larger pattern


A bigger pattern.


Making a customized plastic bag and covering it with fibre-glass sounds trivial but it took me a week of spare time to solves various problems I encountered doing this. In the end I changed my welding methods, my pressure regulator and the type of plastic I used before all my ducks lined up and could I fabricate the bits I need.

January 2009,


I've made an inflated pattern of approximately 3/8 scale of the vessel I plan to make. It turned out reasonably well except the end isn't as round as I'd like it to be.

This may not be the shape I end up using. The round end is for a hatch but this could also be done using a cone.

Hopefully tomorrow I'll do a thin hand layup of epoxy glass over the plastic to form a mandrel for a larger winding test.


This is what the pattern for the end-cap looks like.

Next Day - Disaster. I over-inflated it and popped the big end off. This is partly because I forget I had different grades of blue plastic and I made the end out on thinner material than I had intended.


The first pattern could not be saved and is now in the bin. This is version two which I made in 1/2 the time.
It is a little lumpy in parts put a lot stronger. I changed my welding method for joining the sections together. I learned some things which will make the big one easier so some good has come of this morning's failure.

I'm confident this one will hold. WRONG!!

Notes to self. Pressure test for longer before applying glass. Maybe reinforce the seams with something like duct tape.
Very little pressure is needed - 10 mBar is enough (1/100 of a bar - 10 cm of water).
More than a trickle of bubbles and my shunt reg isn't really working. Remember to clean the surface with solvent.
Mark important positions the plastic - the centre etc. Trace onto the glass before separating the plastic from the FG.

I'm cutting more corners than I would on the big job because these are just test pieces.
Making the end cap pattern is quite easy now - it is joining the cap, cylinder and cone together which is difficult.
I think I will just do the cap separately.
Provided the sizes match - I can join sections of the fibre-glass more easily than plastic.
This is something I wanted to try anyway.

Later, Made another end cap piece and successfully inflated it to 20mBar and covered it with a layer on epoxy facing tissue. I'm not totally happy with the shape - some of the pattern actually went slightly concave.
This may be improved by making the arcs used for welding the seams with a larger radius. This would also make a larger diameter.

Increasing the number of segments would also help - maybe 12 instead of 6.

I'm now inclined to think my problems with the first two patterns are largely due to bad pressure regulation and over pressure.

At the pressure needed a small centrifugal fan with accurate speed control may be a better option than the aquarium pump I'm using.
Air bed inflators should work but they are noisy and not intended to be run for days at a time.

New pressure regulator.

Some web searching indicated that some LPG regulators might be able to be adjusted down to the pressure I need. I had one on hand and found it was in the ball park but wouldn't go as low as I wanted. If I removed the adjustment spring it was a bit too low. I shortened the spring by binding some of it and this seems to work without having to destroy anything. It has only been tested inflating a sandwich bag but so far so good.

Image This is the end-cap after removal from the plastic. The cap is a layer of epoxy and facing tissue. This only needs to be strong enough to hold together while glued to the other sections and to support another layer of glass matt

Curiouser and Curiouser.

I made yet another cylindrical mould and pressure tested it for several hours.
A few seconds after I painted the seam with epoxy the seam popped. This blue plastic is clearly not as inert as I thought.
Why didn't even seam fail this way? Possibly because sometimes I cleaned the plastic with solvent and sometimes I didn't.

The next step is to repeat the process with clear builders plastic. This is what I've always intended to use for the bigger stuff. I used the blue stuff because is it thinner and more "to scale" for the smaller patterns.

I'm getting a little tired of this but hopefully I'm getting close to solving the problems.
A little test piece work fine.

Next day.
Another inflated cylinder, this time made of clear builder plastic. Inflated fine - no leaks. I covered the desired section with facing tissue+epoxy - it worked fine.


The pieces didn't fit together all that well but good enough for attempting to wind it.

Also see Inflated pattern and Filament winder one


Created by eddie. Last Modification: Wednesday 04 of February, 2009 16:58:52 AEDT by eddie.

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