Sandwich-skin material, has any one tried it?

General Discussion
1

Sandwich-skin material, any one try it? It’s called Baltek mat, at tap plastics. Microballoons suspended in a quilted matrix, with thin fiber mat on either side…

http://www.tapplastics.com/shop/product.php?pid=93&

They had some samples where they had laid it up between two 6 oz. carbon layers, and they took the carbon out past the mat so you could see the stiffness difference. It was VERY stiff, and not all that much heavier. (the literature says it takes 4 oz. per square foot to wet out, and it measures .098 inch when wet) The mat dry is very light. it feels/ looks like a thick stiff paper towel.

I got some to play around with, and it wets out very well with epoxy (didn’t try poly) and once saturated is quite easy to lay up…

I was thinking of using it as a stomp patch, or maybe even the back 2/3 of the board, as a lightweight dent-protection option…It would be over stringerless, light EPS, (maybe even 1 lb.cu.ft?) and go, say, a 6 oz carbon underneath and a 4 oz. s-2 glass on top? I could pigment the resin used to wet the mat out white, and wrap the rails, to be trimmed flush with the bottom, strengthening the hard portion of rail…

The hand layup is key for me and my backyard, I’m not gonna get into vac bagging just yet…

2

Haven’t tried it, but anytime you use carbon, it’s gonna be stiff.

Sandwiching carbon is stiffer…with anything in between

Why not just lam 2 layers of carbon, with cloth direction opposing?

3

FWIW I think a thick stiff paper towel an laminated it with carbon on both sides it would be alot stiffer than just the carbon skins, since it brings the two layers of carbon away from each other. IMHO it’s the thickness between the carbon that makes it stiffer.

regards,

Håvard

4

Yes, that’s kind of what I was thinking…It’s not as thick as divinycell, so even if you made a whole baord out of it I don’t think it would be as stiff…And I was thinking of using it only on the deck. Actually, maybe you could control flex in your board by how much it wrapped over your rails–for instance if it was only on the deck-top by the nose and tail, but wrapped over the rails between your stance…then the tail could flex a little… Also, maybe you could lay up the carbon underneath on a 45 bias, so that the board would resist twisting, and the glass outer layer would provide tensile breaking strength–that way, the carbon would help the denting problem (non-directional force) without adding too much stiffness along the length of the board…and the torsional stability would be a fortuitous by-product…Am i correct in assuming that the inside layer of a sandwich-skin mostly feels compressive forces, rather than tensile? sorry, fuzzy fiziks…

Just brainstorming here, I know I could just do any number of regular options, but I want to try something new/unconventional. This stuff would let me explore skin-sandwich construction without needing a Vbag…Maybe those who do Vbag could give me insight on my hybrid lay-up approach, can you control flex this way?

Thanks, wells

5

Kudos for you for trying, but…

You can’t actually make any judgements, because EVERY board you shape is gonnabe different! Blanks, thickness, flow, stringers, taper, are all variables you could never control, plus, your shaping, or anyone’s, can never duplicate boards. Oh, forgot quality of resin, amount used, temps and humidity at the time, and actual batches of epoxy, glass, carbon, can all be different.

But go for the sandwiching idea, to prevent heel/knee dents. Also, you can sandwich regular cloth, or just two layers of carbon.

6

Hi Wells, If the sandwhich material takes 4 ounces per square foot to wet out, then for a nine foot board which might have a surface area of 28 square feet, then you will be adding about 7 pounds of resin, plus the weight of the cloth, if you use it to cover the whole board. If we assume that the cloth when wetted out is a tenth of an inch thick, then a layer of resin without the cloth will take about 7.5 litres to cover the same area. Since 7.5 litres of resin will weigh more than 17 pounds, the laminate appears to be quite frugal on resin but is also relatively thick for a surfboard at nearly a tenth of an inch. Good if you want a battleship!

7

Actually, what I was thinking, was more like just the deck of a short board, about where the deck patch would go, for a total closer to three square feet or so, weighing about 12 oz. This would hopefully be in place of a regular deck patch arrangement, like a 6 oz. piece that usually stretches from tail to 2/3 to nose, so you could subtract that…

I’m 6’0" 200 lb’s (was 190 before this flat spell hit, $#!&$%!) so deck crushing is a problem for me. Outside the deck-pad, I would have a balanced layup of 2x 4oz, full rail wrap, epoxy over lightweight EPS, no stringer.

