Layup on polystyrene

Discussion in 'Materials' started by eberd, Jan 24, 2011.

  1. kroberts
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    kroberts Senior Member

    Not sure what you're trying to accomplish with the vacuum bag. If you try to get a hard vacuum with extruded or expanded polystyrene which is of the same density you use in house insulation (up to 2.5 something, I think it's lbs/cubic foot), then it WILL deform under a hard vacuum. Ask me how I know.

    It's not really going to get you much either IMO, unless you have a hard prefabricated shell that you're gluing to the foam core after it's cured. In that case you're just using the bag as a clamp, which it works great for. You're applying glass to the outside of a plug, so bagging that will scar up the surface of wet resin.

    I've built and helped build several hovercraft hulls using extruded polystyrene with plywood skins, and ideally you get the pressure of maybe 100 to 120 inches water, say 7.3 to 8.8 inhg to get 1/8" plywood to stuck to a moderately curved hull. I think that's about the limit of the stuff I used before it starts to deform. You could get that with a really good vacuum cleaner. If you can suck 8.5 feet of water out of a bucket with it, you're good.

    Frankly though, I would just use a paint roller with a really short nap. You can cover a lot of territory with a roller, and with some practice you basically suck the excess resin right out of the cloth. Not as good as infusion but the best I've done so far with "standard pressure" glassing.
     
  2. bntii
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    bntii Senior Member

    A vacuum set up will buy you extremely good adhesion to the core with a very well controlled resin content.
    Additionally, if using a carrier shell of thin Mylar as I am suggesting, the finished surface is perfectly smooth with no tendency to emboss from the fabric or foam surface.
    Also- the foil is bagged in the 'mold' created by to wire cutting of the foam core. When done this way the mold holds the foil to its correct shape with no tendency to twist or deflect in any direction while curing. This is tough to do with any ease while laying up by hand, and both sides are done in one throw if bagged. The wing sail pops from the "mold" nearly finished and held to the designed form with high integrity.
    This is not an infusion process & so easy/cheap to set up that it is worth pursuit when the benefits are valuable to finished design.

    For reasons stated, I would use a foam of sufficient density that a crush from the draw down will not occur.
    The is provided by a range of inexpensive building foams whose density is fully specified.
    The Dow Hy-Load foams are a decent way to go me thinks.
    http://building.dow.com/na/en/products/insulation/highload60.htm

    Lad. If you take one thing from the vast array of experiences here at BoatDesign.net, take home the simple ploy of making it appear that you are funding your wife's pursuits to a far higher degree than your own.
    With this accomplished- one may pursue the hearts desire with impunity if not full support.
     
  3. bntii
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    bntii Senior Member

    The only real expenses here are carbon or Kevlar skins whose use may not be required as it is dependent on your load case. It could be that the dead cheap finishing cloths combined with a Uni-carbon spar will be a well engineered structure.
    Simple cloth can be stretched on bias while laying up to meet the loads. A bias in the skin can be set to meet twist while the Uni-spar will resist bending
    The cost of material for core is nill and Laszlo doesn't charge much more than the cost of a lunch while you mull this over.
    For small work like this I use one of the Mityvac pumps which can be got for ten bucks off ebay is you look around:

    http://www.autotestsys.com/Merchant...de=ATS&Product_Code=MIT4000&Category_Code=BAT

    I realize I am doing nothing to answer your basic question of how much glass to use in your layup.
    Can you post a design of sail, rig and hull as a starter for how much load the sail will face?
    I am not an engineer either but a design is the starting point to determine loads and layup for when a qualified person weighs in on this thread.

    Just musing but a tri with sponsons of adequate size to resist the heeling forces but small enough so as to allow the boat to right when taking the sails flotation into account?
     
  4. bntii
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    bntii Senior Member

    Ok- I could not find a reasonable tutorial so here are a couple of thoughts.

