production builders and sandwich construction

[G-man]

I just returned from a few days visit to the urban world... pretty different. One thing that they do have, though, is ski stores. So, I got to handle lots of skis that I usually only just get to read about. One thing that I was quite surprised about was that so many production companies are making sandwich skis... or at least dabbling in it. They must be finally giving into the fact that it is so much easier to get a rounder flexing, nicer riding ski with sandwich construction as compared to cap construction. Some of the skis did, indeed, have a very nice flex curve. The ski makers must have also worked out some of the problematic issues with bonding UHMW, since all of them seemed to be using it as a sidewall material.

K2, in particular, had a number of sandwich options. Some of them, upon close inspection, had no visible sign of a glass layer between the top sheet and the sidewall material... just a very tight bond line between the two plastics. However, a slight tri-axial composite pattern was visible in the top sheet, so, it seems that they must be using a pre-preg that is very precisely die cut just a tad smaller than the finished top surface dimensions. Another ski (a capped model) had some sort of rubber top sheet.

The big ski companies have access to polymer engineers and bond specialists who have all of the latest and greatest info regarding materials and production techniques. It'll be interesting to see if, in the long run, they can maintain good durability. Time will tell. I sure saw some nice skis... all made in China.

G-man

[plywood]

just something about the bonding of UHMW:

i worked a few times for a friend who`s machining plastics. as i was on my first ski i asked him if he had something similar to ABS, but better bondable. so he told me that all plastics which have good gliding abilities/smooth surfaces are hard to bond.
but a friend of him invented a technique to treat plastics that they bond. something with plasma, i don`t know exactely. by this method it was also possible to bond teflon.
so there are sure ways to make everything bond well

i also recognized what you were describing: on the rossignol scratch it also seems as if they didn`t use any layer on top of the core. and i wondered how they did it! so your explanation with the die-cut prepreg would be very logical!

[kwetsor]

That's true ! Material engineering and surface technology are obviously one of the main key for good and durable skis ... and also a nightmare for "backyard" skibuilders.
As far as I know, the treatment of plastics for bonding involves typically industrial processes such plasma, corona, flame or chemical treatments.
Not so easy to achieve for an amateur.

[kwetsor]

Well, need three more posts to add an URL...
I have never try flame treatement but it seems that the repeatability and real quality of the treatement could be discussed.
To get good bonding properties with thermoset resins, you need good roughness, wetting property and very clean surface. There is a lot of posts here about dynes and energy of surface wich is related to the "wettability".
...

[kwetsor]

Two posts more ...
The problem with plastics such as Teflon, PE, ABS, PVC, ... and other thermoplastics is the relative high chemical resistance and stability wich is totally unwanted for thermoset bonding (Epoxy, ...). All kind of treatement are about breaking superficially some chemical/moleculare bonds. So new chemical bonds are available to get better chemical compatibility with epoxy, ...

[kwetsor]

"Ten posts" rule against spam is boring !

Maybe I can tell more about chemical treatement with concentrated sulfuric acid wich is typicaly used for neopren or rubber bonding. Interesting buy maybe dangerous for children
This need a chemical resistant tanks with high concentraded acid (60 to 75%). Additional tank with light concentrated caustic soda for neutralising and water for rinsing.
Not as easy that flame treatement but with good quality.
But still need to be improved.

[kwetsor]

So finally. Please take a look at http://www.loctiteaero.com/DownLoads/su ... _guide.pdf
This could be very helpfull for everyone.

I have try this chemical treatement on PVC, ABS and PE, with typical bonding with fiberglass+epoxy laminate => peel test.
No scientific results (yet), but all tests where cohesive failure. This meens that some epoxy remains evenly on the plastic surface.

I will try to do some more tests and post some pictures and results.

A+

[RoboGeek]

you should be able to post links right away now - now that we have moderators to control spam you should be able to post anything first post

I noticed on some teflon impregnated plastics, sodium-ammonia etching was used to get adherance.

[G-man]

Welcome kwetsor and thanks for your perspectives and the PDF. Lots of good info there. I look forward to your test results and pictures.

G-man

[skicore]

K2, in particular, had a number of sandwich options. Some of them, upon close inspection, had no visible sign of a glass layer between the top sheet and the sidewall material... just a very tight bond line between the two plastics. However, a slight tri-axial composite pattern was visible in the top sheet, so, it seems that they must be using a pre-preg that is very precisely die cut just a tad smaller than the finished top surface dimensions. Another ski (a capped model) had some sort of rubber top sheet.
G-man

I don't know which K2 ski you looked at but, K2 has a proprietary system that wraps their cores in fiberglass. Its called triaxial braiding. If you go to their website and click on the "factory team" skis link and then on the "ski technology" link on the right you will see a link for traixial braiding. So even though they look like a sandwich construcion on the outside, they actually have an internal torsion box (cap) construction. They call it a "torsion box sidewall" construction. So you would not see the fiberglass on the top of the sidewall b/c it is between the sidewall and the core.

Pretty interesting........ and yes they must have that UHMW bond to topsheet dialed.

[rockaukum]

Speaking of new skis.. I looked at the G3 Reverand. Have you seen the new top sheet they are using? It is very interesting in that the surface is textured and seems to resist scratching. To rich for my blood (aprox $680).
Another note. I stopped by the local TAP plastic store and inquired about special bonding practices for UHMWPE, flame trating ect., The top dog (owner, I think) said you can not bond this material at all.
rockaukum

[G-man]

skicore,

Thanks for that info tip. It makes more sense than my pre-preg idea. I'll check out the website.

rockaukum,

The owner of the TAP plastics store that I visited and that I asked the same question to regarding UHMW bonding gave me the same answer. No one in the store had ever heard of flame treating. I think they thought I was goofy.

G-man

[rockaukum]

G-Man,
Same thing here. He looked at me after I said, "it is done all the time", with the same "goofy" expression. All I could do was say that I will be bonding it soon and left his store.
Enjoy the new snow!
rockaukum

[G-man]

I checked out the K2 site. The site info states that all of their sidewalls are ABS. When I examined the skis in the store, the sidewalls didn't really look like generic ABS (they may well have been, though), so they might have been one of the 'engineering grades' of ABS. ABS is available in many different grades. Engineering grade looks and behaves much like UHMW. However, engineering grades are very expensive, starting at about $60 per square foot (1/2inch thick). The cost for my 3 foot X 2 foot piece was $450. I got it because I had heard so much about how much better ABS bonded than UHMW. But, I couldn't get it to bond at all. Obviously, there is a way to do it, I just haven't figured it out yet. Flame treating seemed to not work at all. I also tried 'general grade' (about $15 per sq.ft.) material with no better results.

G-man

[mattman]

g-man, i finally was able to get ABS to adhere. What i am working with is thermoforming grade ABS in .030" thick sheets with haircell on side. After searching around and requesting samples, i found some Loctite and Devcon two part epoxies that adhere well. you have to make sure its semi flexable, this is why our resins dont seem to cut it (thats what the technicians told me atleast). i have all the different particular products that work well on ABS listed somewhere, but if you go on either of the companies' websites they have tech sheets that list adhesion strength and durometer for ABS adhesion. i tried solvent welding the sheets on with MEK, and that work descent, but not nearly as good as the treated UHMW we are accustomed to.