Closing and Sealing Long, Thin Vacuum Bags
Half a dozen good ideas for how to wrap and seal long stair stringers for vacuum bag veneering, so you don't have to slide a long heavy piece into a long, slender bag. April 21, 2011
Question
I've recently become interested in vacuum bags and while I'm very happy with the result, I'm not so happy with the process. I do stairwork and want to use a bag for the curved stringers. This requires taking a multi-ply glue-up which may be 20 feet long and a couple hundred pounds and sliding the whole thing into a bag.
This made me wonder why bags are glued to form a seal instead of employing a Ziplock type of system so that the item can be placed on the plastic and then folded over and Ziplocked to make the seal. There may well be a good reason why it isn't done this way, but at first glance I can't see what it would be.
Forum Responses
(Veneer Forum)
From contributor J:
I think Ziploc-type seals require fancier machinery to form, and vac bags are probably made in smaller quantities with simpler equipment. I also suspect a Ziploc seal would be more prone to leak over the long term.
From contributor J:
I've done exactly what you are talking about with custom bags for really big tables. There was no zip lock, but double stick tape works great. You need a helper, but you already have a couple doing laminations like that.
From contributor B:
Boat builders do this all the time using a flexible sticky plumbers sealing tape. It is about 3/8-1/2" diameter and forms to whatever shape you press it into so wrinkles aren't as big an issue. Just lay out your plastic sheet do the glue spreading and fold the sheet over sealing it with the tape along one edge. It works beautiful if you can find the right stuff. It usually comes in long rolls.
From the original questioner:
I'll look into both options. Today I was sliding 10' stringers to the end of a 23' bag and cursing the process all the way.
From contributor A:
The tape is called mastic or bag tape. It is typically yellow and kind of a sticky silly putty. It comes in rolls. I don't have a large press table. So when I've had bigger glueups, I'll wax up a sheet of melamine and bag to it. You can also buy disposable tubes of different sizes. Seal the end with a bit of bag tape.
From contributor L:
I have used 3/4" PVC with a section of 1" thin wall PVC pressed over it. Cut the 1" PVC 1/2 to 3/4 of the circumference. Wrap the end of the vinyl bag over the 3/4" piece and snap the larger size over this. I believe someone sells this closure. Be careful not to tear the membrane. It might work to just roll the end of the membrane bag around the PVC and clamp it.
From contributor D:
I believe there is someone out there doing what you describe. I talked three or four years ago with a stair builder in CA who was having custom bags like that made up
Frankly, I'm so glad not to be clamping dripping bundles of veneers without a vac bag that I don't mind the logistics of sliding it all into a long bag. Are you wrapping the glued-up bundles with tape to hold them together?
Also try sliding a couple long 3/4" x 3/4" sticks into the bag to act as skids. You pull them out after inserting the glued up pack. Then hook up the pump, pull about 5" of vacuum and quickly taking out the hose put a piece of tape over the nipple.
The pack is nicely pulled together, yet flexible enough to place up on your bending form. I've bent with the gumby tape, and for some processes that is absolutely the way to go, but for stair stringers I'm well satisfied with our long bag. Also 200 lb. stringers - why so heavy?
From the original questioner:
Let me be clear that I've only just started using the bags for stringers, so I'm not speaking from much experience here. I've chosen a smaller project to start with in case there was a learning curve. Re-sawing, then book matching risers and stringers seems to work without any surprises.
I like the use of a small amount of vacuum to hold everything in place while setting to the form, as well as the 2 sticks for getting the bundle into the bag. I stretch wrap the entire bundle for glueups with clamps, but was worried about using it with vacuum. So far tape has worked well for the smaller things I've done. As for the weight I was thinking of one that is finishing up that is 4" wide and 23' long - I just couldn't see how to get that into a bag without destroying it.
From contributor O:
I get your concern with the tape. We use the clear packing tape. It will either stretch or break before it puts a crimp or buckle in the bend.
From the original questioner:
Today I received a couple of tube-style bag closures and they reminded me of Pex water line. I had some left over from when I had used it to pipe the vacuum lines throughout the shop. I found that if one was to cut a channel in 3/4", it could be snapped over 1/2" to create a seal. Since the pipe is flexible I believe it can be used as a side closure for a stringer bag. I'll try it out in the next few days.
