Sunday, September 19, 2010

(un)Finished Product

I got the joint unwrapped, pulled off the melted vacuum bag the removed the peel ply. Where it had been compressed the layers were well compacted and had a fine woven impression in the surface from the peel ply. Where the vacuum had failed the fibers were not fully compacted during cure. Looks a little rougher than I'd like but with practice I should be able to tidy it up. Lesson learned, try cure at 200F for 2 hours instead of 250F for an hour.

They Really Mean 250F

Ok, lesson #1 the hard way. Apparently when they rated Stretchlon 200 for 250F they really meant it. I had the joint in the oven with the pump running steady at 25mm Hg. The temperature was ramping up slowly and everything looked good. I noticed the pump speed change right around 240F, glanced over and saw the needle falling on the vacuum gauge. I opened the oven door and head a hissing sound. Doh! The bag had failed where it was most highly stretched in the joint fillet areas. I left the joint in place to fully cure.

Vacuum Bag

This is the joint wrapped in Stretchlon 200, a high stretch vacuum bag. I chose this because it makes it easier to achieve smooth compression, avoid pleating etc. I was able to get 25mm of Hg or about 12psi.

Peel Ply

To vacuum bag a joint you have to apply peel ply over the carbon. It's a fine release coated fabric that can be removed from the cured epoxy. I was trying to minimize wrinkles under the vacuum bag.


Here is an example of the layering process for a joint. I still have to work on making them neater and record the actual layup schedule etc. Trying to get most of the support fibers to be as long as possible and wrap around the head tube.

Prepreg Test Layup

Completed another test of prepreg layup. This time it was used to test vacuum bagging the joint while in the oven curing. Below shows the first few pieces of uni-directional carbon being applied. The lower one wraps all the way around the head tube.

Tuesday, September 14, 2010

Tube-to-Tube Fillets

For these test joints I'm going to use a light weight, high strength fairing compound to add small fillets to the tube joints. The theory is that the carbon won't be making sharp radius turns when it's compressed against the joint. I know "filler" is frowned upon so we'll have to see how the joints perform with it. The photo below is as it was applied, prior to sanding. When finished the fillets will be less than 1/4" radius and the tubes clear of excess material.

The filler used in all the joints depicted below weighs less than 12g prior to sanding.

Preparing to Vacuum Bag

In order to vacuum bag the joints I need to extend the ends of the exposed head tube ends as well as the BB width. That way I can extend the vacuum bag compressed area beyond the ends of the tube. I turned some plugs from UHMW plastic rod since most glues just don't stick well to it. Any expoxy that might migrate onto them during cure "should" (he said hopefully) not stick.

In the background is a turned aluminum tube mandrel. I'm going to attempt to lay up a head tube for use on a frame with 44mm Chris King In-Set headset cups. It was turned with a very slight taper, polished and coated with high temp release wax. In theory a quick dunking in ice water when it comes out of the oven should let the carbon tube slide off...time will tell.

The Shrink Tape Experiment

I was impatient and wanting to test the oven curing process. Instead of carefully setting up a vacuum bag around the test joint I decided to see how the shrink tape would conform to a tube intersection when it was heated. I can say that on round straight sections of tube it works very well. On curved tube joints it conforms not at all leaving large wrinkles, pockets of epoxy, uncompressed fibers and generally a nice mess.

After a little sanding (and only showing the least embarrassing angle) the first wrapping test looked like this...

[Trust me, the underside is not so pretty]

A Quick Test

I did a quick test of the prepreg curing process by wrapping a test joint. I found out that 150g/m^2 uni-directional carbon prepreg is very sticky and hard to work with by hand with gloves on. The fibers come apart very easily. I quickly determined that I need small tools to carefully push and lay the fibers neatly.

A good example of how not to do it...

Carbon Frames

I started experimenting with carbon frame construction in the last few months. After a couple of years of reading and research I settled on using tube-to-tube construction and wrapping the joints in prepreg carbon. In order to use prepreg I built a temperature controlled oven. It's a well insulated sheet metal box heated by two oven elements running on 120V for a total of 1250W. There is an aluminum fan blade driven by an external motor to circulate the hot air. It's all controlled by an Omega Instruments controller to +/- 1 deg at 250F.