bladders, bags and pressure

[fa]

The level of knowledge that is shared in this forum is really impressive and I truly thank you all for your posts.
I hope that if you could stand my terrible English, you ll find some meaning in the following simple thoughts concerning pneumatic pressing.

in the press
In a pneumatic press, an inflated tube squeezed between the top mold and the cat track, is forced to adopt an oval profile.
Considering that the tube has a fixed perimeter, this oval shape may be flatter or rounder,
depending on the distance between the mold and the cat track.
The flatter the oval gets, more area of the cat track comes in contact and gets pressed by the bladder.


The pressure (P1) which the bladder is applying on the cat track, is uniform and equal to the air pressure inside the tube.
But this is not the final pressure that the cat track will apply on the ski …
The pressure applied on the ski is related to one more factor, the contact area between the cat track and the ski, in other words the ski's width.
The final pressure on the ski (P2) is proportional to the ratio of the cat track's upper and lower area in contact, so it will be: P2=P1*d/ds
where P1=air pressure, ds=ski’s width, d= the length of the flat side of the tube's oval cross section.


So if tube diameter, air pressure, and ski width are given, the pressure on the topsheet is depended on the gap between the mold and the cat track.
The board below saws how (d) [the flat side of the oval cross section] is changing according to the gap (h) between mold & cat track, for a 15 cm diameter tube.




even pressure
Even pressure is considered to be a nice thing to have when pressing skis and snowboards.
Personally I think that not all of us really want a "very even pressure".
But does truly the pneumatic press ensures even pressure all over the ski's length?
Well, yes if you are pressing a ski with a constant width tip to tail -no sidecut, straight parallel sidewalls,
and if your top mold is designed to follow the cambered core's profile.
While pressing an average ski, let's say with a 130/86/126 width, the pressure in the waist sould be 130/86=1,51 times higher than the pressure in the tip.
On top of that, because very often the gap between the cat track and the top mold is reduced by a cm in the waist area (as the top mold generally follows the camber profile and not the core profile),
one should expect something like another 112% higher pressure in the waist (according to the board above a drop from 70 to 60mm => 141/126= 1.12).
Hmm…, if you are really after even pressure on your skis, you might start considering that after all,
a few cm of deflection in the middle of the upper beam might not be that bad…



vacuum
Vacuum bagging in its simplest form, where a bag is laid straight on top of the board or the ski, does ensure even pressure all over the pressed surface.
But in this setup, the pressure applied on the board is limited to something less than 1bar, unless the bag is placed in a pressurized environment, for example inside an inflated tube.



Isn't 1bar enough? Honestly I don’t have a clou, but in my mind the advantage of a press over vacuum is not just the higher pressure capability.
However, there is a way to increase the pressure on the ski/board inside a bag that is exposed just to the atmospheric pressure.
If a cat track is inserted between the bag and the pressed object, then the pressure on the object
will be proportional to the ratio of the track's width to the object's width.
So, if the cat track is projecting let's say half the ski's width each side, then the pressure on the ski will be doubled.



If not peel ply & bleeder is used, a cat track inside the bag might be welcome for 2 more reasons.
It will help air bubbles find their way out of the resin (as it leaves empty space along the side walls for the air and the extra resin to flow),
and it will give a nice flat finish in the top surface.
The more the cat track is projecting of the skis, the more rigid it should be.
The negative thing of a cat track inside the bag, especially if it’s a thick one, is that adds extra troubles
when one tries to provide heat to the laminates from outside the bag.

The charming thing about vacuum pressing is its great potential with the minimum means.
Can a piece of cheap PE plastic bag stand tall against the heavy press's frame?
Naaa, not a chance…

[EricW]

Every morning my bladder is a bag full of pressure.

Sorry, couldn't resist. That looks somewhat logical but my understanding of pressure doesn't fit with that. I could very well be wrong though. It seems to me that the pressure on the cat track wouldn't be transferred to the ski the way you describe in a vacuum. I think the track would simply have to bear the weight. The only pressure being transferred to the ski is what is directly in contact with the track.

