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quinn
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Group: Forum Members
Posts: 155,
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hello everyone, im new to this forum as well as working with composites. I am designing and building performance rc helicopters using fusion 360 and a cnc that i designed and built. i have already finished my first prototype 700 class (700mm blades) heli and it was a huge success but there are a couple improvements i would like to make. One slight issue the heli has is tail vibrations at very high rpm. currently for the tail boom i am using roll wrapped 1mm wall, 0/90 degree carbon fiber tube. It is 30mm OD and 800mm long. it has excellent bending strength but because of the 0/90 weave, it is lacking in torsional stiffness which i am convinced is allowing the vibrations at higher rpm. I have actually confirmed this before by trying the same kind of tube on other existing heli designs which had an aluminum boom as stock. In my opinion, the solution is adding 45 degree weave with a braided sleeve to give it torsional strength. Using aluminum is another option but for best performance possible, weight is very important and use of carbon fiber is certainly going to make a lighter part for same strength. Easiest solution would be reinforcing an existing tube with braided sleeve on the inner or outer surface but i would like to take it one step further. usually these booms are braced by 2 rod supports at an angle reaching about 2/3 of the way down the boom, but a couple of manufacturers have made heavier duty tapered carbon fiber booms that dont need boom supports. Overall it looks better and can save even more weight. this is what i would like to do on my heli. I have attached 2 pics, one of the whole heli with the tapered boom on it, and the boom by itself. the boom is 800mm long, 30mm diameter out at the end and 40mm wide by 48mm tall at the base. this isnt the final design, i just quickly modeled it to give a representation of what it will be similar too. I am thinking most of it will be about 1mm thick wall with some extra layers at the base to bring it up to 1.5mm or so. fasteners will pass through the wall of the boom and thread into aluminum pieces placed against the inner wall of the boom so the boom wall is clamped against the side frames of the heli. so here are my questions. Considering i have a cnc at my disposal, what method is going to give the best results for an efficient part? As i mentioned, weight is very important so every gram counts, which means i would like to have as low of ratio possible of resin vs carbon fiber. since i need to have continuous 45 degree weave for torsional strength, that means i need to use braided sleeve which im sure complicates things since i cant just join 2 mold halves together after wetted out fabric has been layed in each side. here is what i picture working best, but i dont know exactly how to do it. 2 female mold halves and an air bladder to push the weave tight to the walls, but since im using sleeve, i need to be able to apply the sleeve layers to the bladder (partially inflated?) before laying it into one mold half, then bolt on other mold half, then inflate bladder. im assuming this means i need to make a custom bladder that can be partially inflated that will be slightly smaller than the final size so it can be layed into the mold after weave has been applied and wetted out. i assume this type of bladder would also be made in a mold of its own, but needs to be smaller than the final mold. I have plenty of other questions about what to mill the molds out of and how to make custom bladders, etc. but i figured i should start by asking if i even have the right idea of how to tackle this, or if theres a better or easier way. any advice is greatly appreciated.  
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Fasta
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Group: Forum Members
Posts: 468,
Visits: 3.5K
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+x+x+x+x+x+xSounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others? yeah thats true, no matter what im gonna need some nice female molds so i guess start there and experiment. maybe even start with fiberglass while experimenting to keep cost down. Exactly, it's more about refining the process. Heated moulds will work great but you may as well build a small oven as then you have a more versatile setup for making other parts too. added an edit to my post, not sure if you saw it, "so if doing this with vacuum, i assume you just have to push the bag through the length with a long stick or something, then the remainder of it gets doubled back over the outside of the mold? so what i really need is a long tubular bag big enough for the mold to fit inside, and a little more than double the length of the mold?" Exactly, or you can lay the bag inside as you close the moulds. This can work fine (with practice and the right pre pregs and cure ramping etc) but will only make about 14 psi pressure whereas the bladder or expanding core techniques can make 90 psi so far more pressure can often mean better part finish with less air bubble issues. ah, makes sense. basically atmospheric pressure with full vacuum. unless you then placed it all in a larger tank and pressurized it, but that sounds complicated. heres something i tried yesterday just for fun. pretty much a 200mm long version of my boom. i glued up a few layers of mdf, machined the negative into it plus a 2mm offset, built the 2mm back up with bondo, machined the finished negative shape into the bondo, sand and clear coat. made some pretty decent molds relatively quick for only a few bucks. definitely wouldnt work for prepreg though and im sure aluminum molds are MUCH nicer, but i might consider this method for some larger stuff that i need like the canopy  It can work for pre preg, I have steel reinforced plywood moulds with a thick filler coating that have made hundreds of parts. Best to stick the adhesive back teflon/PTFE material to the mould as a release surface.
