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PostPosted: April 18th, 2006, 11:06 pm 
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Joined: August 11th, 2005, 9:30 am
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Location: Barrie, Ontario
...continued from part 1

Once laminating is complete, vacuum bagging materials need to be applied in the following sequence:

Peel ply is a fine fabric, polyester or nylon, layed against the wet laminate surface. Its job is to not stick to the cured laminate, and to impart a fine surface texture to facilitate secondary bonding on the cured laminate. It is available in two forms - coated and non-coated. Coated peel plies employ a silicone treatment to assist with release upon removal. Theoretically the silicone can transfer to the laminated part, but this is not likely at room temperature, and not much of a concern for the average canoe builder. Suffice it to say that most any peel ply will work; the release coated should be easier to deal with. Peel ply must be layed over the entire surface. It need not be continuous, but pieces must overlap at least a few inches. The smoother it goes down, the smoother your final inner canoe surface. Be sure it lays flat against the laminate in all areas, as it will not stretch, even under vacuum. It must be laid all the way the edge of the laminate.

Next goes a perforated release film. This is a special plastic film punched full of evenly spaced small holes. While not technically necessary (and not included in a lot of vacuum bagging guides), I feel it is very useful. Its purpose is to control resin loss. I'll explain that more in the next step. It has the secondary benefit of allowing easier release of the entire vacuum bagging material stack on cure. The small holes allow air to pass, along with a finite amount of resin. Again, this layer must lay flat, following the peel ply as closely as possible.

Next is the breather/bleeder layer. This is a non-woven polyester felt type material, similar to quilt batting. It has two jobs - to allow even airflow under the vacuum bag, thereby assuring even evacuation of the bag and volatiles from the resin, and excess resin absorption. This absorption is controlled by the release film layer, for a couple of reasons - the breather is capable of wicking too much resin from an ideal laminate, causing dry spots, and a completely saturated breather layer will not allow air or volatiles to pass, negating the effects of the vacuum in that area. This is why I cautioned against excess resin - the release film will prevent it all from escaping (but this is better than the alternative of a dry laminate). Breather will stretch a bit, so laying it is a little easier.

After the breather layer (we're almost done), the vacuum bag itself. Vacuum bagging films are usually nylons, 1.5 - 2 thousandths thick. Although other plastic films can work, I don't recommend them, for instance, vapour barrier would have to be more than 6 thou. thick to be as airtight, and, at this thickness would be stiff and difficult to work with. Either way, bagging film is the least expensive of the materials used.
Standard bagging film comes in a 60" width off the roll. Other widths are available, but 60" is enough for most canoes. If you are doing a boat larger than this, make sure the film width is at least greater than the surface width at the point of maximum beam. Vacuum bag must fit the contours of the mold completely, but comes in a flat sheet, and must be continuous for an airtight seal. It is made to fit through a series of pleats - sealed folds in the film that take up the excess film in corners. For a canoe shape it is sufficient to make two pleats - one at each end, the full width of the film. (Note that stretch films exist which allow the use of fewer/smaller pleats, but these are fragile films, easily punctured, and a canoe shape will probably still require a pleat - they do not stretch that much.)
Cut the film to the full length of the mold. Start with one corner at the apex of the mold. Peel off a few feet of sealant tape backer, stick the corner of the film at the apex, and proceed to lay the edge of the film down on the sealant tape, as smoothly and with as few wrinkles as possible, around the canoe perimeter. Wrinkles are leak points, so try to avoid them. The film will only stick lightly to the tape if not pressed, so it may be pulled off with a sharp tug if you go awry. One seated smoothly and evenly, press down along the tape, pull off a few more feet of tape backer, and continue until you get to the opposite apex of the mold. Stop, go back to the original end, take the other corner of the film, begin at the original starting point, and cover the other side of the mold. At this point the whole flange should be sealed to the bag, with two large, open folds at the ends, excess bag due to the tapered shape of the mold. These folds will be the pleats.

Now the tricky part (to explain, anyway). Cut a piece of sealant tape slightly longer than the length of a pleat (in this case, about 32", half of the 60" film width plus a couple of inches). Beginning at one apex of the mold flange, attach one end of the free sealant tape, backer attached, to the sealant tape on the flange, where the two corners of the film meet. Hold the pleat taught (or better yet, have a friend do it), while you apply the new tape along the inner surface of one edge of the fold. Like the flange, you don't want any wrinkles in the bag. Follow along until you get just past the top of the pleat. Seat it to the film using your thumb and forefinger. Now, starting at the bottom near the flange, remove a few inches of tape backer and press the second side of the pleat to the first. Again, no wrinkles. Follow along slowly until you get to the end of the pleat. Pull out the rest of the backer, and if you've done it carefully, the end of the sealant tape should fold over on itself in the corner making a perfect seal. (Note that this, being the trickiest part, could be practised on a flat surface like a tabletop well ahead of laminating. In fact, I encourage this heartily.)

Now the opposite end. It goes the same with one exception. Cut a piece of breather a little more than the length of the pleat, about 4" wide. Cut an equal, length of release film, about 6" wide. Place the release film inside the bag, leading from the breather on the mold surface to the apex of the pleat. Lay the strip of breather on top. Place the plate half of a vacuum valve on the outboard end of this breather strip. If you have no vac valve, do worry, skip it, just make sure the breather does not fall back onto the canoe surface.
Repeat taping and sealing the pleat, as done at the other end of the boat.
At the vac valve plate, (very) carefully cut a slit at the top, and insert the mating half of the valve. If you are not using a valve, see a note below. Make sure all materials are still in place and start your vacuum. Depending on your pump, it may take a while to draw a full vacuum. I'll make a note about that in a minute.
Inspect the entire sealed perimeter, looking for wrinkles and folds in the film at the tape surface. Apply more sealant as needed to seal these. If you listen carefully, you should hear any leaks. Pay particular attention to the pleats and corners. Seal as necessary.

If all goes well, you should see the bag draw tight against the mold. Make sure that the bag is drawn tight against the full mold surface, particularly in the fine ends of the canoe. If the bag does not draw down tight it is either leaking, or is bridged. Leaks must be dealt with first. Bridging can be solved be removing the vacuum source until the bag ralaxes and carefully repositioning the bag until there is enough slack to fit the contour. Reapply vacuum and re-check. Again, the fine ends of the boat will likely be the problem areas.

Leave the vacuum source running until the resin cures.

now on to part 3....


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