Springback compensation

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mammuth
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Springback compensation

Post by mammuth »

Some of you say that its necessary to overengineer the mold to compensate springback of tip/tail/camber.

On the other hand some friends say its not necessary on theirs setup, they build with carbon in a pneumatic press with reasonable heat... !? Maybe they build flat(ter) skis ;)

I know it depends on wood, tipspacer, symetrical layup and so on.

My Qs:

1. Is there a difference in springback between vacuum and pneumatic presses?

2. I assume there is a difference if you use heating mats or if u use a ghetto box with vacuum for heating because on the box you start heating the top of the ski till everything is on temp.

I have to redo one mold and build another one and would like to use them for the pneumatic press which i intend to build in summer
Tom

mammuth
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Post by mammuth »

Nobody? :(
Tom

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MontuckyMadman
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Post by MontuckyMadman »

Carbon will help hold the tip rise and camber but there are a crazy amounts of varibles here. Resin to fabric ratio, quality of resin and glass, compsite weights, type of core material and thicknesses in the tips as well as undefoot, length of tipspacer and type, pressure applied and actual pressure at laminate surface, deflection of press. In order to get close to your target I would over build your camber by 5mm and tip and tail rise by 1 cm to start. Its virtually impossible to speculate but you are going to want to add heat during the press cycle in order to get some semblence of consistancy.
Good luck.

mammuth
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Post by mammuth »

Thanks Montucky!

So generally you can say that glassfibre has more springback then carbon.

That was what i assumed

On my vacuum setup with my core & layup i got 2.5cm less at tip and 1.5cm less at tail (big rocker front & rear, ash) and 2-4mm less at camber (different core thickness).

If i redo the mold for compensating this ... would it be the same if i switch to pneumatic later (assuming i have 0 deflection on frame) or will there be a great difference? (i dont care for a few mm's).

I know this Q is little bit like asking for the weather in 1 month ;)
Tom

Richuk
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Post by Richuk »

Before you do anything, take it outside in the snow and confirm the size of the error. You will need to let the ski assume the ambient temperature before taking a measurement. Overnight would be good.

Some of this maybe process, but its due in large part to the relative coefficient of thermal expansion of the material involved.

Factors that muddy the water are

- Fibre used
- Fibre orientation and or misalignment
- Symmetry or a-symmetry of the layup
- Moisture content of the core
- Heat

Things you can discount:

- Fibre content
- Void content
- Pressure

Things you can really discount:

- Thickness of the wood core, especially the relative importance of the thickness at the tips
- Length of the tipspacers
- Type of pressure applied
- Pressure at the laminate surface
- Press deflection

That said, the fix is to design the mold to fix the spring back/forward. Perhaps you could consider using this mold as a test bed for different materials and layups? If you have a heat mat, it is worth using it with a view to getting a close a possible to press conditions. With this experience in hand, you should be in a position to coming close to fixing the issue, with a view to shimming the mold from below in the press. First pressing, define the thickness of the laminate and shim the press as required - a matter of fine tuning )
Last edited by Richuk on Fri Feb 27, 2015 7:23 am, edited 1 time in total.

MadRussian
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Post by MadRussian »

Some of you say that its necessary to overengineer the mold to compensate springback of tip/tail/camber.
I think everybody have a different experience. I have consistently same results. Skis came out exactly as a mold.
Different wood, different time of the year, different moisture content, different shape of the core and it thickness, different size of tip spacers including wood tip spacers, different pressure, different temperature, different time in the press. Every time same results. if anything in skis with flat tail compared to tail rocker creating I'm getting a few mm of increased camber

IMO only one way to find out
I have not failed. I've just found 10,000 ways that won't work.
Thomas A. Edison

gozaimaas
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Post by gozaimaas »

My experience is a thicker or stiffer core will try and stay flat more than a thinner one, makes sense really.

Richuk
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Post by Richuk »

The terminology I've added should help narrow the issue and enable a bit of reading and such like. CTE of carbon and glass fibre, spring-back/forward are all discussed on the web, as are the do's and don't for finding a solution. The trick is to ignore the maths, pick-up on the main points and reach some understanding of the relative importance of each point made within an article. So in this instance, moisture content is not the most important issue, although it will effect how the resin behaves and creep up the ranking, once the ski is used.

I will, per your gentle suggestion, trim my sails )

Goz - thanks I will edit, it was a late post.

The relative mechanical properties of the core material are important, but I'm not clear what core materials you are talking about. I can say, a wood core at 80oC a steaming piece of wood, add a bit more hot water and you could bend it and set the camber and tips, just like they used to make wooden skis. You will have noticed that the tips of the old wooden skis haven't lost there shape?

You could argue that the dominance of the core is clearly observed when a ski starts to lose camber - but even then the fibre continues to dominate the shape of the ski. Discounting the mechanical properties of the laminate, you are left questioning the quality of the laminate and the bond achieved between the face of the laminate and the core.
Last edited by Richuk on Fri Feb 27, 2015 7:53 am, edited 3 times in total.

mammuth
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Post by mammuth »

Thanks for Infos guys!

I already did ski boards out of this mold, so im quite confident that the boards hold their shape in use.

The thickness of the core makes a difference in springback. This was clearly to see for me, thicker core makes less chamber and less rocker.

Changes between the boards was just core profile. The sandwich, tip spacers and so on stayed the same, so it was a good comparsion. Well i changed the epoxy, but imho the strength of the laminate comes mostly from the fibres.

Will do a new mold this WE with some overengineering so i will see soon the outcome.
Tom

Richuk
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Post by Richuk »

Last one:

http://ntrs.nasa.gov/archive/nasa/casi. ... 023033.pdf

http://www.iccm-central.org/Proceedings ... ap1205.pdf


1. Is there a difference in springback between vacuum and pneumatic presses?

The increased pressure is likely to cause only a marginal difference in springback. The effect is likely to show up down the line, once the ski begins to show signs of fatigue.

2. I assume there is a difference if you use heating mats or if u use a ghetto box with vacuum for heating because on the box you start heating the top of the ski till everything is on temp.

The end result will vary according to the type of epoxy. Assuming a heat cure epoxy, then if you are heating from above only, you can expect spring back, as one laminate is being cured to a greater degree over another. A correction would be to include the mat in the vacuum layup (if possible) and apply additional heat from above in the normal way.

3. If i redo the mold for compensating this ... would it be the same if i switch to pneumatic later (assuming i have 0 deflection on frame) or will there be a great difference? (i dont care for a few mm's).

You might be lucky and its only a few mm. All things being equal, it really depends on whether the method of heating you use will be akin to that applied within the press.

Good luck. Enjoy the reading, if that floats your boat. If not, ignore it and have fun!

mammuth
Posts: 348
Joined: Tue Oct 28, 2014 3:48 am
Location: somewhere in the alps

Post by mammuth »

Thanks! I used a cold curing epoxy with post cure temper (20 degrees for 24 hours, then ramp up to 60 degrees). So it "should" have not too much influence. But who knows ;)

Anyways final target is pneumatic + heating maps. The Vacuum setup is ok but i think the pneumatic will make things easier ... and others more complicated
Tom

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