    First off- not trying to force this bagging ideal down your throat.
    I just wanted to lay this out so it is clear what the hell I am talking about...

    The following is a quick sketch of how the core and bagging components look as it is assembled for vacuum:

    [​IMG]


    The way this works is as follows:

    1) A "Mylar" sheet is cut to cover each side of core. The sheet is cut to hold just back of leading edge and past trailing edge by about 1/2" to full length of foil.
    The mylar is any thin sheet plastic which has a smooth surface and epoxy will not stick to. The sheet is rigid enough to form a smooth surface but will still bend to what ever camber is present in work piece. It is found that commonly available .014 sheet Polyester (Mylar) is a good choice.

    2) Cut the layup material to cover each side. The fabric of choice should be held over leading and trailing edges by ~1/4". The bias of the fabric should be used for strength in layup.

    3) Prepare the vacuum bag and pump- 15" bag to longer than foil and short piece of aquarium hose from pump. Seal ends with silicon caulk.

    4) Prepare breather cloth- this is just a sock of light fabric to length of foil.

    5) Layup- place the two pieces of Mylar side by side and use tape to create a 'hinge' on leading edge (Mylars will simply fold around core like closing a book).
    -Place fabric on Mylars and wet out thoroughly.
    -Pad dry to desired degree.
    -Place core on bottom Mylar and fold top over.
    -Place in breather bag.
    -Place in vacuum bag.
    -Seal end with vac tube in and draw down some vacuum.
    -Place whole assembly in the top and bottom "Foam mold" pieces on flat surface and weigh down with phone books.
    -Draw down hard vacuum.
    -Wait for cure and pull from bag.
    -Peel off Mylars and trim edges- done.

    *Though shown thick for clarity all the components wrapping the sail core are quite thin and need no allowance to fit into "Foam mold".

    *The "Breather" and "Vacuum bag" are only shown on top for clarity- they fully wrap core.

    *Placing the spar reinforcement is not described above and is not shown in sketch.

    *All of the above assumes hot wire cores are prepared, and the blocks of foam from which they are cut retained to use as the "Foam molds".

    *The Mylars may be spray painted a nice jazzy color (safety orange) before work and it will be fully bonded to finished sail.

    *While this all may seem like a big complication it is truly less trouble to do than describe.

    Here is a photo of one of Laszlo's beautiful wire cut cores in High-Load 60 for a small sailplane:

    IMG_5357.jpg

    It's difficult to pick up in the photo, but the core surface is very smooth and fair.

    Lets call this installment #1
    Give me a minute & I will cover the spar reinforcement.
     
  5. bntii
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    bntii Senior Member

    Spar reinforcements.

    The way I have installed spars is pictured as follows:

    spar1.jpg

    The uni-carbon is purchased as sheet in the width, length and thickness as required by your sail.
    A block is prepared as shown with a layer of sandpaper glued onto the block to the width and thickness of the CF reinforcement used.
    A straight edge is placed on the foam core and the block used to sand into the core creating the slot for the CF.
    Just before bagging, the CF is epoxied into the slot and then bagged as above.
    This installation results in a perfectly smooth surface and good strength.

    A second option is to use a tapered hollow CF tube to create a spar as follows:

    spar2.jpg

    If this sort of spar is used it seems like a simple means of setting up a rotating sail and the tube can be specified to carry most of the bending load.
    Twist still has to be met but is dealt with in the bias of the layup.
    Tapered CF tubes are available in many sizes and not expensive or one can 'roll your own'.
    I believe this second option is what Doug is referring to in the above post.
    Laszlo can hot wire cut the hole for the tubular spar neatly into the core.
     
  6. kroberts
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    kroberts Senior Member

    Hope you don't mind some questions that I think the OP would ask if he had experience with this stuff. Please don't take any of this as antagonistic, I'm trying to learn something. I'm intensely interested in this, especially if it can be done cheaply. I don't think my questions are off topic.

