Ski Builders Forum

Anyone else having issues with concavity? Can't seem to figure out why mine are coming out with a concave base. Using a pneumatic type press with heat on both sides. The mold appears to be flat. i am rabbiting the core edges for the steel edge to sit flush. Maybe i am not leaving enough depth? i noticed the board is only concave were the edges are. im doing a partial wrap so as soon as the edge ends the base is completely flat. the base material i am using is slightly thicker than the edge step thickness but i don't believe this would do anything. pulling the board out at around 120 F

any ideas?

bobbyrobie wrote:im doing a partial wrap so as soon as the edge ends the base is completely flat.

I think that answers your question right there, doesn't it? If its only bad where the edge is, its the edge pushing it up. Cut out the core a little deeper and see if that helps (do a small test section so you don't press a whole ski/board just to find out its still bad, just make a core section a few inches long)

Have you checked your entire mold with a good accurate metal straight edge to ensure its 100% perfectly flat along the entire length? I had checked my mold quickly for flatness, deemed it good, pressed a ski and ended up with a convex base. Went back and checked again and my mold was just BARELY concave in spots. With a good metal straight edge, you should see no light shining between the mold and the straight edge.

But sounds like you just need to route a little deeper into the core. Try a sample piece

bobbyrobie wrote:the base material i am using is slightly thicker than the edge step thickness but i don't believe this would do anything.

any ideas?

Naw its that problem.

anyone get any change in shape from swapping just from heating bottom to top. pretty sure that would only effect the overall camber of the board, and not effect the edge to edge concavity or or convexity.

bobbyrobie wrote:anyone get any change in shape from swapping just from heating bottom to top. pretty sure that would only effect the overall camber of the board, and not effect the edge to edge concavity or or convexity.

If you have any fibers running anything other than down the length of the ski/board (which presumably you have some at 45deg?) then you bet you can get concavity changes from that. Although the effect is much more pronounced when you have the entire length of the ski (camber change), going across just the width doesn't give much length for it to warp noticably. (concavity change)

How long are you curing it? Try curing longer. If everything gets up to the same temperature and is there for a relatively long period of time, it should cure the same, and cool the same and you won't have any issues.

Is your layup symmetric about the core? In other words the layers are a mirror image on top and bottom of the core. If you're using triax fiberglass, and on the bottom put the longitudinal fibers facing the base, with the diagonal fibers facing the core, you want to do the same thing with the top fiberglass... diagonal fibers facing the core, longitudinal facing the top sheet. You can flip them but you just want to make sure you keep the layers symmetric about the core. This is very important if you are heat curing your skis.

thought i would update everyone. It ended up being the mold. The bottom was not entirely flat, would not recomend using 2x6's for the bottom mold lol. so i took some bondo and flatened it. bases are super flat now. amazing.

Nice work. I had the exact same problem. My mold is made of MDF, and it was just BARELY concave in spots, hard to even tell but enough that it was noticable on the ski base (ended up with slightly convex skis)