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quinn
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Group: Forum Members
Posts: 155,
Visits: 992
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+x+x+x+x+x+x+xSounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others? yeah thats true, no matter what im gonna need some nice female molds so i guess start there and experiment. maybe even start with fiberglass while experimenting to keep cost down. Exactly, it's more about refining the process. Heated moulds will work great but you may as well build a small oven as then you have a more versatile setup for making other parts too. added an edit to my post, not sure if you saw it, "so if doing this with vacuum, i assume you just have to push the bag through the length with a long stick or something, then the remainder of it gets doubled back over the outside of the mold? so what i really need is a long tubular bag big enough for the mold to fit inside, and a little more than double the length of the mold?" Exactly, or you can lay the bag inside as you close the moulds. This can work fine (with practice and the right pre pregs and cure ramping etc) but will only make about 14 psi pressure whereas the bladder or expanding core techniques can make 90 psi so far more pressure can often mean better part finish with less air bubble issues. ah, makes sense. basically atmospheric pressure with full vacuum. unless you then placed it all in a larger tank and pressurized it, but that sounds complicated. heres something i tried yesterday just for fun. pretty much a 200mm long version of my boom. i glued up a few layers of mdf, machined the negative into it plus a 2mm offset, built the 2mm back up with bondo, machined the finished negative shape into the bondo, sand and clear coat. made some pretty decent molds relatively quick for only a few bucks. definitely wouldnt work for prepreg though and im sure aluminum molds are MUCH nicer, but i might consider this method for some larger stuff that i need like the canopy Click To Enlarge It can work for pre preg, I have steel reinforced plywood moulds with a thick filler coating that have made hundreds of parts. Best to stick the adhesive back teflon/PTFE material to the mould as a release surface. hmm, interesting. maybe worth trying then. definitely cheaper than the aluminum molds. Is there something better than bondo for this?
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Fasta
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Group: Forum Members
Posts: 468,
Visits: 3.5K
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+x+x+x+x+x+x+x+xSounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others? yeah thats true, no matter what im gonna need some nice female molds so i guess start there and experiment. maybe even start with fiberglass while experimenting to keep cost down. Exactly, it's more about refining the process. Heated moulds will work great but you may as well build a small oven as then you have a more versatile setup for making other parts too. added an edit to my post, not sure if you saw it, "so if doing this with vacuum, i assume you just have to push the bag through the length with a long stick or something, then the remainder of it gets doubled back over the outside of the mold? so what i really need is a long tubular bag big enough for the mold to fit inside, and a little more than double the length of the mold?" Exactly, or you can lay the bag inside as you close the moulds. This can work fine (with practice and the right pre pregs and cure ramping etc) but will only make about 14 psi pressure whereas the bladder or expanding core techniques can make 90 psi so far more pressure can often mean better part finish with less air bubble issues. ah, makes sense. basically atmospheric pressure with full vacuum. unless you then placed it all in a larger tank and pressurized it, but that sounds complicated. heres something i tried yesterday just for fun. pretty much a 200mm long version of my boom. i glued up a few layers of mdf, machined the negative into it plus a 2mm offset, built the 2mm back up with bondo, machined the finished negative shape into the bondo, sand and clear coat. made some pretty decent molds relatively quick for only a few bucks. definitely wouldnt work for prepreg though and im sure aluminum molds are MUCH nicer, but i might consider this method for some larger stuff that i need like the canopy Click To Enlarge It can work for pre preg, I have steel reinforced plywood moulds with a thick filler coating that have made hundreds of parts. Best to stick the adhesive back teflon/PTFE material to the mould as a release surface. hmm, interesting. maybe worth trying then. definitely cheaper than the aluminum molds. Is there something better than bondo for this? Epoxy resin with fillers will bond better to any wood like materials. The higher pressure techniques will need a strong mould like the alloy.
