endre,
All good points. Thanks.
G-man
alternatives to a peunamic press
Moderators: Head Monkey, kelvin, bigKam, skidesmond, chrismp
If you make one pair of sidewall skis and a one pair with the exact
same composition, but instead of sidewall a cap. The cap ski will be
stiffer. When you bend a sidewall ski in a turn you will get a smooth
transition where longitudinal and torsional stiffness work together.
The cap ski has a much more abrupt way of dealing with that.
Especially if there is a 3D top that stiffens up the front and rear
parts. What they use caps for are to make skis cheaper by reducing
the fiber content and instead make a cap and a 3D top with ridges and
valleys. Remember form stiffness. Then come up with a catchy name like
X-Frame space bar construction. They often use cheap foam cores sometimes injected right into the ski and prepreg laminates to get the right amount of resin. With good moulds and materials exactly preshaped they can make skis with unskilled workers. It is like painting by numbers.
All this said, of cause it is possible to make good skis with cap but
why bother. High performance skis don't sell in the numbers that mass
market skis do and experts are a lot more demanding. It takes a lot
more engineering to make a fine cap ski.
same composition, but instead of sidewall a cap. The cap ski will be
stiffer. When you bend a sidewall ski in a turn you will get a smooth
transition where longitudinal and torsional stiffness work together.
The cap ski has a much more abrupt way of dealing with that.
Especially if there is a 3D top that stiffens up the front and rear
parts. What they use caps for are to make skis cheaper by reducing
the fiber content and instead make a cap and a 3D top with ridges and
valleys. Remember form stiffness. Then come up with a catchy name like
X-Frame space bar construction. They often use cheap foam cores sometimes injected right into the ski and prepreg laminates to get the right amount of resin. With good moulds and materials exactly preshaped they can make skis with unskilled workers. It is like painting by numbers.
All this said, of cause it is possible to make good skis with cap but
why bother. High performance skis don't sell in the numbers that mass
market skis do and experts are a lot more demanding. It takes a lot
more engineering to make a fine cap ski.
Connection the Top-Bottom
Hi. Just read with interest the whole post and have a new angle on the cap/sandwich debate. To start with I'm making snowboards not skis and have no engineering training or knowledge but I'm a cabinetmaker and have worked with wood for a good few years now.
In a ski/snowboard we are building a beam with the top and bottom flange the fiber glass and the web seperating them the corre material. The core (web) does 2 things. 1 it hold the flanges appart a set distance and 2 links the flanges so that they can't move relative to each other when the beam is flexed.
If the top and bottom are not held apart and fixed to each other then the whole beam idea goes out the window and you have effectively 2 thin layers of glass/resin opereating independantly.
In a sandwich construction the only thing that is stoping the top and bottom reinforcements (fibreglass) from sliding past each other when the board is flexed is the shear strength of the wood core. Not only that it's the shear strength along the grain. Now as anyone who has ever split fire wood knows the shear strength of wood along the grain is pretty poor.
So in a cap construction the glass that wraps the side is also locking the flanges which should give a stiffer ski if all the other propetries are the same.
Further, and here I'm really going on intuition, it should make for longer lasting 'pop'. My guess is that when a snowboard/ski loses it's 'pop' it is the wood core shearing on a fibre level allowing the top/bottom glass layers it slip a tiny amount.
Some one with a bit of experience could perhaps comment about weather cap or sandwich board/ski lose their 'pop' sooner?
I've made both styles in a vac press without an autoclave with no problems or delam. I like the look of the sandwich better but reckon that the cap is a better ride and more robust.
see ya, Chris.
In a ski/snowboard we are building a beam with the top and bottom flange the fiber glass and the web seperating them the corre material. The core (web) does 2 things. 1 it hold the flanges appart a set distance and 2 links the flanges so that they can't move relative to each other when the beam is flexed.
If the top and bottom are not held apart and fixed to each other then the whole beam idea goes out the window and you have effectively 2 thin layers of glass/resin opereating independantly.
In a sandwich construction the only thing that is stoping the top and bottom reinforcements (fibreglass) from sliding past each other when the board is flexed is the shear strength of the wood core. Not only that it's the shear strength along the grain. Now as anyone who has ever split fire wood knows the shear strength of wood along the grain is pretty poor.
So in a cap construction the glass that wraps the side is also locking the flanges which should give a stiffer ski if all the other propetries are the same.
Further, and here I'm really going on intuition, it should make for longer lasting 'pop'. My guess is that when a snowboard/ski loses it's 'pop' it is the wood core shearing on a fibre level allowing the top/bottom glass layers it slip a tiny amount.
Some one with a bit of experience could perhaps comment about weather cap or sandwich board/ski lose their 'pop' sooner?
I've made both styles in a vac press without an autoclave with no problems or delam. I like the look of the sandwich better but reckon that the cap is a better ride and more robust.
see ya, Chris.
wow, so i wish i had noticed this discussion sooner...right up my alley! First, to rehash the "stiffness of cap" topic, one important distinction was overlooked. Vertical laminates are added to cores in certain industries (spar webs in planes) to improve compression strength of lightweight core materials. Even if you have a 13mm core, if the foam has a low compression strength, it can act like 12 or 11mm under enough stress. Cap skis essentially add a spar web to either side of the core, assisting the core in performing its task - to resist compression from the opposing facings. For this reason lighter weight materials can be used in cap skis. If you used 3 or 5 lbs/cu.ft. foams that you find in some AT skis in simple sandwich skis, you run the risk of compressing the core.
