Darryl Keil Last Activity 2024-10-10 3:42 PM
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Art Frank

Posts: 3

Joined: 2003-07-28
Location: Chicago

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Subject : Plastic laminate onto MDF with yellow glue
Posted : 2005-06-29 9:57 AM
Post #32294
After constructing a 4'x5'x 3" torsion box covered with MDF I veneered plastic laminate to one face by rolling yellow glue onto the MDF, using 1 oz per sq ft. which provided an even, adequate layer of glue. On top, I used 1/2" MDF as a caul and slid it into the Vacupress bag. Left in the press overnight, it came out of the press well attached and without air pockets, but with a slightly scalloped or wavy surface accross the entire top. As I look across the top at a low angle, the projections are uniformly spaced at the approx location of crossmembers in the torsion box core. Before veneering, the top was uniformly flat and the torsion box assembly had been glued-up the day before.
What causes this?
P.S. I found this discussion on Woodweb:
"but veneer the MDF prior to gluing it to the cores (if you are bagging it) as the vacuum can collapse the MDF at that spacing" Sounds reasonable...
Thanks, Art


 
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Darryl Keil

Posts: 1453

Joined: 2003-05-22
Location: Maine

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Subject : RE: Plastic laminate onto MDF with yellow glue
Posted : 2005-06-29 2:44 PM
Post #32295 - In reply to #32294
Art,

Although I don't know the skin thickness or rib spacing of your torsion box I would say you had dishing between your ribs during pressing which after the laminate bonded to the substrate gave you the wavy surface. Either turn down the pressure a bit during pressing, use a thicker skin and thicker cover sheet, or place your ribs closer together.

If you can still use the other side as the main surface of your torsion box you should be able to salvage it. Do two things. Use a 3/4" cover sheet instead of the 1/2" one you used. Do a test run without glue but with the cover sheet, and place a straight edge across your torsion box. When you start seeing dishing, which you may not get with a thicker cover sheet, run you vacuum press at about 5"HG less than when you first noticed the dishing.

I am assuming you cannot get at the core anymore to add more ribs. If you can this would be the best solution.

Never underestimate the power of vacuum pressure.

Sincerely
Darryl Keil


 
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Art Frank

Posts: 3

Joined: 2003-07-28
Location: Chicago

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Subject : RE: Plastic laminate onto MDF with yellow glue
Posted : 2005-07-04 11:32 AM
Post #32300 - In reply to #32295
Thanks Darryl,

For the record, I used 1/2 Baltic Birch Ply for the core. The rib spacing was approx 8 1/2 on center in both directions. The skins were 1/2 ultra-light MDF.

Per your suggestion, I switched the cover sheet to 3/4 birch ply and continuing with 25 hg max pressure, pressed the laminate onto the other side. Although the results were signifigantly better, some slight telegraphing was still apparent in a couple of areas.
So, next time, would it be better to use standard 1/2 MDF for the skins, along with the 3/4 cover sheet and, or, lower the max hg to 20?
Thanks,
Art


 
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Darryl Keil

Posts: 1453

Joined: 2003-05-22
Location: Maine

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Subject : RE: Plastic laminate onto MDF with yellow glue
Posted : 2005-07-04 12:37 PM
Post #32301 - In reply to #32300
Art,

Its the rib spacing thats the main culprit. For 1/2" ultralight skins I would reduce my rib spacing to 4", but only in one direction. You can leave the 8 1/2" spacing in the other direction.

If you want to keep the larger rib spacing then I would turn down the vacuum to 18"HG max vacuum. You will have no problem glueing on a plastic laminate at that vacuum level. Whether you use regular or Ultralight MDF I dont think would make much of a difference.

Sincerely
Darryl Keil
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