Vacuuming bagging alternative process

Discussion in 'Techniques' started by Curtis Enlow, Apr 3, 2019.

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  1. Apr 3, 2019 #1

    Curtis Enlow

    Curtis Enlow

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    I am lucky to have composite super-shop within reasonable driving distance and yesterday went to pick up some supplies for bagging LOC IV fins (per John Coker's technique). They supply to maritime industry, kayak builders and home built experimental aircraft with everything from glass & resin to carbon/honeycomb & epoxy. (For anyone in NW/W Washington: http://shop.fiberglasssupply.com/)

    I had a great conversation regarding the process with one of the older tech/service people there, and he suggested using a non-permeable film over the peel-ply and then the breather material. Having explained my application it was his position that using breather over the peel-ply would actually leave less-than-optimal ratio of matrix-to-glass, and that, if the lay-up was squeegeed of excess epoxy beforehand, this technique would A) leave the proper ratio of matrix to glass, and B) would also leave a finer finish, requiring less/no weave fill.

    Has anyone tried this squence?
     
    Last edited: Apr 3, 2019
  2. Apr 3, 2019 #2

    Nytrunner

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    (I don't do composites so I'm just here for the answers.)
    Maybe rocketeers want more epoxy removed to reduce weight?
     
  3. Apr 3, 2019 #3

    Curtis Enlow

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    That's true. However, there is an optimal ratio of matrix/substrate, and adding several layers of high-build primer for a smooth finish might eliminate the weight savings advantage. For low speed rockets the difference is debatable, but for high-performance/Mach 1+ rockets the strength-to-weight aspect is probably a critical consideration.
     
  4. Apr 8, 2019 #4

    G_T

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    Peel ply itself causes micro-fractures in the surface when it is removed. That means the surface of the matrix starts out structurally sub-standard. The purpose of peel-ply is to leave the surface already mechanically prepped for another layer(s) of fabric to be added later. Rocketry people mis-use peel-ply all the time. Generally the use of peel-ply is not a good technique.

    If you search for it, you can find an aircraft composites layup and repair technical manual in pdf form. It isn't small but the info is worth reading. Then you can skip a lot of the internet "wisdom".

    I doubt most folks in the composites industry are going to waste time using peel-ply on top of what is to be the final fabric layer, rip it off, sand, prep, fill holes, etc, just to get to an acceptable surface paint quality with many times the effort and time, and still end up with an inferior layup quality. Sorry, just being blunt about it. I've done a little composite work before, where high quality and strength to weight ratio and proper resin content were required. But usually I used a pressurized mold and extruded the excess epoxy out.

    https://www.rocketryforum.com/threads/sprite-6-and-a-baby-o.37382/page-2#post-435909 Starting at this post I show something on simplified vacuum bagging fins. It is more work but you can do the same with the fins already attached to a rocket body. I didn't need to as the fins were removable by design.

    Short generic explanation - when doing composites, you generally do the work from outside in. The mold surface is prepared with a mold release. Then it gets the surface paint coat. Then it gets a prime cote if one is required for compatibility but it usually isn't since you get to pick your paint for compatibility. Then the fine outer surface fabric (a very fine fabric or veil material) is added to alleviate print-through. Then the structural fabric. Epoxy content is set at the correct ratio. Finally the core is added and the mold closed. Vacuum is applied, or pressure, depending on how you are squishing it. The part goes through initial cure in the mold and may even get post-cure in the mold. When you remove the mold (actual mold, or mylars, or ...) then you are done (or ready for post-cure, depending on order of operations and temperature tolerance of your mold). The part is finished.

    Gerald
     
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  5. Apr 8, 2019 #5

    BF Rockets

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    Last edited: Apr 9, 2019
  6. Apr 9, 2019 #6

    Curtis Enlow

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    Hey Gerald,

    Thanks for the advice. your thread on your fin build is great. I will certainly keep it in mind should I get to the point of building high-performance rockets.

