Vacuum bagging from half a century ago...

[Gold Rock]

Check out this terrific vid:
"http://www.britishpathe.com/video/firefly-racing-dinghy/query/dinghy+sailing"><img

[jackster]

Thanks, Chuck, for that trip into the past; great stuff.
Interesting procedure of vacuuming and, apparently, autoclaving!
Not to mention the 'fine' varnishing technique!

[Mrleft8]

Excellent! I believe that method of construction was called "Hot molding" over here.

[JimConlin]

It wasn't vacuum, but steam pressure that pushed the bag against the lamination. The equipment was expensive and had largely been used in WWII aircraft like the DeHaviland Mosquito (http://en.wikipedia.org/wiki/De_Havilland_Mosquito). Molds were expensive, too.

[jackster]

It wasn't vacuum, but steam pressure that pushed the bag against the lamination. The equipment was expensive and had largely been used in WWII aircraft like the. Molds were expensive, too.

Interesting. I first thought that is was steam and pressure instead of vacuum, also.
But that would mean that the narrator had misspoken, yes? Possible, I guess.
Also, I could not figure why the heavy angle iron and machinist clamps were used. Can't think why such a heavy duty arrangement would be needed with steam and pressure?! But maybe.
And, why the heavy rubber assembly coat to begin with?
And then there is the hose hooked to the assembly just as they close the door to the autoclave. What the heck was that for? So many questions.
Any thoughts?

[Gold Rock]

I don't know exactly what the process was Jim. The film states that "a vacuum is applied until the cover is stretched skin tight over the entire surface". It may be that a vacuum is applied only prior to putting the hull in the oven and is used just to 'seat' the rubber cover as finely as possible. They do seem, however, to be taking care to seal the rubber cover rather well to the substrate upon which the hull is sitting and you can see a hose or hoses still attached to this substrate when the hull is positioned and left in the oven. Nonetheless, temperature and pressure within the oven are also noted in the narration so in retrospect I'd have to say that calling the process vacuum bagging, certainly as we all know it today, is a bit of a misnomer. That said, everything I know in the world about the process I gleaned from that short clip alone so..... yeah. Ha ha. I think it's a crackerjack bit of footage though. I love the speed at which the fellows work. They cleary knew their business well.

[Mrleft8]

I think that Luders used that same technique for the 16 for a while.

[Rob Hazard]

If the oven is pressurized they would need a hose leading from inside the rubber bag to exhaust the air so the pressure can compress the glue-up. The hose would have to lead to a vent exiting the oven.

[JimConlin]

As I've understood it, steam under pressure is supplied to the pressure vessel outside the bag.
The glue (resorcinol I think) reacts with the heat of the steam.
The interior of the bag might be sucked down with vacuum or vented to atmosphere.

[jackster]

If the oven is pressurized they would need a hose leading from inside the rubber bag to exhaust the air so the pressure can compress the glue-up. The hose would have to lead to a vent exiting the oven.
Rob,
Is this a statement or (educated) guess?
I have neither done or seen this method, but the film says that the oven IS pressurized.
I have done a fair bit of vacuum bagging, and not much air is extracted in the process, but no air must be allowed in.
I don't know, but it seems an elaborate means of venting to use that hose, but more reasonable, to me at least, to be a suction hose.
But just a guess on my part.
Another thing that comes to mind...
Why the steam, if the laminates are effectively sealed off from any contact with that very heavy rubber skin over the top and the male mold underneath? Hmmm?

[Mad Scientist]

The initial vacuum was intended to get all the air out from between the bag and planks, just like modern vacuum-bagged cold molding. And then, the steam heat is needed to make the glue 'kick'. The steam pressure helps squeeze the whole thing tight, too. The mold is open on the bottom, so the steam pressure pushes 'out' from the mold and 'in' from the rubber bag.

Tom

[peter radclyffe]

we put my fathers ashes in the river outside that yard,his parents had a boat on the hamble

[jackster]

Tom.
I take it that the 'Scientist" half of your moniker means you have some learnin' in this field! :)
Excuse my thick-headedness, but if I understand correctly, the layup is sealed enough to draw a vacuum initially, but open enough through the mold to vent out the steam pressure? That seems contradictory to me,. How do you draw and hold a vacuum when the mold is vented?
And the steam comes in contact with the laminates and the glue to help kick it off?

peter,
Small world, or you are well traveled. Must bring back some pleasant memories.

[John Meachen]

The vacuum bag should be sealed to the mould so that the veneers are pressed against the surface.The additional pressure of the steam further consolidates the laminate and it is the heat of the steam that promotes the curing of the adhesive.Not too dis-similar to the process used to make the composite parts of the Boeing 787 these days.

[Rob Hazard]

Jackster, my post was an educated guess, but I think John has it right. I have done some vacuum bag glue-ups too. The pump generates about 12psi of clamping pressure which is plenty for epoxy but probably not enough for the layups they were doing. Rescorcinol glue needs closer contact between layers. I don't remember if they told how much pressure that vessel gave them, but I'll wager it was a good deal higher than 12psi. You'd still need a hose to vent the trapped air out to atmosphere or there'd be a chance of a void in the laminate somewhere.

[Alan Christopher]

Thanks for posting this! Really interesting video. I especially appreciated the "green" aspect of the reusable rubber vacuum bag. I worked at a composite boatbuilder and we tossed hundreds of yards of mylar vacuum bag material into a dumpster every year. Every rowing shell we built required 2 pieces that were 5' wide x 27' long - one for the deck and one for the hull. Always made me feel really guilty!

[JimConlin]

Thanks for posting this! Really interesting video. I especially appreciated the "green" aspect of the reusable rubber vacuum bag. I worked at a composite boatbuilder and we tossed hundreds of yards of mylar vacuum bag material into a dumpster every year. Every rowing shell we built required 2 pieces that were 5' wide x 27' long - one for the deck and one for the hull. Always made me feel really guilty!
There are nylon and polyurethane vacuum bag materials that are OK for many cycles. For one-off parts that aren't too highly contoured, I use common polyethylene film from the big box. The peelply breather and bag tape cost a lot more than the bag.

[Alan Christopher]

Hi, Jim,
I see you're in Wellesley. I'm in Beverly, MA, and would love to be able to consult with you offline about a boatbuilding project I'm embarking upon. It's a 16' sailing dinghy to be done Stitch & Glue style. Any interest? I'm going to be posting the plans on this site in the next couple of days to get feedback on the design, but I've already made some tweaks based on advice from Paul Gartside. (Super nice guy, BTW.)

[JimConlin]

Hi, Jim,
I see you're in Wellesley. I'm in Beverly, MA, and would love to be able to consult with you offline about a boatbuilding project I'm embarking upon. It's a 16' sailing dinghy to be done Stitch & Glue style. Any interest? I'm going to be posting the plans on this site in the next couple of days to get feedback on the design, but I've already made some tweaks based on advice from Paul Gartside. (Super nice guy, BTW.)

Contact me off-list, pls.