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quinn
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Group: Forum Members
Posts: 155,
Visits: 992
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+x+x+x+x+x+x+x+x+xSounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others? yeah thats true, no matter what im gonna need some nice female molds so i guess start there and experiment. maybe even start with fiberglass while experimenting to keep cost down. Exactly, it's more about refining the process. Heated moulds will work great but you may as well build a small oven as then you have a more versatile setup for making other parts too. added an edit to my post, not sure if you saw it, "so if doing this with vacuum, i assume you just have to push the bag through the length with a long stick or something, then the remainder of it gets doubled back over the outside of the mold? so what i really need is a long tubular bag big enough for the mold to fit inside, and a little more than double the length of the mold?" Exactly, or you can lay the bag inside as you close the moulds. This can work fine (with practice and the right pre pregs and cure ramping etc) but will only make about 14 psi pressure whereas the bladder or expanding core techniques can make 90 psi so far more pressure can often mean better part finish with less air bubble issues. ah, makes sense. basically atmospheric pressure with full vacuum. unless you then placed it all in a larger tank and pressurized it, but that sounds complicated. heres something i tried yesterday just for fun. pretty much a 200mm long version of my boom. i glued up a few layers of mdf, machined the negative into it plus a 2mm offset, built the 2mm back up with bondo, machined the finished negative shape into the bondo, sand and clear coat. made some pretty decent molds relatively quick for only a few bucks. definitely wouldnt work for prepreg though and im sure aluminum molds are MUCH nicer, but i might consider this method for some larger stuff that i need like the canopy Click To Enlarge It can work for pre preg, I have steel reinforced plywood moulds with a thick filler coating that have made hundreds of parts. Best to stick the adhesive back teflon/PTFE material to the mould as a release surface. hmm, interesting. maybe worth trying then. definitely cheaper than the aluminum molds. Is there something better than bondo for this? Epoxy resin with fillers will bond better to any wood like materials. The higher pressure techniques will need a strong mould like the alloy. makes sense, so this could work for vacuum or lower pressure bladder, alloy for high pressure bladder or expanding core. maybe i will experiment with this first since it will basically cost nothing. At least for the prototyping. Once im happy with a design that works, maybe move to alloy molds and more pressure. I think thats a good place to start. thanks for the suggestions
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Fasta
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Group: Forum Members
Posts: 468,
Visits: 3.5K
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+x+x+x+x+x+x+x+x+x+xSounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others? yeah thats true, no matter what im gonna need some nice female molds so i guess start there and experiment. maybe even start with fiberglass while experimenting to keep cost down. Exactly, it's more about refining the process. Heated moulds will work great but you may as well build a small oven as then you have a more versatile setup for making other parts too. added an edit to my post, not sure if you saw it, "so if doing this with vacuum, i assume you just have to push the bag through the length with a long stick or something, then the remainder of it gets doubled back over the outside of the mold? so what i really need is a long tubular bag big enough for the mold to fit inside, and a little more than double the length of the mold?" Exactly, or you can lay the bag inside as you close the moulds. This can work fine (with practice and the right pre pregs and cure ramping etc) but will only make about 14 psi pressure whereas the bladder or expanding core techniques can make 90 psi so far more pressure can often mean better part finish with less air bubble issues. ah, makes sense. basically atmospheric pressure with full vacuum. unless you then placed it all in a larger tank and pressurized it, but that sounds complicated. heres something i tried yesterday just for fun. pretty much a 200mm long version of my boom. i glued up a few layers of mdf, machined the negative into it plus a 2mm offset, built the 2mm back up with bondo, machined the finished negative shape into the bondo, sand and clear coat. made some pretty decent molds relatively quick for only a few bucks. definitely wouldnt work for prepreg though and im sure aluminum molds are MUCH nicer, but i might consider this method for some larger stuff that i need like the canopy Click To Enlarge It can work for pre preg, I have steel reinforced plywood moulds with a thick filler coating that have made hundreds of parts. Best to stick the adhesive back teflon/PTFE material to the mould as a release surface. hmm, interesting. maybe worth trying then. definitely cheaper than the aluminum molds. Is there something better than bondo for this? Epoxy resin with fillers will bond better to any wood like materials. The higher pressure techniques will need a strong mould like the alloy. makes sense, so this could work for vacuum or lower pressure bladder, alloy for high pressure bladder or expanding core. maybe i will experiment with this first since it will basically cost nothing. At least for the prototyping. Once im happy with a design that works, maybe move to alloy molds and more pressure. I think thats a good place to start. thanks for the suggestions Yes it can with the vacuum but even a low pressure bladder will try to push the moulds apart, the tubular vacuum setup does not do this. Also the teflon coat surface gives a good air free surface although it would have the very slight and even smooth texture to it that is from the teflon coated fibreglass. You could also tape the surface with straight teflon tape but even this gets expensive. I recently bought a new 1" roll of it and just that cost AUD$100.