Any math types out there care to weigh in?

[fa]

Every morning my bladder is a bag full of pressure.

lol!!!
Eric, think of a thumbtack
lets say its head is 10x10mm while the point is 1x1mm
then when you push that thing against the wall, the pressure on your thumb
will be 100 times lower than that aplied on the wall.
note that in my drawing the bag is not touching the underside of the cat track, just as the thumbtack head is not touching the wall.
if pressure is allowed under the track, then this area is not helping any more

cheers, faidros

[EricW]

I'm still not getting it. If I exert 10psi into that thumbtack it's still 10 psi no matter the area contacted with the wall, it's just a sharper point. I can shove a sheetrock saw through the wall but not a spoon when applying the same amount of force because the saw takes a lot less force to do so, not because there's more force at the tip.

this is an interesting conversation.

[OAC]

I would say, as little as possible above the ski in the bag. Just stuff that sucks up the redundant resin. Let the bag do the job.

[fa]

I can shove a sheetrock saw through the wall but not a spoon when applying the same amount of force because the saw takes a lot less force to do so, not because there's more force at the tip.

Eric,
the force is the same at tip and head
the pressure is different because the area's ratio is 1/100
the head is not made flat to put more force
its flat to ensure that the pin will go forward -in the wall, and not backwards in the thumb
i referred to the thumbtack because i saw that nail on your hand...
have to go now, ill give you an other example later

oac, just a bag is fine
for my toys 1bar is more than good
airbubles are a common pain when the resin blocks the bleeder around the sidewall
no wories if you make sure that the resine can flow free out of the laminate
but to get a nice flat finish on the top and cover the clothe's weave, that s another story
fa

[skidesmond]

This is sure to get a discussion going. But I would think you'd want even pressure over the ski. Assuming the hose/bladder and cat track cover the area of the ski and the top/bottom forms are mirror image, you'd get evenly dispersed pressure. Wouldn't you? The hose will conform to the shape of the forms, if done right. So at 40psi you'll have 40 pounds of pressure for every square inch of ski surface area in contact w/ the cat track or very close to 40.

I thought that was the reason for the cat track, to even the pressure across the ski surface area.

I know I've read this type of stuff before here in the forum because I was astounded by overall the force being applied, which is why the pneumatic ski presses need to be built super strong with steel.

[doughboyshredder]

nice work.

[Richuk]

Nice one!

[EricW]

according to my friend that's an industrial engineer, the only additional pressure you would get would be from the weight of whatever is on top of the ski.

[doughboyshredder]

I've never met an engineer that wasn't wrong about something.

In fact most engineers that I have met are completely clueless about most anything outside of their area of expertise.

If 1,000 lbs of pressure are exerted on a 1,000 square inch plate above a 1,000 square inch piece of wood then that piece of wood is receiving 1,000 lbs of pressure at 1 psi. If you remove that piece of wood and replace it with a piece of wood that is 10 square inches then that piece of wood is still receiving 1,000lbs of pressure, only now it's at 100psi.
Take it a step further. Replace that 10 square inch piece of wood with an egg. The contact point is so small that if your engineer friend was correct it wouldn't crack, because it would still only be getting 1psi on a surface area 1/10 of that. Of course the egg would explode and get egg all over his face.

[doughboyshredder]

of course, we are assuming that there is no deflection in the cat track.

[Richuk]

With the work done above, we are back to Fibre/Epoxy ratio as a measurable constant - accepting that their is more that can be done to maximise the effect of relevant press methods...which may lead to a greater understanding of what each method can actually achieve!

This is a method I will try. Essentially its a wet layup of relevant fibres cut to the size of the ski - so no overhang. It will be sandwiched between PE sheet, so it does not stick to anything. Weight Fibre before and then after, once trimmed any epoxy without fibre. Will need better scales perhaps ...

