alternatives to a peunamic press

Tassie Boy · Post by **Tassie Boy** » Sat Feb 10, 2007 5:32 pm

29.4 PSI = 2 BAR

That is a good idea robogeek.

A ghetto homemade autoclave!

i like it.

mattym · Post by **mattym** » Sat Feb 10, 2007 5:50 pm

haha what?! so i DIDN'T know the answer to my own question...
It IS possible to use more than one vac pump and pull more than 1 bar pressure!?!?! And it IS possible to run more than one pump off a single compressor?! Strange no one gave this a go yet

Robogeek - can you explain a little further how what u were saying with 16" pipe and 30 odd lbs pressure would overcome the 1bar maximum?!

Im thinking plywoods press idea is the ticket - his is cheaper than a vacuum press and also creates more pressure....

RoboGeek · Post by **RoboGeek** » Sat Feb 10, 2007 6:07 pm

ok - can do..
one bar at normal atmospheric pressure equals 14.7 lbs pressure - the weight of the air over our heads. So with a perfect vacuum you'll have around 15psi pressing every inch of your ski.

So if you keep the same vacuum level, but increase the outside pressure to twice atmosphere, you effectively now have 30psi pressing on every square inch

mattym · Post by **mattym** » Sat Feb 10, 2007 6:42 pm

haha right thanks robogeek - really wasn't that hard to work out ey. Just needed a lamans explanation. Sounds like it could work pretty well then - i guess it would just at least give the vacuum a little more cred in terms of the amount of pressure you can create.

G-man · Post by **G-man** » Sat Feb 10, 2007 7:53 pm

I've been giving this route some thought lately myself. Someone correct me if I'm wrong, but I think that if you put your mold assembly into a vacuum bag that is drawn down to 1 BAR, insert the bagged mold into the autoclave set-up, then pressurize the autoclave to 30 psi, you'd end up with a total pressure of 3 BAR, or 45 psi... give or take a little.

I have a vacuum generator set-up on a fairly large volume compressor. I can't see why the compressor couldn't run the vacuum and autoclave at the same time, as long as both systems were relatively leak free.

I was initially thinking of using some large diameter schedule 80 PVC irrigation pipe for an autoclave, but because I'd be using the system mainly for heat forming top sheet caps, I don't think that the PVC could handle the pressure and the heat at the same time. So, I'm not sure what material to use... maybe a piece of steel culvert.. there seems to be a lot of it laying around on some of the ranches around here.

G-man

RoboGeek · Post by **RoboGeek** » Sun Feb 11, 2007 5:46 am

You should be able to run both at once - you can even use the pressure side as a huge air tank for the vacuum side - just run them in series

Only thing is making sure whatever vessel you use can hold the pressures you want. There is piping for high pressure out there, also old boilers might be able to be gutted, depending on their design

plywood · Post by **plywood** » Sun Feb 11, 2007 6:41 am

guys, can you tell me the advantage of an autoclave? well, higher pressure than with vacuum, i got that.
but the advantages for skibuilding?
for making cap constructions? (then; what`s the advantage of a cap construction?)
for some kind of 3D-surfaces on the skis? well, this point i would understand. but at the same time i had to ask: who can build skis with the needed precision for such plays?(since we all are some kind of hobbyists and tinkerers).

or are there any advantages i haven`t seen?

RoboGeek · Post by **RoboGeek** » Sun Feb 11, 2007 10:39 am

I just had an idea for high pressure tanks - used propane tanks - I think they are 100lb ones - cut the tops off 2 and weld in the middle. Just add an access door at one end.

Another thing.. it wouldn't be a bad idea to use 3 - 2 for the actual device and a 3rd for a pre-pressurized reserve tank so you don't have to wait an hour for full pressure to build, using all the compressors air.