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quinn
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Group: Forum Members
Posts: 155,
Visits: 992
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+x+x+x+x+x+x+x+x+x+x+xSounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others? yeah thats true, no matter what im gonna need some nice female molds so i guess start there and experiment. maybe even start with fiberglass while experimenting to keep cost down. Exactly, it's more about refining the process. Heated moulds will work great but you may as well build a small oven as then you have a more versatile setup for making other parts too. added an edit to my post, not sure if you saw it, "so if doing this with vacuum, i assume you just have to push the bag through the length with a long stick or something, then the remainder of it gets doubled back over the outside of the mold? so what i really need is a long tubular bag big enough for the mold to fit inside, and a little more than double the length of the mold?" Exactly, or you can lay the bag inside as you close the moulds. This can work fine (with practice and the right pre pregs and cure ramping etc) but will only make about 14 psi pressure whereas the bladder or expanding core techniques can make 90 psi so far more pressure can often mean better part finish with less air bubble issues. ah, makes sense. basically atmospheric pressure with full vacuum. unless you then placed it all in a larger tank and pressurized it, but that sounds complicated. heres something i tried yesterday just for fun. pretty much a 200mm long version of my boom. i glued up a few layers of mdf, machined the negative into it plus a 2mm offset, built the 2mm back up with bondo, machined the finished negative shape into the bondo, sand and clear coat. made some pretty decent molds relatively quick for only a few bucks. definitely wouldnt work for prepreg though and im sure aluminum molds are MUCH nicer, but i might consider this method for some larger stuff that i need like the canopy Click To Enlarge It can work for pre preg, I have steel reinforced plywood moulds with a thick filler coating that have made hundreds of parts. Best to stick the adhesive back teflon/PTFE material to the mould as a release surface. hmm, interesting. maybe worth trying then. definitely cheaper than the aluminum molds. Is there something better than bondo for this? Epoxy resin with fillers will bond better to any wood like materials. The higher pressure techniques will need a strong mould like the alloy. makes sense, so this could work for vacuum or lower pressure bladder, alloy for high pressure bladder or expanding core. maybe i will experiment with this first since it will basically cost nothing. At least for the prototyping. Once im happy with a design that works, maybe move to alloy molds and more pressure. I think thats a good place to start. thanks for the suggestions Yes it can with the vacuum but even a low pressure bladder will try to push the moulds apart, the tubular vacuum setup does not do this. Also the teflon coat surface gives a good air free surface although it would have the very slight and even smooth texture to it that is from the teflon coated fibreglass. You could also tape the surface with straight teflon tape but even this gets expensive. I recently bought a new 1" roll of it and just that cost AUD$100. so if i try the vacuum method, will i just have the vacuum bag right up against the lay up? or will i want to do peel ply and breather, then vacuum bag? Also would this differ for prepreg vs wet lay? ill try to do prepreg as its just going to give better results, just curious in case i try wet lay. I imagine with wet lay, the peel ply and breather is gonna be needed to let extra resin out. also, the teflon tape sounds nice and convenient with not much mold prep work, but would i be at much disadvantage to just use something like the s120 mold sealer and mold release wax? Im sure its more work, but maybe potentially cheaper and better surface finish?
