Tubes after clear
I don't think I ever posted in here the process to make these tubes. Enclosed shapes are a challenge with composites, especially if the mold side is the outside of the shape. There's several ways to do it, lost foam, wet layup bladder molding, pultrusion, etc. Because the parts coming out of the mold would be visual and needed an exact OD, I wanted a process that molded from the outside and would leave a void free finish. I wanted to infuse the tubes for strength and to have a minimal void finish for the clear coat, however there isn't a good way to bag and seal the fabric from the inside of the tube. To solve this I used a modified version of wet bladder molding that would allow resin to be infused around the outside of the bladder and inside of the tube.
The molds were designed in CAD and CNC machined from 6061. The mold has a recessed section to allow tacky tape to seal the outside. There are three openings, one for the resin port, another allows the stem of the innertube to be accessed when the mold is closed, and the last is for the vacuum line.
Visual fabric is laid into one half first, then a NCF quadraxial, followed by a 2mm core. A innertube is cut to length, sealed on the ends and partially inflated. Another quadraxial layer then has a peel ply and flow media is attached to it and rolled around the semi inflated innertube. The innertube is placed in the mold and the other layers are wrapped around it. Mold is then sealed with tacky tape and the top half is secured with bolts.
Ports are sealed on the outside and the bladder is inflated. Once inflated, a vacuum is pulled on the mold, then resin is injected.
Since this isn't a process that I found a lot of information on, almost all of it I had to work out myself. While in theory it should work, I found in practice it wasn't nearly so straight forward. For one, instead of two pressure/vacuum environments, you have three: atmospheric pressure, the vacuum inside the mold, and the pressure applied to the bladder. The biggest challenge was that you can't see inside the mold. Normally you can work with your fabrics, consumables and bagging film, and you can see what is happening through the bag the whole time. You know where your resin front is, how it's flowing, if you had any bridging of the bag, etc. With this process, you're effectively blind. The only indications you have about what is happening inside the mold is your vacuum level, pressure level, and the flow rate of the resin leaving the cup.
The first attempt resulted in a part with 5 times the resin consumption needed. I found that the amount of spray adhesive used during the layup makes a huge difference in the bladder's ability to push the fabric tightly against the mold walls. Also that I needed almost 50 PSI to get the fabric to set in place. The second attempt the resin almost didn't move at all. The result was a dry part.
Again, without being able to see what is happening inside the mold, you have to take an educated guess at what the cause was for the failure. After another couple of attempts I came to realize that the bladder pressure was so strong that it actually filled the voids in the flow media so well that the resin couldn't flow through it. I think in total there were 8 or 9 failed tubes trying to dial this in. In the end, I had to use three flow medias, and pull the bladder pressure back to 18-22 PSI after bringing it up to 50 and pulling the vacuum.
As frustrating as the process with these tubes was, it's also the reason I enjoy composites so much. I learned a ton in the process and has given me ideas on how to mold even more complex parts such as pumps, reduction nozzles, and trim rings.
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