Help with print through on prepreg part

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#1

Hello everyone. For a while now i have been trying to roll prepreg tubes for my hobby which is high power rockets. I’m using dunstone 120R sample tape for compression and a homemade curing oven with an arduino based temp controller.
I bought some 5.7 ounce 2x2 twill prepreg to roll tubes with. The manufacturer said it has a 38% resin content. But i calculated more at around 40-45%. Here is the prepreg data sheet.
http://www.orcacomposites.com/Orca%201425%20Pre%20Preg%20Resin.pdf

Yesterday I rolled my first tube with 3 layers to try and get a nice smooth and uniform finish with heat shrink tape for compression. My results were was not what i hoped for. I got significant dry spots in between the fibers at the surface of the tube. I believe it is called print through, but i am not sure. Also the mylar was not peeling off very well so i don’t think I’ll use that again.

I have a couple of theories:
1)There isn’t enough resin to get to the surface of the tube.
Or maybe the resin is curing before it can run fully throughout the part.
I followed the highest curing cycle the best I could. Ramp up, hold at 310F for 1 hour, ramp down.
The three pictures are shown, the first one is the surface of the tube. The second one is the prints the resin made on the mylar. The third is the 6" long tube in its entirety.

Do you guys have any suggestions on how get a nice smooth finish on these tubes? I would hate to add more resin to the tubes.

Thanks,
Tom

#2

If you are wanting to get a smooth exterior finish then you will want to make the parts in a female mold. Bladder molding would work very well.

It looks like you are not compressing the fibers enough for the resin content of the prepreg. The lower the resin volume the higher the required compaction…unless porosity is acceptable.

#3

Thanks for the reply. In your opinion, can i get a smooth finish using fabric without the use of an autoclave? A high vacuum system? They do it al the time with uni i think.

Using a female mold would be a little complicated and would mean i would have to do it in two parts. Right?

#4

You could easily do them in one shot with a two piece mold.

#5

More pressure needed indeed. Is the system you use intended for out of autoclave use? (and did you use release on the mylar? )

#6

If you have enough pressure, you could use a “dwell time” at a lower temp, at which the resin is flowing, but not curing yet.

#7

I’ve used dunstone tape often, and never get a tube like that. So i can only advise the following:
Make sure the prepreg and tape are wrapped tightly. I personally use 2/3rd overlap (meaning, I wrap the tape around itself so that only 1/3" is not overlapped. This allows excess resin AND gasses to escape. Wrap too many times, and the entrapped air/gas/moisture has no where to go.
Make sure your prepreg is good, and stored correctly. In picture one, it can be a moisture problem when heating the part.
Try flashing the tube at 350f for 5min to shrink the tape, and quickly lower the curing temp to a lower and longer dwell. This will allow the tape to shrink, and a longer time for the resin to infuse through the fabric (if you have the one-side coated fabric, you still need to allow the resin to infuse!)

#8

Thanks guys for the reply’s.

Susho-I added a few layers of Partall High temp wax on the mylar. My theory is that mylar is at the temperature where it starts to get brittle, ,making it difficult to pull off.

Riff42-As far as overlap, I think it was about a half overlap. I’ll give more wraps per inch a try and a dwelling period to around 150F for an hour after the tape has been shrunk to max pressure.
It did say that you do not have to use an autoclave or vacuum bag system with this prepreg.
The prepreg I am using has a year shelf life at 75F. I store it in my cold garage wrapped in saran wrap and placed it in the shipping carboard box it came in. I don’t think there is any moisture on it. It is single sided too.
How does your surface finish come out after using the tape?

Thanks,
Tom

#9

My finish is spot on. A few voids, and the resin flash line, but very good finish. I’ve used many different prepregs and the only issue I have is accidentally not using the release side.

#10

Cool. Well it sounds like I need more pressure from the tape and a dwell period for the resin. I’ll keep you guys posted on my next attempt

#11

I would say you have possibly 2 problems

1-the more obvious lack of pressure

2- you may be trying to cure the prepreg too fast for the resin to flow on a 1 hour cure…
i would say go for a lower cure temp with a dwell period.

eg ramp up to 80 degrees C @ ±2C/min dwell at 80C for 8 hours and cool to room temp @ ±2C/min

works for me :wink:

#12

looks like a bag leak almost. probably exotherm, I’d say slow it down.

#13

I wasn’t using a vacuum bag. Just heat shrink tape. Im going to change the number of wraps like Riff42 said and see if i can get more pressure that way. It looks like slowing down the curing process will help a lot too.

#14

oh, I understood you just used shrink wrap. I was just saying what it looked like.:wink: I do agree though you must have temp ramps for most all prepregs. You mention an oven. What kinda setup do you have? I’m have an old oven that would work for small stuff but I’d have to come up with some sort of controller system.

#15

I made the oven from scratch. The outer shell is plywood, then mineral wool board. And the very inside is aluminum flashing. I have taken it up to 325F for extended periods of time.
My friend who is into programming and electronics helped me build a temp controller on the cheap using an arduino. The program is still under development and does need some work as it likes to over or under shoot the desired temp a little. Other than that it works pretty well.
The heating elemnent is from an old black and decker space heater that puts out 1500W and came with a metal fan blade/ electric motor system. All that for $8 at a thrift store. :stuck_out_tongue:

I am in about $200 into the oven. Not bad for being over 6 feet long!

-Tom

#16

With such an oven you won’t get an even distribution of the heat: the fan side will always be much hotter then the opposite side.

#17

Very nice. Is the heating element built into the fan unit?

#18

Roberto-
Yeah, it isn’t perfect yet. How would you go about fixing this problem? Another fan/heating element unit?

Extreme-Thanks, no the electric motor for the fan is mounted on the outside and the shaft goes in the middle of the seperately mounted heating element.

-Tom

#19

Yes, another heater and fan on the opposite side can help a lot.
Of course they have to be controlled by a means of thermostat.

#20

Yeah, ill look into it. I’ll have experiment with other ways to get a more consistent temp first before taking the plunge and trying to get a whole other unit on the other side. Tearing apart the old space heater was a pain in the butt, and i don’t have another one.

-Tom