Ski profile (across the width)

wannabuild · Post by **wannabuild** » Tue Jun 10, 2008 8:59 am

Has anyone tried to put a shape (in the width) to the skis. I know a little bit about molds. Does a profile in the width create too many problems with creating voids?

alloyguitar · Post by **alloyguitar** » Tue Jun 10, 2008 9:42 am

I'm not really sure that I understand your question...

...profiling the width as in, giving your ski an arch that runs lengthwise?

brianmwaters · Post by **brianmwaters** » Tue Jun 10, 2008 6:28 pm

it's been done before. read here:

http://www.skibuilders.com/phpBB2/viewt ... =honeycomb

Wannabebuilder · Post by **Wannabebuilder** » Tue Jun 10, 2008 10:26 pm

Yeah, don't really follow and you stole my name.

wannabuild · Post by **wannabuild** » Fri Jun 20, 2008 9:06 am

I'm sorry about not being clear. I'll try again. It looks to me as though most if not all of the skis are being built flat (smooth or straight) across the width (on the top) of the ski. My question is a 2 part question. First, is that the case? If the answer to the first is yes, than why?

brianmwaters · Post by **brianmwaters** » Fri Jun 20, 2008 9:18 am

yes, most of the skis here are like that. there's two reasons:

first off, only a few people here actually have the fancy and expensive CNC machines used to machine out a lumpy core like that. so we just make regular flat cores.

secondly, in order to press those skis, you need some kind of fancy-schmancy top mold that fits accurately into recess created by the core profile. fire hose, which is what most of us use, just won't cut it.

that said, i'm sure somebody's done it. a few people have build their own CNC machines, and those people are obviously exceptionally crafty, so it is possible. there's an ancient thread somewhere in here about this.

Cadman · Post by **Cadman** » Mon Jun 30, 2008 8:17 pm

I've done quite a bit of "3d" shaped molds for snowboards and wakeboards. To concur with some of the above posts, flat skis and snowboard are the norm because they are strictly sandwiches. In other words, for the most part, you want a stiffer ski, you just add some 'meat" ( thickness).
With 3d molds the core has to be CNC machined, The tip and tails have to be scaled so they stretch to the correct length when bent, your thickness profile has to be correct within a very tight tolerance or you get pressure problems. If your top has little islands and recesses, the top mold has to match them very closely so you must have a way to align the top and the bottom so it does not shift. If the core shifts on a laminate ski, you can possibly get by using it but on a cap or 3d laminate ski you will probably have pressure problems. If the islands or recesses don't match up, the mold usually stays open and the ski is junk. If the core
is too big, the mold won't close. If the core is too small, you get resin rich areas that make the ski heavy and big bubbles where there is too much space since the top mold normally closes down on the glass and top sheets that come out the side of the mold.

Cap skis are a little bit easier to finish since the only thing you really need to do on the sides is sand it flush with the edges. However, there is a ton
of things that can go wrong when pressing the skis so you have to have that all dialed in to build good skis. I would suspect that most people who are novice ski builders make sidewall sandwich construction because it is easier, less expensive (tooling) and less rejects.

Good tooling is not cheap and cheap tooling is not good!

ben_mtl · Post by **ben_mtl** » Wed Jul 02, 2008 4:21 am

Cadman -> Don't you think more of those concerns could be removed just by vacuum-pressing your skis instead of using a press ?
Sure there are some pros and cons about vacuum vs press but except maybe for the processing time I don't see many major differences.
Pressure is different, OK, but the pressure obtained with a good vacuum system should be enough anyway, epecially if cores and other materials are pre-bent, right ? If too much pressure the fibers would become "dry" in my opinion.
Thanks

knightsofnii · Post by **knightsofnii** » Wed Jul 02, 2008 5:40 pm

with a wide bladder fire hose, a router, and some skill with your hands, it is quite possible. If capped boards can be made flawless this way so can 3d tops.

mpm32 · Post by **mpm32** » Thu Jul 03, 2008 5:08 am

Why not have your set-up in vacuum bag and throw that into a press - using both the benefits of vacuum and a press?

knightsofnii · Post by **knightsofnii** » Thu Jul 03, 2008 8:21 am

press would probably pinch off the vacuum. you'd have to get it sucked down good and tight then load it in and press for like 15-20min with heat and hope the vac film doesnt deform and lose its suction.
plus any ripples/wrinkles in vac film or breather would translate right into the topsheet from the press.

knightsofnii · Post by **knightsofnii** » Thu Jul 03, 2008 8:25 am

you could just build blocks cut to the shape of the 3d parts and place them over the board prior to pressing, no? Kind of ghetto but beats spending 1000 on a 3d mold if you dont know if you have the design nailed.