I think this stuff would be a little too dense to use as a full board wrap, the pamphlet gave the cured density at 40 or 50 lb’s per cu. ft., about like some dense hardwoods, if I’m not mistaken…the upside would be that you wouldn’t need a boardbag!(grin)

It’s similar to laying down a layer of glass, spreading a thin layer of microbubble paste, then another layer of glass, except it’s stiffer and stronger, because the quilt-holes serve to attach the top skin to the bottom…I’m trying to deviate, so I can make progress…

I wrapped a newspsper round my head,

so id look like I was deep,

I said some mumbo jumbos then,

an told him he was goin to sleep.

I robbed his ring and pocketwatch,

and everything else I found,

I said,

The price of meat has just gone up,

and yo old lady has just gone down…

Look here brother, who you jivin with that cozmik debris?

you could make more money as a butcher, so don’t you waste yo time on me.

Zappa for president.
8

the thinking that 4 oz. per square foot will add up to x amount of poundage is slightly flawed.

fluid ounces are a volumetric measurement, not a weight measurement. a gallon of water weighs what, 6 lb? that’s 144 fluid ounces wheras 6 lbs of weight equals 96 oz. my point is that you need to see what your resin weighs per oz (or gallon) to get an idea of what it’ll weigh in the final layup. it may be lighter, or it could be way too heavy.

9
Quote:

…Am i correct in assuming that the inside layer of a sandwich-skin mostly feels compressive forces, rather than tensile? sorry, fuzzy fiziks…

I’m not sure what you mean by “inside layer of a sandwich-skin”. But for the sake of argument, let us assume you have a standard sandwich type surfboard with an upper skin and a lower skin, and a core. When riding on it, the upper skin is in compression, the lower skin is in tension, and the core is primarily in shear (plus some localized compression). Now let us assume that the upper skin is itself a sandwich (and the core has greater compressive strength than the main core of the board). Then when riding on the board both the upper and lower skins in the deck sandwich are generally in compression overall. However, locally (i.e. in the vicinity of the rider’s feet–and particularily the heel), the loading on the upper skin of the deck sandwich has a localized compressive load in addition to the basic overall compressive load for the upper skin of the board as a whole. The lower skin on the deck sandwich is carrying a localized tensile load combined with the overall compressive loading of the upper skin of the board as a whole. Hence the loading in the lower deck skin changes with position, and while most of the lower skin in the deck sandwich will be in compression, there may be localized areas where the combined stresses may be in tension. The deck sandwich core carries both shear and compressive loadings in localized areas. The deck sandwich should not only help avoid heel depressions, but also significantly increase the resistence to buckling of the upper skin of the board. MTB

10

I just realized it’s also called coremat, and there’s some stuff in the arachives, but my questions still stand…

Wells

11

the stuff you first decribed wells , sounded like this german stuff called spheretex…its a powdery fabric …filled with Q cell looking stuff

works great for bagging as long as you dont apply resin directly to it , using a wet out table , you wet your glass , then carry it to your job…

while being vacumed , resin penetrates the holes and joins the loadbearing skins together through the holes, still keeping the material relatively dry and light…

mtb was basically right about the tension on the inner layer of fabric under the cored skin…

thats why sandwich boards are so hard to snap…

coz when its getting load like it should snap , and one whole side of the board has compression load on it…then the skin on that side of the board will want to bend , as soon as that happens either one of those layers surrounding the skin core will be under tension again…

as a general rule the layer underneath mostly recieves tensile loads,both on the deck and the bottom of your board…

also wells if you can support your rocker …like if you have a sand pit nearby…

you could use divinycell without vacumn…

get a sheet of plastic and bed your board into the sand , so its supported evenly under the whole board, (or at least where you want to put divinycell)…

then glass the back of a sheet of divinycell ,place on your foam blank , where youve shaped it to fit , then lay a sheet of plastic over it and cover it with sand …it gives a real even pressure and is quite heavy…

thats how we do repairs on small jobs where it doesnt warrant using the pump…but i once had to do a whole deck that way in an emergency…

the motor on our vac pump burnt out while it was in use…

our glassing area had a sand floor , so i moved a few glassing stands and it became a temporary sandwich application area…

a couple of customers came in , looking real confused why we were burying a half made board in our glassing bay, you had to be there…

regards

BERT

12

Hello Wells, I’ve not used coremat so forgive my urge to post here which is a result of work pressures finally relenting and i’m on holiday in thailand with plenty of time to browse swaylocks for relaxation.

