Resin impregnated cloth... and pop

[Nick's Sticks]

************ I moved the "pop" section of this discussion to the design/layup section*************


I am interested in procured resin impregnated fabric. Has anyone experimented with using their press to make precured laminates, either for topsheets or structural layers? I saw that snowboard materials is selling precured fiberglass and carbon fabrics. I understand that using these as laminates can greatly improve torsional rigidity. It seems like one could make their own by laying up a single layer of fabric in their press, or layers with custom carbon stringers. The same principle could be used to make topsheets. I can’t come up with any good reason why a precured topsheet would be better or more durable than just laying it up with everything else, but I feel like it might be. You don’t have to have a reason for everything right?

[doughboyshredder]

The biggest benefit to prepreg is that the resin/glass ratio is perfectly controlled and in the laminating process you are only having to use a small amount of resin for adhesion between the layers.

[Head Monkey]

There are two different things out there: “precure" and “prepreg”.

Precure, which is what I’m guessing you’re asking about Nick (Outlook is autocorrecting “precure" to “procure” for me, too ), is a glass laminate that is impregnated with resin and cured under pressure and tension. You end up with a thin fiberglass sheet that is ready to be placed in a larger laminate just like any other component, like the base, core, or top sheet. People who use precures assert that they yield more “pop” because they have a lot of stored energy in them from their formation. The glass fibers are tensioned, and then pressed with heat. The epoxy cures, holding the tensioned fibers in place. I know a guy who uses thin strips of precured glass along with normal glass in construction of custom, hand-made long bows and he swears that this gives him far better properties than using only regular glass. Search this forum, and grafsnowboards.com, for “precure" and you’ll get a lot of hits. Over on grafsnowboards you’ll find posts from a guy named “Broz”, who has made really nice boards using precured layers. There is also some discussion of how to make your own on one of these forums… I don’t remember which one off the top of my head. Making your own would be a very non-trivial process… the key is tensioning the fibers, not just pressing a piece of glass with resin ahead of time. While the glass to resin ratio should be well known for any procure (it’s just simple math and weighing), because you are still using wet layup techniques to put it into your board, and perhaps even other layers of normally wet-out glass, you can still end up with great variability in the final resin to everything else ratio of the entire laminate.

Prepreg is fiberglass impregnated with epoxy, but it is not yet cured. They use a special epoxy formulation that cures very, very slowly until heated to a specific (usually high) temperature. Prepregs are kept cold, usually in a sub-zero freezer, until needed for a layup and may remain viable for a year or more. The cloth is still flexible, and you cut it like normal. You use prepregs in your layup without adding any other epoxy: there is enough epoxy in the glass to bond all the layers together. You have time to work with prepregs at room temperature before they start to gel. Depending on the specific prepreg, this may be hours or days. The advantage here is that you get extremely good control over the epoxy/glass ratio and very consistent distribution of resin throughout the laminate. Prepregs are commonly used in extremely complicated aerospace layups, things that take days to lay out and get vacuum bagged before being placed in an autoclave and cured. I also understand that some large manufactures of skis and boards use prepregs to eliminate the skill needed for (and the variability caused by) wet layup in their production process. Again, search both forums for “prepreg” and you’ll get a lot of hits, including various discussions of the pros and cons to using prepregs.

Good luck!

[doughboyshredder]

thanks for the great info Mike.

I guess what I had used in the past that I thought was prepreg was actually precure. Interesting.

[Nick's Sticks]

Thanks for the info and for pointing out my spelling errors. Sometimes I hate freaking Microsoft.

I was mostly referring to the precured composites. Tensioning the fibers makes a lot of sense and explains a lot of what I already read. I guess my next step is to figure out how to make my own. I would still be interested to see how other people have accomplished this. For purely masochistic reasons I'd rather make my own than buy them premade.

My next pair of skis are going to have tip rocker and maybe some tail rocker too. My biggest pet peeve with rockered and zero camber skis is their lack of spring. If I wanted dead skis I could have just pulled an old pair out of the closet. It sounds like precured sheets might help. Off to the drawing board. Cheers.

[Kevin6q]

Hey Mike,
Nice explanation and I'd like to add to it. Any fiber matrix can be prepreg not just FG. The resins in prepreg are not necessarily better or of the optimal resin/cloth ratio. Some wet layups can have better resin/cloth ratios than prepregs if the builder is skilled. The biggest reason the aerospace inustry uses prepregs is there is less skill required to get a proper layup. With wet layup (what most of us on SB use) there are many ways to screw it up. Most of them being too much or too little resin. The prepregs can also be CAD cut so the fibers are aligned properly and the layup schedule is very cookbook. Many of the prepregs require pressure in addition to heat in order to properly cure. An autoclave is beyond most homebuilders. The prepregs are also vac bagged while in the oven to remove excess resin as it liquefies.

