First prepreg attempt - surface imperfections

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#1

I just release a OOA prepreg I tested a surface layer on the glass.

The surface doesnt have typicall resin voids, it is rather resin rich but has some kind of imperfections which I belive where caused by entrapped air while laying prepreg on glass. When i lay prepreg on glass I check out the under side and there was many air pocket closed by prepreg , I tried to push them out by finger but that didnt help.

So i left it like it was (with this air pockets) connected vacuum , compressed it for 30 minutes - but still nothing change , so I left the part under vacuum , turn off the pump and put the part to oven.

The part was cured in oven for like 10 hours at max 90 degree, after demoulding it was stiff and strong but surface look like this :

#2

what’s your ramp rate? Did you have the part in the oven connected to vacuum?

the ramp rate is important because of your resin getting liquid. if your ramp is too fast, it gels too fast, keeping the air in. If you don’t have enough vacuum, the air is kept in. How many layers? Did you debulk the first one?

#3

Ramp up was like 10C / hour starting at 30C

I didnt connect part to pump while in oven instead the part was closed and lefted vacumed in bag - should the pump be turned on at whole cure cycle ?

I used only 2 layers , number of layers can affect surface of first one?

After layup vacum was aplied for 30mins before putting to oven - this you mean as debulking?

#4

Did you have a perf film and breather over the laminate? This will allow air to be pulled through the laminate and out the vacuum, if not air can get trapped.

Leaving the vacuum on is a good idea as when the resin cures some violates are released which will drop the vacuum in the.

If the cloth has a dry side and a glued side, try placing it dry side against the mould. It will be a bit of a front bum to stay in place, but if you are just making a flat plate it should be to bad. The idea is that the air trapped against the mould will travel out the carbon as if it was a breather and when heated the resin will be pushed/drawn through to the mould.

#5

The part doesn’t look too bad to me in the photo but if you are looking for perfection in the oven the following will help:

i) Pull a strong vacuum throughout the cure. Even a small leak in a bag can cause issues especially if the vacuum is disconnected for 10 hours. Also any volatiles, e.g. release agent solvents etc., will boil under the low pressures associated with a vacuum. By leaving the pump connected the volatiles will be sucked out provided they can escape the laminate stack.

ii) Follow the manufacturers recommended ramp-rate. 10°C / hour sounds too slow and is a postcure type ramp-rate to my eyes. As others have said ramping too quickly will narrow the time for air to escape as gelation will occur too soon but ramping too slowly will mean the minimum viscosity attained will not be low enough to ensure appropriate resin flow. The ramp-rate is an optimum and trial and error are important which is where the prepreg manufacturer should be able to help.

iii) debaulk the laminate stack during lay-up with perforated halar. Do this after the first ply and every 2 or 3 subsequent plies in the stack.

iv) follow the manufacturers guidelines for edge breathing strategies, e.g. peel-ply round the edges extending into the breather, to allow for air evacuation paths

#6

+1!

I totally agree… I have found in my new oven the ramp rate on the part (not just the oven) made a huge difference and I suffered pinholes and or minor air entrapment in my part for the first time since I had worked out how to get a great finish. The lay up was correct and so was the breather stack. The problem was purely down to ramp rate and cure temperature I was running. I ramp up at about 1 to 2 degrees per minute. I also have dwell points in the cure schedule where the resin would be at its most fluid before ramping up as the cure starts to gel off and finally if I need a third point which will speed up the final cure and allow me to take the part out of the mould when I want not just leaving it for up to 16 hours which could stop me from turning round that mould for another part that day.

I always keep the pump running on the bag when curing. For the reasons mentioned above. I have cured parts in another oven that a friend let me use and he always turns off the pump and runs connected to a vacuum bottle with a gauge attached so he can pull down the bag from time to time. But I personally do not think this is enough and means you have to keep a constant check on the vacuum levels.

overall the finish looks OK it is just fine tuning the way you cure that material.

#7

I have found in my new oven the ramp rate on the part (not just the oven) made a huge difference

Yes, oven and part temperature can differ quite a bit, especially with thicker (cored) layups. (or thick moulds, bigger metal moulds, etc.) Adding a sensor on the mould surface can help solving a lot of problems. A dwell period doesn’t only help with resin flow and air evacuation, but with equalizing the oven and mould temperature too.
Oven temperature isn’t very significant in the process, which is overlooked quite often. (I found out the hard way too, haha)

#8

I always use a remove meat thermocouple taped to the part to check mould temperature then when the part it up to temp I take my cure times from that… I use an ET72 Maverick RediChek Remote/Wireless Meat Probe Thermometer.