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Thread: Vacuum Bagging Body Molds - male / female ?!?

  1. #1
    I am curious what percentage of teams are using male vs. female body molds, and what some nuiances might be working with either. I imagine that the female molds require more work initially (usually made from a male plug mold) but result in a cleaner product with less finishing required.

    Are there tricks to using female molds when vacuum bagging fibreglas ?? My experiance has been restricted to male molds, and quite limited at that, but I imagine that it may be more tricky to get the release film / bag materials to conform to the contours of, say, a nose cone. What kind of large polyethelene material (and what kind of sealing technique)can be used to create the 'bag'?

    thanks,
    Funh
    CCS #608
    01 sv650

  2. #2
    I am curious what percentage of teams are using male vs. female body molds, and what some nuiances might be working with either. I imagine that the female molds require more work initially (usually made from a male plug mold) but result in a cleaner product with less finishing required.

    Are there tricks to using female molds when vacuum bagging fibreglas ?? My experiance has been restricted to male molds, and quite limited at that, but I imagine that it may be more tricky to get the release film / bag materials to conform to the contours of, say, a nose cone. What kind of large polyethelene material (and what kind of sealing technique)can be used to create the 'bag'?

    thanks,
    Funh
    CCS #608
    01 sv650

  3. #3
    We're using the male plug/female mold technique this year. Of course it's longer but when well done, it gives the best finish. No bondo is needed on the body itself if the female mold is well finished.

    As for vacuum bagging, there's no real difficulty in making one for a female mold. You just have to make sure you use enough bagging film for it to go in every corner of your mold when you turn on the vacuum. Also, I don,t recommend release film for complex forms, it kinda messes up with the finish as it is almost impossible to position without any wrinkle. Just use many layers of wax or some freekote. You can also use PVA, but be aware you finish won't be as glossy. You should use vacuum bagging sold by composites company, although I'm not aware of the exact material. Maybe some cheaper bagging could do the trick, but you have to ake sure it won,t react with the resin.

    As for sealing technique, hi-tack sealer is the only option. Put some masking tape strips on the edges of your mold to make sure you get non-contaminated areas for your sealer to adhere. Try not to drop any resin on the sealer, I can tell you it won't stick anymore afterwards, and it will be a mess to seal the bagging.

    I hope everything's clear.
    Didier Beaudoin
    École Polytechnique de Montréal 2005-2008
    École nationale d'aérotechnique 2004

  4. #4
    Male pieces are easy to sand. Female pieces are tough. It's best to make the male plug perfect, then take a mold off of that. Then when you lay up your part, your finish will be perfect. Experiment with laying parts up by hand, it isn't that hard and requires a lot less tooling than vacuum bagging.
    ----
    Mike Cook
    It's an engineering competition, not an over-engineering competition!

  5. #5
    I agree with the above. A male plug, then a female mold, is the way to go, and make it as perfect as you can at each step. Vaccum bagging shouldn't be needed for wet layup glass or carbon.
    I used the PVA (actually I was not up to applying it myself, but another guy was more of a PVA ninja than me, and got it working, the stuff is water based, and you are applying it over mold release wax.... go figure that it's a tad difficult to apply) ANYWAYS... it's true that the PVA will give you a bit of a pebbly texture on the part, but nothing a coat of primer can't fix.
    I am interested, for the teams who used no PVA, how much, and what kind of wax did you use, and were there any problems with it letting go from the mold?
    Dan Dyck
    Tech. Director
    University of Saskatchewan

  6. #6
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Big D:
    Vaccum bagging shouldn't be needed for wet layup glass or carbon. </div></BLOCKQUOTE>

    What makes you say that? I don't agree with it at all.
    James Waltman
    VRI at WWU Alumn
    FSAE 01 to 05
    http://dot.etec.wwu.edu/fsae/

  7. #7
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by James Waltman:
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Big D:
    Vaccum bagging shouldn't be needed for wet layup glass or carbon. </div></BLOCKQUOTE>

    What makes you say that? I don't agree with it at all. </div></BLOCKQUOTE>

    I don't agree either... Vacuum bagging gets the extra resin out of the part, and thus lowers the part's weight without changing its mechanical properties. It's also useful to make sure the layers of composite stay in place and that there are no space between them.
    Didier Beaudoin
    École Polytechnique de Montréal 2005-2008
    École nationale d'aérotechnique 2004

  8. #8
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Didier Beaudoin:
    Vacuum bagging gets the extra resin out of the part, and thus lowers the part's weight without changing its mechanical properties. </div></BLOCKQUOTE>

    doesnt a higher fibre/matrix ratio also give greater strength? depending on the fibre/matrix type, i think there is an optimal value of fibre/matrix ratio that gives maximum mechanical properties?

    if thats right (im 90% sure it is), then a part that has been vacuum bagged (or had some sort of compression) will not only be lighter but will also be stronger than a part that uses the same amount of carbon fibre but just a wet lay-up technique.

    of course, i could be getting mixed up with strength to fibre weight figures and strength to final composite weight figures...

    anyways, you can get away with not vacuum bagging a body mold (we have, and got voted the best looking car there!). i would say its worth it if you could find the time though.

    oh yeah, if you are getting your resin from a boat manufacturer (or anywhere else for that manner, but boaties always have this problem), check the data sheet on it. a lot of resins will have a glass transition temperature (Tg) or a heat distortion temperature (HDT) (not the same thing, but similar) of about 40 degrees C. while its not too noticable when they are mounted to the car, if you plan on getting the parts painted and then getting the paint baked on in an oven, 40 degrees C HDT isnt good enough and the part will seriously distort. thats what happened to ours and they had to have the paint baked on at a lower temperature.

    btw., what sort of paint do you guys use on your shells? we had 2-pack automotive paint, but i think it actually weighed a fair bit and there might have been lighter options.
    - ARC '04 member (now retired ) - Bling Bling Competition winners FSAE-A '04 (and design winners)

  9. #9
    Vaccuum bagging is NICE, but is not strictly needed. It does, however, provide some advantage, although in a non structural body application, you'll most likely be fine without said advantage.
    So, if you don't have the wherewithall/equipment/facilities, or if this is your first time, don't worry about it. Squeegee as much resin out of the cloth as you can, and be happy. I had never done a body before, and that's how I managed. If you have more experience/resources, by all means, bag it, if not, it is a step that you can survive without.
    Another great way to reduce weight is to use 'micro-balloons' in your resin. These are TINY hollow spheres, and you pour a bunch into your resin, so your resin goes further, with less weight.
    Dan Dyck
    Tech. Director
    University of Saskatchewan

  10. #10
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by gug:
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Didier Beaudoin:
    Vacuum bagging gets the extra resin out of the part, and thus lowers the part's weight without changing its mechanical properties. </div></BLOCKQUOTE>

    doesnt a higher fibre/matrix ratio also give greater strength? depending on the fibre/matrix type, i think there is an optimal value of fibre/matrix ratio that gives maximum mechanical properties? </div></BLOCKQUOTE>

    Yeah, you're right.
    Didier Beaudoin
    École Polytechnique de Montréal 2005-2008
    École nationale d'aérotechnique 2004

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