i did spend a bit of time searching the net on this stuff because i’ve been looking for means of extracting even more stiffness out of the deck/hull non wrapped rail sandwich which i’ve had success with. So i was thinking i could easily produce a sandwich rail with this stuff. The search yielded much contradictory methods of using this stuff. The makers Baltek say that its non woven fibre is bonded like chopped strand with an agent that is dissolved by polyester only. However TAP say it works with either and most of the info comes from model aircraft builders who use only epoxy. However youve already discovered it works with epoxy. The intended use where they quoted 4 oz a square foot seems to be by saturating it and using the balloons to keep the resin ratio down. The aeromodellers seemed to say don’t saturate it, but let resin from the layed up cloth either side of it soak in and set it hard - they are very weight conscious but i wonder if this resin dry technique would be strong enough?

anyway i gave up on this idea and went for wood rail reinforcement instead…

seems like a good idea for your stomp patch reinforcement though and the materials you are pursuing - btw i read with interest your rail channelled blue foam experiment and i think youve cleared up something that had been puzzling me - why is it that some innovative manufacturers have thrown considerable research into this foam and failed yet i seemed to “succeed” at hobbyist level. The answer i think is because normal weight monolithic glass jobs were applied. I’ve got one monolithic xps board, but with complete double s-glass layup on both sides with an extra stomp - making up to 18oz of glass in the tail area and even more on the rails - i lapped every layer (this is significant as i found with the white eps that the hand area caved in with a regular weight glass job). Result is a board that is simply too hard to dent thus i’m not getting any delams. As you can imagine its not light but not as bad as it sounds. I squeegeed out a lot of resin - with the penalty of foaming the epoxy and creating a board riddled with pinholes - but from a water absorbtion point of view that doesn’t seem to matter with hydrophobic xps. This board is a twin fin cruiser so the weight isn’t a big issue, but this method is not going to be an acceptable result for a hp shortboard - or if you take any sort of pride in the appearance

I had a much more superior result with sandwiching it with corecel. i mean superior in terms of strength to weight ratio, there are some issues with its appearance which i’m hoping to partially overcome. But again a board simply too hard to dent, thus no delams. Although this board is dual density and combined with home depot grade EPS on the hull surface. But the deck which takes the rider weight is the xps side. I showed it to the supplier of some of my materials and although i’m sure there was a certain amount of humour the customer in his reaction, he did seem genuinely surprised at how it resisted the thumb pressure test.

on the subject of sandwiches there is a great article here -

http://boatdesign.net/articles/foam-core/

what the boatbuilder calls a sandwich is not the same thing as the use of the term by a surf/sailboard sandwich builder though, which leads some swaylocks posters into confusion when they think that ordinary surfboards are sandwiches (this is technically true but not what is generally known as a surf/sailboard sandwich). A section thru a true sandwich surfboard will in fact reveal a triple stacked sandwich, with the top deck being a glass and pvc/SAN or whatever sandwich, the middle sandwich a glass and eps sandwich and the hull layer being another glas and SAN/pvc sandwich.

so the equation

D = Ebtdd/2 would have to be applied three times i suppose to get the stiffness of a true sandwich surfboard. Also they say that equation only applies to cores of < 5 lb/cu which is about the density of divinycel or corecel i think, so maybe need to use their more complicated equation. I’m no mechanical engineer or physicist so don’t ask my how the boffins came up with those equations, but the effects of those numbers can be easily seen from the simpler equation as explained by boatdesign.net:

“From this equation, it is apparent that the core material does not directly contribute to the stiffness of the panel or beam, (at least in lower density cores) but it’s the distance between the skins that is the overwhelming factor. Increasing the “d” variable will have a much greater effect on the flexural rigidity than any other component in the equation, since every other variable has a linear contribution.”

(i wrote down “d” twice as the puter keyboard doesn’t have the squared character)

So although i had the opportunity to produce a quadruple stacked sandwich when i put corecel over xps over eps over corecel, i probably did the right thing by not wasting glass in between the xps and eps and instead reserved my boardweight glass budget for the 2 1/2" inner sandwich rather than a couple of 1"+ inner sandwiches.

13

From mr. J

I’ve got one monolithic xps board, but with complete double s-glass layup on both sides with an extra stomp - making up to 18oz of glass in the tail area and even more on the rails - i lapped every layer (this is significant as i found with the white eps that the hand area caved in with a regular weight glass job). Result is a board that is simply too hard to dent thus i’m not getting any delams. As you can imagine its not light but not as bad as it sounds.

