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Gaurav Kumar Thakur
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What could be the reason for the Crack, pin holes or dry kind of surface occurring on the surface of CFRP sheets? The plate is made of out of autoclave pre-preg system,using 10mm thick glass as a mold. 
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Hanaldo
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It is very difficult to see the issue in those pictures, but I can sort of see that the surface maybe looks a little bit lean. Which to me indicates a flow issue, which means your cure cycle likely needs some tweaking.
What pre-preg are you using and what cure cycle did you use? Is your PID in charge of the ramp rates and dwell or are you doing it manually with a switch?
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Gaurav Kumar Thakur
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Group: Forum Members
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+xIt is very difficult to see the issue in those pictures, but I can sort of see that the surface maybe looks a little bit lean. Which to me indicates a flow issue, which means your cure cycle likely needs some tweaking. What pre-preg are you using and what cure cycle did you use? Is your PID in charge of the ramp rates and dwell or are you doing it manually with a switch? Its 200 GSM epoxy based pre-preg system.Yes we can set up the ramp rate and dwell time in oven: ramp rate : 2 degree/C and the dwell time is 50 degree -10 min,80 degree -10 min,120 degree -60 min. I am attaching some more pic please check;  
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KLComposites
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Need to know the brand and model number of the prepreg. In my personal experience, if you're trying to use anything but EC XC110 or one of the SHD OOA prepregs, you're going to have problems like this no matter what you do.
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Hanaldo
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Group: Forum Members
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+xNeed to know the brand and model number of the prepreg. In my personal experience, if you're trying to use anything but EC XC110 or one of the SHD OOA prepregs, you're going to have problems like this no matter what you do. 5 years ago I would have agreed with you, but since Covid made reliable supply from England tough for me I've actually spent the last year sourcing and testing 5 or 6 different OOA pre-pregs that work just as well if not better than the XC110. Some even with cure temp ranges as wide as 50c to 150c. Chris Rogers helped me a lot initially, luckily he knew one of the first systems I tried very well and was able to coach me through it. From there I was able to take that advice and apply it to all the different pre-pregs, and get some very good results. There's honestly some brilliant pre-pregs on the market right now, the technology has come a very long way. That said, only one of those systems delivered perfect results straight off the bat without having to dial the cure cycle in. The others I eventually got to work very well, but I needed to spend time doing tests and getting the cycle locked in. There are of course still some rubbish pre-pregs out there as well. The real benefit of the XC110 and the SHD pre-pregs is Easy Composites and SHD have already done that testing etc. for you, so there is very little guess work and they tend to just work straight out the box. There is value in that. The companies I am dealing with haven't put that legwork in, a lot of them are finding my feedback and results very useful and I'm teaching them about their own product. Annoyingly, most of them are pretty far off with their suggested cure cycles. So if they don't provide the actual resin data, then you can still be quite lost even if they are telling you how to use it. Anyway, OP - yes, your second set of photos back up my opinion that the surface is dry and you have a flow issue. If you have the datasheet for the pre-preg and can send us that, that will help. Every pre-preg has a viscosity curve, where the resin starts to flow and then reaches it's lowest viscosity, and then it starts to gel and it's viscosity increases again. Ideally you want your initial dwell to be just before the lowest point of the viscosity curve, as this will allow the resin the most time to flow before it starts to gel. If you don't hit that point on the viscosity curve, your resin either may not become thin enough to flow nicely, or it may gel too quickly and not have enough time to flow. Similarly, if you don't dwell for long enough then your resin might not have enough time to flow before the temperature ramps up and it gels, and if you dwell for too long then you may bleed too much resin. Obviously you can see it gets fairly complex and guess work is difficult, so a detailed datasheet can do wonders. If you haven't got that, my advice would be that MOST pre-pregs won't really be flowing at 50C, and all you're really accomplishing there is a warm debulk. So I would increase that initial dwell to 65C. Then, 10 minutes is not long enough to allow the resin to flow nicely. So I would increase that dwell time as well, probably start at 1 hour and see how that changes things. The second dwell at 80C may not be necessary, it depends on your specific pre-preg and when it reaches it's lowest viscosity point. I would probably drop it for now for simplicity, and see how you go with just the 65C dwell. Then ramp straight to 120 with your 2C/min ramp rate. If you get similar issues, increase the initial dwell to 80C and try that, it may be that your pre-preg has quite a high viscosity curve onset.