Cadman · Post by **Cadman** » Thu Jul 03, 2008 8:01 pm

There was no intent to disscourage anyone from using a vacuum bag over a 3d shaped core. However, Vacuum has a limit of what kinds of pressure can be applied so bubbles and resin rich areas are typical problems with vacuum bagging. The curing time is also a factor. If you can control temperatures you will get better results. In most cases, compression molding 3d shapes is more expensive and requires more
precise tooling and process (my main emphasis). Most vacuum bagged skis that I have seen do not have the strength that a good compression molded ski has. Maybe some of the more experienced mold experts can comfirm or argue this point. Raising the temperature of wet layups helps with the crosslinking of the resin from what I have been told by resin experts. This leads to a much stronger , longer lasting ski. Most of us don't have the resources to build expensive tooling. I was lucky to
work for a pretty good tooling company in the 90's so I got to see how
the some of the big boys did it. Now the Chinese are doing it all for us
That is why I don't work for a tooling company any more. They all went
away!

hugocacola · Post by **hugocacola** » Fri Jul 04, 2008 4:56 am

CADMAN for vacumm is more simple using a very thin topsheet, like 0.4mm thickness, it can ajust to the 3d profile...

Craig · Post by **Craig** » Wed Jul 09, 2008 2:49 am

I've been thinking for awhile now about a way to built a top mold for a 3D core for a budget price. I have an idea to do a cheap version of a top mold, and even though i already see some problems here, I think it might be a step towards finding a good solution. I should stress here that I've never tried it, but I think it could work.

The top mold would be made of multiple layers of glass, and given its shape by pressing these layers to the top of your 3D core in a vacuum bag lamination. Basically, you would be building a 3D negative of your ski. I envision using this in a sandwich construction (not cap) where the only shape is needed to press the top fiber layers and topsheet to your 3D core.

I would plane a piece of core to the proper core thickness/impart the 3D shape to it (however you intend to do that), but not cut the sidecut into the core - you basically have a rectangular piece of wood planed to the proper thickness. Whether I would add tip and tail spacers, or actually just plain the wood for the entire length of the ski, I do not know, since structural integrity is irrelevant. I like the idea of using only wood for continuity of the top mold (no crack between materials it can push down into). This 3d piece will constitute the positive.

Next, you would prep the surface to be as smooth as possible, and then apply wood sealers and release agents to the piece (there are many possible materials/methods to do this - it just has to be a way to release the negative mold being laminated from the core model with a smooth finish). This piece would then be situated on top of your bottom camber mold, and then laid up with multiple fiberglass layers. The whole getup would then be vacuum bagged.

I'd hope that the resulting fiberglass piece would then take on the identical form of your 3D shape, and could now be used in a pneumatic press as the top mold (while this method could just be done to laminate your final ski, I have the goal of gaining the structural integrity possible from the higher pressure of a pneumatic press). However, i don't know how well it will work at this point.... questions I have:

What kind of release agent would need be applied to prevent the mold from sticking? Or maybe just some nonadhesive barrier layer?
Will the mold be strong enough to impart structural integrity to the ski?
Will the finish on the ski be decent?
How will this mold hold up to heat/pressure?

I'd like to know if anyone thinks this is a decent idea for a cheap/simple top mold. Please let me know if anyone has tried it, or has any thoughts on ways to improve it. I am traveling this summer, so I won't get to try this until the fall.

Hope this isn't too terrible an idea....

Ski Builders Forum

Ski Builders Forum

Ski profile (across the width)

Ski profile (across the width)