    1. How can you get a truly straight smooth and fair cut with a hot wire? I have made spring-loaded handles and have a really good variac, I have always had line sag or bending because of the resistance through the foam. As you go around the form, your wire in the middle takes a shortcut and you wind up with a poorly shaped part.
    2. I see you specified a foam that is able to prevent crushing. Is this an open or closed cell foam? Do you have something specific in mind?
    3. The outer form you have shown, not really sure what good that's doing except maybe making it easier to lay the glass up by putting the project back in it when you're done with one side, until you get the pump going. What am I missing?
    4. How tight a radius does that mylar curve around? I'm thinking of a rudder with an airfoil shape, an inch thick and a foot or so chord and a relatively tight leading edge. 3/16" or less radius maybe? Do you have a source?
     
  7. bntii
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    bntii Senior Member

    I will answer these in turn:

    1) Straight cuts with hot wire.

    My answer to this is just a deferral to the product as provided to my by Laszlo Horvath who makes the cores- They are perfect.

    He is using a CNC set up and I am seeing perfect integrity to in producing the airfoils I have sent to him to be cut. He states that the accuracy provided by his gear is plus/minus .005". Its remarkable to see a foil cut to such a fine trailing edge that you can almost see through it.
    Again- I would call him if at all interested in this technique. He is the nicest guy in the world and will talk your ear off about this topic. One thing to remember is that compound curves are not going to happen with a hot wire.

    2) I am using Dow High-Load foams as referenced here:

    http://building.dow.com/na/en/products/insulation/highload60.htm


    3) The outer form

    When laying up thin structures some means of keeping the part from deforming has to be used. This can be a flat table, piece of glass, tube, bowl etc. It all depends on the shape of the part one is making.

    When doing foils, the part is extremely flexible when wet out. It must be held flat in several axis one of which passes through a center which has not flat reference on the outer side. In other words if you with to maintain the form integrity it is not possible to simply lay it down on a flat table to cure.

    By placing the vacuumed sail into the "form" from which the foil was cut, the part cures in the exact shape in which it was designed.

    From this:

    "not really sure what good that's doing except maybe making it easier to lay the glass up by putting the project back in it when you're done with one side,"

    I believe I am not clear on how the "Mylar" carriers work.

    You have a sheet of plastic for both sides of the part. Top and bottom. The layup is done on these 'carriers' then they are wrapped over the part and placed into the vacuum bag. Since they do not stick to the epoxy, after cure they are peeled off and the laminate is left bonded to the part.


    4) Radius

    The technique does not carry over the leading edge so the most any sheet has to do is the radius on any given side. This can be very tight- 1/4" or so with the .014" mylar, and thinner sheet can be used or the leading edge sanded on outside to thin in a tight radius area.


    "Hope you don't mind some questions that I think the OP would ask if he had experience with this stuff. Please don't take any of this as antagonistic, I'm trying to learn something. I'm intensely interested in this, especially if it can be done cheaply. I don't think my questions are off topic."

    I already committed the forum sin of galloping off topic with my area of interest... you may be compounding the error. ;)
     
  8. eberd
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    eberd Junior Member

    Thanks a lot for the beautiful drawings bntii! Doing the vac bagging does seem doable based on your description, but since the wing I'm planning to use is more blunt (NACA0020), I'm thinking maybe I could do the layup in one piece and just bend it around the leading edge.

    But what do you think - how much difference does the vacuum make? What if I did the layup the same way, put the wing back into its mold on a flat surface, then put a robust flat piece of something on the top and lots of weight (making sure it doesn't deform the core of course). The mylar would still give a nice surface I guess, the only thing is that with the vacuum you can probably get more resin out.

    About strengthening the wing - I had a similar idea, but I was thinking of using unidirectional CF tape instead of prelaminated pieces and because of the a lighter foam, I thought I would put a layer or two glass connecting the carbon tapes perpendicular to the chord to make sure the two sides stay apart when the wing is under bending loads. Something like this:

    [​IMG]

    I guess with a more rigid foam it's not necessary.