Ahem…The board with the rail channels had 2x4 oz cloth on bottom, and 3x4 oz on deck with a 6 oz. patch from tail to front foot, so it was (Is) plenty beefy. It took all summer to break down the foam under my back foot, but now it’s delammed all the way across. (keep in mind I’m 200lb’s and I like trying to land big floaters…) That stuff does dent, and if it doesn’t tear right at the surface, eventually, it will deeper down. I think that may be a big draw to the white stuff, is that it will show more resiliency and be able to flex back up with the epoxy.

I squeegeed out a lot of resin - with the penalty of foaming the epoxy and creating a board riddled with pinholes - but from a water absorbtion point of view that doesn’t seem to matter with hydrophobic xps. This board is a twin fin cruiser so the weight isn’t a big issue, but this method is not going to be an acceptable result for a hp shortboard - or if you take any sort of pride in the appearance

That’s kind of spooky, because that’s pretty much what happened to mine in a coulpe of areas, since on the deck i was wetting out a total of 18 oz. in 4 layers, I had some bubbles within the lam…It seemed like you could definitely drag the stuff (epoxy) too hard…even with all that glass, it was a little lighter than a poly shortboard of similar shape.

BTW even though the tail is delammed all the way across, the board is so strong that I have been surfing it no problem…I’m gonna fix it next week, but you can hardly flex the delam up with all your weight…stuff’s tuff. The delam isn’t spreading either, so I did something right…

Regarding sandwich construction, funny how there’s always some semantics problems surrounding this subject…yes, I was in line with your definitions…my proposed project would be a monolithic lam everywhere but the deck, where it would be a sandwich made with carbon on the inner surface next to the foam, then the coremat, then s-2 glass, which would continue past the coremat to wrap the rail.

It definitely works with epoxy. generally, I found best results were to wet it out on a hard surface, scraping the resin into the cloth, then transfer the wet cloth to the surface, which is also presaturated with epoxy. Then it lays right down. You definitely can’t do it like a normal piece of glass, and expect it to soak in by itself.

One neat thing I did was cut a 2" wide strip, dremelled a shallow groove into some foam, wet out the strip, used a scraper to push it into the groove from the center, and flattened out the remaining edges, to form a T-beam. It was pretty strong, too, and the side-flaps laid down onto the foam flat, with just a little encouragement, by hand.

Thanks for your post, it’s quite informative. sounds like we’re on similar paths, trying to use the lightest possible combo for a strong board/human interface.

Wells
14

hey guys check this out…

i guy came in today with a repair, i look at this board as he puts it down and dont give it a second look…i blow some dust off , take off my mask and muffs, then go to quote this repair…

im putting 3 photos in youll be impressed , specially you mr j …

ok so im a bit partial to my own work, but if your not into wood , then this is the best sandwich custom work ive seen …

looks just like a normal board , even the guy who owned it didnt even know it was a sandwich board…

as im inspecting it saying "wow , how well is this board done , this is the best sandwich epoxy work ive seen "

the guy says “is it epoxy?”

i give him the run down on how its built …

he says " i always wondered why it was so light and strong"…

he bought it secondhand…

judging by the weight i would say its sl grade eps core, its got 4oz /epoxy/4mm divinycell /6oz/polyester on the bottom…

cant say for sure but judging on flex its got 6oz/epoxy/4mm divinycell/6x6oz/polyester on the deck…

as you can see in the photos , its been taped up down the middle and sprayed blue , leaving the uncovered divinycell to look like a stringer…

the sandwich wrap is really well done…theyve wrapped the deck first, then vacumed on the bottom to leave a real clean join line…

its got one of those new gortex vents…

i was real impressed with the work on this board …

its made by D S surfboards in taranaki new zealand…shaped by biggie smithers…

i thought it was interesting that the whole out side was done just like a conventional board with polyester…that was probably the only aspect i didnt like other than that …yea good to see great quality sandwich work getting out there…

check pics

regards

BERT

15

Bert,

Have you tried using a barrel roller for wetting out the Spheretex?

16

na i dont use the stuff , wood works fine for me…

i know what your saying tho, you just want the bare minimum amount of resin either side of your sandwich medium…

im always still looking all the time , keep trying new stuff until the next breakthrough…

i havent really found any significant construction improvements for over a decade now…keep trying … but always fall back to what im doing now…

regards

BERT

17

Mark,

I got the impression that working the stuff a little is required to get it evenly and thoroughly wetted-out. Specifically saturating it a little on the heavy side, working it through, then removing excess with a scraper. in the limited testing I did, you could scrape it pretty light, and it would still be strong. Like Bert says, it’s best to use a wet-out surface to work in the resin, then transfer it to the workpiece.

