I am utilizing OOA prepregs and also have used regular prepreg materials to test in an electric oven. I have issues with pinholes and what I would call a dry finished product look where you can feel the weave. When I use aluminum molds I always have better results with very minimal pinholes, but when I use gelcoat molds is when I have major issues. I have tried changing my curing cycle with no help. My oven is not convection just electric heat, would this be causing it? I’m sick of having to clear my parts 5-7 times to get rid of pinholes, any help would greatly be appreciated.
Can you measure your temperature on the part and is your oven accurate in temperatures and ramps?
I don’t have a PID controller on it so every 15mins I turn the temp up 10 degrees F. I know this isn’t the ideal situation but I’m not good enough on the electrical side to install the ramp/soak PID. I do have a gun to check the part on temp and the oven is accurate. Do you think the ramp is my trouble? Why would I have more issues in gelcoat molds than aluminum molds?
Have you talked to the manufacturer of the prepreg?
What mold release are you using?
Many issues can cause pinholes. Prepreg is not usually a perfect solution, but sometimes can help.
Do you get a better surface in your aluminum mold with wet layup?
Yes get a controller… if you don’t have a well controlled oven, then you’re not going to have the best results.
Proper temperature control will greatly effect the quality of the parts. You can get a controller for pretty cheap on ebay. I’m sure you could rig your oven up to run off the controller.
Metal molds will generally give better results as they tend to heat nicely and can take a good polish.
The ramp can be important, too fast and the resin has no time to flow properly. Metal moulds can heat up better and more evenly, and have a different surface tension. Mould release can make a difference too. And offcourse your air evacuation, humidity, handling, formulation of the resin, etc, etc.
But given the fact that there is a difference in metal versus gelcoated moulds, I would bet on release and/or temperature related issues.
Is a fan in the oven a necessity along with the controller? I utilize pmr90 release and put a sealer on the mold prior to the release as well. When I layup in the gelcoat molds I have even tried resin film behind the surface layer which helps, the only problem is it adds a white tint to the finished product.
It’s always better to have a fan, but how much fan does something depends on how big the oven is.
Gilbert
I now consider this kind of issue as normal since I have been having the same problem for 10 years.
While some will say they can achieve a porosity free finish from a gelcoat surface mould I think it is very difficult without really knowing the exact material, release agent and cure schedule to make it work. These people just need to publish it in this thread and everyone will be happy.
For now I still have the same issues where I get some porosity from a gelcoat mould and always a much better finish from a metal mould. Many of my fibreglass moulds are covered with sticky backed PTFE and this gives me a porosity free finish although with a slight texture of the PTFE.
Might be a stupid question but what is PTFE? When you have pinhole issues how do you go about having a nice smooth cosmetic look without clearing and sanding 5-7 times? Is it possible to use duratec top coat as an IMC and let it cure then layup the prepreg and bake without having issues?
PTFE is teflon. You can buy sheets of it with pressure sensitive adhesive on the back with or with out fiberglass in it to reinforce it. Usually the fiberglass reinforced is easier to use. Airtech sells it as Tooltec but the best place it buy it in the is Taconic Industries who actually manufacture it and I believe will sell cut yardage, though I always bought full rolls.
PTFE is teflon, the one I use is a teflon coated fibreglass. It’s does not stretch in any way so can only be used in a two dimensional fashion and can not do compound shapes. Quick and easy to apply and gives many many releases off pre preg.
http://image.made-in-china.com/43f34j00ysVQzcbFyjoZ/PTFE-Coated-Fiberglass-Self-Adhesive-Tape.jpg
I have not tried duratec as an IMC, IMC and pre pregs can be risky since the tacky pre preg can grab your IMC and pull it off when tailoring your first layer.
To finish I brush coat duratec and spray coat also. Sand smooth and spray again with the same or any other clear coat material, pref also UV.
The key to pinhole free parts is mostly in the material. But you also need to have an oven that flows a lot of air round the oven for even heat and a good controller. I use a ramp and dwell controller that I got on eBay. The program can be critical to 5 degrees with the material I use. If I am off by 5 degrees it has pinholes. The dwell time and stages are also critical too. I have a 3 step cure cycle with a lower level and dwell which then steps up to a mid point and dwell before going up to the final cure temp. Obviously lay up and the breather stack is important as i do have a specific breather stack which is one I have had to adapt to help get air off the surface too.
So what’s your cure schedule, temp, dwell etc?
I got the oven, air flow and controller.
210 degrees F for 4hrs with a ramp of 1-2 degrees a minute. I know for a fact my ramp is not even close to accurate. The soak at 210 I am ok with.
This is not the best one to have but will work OK… Its made for soldering circuit boards so works in seconds not minutes or hours. So you need to use 3600 seconds for an hour. Ramp rate for the slowest speed would be 1 degree every 90 seconds if you set it for degrees C. I have not looked at what the speeds would be for degrees F. It will make s huge difference even for a first ramp and dwell controller. It needs to have a STOP switch wired in or it has be to bridged ( I learnt this the hard way) But it is a good start. Or you can but a West controller.
210F (99C) for 4 hours is my regular full cure cook. My controller is always set at 2 degrees ramp but the actual ramp depends on the moulds/mass being used. Ramp is typically between 1 and 2 degrees.
Is there not a dwell at a temp lower ? 75C ??
I had good results with delta prepreg(cant remember which one it was…), Marbocote 227cee, in an epoxy gelcoated mould (axson stuff) with a ramp of 2 degrees/minute, half an hour dwells at 65 degrees celcius and 85 degrees celsius, and final cure at 110 dergrees celcius. The temperature of the moulds would stay behind untill the dwell periods. Foam cored products or heavy moulds need a lot more dwell time and slower ramp, temperature under the foam could be behind as much as 20 degrees or more.
The material I use I dwell at 50 degrees C for an hour. Its under the temp where it actually starts to cure and the mould gets an hour to catch up so the next step up is 60 degrees C which is when the resin is most fluid and again the mould gets to equalize and get a good surface before I then go up to 75 degrees for the rest of the cure. I could keep it there until it gels of and ramp up to cure faster but as I do it overnight there is no need to speed it up.
I had to do a single layer of pre-preg for a guy. No idea why he wanted it that thin as a single layer can be more prone for pinholes as there is very little excess resin to fill the holes where multiple layers will allow the excess to flow into them. But it came out flawlessly. No pinholes and the only marks were the ones on the old sheet of glass I used.
Get a controller and stick a good fan into the oven… Get the air flowing and recirculating quickly. If the air can flow fast and create turbulence it should get rid of any hot or cold spots. Here it was about 3 degrees C yesterday and I had to cure this sheet so the guy could collect it on Monday. My oven is about 1.8 cubic meters or 64 cubic feet and I can run it with 1, 2 or 3Kw heater elements depending on what I want to do. I cured it easily still set on 2kw even though it was only 3 degrees above freezing outside and my oven is in my garage so it was about 6 degrees in there. If you have a good oven and good control then if you are still not getting good results on sheets of glass (a good way to test material)then you need to find a cure cycle that will work or a new material.
These ramps and different dwell times, can they all be programmed into a good controller?