Divinycell vs Core Cell

Discussion in 'Materials' started by CatBuilder, Dec 23, 2010.

  1. AndrewK
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    AndrewK Senior Member

    When thermoforming it is a time versus temperature relationship.

    For corecell A I was told the temperature at which the foam starts to expand is 73'C.
    My experience is if you want to shape a reasonable size piece then 75'C was about the minimum temperature at which it was sufficiently pliable to bend into a complex shape.
    Also found that 85'C was about as high you want to set the oven at. Anything higher meant that if you got held up for more than a few minutes the expansion becomes obvious.
     
  2. War Whoop
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    War Whoop Senior Member

    I got a little different information on this from SP engineers, looking for both ends of the heat range and their recommendations on the process.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    RX…thanks for those.

    Those figures in the attached word doc, are the “nominal” figures I quoted from my ref book. Same shape graph, fig.3, just slightly different values. The interesting graph is the “micro-cracking” graph, fig.2 in your word doc.

    This leads to AndrewK’s comments:

    Ostensibly this is the micro-cracking. This is where long before the UTS is achieved the laminate will reach a load where the resin will begin to crack away from the reinforcement and thus spreads through the resin.

    An old SPSystems internal document I have states:-
    “…for a resin to exhibit good performance with respect to resistance to micro-cracking it requires good adhesion to the fibre reinforcement – largely demonstrated by the resins’ shear strength, and it ability to stretch under load and then fully recover – demonstrated by its elongation at break……Work from various sources has shown that the main factors which affect the micro-cracking performance are the resin toughness, resin fibre inter-facial adhesion, level of residual stresses on cure due to shrinkage on cure, and weave depth and fibre crimp..”
    This is the problem with composites. There is no real “fixed” standard. Which is the beauty of it, it can me “made” to be what you need for your application. Downside is, far too many variables to be qualitative, can only ever be quantitative.

    Thus everyone has an opinion regarding strengths of composites and no one can ever really be “right”. But the more independent published data there is, helps us to ascertain who is telling porkies and who isn’t!
     
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  4. War Whoop
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    War Whoop Senior Member


    Yes Sir ,every resin system we used I was not afraid to call the lab they spoke a different language but I eventually got the info, Now the Peroxide guys are on another level completely , the Airex people here, I had a very good relationship with them (sponsored) and they started the corecell manufacturing which got eventually bought out by SP Gurit.
     
  5. rberrey
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    rberrey Senior Member

    As the elongation of E-glass is 4.8, epoxy and vinylester is 5.5, will the fiber not fail before the resin ? Arn't we ok using either resin as far as elongation is concerned? To achive a better elongation layup would'nt we have to use S-2 glass in a unidirectional, bias or triax cloth?
     
  6. rxcomposite
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    rxcomposite Senior Member

    We used to form 1/4" Divinycell HT (high Temperature) with a heat gun, dual setting and a piece of mailing tube to coax it into form. It takes some patience anda little skill but with minimum of investment.

    If you are thinking of thermoforming, then you will need an oven and a form plus a vacuum bag system. And you still have to worry about the "springback" of the foam. It won't form exactly to the mould.
     
  7. rxcomposite
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    rxcomposite Senior Member

    Ad Hoc,
    Interesting discussion. I have ran across this before but I thought it was impractical for boats, that is predicting laminate strength by the choice of resin. There are two formulas I have come across with, Lloyds Register for predicting sterngth of Uni and the Tsai Hill formula. My reference book has some explanation.

    I am interested in how much elongation the resin should have. All my references points only to impact resistance. What about usable strength? The one in elastic zone.

    Azok Fig. 2 on microcracking seems far off on the ultimate strength. I think the laminate test result is more realistic because when microcraking starts, the laminate starts degrading. Not good for the marine industry but other application (high pressure vessels and rocket nozzles) takes advantage of this. They use microcracking to "normalize" the laminate. Similar to stress releiving.

    Here is an excerpt from Filament Winding by SAMPE;

    "In a composite laminate, operation at strain levels sufficient to cause matrix cracking allows more effective use of the materials. Usually, through ply matrix cracks form first, aligned with the fiber direction. At each crack, there is a shear lag redistribution of load around the crack and back into the matrix. At a distance from the crack depending on laminate parameters, the stress builds up to a level necessary to cause another crack, and subsequent redistribution. This process continous until the cracks reach equilibrium (saturation) spacing."