I'm not sure of any advantages or disadvantages, other than it could be lighter and cheaper to build. Other than the compressor, you could stand it vertically in a corner when not in use and only take up a small area, Or even use it indoors and store it in a closet or something

It would also be pretty easy to heat up for faster cure times

G-man · Post by **G-man** » Sun Feb 11, 2007 11:21 am

Hi plywood,

The only advantage to using an autoclave for myself, at this point in time, is in making pre-formed top sheet caps. Yes, it's true that I have always posted that I don't like the flex characteristics of a capped ski, and I still don't. But, here's some of my current thinking. Because I use clear top sheets almost exclusively (so that I can more easily monitor whats going on in the top layers of the ski with ongoing use/abuse), I have always noticed the occurrence of little top glass layer fractures as the skis get banged together while skiing. I think that this probably happens on all sandwich skis, but it isn't noticeable with a colored top sheet. It's kind of like a little 'ding' in a surfboard... the resin is inherently brittle and micro-fractures when it sustains an impact. It's probably not a very big deal, but I still don't like seeing them and I think a wrap-over-the-edge top sheet would absorb some of the impact and reduce fracturing. Then there's the sidewall delamination issue. Because the top sheet comes from the factory with a very good surface treatment, theoretically, it should bond well at the joint with the bottom glass layer, thereby reducing delam problems. In order to mitigate the negative flex issues associated with cap construction, I'd just machine away the sidewall fiberglass through the waist of the ski, much the same that Rossi does on many of their skis. Of course, this would require that the waist area have plastic sidewall attached to the core so as to prevent moisture infiltration into the core. Those are the advantages to a 'modified' cap construction, as I see them. I don't know if I'll really ever actually pursue it though... only so much time in a day... only so much time in a life.

I do think that it can be relatively easy and inexpensive to build accurate top molds for pressing cap skis. Much of the auto industry uses tooling resins (that are poured) to make dies for stamping sheet metal body parts. I see no reason that this same process couldn't be used for ski molds. The resins have a very high working temperature range.

The real advantage of cap construction (to production manufacturers) is durability (resists delam) and rapid replication of a particular design. Cap construction doesn't need an attached sidewall (except in cases like Rossi's approach) so it saves a lot of production time/cost. Cap molds can lock you into a specific design for a season or two, just because they can be time consuming and expensive to make... so, they aren't very practical if you want to be consistently varying your designs/dimensions. I think this is why were seeing more production builders going to sandwich construction.

I think that an autoclave set-up might be considered a bit safer than an open pneumatic press design (if the autoclave is properly constructed)... less potentially flying parts (except for the autoclave door!?). Also, overall operating pressures would be somewhat less. RoboGeek's last post caused me remember that I have a couple of old water system pressure tanks laying around (rated for over 100psi and 22 inches in diameter) that I could weld together to make a fine lookin' autoclave... or mini-submarine.

G-man

endre · Post by **endre** » Sun Feb 11, 2007 11:22 pm

G-man, you'r so way off I don't know where to start

-do you have any logical explonations for your theories about why a capped ski should have different flex characteristics from a sandwich ski? I have done some work on this subject, but have not found any coherence. (And to be honest, why should it be? The only difference is the sidewall!)
-You have a piont with the delam issues of sandwich, but the downsides to a ski without real support on the edge is a worse problem. if you hit a rail/rock etc. with a capped ski, there is nothing but glass and topsheet to support it.
-the ski industry would deffinately choose capped skis if they could choose, they all say so. The reason they don't is because we don't like capped skis. People think they look cheap! (which is probably why some tend to complain about their flex characteristics)
-I think you have a good point with the cheaper capped molds, you should try it out. (But hearing this from you was a surprise!

Good luck on your edgeless skis, sounds like a good idea!

G-man · Post by **G-man** » Mon Feb 12, 2007 1:44 am

Hey endre,

You know I like it when you call me on this stuff, but man, but I'm not sure how well I'll do with trying to be logical. I'll give it my best shot. Just remember I'm not even close to being an engineer, so I just have to stumble my way along through much of this stuff.

