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quinn
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+xA word of warning: this isn't an easy technique. Be prepared for a LOT of wastage until you nail it. I've played around with this quite a bit, but unfortunately it's quite tricky to do on anything but very very simple shapes where the expansion is even and very predictable. Boxes and straight tubes: dead easy. Complex shapes: significantly harder. There's a bit of maths to do, it isn't quite as simple as just saying your layup should be about 1mm thick so you just make a silicone plug 1mm smaller and presto, perfect results. You need to find the formula for the thermal expansion of silicone and use that to calculate very precisely how much smaller you need to make the plug. The hard bit is that most silicone manufacturers don't provide the CTE of their product in the data sheet, and silicone can have a linear expansion anywhere between 5.9x10-4/C and 7.9x10-4/C. This means that you don't really know how much your silicone is going to expand across a shape of varying size. It also isn't very forgiving. The rate of thermal expansion for silicone might be relatively huge, but if you have a 10" long part you may have a linear expansion along the length anywhere between 7mm and 9.5mm - a rather hefty difference to just be guessing. So on something like an RC helicopter boom, which tapers from relatively large at one end to relatively small at the other, across a relatively long distance; it may well be that you need to make the plug 5mm smaller along the length, but only 0.001mm smaller at the small end and 0.865mm smaller at the large end, and some varying number in between. Those numbers are just random to demonstrate my point. You will probably find over your first few attempts that you end up with a lot of uneven compaction. One part may be under immense pressure, whilst somewhere else the silicone barely expands at all and you get no compaction. Or it could go the other way, you make your silicone plug too big, and apply such huge forces that it breaks your mould (admittedly very unlikely if you're machining an aluminium mould, but still possible). I don't mean to put you off, as I do think this is a very repeatable technique once you get it right. It just isn't as easy as you might be hoping, hence why you can't find a lot of information about it. My approach, given I'm not an engineer and not superb at maths, would be to use the formula to work out a rough idea of the range of expansion you can expect. Then, layup a carbon part seperately in each of your mould halves using vacuum consolidation (pre-preg would be easiest here). Once that is cured and you can measure it's exact thickness rather than just make a rough guess, then you can use sheet wax to add thickness to sections where the silicone will have more expansion. Then bolt the moulds together, and cast your silicone into the shell. Then, rinse and repeat when it doesn't quite work, making changes as needed. I was under the impression that as long as the plug fits tight enough, no loose areas, that was all that really mattered. I wasn't aware there could be "too much" expansion. I will be using quite beefy aluminum molds (no less than 20mm wall thickness) being held together with plenty of large bolts. Do i really need to worry about too much pressure? I also plan to contain the mold 100%, as in thick aluminum caps bolted on the ends. Because of the physical properties of the silicone, I would expect it to somewhat displace itself and give relatively similar pressure throughout. For example let's say the plug was .1mm away from all walls of the mold including the ends. as it starts to expand it will obviously touch the ends first because of the linear expansion, but will it not then start displacing itself outwards towards the walls? If it was a completely rigid plug I can see how there would need to be very careful calculation for expansion in each direction but a chunk of rubber I would expect to take the form of its container as it expands. This is of course assumining the container is strong enough. If you were to take a cylinder of silicone and push on the ends between your hands, it would squish in and push the walls out at the same time right? I would think it would have those similar displacement properties inside a mold as it expands. So with an aluminum mold that has minimum 20mm walls and plenty of bolts, do I need to worry about not allowing enough room for expansion in specific directions?
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oekmont
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You are not dealing with ordinary forces here. Heat expansion is an atomic force. Once the plug filled the cavity completely, there is nothing except neutron star like gravity, wich will stop the plug from bursting your mold like it was nothing. However, in your case the plug will expand through the ends. But because the boom is tapered, this will mainly happen into the wider direction. This, however will likely lead to high pressures on the tighter side (the plug can't really expand in this direction) and low pressure on the wider side (the plug is pushed in this direction, and therefore ever mould "section" gets a thinner plug section than planned, resulting in lower pressure).
Because of that I would advice you to close both mould ends, and make a silicone bladder. Either with rotation casting, or with silicone sheets. with a pressure inlet on one side of course (bladder and mould). That way you can perfectly control the pressure, and get a evenly pressure along the part.
