Hello all,

              I am currently having a problem with my Xpreg XC110 prepreg. In my last test the prepreg did not cure and was still  gooey once the cure cycle was complete. My oven, which is home is set up with a proper PID controller complete with the thermo coupling. Its shows to have reached the temperatures that are recommended for the XC110 system. My mold is a CNC'ed piece of aluminum 2" thick. The mold itself is 20" x 46" x 2" With basically the back and sides of a guitar cnc'ed into it. When I did the trial run once it had soaked for an hour at the higher temp I lifted the oven off the mold so it could room cool. So, what would be my problem? 
A. I should have left the oven over the part to let it slow cool
B. the aluminum is taking to long to reach temperature
C. the oven isn't actually reaching the temperature the thermo coupling says it is. 
Any other ideas on what it may be would be of great help.

Also, unlike the video my surface finish is almost like dry carbon, there is no clear looking finish on it at all. Does the Paper side of the pre-preg go down first or the plastic side? or does it matter? and could it be the either over cooked or under cooked problem that could have also caused that?  Any help would be greatly appreciated. Thanks!

JC

Could just be your aluminium mould is sucking up much of the heat so assuming you are just measuring the air temp and timing from this your part is under cooked as it takes 45min to heat a mould like that.

I usually put my oven controller thermocouple in the air and just set my cure time to be 1 hour more than the cure time stated. You can't overcook it but you can under cook it. 

Definitely not reaching cure temp.

How is your oven built? Are you sure that the temperatures are even throughout the oven? When I first built my oven, I had an issue where my thermocouple probe was right in a slight hot spot due to a draught caused by a 10mm section of my door seal that wasn't seated properly. So the PID thought the temperature was 80C or whatever - and it was where the probe was positioned - but the draught was cooling the middle of the oven to below the cure temp. It took a bit of playing around with an infrared thermometer to work that one out, but since then my oven has worked really really well and I've never had any cure issues.

As for the dry finish, that will be due to the under cure for sure. When it is cured properly, it will be coming out exactly like your mould finish. So if you have a gloss mould it will come out glossy, if you have a matte mould it will come out matte. If you have a glossy mould and it is coming out matte, that is a sign of either a curing issue (either under or over, but it would have to be WAY over for it to lose its gloss) or a reaction (which really shouldn't happen with epoxy).

A number of factors could play an impact here but a large aluminium tool seems the most obvious issue in that it will take a long time to heat up and cool down so although the oven air temperature may have followed the cure cycle, due to the extra energy needed to heat the aluminium tool, the tool is probably lagging behind temperature wise by anything up to an hour or maybe longer. Ideally you would want a thermocouple as close to the mould surface as possible so you can monitor how long it takes to actually get to temperature.

We would suggest that the paper side goes against the mould surface.  In cosmetic terms, the side against the backing paper, fractionally has more resin on it and is slightly more tacky. This means applying that side to the mould may help to slightly improve the cosmetic finish of the part.

If the part has not properly cured, then the surface finish is likely to be degraded hence the matt  finish and potentially even print through.  Demoulding before the part has cooled down can sometimes cause this effect too.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thank You Gentlemen for the replies, you have confirmed my suspicions. My oven is crude at best but I thought it seemed to work well. The thermo-couple is mounted in the dead center of the over, running parallel underneath the surface. once the oven is lowered down over the mould it is approximately 1" away from the surface of the carbon. However, I plan now to run test on the system to see what the surface temperature is at various points on the mould and at different times during the cure cycle. Don't make to much fun of my oven, I can't afford one large enough to get a 150 lb. mould in at this point. I have the oven on a wench system that I raise and lower  over the moulds, which are mounted to there own rolling table. I polished the moulds to a high gloss finish after these pic were taken, but I believe the "print through" , which is exactly what I am getting, is due to my cure temps of the mould. As you can see, the mould is directly laying on a wood table. Would you suggest raising the mould up so there is an air gap underneath so that the mould itself would heat more evenly? I greatly appreciate your comments and suggestions. Just as a starting point would anyone
have a suggestion for a modified cure cycle to fit my circumstance? Thanks again!!



JC

Aluminium will heat very evenly, it will just take longer to do so. So you should increase the soak cycles of your cure to compensate and give the mould a chance to catch up, eg. instead of soaking at 70 for 4 hours, make it 70 for 5 hours. You may also consider adding a couple more soaks, so do a soak at 50 for half an hour and then a soak at 60 for half an hour, just so the oven temps don't get too far away from the mould temps. 


Your oven is a very interesting design, certainly not the usual! Not making fun of it at all, but I do wonder how even the air circulation is, and I feel like it may have something to do with your issues. 

Air circulation probably won't help. You might want to add some insulation to help improve efficiency.  Also a rubber seal around the base to help reduce the escape of hot air.  Knowing the temperature at the mould face would be ideal as you can then taylor the cure cycle to take into account how the mould heats up.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thanks Warren,

            I'll get those things done. Do you agree with the statement " You can't overcook it but you can under cook it." ? Also, when it says at the end of the cycle to let everything air cool, would you suggest leaving the oven over the mold and waiting till its all cool, or raising the oven off so it can cool quicker. I raised it off as soon as the high temp cycle was done, and I'm thinking that may have been the wrong thing to do.

JC

Yes you can't really cure for too long. You can "overcook" it in the sense that you use too high a temperature for the resin system or you ramp up too quickly for the resin to flow properly.

Leave it in the oven to naturally cool before demoulding.  it is possible that  with a large mass billet tool at high temperature and a cool air the other side of the carbon, it sets  up some slight cooling warping causing pre-release and potentially some surface dulling or print issues. Although it would be hard to be sure if this is significant without testing it.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

I'd also recommend buying a remote thermocouple unit. You can buy these fairly cheaply on places like Ebay

If you get one that has two probe wires, long enough to extend outside of your oven, you can gum tape the ends onto selected places on the part in the mould and monitor actual temperatures there rather than reply upon the fixed thermocouple on your oven set-up

These will, hopefully, provide you with much better assurance of the temperature your mould is running at