vacuum bagging

[AndreyN]

Hi everyone!

Last week I start experiment with vacuum bugging.
To close peace of canvas inside the leak-tight bag, at one side of bag connect to the vacuum pump and other side of bag connect to vessel, with epoxy resin at temperature approx. 50 C. Turn on the pump and …. Epoxy soaking at 7 cm. length inside the canvas and stop soaking.

Anybody know a solvent for reducing epoxy viscosity?
Must I made some kind of channels inside the bag for promoting epoxy current through the canvas?

Andrey.

[Tim]

You need a breather fabric inside the bag; this is like a bubble wrap that allows resin to flow while still allowing full vacuum.

However, a standard vacuum setup is not designed to pull resin through the material, like a SCRIMP process. Standard vacuum pumps are for compressing fabric or cores into place, and for eliminating excess air and resin. To pull resin from an outside source into a vacuum bag and saturate the material is a whole different process.

[Tim]

Here's a link to a West Systems document discussing the possibility of thinning the epoxy resin.

http://www.westsystem.com/ewmag/14/pdf/ ... gEpoxy.pdf

[Tim]

Here's a link describing the SCRIMP process:

http://www.tpicomp.com/technology/advantage.htm

[Guest]

Thank you very much, for comprehensible answer!
I’ll come back after some experiments :-)

Andrey.

[JonnyBoats]

Tim, would you be willing to discuss the merits of the various fabrication methods?

If memory serves me, Lyman Morse uses the SCRIMP process, Hodgdon Yachts uses cold molded (well OK, it's not plastic, but they sure look classic) while other high quality builders use other methods.

I would also like to know which methods are more suited to large shops only as opposed to home use.

Thank you.

[Tim]

Cold molding is well suited to the home shop, and is one of the more commonly-used techniques for the home builder. Cold molding is a broad term that can cover several different building techniques, including strip planking and plywood construction, as well as the directional veneer-type construction, such as that used in the Hodgton projects and others. Cold molding is strong, relatively lightweight, and well worth considering for custom boats.

Vacuum bagging is well within the reach of the dedicated and serious home builder. Beginning kits aren't that pricey, and I think that it would make sense for someone involved in building a large cold-molded project to also invest in simple vacuum equipment, since cold molding involves wrapping the completed hull in fiberglass. The vacuum would help ensure good bonding and a better fiber/resin ratio. I would look at vacuum systems myself before getting into another large recore job or similar.

SCRIMP, pre-preg, and other similar methods are for commercial shops only, as each requires a large investment in special and expensive equipment. I imagine SCRIMP in particular must be very expensive to start up, so the ability to amortize the cost over many boats is required.

Hand lamination is still a viable and effective means of fiberglassing for most needs. The ultimate strength/weight ratio is lower than with the high-tech methods, but most boats are still build with basic hand-laminating techniques. With moderate care and experience, excellent results can be had.

[JonnyBoats]

I am sure I don't understand all the complexities, and certanily 60 ft hulls etc would require some big equipment.

I am wondering however why it wouldn't be possible to make things like dorade boxes and hatch covers etc at home using low tech equipment like a shop vac from Sears etc.?

[Tim]

I imagine you could, in theory. Usually, you need to pull something like 15" Hg (I think) to acheive a successful vacuum lamination. If you can do that with a shop vac, or similar, then fine. People have made good vacuum systems out of a wide variety of materials and parts from the scrap heap. Not being one who enjoys building machinery, nor having the skills to do so, I don't usually look toward this avenue. My time is best spent on those things that I know I can do, and I tend to leave certain machines for someone else to conceive.

One problem with something like a shop vac might be that the motors in these sorts of items are not meant for continuous duty. To pull a vacuum on a part, you'd need to run the motor continuously until the resin cured, usually 12+ hours (epoxy). I don't see a universal Shop Vac motor handling that. You could run hot polyester parts in fewer hours.

Frankly, for small parts, I see little benefit to vac-bagging. Its main benefits are realized when doing cored structural items, veneered or layered panels, and larger laminations where a more consistent pressure is desired. That said, the small parts would be fun to experiment on.