In the future I will need to discount for the fact that wood will absorb some epoxy, epoxy between the tip spacers and rubber sheet etc ... but its provide an accurate initial result.

Thks again fa!

[fa]

I'm still not getting it. If I exert 10psi into that thumbtack it's still 10 psi no matter the area contacted with the wall, it's just a sharper point. I can shove a sheetrock saw through the wall but not a spoon when applying the same amount of force because the saw takes a lot less force to do so, not because there's more force at the tip.

Eric, try that:
rest your arm with its own weight, on a nails head
now do the same on a nails tip
the force in both cases is the same: your arms weight
but that force may be spread over the heads greater surface area (lower pressure, no pain), or can be concentrated in the tips minute surface area (big pressure, yes pain)
pressure is defined as Force/Surface, so the same force exerted in a bigger surface area gives lower pressure than when exerted in a smaller surface.
pressure and force are often confused
theyare 2 different things, just related to each other


"Assuming the hose/bladder and cat track cover the area of the ski and the top/bottom forms are mirror image, you'd get evenly dispersed pressure. Wouldn't you?"
Skidesmond,
a common cat track has a rectangular shape, wider than the ski. Assume that a track like that, sits nicely in place in a constant distance for the top mold. In the uniform gap the hose will take an even shape tip to tail, touching equal area on every track’s member. The air pressure is uniform, all members are identical, so equal load is applied on each one of them.
If the ski has a constant width, even pressure is done tip to tail.
But if there’s a sidecut in the ski, the pressure will be higher on its waist than on its tip, as the same load will be distributed in a smaller area.
A way to achieve even pressure on the ski with the track kept in place, might be to alter the hose’s footprint on the track to match the skis outline (for example, by altering the gap where the tube fits).
But at the end of day, do we really care that much for “even pressure”?
Not me, for sure.

doughboyshredder, thanks!
"of course, we are assuming that there is no deflection in the cat track."
True. Stiff is good. Deflection on the track will mess things up.
about deflection on the track in the bag:
If deflection is such that the track touches the mold, more or less of the extra pressure gain is lost.
As long as the track is overhanging with out resting on anything else but the pressed ski, the extra load is still there pressing the ski, even if the track is deflected.
But then, the more the track deflects, the more the loads are gathering along the sidewalls.
Deflection is bad, uneven pressure distribution again…


Richuk, my pleasure. Glad if it was any helpful.
Following your great work through your posts was such a joy!

cheers, fa

[skidesmond]

This is where my little pea brain begins to struggle so bear with me... btw- I turned 50 recently ( I got my official FOG card in the mail ) so add that w/ my pea brain..... Its seems to me...

Wait a minute I think I just had an ah-ha moment... regarding pressure and force.... Let me back track for a moment.... From an observational point of view (because I'm not an engineer) I see the bladder exerting 40psi in all directions because my pressure gauge tells me so. So whether it's in a press, sitting on a table or floating in the air, it's exerting 40 psi.

Now assume we have the hose w/ 40psi in a press with a cat track measuring 72in x 12in. The cat track will have 34,560 lbs of force exerted on to it ( 72*12*40=34560) which will be directed to the ski. Assume the ski measures 72x5 which gives it 360 sq inches to absorb 34,560 lbs of force. The ski will be absorbing 96 psi (34560/360=96). Now we have a ski that measures 72x4=288 sq inches. This ski will be absorbing 120psi (34560/288=120). One more example, a ski measures 72x12 w/ 40psi (72*12=864, 34560/864=40) the ski will have 40 psi exerted on to it. Why? because the force in this example is always constant, 34,560.

Now, the formulas maybe more sophisticated than what I'm trying to describe here.. but the bottom line is yes a ski can have more/less forced on to it depending on it's shape.

DBS, FA- Are we saying the same thing, if so, I think I got it. If not I'm going to walk into traffic. (and delete this post as not to confuse others before J walk).