bent lam + vacuum bag ... needs help. i'm doing something stupid!

[Ben Fleis]

Hi,

I have a vacuum bag system which I've used on veneer and other sheet laminations, but never for a large scale bent lamination. This one is 2 meters long, 70mm wide by 110mm tall; 28 lams of ~4mm. (very roughly 3" x 5.5" x 6.5", 5/32" lams) My problem is simple -- the ends don't get pulled down by the bag system. I took some pics of my last dry run, but unfortunately my battery died on the last shot, so I can't get any more until monday.

It looks like the problem, somewhat visible in the 2nd pic, is that the bag sucks onto the mold end, and there's no "bag space" to remove, and therefore no inward pressure. In this pic I have clamps on the end, which I can use if needed, but I would prefer to understand how to do this with the bag, since I will need to make a few more of these in the future.

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The mold is show below. The dark brown is the fixed mold, the orange is a floating piece to keep all the lams front aligned. The red area marks what I was thinking to cut off, from both the floating piece and the fixed back. My thinking is that then the bag is directly pulling on those ends, and as long as I can pull the slack out during bag evacuation, that it should actually provide the needed inward pressure. My other thought was to make the top caul on the ends super tall.

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Can anybody tell me where I'm going wrong? This feels like something that should be simple, and that I'm making a fundamental mistake. Thanks,

ben

[john bateman]

I think for that to work the laminations might need to extend past the end of the form...just slightly.

[Jamie Buxton]

One thing you might do is to put the bending form outside the bag. The bag just serves to suck all the laminates together for a nice tight glue line. Use clamps, as big you want, to pull the workpiece to the form. This approach would allow you to get more pressure at the end of the workpiece than just atmospheric pressure.

[Thomas S Stockton]

Ben
I think part of the problem is that you don't have a way of getting the air out of the top of the lamination and it is sealing itself before you reach maximum vacuum. I see you have cut grooves for air on the sides and top of your form but it looks like the bottom piece is wider than the laminations which could seal the airways off also. I use a product called evacunet on top of my bent laminations works well at moving the air out, I don't know if it is available in Europe but I know people who have used burlap. Both vacupressing systems and veneer supplies carry the stuff so you can get an idea of what it is and maybe find an alternative because all it is is a flexible heavy screen like found in crafts shops for doing some type of needle point.
good luck
Tom

[Wes Grass]

I've never done any of this, but my gut feeling is yes, cut the ends of the alignment fences down like you've shown. And I'd think it would work better if the form extended past the ends of the lamination by several inches so the bag doesn't fold up there.

[Kent Chasson]

Ben
I think part of the problem is that you don't have a way of getting the air out of the top of the lamination and it is sealing itself before you reach maximum vacuum. I see you have cut grooves for air on the sides and top of your form but it looks like the bottom piece is wider than the laminations which could seal the airways off also. I use a product called evacunet on top of my bent laminations works well at moving the air out, I don't know if it is available in Europe but I know people who have used burlap. Both vacupressing systems and veneer supplies carry the stuff so you can get an idea of what it is and maybe find an alternative because all it is is a flexible heavy screen like found in crafts shops for doing some type of needle point.
good luck
Tom

Most people call it "breather mesh". Here's a link to the Joe Woodworker site http://joewoodworker.com/veneering/breather-mesh.htm

Another possibility is that you just aren't getting enough clamping pressure. You are lucky to get 14 psi with vacuum. Probably more like 10 - 12 psi.

[Ben Fleis]

Thanks for the suggestions. I ended up doing 3 things:

- cut the corners, as I showed in the diagrams
- add more air channels to my mold
- used end clamps

It worked quite well, aside from one of the clamps popping off, but once the shape was formed and the bag tight, it was locked into position.

-b

[Jeff Bratt]

Here's why you're having trouble... this is not a good application of vacuum clamping. Vacuum presses work really well for larger panels, but your bent lamination piece is relatively tall and skinny. Here's a similar bent lamination glue-up (chair rockers) that uses mechanical clamps - http://www.russfilbeck.com/Images/chairmaking/IMG_0874.jpg.

Compare the total clamping force (imperial units - you can convert)
a) mechanical clamps - 15 clamps x 600 lbs each = 9,000 lbs total
a) vac press - 80" (long) x 3" (wide) x 10 lbs/sq inch - 2,400 lbs total (12 lbs/sq inch = 2,900 lbs total)

You said you used mechanical clamps to hold down the end(s) - this is because you need more clamping force. In addition, the vacuum bag is putting even more pressure on the sides of the bent laminations than on the top, so you've also got 3,200 lbs of force squeezing on the sides of the laminations which impedes them bonding together. The mechanical clamps only apply pressure in the direction that you really want.

I'm glad you got it to work, but I don't think this is a very good setup setup for the size and shape the piece you are gluing.

[Jeff Duncan]

Yup, your form is too short. You want it to extend several inches past the parts. How much pressure are you using? When I'm using solid stock on a solid well made form I shoot for about 20in. That should easily pull your parts together for a nice clean glue up.

good luck,
JeffD

[Ken Masco]

I think you have to put a tall stand under the form so the bell shape starts after the end of the form.