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John Hansen
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In another thread I asked about resin uptake of Balsa when used as a core. I have received various answer that are not answering the question, so let me go at it a different way.
If identical composite parts are made using any available method such as - but not limited to - hand layup, resin infusion, or prepreg, with a core of unsealed balsa and sealed balsa, Which would be the lightest and by how much would the weight difference be?
On another forum, I have received replies that seem to presume that it is common knowledge that resin infusion will weight a LOT MORE than if the parts are made with prepreg. Maybe all the old timers know this to be true and never use resin infusion to make light balsa core products.
But really now.... How much more is a LOT MORE?
Is there any data that will tell me how much heavier one is compared to the other?
For this discussion, please do not confuse the question by bringing up other core materials that you might think are better, lighter, stronger. That discussion will be a separate topic. First I want to know about balsa core.
A Lifelong Learner
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Warren (Staff)
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The problem you have is not that many people are using a balsa core, especially in the more technical composite processes. Sure it is more common in hand layup on rc aircraft etc but not so common on carbon composite infusions.
Ignoring the balsa or assuming it is sealed and therefore has a negligible resin uptake may be more sensible to getting a answer at least to start with.
In this case you also need to assume the same basic layup with same fabric weights.
Like for like I would expect the lightest layup to be pre-preg, followed by resin infusion, then wet lay vacuum bagging with, finally, wet lay up as the heaviest.
This is mostly down to the fibre resin ratio achievable in each technique. There is some overlap with all of them, eg a resin rich pre-preg part may be slightly heavier than a relatively dry (but still structurally sound) resin infused part. A well vacuum bagged wet lay up can be almost as light as a fairly resin rich (for cosmetic reasons) infusion. Some expert wet lay laminators can get their parts almost similar weight to resin infused - all down to skill and experience. So there is cross-over but generally the Fibre resin ratio changes with each process.
Typically in a out of autoclave pre-preg part you would be around 60:40 fibre resin ratio - some fractionally dryer.
A resin infusion you always aim for 60:40 and most typical layups can get reasonably close to that , in practice I have seen a lot around 55:45 or a smidge closer to the ideal.
Wet lay and wet lay vacuum bagging is typically starting on the heavy side at 50: 50 although with a bit of experience you can get it closer to 55:45 with careful vacuum bagging. Complete newbies tend to be quite resin rich in their wet lay ups so could have an even higher resin ratio.
That is all fairly "broad brush" figures to get most people started.
Without testing of your specific core materials, you will struggle to find specific data on the impact of the balsa core because, I just don't think the data exists or is not common enough to be easily accessible. In the seven years I have been with EC and my composites experience before, I have only heard of balsa cores in this way generally for wet lay ups.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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oekmont
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I would disagree in some points:
Prepreg and hand layup could be equally light, if you don't aim for a perfect finish. And if there is any resin uptake, you won't get a perfect finish with prepreg either. Infusion will be heavier, even if you completely seal the balsa, because of the resin in the grooves and holes you have to make for the process. The resin uptake will increase the difference further, because the machined infusion ready balsa has more surface than the simple flat balsa.
You won't find any universally valid numbers to that question.
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John Hansen
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+xThe problem you have is not that many people are using a balsa core, especially in the more technical composite processes. Sure it is more common in hand layup on rc aircraft etc but not so common on carbon composite infusions. Ignoring the balsa or assuming it is sealed and therefore has a negligible resin uptake may be more sensible to getting a answer at least to start with. In this case you also need to assume the same basic layup with same fabric weights. Like for like I would expect the lightest layup to be pre-preg, followed by resin infusion, then wet lay vacuum bagging with, finally, wet lay up as the heaviest. This is mostly down to the fibre resin ratio achievable in each technique. There is some overlap with all of them, eg a resin rich pre-preg part may be slightly heavier than a relatively dry (but still structurally sound) resin infused part. A well vacuum bagged wet lay up can be almost as light as a fairly resin rich (for cosmetic reasons) infusion. Some expert wet lay laminators can get their parts almost similar weight to resin infused - all down to skill and experience. So there is cross-over but generally the Fibre resin ratio changes with each process. Typically in a out of autoclave pre-preg part you would be around 60:40 fibre resin ratio - some fractionally dryer. A resin infusion you always aim for 60:40 and most typical layups can get reasonably close to that , in practice I have seen a lot around 55:45 or a smidge closer to the ideal. Wet lay and wet lay vacuum bagging is typically starting on the heavy side at 50: 50 although with a bit of experience you can get it closer to 55:45 with careful vacuum bagging. Complete newbies tend to be quite resin rich in their wet lay ups so could have an even higher resin ratio. That is all fairly "broad brush" figures to get most people started. Without testing of your specific core materials, you will struggle to find specific data on the impact of the balsa core because, I just don't think the data exists or is not common enough to be easily accessible. In the seven years I have been with EC and my composites experience before, I have only heard of balsa cores in this way generally for wet lay ups. Thank you Warren, Your answer is both useful and perplexing. And oekmont's answer is equally useful. When we look at the reason that Balsa is used as a composite core for its properties of light weight and strength, I am puzzled that the most commonly used method of fabrication is potentially the heaviest. But neither of you have used the phrase "a lot heavier" as it relates to the difference in weight between resin infusion and prepreg. With so much balsa going into transportation and aerospace, I simply expected there to be data available on the topic. It is possible that the majority is skinned with fiberglass rather than CF. ,