When a core is completely wrapped in a laminated composite, the composite matrix forms a rectangle... thinner and wider at the tips and tails, and narrower and taller at the waist. As this rectangle becomes narrower and taller, it becomes more and more resistant to deflection through the vertical axis. Much of this is due simply to the fact that the bottom and top layers of fiberglass are moving increasingly farther apart and that the core itself is increasing in vertical cross-section, but it seems to me that it is also somewhat related to the fact that vertical surface of sidewall fiberglass is at the same time increasing in its vertical dimension. If we take a wooden yardstick (or meter stick) and place it flat upon a table top with the tip protruding over the edge a bit, push down on the center of the stick with one hand, then grasp that tip and lift upward with the other hand, the stick flexes upward rather easily. But if we turn the stick 90 degrees onto it's edge and lift up on the tip, it is quite resistant to flexing. I think that the glass layer that drapes over the sidewall at the waist of the ski responds to flexing forces in a similar manner. Look at the Goode 'carbon core' skis... theoretically, they are constructed with numerous vertical 'webs ' of carbon fiber sandwiched between corresponding vertical sections of foam. The company's claim is that these vertical webs contribute significantly to the ski it's stiffness.

Subjectively, when I do the hands-on flex arc testing at the ski shops, it always seems that the cap skis are much flatter through the waist than are the sandwich skis... in general. I recently found a Rossignol patent where Rossi explained in depth how the fiberglass wrap of the capped ski interfered with the smooth, round flex of the ski, so their new patent design had some strange stepped down section through the binding area that was supposed to mitigate the problem. As I described before, Rossi has, on some models, simply machined away the sidewall fiberglass through the waist area which I have assumed was an effort to reduce the stiffness though that section of the ski. In order to protect the core from moisture, they have had to add polymer sidewalls under the cap material. That seems like a lot of expense to go to unless they felt that it was definitely worth it to get rid of that sidewall fiberglass.

Ah, found it:

USPAT6637766 B2 from Oct. 28, 2003... states under "Detailed Descriptions of the Invention"... "The decrease in the rigidity of the center part of the ski is achieved by local elimination of the lengthwise reinforcing element." Then, there is an illustration (figure 11 in the patent document) of the technique that I describe above where they machine away the sidewall fiberglass at the waist of the ski.

Anyway, I know that you have done a boatload of work flex testing skis and I definitely value the information the you have assembled from those studies. From an engineering perspective, maybe the vertical layer of sidewall fiberglass is not considered to be a factor in the stiffness of the ski. And in controlled laboratory testing, cap skis may not show any different flex characteristics than sandwich skis. At the same time, various factions of the ski industry have, from time to time, certainly promoted the 'torsion box construction' as a major positive influence in increasing 'beneficial' stiffness.

Jeez... it's 1:30 am... this is all going to sound so dumb when I read it in the morning.

G-man

Lennart · Post by **Lennart** » Mon Feb 12, 2007 3:21 am

Endre I have to go with G-man on this one. The reason a capped ski is
stiffer under foot is form stiffness. A curved surface is stiffer than
a straight one. Compare it with a boat hull. A straight sided one is
much more flimsy then a curved one if the dimension is the same. A ski
is even more complicated while the thickness increases from both ends
to the middle causing the stiffness of a cap ski to increase at a faster
rate then in a sandwich ski. Have you any idea if the skis you have
measured are true cap skis or merely cosmetic caps where the top layer
of fiber don't go down to meet the bottom layer over the edges.
All racing skis for speed events are sandwich skis. In slalom and GS
there are exceptions. Especially in slalom a cap skis firm mid section
combined with a short length could be beneficial when skied on hard
surface. In that case you really have to dig the edges into the ice.
So if they flex nicely doesn't matter much.

mattym · Post by **mattym** » Mon Feb 12, 2007 9:19 am

Ok wow, this has kind of confused me. I have been working in ski shops for a few years now, and I have always been told/under the impression that skis with full sidewall were more durable, stiffer, but yet slightly heavier and more expensive to produce. Cap construction has always been associated with being a cheaper, less durable method of production. I agree with G-man and endre to some extent. I think delam issues are just as much a problem with both construction techniques- just in different ways.