Is a barrel roller the same as a roller paintbrush? if so, it might be ok for putting the resin on initially, but I don’t think it would really work the resin into the microbubble matrix. I used a bondo spreader with good results. The better saturated it is, the more malleable it becomes.

wells

18

Wells,

I havnt used Spheretex myself. I saw some samples laid up last month by a Florida shaper who is experimenting with it’s use. He is tesing the product for the best layup technique. I was not there for the layup because he did that around 1 am and I was with a team rider and a couple girls and we were busy getting thrown out of our hotel pool. But I saw the results the next day and was impressed.

A barrel roller is specifically designed for resin and glass layup. The barrel itself can be either metal or plastic. I’ve used them for laying up fiberglass mat in industrial and automotive applications. The design allows for control over the amt of resin being evenly distributed throughout and through the mat. It’s a grooved roller barrel about 3/4" to 1" in diameter and 4-5" long.

19

yes Wells we do seem to have taken very similar approaches with our blue XPS boards. I should add that I’m a flyweight and used 3.3 lb/cu ft hi density foam for mine. I got it from acp-composites.com who supply mainly aeromodellers. They call it spyder foam and say it has a compression resisting grain, but i’m not sure about that. I used it about 2 - 3 times a week during the summer of its construction and only occassionaly after so i think it hasn’t been absolutely tested yet. However my vacuumed home depot grade XPS/EPS has had much more testing and i’ve got a lot of confidence in it. In comparison i managed to put soft spots in the tail and a number of pressure dents in my polyester board after similar use.

I knew exactly what you meant about the coremat forming a sandwich skin - the balloon filled inner layer forms a low density core and the carbon cloth both sides form the facings.

Perhaps MTB you were not drawing the distinction between what the sailboard builders call a sandwich?

20

I managed to put this pic together on my sister in law’s macintosh while i’m here in Thailand. No waves in Bangkok, but this is where my wife lives and works, so that is a good reason to be here. I was hoping to visit the manufacturer of my wonderful surftech, but the aquaintance of my wife’s friend no longer works for Cobra so unfortunately i’m not going to get the chance. My surftech is a delight to my senses and what provided me with the motivation to attempt the elaborate sandwich procedure at home.

so if the corecel has a density of 5lb/cu ft then following http://www.boatdesign.net recommendations the more complicated equation 1 should be applied to sandwich A and C. Sandwich B consisting of the mixture of 0.9 and 1.8 lb/cu ft can use equation 2.

Equation 1 includes Ebccc/12 in its formula. I’m not really sure about how to calculate stiffness of the entire stack. So I can only attempt to get a vague feel for why the “true” sandwich works so well for surfboards, from just looking at the equations.

D = EI

so calculate D for sandwich A or C.

from

http://darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture28/461_lecture28.html

Then coefficient of momentum

I = b * hhh/12

b = width of sandwich C

h = height of sandwich C

E = D/I

i’m a bit out of my depth here and got a feeling there is a better way, but i’m thinking could mulitply calculation for D by 12/ (b * hhh) to get elastic modulus E of sandwich a. then feed into equation 2 as the elastic modulus of the facings of sandwich B to get a calculation for total board stiffness.

looking at equation 1, E the elastic modulus of the skin cores (1/8" corecel in my boards) does have a power of 3 component. So I guess this is what allows sandwich A and C to get away with such a small core thickness and still be a significant contributor to the overall board stiffness. In practical terms the hi density cores of the skins is what resists the deck dents too.

glass lam 3 (the deck surface) was 6oz s-glass in my construction and all other lams were 4oz. So it would be interesting to be able to calculate whether it is worth it or not to reduce lam 3 to 4oz and putting say a 2oz layer in between the XPS and EPS instead of the lightweight micro-balloon glue. This would turn it into a stack of 4 separate sandwiches. Because the strain (a measurement of elongation under load i believe) would be less the closer we get towards the centre of the stack it would not require so thick a glass as the outer glass lams. As I said before i’ve already used up my weight budget with my present glass schedule- when i add glass somewhere i have to take away from somewhere else.

you can see what we mean by inner lam of sandwich skin in my pic. they are glass lams 1 and 2. the numbers are the order in which i lammed. I think this method is good coz all glass layers tie together at the rails, thus helping to keep glass and skin core together as an integral unit.

anyone out there done this sort of calculation?