I've done quite a lot of vac bagging for large models, wings and bits and
pieces of 49ers and other epoxy fixup projects. I find it works for me and I
love to work with epoxy in a nice clean bag. But it isn't a panacea for all
things that go wrong in a lay-up. I sometimes use the technique just as a
clamp with a plastic bag and light vacuum just to hold the lay-up on the
job. Especially if I am working with carbon which sometimes doesn't want to
bend around the job.
The advantage of the vacuum technique is that you get better control of
resin content. i.e. you can get rid of more resin especially if handling
multiple layers of reinforcement or both sides of a piece simultaneously.
This makes things very quick. Apart from the resin control, in a production
environment, it speeds the molding process and downstream processing. For
home builders like us its advantages come from removing excess resin.
Removing excess resin is done by using a piece of nylon woven cloth placed
over and directly in contact with the lay-up. Epoxy sticks poorly to nylon
and that sheet of nylon, called peel ply, can be removed when the epoxy is
at the leathery stage (depending on hardeners and epoxy 4-6hrs) after kick.
As the resin will pass through the peel ply you need something to absorb the
excess. This is the breather layer. I have use old towels and more expensive
felt, but anything that is absorbent, available and will let air flow freely
through (even when crushed) will do. Cover the whole lot with a sheet of
plastic and seal the edges, then a vacuum can be used to press the lay-up
down and express as much resin as possible.
Vacuum is not always necessary! Especially when making smaller parts. Some
resin will seep out (or even drip off if you do nothing!) into the peel ply
and breather layer if you tape wrap the lay-up. This is a method I use for
long thin things. You can use saran wrap as a kind of tape for larger
things. I do this if I am not really worried about weight. For example this
would be a good technique for transom bars, vang levers etc. Then you don't
have to have hanging weights etc. Sometimes if I want a nice thick layer of
resin on top or I use wound glass tape with selvaged edges that stick up I
use just an outer bag alone with a gentle vacuum. When it is removed the
job has a high shine and nice surface that contains adequate resin in the
lowspots, particularly around the tape edges. That is if I have removed all
wrinkles! This greatly reduces downstream work on something that just has to
look good, Perhaps center rails?
For one-off hulls its a lot of work and expense to make such a multi
layered setup. Peelply, breather and bag will almost double the glass cost
of the hull. Time to setup will be tripled. In addition, a larger pump is
needed and also additional breather layers (bubble wrap etc.). Vacuum
bagging of hulls is easier to accomplish in a female mold. It can also be
used fairly easily on strong male molds. It is complicated to use on
stripped hulls and could cause a massive failure if you have not much
experience with it. (I suggest bagging your bulkhead lay-up for starters if
you are determined to use this technique).
However, best resin control is obtained with vacuum. Moreover, It can hold
all the straggly bits of glass in the right place and make a lighter piece
of the same strength. There are other vacuumed techniques such as scrimp or
resin infusion that can only be applied to high volume production operations
because of the high overhead of the setup stage (read throwaway prototype
hulls). One can obtain stretchy bagging film that can be sealed on the
gunwales and allow the inside of our dish shaped hull to be bagged nicely.
Bagging the outside and inside of a stripped hull must be done in separate
operations making sure the vacuum cannot apply stress to the hull and cause
deformations. The hull must be sealed first. There must be an adequate
flange to bond and seal the outer ply. If you are going to do the inside of
the hull with this technique you might consider the film type peel plies
(perforations on the 1/2inch) which leave a little more resin in the
reinforcement but have little or no processing downstream as they leave
glassy smooth surface (with some nipples that can be knocked off).
I have a small "diaphragm and reed" pump that is adequate for medium size
jobs like rudders and centerboards. For larger or more important jobs I use
a "backing pump" which can be a vacuum cleaner (my wife loves that!) or shop
vac to get most all the air out and allow me to adjust/remove wrinkles and
get the setup right. I then turn on the reed pump and seal off the shopvac.
After a while a vac will develop ~ <=20 in Hg.
This has been a long post I hope it has helped. I do want to say, however,
that the weight limit of the boat is set for hand lay-up. It is already, by
design, a light boat (no excess structural components inside). Shaving more
weight by using difficult building techniques is 1) not really in the
interests of the class at the moment (IMHO) 2) possibly counterproductive if
you remove inertia that is useful for smooth tacking. 3) not going to matter
if the rules make you add lead spaced all over the deck to meet the Lamboley
test and minimum weight requirements.
The debate about weight will rage to be sure. Lets get the boats built and
support Bram and others in the efforts to reduce weight where it really
counts. The single biggest contribution to the speed and friendliness of the
boat will come from reducing weight aloft in the mast and sails. Mast weigh
reduction and the chop top main will go a long way to maintaining suction at
the top of the sail in real conditions, making the boat pleasant and steady
and fast. I am sitting on the edge of my seat literally, waiting for the
results from this weekends trials against the International 14s.
Greg
Swift Solo USA009 - "Rose"
49er USA023. "I've never bothered to name it".
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