Vacuum press bent lamination...bag getting stuck between the work and the form?!

[David Kuzdrall]

I am dry fitting a series of 1/4" mdf panels over a bending form which is convex. I believe that i have plenty of vacuum to bend the mdf to the form but I keep getting the bag and breather mesh sucked under the edges which then prevents full contact with the form.

Any ideas on what is the best solution to this problem?

thanks,

[Bob Vavricka]

Things I have done to help prevent this. First I put some 1/4 inch masonite pieces on the side of my form that stick up a little less than the total height of my lamination. These were cut to the contour of the form and applied at the ends where I was having the same problem. Second, stop the vacuum occasionally and take time to stretch the bag and breather mesh out from under the form and laminations. Third and I think this helped the most, hand bend the lamination as close to the form as you can and use some plastic stretch wrap around the lamination and form to hold it there before putting it in the bag. This gives less space for the mesh and bag to suck under the lamination. I also glue up my lamination on some saran wrap and wrap the lamination with saran wrap so the glue that squeezes out doesn't get in the mesh. It will not generally stick to the bag, but it sure sticks to the mesh.
How long and wide are your laminations? I have done some about 7" wide and 66" long using 1/8" walnut strips to build a 1" thick cap rail for a crib I made recently. I was pleased with the results.

[Jamie Buxton]

One fix is to put the bending form outside the bag. You design the setup so that the form is outside the bag, and you use mechanical clamps and cauls to pull the bag-encased laminates to the form. Then you pull vacuum inside the bag to clamp the laminates tightly together. Because the laminates are already in place before you pull vacuum, the bag doesn't get caught between them.

[David Kuzdrall]

Things I have done to help prevent this. First I put some 1/4 inch masonite pieces on the side of my form that stick up a little less than the total height of my lamination. These were cut to the contour of the form and applied at the ends where I was having the same problem. Second, stop the vacuum occasionally and take time to stretch the bag and breather mesh out from under the form and laminations. Third and I think this helped the most, hand bend the lamination as close to the form as you can and use some plastic stretch wrap around the lamination and form to hold it there before putting it in the bag. This gives less space for the mesh and bag to suck under the lamination. I also glue up my lamination on some saran wrap and wrap the lamination with saran wrap so the glue that squeezes out doesn't get in the mesh. It will not generally stick to the bag, but it sure sticks to the mesh.
How long and wide are your laminations? I have done some about 7" wide and 66" long using 1/8" walnut strips to build a 1" thick cap rail for a crib I made recently. I was pleased with the results.

Thank you for the reply.

I was able to apply your wrap method to contain the mess and ended up making the form falsely larger by padding out the perimeter with scrap lumber; this gave the bag something to rest on an has kept it out from under the work.

But now I have a different problem...calculating the rebound of the material. I did make the forms a bit smaller in radius to compensate for spring back but I have seriously underestimated how much; is there data out there for this? I am doing a 3-layer laminate of 1/4" MDF with West System Epoxy as the glue.

Thx

[Steve Jenkins]

You don't say what the radius is but I try to make my plys thin enough that I can easily pull them to the form by hand. this will almost totally eliminate springback. 1/8" mdf is available and might be a better choice. there is also a bendable plywood(somtimes called wigglewood) that comes in 3/8 and 1/4' thicknesses and maybe others. It is easily bent and holds its shape well. You can get it in either 4x8 or 8x4 sizes.

[David Kuzdrall]

You don't say what the radius is but I try to make my plys thin enough that I can easily pull them to the form by hand. this will almost totally eliminate springback. 1/8" mdf is available and might be a better choice. there is also a bendable plywood(somtimes called wigglewood) that comes in 3/8 and 1/4' thicknesses and maybe others. It is easily bent and holds its shape well. You can get it in either 4x8 or 8x4 sizes.

The tightest one is a 25" radius and there is NO way to bend the 1/4" MDF like that by hand. The vacuum gets it down pretty good but I think the edges are not 100" on the form and create the spring.

Where can one find 1/8" MDF, I have only seen 1/8" HDF?

[Richard Coers]

You can also make the form and pieces longer (something like 10-12") than needed, adding more "leverage" to get the pieces tight to the form. Then trim the pieces to size after cured.