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quinn
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+xYou are not dealing with ordinary forces here. Heat expansion is an atomic force. Once the plug filled the cavity completely, there is nothing except neutron star like gravity, wich will stop the plug from bursting your mold like it was nothing. However, in your case the plug will expand through the ends. But because the boom is tapered, this will mainly happen into the wider direction. This, however will likely lead to high pressures on the tighter side (the plug can't really expand in this direction) and low pressure on the wider side (the plug is pushed in this direction, and therefore ever mould "section" gets a thinner plug section than planned, resulting in lower pressure). Because of that I would advice you to close both mould ends, and make a silicone bladder. Either with rotation casting, or with silicone sheets. with a pressure inlet on one side of course (bladder and mould). That way you can perfectly control the pressure, and get a evenly pressure along the part. Not quite neutron star gravity forces lol. Solids are compressible and bulk modulus of rubber is lower than most solids. But yeah, you're probably right, still too much force for my mold. So the bladder was actually my original idea when first thinking about this boom, but couldn't find much info on that either. What do you use for rotation casting? I imagine you want pretty runny stuff to evenly coat the walls as you spin it around. One other thought to make the silicon plug possibly more controllable. Let's say before pouring the silicone, you have a small plastic thin wall container than can hold let's say 40psi. This little container is suspended in the middle of the mold while casting the silicone. So what you end up with is a silicone plug that has a small 40psi cavity in it. When baking the layup, silicone will expand until pressing against all walls with 40psi of pressure, then any expansion left over will compress the 40psi capsule in the middle. Obviously pressure goes up a bit as volume of capsule goes down, but can be accounted for. I realise the silicone is going to try and expand differently in different directions, but it's elastic, I honestly think it would just displace itself to give pretty consistent pressure against all walls and excess pressure compresses cavity in the middle.
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Hanaldo
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You've got some unique approaches to problems, and I like that - but honestly I think you need to get a little bit of first hand experience with these materials. My advice would be to make a small scale version of what you are wanting to do, go out and buy a kg of 3428 RTV silicone, and give the technique a go. You will see what I was talking about, that for anything more than the most simple of shapes, the expanding plug is not an easy technique. Theres a lot of things that seem so simple about it, until you try it and you realise how exact you need to be and how many things you need to think about and get right. The overlap between your two mould halves for example - how do you account for this in the silicone plug, and then make sure that for the next 100 parts you make you get that overlap in the exact same place every time? Little things like that catch you out and result in failed components and having to start again from scratch.
I feel you picked up the wrong piece of info from my previous post - I consider it quite unlikely that you will break your mould from having too much pressure. Rather you will much likely just get a lot of poor results from not enough pressure in some areas. As I said, it's not as easy as just making the plug slightly smaller so it's a tight fit and it works - but I only know that because I've tried it. Dozens of times. I've got 20 or 30 fails under my belt. I did eventually manage to successfully make a little complex carbon fibre tube using an epoxy tooling block mould, so it's certainly a doable technique even with softer mould materials. But you have to be committed to fine tuning the process to get there.
For me, I wouldnt be doing it unless I had a paid order for 100 or 200 parts. It's way easier to just get off the ground using vacuum, and once you've done 10 parts and people are buying them and it is worth trying to reduce your labour time, then it's worth messing around with this sort of process.
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quinn
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+xYou've got some unique approaches to problems, and I like that - but honestly I think you need to get a little bit of first hand experience with these materials. My advice would be to make a small scale version of what you are wanting to do, go out and buy a kg of 3428 RTV silicone, and give the technique a go. You will see what I was talking about, that for anything more than the most simple of shapes, the expanding plug is not an easy technique. Theres a lot of things that seem so simple about it, until you try it and you realise how exact you need to be and how many things you need to think about and get right. The overlap between your two mould halves for example - how do you account for this in the silicone plug, and then make sure that for the next 100 parts you make you get that overlap in the exact same place every time? Little things like that catch you out and result in failed components and having to start again from scratch. I feel you picked up the wrong piece of info from my previous post - I consider it quite unlikely that you will break your mould from having too much pressure. Rather you will much likely just get a lot of poor results from not enough pressure in some areas. As I said, it's not as easy as just making the plug slightly smaller so it's a tight fit and it works - but I only know that because I've tried it. Dozens of times. I've got 20 or 30 fails under my belt. I did eventually manage to successfully make a little complex carbon fibre tube using an epoxy tooling block mould, so it's certainly a doable technique even with softer mould materials. But you have to be committed to fine tuning the process to get there. For me, I wouldnt be doing it unless I had a paid order for 100 or 200 parts. It's way easier to just get off the ground using vacuum, and once you've done 10 parts and people are buying them and it is worth trying to reduce your labour time, then it's worth messing around with this sort of process. That's the thing, my helicopter design has been prototyped, and now being tested by a pro level pilot that is currently the US distributor for a very large brand. He is 100% confident that if we did a run of 100 to 150, it would sell out very quick