Sunday, February 22, 2009

First vacuum bagging project

Having watched Dave Triano's composite videos twice I've been ordering all the goodies necessary to create high-strength composite materials with optimal weight.  Several mistakes are likely but I embraced my enthusiasm and decided to invent on my very first vacuum bagging project today.  My idea was to build a fin profile by stacking progressively smaller fiberglass shapes on top of each other. This way I'm building some shape as I stratify rather than building a thick rectangular block, cutting it to shape, and grinding the airfoil. This method should (eventually with practice?) yield two fin halves that can be epoxied together to form a rigid, aerodynamic stabilizing structure.  Something along the lines of the following:

I designed both right and left halves in Adobe Illustrator, printed them all on 13x19" HP Professional Satin Photo Paper (300 gsm), cut them out, marked up the fiberglass cloth, and cut all the chunks using a razor blade.  Late in the game I'd also decided to apply a sixth layer of gorgeous carbon fiber/red Kevlar™ hybrid fabric.  There's a reason why Kevlar™ is used for modern bullet-proof vests... that shit is nearly impossible to cut with regular scissors!  I was able to cut the tiny swatch I needed for this first effort after several minutes but then ordered some Kevlar shears to make it easier next time.  Here's the vacuum bag setup:
Here's the layering
  • Well cleaned 1/8" mirror glass (12x12") from Home Depot.
  • Border of yellow vacuum bagging tape (but leave the liner on the top side until ready to apply the bagging film)
  • Apply Teflon release spray inside the tape border to facilitate removal of the cured part
  • Lay down a layer of West Systems resin  on the mirror/Teflon surface (this is just a test part and I plan to use high-temp Aeropoxy for the final design).
  • Lay down an oversized swatch of nylon peel-ply fabric (to facilitate bonding of the two fin halves later)
  • Lay down the shaped but oversized layer one of 5.8 oz fiberglass cloth (45˚ fiber orientation) and pat down with more resin
  • Lay down successively smaller layers 2-5 (alternating orientation between 0˚ and 45˚) with resin in between
  • Apply perforated bleeder film to allow resin to leave the structure for optimal weight.
  • Top that with breather fabric to absorb excess resin.
  • Punch a small hole in the upper right of a 12x12" chunk of vacuum bagging film (damn that plastic is tough!) and mount the vacuum plate into the sheet.
  • Progressively peel the yellow tape liner and adhere the film to form the bagging volume.
  • Turn on the vacuum and listen for hissing.  I only found one leak and pressing the film down into the tape sealed that up.  
  • Leave this setup to cure for two hours then turn the pump off.  You can see that resin has progressive sucked out of the strata, through the perforated film, and into the breather/absorber layer.  (See how awesome that red/black fabric will be as a fascia layer?!):
I'll pull this apart tomorrow morning and see how things cured.  In the mean time I've already learned a few things.
  1. That vacuum bleeder valve assembly might not be necessary.  I wasn't able to approach the 15 microns of mercury theoretically possible with the pump.  I'll probably hook the pump up directly next time.
  2. The vacuum pump constantly vaporizes oil!  I walked into my garage to see a cloud of fine mist dispersed throughout.  Duh. Next time I'll run this outside or with a fan blowing the effluent out the window.
  3. I'll probably use more resin in the center of the stack next time.  You can see that the breather layer is lean near the center.
  4. I'll need to cut the mirror to fit into the curing oven I just built (10" wide).  I should be able to fit two fin halves per mirror and cure/post-cure the Aeropoxy in the oven (1.5 and 3-4 hours, respectively).
I'll post an update tomorrow when I excavate the part.

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