Increase in camber of the ski after getting out of the press.

[antoxasam]

I ask for advice from knowledgeable people or who have come across a similar one. I searched the forum for information, but did not find an answer. Perhaps I was looking badly, I don’t exclude
I glued the ski under a pneumatic press (I use a single ski press). The camber of my mold was 7 mm. When I took the ski out of the press, its camber became about 12 mm. Ash core, liquid plastic sidewalls, ash veneer as top sheet. I glued with silicone heaters in the following mode: after clamping the ski in the press, I increased the temperature from 30C(80F) to 80C(176F) for 25 minutes (that is, I added 2C degrees per minute). Then left for one hour at a temperature of 80C(176F) under preassure 4 atm. After that, I turned off the heating and left the ski to cool down clamped in the press for 24 hours.
Before that, I glued the ski in the same mode without heating(4 atm, 24 hours clamped press) and instead of ash veneer I used Isosport nylon topsheet. Then the camber corresponded to the height of the mold and there was no such bend. If I understand correctly, the reason is the different thermal expansion of the materials. But how to control this process, I do not understand. I really hope for your advice.

[chrismp]

This is a tough one to answer as this could have many reasons. First off, did you use the same fibers on top and bottom of the core?

Another issue that could be causing this might be uneven heating - i.e. the top and bottom heat blanket do not heat at the same rate. This again can have many causes. Sometimes one heat blanket heats faster than the other even though they have the same specs. Another thing you should be careful with is the placement of your thermocouples - for example placing the thermocouples on the edges of your aluminum sheets will give you much lower readings than placing them directly where the ski sits. One more issue could be a faulty thermocouple or PID controller that gives you incorrect readings - test your thermocouples and PID against another reference thermometer by measuring the temperature of water at different temperatures up to boiling.

For a good guide on which thermocouples to use and where to place them, check out HeadMonkey's wiki here: http://www.happymonkeysnowboards.com/Mo ... rmocouples

[mammuth]

You stay with one process (hot or cold cure) and dial in your workflow and molds (more or less pre camber). You need to do the same heat & cool down process (including time of pull) every time for heated systems.

[mammuth]

i add to Chrismp post: You need to calibrate both your pids with a proper setup, put a old ski/board inside the press and run the calibration. Main reason is the asymetric heat sink of a press and its molds. E.g. on bottom you have just the mold, on top you have the cattrack which reacts completely different thermal wise. My press is logging the complete cycle and is displaying the programmed and real heat curves, so i could see this effects quite nice. If not calibrated the pids are doing different overshoots when the max temp is reached, same for heat up and cool down.

[antoxasam]

Thank you for your responses. Today I did some tests and it turned out that the thermostat of one of the heaters is showing the wrong temperature. The difference between the bottom and top heaters is about 10C. I will continue to test.

[Idris]

Thank you for your responses. Today I did some tests and it turned out that the thermostat of one of the heaters is showing the wrong temperature. The difference between the bottom and top heaters is about 10C. I will continue to test.

Bingo - this is the reason for your camber change!
Many moons ago I played with using differential heat to change a skis camber - result unreliable and ski is potentialy variable camber due to temperature of use!...

[antoxasam]

I ran several tests and achieved a certain result. Let me remind you that my bottom mold has a 7mm cam. I will describe the process: I put the ski in the press (pressure 3 atm), within 20 minutes I raise the temperature from 22C (71F) to 80C (176F), hold 80C(176F) for one hour, then turn off the heating and cool it to 30C for 4 hours. Then I take the ski out of the press. As a result, after getting out of the press, the ski has a camber of 4.5 mm (at 7 mm on the mold). After 2 weeks, the camber dropped to 3 mm. What do you think, if get the ski out of the press earlier (or later), how will this affect the camber's height?

[mattman]

Cooling in the press doesn’t necessarily mean it cools evenly. The mold has a lot of thermal mass. We always pull our skis hot and immediately “tune” the flex as they cool. Got the idea from a Volkl video and it works great. Once it holds the desired camber without springback, the pair is clamped back-to-back with a spacer block to finish cooling and curing for ~24hrs.
I am sure the same result could be had other ways, like with an extra-cambered mold, but I like that the tuning method removes guesswork.

[rushskis]

Cooling in the press won't necessarily make much difference. I use one heat blanket and if I place it on top my camber comes out exactly like the mold or slightly flat. If I place it on bottom it always comes out 10-15mm higher than the mold. I'm no materials engineer so not exactly sure how all the different materials are changing shape with heat and cooling, but my guess is the epoxy is curing faster where it is closer to the heat and this locks in the base shape, but still allows the upper layers to expand a little more before they set up, causing the increased bow.