Laminate Gluing Pressure

[Overs Pianos]

>I am toying with the idea of laminating spruce ribs with Titebond 
>and providing clamping pressure with the vacuum bagging technique. 
>Of concern to me is the relatively low pressure attainable with 
>vacuum bagging. With a perfect vacuum, you will only get atmospheric 
>pressure - 14.7 psi. Del Fandrich has reported good bonding with 40 
>to 50 psi and the Titebond web site recommends over 100 psi. 
>Clearly, less than 15 psi is reason for concern. However, Ron Overs 
>reports that he is using vacuum bagging to manufacture his laminated 
>soundboard panels. I presume atmospheric pressure on the east coast 
>of Australia is about the same here in Florida. How does one reason 
>all this out - vacuum bag a rib and then watch it for 75 years?
>
>Terry Farrell

Hello Terry and all,

We are using 0.5 atmospheres of vacuum to glue the three laminates 
together to form the laminated panel. While this might not seem like 
a lot of atmospheric pressure to place on the panel, you must 
remember that a 2.8 mm thickness of spruce, on its own, flopps around 
like a piece of oversized cardboard. So this small vacuum is more 
than adequate for the purpose. Furthermore, if you calculate out the 
actual vacuum of terms of tons per sq. ft, it comes out to 0.47 
tons/sq. ft.

When gluing the panel to the ribs, we are using 0.25 atmospheres. 
However, the effective pressure at the interface of the glue joint is 
actually the sound board area divided by the rib glue joint area.

	Actual gluing pressure = (adjacent sound board area)/(rib area)

For example, if you were to use 0.25 atmospheres to glue a 25 mm wide 
rib set onto a panel at spacings of 125 mm, the psi. at the glue 
joint would be;

	Pressure = (0.25 x 14.7)/(125/25)

	Which equals = 18.3 psi

We could use a higher vacuum to glue the panel to the ribs, but the 
higher the vacuum the greater the risk of distorting the panel down 
between the ribs. We have destroyed test joints, after being glued 
together in the vacuum press, to check their strength. The results so 
far have been very good.

For those of you who haven't seen our vacuum press, go to;

http://overspianos.com.au/vacpress.jpg

Ron O.
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OVERS PIANOS - SYDNEY
      Grand Piano Manufacturers
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Web http://overspianos.com.au
mailto:ron@overspianos.com.au
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