and he is ready to throw a lot of money at me lol. I declined for now and need much more time for developing faster processes for the whole heli. Currently for the boom im using premade 30mm tube that I reinforced from the inside with braided sleeve (to give torsional strength needed) using a bladder. I inflated large surgical tubing, slid a peice of 24mm pvc inside it to form a rigid undersized plug. This allows the braided sleeve to be applied to outside of plug, wetted out, then placed in the premade tube and inflated. This design already works but the tapered boom will reduce weight by eliminating other parts, improve cg, and add value to the helicopter. So in this case Its definitely worth experimentation to fine tune a quick process. I believe you when you say it's a tricky process, I'm just trying to pinpoint why and come up with a solution that could work with less trial and error. No matter what I'll be attempting to make the boom so the molds will be milled and material cost for one boom worth of prepreg is about 6 bucks, so I don't really see an advantage to experimenting at smaller size. As for getting consistent overlap size and location, the prepreg pieces will be cut with drag knife on my cnc so very consistent from one to another. One edge of prepreg section is flushed to edge of mold, remaining overlap at other side should be consistent in width. What I'm trying to wrap my head around is why the silicone would not displace itself to give similar pressure throughout. As pressure builds in one area, it should displace to other areas with less pressure if the mold is strong enough. If you squeeze a chunk of rubber in your hand, it displaces and squeezes out where it can. Maybe a softer more flexible silicone will help with this. Out of curiousity, have you tried this with milled aluminum molds? With less rigid molds, I can see it being a much more sensitive process. In the end I'm sure it will take experimentation, just trying to not make it harder than it needs to be. There has to be a way other than trying and failing 20 times. The cad software I use can simulate thermal expansion of a complex part. Maybe I can purchase the silicone, pour a small measurable shape, heat it, and measure to get an accurate cte to plug into cad simulation.
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oekmont
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Well I wasn't sure about the actual bulk modulus of silicone rubber. Hard to find actual data. I was just assuming from the formula k=E/(3-6ny) that it's bigger than you might think, since poissons number is close to 0,5 for elastomers.
Now I looked it up, and I found K~2 Gpa for basic silicone rubbers.
Considering that the thermal expansion is about 10 times greater compared to aluminium, you can easily get pressures that aren't optimal for composite compression.
A bladder seems the most reliable an easiest way in my opinion. That's basically how it has been done since the beginning of small diameter composite tubes, and still how it is done today. Seems like a good process to me.
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f1rob
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  can't get any better pics at the moment This is the vac innovations silicone You just mould it like plastacine, can be used in closed or open moulds Solid or have it hollow an through bag Can mould off the mould surface,offset surface or off the laminate itself Part pictured is a former to create a double a surface on the inside of a part,fractionally undercut and carbon would lock in but half silicone gives enough flex to release. Have got some silicone turbo pipe forms that are made hollow and we through bag it but if I post those pictures I might get in trouble
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Hanaldo
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Stuff looks interesting, would love to try it. Waiting on a reply from Vac Innovation. Do they have a MOQ Rob, do you know?
Quinn, I wasnt saying do a small scale part to save on pre-preg costs, I know pre-preg is cheap in this instance. But at around AU$85 a Kg, those silicone plugs are going to cost you a packet if you need to make them several times in trial and error. Your labour time in laying up each piece is also worth something, and it would be quicker to layup a smaller scale piece.
Anyway, there is a better way than 20 trial and error processes - do the maths, work out the actual expansion to expect for your silicone across its specific form, put that into CAD and then CNC machine two moulds. One that you use as your actual pre-preg mould, and one that you use as the mould for casting your silicone plug. If you've done your maths right, that will be accurate enough to work reliably. But to get the maths right, you'll need to get some silicone and test it for its expansion rate.
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f1rob
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+xStuff looks interesting, would love to try it. Waiting on a reply from Vac Innovation. Do they have a MOQ Rob, do you know? Quinn, I wasnt saying do a small scale part to save on pre-preg costs, I know pre-preg is cheap in this instance. But at around AU$85 a Kg, those silicone plugs are going to cost you a packet if you need to make them several times in trial and error. Your labour time in laying up each piece is also worth something, and it would be quicker to layup a smaller scale piece. Anyway, there is a better way than 20 trial and error processes - do the maths, work out the actual expansion to expect for your silicone across its specific form, put that into CAD and then CNC machine two moulds. One that you use as your actual pre-preg mould, and one that you use as the mould for casting your silicone plug. If you've done your maths right, that will be accurate enough to work reliably. But to get the maths right, you'll need to get some silicone and test it for its expansion rate. Don't think they do Comes in a flat sheet,about A5 size Thickness is variable between about 10-15mm. Very handy on detailed parts you just fill or cover the detail an you have a smooth area to bag an don't have to worry really
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Hanaldo
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It sounds great. I didnt realise you could mould it like plasticine, I thought it was a 10mm sheet and you stuck it where you wanted it and that's it. I like the idea of using it to fill voids on the outside of moulds where the bag might bridge and fail during cure as well. Got a couple of molds with deep recesses on the outside that are a pain to bag without nagging or bridging. I shall be getting some if they'll ship to Aus.
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