Vacuum bag press

[Giff]

Thanks for the explanation Custard. I did know about maximum pressure, which is quite possible with cauls and clamps, but not about minimum pressure. I have learnt a lot here, 2 main points being Slainte's pyramids and the electric blanket trick in cooler weather. Thanks everyone Geoff

 

[Steve Maskery]

Richard, I've been thinking about this.
Whilst I see exactly how this helps, I don't see how it is any better than the more usually practice of having a top caul over the work. I mean, how is having a pyramid of three (say) smaller size plates over the top more effective than having just one?

 

[Sgian Dubh]

Richard, I've been thinking about this.
Whilst I see exactly how this helps, I don't see how it is any better than the more usually practice of having a top caul over the work. I mean, how is having a pyramid of three (say) smaller size plates over the top more effective than having just one?

It seems to help apply pressure in the middle of the panel first, then the pressure spreads from there towards the panel edges. I've no scientific or engineering proof that this is actually what happens, but back in the 70s/80s when I used a two platen Interwood press with a very stretchy black rubber membrane (easily puncture too by the way), and a heated hood that rolled from one side to the other, this method became the default technique for us after some disasters without a stack of boards because the improved results spoke for themselves. I simply carried the technique forward into the more modern vac presses and vac bags reckoning the same thing would apply with with those system too.

I sometimes go along with the motto of "If it ain't broke, don't fix it", and this is one of those cases. Of course I could just be opening my mouth and letting my belly rumble to no good purpose, and someone might be along soon to show just how wet I am about the topic. Slainte.

 

[Steve Maskery]

It would be a brave man who did that, Richard. I'm still not sure I understand, and it seems that you are not certain either, but, as you say, if it ain't broke...
TVM
Steve

 

[Giff]

Isn't the" Pyramid technique" working on the stiletto heel principle but upside down ?

 

[Steve Maskery]

I don't think so, not really. It would be if pressure was being applied only to the top of the pyramid, but the air presses equally in all directions. That is why I have difficulty in understanding. Clearly Richard finds it beneficial, and he has done a lot more of this than I have. But I like to understand things and this is eluding me for the moment.
S

 

[Giff]

If you apply pressure to a point isn't this spread out through the pyramid. ie if you put a 10kg weight on a 1000mm board there would be very little pressure at the edges of the boards. If you do Richard's pyramid there may be less pressure on one given spot, but it would be even across the board?

 

[Steve Maskery]

But there isn't less pressure on one given spot, that's the point.

If you really were putting 10kg in the centre of the board, then what you suggest would indeed be the case, but that is not what happens with air. It's the same pressure all over, 14 PSI. The air is not pressing only on the top of the pyramid.

 

[Giff]

I thought you would say that ….I am definitely not a physicist..but I know a man who is. I am going to ask. Something to scratch his head about ! Geoff

 

[custard]

Richard, I've been thinking about this.
Whilst I see exactly how this helps, I don't see how it is any better than the more usually practice of having a top caul over the work. I mean, how is having a pyramid of three (say) smaller size plates over the top more effective than having just one?

It seems to help apply pressure in the middle of the panel first, then the pressure spreads from there towards the panel edges. I've no scientific or engineering proof that this is actually what happens, but back in the 70s/80s when I used a two platen Interwood press with a very stretchy black rubber membrane (easily puncture too by the way), and a heated hood that rolled from one side to the other, this method became the default technique for us after some disasters without a stack of boards because the improved results spoke for themselves. I simply carried the technique forward into the more modern vac presses and vac bags reckoning the same thing would apply with with those system too.

I sometimes go along with the motto of "If it ain't broke, don't fix it", and this is one of those cases. Of course I could just be opening my mouth and letting my belly rumble to no good purpose, and someone might be along soon to show just how wet I am about the topic. Slainte.

The pyramid idea makes sense to me, and it seems to directly address a key problem of a vac bag versus a rigid platten/press. If you veneer a flat panel in a vac bag and then cut it down the middle, you'll sometimes find the adhesive has pooled in the centre but is starved at the edges, so there's a thicker glue line in the middle. I'd agree that a pyramid will act like a curved caul. Seems a sensible idea and as it's easy enough to do I'll definitely be giving it a go.

 

[woodbrains]

Hello,

Actually, the pyramid IS a way of increasing pressure over the surface area of the ground. Think about it, if 14 psi is spread evenly over the whole surface area, then the pyramid sides have a bigger surface area than its base, but it transmits it all through the base, which is the area of the ground. The more layers to the pyramid, the bigger the surface area, so more pressure available to the ground, which remains the same. Simples!

Mike.

 

[woodbrains]

Hello,

Just done a rudimentary sum to illustrate. If the ground is a 10 inch square, it will have an area of 100 sq in. If we make a pyramid with a length of side of 10in, each pyramid side will have a surface area of over 40sq in. Four sides will give a total surface area of 160 sq in, all exposed to the atmospheric pressure of 14 psi. This will all be transmitted to the same 10 in sq ground. 160 sq in will press 2240 pounds over the ground, which could only exert 1400 by itself.

Mike.

 

[Steve Maskery]

Sorry, while your maths is accurate your assumption is flawed. The only force that matters is the downward force. Any sideways force is irrelevant. In your example, IIUYC, yo are calculating the face of an angled surface. You can only use that part of the vector which pushed downwards, not the sideways vector.
Not so simples!

 

[woodbrains]

Sorry, while your maths is accurate your assumption is flawed. The only force that matters is the downward force. Any sideways force is irrelevant. In your example, IIUYC, yo are calculating the face of an angled surface. You can only use that part of the vector which pushed downwards, not the sideways vector.
Not so simples!

Hello,

You're right, I was wrongly assuming that the increased surface area was condensing more force into the base, but it is countered by the force on the opposite pyramid faces.
Sadly, the pyramid idea is Bol€%¥#s!

Mike.

 

[marcros]

Could the answer lie in the order that pressure is applied- the top of the pyramid will clamp first?

 

[doctor Bob]

some serious witch craft and hob gobblingness going on here, I just put it in and switch on and put a lucky rabbits foot pointing north on top.

 

[Sgian Dubh]

some serious witch craft and hob gobblingness going on here, I just put it in and switch on and put a lucky rabbits foot pointing north on top.

I hope you put it on top of your iconic altar like pyramid thingy Bob to engender the full shamanic effect. Slainte.

 

[gavinr]

i can comment on bagpress... i have a bagpress system that was a virtually unsused and just few months old when i bought it but after about 10 months the pump failed. i called bagpress who said that it would have been in warranty had i bought it new from them, however as i bought it used their only offered suggestion was an all new unit from them but they would give me free post.

on the offchance i then called the pump manufacturer Becker to ask if there was any way they could help, they were really helpful, told me they had a repair department, and after talking with them they fixed it for 60quid (25 of that was their standard carriage charge for it to come back to me).

so in terms of good customer relations i simply cannot understand why bagpress did not suggest calling becker, in my book if you can't help its better to be helpful than try to grab a sale, and when you get down to it the bagpress system i have is nothing special, the fittings (at least on mine) are cheap plastic, the bag seal is electrical tube and conduit and the bag is a constant battle to keep the seams from leaking.