As far as why do manufacturers push sandwich skis as the high end and cap as the low end...which type do they sell more of? If they sell 10 times more cheap rental skis, its easier to cover the cost of the cap tooling. Even if they knew cap to be the better method, they would be shooting themselves in the foot to admit it. Imagine if every high end, race stock ski that they sell low quantities of needed its own expensive cap tooling because they admitted that the skis were more durable...they would make much less $. Same goes for the tons of new startups...most likely they all have vertical presses and it is simply easier to get into the business by building only sandwich. With the exposed seam of the topsheet alone, arguing that sandwich is more durable seems absurd. Instead they usually claim "better edge grip". The odd thing about this is that the weakest link in the sandwich ski is the poor bonding sidewall...supposedly supporting the edge better. Both styles have equal overhang when made properly, needed so that after many edge grinds, you arent cutting into the ski. For a sidewall ski controlling the depth of the overhang is simple (with the orientation of the router bit), but for a cap ski it relies on the accurate cutting of the core's shape. For this reason, poorly made cap skis end up with excessive overhangs (maybe due to too high of press pressure and compressable core material) and end up giving the process a bad name...still not hurting the manufacturer that wants you preferring the "high end" sandwich skis.
Clearly I am a fan of cap...but do like the idea of sandwich with bamboo sidewalls (finally a hard sidewall with good bonding). The goal is to eventually have an autoclave to bag in. I am picturing 12" diameter lay flat hose with an industrial strength version of a bag clip at the ends.
G-man: why preform your topsheets as apposed to forming them during the press? I currently preform my topsheets and am hoping to eventually eliminate this process with a female die (like draw forming, but on the ski). The problem I have had with thermoforming the topsheets ahead of time is that UHMW LOVES to shrink after forming. Over the length of a 5ft ski, small shrinkage rates turn into large differences. There are ways to take the shrinkage into consideration ahead of time (add "spacer" layers) but its still tough to predict.
Sorry its so long, I just had three pages worth of discussion that I really wanted to get in on!
As far as why do manufacturers push sandwich skis as the high end and cap as the low end...which type do they sell more of? If they sell 10 times more cheap rental skis, its easier to cover the cost of the cap tooling. Even if they knew cap to be the better method, they would be shooting themselves in the foot to admit it. Imagine if every high end, race stock ski that they sell low quantities of needed its own expensive cap tooling because they admitted that the skis were more durable...they would make much less $. Same goes for the tons of new startups...most likely they all have vertical presses and it is simply easier to get into the business by building only sandwich. With the exposed seam of the topsheet alone, arguing that sandwich is more durable seems absurd. Instead they usually claim "better edge grip". The odd thing about this is that the weakest link in the sandwich ski is the poor bonding sidewall...supposedly supporting the edge better. Both styles have equal overhang when made properly, needed so that after many edge grinds, you arent cutting into the ski. For a sidewall ski controlling the depth of the overhang is simple (with the orientation of the router bit), but for a cap ski it relies on the accurate cutting of the core's shape. For this reason, poorly made cap skis end up with excessive overhangs (maybe due to too high of press pressure and compressable core material) and end up giving the process a bad name...still not hurting the manufacturer that wants you preferring the "high end" sandwich skis.
Clearly I am a fan of cap...but do like the idea of sandwich with bamboo sidewalls (finally a hard sidewall with good bonding). The goal is to eventually have an autoclave to bag in. I am picturing 12" diameter lay flat hose with an industrial strength version of a bag clip at the ends.
G-man: why preform your topsheets as apposed to forming them during the press? I currently preform my topsheets and am hoping to eventually eliminate this process with a female die (like draw forming, but on the ski). The problem I have had with thermoforming the topsheets ahead of time is that UHMW LOVES to shrink after forming. Over the length of a 5ft ski, small shrinkage rates turn into large differences. There are ways to take the shrinkage into consideration ahead of time (add "spacer" layers) but its still tough to predict.
Sorry its so long, I just had three pages worth of discussion that I really wanted to get in on!
I am in the process of building an "autoclave". It is actually going to be just a heated pressure chamber. I am going to have to pull in all my favors from my sheet metal/welding friends to get it finished. I am spec'ing it at 100psi hoping to give myself a FOS of at least 2.
This will be my first snowboard construction. My theory in terms of choosing this route over the traditional press is primarily because of the well defined and uniform pressure distribution. I get a little anal about experimentation and keeping good notes so this will allow me to accurately quantify how much pressure I am pressing with. With this data, I am hoping to get a nicely repeatable process. Furthermore, because a ski is essentially a laminate composite structure, I felt the vacuum/pressure system is favorable primarily because it is designed to remove voids from such composite structures. Can't wait to get this thing up and running!
This will be my first snowboard construction. My theory in terms of choosing this route over the traditional press is primarily because of the well defined and uniform pressure distribution. I get a little anal about experimentation and keeping good notes so this will allow me to accurately quantify how much pressure I am pressing with. With this data, I am hoping to get a nicely repeatable process. Furthermore, because a ski is essentially a laminate composite structure, I felt the vacuum/pressure system is favorable primarily because it is designed to remove voids from such composite structures. Can't wait to get this thing up and running!