vacuum bagging canoe - part 3

[andrzej]

Notes:
A vacuum bag must be attached to an airtight surface. If your mold is not airtight (ie, mdf or strip plank) a vacuum bag can be used around the whole mold, in a technique called envelope bagging. This involves laying a vac bag (film only) on the backside of the mold, and sealing the top bag to it. Since canoe molds are large and unwieldy, this would be very awkward, and the outer bag easily damaged. I don't recommend this, I only mention it because it is possible.

A vac valve is a two piece coupler device used to apply vacuum through a bag. A metal plate goes on the inside, a hole is cut on top, and the top half attaches with an airtight gasket, usually with a half twist. If you do not have a vac valve, a vac hose can be inserted through the bag as such: on the bag at the entry location, apply a circle of sealant tape with an inside diameter equal to the O.D. of the vac hose. Carefully cut a hole through the bag in the center. Apply a loop of sealant tape around the circumference of the vac line, a few inches from the end. Push the hose through the bag until the tape on the hose and the bag meet. Squeeze tape together until sealed. Note that, inside the bag, the hose must end on the piece of breather in the pleat that I referred to in the previous post.

If a fast cure resin system is all that is available, you can still vacuum bag, but you must still get the bag on before the resin gels. Therefore apply the entire process to the first or second layer only. This will get time consuming and expensive, but it might be good practice (???). Use only non-coated peel ply in this case to avoid interlaminar silicone contamination. If there is no time to apply even one full laminate layer and bag it before the resin gels, I'd find slower resin, or skip the bagging procedure.

A good way to ensure full laminate cure before removing vacuum is to make a small laminate coupon on a work bench somewhere - a few square inches with an identical laminate schedule. When the coupon is cured, the boat is cured.

A partial solution for a slow vacuum source is to use a shop vac to draw down the bag. Before sealing the final pleat all the way insert a shop vac hose and pull as much air from the bag as possible. Then remove the shop vac and quickly finish the pleat seal. Apply vacuum source as normal.

I've assumed the use of a proper vac source - I know electric vac pumps can be expensive, but a good venturi vacuum generator can be had for less than $100. These connect to an air compressor, and can generate very good vacuum, albeit slowly. Do not attempt to use a shop vac or air compressor inlet as a vac source - these devices rely on airflow for cooling, and (if you do it correctly) a vacuum bag will ultimately have no airflow at all. There is a method for using an old refrigeration compressor however....

One last note - if using a vinylester (or some polyester) resins, you may experience styrene inhibition. This will cause you to remove the bagging materials and find what appears to be a sticky, undercured laminate. Fear not, with an hour or so of open-air circulation, it will cure tack free quite nicely. Assuming the resin was catalyzed and cured properly to being with, of course (see above note about a laminate coupon).

Bagging materials after the peel ply have a tendency to fall from vertical surfaces; keeping these dry materials in place can be a real pain. Vacuum bag manufacturers product various aerosol tackifiers that can be sprayed between layers to hold these materials together. It's similar to spray on contact cement.

Above all, I recommend practise. Get some bagging materials, even a yard of each, and practise bagging something simple, like a phone book, to a table top. Better to learn slowly, when you are not concerned with resin curing.

Whew. That's about it. I've covered everything I can think of. If anything sounds unclear to anybody, I'll see if I can elaborate. Maybe at a future date. I need some sleep.

If there's any interest in resin infusion techniques, I'd be happy to take a shot at composing a post on that, as well.