Take for example the Dynastar legend series. The pro rider and pro rider XXL use full length vertical sidewall - and these are their burliest, heaviest, stiffest skis (ie. designed to be the most durable too). The 8800 is designed with this Autodrive SD technology - so there is a vertical sidewall underfoot, and capped in the tips and tails. This is to give a stiffer flex underfoot, and softer in the tips and tails for easier turn initiation!?!? Also, cap skis are generally lighter and cheaper - period. Manufacturers can produce high quality ones yes - but they are surely generally less durable (not saying that cheaper construction is necessarily less durable but it seems to be in this case). Take the pontoon as another example - made with cap construction (torsion box) to make it lighter weight, and as it is a 99% powder ski - there is no need for the supposed enhanced durability of a sidewall.

Can someone point me in the right direction here? Haha im being told two completely opposite things?!

endre · Post by **endre** » Mon Feb 12, 2007 10:46 am

Yo G

When a core is completely wrapped in a laminated composite, the composite matrix forms a rectangle... thinner and wider at the tips and tails, and narrower and taller at the waist. As this rectangle becomes narrower and taller, it becomes more and more resistant to deflection through the vertical axis. Much of this is due simply to the fact that the bottom and top layers of fiberglass are moving increasingly farther apart and that the core itself is increasing in vertical cross-section,...

this is a very good description.

If we take a wooden yardstick (or meter stick) and place it flat upon a table top with the tip protruding over the edge a bit, push down on the center of the stick with one hand, then grasp that tip and lift upward with the other hand, the stick flexes upward rather easily. But if we turn the stick 90 degrees onto it's edge and lift up on the tip, it is quite resistant to flexing. I think that the glass layer that drapes over the sidewall at the waist of the ski responds to flexing forces in a similar manner.

this is correct, but at the same time where something goes wrong.
your analogy with the yardstick is logical for the yardstick alone, not for fibreglass around a core. The whole piont with the core (as you so well describe it yourself) is to build distance between the top- and bottom layer of strengthening material (most often fibreglass) The glass sidewall is placed between the two sheets, closer to the neutral axis of the ski. This makes the glass in the sidewall less exposed to the forces in a ski.

A vertical rib contributes conciderable less to the overall stiffness than the same amount of glass on each side of the core.

And if this does make the ski a bit stiffer in the middle, it is very easy to compensate for this by making the core slightly thinner in the middle. -So what's the problem?

Look at the Goode 'carbon core' skis... theoretically, they are constructed with numerous vertical 'webs ' of carbon fiber sandwiched between corresponding vertical sections of foam. The company's claim is that these vertical webs contribute significantly to the ski it's stiffness.

these ribs keep the top- and bottom layer together, the biggest problem in a carbon-foam construction, where the differences in material stiffness is huge. Dave Goode has got no problem making his skis stiff enough (he is using carbon fibre!), his problem is making them soft enough! That is why he puts most of the fibres in the 45deg. direction, this makes way too tortionally stiff skis. (about twice as stiff as normal skis).

Subjectively, when I do the hands-on flex arc testing at the ski shops, it always seems that the cap skis are much flatter through the waist than are the sandwich skis... in general.

Take a look at the curves in the PDF: http://www.endrehals.no/Flexcurves%202007.pdf, the stiffest mid skis are all sandwich, mostly because these skis are stiffer in general, of several reasons. Visualize all the Salomon curves, the Atomic Big Daddy. As you know, all these skis are cap construction. Then visualize all the Armada skis, The Kneissl, The Atomic Thug. All these are sandwich skis. These are just some examples that your theory has no coherence with reality. All these capped skis are softer in the middle than the sandwich ones.