    For now, my LOC IV clocks in at well under 200mph in the sim, and - being my first rocket build in quite a while ('D' motors were monsters in my day...) I am trying to keep the demands on my skills and materials at a low roar, and so compromises must be made.

    As far as my fins go, I just completed the finish trim/sand on my fins and I am pretty happy with the results. I added a simple tapered airfoil with shaped balsa skins on both sides and styrene leading & trailing edges (for added car-rash toughness and because they are cosmetically better than anything I could achieve by hand-sanding). I covered it all with 2 oz cloth and epoxy. They are considerably stiffer & stronger than the stock fins (which seem to the be IV's weak ink and do not make good landing gear). The slab-side 1/8" ply stock fins could be bent easily with three fingers and I cannot flex these at all. The airfoil adds an edge on stability as opposed to a slab-side fin at lower airspeeds and actually moves the CP aft a bit. The stock fin was 31g and these weigh in at 51g so I think the 20g difference per fin is an acceptable price to pay for the added benefits, and - certainly compared to some build videos I have seen online - I feel I can make up for the weight increase in other areas - micro-balloons in aerodynamic fillets, etc.. Also, looking to the near future, I am going to build a dual-deploy HP version of the IV with a e-bay coupler and extended body tube, and I think the more effective fins will be a benefit for the stretched IV-B.

    [​IMG]
     
  7. Apr 9, 2019 #7

    G_T

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    Be very careful not to confuse stiffness with strength. Stiffness is deflection vs force. Strength is when it breaks. Composites are generally much stiffer than wood. But if you don't have enough composites, it may break easier than the core by itself would break!

    As an aside, a little food saver can be used as a vacuum pump, sealing the part in the bag until it is cured.

    Gerald
     
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  8. Apr 11, 2019 #8

    Curtis Enlow

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    That's what I used :) Also, Foodsaver canisters, a 60mL horse syringe and some aquarium tubing can be used to test recovery electronics.

    [​IMG]
     
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  9. Apr 16, 2019 #9

    Chad

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    I've done permeable film (perforated Teflon) over the wetted fg and then pealply (Dacron cloth) and it worked well. If you put on non-permeable film then does it make sense to put anything over it? After all, it's non-permeable.
     
  10. Apr 16, 2019 #10

    Steve Shannon

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    Why put peel-ply over perforated teflon? I could understand putting breather to absorb the excess epoxy, but peel-ply is intended to leave a surface which will accept additional layers of composite.
     
  11. Apr 16, 2019 #11

    Chad

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    Don't remember the source but I read/watched that Dacron worked fine as a material to absorb the excess resin. It worked for me but may not be optimal, I was starting with zero prior experience.
     
  12. Apr 16, 2019 #12

    Steve Shannon

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    Ahh,okay, I think I understand. You just used Dacron fabric as an absorbent layer rather than paying extra to buy real peel-ply. That’s not a bad idea.
     
  13. Apr 16, 2019 #13

    Curtis Enlow

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    Chad, after explaining my application and how I was going to approach it - glass, peel-ply, breather, impermeable - and that my concerns were weight, the tech gave it some thought and advised me to use the impermeable over the peel-ply. It was his view that, providing excess epoxy was squeegeed off after wetting the glass, this would leave a better ratio of glass to matrix, and that it would leave a finer finish that would require less additional steps to finish, and that seems to be the case; there is a nice 'shell' to the shaped balsa and core, and yet it feels light, and there is also a fine finish, not quite gloss, but with no discernible weave at all, and with just enough tooth that it should form a nice base for primer. It worked well.
     
  14. Apr 16, 2019 #14

    Chad

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    oh ok i must have read it wrong, i thought they were saying impermeable then peel ply on top of that.
     
  15. Apr 16, 2019 #15

    Nytrunner

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    For clarity, did he advise you to use the impermeable "instead-of" the peel-ply?
     
  16. Apr 16, 2019 #16

    Curtis Enlow

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    No, impermeable OVER the peel-ply.

    I think his view was that the peel-ply would absorb some excess, but that breather over the peel-ply would absorb excess amounts.
     

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