Boat Forum / Building / January 2008

No personal experience. Here's an unqualified opinion:
epoxy bonds to PVC.   PVC pipe is a little wavy.
Taken together, I think this means removing the glass from the plug
would be tough.
Saran wrap is one avenue to reliable release I hear - or maybe a
good release agent - or aluminum foil I have heard suggested.....

Brian Whatcott    Altus OK

.

NO.

Depends.

Easiest solution is to glass pvc tube and leave it in place.

It what I did to make a shaft tube.

Next approach was using an 8" PVC pipe for a mold to fabricate a bow
thruster tube..

First you slit the PVC pipe on one side using a table saw.

Then you plug the ends of the slit tube with wooden plugs.

Next, cover slit pipe completely with clear contact paper.

Support tube on each end.

Next, using light weight glass, say 17 OZ, double bias, wrap tube in a
continuous wrap until yo get req'd layers.

When cured, cut off plugs and break PVC pipe in pieces to remove.

Doubtful you can do this on a small pipe such as 2-1/2".

If you must remove PVC tube, the cover it with contact paper and wrap
2-3 layers of glass.

When cured, slit both glass and PVC tube with a saw, the spring
lamination open to remove PVC tube.

Repair cut in glass, then use a mold to finish layup.

I've done all of them.

Have fun.

Lew

Interestingly I've done pretty much exactly this recently. I think the
plastic sheet is essential and you need few wraps. One suggestion on another
forum was to put several long thicker strips of plastic under the wrap
...protruding from the end so they could be pulled out and give a little
room for the wrapped plastic to slide and twist as you remove the glass
tube.

Peter HK

It will release but only with great difficulty.  The GRP part will shrink as
it cures and that will make the release VERY difficult even though the GRP
is not stuck to the mould.  What we normally do is engineer some flanges
into the layup such that we have something to pull against because we will
be exerting great amounts of force with clamps, presses, wedges, compressed
air etc.  The part needs to be able to witstand these forces and you need
some way and place to apply these forces.  The flanges or metal chains laid
into the part can later be cut off.  Likewise, you may need to engineer some
sort of bearing piece in your PVC pipe.  Or maybe use a 6' long theaded rod
with nuts to apply force like a bearing puller.  Still, the part needs to be
strong enough to not buckle.

Instead of wrapping with plastic sheet, we frequently use clear packing tape
in 2" widths as a mould release.  You tape up the part with clear tape, then
wax then lay up.  Do not use masking tape - sometimes the wax on the masking
tape inhibits full cure of the poly resin.

The easist way would be to buy polystyrene foam rods of the correct diameter
from a craft store.  Tape up and lay up.  Melt out the foam with acetone or
lacquer thinner when cured and pull out the clear tape.  This is the most
stress free way.  Only problem is that the foam rod may deform (bend) during
curing.  Same as with PVC tubes.  .... I just re-read that you will be using
epoxy.  In that case, with foam rods, don't even bother with a release agent
or tape since epoxy resin will not dissolve your foam.  And if you are using
a low shrink epoxy, you will not get much deformation if at all.

Or do what Peter hk said.  That'll work.

Here's a tip - we've all been taught to "wax on, wax off".  I do it
different - "wax on, wax on, wax on" let the wax harden up and do not wipe
off.  Layup over that.  The part comes up with alot of wax transfer and
swirls marks but those are easy to polish out.  I do this even when making
production molds because I refuse to risk a stuck plug and mold.

Arnold sg

You've had many good suggestions for dealing with the shrinkage of the
glass-epoxy laminate.  Here's one more.

With a tablesaw, cut one lengthwise slit in the pipe and cover the slit with
packing tape.
Wax the pipe thoroughly and do your layup.  Bond a couple of blocks or other
attachment points to one end the part.
Twisting the pipe vs. the part should break the bond between the pipe and
the part.
Drill the other end of the pipe for  some attachment and pull the part from
the pipe with a come-along.

Ok, Garland,

BTDT.....

If you are going to do this, do not wrap anything around the pipe,
without some other plan.  The wrapping will ball up when you try to pull
the two apart.  I have not tried the PeterHK plan, but I'm not sure how
well the 6' strips will pull out.  I even tried a teflon family sheeting
in one try and it was still tough.  That one had a stainless mandrel
because the mandrel would later be the rudder shank.

Cut the mandrel pipe too long so you can hold it.  Make the layup too
long and fabricate some feature at the end to allow good purchase.

Shrinkage is epoxy specific, talk you the manufacturer about both that
and the best mold release.

This is not going to be easy.  If you can create a mandrel that can be
unloaded, that will be a great advantage.  I have never figured out how
to do this without a surface artifact that leaves an internal
longitudinal ridge in the layup.

Good Luck Guy

Matt Colie

I want to thank everyone who has given me all this good advice. I suspected
it might be a problem, so I thought I'd ask.

You have saved me a lot of frustration. Thanks again.

I don;t believe that you could get the glass off the PVC. I have made
fiberglass tubing by molding two halves inside PVC pipe split
lengthwise and using several coats of mold release it still took
significant prying to get the halves out of the molds.

Bruce-in-Bangkok
(Note:remove underscores
from address for reply)

After the goo sets up, running a source of heat inside the pvc could
soften it enough to pull out, or even to melt it out.  Epoxy wouldn't
be bothered by the heat required, I suspect.

Alternatively, perhaps a plug of wax wrapped in a seperator would more
easily be melted out.  It's called the "lost wax" process.

Or, a spiral coiled cardboard tube could be pulled out after set up.

When I relined my rudder tube, I used epoxy with graphite shaped with
a pvc pipe pulled out before setup and sanded inside after set up
using a balloon and sand paper.  I was worried that I might need to
cut a tooth in the rudder post and use it as a tool to bore out the
hole, or to use a hole saw and guide setup.

I am told the "proper" way was to install top and bottom rudder
bearings, but that would have cost quite a bit.

Seems to have worked out, no more jamming or juddering under moderate
loads.

Terry K

One solution that I have tried on a smaller scale.  Slit the PVC pipe down
one side on a table saw.  The pipe should close up a little so the kerf will
be a little less than 1/8".  Cut a strip of 3/16 or maybe 1/4" polyethylene
sheet maybe 1/2" wide and fit it in the kerf so that the edge is flush with
the pipe's outer diameter.  That will expand the pipe over it's normal
diameter. Wax everything and spray on a couple of coats of PVA mold release.
Mist coat first then a fill coat.

Wrap your glass.  If it is uni-directional, diagonally in both directions
then straight once or twice then a top ply of woven on the bias.  If you are
vacuum bagging you have to pull all the diagonal and bias plies TIGHT and
tape them well or the fiber will bunch up as the bag pulls it down.

Once it is set up, knock out the poly strip out of the kerf and whack the
outside of the tube lightly to break the release agent and let some air in.
The pipe will spring back to its original diameter and drop out.  If it
still sticks soak it in water to dissolve the PVA.

There was an article in Good Old Boat where the guy built a water lift
muffler. To do what you are asking, the guy wrapped one layer of glass
and epoxy around a well-waxed pipe. After it cured, he slit the glass
and removed it from the pipe. A little epoxy and he glued it back
together again and continued with the lay-up.
Article 3367 May/June 2007. Pages 33 - 36.