    Thanks again!
     

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  9. bntii
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    bntii Senior Member

    I am being called to other tasks so only a couple of quick thoughts:

    Spar- I am going to defer completely on the spar question.
    I am really hoping that a qualified person will weigh in to better sort out the load your sail will face and how to meet that load with the skin lay up and spar.

    Also- the carbon I have used is precast, but very thin.
    I made the element thick in the drawing for clarity.

    Vac worth it?
    Once the few materials needed are assembled, it is so easy and has so many benefits that I would not consider forgoing for this sort of work.

    Got to run..

    Opps- I see your condition on spar is for lighter foam.... use the Dow 60 :)
     
  10. kroberts
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    kroberts Senior Member

    bntii,

    Hopefully that sort of thing can be found locally in some quantity less than a pallet.

    eberd,

    Taking your 5.5x1 foot sail, convert to square inches: 792

    Now multiply by 14.5 psi and you get 11,484 pounds of force down on your sail, perfectly distributed. You won't get a perfect vacuum, but maybe you can see what all the fuss is about. I bet your entire collection of automobiles won't weigh in that high.

    You Will Not Get Better Clamping. Then consider the cost of whatever you might come up with.

    If you can rig the pump then it's incredibly worth it. It doesn't take much. If you can't find a good vacuum pump then a good shop vac is better than anything else you might find.
     
  11. eberd
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    eberd Junior Member


    Hmmm air is heavy! :)
     
  12. kroberts
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    kroberts Senior Member

    1. Get a rectangular gallon paint thinner can.
    2. Get a plain old house vacuum cleaner.
    3. If the hose won't fit in the opening of the can, then chop a hole in one end and fold the extra metal inward.
    4. Put the hose into whatever opening works.
    5. Wrap the whole thing in a towel.
    6. Put the whole thing into a garbage bag.
    7. Tape the lip of the trash bag to the hose so that it's air tight.
    8. Turn on the vacuum cleaner.

    For a more impressive display, do this with something that can get a hard vacuum.

    Edit:
    Ok I confess to having spent hours with a vacuum cleaner and a plastic bag finding out what I can make smaller. So sue me.
     
  13. bntii
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    bntii Senior Member

    Opps- I had forgotten, the leading edge does get a wrap of laminate before the Mylars are placed on the foil.

    As below:

    na20block.jpg

    That is your NACA0020. I also do blunt edge foils- the earlier drawings were just the fastest thing I could draw...
    More on the advantages of vac later..
     
  14. eberd
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    eberd Junior Member

    Is it necessary to have separate pieces for the two sides? Can I just use continuous layers of cloth that wrap the wing around? It seems stronger and easier unless I'm missing something. Do you need a gap in the mylar to let the resin and trapped air escape?
     

  15. bntii
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    bntii Senior Member

    This is perhaps a hard one to describe..

    Take a look at this drawing of what you are proposing:

    360mylar.jpg

    The thing to notice is that the circumference of the Mylar carrier sheet is greater than that of the foil over which it is positioned.

    This is how it will enter the bag. The Mylar/laminate is placed over the foil but it is not yet tight to the surface.
    Now imagine what must happen to have the Mylar absolutely tight to the surface.
    The extra length of the Mylar must be shifted to the right as the front of the Mylar tightens over the 'nose' of the foil.
    This will not happen. As the Bag clamps down a gap will remain in the nose of the foil and the vacuum will fail to clamp the skin to the foam. The vacuum will attempt to crush the extra length out of the Mylar but will fail.

    There is no 'which is stronger' issue here. The fabric is at even a 1/4" of overlap will have no loss of strength in these fine cloths.

    I have a bit more on the whole vac or not to vac issue floating around in my head. I will post a bit more later today.
     
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