    And impact damage from Fundamentals of Composites Manufacturing, with reference to AndrewK info;

    "In all the matrix materials the trend is consistent and shows the matrix choice can double the compression-after impact or reduce the damage area after impact by a factor of 6". Toughened epoxy was mentioned.

    Seems Catbuilder and War Whoop is on the right track by their choice of resin.
     
  8. rxcomposite
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    rxcomposite Senior Member

    rberrey,

    The elogation is measured along the direction of the fiber. When off axis such as the biax, the matrix takes over. If you pull a uni sideways, you are just separating the fibers from the matrix.
     
  9. CatBuilder

    CatBuilder Previous Member

    I got lucky!

    My resin was chosen on working criteria only:

    *Able to be used down to 35F and in any humidity
    *Thin viscosity so it wets out heavy triaxial better
    *72 hour time to add another coat without sanding
    *Workable in hot summer at 100F (though fast)

    I got lucky on the other characteristics.
     
  10. rxcomposite
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    rxcomposite Senior Member

    What is the Ultimate Tensile Strength (UTS) of the Silver Tip?
     
  11. CatBuilder

    CatBuilder Previous Member

    From the data sheet:

    Tensile Strength, psi (ASTMD638)..7,900
    Tensile Elongation (ASTM D638)............8.0%
    Tensile Modulus, psi(ASTM D638)...........390,000
    Flexural Strength, psi(ASTM D790)...........13,000
    Flexural Modulus, psi(ASTM D790).........420,000
    Heat Deflection Temperature, ° F...................128
    CompressiveStrength, psi(ASTM D695): at yield.......13,000
    at failure.............................26,000

    More data can be seen on the sheet itself at:
    http://www.systemthree.com/reslibrary/tds/Silvertip_C&L_Resin_TDS.pdf
     
  12. AndrewK
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    AndrewK Senior Member

    War Whoop, which VE resins have the 6% elongation? I am looking for a low viscosity infusion grade.

    Cat Builder, statements by resin suppliers claiming that their resin is not effected by humidity are misleading. While true it does not mean that you can laminate in high humidity. Primary job of the resin is to surround and bond to individual fiber filaments so that they all co-operate together. Reinforcement fibers are chemically treated to improve the wet out and bonding. Laminating in high humidity compromises this bonding which leads to fiber pull out at lower strain levels.
    I would also ask for the properties for the slow hardener you are using rather than the generic given in the data sheet.

    rberrey, rx has already pointed out that laminates are not just loaded along the fiber so no point in spending extra money on S glass if you are not going to match this with high elongation and high strength resin.
    The simplest and cheapest way to improve the bulk strain a laminate can take is to reduce the typical 90 deg angle between neighbouring plies. Take a triaxial for an example +45/0/-45 one only has 45 deg between each plie but +45/-45/0 has 90 deg between the +45/-45 plies.
    Also as infusion and vacuum bagging is becoming more common the increase in fiber volume fraction from 30% to 50% also increases the local strain between the fibers but a lot of fabricators still choose to use pe resin. This is another case where you should be matching resin properties to the process being used.
     
  13. rberrey
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    rberrey Senior Member

    Andrew I bought silver tip with slow hardner as well, +45/-45, 0/90 biax, I will be using some light S glass for a finish cloth.What I was wondering was if we are getting any benifit from the greater elongation of the high strength epoxy if we are not using S glass. rick
     
  14. CatBuilder

    CatBuilder Previous Member

    Thanks, Andrew. I realize the effects of humidity on lamination (and wood). When they say I can use the epoxy in higher humidity, they mean it is not going to blush on me in cooler temperatures and higher humidity. There is still the concern of bonding properly on wet fiberglass. I will likely use a small hot box to keep the glass dry.

    The properties of the epoxy I listed are for the hardeners listed... there are two. They are in the link.
     

  15. War Whoop
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    War Whoop Senior Member

    HUH?? The Derakane Momentum 411-350 Epoxy Vinylester should be 6% on the high side, if memory serves, it comes in a infusion grade as well.
     
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