A Lifelong Learner
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Matthieu Libeert
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+x+xThe problem you have is not that many people are using a balsa core, especially in the more technical composite processes. Sure it is more common in hand layup on rc aircraft etc but not so common on carbon composite infusions. Ignoring the balsa or assuming it is sealed and therefore has a negligible resin uptake may be more sensible to getting a answer at least to start with. In this case you also need to assume the same basic layup with same fabric weights. Like for like I would expect the lightest layup to be pre-preg, followed by resin infusion, then wet lay vacuum bagging with, finally, wet lay up as the heaviest. This is mostly down to the fibre resin ratio achievable in each technique. There is some overlap with all of them, eg a resin rich pre-preg part may be slightly heavier than a relatively dry (but still structurally sound) resin infused part. A well vacuum bagged wet lay up can be almost as light as a fairly resin rich (for cosmetic reasons) infusion. Some expert wet lay laminators can get their parts almost similar weight to resin infused - all down to skill and experience. So there is cross-over but generally the Fibre resin ratio changes with each process. Typically in a out of autoclave pre-preg part you would be around 60:40 fibre resin ratio - some fractionally dryer. A resin infusion you always aim for 60:40 and most typical layups can get reasonably close to that , in practice I have seen a lot around 55:45 or a smidge closer to the ideal. Wet lay and wet lay vacuum bagging is typically starting on the heavy side at 50: 50 although with a bit of experience you can get it closer to 55:45 with careful vacuum bagging. Complete newbies tend to be quite resin rich in their wet lay ups so could have an even higher resin ratio. That is all fairly "broad brush" figures to get most people started. Without testing of your specific core materials, you will struggle to find specific data on the impact of the balsa core because, I just don't think the data exists or is not common enough to be easily accessible. In the seven years I have been with EC and my composites experience before, I have only heard of balsa cores in this way generally for wet lay ups. Thank you Warren, Your answer is both useful and perplexing. And oekmont's answer is equally useful. When we look at the reason that Balsa is used as a composite core for its properties of light weight and strength, I am puzzled that the most commonly used method of fabrication is potentially the heaviest. But neither of you have used the phrase "a lot heavier" as it relates to the difference in weight between resin infusion and prepreg. With so much balsa going into transportation and aerospace, I simply expected there to be data available on the topic. It is possible that the majority is skinned with fiberglass rather than CF. , Like Warren says it's all a matter of how it's done, I for example get lighter parts with resin infusion than prepreg. I get a fibre to resin ratio of 37.5 (resin) tot 62.5 (fibre) but keeping in mind that the infusion resin is less "strong" than a prepreg resin, so again more variables  It's not a general rule and you just have to find what fits you/the customer best. Money wise/weight wise/strenght wise/application wise/.........
Founder MAT2 Composites X Sports
website: www.mat2composites.com  
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Fasta
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When talking about infusing only a solid laminate it is likely you can get an equal resin content to pre pregs. As mentioned above though when you add in any cores, with their edges, open surface cells of foams, bridging from a vac bag then all these gaps will fill with resin too and can make for heavier parts. I have built the same small boat design in both pre pregs and infusion. The infused hull was exactly 10% heavier. Pre preg = 10kg , Infused = 11kg. The laminates will still have a great resin/fibre ratio but all the gaps will be full of resin. With balsa there maybe a chance that you infuse the wood too. Super strong but heavy. Balsa is cheap, that's why they use it !
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John Hansen
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+xWhen talking about infusing only a solid laminate it is likely you can get an equal resin content to pre pregs. As mentioned above though when you add in any cores, with their edges, open surface cells of foams, bridging from a vac bag then all these gaps will fill with resin too and can make for heavier parts. I have built the same small boat design in both pre pregs and infusion. The infused hull was exactly 10% heavier. Pre preg = 10kg , Infused = 11kg. The laminates will still have a great resin/fibre ratio but all the gaps will be full of resin. With balsa there maybe a chance that you infuse the wood too. Super strong but heavy. Balsa is cheap, that's why they use it ! Thank you Fasta. This is very valuable information. If 10% is an attainable figure for my project which is a UAV frame/hull, it will round out to about 100 to 150 gram difference. So, all things being equal, the powerplant and battery system will need to lift that extra weight 100% of the time. It does not seem to be enough of a difference to not consider using infusion. This encourages me to move forward.
A Lifelong Learner
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