(K2 sandwich skis have vertical walls of fibreglass behind the sidewall, in case you use these ones for comparison.)

From an engineering perspective, maybe the vertical layer of sidewall fiberglass is not considered to be a factor in the stiffness of the ski.

It is definately a factor, but it is so easy to compensate for that it is not really a problem (make the core 1/10 of a mm thinner)

That thing about boat hulls I am not going to start on right now Lennart, this post is too long allready!

G-man · Post by **G-man** » Mon Feb 12, 2007 11:28 am

Okay, it's morning and, hopefully, I'm thinking more clearly... although that may not mean much. Most of my perceptions are based in intuition and not a formal education in physics, which, of course, makes them pretty worthless from the start. But one has to work with what one has available.

Regarding stiffness of a ski, I think it is most effected by the cross-sectional thickness of the core itself, and by the distance that the top and bottom layers of composite are held apart, again a function of the thickness of the core. As an example, if you take two sheets of thin card stock ( by themselves pretty flimsy), and glue/laminate them with a 1/4 inch (or 6.5mm) layer of light foam board in between (also flimsy), you end up with a quite stiff structure. The thinner the foam layer is, the more flexible the structure will be, and the thicker the foam layer, the stiffer the structure will be. At the same time, if a ski builder adds foam supported longitudinal ridges in a ski's fiberglass top surface, this also stiffens the the ski flex in the area of the ridges (I've seen this on many Atomic foam core skis). Whenever I pick up a ski that has dual ridge formations emanating out from the flat of the binding area and gradually fading into the tip and tail curves, I know that this ski is going to have a stiff tip and tail flex... and it pretty much always does. This increased stiffness results from the change in the direction (horizontal to vertical) of the composite layer... the same basic structural change the we see with the glass layer wrapping over the sidewall in cap construction. It's the same principle that is used when stamping stiffening ridges into sheet metal automobile parts in order to give them form and to provide controlled resistance to crumpling under impact.

An engineering friend of mine often reminds that the only purpose of the vertical member/web of an I-beam is to hold the top and bottom horizontal load bearing members a particular distance apart. Then I always have to add that, if the top and bottom horizontal members were removed, and the vertical member was stabilized in such a manner as to prevent it's deflection, it could still support a pretty substantial load. He usually just looks disgusted and grunts. I guess that's the difference between formal education and intuation.

Lennart, your comment regarding true cap versus cosmetic cap is interesting. I had wondered if some builders used cap construction without draping the glass layer over the sidewall area. It sounds that you are confirming that it is sometimes done.

mattym, when you use the term 'full sidewall', is that synonymous with sandwich construction? If so, I agree that sidewall construction is a much more involved process than cap, once the cap molds have been created. In terms of durability, I guess that it mostly depends on how well one builder vs. the next is able to surface treat the UHMW (or other polymer) components to get them to bond well. My sandwich skis sure are not very durable.

Regarding weight differences , I think it often depends on various factors. If a set of cap molds are not built very well, then a greater amount of resin may be retained in the ski structure, making it heavier. If a builder gets his (/her) sandwich tolerances refined well and uses a very thin sidewall strip, then that ski can actually be lighter than a cap ski because of the sandwich ability to displace more unnecessary resin. I think that you are correct that delam issues may be common with both methods... I had a pair of production skis delam after two outings last year. I think that the stiffness of cap ski is primarily a result of overall thickness of the core (profile) and varied use of composite types and weights. I think I'll bow out of any more discussion regarding stiffness of cap skis

. Lastly, my thoughts are in no way meant to encourage or point anyone in what I might think is a 'right direction'. I, too, am just tossing around ideas, trying to learn what can from folks who know more than I do.

G-man

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alternatives to a peunamic press