The pretensioned fabrics have more pop since the fibers are all running in the same direction and are straight. In any weave there are kinks which weaken the overall resin/cloth matrix. When cured under tension, these kinks are removed. Any kink changes the way the forces are transmitted and carried by the fibers. Even most uni fabrics have a polyester or polypropylene thread to hold the uni fibers together and in alignment. These fibers are weaker and allow the designer to specify which directions the stiffness should be.

Depending on how the fibers are run, the ski can be very soft in longitudinal flex but very stiff in twist or the other way around. The strength of the fibers runs along the fiber. Run uni the length of the ski and it will be stiff but very twisty. Run the uni at 45 to the length it will be soft and torsionally rigid in one axis and super twisty in the other. The tri axial fabrics have fibers running along the ski (0 degree) and at a bias, usually 45 degrees in each direction to the uni (+45 and -45). This gives resistance to flex and torsional rigidity to twisting in both directions.

The precure uni will be very stiff but very soft in torsion. The longbow only has force applied in one direction. Using the precure might yield stiff, poppy skis that are prone to twisting/washing out when turned hard in firm snow.

Making the precure can be done but after thinking about it pretty equipment intensive depending on what you already have available and what you hope to achieve in terms of resin/cloth ratios and if you can equally distribute the fibers across the width and thickness. By the time you add in some torsional fabric to the ski, I think the overall package would be heavy. Adding a mm or so to the core would make the ski stiffer and add much less weight than the layer of precure.

[knightsofnii]

i'm going to throw a monkey-wrench into all this, and say there's also a third variation, sortof:

pre-PRESSED.

I dont know if it's still done, but Salomon used to press their boards twice.
The first pressing was completely flat, or flat plus camber, i forget.

The second pressing was to bend the nose and tail. Apparently it probably did a bit of what using precures does. It made the board rigid one way, and so when they press the second time the glass layers are stressed and there is more tension on them already without even leaning on the nose and tail, and the board ends up with more 'pop'.

[davide]

I made some pre-preg carbon: put under vacuum for about 1 h (full curing time > 20 hours), then I used the material to make some complicate shapes (helmet shell and boot shell).
Pre-preg worths only for making 3D shapes. For home made skis, it is useless and too complicate.

Salomon press the skis only once, but they put layers of some special plastic film under and below the core at tip and tail. Then they insert tips and tails in a heated press, the plastic get soft and the cured glass layers can shear with respect to the core. After cooling down tips and tails keep the shape.
Again, great for speeding up factory production, but too complicate for home building.

Just one more thing. Can anybody give me a definition of "pop" in terms of engineering quantities (like force or Young modulus)?
Thanks.

[knightsofnii]

nope, "tons of pop" is too vague a thing still.

to me it is the ability of the nose/tail to spring allowing you to pop off jumps or ollie just a pinch higher. I dont know what other people think it is and i cant quantify it.

i guess it's the ability of a board to rebound when flexed, without being underdamped so that it wobbles or too stiff.

[knightsofnii]

oh and like i said i'm not sure what salomon's doing NOW with their production methods, that sounds pretty interesting what you described with the sheets.

I just know that a few years back they were pressing their snowboards twice, once flat then the 2nd time to bend it.

[Nick's Sticks]

one unit of 'Pop' is equal to or grater than the force necessary to transform an ordinary smile into a sh** eating grin...

[davide]

one unit of 'Pop' is equal to or grater than the force necessary to transform an ordinary smile into a sh** eating grin...

My skis have plenty of it.

[Nick's Sticks]

I am not an engineer but like most scientists I like to try to quantify qualitative elements. I agree that something like resistance to deflection would be one way to measure 'pop.' I tried to figure out the 'pop' of an old pair of k2 x15's. I measured the deflection of the ski when loaded with a 31.8kg weight at its midpoint.



I used the deflection of a beam formula: deflection=(W*L^3)/48EI.
Since calculating I would be hard, at least for me, and since I was being less than scientific, I used EI as my unit of 'pop' (P).

where:
defleciton=.06m
W=312N
L=1.86m

.06=(312*1.90^3)/48EI
in this case EI=P=743

When I did the same test to a flexier pair of skis that I built I got P=637. It should be noted that I eyeballed a tape measure for all my measurements and did a lot of rounding to make things simple. But it was clear that the softer ski had less pop on the snow and deflected much more under weight. If anyone else has a way to calculate 'pop' stiffness, etc I would be interested in hearing it.

This said, I think there is a lot more that goes into the feel of poppyness than the physical ski itself. The technique, strength, and mass of the rider have as much or more to do with how a ski behaves than the ski's parameters. Flex of a ski is also dynamic since one can flex a ski in many different ways: leaning back, stomping with the heal, etc. Not to mention that the shape of the tail will change how the ski compresses. Even if a rockered tail is stiff as a board you will have to lean much farther back to get it to load than a flat tail.

Just some thoughts.


****This post is now also in the "...Pop" part of the design/layup forum. If you have anything to say about pop please put it there. Thanks.*****