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Fasta
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Group: Forum Members
Posts: 468,
Visits: 3.5K
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+x+x+x+x+x+x+x+x+x+x+x+xSounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others? yeah thats true, no matter what im gonna need some nice female molds so i guess start there and experiment. maybe even start with fiberglass while experimenting to keep cost down. Exactly, it's more about refining the process. Heated moulds will work great but you may as well build a small oven as then you have a more versatile setup for making other parts too. added an edit to my post, not sure if you saw it, "so if doing this with vacuum, i assume you just have to push the bag through the length with a long stick or something, then the remainder of it gets doubled back over the outside of the mold? so what i really need is a long tubular bag big enough for the mold to fit inside, and a little more than double the length of the mold?" Exactly, or you can lay the bag inside as you close the moulds. This can work fine (with practice and the right pre pregs and cure ramping etc) but will only make about 14 psi pressure whereas the bladder or expanding core techniques can make 90 psi so far more pressure can often mean better part finish with less air bubble issues. ah, makes sense. basically atmospheric pressure with full vacuum. unless you then placed it all in a larger tank and pressurized it, but that sounds complicated. heres something i tried yesterday just for fun. pretty much a 200mm long version of my boom. i glued up a few layers of mdf, machined the negative into it plus a 2mm offset, built the 2mm back up with bondo, machined the finished negative shape into the bondo, sand and clear coat. made some pretty decent molds relatively quick for only a few bucks. definitely wouldnt work for prepreg though and im sure aluminum molds are MUCH nicer, but i might consider this method for some larger stuff that i need like the canopy Click To Enlarge It can work for pre preg, I have steel reinforced plywood moulds with a thick filler coating that have made hundreds of parts. Best to stick the adhesive back teflon/PTFE material to the mould as a release surface. hmm, interesting. maybe worth trying then. definitely cheaper than the aluminum molds. Is there something better than bondo for this? Epoxy resin with fillers will bond better to any wood like materials. The higher pressure techniques will need a strong mould like the alloy. makes sense, so this could work for vacuum or lower pressure bladder, alloy for high pressure bladder or expanding core. maybe i will experiment with this first since it will basically cost nothing. At least for the prototyping. Once im happy with a design that works, maybe move to alloy molds and more pressure. I think thats a good place to start. thanks for the suggestions Yes it can with the vacuum but even a low pressure bladder will try to push the moulds apart, the tubular vacuum setup does not do this. Also the teflon coat surface gives a good air free surface although it would have the very slight and even smooth texture to it that is from the teflon coated fibreglass. You could also tape the surface with straight teflon tape but even this gets expensive. I recently bought a new 1" roll of it and just that cost AUD$100. so if i try the vacuum method, will i just have the vacuum bag right up against the lay up? or will i want to do peel ply and breather, then vacuum bag? Also would this differ for prepreg vs wet lay? ill try to do prepreg as its just going to give better results, just curious in case i try wet lay. I imagine with wet lay, the peel ply and breather is gonna be needed to let extra resin out. also, the teflon tape sounds nice and convenient with not much mold prep work, but would i be at much disadvantage to just use something like the s120 mold sealer and mold release wax? Im sure its more work, but maybe potentially cheaper and better surface finish? Ok It will be difficult to do the full release film and breather inside such a small mould and maybe worth trying the simple bag straight against the inside although sometimes the bag may not release that easy especially pulling from inside a tube like this. Maybe use a proper release film and bag with just a 30mm strip of breather? No peel ply needed, that would also be hard to remove from within the tube. The proper release films would pull off the cured resin best. The S120 looks really impressive and I hope to try it someday, sounds like this will also need the easy lease too. Using the semi permanent release systems is the normal thing with pre pregs, waxes are not generally used. I still like the teflon tape as this makes a true impermeable barrier and foolproof release to save any damage to the mould, especially when releasing off a questionable mould surface material. If the taped mould surface gets any damage then you just stick some new stuff on, it is soft. The tape is very thin and would leave just a few lengthwise edges. Cover the flange areas too. https://www.findtape.com/JVCC-PTFE-2HD-Skived-Teflon-Tape/p256/I think you probably could still consider a pressure technique with having some steel support each side of your moulds and a bunch of clamps holding the moulds shut.
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