infusion advice on CF ukulele

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#1

Hi all, merry Christmas!

First post here, I’m stuck at work on christmas so this is the most productive thing I could think to do.

I am looking for some advice on vacuum infusing a carbon fiber ukulele shell. This will be my first experience with infusion, but I am very comfortable with vac bagging wet layups (surfboards mostly). I have previously made one from this mold using a vac bagged hand layup, and I was not happy with the process or results. In a nutshell, it was too messy, and the fitting of release film on the inside of the mold after applying the epoxy was a nightmare.

Here’s a photo of the part to be molded:

Maximum width of the body is like 10" if I recall correctly, and length is about 26".

I already have some infusion resin, resin research 2070 (800 cps), and most of what I think I’ll need for the bagging.

I’m planning on doing 5-7 layers of 5.7oz carbon twill fabric.

My question mostly involves how to set up the vacuum, flow mesh, infusion port, etc… for ease of execution.

The problem I run into in my mind is laying down the peel ply and flow media on top of the dry stack. The twill CF drapes very well, but the peel ply and flow mesh definitely won’t. The only way to get those to lay flat inside the mold would be to make a bunch of relief cuts and use small pieces, but then I’m looking at a nightmare to remove all that stuff from the finished laminate, plus it’d be impossible to avoid overlapping pieces. I’m not hugely concerned about the cosmetics of the inside of the laminate, they’ll be mostly hidden by the top of the ukulele… HOWEVER, the cosmetics of the last one I did were BAD (stuck chunks of breather, big lines from pleats, pools of resin), so I’d like it to be a bit better at least.

Could I get away with just using a flow media on the flat wide part of the body and let it just infuse through the carbon the rest of the way, taking vacuum all around the perimeter of the mold? Where would you suggest placing the vacuum ports and infusion ports? Other ideas/suggestions on making this work as efficiently as possible?

Great site everyone, thanks for any and all advice, and once again, MERRY CHRISTMAS!!

#2

Hello and mery Christmas,

you will have to make cuts in the peel and the flow media. I most times do not cut the peel ply, I make crinkles.
Playe a layer of ferforated release film between the peel ply and the flow media, but there you have to be sure that it is not layed double, because than the holes in the film are closed.
If you make cuts in the flow media and make a partiel double layer of flow media be careful not to build a resin highway that cuts the airflow, that can produce pinholes in the surface.
Depending on the way you infuse you have to use big brake zones or the MTI hose.
First: classic infusion

I would make a resin inlet via spiral tube (yellow marked) and connect it via flow media. Place flow media in the hole part til the part lip. Place a spiral tube as vacuum hose where the green line is painted and only connect it with peel ply to the part, it is your resin brake.

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Second: MTI infusion
You can read my articel about the MTI hose and take a look at the video
http://www.compositescentral.com/showthread.php?t=6530
Place two layers of flow media below the siral tube you use as resin inlet to be sure that there will be no print through. Place the MTI hose direct to the part lip and connent it with peel ply. There should be 1/2" space between flow media and MTI hose.

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In both cases degas your resin, be sure that your mould is tight and you should have a vac level of 0.6in hg / 20mbar.
In normal infusion you should reduce your vac level after infusion. Using the MTI hose you don’t have to care about anything. To get a perfect fibre to volume ratio place you resin pot at the same hight than your mould. When the flow media is filled complete place the resin pot 3 feet below you mould and wait until the resin reaches the MTI hose everywhere. Than close the resin inlet and let the full vacuum work until your part is cured.

Wait until next year Christmas and demould :wink:

Merry Christmas Dominik

#3

Thank you, Dominik!

I think that answers pretty much all the questions I had. Well, maybe just a couple more, to what pressure should I reduce a regular infusion after it’s fully infused?

also, where can I find the MTI tubing in the US if I want to try that?

#4

To about 50 mbar. This is to prevent any residual moisture to boil off, creating voids.

What materials do you have available?

Make sure you have a slow infusion mesh, and preferably a small diameter feed line (1/4" OD) to prevent the resin from “overrunning” the fabric, also leaving voids. Carbon infusion is not about to speed, but about slow speeds, high quality.

I am a total nono about musical instruments (although in a previous life I played piano quite well) but consider using quite some UD fibers in the neck of the thing. This will make the ukelele stiffer, so the tension of the strings will not bend it. The strings as a result will be closer to the neck, and the ukelele can be played “faster”. This is a trick that the builder of guitars for Jan Akkerman told me.

#5

THanks for the reply, herman

My gauge reads in inHg, according to the conversion I found online 50 mbar is only like 1.5 inHg, does that sound right? Seems pretty low.

As for materials, I have my CF already, release film with .015 diameter holes staggered on 1/4" centers. I was holding off on getting peel ply and breather mesh until I had a better idea of which of each I should be using. I planned on getting spiral wrap at the same time I get the peel ply and mesh.

with regards to the layup, I did about 6-7 layers of twill 5.7 cf on my first ukulele, and it was plenty stiff enough. Ukuleles are relatively low-tension stringed instruments and the neck didn’t bend a bit once I had it strung up.

#6

your gauge is reading vacuum, his pressure. that means the conversion would be 50mbar pressure= -28.42" HG Vacuum

#7

He means the absolut pressure. The atmospheric pressure is 1013mbar, so if you have a vacuum you have less than the ambient pressure.
I prefer 20 mbar / 0.59inHg absolut pressure for infusion. Do you work backwards in the US? Means ambient pressure is 0inHG so absolut vacuum is -29.92inHG? Thats tricky if your ambient pressure is unsteady.

#8

Ahh, I see!

I honestly don’t know if we do things backwards here in the US. I’ve learned everything about vacuum bagging composites through trial and error, so I really don’t know how things are supposed to be done.

It’s lseeming like the tolerances are a lot tighter in infusion than in bagged wet layup… Should I just look again at the wet layup process and see if I can’t make that work for me?

#9

You’re right about that. You have to be a lot more careful with infusion, things like vacuum pressure and leaks affect it much more radically than in vacuum bagging, it’s a much less forgiving process.

However it does provide more consistent and superior results. While it’s more finicky I encourage you to practice it, once you get it down you won’t look back :smiley: