Ski Builders Forum

138-106-122
12oz biax and 4.5oz uni carbon.

Flex feels and skis right. The skis are super light. Like 7lbs per pair.

I do however have some trouble.
The base is really concave, more than ever and the top of the ski is actually convex.
This is weird because the base was over run cheap stuff and 1.5mm thick and my edge was a 1.3mm offset.
I should be able to grind flat, and I thought it was a few days after I pressed, then I skied them and now I look and they are very concave.
I swear it wasn't like that. Best to wait 7 days till you cut and grind?

Heating issue? New double press issue?
Carbon issue?
Really weird.

did i miss something? what's this BTSR everyone's writing about?

BeerToSkiRatio

Concave base has been a little bit of a battle for me too...it seems to decrease the longer (4 hours+) I leave the boards in the press to cool under pressure. I have a sense that if we left the cutting and grinding for a few days post cure it may resolve to a more acceptable range. The grinding for sure heats the base and may be an additional issue. I'm actually thinking that a certain amount of "edge high" may be unavoidable unless I create a slightly convex base mold system to "engineer' the issue out.

Maybe the high pressure is an issue? I don't have that problem with vacuum...

Yeah, the lower pressure makes it less of an issue. Temperature is also critical...I lowered the temp in the press to bring the various materials more into a balance with their shrinkage potential and that helped quite a lot. I.E. , topsheet shrinks 4% base shrinks 10% blah blah blah.

55psi
180F to temp in 45 min, soak for 15 min and shut off heat top and bottom. Maybe I over cooked exacerbating the thermal expansion problem?
I left them in the press over night under pressure to cool.
I have never had it this noticeable with thicker AL and heat only bottom. Used to be 1 hour to temp 180F and 45 min cook to get it hot all the way.
Grinding gets the base and edge slightly warm to touch but not over 100F.

They were flat when I ground them and I swear it happened days later.

I thought we had this licked. Damn.

EDIT this must be the carbon and rate of cooling causing this warpage or post cure issue?

Gonna press another without carbon and see if the same problem happens.
B, should I take the temp down to 165?
My resin says 175.

Since I haven't experienced this, but an observation(my) after what you told us.
1. too much heat compared to recommendations. Try another resin.
2. too little resin/(thin)CF between core and base. I remember you saying it was difficult wetting the CF? Maybe it soaked in during pressure? And "sucked" the base with it?
3. Don't underestimate after-curing. (I don't always follow that rule myself... = delam)

Just my thoughts....

we used extra resin so too little was not a problem, too much if anything.
We always heated to 180 just not on both sides. I will try 165 or so.

... hmmm got me thinking, ... if heat is used to induce camber, does it follow too much heat too quickly will produce a concave base? If so, I have been lucky and avoided this issue because: low watt heater when using 1.97mm cassette and then, 3 mm cassette when using high watt heater.

I think, once you have reached your required Tg, then further change require additional energy, so if changes happen post cure, it points to the required Tg not being met or not being met throughout the ski. If so, it might be worth considering whether using 3mm AL is preferable because it will provide a better heat map during the ramp and soak cycle?

Just thoughts ... adding to what Brazen has said

Could this be an issue with too much epoxy? As in it getting trapped in the base and curing before being able to flow out of the laminate?

Vinman wrote:Could this be an issue with too much epoxy? As in it getting trapped in the base and curing before being able to flow out of the laminate?

There was a bit more than usual on these bases but very thin and not all the way across. Maybe this was it. But I used more epoxy than normal. I would assume this would flow out. I had it under pressure for 15 min before I turned on the heat.
I think the base being thicker than than the step caused an issue as well maybe?

EDIT: I think i got excited with double heater and I was bleeding and drunk and it was late and I cooked them to fast maybe causing the concavity on bottom and convexity on top.

Hahaha, I can't help but relate. The last board I made at a reduced temp was pretty good to the concavity...it was somewhat arbitrary but I set top and bottom at 148f. I have always brought the temp up in <5 minutes, I guess I'll figure out those ramp settings on my PID's now...the slower climb to temp feels right to me.

stop leaving your skis and boards in the press to cool.

There is ZERO benefit to doing so, and the pressure is not allowing things to cool down properly.

I am 95% positive that leaving your boards in the press to cool is causing your problems.

i don't leave my boards in the press to cool either and never had problems with warping whatsoever.