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Gaurav Kumar Thakur
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Group: Forum Members
Posts: 9,
Visits: 273
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+x+xNeed to know the brand and model number of the prepreg. In my personal experience, if you're trying to use anything but EC XC110 or one of the SHD OOA prepregs, you're going to have problems like this no matter what you do. 5 years ago I would have agreed with you, but since Covid made reliable supply from England tough for me I've actually spent the last year sourcing and testing 5 or 6 different OOA pre-pregs that work just as well if not better than the XC110. Some even with cure temp ranges as wide as 50c to 150c. Chris Rogers helped me a lot initially, luckily he knew one of the first systems I tried very well and was able to coach me through it. From there I was able to take that advice and apply it to all the different pre-pregs, and get some very good results. There's honestly some brilliant pre-pregs on the market right now, the technology has come a very long way. That said, only one of those systems delivered perfect results straight off the bat without having to dial the cure cycle in. The others I eventually got to work very well, but I needed to spend time doing tests and getting the cycle locked in. There are of course still some rubbish pre-pregs out there as well. The real benefit of the XC110 and the SHD pre-pregs is Easy Composites and SHD have already done that testing etc. for you, so there is very little guess work and they tend to just work straight out the box. There is value in that. The companies I am dealing with haven't put that legwork in, a lot of them are finding my feedback and results very useful and I'm teaching them about their own product. Annoyingly, most of them are pretty far off with their suggested cure cycles. So if they don't provide the actual resin data, then you can still be quite lost even if they are telling you how to use it. Anyway, OP - yes, your second set of photos back up my opinion that the surface is dry and you have a flow issue. If you have the datasheet for the pre-preg and can send us that, that will help. Every pre-preg has a viscosity curve, where the resin starts to flow and then reaches it's lowest viscosity, and then it starts to gel and it's viscosity increases again. Ideally you want your initial dwell to be just before the lowest point of the viscosity curve, as this will allow the resin the most time to flow before it starts to gel. If you don't hit that point on the viscosity curve, your resin either may not become thin enough to flow nicely, or it may gel too quickly and not have enough time to flow. Similarly, if you don't dwell for long enough then your resin might not have enough time to flow before the temperature ramps up and it gels, and if you dwell for too long then you may bleed too much resin. Obviously you can see it gets fairly complex and guess work is difficult, so a detailed datasheet can do wonders. If you haven't got that, my advice would be that MOST pre-pregs won't really be flowing at 50C, and all you're really accomplishing there is a warm debulk. So I would increase that initial dwell to 65C. Then, 10 minutes is not long enough to allow the resin to flow nicely. So I would increase that dwell time as well, probably start at 1 hour and see how that changes things. The second dwell at 80C may not be necessary, it depends on your specific pre-preg and when it reaches it's lowest viscosity point. I would probably drop it for now for simplicity, and see how you go with just the 65C dwell. Then ramp straight to 120 with your 2C/min ramp rate. If you get similar issues, increase the initial dwell to 80C and try that, it may be that your pre-preg has quite a high viscosity curve onset. Thanks for your reply!! Actually the vendor has given the curing cycle and ramp time, which I was following.But now I will try out the suggestion given by you. Meanwhile, I have shared the data sheet of pre-preg via personal msg to you.Also i would like to tell that material is 8 month old its kept in -18 degree for maintaining the pre-preg life and the vendor has told we can store it for 12 month if we are keeping in -18 degree C. Please look in to it and give your suggestion it will be very helpful for me.
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Hanaldo
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Group: Forum Members
Posts: 2.5K,
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Took a look at the data sheet - they don't expressly say that it is an out-of-autoclave pre-preg? They mention vacuum bagging, but it does technically still work with vacuum only pressure it is just difficult to reduce the void content. Would explain your flow issues for sure, those are about the results you will get if you use a regular pre-preg in an out-of-autoclave process.
Anyway, assume it is an ok pre-preg for OOA use and keep trying. Did you give my suggested cycle a go?
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Gaurav Kumar Thakur
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Group: Forum Members
Posts: 9,
Visits: 273
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+xTook a look at the data sheet - they don't expressly say that it is an out-of-autoclave pre-preg? They mention vacuum bagging, but it does technically still work with vacuum only pressure it is just difficult to reduce the void content. Would explain your flow issues for sure, those are about the results you will get if you use a regular pre-preg in an out-of-autoclave process. Anyway, assume it is an ok pre-preg for OOA use and keep trying. Did you give my suggested cycle a go? Actually they have claimed that this OOA prepreg & also I am applying vacuum pressure while giving the heat cycle through out the process. I will try out the suggestion given by you and let you know in few days...
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