Infusion is in comparison an art. The process allways has to be adapted to the shape of the part. To get it right on first try, you need experience. You have convinced me that prepreg is the way to go, cheers.
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oekmont
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The problems with difficult infusions are:
It is much harder to get a frayless finish in places where you need to cut the cloth. With painting, this isn't a problem for you.
The flow media is much harder to get into complex shapes, than everything used in a prepreg process.
on complex 3d shapes the resin front easily flows around some higher or lower shapes, trapping some voids, resulting in pinholes or (In worst case) unimpregnated areas.
Split mold infusion is a different topic on its own.
If you are careful during the layup, get the bag tight into the corners, and follow the heating instructions, prepreg parts always come out nicely. I never had a failed part.
Infusion is in comparison an art. The process allways has to be adapted to the shape of the part. To get it right on first try, you need experience.
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Steve Broad
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+x
Have you the facilities to cure the pre-preg...either a temperature controlled oven or access to an autoclave?
If so and you are planning to paint the panels afterwards, then this feels like the right way to go as you'll have the advantage of having a fabric that can be applied to the mould relatively easily and the final surface finish might not be a big deal. You'd also have the in-built heat tolerance ideally required for a bonnet. If going down this route, then look to build in as many splits in the mould as you need to gain a stress-free release, as any visible signs of the mould can be flatted back afterwards
However, infusing that mould is indeed possible and using a good spray tack will allow you to lay in cut sections easily also. I use Cytec's Aerofix 3 all the time and, used sparingly, it tends not to leave any residue when used with EC's infusion resin (although I use an in-mould coating on most parts).
Infusion will also offer you a greater choice of fabrics and also allow you to build in various core materials if added stiffness is needed (although at the cost of a little additional weight). Be aware, though, that an elevated post-cure would likely be needed to help cope with the under-bonnet heat and this will, most likely, lead to some shrinkage which will show up as unevenness on the surface. This would likely be even more apparent if using a product like Soric, where the matrix tends to show through on all but the thickest laminates. This can be addressed by flatting the panel down afterwards and clear-coating/painting, however.
An infusion would also likely need the mould to be envelope bagged if it's a multi-part one, which can make controlling the flow of resin to the outer section of the mould tricky. If the part's a one-off, then not so much of a concern. Conventional bagging is possible, but sealing such a large mould would take some skill and thought in the mould design.
Cheers, Mark. I have just built an oven big enough to take the bonnet -1.8m x 1.8m x 0.9m, with thermostacially controlled temperature so cooking isn't a problem :-) The car is away being wired so I have plenty of time to come to a decision on whether to infuse or prepreg. Even when I start I will build the mould in materials that are suitable for both methods. The final result will probably end up crap as, as oekmont points out, I lack the necessary 90% skill level but I will give it my best shot :-)
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MarkMK
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Have you the facilities to cure the pre-preg...either a temperature controlled oven or access to an autoclave? If so and you are planning to paint the panels afterwards, then this feels like the right way to go as you'll have the advantage of having a fabric that can be applied to the mould relatively easily and the final surface finish might not be a big deal. You'd also have the in-built heat tolerance ideally required for a bonnet. If going down this route, then look to build in as many splits in the mould as you need to gain a stress-free release, as any visible signs of the mould can be flatted back afterwards However, infusing that mould is indeed possible and using a good spray tack will allow you to lay in cut sections easily also. I use Cytec's Aerofix 3 all the time and, used sparingly, it tends not to leave any residue when used with EC's infusion resin (although I use an in-mould coating on most parts). Infusion will also offer you a greater choice of fabrics and also allow you to build in various core materials if added stiffness is needed (although at the cost of a little additional weight). Be aware, though, that an elevated post-cure would likely be needed to help cope with the under-bonnet heat and this will, most likely, lead to some shrinkage which will show up as unevenness on the surface. This would likely be even more apparent if using a product like Soric, where the matrix tends to show through on all but the thickest laminates. This can be addressed by flatting the panel down afterwards and clear-coating/painting, however. An infusion would also likely need the mould to be envelope bagged if it's a multi-part one, which can make controlling the flow of resin to the outer section of the mould tricky. If the part's a one-off, then not so much of a concern. Conventional bagging is possible, but sealing such a large mould would take some skill and thought in the mould design.
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Steve Broad
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+x+xTwo things: 1st: I really want to help you. If the parts you have done so far are like 20% skill level, this would be like 90%. It is even harder than you can imagine. At least in infusion. I just want to save you from using 400£ worth of material to get a part you are not satisfied with. Be live me, I went that path. Better make slow progress with your part complexity. Said so, prepreg is definitely the easier way for a part like this. 2nd: As you realised yourself, the double wishbone chassis is not nearly as smart as everybody says it is. When it comes to torsional stiffness, the elan has to rely on the additional resistance from the body. By converting it to a flip front, you pretty much cutting away all of its moment of inertia. I am always happy to listen to others, so thanks for the advice.. If the infusion attempt goes wrong it is a few hundred pounds, with the prepreg it is over a grand! However, the jury is still out on the way forward as there is no rush. The body of the DHC adds almost no torsional stiffness to the chassis, which is why I fitted a full roll cage, from suspension uprights to suspension uprights. It is pretty stiff now :-) Yeah, the more I look at it, especially the nose area, prepreg seems the less hard (rather than easier) option. And, yes, I still plan to hide the carbon :-)
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oekmont
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If prepreg goes wrong, you could likely fix it with a fair amount of 2k filler (If you are still planning to paint it). At least if you get the bagging right, for wich you have several tries for a few pounds.
Infusion can result in parts, wich are half unimpregnated.
It could be infused, no question. But it is a complicated part. Prepreg scales more user friendly when it comes to bigger and more complex parts.
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Steve Broad
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Group: Forum Members
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+xTwo things: 1st: I really want to help you. If the parts you have done so far are like 20% skill level, this would be like 90%. It is even harder than you can imagine. At least in infusion. I just want to save you from using 400£ worth of material to get a part you are not satisfied with. Be live me, I went that path. Better make slow progress with your part complexity. Said so, prepreg is definitely the easier way for a part like this. 2nd: As you realised yourself, the double wishbone chassis is not nearly as smart as everybody says it is. When it comes to torsional stiffness, the elan has to rely on the additional resistance from the body. By converting it to a flip front, you pretty much cutting away all of its moment of inertia. I am always happy to listen to others, so thanks for the advice.. If the infusion attempt goes wrong it is a few hundred pounds, with the prepreg it is over a grand! However, the jury is still out on the way forward as there is no rush. The body of the DHC adds almost no torsional stiffness to the chassis, which is why I fitted a full roll cage, from suspension uprights to suspension uprights. It is pretty stiff now :-)
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oekmont
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I rewatched the photos. My point about the moment if inertia might not be that important, as the front will be entirely ahead of the front axle.
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