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hi
im loking at the pros and cons of pre preg as it cost more to bye but you don't need to bye the resin so does it work out the same price? obviously ill need to make an oven and vac pump but just wondering how it all works out
so give me your views on pre preg against resin infusion
cheers Rob
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On the face of it, they are much of a muchness. Like everything in composites it depends how much you are buying, and more importantly, what you are making and how many parts you expect to make. Overall, pre-preg probably works out to be a bit more expensive. There are other advantages and disadvantages to each though.
Obviously with infusion, you have a lot more material and a lot more waste. So if storage is a concern, or you are environmentally conscious, then pre-preg might be the better bet. On the other hand, pre-preg requires the use of both an oven AND freezer. So while you've got less materials to store, you still require the space for both those appliances. By the time you've spent money on a freezer and an oven, you have easily outspent what you would have paid for the extra materials for infusion. You also need to think about how much it costs to run a freezer and an oven. These are costs that you don't necessarily have with infusion.
The biggest advantage for me with infusion is its versatility. You can essentially make anything you want with infusion, whether it is a carbon laminate, a kevlar laminate, a fibreglass laminate, a combination of those, etc. You can use whatever fabric you like, so you open yourself up to more exotic fabrics like coloured carbon/fibreglass, Zylon, Innegra, uni-directional, stitched multiaxial, etc. You can also make larger parts like vehicle bonnets without requiring a large oven to fit them in.
On the other train of thought is if you are making small, complex parts (especially in complex multi-piece moulds), then pre-preg is the go. It can be cut extremely accurately, so it becomes possible to make templates of your mould and cut out exact shaped pieces of fabric to lay up. Not only does this make it much quicker and easier to lay up, it reduces waste. What would take you half a day to lay up and infuse could be done in an hour with pre-preg. Then you could consider the resins and their applications. Infusion resins with properties similar to pre-pregs are likely to be very expensive and hard to come by. So if you need a high Tg, pre-preg again is the obvious choice. However if you don't need an especially high Tg (and don't get me wrong here, you can get high Tg infusion resins), you have more control and versatility with your choice of resins for infusion. I could go on all day. There is no obvious winner, that's why BOTH these process are used regularly in modern composites. What it comes down to is what YOU need. What are YOU going to be making. Do you need the versatility and range that infusion can give you, or is what you are making likely to be very simple? Do you need a 3x3 metre oven to build what you want to build, or are you going to be building small pieces that don't require such a large oven? Are your pieces complex and difficult to lay up, or are they large simple pieces?
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In the past I had always favoured pre pregs since it is mostly superior if you are building high strength structural parts and weight matters. I feel it is a far easier process than infusion especially for newcomers to composites. There is less materials involved in preparing the laminate stack with all consumables, no meshes, no pipes or plumming. Having the oven is a once off thing and then you are off and running with it. These days infusion has come a long way with many new materials, cores and other that are made specifically for the process so it just much more understood and there is plenty of advice on line. It's still a tricky process though and easy for newcomers to underestimate the attention and detail needed for success. As Hanaldo said, with infusion you can then just pick up any fabric material and infuse so the available palate of materials is endless, carbon, glass, Kevlar, wood, natural fibre, printed cotton, anything can be encapsulated in the laminate. Getting great cosmetic finishes with pre preg is not easy without a real pressurised autoclave whereas infusion can give you a great finish with UV stable gelcoats/paints straight from the mould every time assuming all went well with the process.
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cheers for the reply's im planning on making body panels for my project car ie bonnet doors roof ect ect
ive worked with fiberglass for years but new to carbon im thinking of using carbon Kevlar.
ive got a good size workshop so space isn't a problem and was thinking that making an oven would be a nice little project
cheers Rob
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k9 (22/05/2015)
cheers for the reply's im planning on making body panels for my project car ie bonnet doors roof ect ect
ive worked with fiberglass for years but new to carbon im thinking of using carbon Kevlar.
ive got a good size workshop so space isn't a problem and was thinking that making an oven would be a nice little project
cheers Rob I would go with infusion for this project, and avoid the kevlar.
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Yeh for those sorts of things, infusion every time. Pre-preg would be a mega-expensive way of doing it.
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ok thanks for the help but why avoid the Kevlar???
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k9 (23/05/2015)
ok thanks for the help but why avoid the Kevlar???It's just very hard to use and cut, more so when it's dry and you are preparing the laminate (pre preg Kevlar is easier). Unless you have a really specific reason for needing it then I always avoid it. Once cured it will cut ok but leave you with a furry edge, if it comes through the surface of your parts with sanding then you get more fur and more painting issues too. When I do use it I try to use very little of it and make sure it is not in your moulded outer layer, best put further into the laminate somewhere, for when you want a real resistance to punctures or tears etc. PITA.
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