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Ssonerai
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08-13-2019, 09:12 PM

Martin -
My apologies - I was posting the same time you were, then went out to close up the shop without reading your note immediately above mine.

Quote:
As I said, my homework is not done yet but from what I've seen I was thinking to put the whole thing in a bag. I think I understand what is the bleeder layer (fluffy material near the vacuum port to let the air bleed) but I have no clue about the peel layer purpose.
The peel ply is so the bleeder and anything else can be stripped easily, if the glue wicks through the top veneers. Or gets smeared on surfaces, as it often does. It is a slick material the glue can wick through, but not stick. The bleeder is so there are no sealed off airbubbles that can't be drawn down by the vacuum. It is a method to let the vacuum permeate thoroughly without restriction. Possibly not necessary on a part the width shown; but also will not hurt.

What happens with only the bag mebrane on a larger assembly, is the bag can seal itself to the surface in areas blocking others off, and no further vacuum can be drawn in them. The diffuser prevents the bag from sealing to the surface of the part. The peel ply prevents a non-synthetic bleeder/diffuser from sticking, as burlap would without the peel layer between it and the part, especially at the edges.

Quote:
Did you look at the material i want to use ? It's 7 ply 1/16 maple veneer kit. I wonder if bending perpendicular to the grain is a problem ? Probably not since the core layers have opposite grain but I'm not sure.
How wide is your part, and how long? I'm going to have to see what i think bending 7 layers to the inside curve. with half of them being long grain across the S-curve. A perfect vacuum is 14 PSI on most days at sea level, for practical purposes. Depending on your vacuum method perhaps you will net 12 or 13? That's actually quite a lot of pressure, but taking a stack of 1/16" maple veneers 1" wide, and estimating how difficult it is to bend them to the radius is often informative.

Another factor - leverage. If the veneers and form are only exactly the size of the part, it will be difficult to pull the edges down. The form needs to be wider across the bends, so there is leverage on the ends (edges, actually) to force them around the curve. Try pressing 4 layers of long grain tight to the form right up to the ends with your fingers. Then try adding an inch or so, and pressing . Even if the ends still don't go down to the extended radius of the form, they will easily do so further inboard, at the actual part dimension.

Quote:
I will also need some advice for the form. I've seen some example made of foam, I dont know if it's really suitable for my application.
That is a fascinating and innovative program that guy has developed!
I would be leery of trying 1/16" thick maple cross-laid into the short S-curve shown by your part on a foam form. That said, it never would have occurred to me to try it.

How thick is the actual part?

The hollow section of the curve is the difficult area to bend solid and true; all depends on radius and veneer thickness.

smt

PS: If you do try a foam form, it will have to be backed up by a solid flat or the net curve will be a result of the foam curving partially to accommodate the stresses induced in the veneer stack. It will probably curve both widthwise and lengthwise. I find it easier to shape wood than foam, but again, have not actually tried the latter.

Last edited by Ssonerai; 08-13-2019 at 09:19 PM.
  
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